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Flat subduction can significantly influence the distribution of volcanism, stress state, and surface topography of the overriding plate. However, the mechanisms for inducing flat subduction remain controversial. Previous two-dimensional (2-D) numerical models and laboratory analogue models suggested that a buoyant impactor (aseismic ridge, oceanic plateau, or the like) may induce flat subduction. However, three-dimensional (3-D) systematic studies on the relationship between flat subduction and buoyant blocks are still lacking. Here, we use a 3-D numerical model to investigate the influence of the aseismic ridge, especially its width (which is difficult to consider in 2-D numerical models), on the formation of flat subduction. Our model results suggest that the aseismic ridge needs to be wide and thick enough to induce flat subduction, a condition that is difficult to satisfy on the Earth. We also find that the subduction of an aseismic ridge parallel to the trench or a double aseismic ridge normal to the trench has a similar effect on super-wide aseismic ridge subduction in terms of causing flat subduction, which can explain the flat subduction observed beneath regions such as Chile and Peru.

[Display omitted] • The Jurassic granite in the Korean Peninsula and Japan are grouped into two fields • Early- and late-stage magmatic fields due to steep and flat subduction, respectively • Early-stage magmatic fields are defined by positive ε Hf (t), higher T, arc character • Late-stage magmatic fields are defined by negative ε Hf (t), lower T, adakitic nature • The Hida Belt was located in the early-stage magmatic field during the Jurassic Jurassic granitoids in the Korean Peninsula and Japanese Islands are key for deciphering the tectonic evolution of East Asia and the tectonic correlation between them. Zircon U–Pb age dating of seven granitoids in Southwest Japan gives 247.3 ± 1.8 and 192–191 Ma for the Ebi Granites and 200.8 ± 2.1 and 184–183 Ma for the Hida Granites. On the east coast of the Korean Peninsula, 188–186 Ma, 183–177 Ma, 167.7 ± 1.5 Ma, and 173–166 Ma are obtained for the granitoids in the Gyeongsang Basin, Yeongnam Massif, Taebaeksan Basin, and Gyeonggi Massif, respectively. All granitoids show depletions in Nb, Ta, P, and Ti in whole-rock compositions, suggesting an arc environment. The studied granitoids can be divided into two types. The early-stage (∼201–183 Ma) granitoids from Southwest Japan and the Gyeongsang Basin are characterized by the absence of Precambrian inherited zircons, high ε Hf (t) values of +13.0 to −0.8, and higher crystallization temperatures of ∼750–830 °C. The later-stage (∼183–166 Ma) granitoids from the remaining areas have abundant Paleoproterozoic inherited zircons, low ε Hf (t) values of −13.9 to −25.0, and lower crystallization temperatures of ∼680–750 °C. The early-stage granitoids formed during a steep subduction regime under higher temperatures due to the heat supply from the asthenospheric mantle. In contrast, later-stage granitoids mostly have an adakitic nature and formed by the partial melting of garnet- and hornblende-bearing thickened Precambrian lower crust under lower temperature conditions during flat subduction. The Ebi and Hida Granites in Southwest Japan formed along the continental arc passing through the Gyeongsang Basin to the Dumangang Belt of the Korean Peninsula, indicating their paleopositions. A change in the subduction angle from steep to flat caused the migration of magmatism to the northwest or west throughout Northeast Asia, including Japan, Korea, and China. [ABSTRACT FROM AUTHOR]

Syn-subduction basins bear significant implications to understand tectonic evolution of any fossil subduction zone. The late Paleozoic to early Mesozoic (Paleo-Tethyan) tectonics of the eastern and southeastern Tibetan Plateau (i.e., the Sanjiang Orogenic Belt) is featured by ocean-continent subduction systems. A huge pile of volcanic-absent sedimentary succession developed in the middle segment of the Sanjiang orogenic belt, its age and tectonic nature remain unclear. Detailed geological mapping and zircon U-Pb dating results demonstrate that the early Late Triassic volcanic-absent succession comprises the nonmarine Maichuqing Formation in the lower part and the shallow marine Sanhedong Formation in the upper part. The Maichuqing Formation consists of coarse to fine-grained sandstone, siltstone and mudstone with abundant basal erosional surfaces, trough and planar cross-beddings, ripples, mudcracks, and plant fragments. The Sanhedong Formation comprises predominantly bioclastic limestones interlayered with marl, calcareous-muddy siltstone, and calcareous sandstone with abundant bivalve fossils. Syn-sedimentation deformation structures, such as slump folds and associated normal faults are common, suggesting intense tectonism during deposition. Synthesizing sedimentary data, paleocurrent and provenance results, combined with other available data, demonstrate that the volcanic-absent succession deposited within a retro-foreland basin along the rear part of the Permian-Triassic Jomda-Weixi-Yunxian arc in response to flat-subduction of the Paleo-Tethyan Ocean during the early Late Triassic time.

This study tests the hypothesis of Cretaceous Turn of Geological Evolution (CTGE). It uses the large dataset on mineral deposits of NE Asia compiled by the US Geological Survey in collaboration with Russian, Mongolian, Korean, and Japanese geological institutions. As predicted, the Triassic–Early Jurassic and Late Cretaceous–Paleogene geodynamic activities in NE Asia were simple, producing a relatively small amount of mineral deposits (94 and 132, respectively). In contrast, the greatly increased geodynamic activity around CTGE produced a huge amount of mineral deposits (288). The Jurassic–Early Cretaceous superplume-related melts were injected into accretionary wedges that formed along the Pacific–Eurasian margins, whereas adakitic and granitic magmas derived from the shallow slab and lower crust were intruded into the huge intracontinental region. The characteristic mineral deposits are represented by the unique Jurassic–Early Cretaceous plume-related Ti-Fe-V (+P + Cr-PGE + Au + diamond) ores. Other CTGE representatives are the porphyry Cu-Mo and Au (+Ag)-vein deposits, which formation, however, continued into the Late Cretaceous–Paleogene epoch. These deposits were generated by the slab- and crust-derived adakitic and granitic melts formed under influence of the expiring superplume and intensifying subduction. The Late Cretaceous–Paleogene epoch is indicated by a decreasing metallogenic activity in general, and an increasing role of subduction-related deposits in particular. [ABSTRACT FROM AUTHOR]

Paleozoic magmatic rocks are widespread in the western Middle Tianshan. Their petrogenesis can provide important insights into the geodynamic evolution of the southwestern Altaids. Here, we present an integrated study of U–Pb zircon geochronology and geochemical and Lu–Hf isotopic compositions for the Late Paleozoic shoshonitic Chorukhdairon pluton and genetically and spatially related quartz porphyry in the southern Chatkal–Kurama terrane, western Middle Tianshan. The Chorukhdairon pluton mainly comprises monzodiorite and quartz monzodiorite (first phase), quartz monzonite (second and main phase), monzogranite (third phase), and leucomonzogranite (fourth phase). LA–ICP–MS zircon dating yielded magma crystallization ages of 294–291 Ma and 286 Ma for the Chorukhdairon pluton and quartz porphyry, respectively. All the rocks possess high K2O content (3.29–5.90 wt.%) and show an affinity with shoshonite series rocks. They display similar trace element compositions characterized by the enrichment of large ion lithophile elements (e.g., Rb, Th, U, and K) and depletion of high-field strength elements (e.g., Nb, Ta, P, and Ti), compatible with typical arc magmatism. Combined with zircon Lu–Hf isotopic data, we suggest that the Chorukhdairon pluton was produced by partial melting of the enriched mantle, followed by fractional crystallization of pyroxenes, amphibole, plagioclase, biotite, and accessory Fe–Ti oxides, apatite, and zircon. The quartz porphyries are similar to highly fractionated I-type granitic rocks, and their parental magma could result from the mixing of different batches of mantle-derived magmas or magmas derived from the mantle and juvenile lower crust. Considering the continuousness of the Middle Carboniferous to Early Permian magmatism in the western Middle Tianshan and other regional geological data, we suggest that the Chorukhdairon pluton and related quartz porphyry probably formed in an oceanic subduction setting. Furthermore, the temporal and spatial evolution of the Paleozoic magmatism imply that the flat-slab subduction that was induced by the subduction/accretion of seamounts probably occurred beneath the Middle Tianshan during the Middle Devonian to Early Carboniferous, after which the southeastward slab roll-back occurred during the Middle Carboniferous to Early Permian. The late slab roll-back was responsible for the southeastward arc magmatism migration and magmatic flare-up in the Chatkal–Kurama terrane, western Tianshan, and led to the formation of arc-related extensional basins and significant crustal growth in the southwestern Altaids.

[Display omitted] • Five-stage deformation identified in the Amdo micro-block since Mesozoic. • Flat subduction occurred beneath the Amdo micro-block. • Two collisions between the Amdo micro-block and Qiangtang Block. The assembly of East Asia involved multi-directional convergence of several micro-blocks during the Mesozoic. However, the geodynamic and tectonic elements associated with the southwestern convergent tectonic system of East Asia involving the Bangong-Nujiang subduction-collision system remain unclear. The Bangong-Nujiang Suture Zone in central Tibet shows a prominent east–west-trending differentiation divided by the Amdo micro-block. Here we evaluate the structural architecture of the Amdo micro-block, together with stratigraphic and paleomagnetic data, geochronological and geochemical data on the multi-stage magmatic and metamorphic rock suites with a view to trace the subduction and closure history of the Bangong-Nujiang Ocean as well as the collisions among micro-blocks. We also present new data from outcrop structural analysis which suggest that the micro-block has undergone five episodes of deformation (abbreviated as D 1 to D 5), which are represented by respectively NW-striking isoclinal tight folds, NE-striking tight folds, E-trending asymmetric folds, V-type conjugate strike-slip shear zones, and top-to-the-southwest thrust faults. In the Early Jurassic, initial active collision occurred between the Amdo micro-block and the Qiangtang Block followed by rifting and back-arc extension of the Bangong-Nujiang Ocean south of Amdo and passive collision between the Amdo micro-block and the Qiangtang Block in the late Early Jurassic. In the Middle-Late Jurassic, the Bangong-Nujiang Oceanic lithosphere underwent flat subduction beneath the Amdo micro-block resulting in slab tearing along the western border of the micro-block, large-scale intracontinental orogeny and thrusting. The slab tearing induced eastward mantle and crustal flows, and along with the N-S-directed compression, V-shaped conjugate shear zones formed in the interior of the Amdo micro-block. The intracontinental destruction and deep mantle lithospheric delamination in East Asia triggered the coeval convergent drops of deep materials in late Mesozoic, which drove further collision between the Lhasa Block and the East Asian continent in the Early Cretaceous. [ABSTRACT FROM AUTHOR]

A new seismic model for crust and upper mantle of the south Central Andes is derived from full waveform inversion, covering the Pampean flat subduction and adjacent Payenia steep subduction segments. Focused crustal low‐velocity anomalies indicate partial melts in the Payenia segment along the volcanic arc, whereas weaker low‐velocity anomalies covering a wide zone in the Pampean segment are interpreted as remnant partial melts. Thinning and tearing of the flat Nazca slab is inferred from gaps in the slab along the inland projection of the Juan Fernandez Ridge. A high‐velocity anomaly in the mantle below the flat slab is interpreted as relic Nazca slab segment, which indicates an earlier slab break‐off triggered by the buoyancy of the Juan Fernandez Ridge during the flattening process. In Payenia, large‐scale low‐velocity anomalies atop and below the re‐steepened Nazca slab are associated with the re‐opening of the mantle wedge and sub‐slab asthenospheric flow, respectively. Plain Language Summary: Taking advantage of the abundant information recorded in seismic waveforms, we imaged the seismic structure of the crust and upper mantle beneath central Chile and western Argentina, where the oceanic Nazca slab is subducting beneath the South American plate. The subducted Nazca slab is almost flat at a depth of 100–150 km in the north of the study area below the Pampean region, where the Juan Fernandez seamount ridge is subducting as part of the Nazca slab. The slab steepens again in the south in the Payenia region. Our model reveals pronounced low‐velocity anomalies within the Pampean flat slab along the inland projection of the Juan Fernandez Ridge, indicating that the Pampean flat slab is thinned or even torn apart. A high‐velocity anomaly is imaged beneath the flat slab, representing a former slab segment that was broken off during the slab flattening process and was overridden by the advancing young slab. Our model suggests a causal relationship between the oceanic ridge subduction and the flat slab formation. In the Payenia region, the slab re‐steepening resulted in the re‐establishment of the mantle wedge and induced hot mantle flow below the slab, which are characterized by low‐velocity anomalies in the model. Key Points: A new seismic model for the crust and upper mantle beneath central Chile and western Argentina is presentedThinning and tearing within the Pampean flat slab is detected along the inland projection of the Juan Fernandez RidgeA relic slab is imaged beneath the Pampean flat slab, reflecting slab break‐off during the flattening process [ABSTRACT FROM AUTHOR]

The existence of historical flat slabs remains debated. We evaluate past subduction since 200 Ma using global models with data assimilation. By reproducing major Mesozoic slabs whose dip angles satisfy geological constraints, the model suggests a previously unrecognized continental‐scale flat slab during the Late Cretaceous beneath East Asia, a result independent of plate reconstructions, continental lithospheric thickness, convergence rate, and seafloor age. Tests show that the pre‐Cretaceous subduction history, both along the western Pacific and Tethyan trenches, is the most important reason for the formation of this prominent flat Izanagi slab. Physically, continuing subduction increases the gravitational torque, which, through balancing the suction torque, progressively reduces dynamic pressure above the slab and decreases the slab dip angle. The flat Izanagi slab explains the observed East Asian lithospheric thinning that led to the formation of the North‐South Gravity Lineament, tectonic inversion of sedimentary basins, uplift of the Greater Xing'an‐Taihang‐Xuefeng mountains and the abrupt termination of intraplate volcanism during the Late Cretaceous. Plain Language Summary: Flat subduction refers to a downgoing slab whose dip angle is smaller than 15°. Although we can observe the present‐day flat slabs through seismic imaging, it is harder to detect the ancient ones. In this study, we use global geodynamic models to simulate subduction during the past 200 Ma and find that there was a flat Izanagi slab beneath East Asia during the Late Cretaceous. This previously unrecognized flat slab provides an alternative and unified explanation for multiple synchronous geological events in East Asia, including the thin lithosphere to the east of the North‐South Gravity Lineament, inversion of sedimentary basins in the region, uplift of the Greater Xing'an‐Taihang‐Xuefeng mountains and the termination of intraplate volcanisms. We also evaluate different mechanisms for the formation of the flat Izanagi slab and find that the most important reason is the long‐lasting Mesozoic subduction along the western Pacific margin. Key Points: Global data‐assimilation models reproducing past subduction discovered a previously unrecognized continental‐scale flat Izanagi slabThe flat Izanagi slab caused the unique East Asian lithospheric structure, basin inversion and regional uplift during the Late CretaceousThe key mechanism of this flat slab is dynamic suction due to long‐lived prior subduction along the west Pacific and south Asian margins [ABSTRACT FROM AUTHOR]

Several mechanisms have been suggested for the formation of flat slabs including buoyant features on the subducting plate, trenchward motion of the overriding plate and the thermal or cratonic structure of the overriding plate. Analysis of episodes of flat subduction indicate that not all flat slabs can be attributed to only one of these mechanisms and that it is likely that multiple mechanisms work together to create the necessary conditions for flat slab subduction. In this study we examine the role of localized regions of cratonic lithosphere in the overriding plate in the formation and evolution of flat slabs. We explicitly build on previous models, by using time-dependent simulations with three-dimensional variation in overriding plate structure. We find that there are two modes of flat subduction: permanent underplating occurs when the slab is more buoyant (shorter or younger), while transient flattening occurs when there is more negative buoyancy (longer or older slabs). Our models show how regions of the slab adjacent to the sub-cratonic flat portion continue to pull the slab into the mantle leading to highly contorted slab shapes with apparent slab gaps beneath the craton. These results show how the interpretation of seismic images of subduction zones can be complicated by the occurrence of either permanent or transient flattening of the slab, and how the signature of a recent flat slab episode may persist as the slab resumes normal subduction. Our models suggest that permanent underplating of slabs may preferentially occur below thick and cold lithosphere providing a built-in mechanism for regeneration of cratons.

The present study provides new petrological and geochemical data of the dredged rocks from submarine volcanoes along the Andaman arc and describes the petrogenetic evolution of the arc system in terms of mantle wedge characteristics, nature and quantitative input of subducted slab components, and fractionation processes of precursor magma. The studied rocks include basaltic andesite, andesite, dacite and rhyolite. These volcanic rocks exhibit LILE, LREE enrichments and HFSE depletion, corroborating their generation through subduction processes. High abundances of Th/Nd, La/Sm (N) , LREE/HFSE than LILE/HFSE, LILE/LREE suggest a substantial contribution of sediments from the subducting slab over slab-dehydrated aqueous fluids to the mantle wedge. The 87Sr/86Sr-Ba/La mixing model suggests 0.6–0.8% addition of slab fluid (90:10 AOC: sediment fluid) to account for the fluid signature, whereas the 143Nd/144Nd-La/Sm (N) mixing model envisages ∼3–4% addition of sediment melt to the mantle source, reconciling the sediment signature in Andaman submarine volcanic rocks. The presence of N-MORB type mantle is attributed to the absence and/or inefficient convection of asthenospheric material from the Andaman back-arc basin to the mantle wedge. This ineffective convection can be equated with the flat subduction of the Indian Plate, caused by the convergence of the aseismic Ninety East Ridge. The non-modal batch-melting model suggests that 13–24% partial melting of the spinel lherzolite mantle beneath the Andaman submarine volcanic arc formed the parent magma. The crystallization model invokes up to 60–70% of fractionation of olivine, plagioclase, clinopyroxene, orthopyroxene, sanidine and magnetite in all the rock types with subordinate proportions of amphibole, biotite, apatite, ilmenite, and sanidine in rhyolites. The basaltic andesites, andesites and dacites do not show upper crustal input, while rhyolites indicate crustal contamination from an upper crust and/or arc crust. [Display omitted] • First report of Sr-Nd isotopic data for submarine volcanic rocks of Andaman arc. • Mixing model suggests 3–4% sediment melt influx over 0.6–0.8% slab dehydrated fluids. • Partial melting of metasomatized mantle wedge. • 20–70% degree of fractionation and generation of B-A-D-R spectrum. [ABSTRACT FROM AUTHOR]

The Circum-Pacific Late Albian–Cenomanian orogenic belts (including the Sikhote-Alin–Western Sakhalin belt) were formed as a result of the deformation of mainly epioceanic terranes as fragments of Jurassic–Early Cretaceous accretionary wedges with ophiolites and other fragments of oceanic crust, turbidite basins, and island-arc systems. To the west of the Sikhote-Alin–Northern Sakhalin belt and orthogonally, the previously consolidation structures include the Bureya–Jiamusi–Khanka fragment of the orogenic belt of the Late Cambrian–Early Ordovician consolidation of the Late Proterozoic–Cambrian complexes. Within this belt, four isolated outcrops of the Heilongjiang complex are mapped. This complex combines metamorphic rocks of the epidote–amphibolite and glaucophane–schist facies and represents a fragment of a Jurassic accretionary wedge. It was assumed that these outcrops marked a suture; in particular, they represent the remains of the closed Mudanjiang paleoocean, separating the original Jiamusi terrane (and the Bureya–Jiamusi–Khanka belt) located to the west of Central Asia structures. This paper provides data that indicate that the Heilongjiang complex does not mark a suture, but is an underground near-horizontal continuation of the marginal continental accretionary wedge of the Nadanhada–Bikin terrane (flat subduction model) brought to the surface at the antiform bending site. The unity of the compared parts of the accretionary wedge is emphasized by the close matrix ages, the similarity of detritus zircon populations, and similar composition and age of allochthonous inclusions (limestone, chert, Late Paleozoic, and Early Mesozoic basalt). One important common feature is that Late Paleozoic and Early Mesozoic basalts from allochthonous inclusions occur as the N-MORB and OIB types in both cases, without any suprasubduction volcanism traces in the matrix. The Heilongjiang complex forms, according to this interpretation, a tectonic window among the more ancient structures of the Jiamusi terrane. There is no need to assume the existence of a Mudanjiang Ocean to explain the formation of the Heilongjiang complex. The structural features of this complex and its bedding conditions can be explained by the flat subduction processes of the Pacific slab in the Jurassic and its deformation in the Early Cretaceous. [ABSTRACT FROM AUTHOR]

Abstract The Caribbean plate subducts beneath northwest South America at a shallow angle due to a large igneous province that added up to 12 km of buoyant crust. The overriding plate lacks volcanism and exhibits Laramide‐style uplifts over 500 km from the trench. Here, we illuminate the subduction structures through finite frequency teleseismic P‐wave tomography and connect those structures to the Laramide‐style deformation on the overriding plate. We use a new data set collected from the Caribbean‐Mérida Andes seismic experiment comprised of 65 temporary broadband stations integrated with permanent stations from the Colombian and Venezuelan national networks. We identify three segments of subducting Caribbean plate with one segment completely detached from the surface. The timing of the detachment aligns with other regional events, including the uplift of the Mérida Andes, about 10 Ma. Slab buoyancy post‐detachment likely resulted in recoupling with the overriding plate, reactivation of Jurassic‐aged rift structures and subsequent uplift of the Mérida Andes. Mantle counterflow over the broken segment induced by rollback of the attached slab likely contributed to the uplift of the Mérida Andes. We conclude that the northern limit of subduction lies south of the Oca‐Ancón fault, though the fault itself may be the surface expression of the boundary. The southern limit of subduction lies south of our study area.

The Caribbean plate subducts beneath northwest South America at a shallow angle due to a large igneous province that added up to 12 km of buoyant crust. The overriding plate lacks volcanism and exhibits Laramide‐style uplifts over 500 km from the trench. Here, we illuminate the subduction structures through finite frequency teleseismic P‐wave tomography and connect those structures to the Laramide‐style deformation on the overriding plate. We use a new data set collected from the Caribbean‐Mérida Andes seismic experiment comprised of 65 temporary broadband stations integrated with permanent stations from the Colombian and Venezuelan national networks. We identify three segments of subducting Caribbean plate with one segment completely detached from the surface. The timing of the detachment aligns with other regional events, including the uplift of the Mérida Andes, about 10 Ma. Slab buoyancy post‐detachment likely resulted in recoupling with the overriding plate, reactivation of Jurassic‐aged rift structures and subsequent uplift of the Mérida Andes. Mantle counterflow over the broken segment induced by rollback of the attached slab likely contributed to the uplift of the Mérida Andes. We conclude that the northern limit of subduction lies south of the Oca‐Ancón fault, though the fault itself may be the surface expression of the boundary. The southern limit of subduction lies south of our study area. Plain Language Summary: At the convergent boundary between the southern Caribbean and northwest South America the Caribbean plate dips, or subducts, beneath South America at angles less than 30° for up to ∼300 km inland before steepening. Such low‐angle subduction systems are unique because they produce mountains hundreds of kilometers inland. Using new data collected by the Caribbean‐Mérida Andes seismic array, which continuously recorded earthquakes for two years, we imaged the Caribbean plate to depths of 660 km and identified at least three segments, including a segment detached from the surface. The timing of the detachment ∼5–15 million years ago aligns with the uplift of the Mérida Andes of Venezuela ∼500 km inland. Both the attached and detached segments interacted with the underside of South America, compressing parts of it, and causing uplift of the Mérida Andes. The northern limit of subduction lies south of the Oca‐Ancón fault of northern Colombia and Venezuela. The southern expanse of subduction includes the Bucaramanga earthquake cluster, but the boundary lies south of our study area. By understanding how the actively subducting plate, the Caribbean, deforms the upper plate, South America, we can better assess seismic risk and understand similar systems that are no longer active. Key Points: At least three Caribbean segments subduct under South America, one detached under the Merida Andes. Detachment due to lithospheric weaknessPost‐break recoupling and rollback of slab contribute to uplift of Mérida AndesNorthern subduction boundary identified [ABSTRACT FROM AUTHOR]

Continents grow mainly through magmatism, relamination, accretionary prism development, sediment underplating, tectonic accretion of seamounts, oceanic plateaus and oceanic lithosphere, and collisions of island arcs at convergent margins. The modern Pacific–Rim subduction zone environments present a natural laboratory to examine the nature of these processes. The papers in this special issue focus on the: (1) modern and ancient accretionary margins of Japan; (2) arc–continent collision zone in the Taiwan orogenic belt; (3) accreting versus non-accreting convergent margins of the Americas; and (4) several examples of ancient convergent margins of East Asia. Subduction erosion and sediment underplating are important processes, affecting the melt evolution of arc magmas by giving them special crustal isotopic characteristics. Oblique arc–continent collisions cause strong deformation partitioning that results in orogen-parallel extension, crustal exhumation and wrench faulting in the hinterland, and thrust faulting–folding in the foreland. Trench-parallel widths of subducting slabs exert major control on slab geometries, the degree of coupling–decoupling between the lower and upper plates, and subduction velocity partitioning. An initially large width of the subducting Palaeo-Pacific Plate against East Asia caused flat subduction and resistance to slab rollback during the Triassic Period. These conditions resulted in shortening across SE China. Foundering and delamination of the flat slab during the Early Jurassic Epoch led to slab segmentation and reduced slab widths, followed by slab steepening and rollback. This pull-away tectonics induced lithospheric extension and magmatism in SE China during Late Jurassic – Cretaceous time. Melting of subducted carbonaceous sediments commonly produces networks of silicate veins in CLM that may subsequently undergo partial melting, producing ultrapotassic magmas. [ABSTRACT FROM AUTHOR]

Our field-based geochemical studies of the Triassic, Jurassic and Cretaceous granitoids on Hainan Island indicate that their magmas had different geochemical affinities, changing from alkaline in the Triassic through ocean island basalt (OIB) in the Jurassic, to calc-alkaline in the Cretaceous. We show that these changes in the geochemical affinities of the Mesozoic granitoids on Hainan and in SE China reflect different melt sources and melt evolution patterns through time. Our new geodynamic model suggests that: (1) Triassic geology was controlled by flat-slab subduction of the palaeo-Pacific plate beneath SE China. This slab dynamics resulted in strong coupling between the lower and upper plates, causing push-over tectonics and contractional deformation in SE China. Flat subduction-induced edge flow and aesthenospheric uprising led to the production of high-K granites, syenites and mafic rocks. (2) Slab foundering, accelerated subduction rates and subduction hinge retreat in the Early Jurassic caused rapid rollback of the downgoing slab. Strong decoupling of the upper and lower plates resulted in pull-away tectonics, producing extensional deformation in SE China. Decompression melting of the upwelling aesthenosphere produced OIB-type melts, which interacted with the subcontinental lithospheric mantle (SCLM) to form A- and I-type granitoids. (3) Segmentation of the palaeo-Pacific plate in the Early Cretaceous resulted in steeply dipping slabs and their faster rollback, facilitating lithospheric-scale extension and oceanward migration of calc-alkaline magmatism. This extensional deformation played a significant role in the formation of metamorphic core complexes, widespread crustal melting and development of a Basin and Range-type tectonics and landscape evolution in SE China. [ABSTRACT FROM AUTHOR]

Flat subduction refers to low-angle (<10°) or sub-horizontal subduction of oceanic slabs. Flat subduction is only recognized in ~10% of present-day subduction zones, but its impact on the behavior of the overriding plate is particularly strong. For example, flat subduction zones are typically associated with stronger earthquakes. The deformation caused by typical flat subduction will transfer from the trench to the overriding continental interior and form a broad magma belt. The formation mechanism of flat subduction has been linked to the relative buoyancy of subducted oceanic plateaus, overthrusting of the overriding plate, hydrodynamic suction, and trench retreat. However, these mechanisms remain debated. This paper systematically analyzes and summarizes previous studies on flat subduction, and outlines the possible geological effects of flat subduction, such as intracontinental orogeny and magmatism. Using examples from numerical modeling, we discuss the possible formation mechanisms. The most important factors that control the formation of flat subduction are associated with overthrusting of the overriding plate and the arrival of an oceanic plateau at the subduction zone. In addition, trench retreat is necessary to enable flat subduction. Hydrodynamic suction contributes to the reduction of the slab dip angle, but is insufficient to form flat subduction. Future numerical modeling of flat subduction should carry out three-dimensional high-resolution thermo-mechanical simulation, considering the influence of crustal eclogitization (negative buoyancy) and mantle serpentinization (positive buoyancy) of oceanic lithosphere, in combination with geological and geophysical data. [ABSTRACT FROM AUTHOR]

This paper investigates the causes of the Late Cretaceous transition from "Sevier" to "Laramide" orogenesis and the spatial and temporal evolution of effective elastic thickness (EET) of the North American lithosphere. We use a Monte Carlo flexural model applied to 34 stratigraphic profiles in the Laramide province and five profiles from the Western Canadian Basin to estimate model parameters which produce flexural profiles that match observed sedimentary thicknesses. Sediment thicknesses come from basins from New Mexico to Canada of Cenomanian–Eocene age that are related to both Sevier and Laramide crustal loads. Flexural models reveal an east‐to‐west spatial decrease in EET in all time intervals analyzed. This spatial decrease in EET may have been associated with either bending stresses associated with the Sevier thrust belt, or increased proximity to attenuated continental crust at the paleocontinental margin. In the Laramide province (i.e., south of ~48°N) there was a coeval, regional decrease in EET between the Cenomanian–Santonian (~98–84 Ma) and the Campanian–Maastrichtian (~77–66 Ma), followed by a minor decrease between the Maastrichtian and Paleogene. However, there was no decrease in EET in the Western Canada Basin (north of ~48°N), which is consistent with a lack of Laramide‐style deformation or flat subduction. We conclude that the regional lithospheric weakening in the late Santonian–Campanian is best explained by hydration of the North American lithosphere thinned by bulldozing by a shallowly subducting Farallon plate. The weakening of the lithosphere facilitated Laramide contractional deformation by focusing end‐loading stresses associated with flat subduction. Laramide deformation in turn may have further reduced EET by weakening the upper crust. Finally, estimates of Campanian–Maastrichtian and Paleogene EET are comparable to current estimates indicating that the modern distribution of lithospheric strength was achieved by the Campanian in response to flat subduction. Plain Language Summary: The western United States and Canada share an organized mountain belt which runs their length. However, in the United States there is also a series of isolated mountain ranges to the east of that organized belt. This difference is often attributed to the shallow descent of an oceanic tectonic plate beneath the western United States between about 90 and 45 million years ago (Ma), and steeper descent beneath Canada. However, it is unclear why this would produce the differences observed at the surface. We examine this question by reconstructing the strength of the North American tectonic plate from 90–45 Ma, by modeling its rigidity (resistance to bending). In the model we apply random loads and stiffnesses to observe how the modeled plate bends to form sedimentary basins and accommodate sediment accumulation. We then compare the results of this Monte Carlo model to observed sediment thicknesses in real basins to determine which set of parameters match observations. Our results are most consistent with weakening of the USA portion of the North American plate by incorporation of fluids from the shallowly descending oceanic plate. This weakening focused deformation in the region directly above the shallow plate leading to spatially limited deformation. Key Points: Monte Carlo flexural modeling shows temporal and spatial variations in effective elastic thickness of the North American lithosphere from Late Cretaceous–EoceneSantonian–Campanian lithospheric weakening is best explained by introduction of fluids to the North American lithosphere during flat subductionLithospheric weakening enhanced Laramide deformation by focusing end‐loading stresses associated with flat subduction [ABSTRACT FROM AUTHOR]

The ~545 Ma-old syn-collisional Otjimbingwe alkaline complex is composed of pyroxene-amphibole-biotite-bearing, mildly nepheline-normative to quartz-normative rocks ranging in composition from monzogabbro to monzonite, syenite and granite. The alkaline rocks have moderate to high SiO 2 (50.5–73.0 wt%) and Na 2 O + K 2 O (5.1–11.5 wt%) and moderate to low MgO (6.6–0.2 wt%) concentrations. All samples have high large ion lithophile element (LILE: Ba up to 4600 ppm) and high-field-strength element contents (HFSE; Zr: 155–1328 ppm; Nb: 16–110 ppm; Ta: 1.4–7.1 ppm and Hf: 4–24 ppm) and have strongly fractionated LREE patterns ((La/Yb) N = 14–51). The most primitive members lack significant negative Eu anomalies. Mantle-normalized multi-element diagrams show depletion in Ba, Rb, Nb (Ta), P and Ti. The alkaline rocks have moderate radiogenic initial 87Sr/86Sr ratios (0.7061–0.7087) and unradiogenic initial ɛNd values (−3.9 to −6.1). This isotope signature, associated with high LREE/HFSE ratios indicates that the parental melts were generated in enriched portions of the shallow lithospheric mantle, which was probably affected by previous subduction zone processes. In addition, correlations between Sr and Nd isotopes indicate that some of these variations result from combined crustal assimilation and fractional crystallization (AFC) processes. A new model of flat subduction is presented that explains most of the unsolved problems in the orogenic evolution of the Damara orogen, namely (i) the absence of early intrusive rocks with a clear subduction zone setting, (ii) the absence of high-pressure rocks such as blueschists and eclogites, (iii) the unusual distribution of igneous rocks with a clear predominance of granite and granodiorite and (iv) the need for a asthenospheric window during a classical subduction to explain the high T/moderate P granulite facies conditions in the overriding plate. Unlabelled Image • The syntectonic Otjimbingwe alkaline complex (Namibia) is ca. 545 Ma old. • Metasomatized mantle-derived melts are modified by AFC and accumulation processes. • The alkaline melts intruded during extensional tectonics during flat subduction. [ABSTRACT FROM AUTHOR]

We have estimated the DBML (depth to the bottom of the magnetic layer) in South America from the inversion of magnetic anomaly data extracted from the EMAG2 grid. The results show that the DBML values, interpreted as the Curie isotherm, vary between ∼10 and ∼60 km. The deepest values (>∼45) are mainly observed forming two anomalous zones in the central part of the Andes Cordillera. To the east of the Andes, in most of the stable cratonic area of South America, intermediate values (between ∼25 and ∼45 km) are predominant. The shallowest values (

A wedge subduction can initiate after a ridge subduction and is characterized by the young and thin part of the plate subducting first. A series of 2D geodynamical models were built to investigate the factors in the development of a young flat-slab during the wedge subduction process after a ridge subduction. The factors include the convergence rate, the half spreading rate of the ridge, the initial subduction angle, and the thickness of the overriding plate. The results show that a flat-slab generally forms before the slab age reaches 40 Myr, except in special cases, for instance, when convergence rate ≤ 1 cm/yr, the half spreading rate is low, or the initial subduction angle ≥ 55°. The wedge subduction slab is prone to flatten because the young wedge subduction slab is naturally accompanied by buoyancy anomaly and has the potential to flatten at the relatively low convergence rate. There is no evidence as to whether a flat-slab exists south of Chile Triple Junction. According to the simulation results, the tomographic evidence, and the oceanic age distribution, it can be inferred that, from 46.5°S to ∼50°S, the wedge subduction is in the initiation stage and the formation of a flat-slap is impossible. Near 50–52°S, the slab possibly flattens when the initial subduction angle is 20–30°. From ∼52°S to ∼55°S, when the initial subduction angle is 20–40°, a flat subduction is likely to occur. • A wedge subduction is easier than a common subduction to form a flat-slab. • The wedge subduction zone exists in a post-ridge-subduction area south of Chile Triple Junction. • There is no evidence as to whether there is a flat-slab south of Chile Triple Junction. • A flat subduction is possible south of 50°S. [ABSTRACT FROM AUTHOR]

Abstract: The geometric transformation of a descending plate, such as from steep to flat subduction in response to a change from normal to overthickened oceanic crust during subduction, is a common and important geological process at modern or fossil convergent margins. However, the links between this process and the metamorphic evolution of the exhumation of oceanic (ultra)high‐pressure eclogites are poorly understood. Here we report detailed petrological, mineralogical, phase equilibria, and secondary ion mass spectrometry zircon and rutile U‐Pb age data for the Dong Co eclogites at the western segment of the Bangong‐Nujiang suture zone, central Tibet. Our data reveal that the Dong Co eclogites experienced peak eclogite‐facies metamorphism (T = 610–630°C, P = 2.4–2.6 GPa) and underwent multiple stages of retrograde metamorphism. P‐T pseudosections and compositional isopleths of garnet define a complex clockwise P‐T‐t path (including two stages of decompression‐dominated P‐T path and one of isobaric heating), suggesting varying exhumation velocities. Combining previous studies with our new results, we suggest that the transformation from rapid to slow exhumation is dominated by the transition from steep to flat subduction. The flat‐slab segment, caused by subduction of buoyant oceanic plateau, led to an extremely slow exhumation and a strong overprinting of HP granulite facies at a depth of ~50 km at ~177 Ma. The slab rollback that followed in response to a substantial density increase of the eclogitized oceanic plateau resulted in another rapid exhumation process at ~168 Ma and triggered the formation of abundant near‐simultaneous or later magmatic rocks. [ABSTRACT FROM AUTHOR]

Collision with and subduction of an oceanic plateau is a rare and transient process that usually leaves an indirect imprint only. Through a tectonostratigraphic analysis of pre-Oligocene sequences in the San Jacinto fold belt of northern Colombia, we show the Late Cretaceous to Eocene tectonic evolution of northwestern South America upon collision and ongoing subduction with the Caribbean Plate. We linked the deposition of four fore-arc basin sequences to specific collision/subduction stages and related their bounding unconformities to major tectonic episodes. The Upper Cretaceous Cansona sequence was deposited in a marine fore-arc setting in which the Caribbean Plate was being subducted beneath northwestern South America, producing contemporaneous magmatism in the present-day Lower Magdalena Valley basin. Coeval strike-slip faulting by the Romeral wrench fault system accommodated right-lateral displacement due to oblique convergence. In latest Cretaceous times, the Caribbean Plateau collided with South America marking a change to more terrestrially influenced marine environments characteristic of the upper Paleocene to lower Eocene San Cayetano sequence, also deposited in a fore-arc setting with an active volcanic arc. A lower to middle Eocene angular unconformity at the top of the San Cayetano sequence, the termination of the activity of the Romeral Fault System, and the cessation of arc magmatism are interpreted to indicate the onset of low-angle subduction of the thick and buoyant Caribbean Plateau beneath South America, which occurred between 56 and 43 Ma. Flat subduction of the plateau has continued to the present and would be the main cause of amagmatic post-Eocene deposition. [ABSTRACT FROM AUTHOR]

We present field, U-Pb zircon geochronology and geochemical data from grey gneisses and amphibolites from Tróia Massif of Northern Borborema Province, NE-Brazil. U-Pb zircon dating of the gneisses indicate two different episodes of Archaean crust formation, represented by the Mombaça (2.85–2.77 Ga) and Pedra Branca (2.70–2.68 Ga) units. Gneisses from Mombaça unit have high content of SiO 2 (64.4–75.4 wt.%), Na 2 O (3.28–5.01 wt.%), Al 2 O 3 (13.7–17.2 wt.%) and low MgO (0.24–2.60 wt.%) and ∑HREE combined with (La/Yb) N ratios varying from 6 to 118. In general, these geochemical features are in agreement with patterns of Archaean TTGs younger than 3.5 Ga and high-silica adakites, generated by partial melting of hydrous mafic rocks having garnet as a residual phase. Although high-SiO 2 samples of the Pedra Branca grey gneisses have similar geochemical characteristics to those of Mombaça unit, most samples have higher values of MgO, Sr and compatible elements (Ni-Co) indicating contrasting sources and/or petrogenesis, and thus should be referred to TTG-like gneisses. The lower SiO 2 content of the Pedra Branca TTG-like gneisses coupled with higher values of MgO, Ni, and Co preclude direct melting of meta-mafic rocks, but is suggestive of interaction with peridotitic mantle components. The Mombaça TTGs are better classified as medium-pressure TTGs derived from melting of a garnet-rich, but plagioclase-poor amphibolite. We argue that the higher geothermal gradient required for the formation of these medium-pressure TTGs could be achieved in a flat subduction setting, explaining not only the its geochemical features but also the absence of a constituted mantle wedge. The residual garnet within the subducting slab coupled with eclogitization would raise the density and trigger the slab retreatment and deepening the subduction zone. This would allow the formation of an incipient mantle wedge, and continuous TTG magmatism would fertilize the mantle, that was subsequently involved in the petrogenesis of the younger Pedra Branca unit. Mafic dykes with E-MORB composition yielded a Paleoproterozoic age at ca. 2.03 Ga with inherited zircons from the host gneiss at ca. 2.85 Ga, indicating thus no relation with TTG magmatism. [ABSTRACT FROM AUTHOR]

Geochronological data from volcanic rocks aid in reconstructing the Silurian-Devonian evolution of the northern Appalachians of New Brunswick in the context of Salinic (Silurian) and Acadian (Devonian) orogenesis. Late Silurian to Early Devonian sedimentation, volcanism and deformation in northern New Brunswick is complex, and characterized by transgressive-regressive sedimentary cycles, local disconformities and angular unconformities, and two stages (ca. 422-419 Ma, and ca. 417-407 Ma) of typically bimodal, within-plate magmatism. These events overlapped the end of the Salinic orogenic cycle, related to the ca. 430 Ma collision of the Ganderian passive margin with composite Laurentia, and the beginning of the Acadian cycle, which is associated with subsequent (ca. 424-422 Ma) collision and underthrusting of Avalonian crust beneath composite Laurentia (now including the Gander margin). Following Ganderia--Laurentia collision, the earlier (Pridolian) stage of withinplate magmatism is interpreted to result from ca. 425 Ma detachment of the subducting Tetagouche backarc lithospheric slab and is linked to the Salinic cycle, along with local, pre-Acadian (Salinic C) deformation of Ludlow--Pridoli rocks. Break-off of the Tetagouche slab also led to rapid uplift and subsequent extensional collapse of the Salinic metamorphic core, resulting in latest Silurian--earliest Devonian D3 vertical shortening and flat belt formation in the Brunswick subduction complex. Precise ID-TIMS U-Pb (zircon) dating of volcanic rocks reveals the existence of a 2.2 myr hiatus in the stratigraphic record, corresponding to an interpreted latest Silurian--earliest Devonian erosional surface between volcanic rocks identified with the Salinic cycle (Dickie Cove Group and lower part of Tobique Group) and those associated with the Acadian cycle (Dalhousie Group). The disconformity coincides with a widespread marine regression recorded by roughly coeval deposition of shallow-water to intertidal redbeds across the northern Appalachians. Marine regression is closely followed by deposition of Lochkovian deep marine sedimentary rocks (turbidites) in the Seboomook foredeep. This regressive-transgressive sequence immediately precedes arrival of the northwest-migrating Acadian deformation front, and is interpreted as evidence of a migrating Acadian forebulge-foredeep system associated with flexural loading of Ganderian crust by the Acadian orogenic wedge situated farther southeast in southern New Brunswick and coastal Maine. Early Devonian volcanism and Acadian retro-arc foreland deformation are linked to flat-slab convergence of Avalonia after underthrusting of buoyant Avalonian continental crust began ca. 418 Ma. It is proposed that the latest Silurian--earliest Devonian disconformity and marine regression mark the beginning of the Acadian orogenic cycle, implying that, in northern New Brunswick, the effects of waning, late Silurian (Salinic) deformation temporally overlapped the early, far-field effects of Acadian orogenesis. [ABSTRACT FROM AUTHOR]

Stable isotopes combined with pre-existing40Ar/39Ar thermochronology at the Gavilan Hills and Orocopia Mountains in southeastern California record two stages of fluid–rock interaction: (1) Stage 1 is related to prograde metamorphism as Orocopia Schist was accreted to the base of the crust during late Cretaceous–early Cenozoic Laramide flat subduction. (2) Stage 2 affected the Orocopia Schist and is related to middle Cenozoic exhumation along detachment faults. There is no local evidence that schist-derived fluids infiltrated structurally overlying continental rocks. Mineral δ18O values from Orocopia Schist in the lower plate of the Chocolate Mountains fault and Gatuna normal fault in the Gavilan Hills are in equilibrium at 490–580°C with metamorphic water (δ18O = 7–11‰). Phengite and biotite δD values from the Orocopia Schist and upper plate suggest metamorphic fluids (δD ~ –40‰). In contrast, final exhumation of the schist along the Orocopia Mountains detachment fault (OMDF) in the Orocopia Mountains was associated with alteration of prograde biotite and amphibole to chlorite (T ~ 350–400°C) and the influx of meteoric-hydrothermal fluids at 24–20 Ma. Phengites from a thin mylonite zone at the top of the Orocopia Schist and alteration chlorites have the lowest fluid δD values, suggesting that these faults were an enhanced zone of meteoric fluid (δD < –70‰) circulation. Variable δD values in Orocopia Schist from structurally lower chlorite and biotite zones indicate a lesser degree of interaction with meteoric-hydrothermal fluids. High fluid δ18O values (6–12‰) indicate low water–rock ratios for the OMDF. A steep thermal gradient developed across the OMDF at the onset of middle Cenozoic slip likely drove a more vigorous hydrothermal system within the Orocopia Mountains relative to the equivalent age Gatuna fault in the Gavilan Hills. [ABSTRACT FROM AUTHOR]

Numerical experiments are used to investigate the thermo-mechanical controls for inducing flat subduction and why flat subduction is rare relative to normal/steep subduction. Our modeling results demonstrate that flat subduction is an end-member of a steady state subduction geometry and is characterized by a curved slab with a nearly-horizontal slab section. Intermediate cases between normal/steep and flat subduction appear to be transient in origin and evolve toward one of the stable end-members. Physical parameters inducing flat subduction can be classified into four categories: buoyancy of the subducting oceanic lithosphere (e.g., slab age, oceanic crustal thickness), viscous coupling between the overriding and downgoing plates (e.g., initial subduction angle), external kinematic conditions, and rheological properties of the subduction zone. On the basis of parameter sensitivity tests and the main characteristics of present-day flat subduction zones, positive buoyancy from either the young slab or the thickened oceanic crust is considered as the primary controlling parameter. Our results show that the possibility of flat subduction is directly proportional to oceanic crustal thickness and inversely proportional to the slab age. Furthermore, oceanic crust must be thicker than 8 km to induce flat subduction, when the slab is older than 30 Ma with an initial subduction angle of ≥ 20° and without absolute trenchward motion of the overriding plate. The lower the initial subduction angle or the thicker the overriding continental lithosphere, the more likelihood for flat subduction. The initial subduction angle is more influential for the development of flat subduction than the overriding lithospheric thickness, and a thick overriding lithosphere induces flat subduction only under the condition of an initial subduction angle of ≤ 25°, with a slab age of ≥ 30 Ma and without absolute trenchward motion of the overriding plate. However, when the initial subduction angle is increased to > 25°, no flat subduction is predicted. All the parameters are evaluated within the constraints of a mechanical framework in which the slab geometry is regarded as a result of a balance between the gravitational and hydrodynamic torques. Any factor that can sufficiently reduce gravitational torque or increase hydrodynamic torque will exert a strong effect on flat subduction development. Our results are consistent with the observations of modern flat subduction zones on Earth. [ABSTRACT FROM AUTHOR]

On the eastern margin of the Himalayan orogenic belt, the rapid uplift of the Namche Barwa metamorphic terrane and significant bending of the Yarlung Zangbo suture zone occur. The formation mechanism and dynamics of the Eastern Himalaya Syntaxis (EHS) is still debated. In order to better understand the deep structures beneath the EHS, we deployed 35 broadband seismic stations around the Namche Barwa Mountain. The data were integrated with existing datasets for a 3-D teleseismic P-wave tomography. The results demonstrate complex deep structures and significantly contrasting Indian subduction styles in the eastern Himalaya. In the western region of the EHS, the Indian slab flatly subducts under southern Tibet and might extend to the Bangong-Nujiang Suture. In contrast, a (north)eastward steep subduction occurred in the eastern region of EHS. The contrasting subduction styles result in tearing and fragmentation of the Indian lithosphere between the flat and steep subducting slabs beneath the EHS. Consequently, the hot asthenospheric mantle may rise through the slab window, which might further lead to the rapid uplift of Namche Barwa and the formation of EHS. The lateral variation in subduction/collision mode and slab tearing induced asthenospheric mantle upwelling is similar to that observed in the Hellenide and Anatolide domains of the Tethyan orogen. [ABSTRACT FROM AUTHOR]

The Laramide orogeny (~ 80–50 Ma) was an anomalous period of mountain-building in the western United States that occurred more than 1000 km inboard of the Farallon Plate subduction margin. It is widely believed that this orogeny is coincident with a period of flat (subhorizontal) subduction. However, the factors that caused the Farallon Plate to evolve from a normal (steep) geometry to flat subduction are not well understood. Three proposed factors are: (1) a westward (trenchward) increase in North America motion, (2) an increased slab suction force owing to the presence of thick Colorado Plateau lithosphere, and (3) subduction of a low-density oceanic plateau. This study uses 2D upper mantle scale numerical models to investigate these factors. The models show that trenchward continental motion is the primary control on subduction geometry, with decreasing slab dip as velocity increases. However, this can only create low-angle subduction, as the Farallon Plate was old (> 100 Myr) and denser than the mantle. A transition to flat subduction requires: (1) subduction of a buoyant oceanic plateau that includes an 18-km-thick crust that does not undergo metamorphic densification and an underlying depleted harzburgite layer, and (2) a slab break-off at the landward side of the plateau. The break-off removes the dense frontal slab, and flat subduction develops as the buoyant plateau deflects the slab upward. The slab suction force has only a minor effect on slab flattening, but the thickness of the Colorado Plateau lithosphere controls the depth of the flat slab. With a continental velocity of 4 cm/yr and a 400-km-wide oceanic plateau, flat subduction develops within 15 Ma after plateau subduction. The flat slab underthrusts the continent at ~ 200 km depth, eventually extending > 1500 km inboard of the trench. [ABSTRACT FROM AUTHOR]

The Halasu porphyry copper belt situated in the East Junggar is one of the major porphyry copper belts in Xinjiang Uygur Autonomous Region, northwest China. Copper and molybdenum mineralization occurs as disseminated sulfides or veinlets mainly in granodiorite porphyry and diorite porphyry, with the intense development of zoned alteration from potassic, through sericitic to an outer zone of propylitic alteration. New LA–ICP-MS zircon U–Pb dating reveals that magmatism in the belt can be divided into three periods during the Middle Devonian and Early Carboniferous, namely the pre-mineralization stage of 390 Ma, syn-mineralization stage of 382–372 Ma, and post-mineralization stage of 350–320 Ma. The syn-mineralization intrusions are calc-alkaline, whereas pre- and post-mineralization intrusions are shoshonitic and high-K calc-alkaline. The syn-mineralization intrusions are enriched in highly incompatible trace elements but depleted in Nb, Ta, Hf and Ti relative to the pre- and post-mineralization intrusions. Zircon trace elements analyses demonstrate a negative correlation between Ti-in-zircon temperatures and oxygen fugacity. Ore-bearing syn-mineralization granitoids are characterized by higher water content, oxygen fugacity and low temperatures with higher mineralization potential than pre- and post-mineralization ones. These characteristics, together with the geochemical signature of the intrusions, suggest that the ore-bearing porphyries are derived from relative high ƒH 2 O magma reservoir. The remarkably homogeneous Hf isotopic compositions (εHf(t) = 8 to 13) from syn-mineralization intrusions span over 10 m.y., suggesting the existence of a long-lived reservoir beneath Halasu belt during the Middle Devonian. All the intrusions have low initial 87 Sr/ 86 Sr values (0.703935 to 0.707172), high εNd(t) values (4.7 to 5.5) and young crustal model ages (650 to 750 Ma). Combined with the mantle-derived Pb isotope characteristics, the Sr–Nd–Hf data suggest that the parental magma was probably derived from flat subduction triggered partial melting of juvenile crust generated during subduction–accretionary process with no significant input of old crust, whereas pre-mineralization and post-mineralization intrusions are supposed to emplaced in immature island arc setting and post-orogenic setting, respectively. [ABSTRACT FROM AUTHOR]

The Cenozoic evolution of South American topography is marked by episodes of large-scale uplift and subsidence not readily explained by lithospheric deformation. The drying up of the inland Pebas system, the drainage reversal of the Amazon river, the uplift of the Sierras Pampeanas and the uplift of Patagonia have all been linked to the evolution of mantle flow since the Miocene in separate studies. Here we investigate the evolution of long-wavelength South American topography as a function of subduction history in a time-dependent global geodynamic model. This model is shown to be consistent with these inferred changes, as well as with the migration of the Chaco foreland basin depocentre, that we partly attribute to the inboard migration of subduction resulting from Andean mountain building. We suggest that the history of subduction along South America has had an important influence on the evolution of the topography of the continent because time-dependent mantle flow models are consistent with the history of vertical motions as constrained by the geological record at four distant areas over a whole continent. Testing alternative subduction scenarios reveals flat slab segments are necessary to reconcile inferred Miocene shorelines with a simple model paleogeography. As recently suggested, we find that the flattening of a subduction zone results in dynamic uplift between the leading edge of the flat slab segment and the trench, and in a wave of dynamic subsidence associated with the inboard migration of the leading edge of flat subduction. For example, the flattening of the Peruvian subduction contributed to the demise of Pebas shallow-water sedimentation, while continental-scale tilting also contributed to the drainage reversal of the Amazon River. The best correlation to P-wave tomography models for the Peruvian flat slab segment is obtained for a case when the asthenosphere, here considered to be 150 km thick and 10 times less viscous than the upper mantle, is restricted to the oceanic domain. [ABSTRACT FROM AUTHOR]

Subduction exerts a strong control on surface topography and is the main cause of large vertical motions in continents, including past events of large-scale marine flooding and tilting. The mechanism is dynamic deflection: the sinking of dense subducted lithosphere gives rise to stresses that directly pull down the surface. Here we show that subduction does not always lead to downward deflections of the Earth's surface. Subduction of young lithosphere at shallow angles (flat subduction) leaves it neutrally or even positively buoyant with respect to underlying mantle because the lithosphere is relatively warm compared with older lithosphere, and because the thickened and hence drier oceanic crust does not undergo the transformation of basalt to denser eclogite. Accounting for neutrally buoyant flat segments along with large variations in slab morphology in the South American subduction zone explains along-strike and temporal changes in dynamic topography observed in the geologic record since the beginning of the Cenozoic. Our results show that the transition from normal subduction to slab flattening generates dynamic uplift, preventing back-arc marine flooding. [ABSTRACT FROM AUTHOR]

Gold and silver ores in the vast majority of Andean high-sulfidation epithermal Au–Ag deposits occur at high present day elevations and typically 200–500 m below low relief landforms situated at 3500 to 5200 m a.s.l. Most deposits are middle Miocene and younger and include, El Indio, Tambo, Pascua–Lama, Veladero (El Indio belt, Chile/Argentina), Cerro de Pasco (Central Peru), Pierina, Lagunas Norte, Yanacocha (northern Peru), Quimsacocha (Ecuador), and the California–Vetas mining district (Santander, Colombia), jointly accounting for > 130 Moz Au resources. Slightly older examples are only preserved in the Atacama Desert and include the middle Eocene El Guanaco and El Hueso and the late Oligocene/early Miocene La Coipa deposits. The absence of Paleocene and older high-sulfidation epithermal deposits can be explained by limited preservation potential imposed by transpressional tectonics within overall contractile episodes and surface uplift. These conditions prevailed predominantly in segments of shallow-angle subduction of the Nazca or Caribbean plate below the South American continent, a tectonic setting also common for porphyry-style Cu (–Au, Mo) deposits. Stratovolcanoes are uncommon ore hosts and volcanic rocks coincident with mineralization are in most cases volumetrically restricted or absent, recording the terminal stages of local arc magmatism. However, dacitic domes are important at, e.g., Yanacocha and La Coipa. At Lagunas Norte, a small stratovolcano largely pre-dating but temporally overlapping with mineralization occurs immediately east of the deposit and volcanic sector collapse may have occurred during hydrothermal activity. Mineralization is typically located near the backscarp of pediments or the heads of valleys incising now high-elevation, low-relief surfaces. In the California–Vetas Mining District and El Indio belt, hydrothermal alunite ages become generally younger upstream along the incising valleys, indicating that hydrothermal activity and, by inference, ore deposition were facilitated by erosion. The lowering of the water table and reduction of hydrostatic and lithostatic pressure at these sites of high local relief are believed to have enhanced both boiling and mixing of magmatic with meteoric fluids, ultimately enhancing ore deposition. The host rock composition, permeability and location of the water table control the distribution of alteration zones and ore. Intermediate volcanic rocks are the most common ore-hosts but they typically pre-date mineralization by several Ma. However, high-sulfidation epithermal mineralization can be hosted in any conceivable rock type including high grade metamorphic rocks (California–Vetas mining district), significantly older plutonic rocks (Pascua–Lama) or quartzites (Lagunas Norte). Large vuggy quartz alteration zones and commonly oxidized low-grade large-tonnage mineralization are best developed in relatively permeable volcaniclastic rocks or hydrothermal breccia bodies, whereas coherent volcanic, plutonic, or metamorphic rocks may host fault- and breccia-controlled ores. The near-surface steam-heated zone can attain a thickness of several hundred meters in dry climates (e.g. Veladero, Pascua–Lama, Tambo) but is typically poorly developed and less than 20 m thick in humid climatic zones. The physiographic and tectonic settings of high-sulfidation epithermal deposits are distinct from low-sulfidation epithermal districts such as those of Patagonia, El Peñón (Chile) or Fruta del Norte (Ecuador). The latter range to significantly older ages (Jurassic to early Eocene) occur at mainly lower elevations and were emplaced in extensional settings. A temporal coincidence between uplift, erosion and mineralizing processes as well as a spatial and temporal association with porphyry style mineralization is not evident for these low-sulfidation districts. [ABSTRACT FROM AUTHOR]

Abstract: Southern Peru is located in the northern Central Andes, which is the highest plateau along an active subduction zone. In this region, the Nazca slab changes from normal to flat subduction, with the associated Holocene volcanism ceasing above the flat subduction regime. We use 6 s to 67 s period surface wave signals from ambient noise cross-correlations and earthquake data, to image the shear wave velocity ( ) structure to a depth of 140 km. A mid-crust low-velocity zone is revealed, and is interpreted as partially molten rocks that are part of the Andean low-velocity zone. It is oblique to the present trench, and possibly indicates the location of the volcanic arcs formed during the steepening of the Oligocene flat slab beneath the Altiplano plateau. The recently subducted slab beneath the forearc shows a decrease in velocity from the normal to flat subduction regime that might be related to hydration during the formation of the Nazca ridge, which in turn may contribute to the buoyancy of the flat slab. The mantle above the flat slab has a comparatively high velocity, which indicates the lack of melting and thus explains the cessation of the volcanism above. A velocity contrast from crust to uppermost mantle is imaged across the Cusco–Vilcanota Fault System, and is interpreted as the boundary between two lithospheric blocks. [Copyright &y& Elsevier]

Abstract: Nanling is the largest W–Sn mineralization belt in the world, the formation of which remains obscure. In contrast to most other deposits in the Nanling region, Dabaoshan is a polymetallic deposit, located in north Guangdong province, southeastern China. Porphyry Mo deposit was found in 2008 in the north part of Dabaoshan ore district. Here we report zircon and molybdenite ages and geochemistry results of zircon and apatite. Zircon U–Pb LA–ICP-MS dating shows that the porphyry Mo deposit formed at 167.0±2.5Ma (2σ), which is identical to the molybdenite Re–Os age for the ore deposit (166±1Ma) within error. These ages are marginally older than the major W–Sn mineralization event in the Nanling region (160±5Ma). Zircon grains associated with the Dabaoshan porphyry Mo deposit have high Ce(IV)/Ce(III) values (356–1300), which indicate high oxygen fugacity, likely associated with plate subduction. Apatite from the Dabaoshan porphyry has high and varied F with low Cl concentrations, suggesting that it formed in a F-enriched environment with high F/Cl components in the magma source. This is consistent with abundant high-F granites in the Nanling region. Chlorine is highly mobile at the early stage of plate subduction. In contrast, F is mainly hosted by minerals that are fairly stable at shallow depths, e.g., apatite, phengite, such that is much less mobile than Cl before phengite decomposition. Therefore, the F/Cl ratio increases with increasing distance from the subduction zone. Compared to the Dexing porphyry deposit to the northeast, the Dabaoshan porphyry has lower Ce(IV)/Ce(III) and high F/Cl. It is also about 5Ma younger than the Dexing porphyry Cu deposits. All these phenomena can be plausibly interpreted by slab rollback of the obliquely subducted Pacific plate in the Jurassic. We propose that the subducting slab reached the Dabaoshan region before ~167Ma, through a “flat” subduction regime, resulted in high oxygen fugacity in the magmas, which is followed by abrupt slab rollback. The onset of slab rollback is the most favorable event that leads to decomposition of phengite in the subducting slab because of abruptly elevated temperatures, releases F and consequently increases F/Cl in magmas. The Ti-in-zircon temperatures of the mineralized porphyry range from 608 to 641°C, with an average of about 630±15°C, whereas average temperature of the barren granodiorite nearby is 700±9°C. Considering that the Mo porphyry deposit appears as an egg shell surrounding the granodiorite and the geochemical behavior of Mo, we propose that the mineralization was related to the assimilation of metamorphosed sedimentary rocks by the granodiorite. [Copyright &y& Elsevier]

Abstract: South China is famous for its polymetallic deposits, with more than 50% of the world''s W and Sb reserves and 20% of the Sn reserves, as well as abundant Cu–Mo–Pb–Zn–Au–Ag, Ta, Nb, HREE and U. This special issue reports recent progress on the tectonic evolution, magmatism and ore deposits of South China, which leads to a more comprehensive understanding of the Mesozoic large scale magmatism and mineralization events: (1) South China has been located in backarc settings during multiple subductions since the Paleozoic, which may have contributed to pre-enrichments of the ore-forming materials. (2) The Pacific plate subduction was a principle factor that controlled the late Mesozoic large scale magmatism and mineralization in South China, through flat subduction, ridge subduction/slab tearing and slab rollbacks. [Copyright &y& Elsevier]

Abstract: In the São Félix do Xingu region, southern portion of Amazonian craton, voluminous well-preserved Late Paleoproterozoic volcanic and plutonic rocks are grouped in the Sobreiro and Santa Rosa formations. The basal Sobreiro Formation (1.88 Ga) is composed of massive andesitic to rhyodacitic flows and volcaniclastic facies that are high-K and metaluminous, geochemically similar to calc-alkaline granitoids of volcanic arcs. Fractional crystallization of magnetite + augite + magnesiohastingsite (up to 20 vol. %) and zircon + plagioclase in most evolved rocks controlled the magmatic evolution of this unit, under oxidizing condition. The Santa Rosa Formation (1.87 Ga) comprises: 1) massive, bedded, and foliated rhyolitic lava flows, and large dikes of banded rhyolite and ignimbrite; 2) felsic ignimbrite associated with thin unwelded ash-fall tuff and highly rheomorphic felsic ignimbrite; 3) felsic crystal tuff, lapilli-tuff, and breccia; and 4) stocks and dikes of granitic porphyry, and subordinate equigranular granite intrusions. This unit was in part erupted along major fissures and has A-type intraplate geochemical affinity, peraluminous composition, and transitional subalkaline to alkaline characteristics. The magmatic evolution was predominantly controlled by fractional crystallization of zircon + apatite + Fe-Ti oxides + feldspars (up to 30 vol. %) with subordinate biotite, probably under intermediate oxidizing conditions. Nd model ages (3.0–2.49 Ga) for the Sobreiro Formation strongly suggest its generation by mixing of mantle-derived and anatectic melts of Archean rocks beneath the volcanic sequences in the São Félix do Xingu region. The Santa Rosa Formation could have been originated from several Archean crustal sources (T DM 3.12–2.56 Ga), involving assimilation/contribution of juvenile mantle components. The integrated data point to a narrow transition from Andean-type subduction to a dominantly extensional tectonic setting for the volcanic and plutonic event of the São Félix do Xingu region. A stage of flattening in the subduction angle of the subducted plate during the Paleoproterozoic ocean–continent orogenesis in the Tapajós Gold Province might explain the volcanism and plutonism in the São Felix do Xingu region as consequence of the arc migration, resulting in the ∼2.0–1.87 Ga geochronological and metallogenetic zoning observed in southern Amazonian craton. [Copyright &y& Elsevier]

The Vinchina Foreland Basin, western Argentina, contains a ∼7 km thick nonmarine stratigraphy, chronologically constrained within the Mio-Pliocene (circa 19-3.4 Ma), and where distribution of Illite/Smectite interstratified phases has shown a progressive smectite-illitization progress (R0 → R1 → R3), is consistent with an incipient burial history. R0 represents randomly mixed-layered illite/smectite normally found at shallow depths, as this ordering is not stable at ∼120°C. In the Vinchina Basin, however, the R0 is still persistent at ∼7 km depth, and its appearance even in the deepest levels is consistent with previous interpretations of low burial temperatures based on thermochronologic studies of detrital apatites. The maximum paleotemperature estimation and basin depth imply geothermal gradient as low as ∼15°C/km, which allowed an estimate of heat flow values between 33 and 42 mW/m2, that would rise to between ∼40 and 51 mW/m2 when the sedimentation rate (thermal blanketing) is taken into account. These values were only reported for cold basins and represent a paleothermal state of a refrigerated lithosphere. We suggest the central Andes were dominated since the Miocene by heat transfer derived mostly from crustal contributions with a minimum input from the asthenosphere. This refrigerated lithosphere is typical of segments affected by flat subduction. Preliminary thermal models based on previous geodynamic approaches support our conclusions. [ABSTRACT FROM AUTHOR]

Abstract: Flat and steep subduction are end-member modes of oceanic subduction zones with flat subduction occurring at about 10% of the modern convergent margins and mainly around the Pacific. Continental (margin) subduction normally follows oceanic subduction with the remarkable event of formation and exhumation of high- to ultrahigh-pressure (HP–UHP) metamorphic rocks in the continental subduction/collision zones. We used 2D thermo-mechanical numerical models to study the contrasting subduction/collision styles as well as the formation and exhumation of HP–UHP rocks in both flat and steep subduction modes. In the reference flat subduction model, the two plates are highly coupled and only HP metamorphic rocks are formed and exhumed. In contrast, the two plates are less coupled and UHP rocks are formed and exhumed in the reference steep subduction model. In addition, faster convergence of the reference flat subduction model produces extrusion of UHP rocks. Slower convergence of the reference flat subduction model results in two-sided subduction/collision. The higher/lower convergence velocities of the reference steep subduction model can both produce exhumation of UHP rocks. A comparison of our numerical results with the Himalayan collisional belt suggests two possible scenarios: (1) A spatially differential subduction/collision model, which indicates that steep subduction dominates in the western Himalaya, while flat subduction dominates in the extensional central Himalaya; and (2) A temporally differential subduction/collision model, which favors earlier continental plate (flat) subduction with high convergence velocity in the western Himalaya, and later (flat) subduction with relatively low convergence velocity in the central Himalaya. [Copyright &y& Elsevier]

Abstract: It has been suggested that much of the lithopheric mantle beneath the Colorado Plateau was hydrated by the dehydration of the Farallon plate when it was undergoing low angle subduction during the Laramide orogeny. If correct, low angle subduction could be a viable mechanism for weakening laterally extensive regions of continental lithosphere, allowing such lithosphere potentially to be recycled back into the Earth''s interior and into the asthenospheric mantle wedge. To test this hypothesis, we model the release of water during prograde metamorphism of a flat-subducting Farallon plate by considering a thermal model for flat subduction and tracking open-system metamorphic phase equilibria. Our model indicates that significant amounts of water can be laterally transported ∼700km inboard of the trench, close to the width of the North American Cordillera. The amount of water released is shown here to have been large enough to influence the rheology of the overriding North American lithosphere and the potential for melting at its base. Anomalously high S-velocities in the lithospheric mantle supports our modeled calculations of laterally extensive weakening at the base of the continental lithosphere. [Copyright &y& Elsevier]

Abstract: The Late Archean Taihua complex, mainly composed of amphibolite and TTG and TTG-like gneisses, is volumetrically most important metamorphic rock suites scattered along the southern margin of the North China Craton (NCC). Zircon SHRIMP U–Pb dating shows two episodes (2765±13 and 2723±9Ma) of Archean magmatism in the Lushan area with distinct geochemical features. The early (2765±13Ma) suite (TTG-like gneisses) has low-SiO2 (52.5–66.1wt%), high-Mg# (0.47–0.68), low HREE (YbN =3.0–5.4) and Y (8.07–13.9ppm) with low to moderate (La/Yb)N (6.7–37.1) and Sr/Y (25.9–119.3). The younger (2723±9Ma) suite (TTG gneisses) has high-SiO2 (63.5–74.3wt%), low-Mg# (0.13–0.52), very low REE (YbN <1.8) and Y (<4ppm) with a wide range of (La/Yb)N (5.2–86.6), Sr/Y (71.4–949) and showing Eu/Eu*>1 (1.20–2.43). Both suites show pronounced negative Nb–Ta and Ti anomalies on the primitive mantle-normalized spidergram. The TTG-like gneiss suite has similar bulk-rock Nd and zircon Hf model ages (∼3.0Ga) with ɛ Nd(t)>0 (0.26–1.46), and is interpreted as resulting from melt of mantle interactions with the melts derived from partial melting of subducted ocean crust with a residual assemblage of garnet+clinopyroxene+rutile±amphibole, which favors subducted slab model for the late Archean TTG. The TTG gneiss suite has abundant relic zircons (2.95–2.80Ga) with ɛ Nd(t)<0 (−1.31 to −0.23), which is best interpreted as derived from partial melting of thickened lower continental crust with a garnet-amphibolite residue (garnet+amphibole±rutile). Significant high-pressure fractional crystallization (garnet±amphibole) and accumulation (plagioclase) are also required in the petrogenesis. The Lushan amphibolite with nearly flat primitive mantle-normalized trace-element pattern is interpreted to represent early ocean crust metamorphism. These observations suggest a possible model of late Archean crustal accretion from ocean crust to continental terrain in the southern North China Craton. [Copyright &y& Elsevier]

Abstract: We analyze the Pampean foreland (the Pampas) along the modern flat-slab segment of the south-central Andes between 31° and 33° South latitude and to the east of the Argentine “flat-slab” province, using flexural and gravity studies and computations of dynamic topography. Bouguer anomalies and flexural analysis predict a foredeep of ∼250km width and a peripheral bulge amplitude of ∼25m, which match the regional morphologies of the modern Argentine Pampean Plain. However, these studies do not account for the subsurface Miocene–Quaternary basin preservation, represented by sedimentary thicknesses >400m and with depocenters >400km eastward with respect to flexural models. The discrepancy suggests that two mechanisms, acting at different wavelengths, influence the Argentine Pampas. The basin preservation is likely the result of a large-scale geodynamic forcing. Models of mantle flow, driven by realistic flat-slab subduction geometry and density contrasts, reproduce the depocenter location and the wavelength of subsidence as well as most of the remaining amplitude. Nonetheless, more sophisticated studies (e.g. considering lateral viscosity variations in the mantle wedge) might help reduce the dynamic amplitudes and better reproduce the observed geological record. [Copyright &y& Elsevier]

North China craton destruction (i.e., NCC destruction) during the Early Cretaceous is typically considered within the context of continuous, westward subduction of the Izanagi /paleo-Pacific plate beneath eastern Eurasia (i.e., Andean-style subduction). However, geological evidence indicates intra-oceanic arc accretions along east Eurasia during the Early Cretaceous that are incompatible with Andean-style plate tectonics. Here we review oceanic terrane accretions along NE Asia during Cretaceous times from published magmatism, stratigraphy, and paleomagnetism. We synthesize an alternative 'intra-oceanic subduction'-style NE Asian plate tectonic model between ~15–40° N latitudes during the Early Cretaceous (130–100 Ma) and discuss implications for NCC destruction. Well-known NE Asian magmatism migrated >1000 km inboard to NE China during the Jurassic, and then >1000 km outboard during early Cretaceous (140–110 Ma). Early Cretaceous NE Asian igneous rocks include: (1) arc-related igneous rocks, (2) 132–99 Ma adakites in Japan and Sikhote-Alin, and (3) 145–120 Ma K-rich adakites in NE China. Roughly co-eval to these periods (130 to 100 Ma), intra-oceanic arcs accreted diachronously along the Sambagawa belts, SW Japan, Oku-Niikappu belts, NE Japan, and Kema and Kiselevka-Manoma, Russian Far East. Based on the adakite geochemistry and spatiotemporal overlap between the arc accretions and adakites, we link the NE China adakites to lower NCC crustal melting, whereas the Japan-Sikhote Alin adakites originated from oceanic slab melting. We show that eastern Eurasia-NW Panthalassan plate tectonics during the Early Cretaceous was more complex than generally recognized, involving intra-oceanic subduction zones and multiple oceanic plates. The Early Cretaceous-aged NE Asian adakites were generated within elevated mantle geotherms during 140–110 Ma slab rollback and 130–100 Ma intra-oceanic arc accretions. Oceanic mantle emplaced during these events replaced the NCC subcontinental lithospheric mantle with more juvenile mantle during final NCC destruction at 115 Ma. The more complete plate tectonic picture provided here suggests that NCC destruction models that rely on straightforward Andean-style subduction are likely oversimplified. Instead, future NCC destruction studies should include more complex geodynamics with intra-oceanic subduction and additional plates that will alter boundary conditions for geodynamic modeling, petrogenesis, and magmatic mixing models. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: Eocene to late Miocene magmatism in the central Peruvian high-plain (approx. between Cerro de Pasco and Huancayo; Lats. ∼10.2–12°S) and east of the Cordillera Occidental is represented by scattered shallow-level intrusions as well as subaerial domes and volcanic deposits. These igneous rocks are calc-alkalic and range from basalt to rhyolite in composition, and many of them are spatially, temporally and, by inference, genetically associated with varied styles of major polymetallic mineralization. Forty-four new 40Ar–39Ar and three U/Pb zircon dates are presented, many for previously undated intrusions. Our new time constraints together with data from the literature now cover most of the Cenozoic igneous rocks of this Andean segment and provide foundation for geodynamic and metallogenetic research. The oldest Cenozoic bodies are of Eocene age and include dacitic domes to the west of Cerro de Pasco with ages ranging from 38.5 to 33.5Ma. South of the Domo de Yauli structural dome, Eocene igneous rocks occur some 15km east of the Cordillera Occidental and include a 39.34±0.28Ma granodioritic intrusion and a 40.14±0.61Ma rhyolite sill, whereas several diorite stocks were emplaced between 36 and 33Ma. Eocene mineralization is restricted to the Quicay high-sulfidation epithermal deposit some 10km to the west of Cerro de Pasco. Igneous activity in the earliest Oligocene was concentrated up to 70km east of the Cordillera Occidental and is represented by a number of granodioritic intrusions in the Milpo–Atacocha area. Relatively voluminous early Oligocene dacitic to andesitic volcanism gave rise to the Astabamba Formation to the southeast of Domo de Yauli. Some stocks at Milpo and Atacocha generated important Zn–Pb (–Ag) skarn mineralization. After about 29.3Ma, magmatism ceased throughout the study region. Late Oligocene igneous activity was restricted to andesitic and dacitic volcanic deposits and intrusions around Uchucchacua (approx. 25Ma) and felsic rocks west of Tarma (21–20Ma). A relationship between the Oligocene intrusions and polymetallic mineralization at Uchucchacua is possible, but evidence remains inconclusive. Widespread magmatism resumed in the middle Miocene and includes large igneous complexes in the Cordillera Occidental to the south of Domo de Yauli, and smaller scattered intrusive centers to the north thereof. Ore deposits of modest size are widely associated with middle Miocene intrusions along the Cordillera Occidental, north of Domo de Yauli. However, small volcanic centers were also active up to 50km east of the continental divide and include dacitic dikes and domes, spatially associated with major base and precious metal mineralization at Cerro de Pasco and Colquijirca. Basaltic volcanism (14.54±0.49Ma) is locally observed in the back-arc domain south of Domo de Yauli approximately 30km east of the Cordillera Occidental. After about 10Ma intrusive activity decreased throughout Central Perú and ceased between 6 and 5Ma. Late Miocene magmatism was locally related to important mineralization including San Cristobal (Domo de Yauli), Huarón and Yauricocha. Overall, there is no evidence for a systematic eastward migration of the magmatic arc through time. The arc broadened in the late Eocene to early Oligocene, and thereafter ceased over wide areas until the early Miocene, when magmatism resumed in a narrow arc. A renewed widening and subsequent cessation of the arc occurred in the late middle and late Miocene. The pattern of magmatism probably reflects two cycles of flattening of the subduction in the Oligocene and late Miocene. Contrasting crustal architecture between areas south and north of Domo de Yauli probably account for the differences in the temporal and aerial distribution of magmatism in these areas. Ore deposits are most abundant between Domo de Yauli and Cerro de Pasco and were generally emplaced in the middle and late Miocene during the transition to flat subduction and prior to cessation of the arc. Eocene to early Oligocene mineralization also occurred, but was restricted to a broad east–west corridor from Uchucchacua to Milpo–Atacocha, indicating a major upper-plate metallogenetic control. [Copyright &y& Elsevier]

We present lithospheric-scale analog models, investigating how the absolute plates' motion and subduction of buoyant oceanic plateaus can affect both the kinematics and the geometry of subduction, possibly resulting in the appearance of flat slab segments, and how it changes the overriding plate tectonic regime. Experiments suggest that flat subductions only occur if a large amount of a buoyant slab segment is forced into subduction by kinematic boundary conditions, part of the buoyant plateau being incorporated in the steep part of the slab to balance the negative buoyancy of the dense oceanic slab. Slab flattening is a long-term process (∼10 Ma), which requires the subduction of hundreds of kilometers of buoyant plateau. The overriding plate shortening rate increases if the oceanic plateau is large enough to decrease the slab pull effect. Slab flattening increases the interplate friction force and results in migration of the shortening zone within the interior of the overriding plate. The increase of the overriding plate topography close to the trench results from (1) the buoyancy of the plate subducting at trench and (2) the overriding plate shortening. Experiments are compared to the South American active margin, where two major horizontal slab segments had formed since the Pliocene. Along the South American subduction zone, flat slab segments below Peru and central Chile/NW Argentina appeared at ∼7 Ma following the beginning of buoyant slab segments' subduction. In northern Ecuador and northern Chile, the process of slab flattening resulting from the Carnegie and Iquique ridges' subductions, respectively, seems to be active but not completed. The formation of flat slab segments below South America from the Pliocene may explain the deceleration of the Nazca plate trenchward velocity. [ABSTRACT FROM AUTHOR]

Abstract: Dehydration of subducting lithosphere likely transports fluid into the mantle wedge where the viscosity is decreased. Such a decrease in viscosity could form a low viscosity wedge (LVW) or a low viscosity channel (LVC) on top of the subducting slab. Using numerical models, we investigate the influence of low viscosity wedges and channels on subduction zone structure. Slab dip changes substantially with the viscosity reduction within the LVWs and LVCs. For models with or without trench rollback, overthickening of slabs is greatly reduced by LVWs or LVCs. Two divergent evolutionary pathways have been found depending on the maximum depth extent of the LVW and wedge viscosity. Assuming a viscosity contrast of 0.1 with background asthenosphere, models with a LVW that extends down to 400 km depth show a steeply dipping slab, while models with an LVW that extends to much shallower depth, such as 200 km, can produce slabs that are flat lying beneath the overriding plate. There is a narrow range of mantle viscosities that produces flat slabs (5 to10×1019 Pa s) and the slab flattening process is enhanced by trench rollback. Slab can be decoupled from the overriding plate with a LVC if the thickness is at least a few 10 s of km, the viscosity reduction is at least a factor of two and the depth extent of the LVC is several hundred km. These models have important implications for the geochemical and spatial evolution of volcanic arcs and the state of stress within the overriding plate. The models explain the poor correlation between traditional geodynamic controls, subducting plate age and convergence rates, on slab dip. We predict that when volcanic arcs change their distance from the trench, they could be preceded by changes in arc chemistry. We predict that there could be a larger volatile input into the wedge when arcs migrate toward the trench and visa-versa. The transition of a subduction zone into the flat-lying regime could be preceded by changes in the volatile budget such that the dehydration front moves to shallower depths. Our flat-slab models shed some light on puzzling flat subduction systems, like in Central Mexico, where there is no deformation within the overriding plate above the flat segment. The lack of in-plane compression in Central Mexico suggests the presence of a low viscosity shear zone above the flat slab. [Copyright &y& Elsevier]

The subduction of an aseismic ridge has important consequences on the dynamics of the overriding upper plate. In the central Andes, the Nazca Ridge subduction imprint can be tracked on the eastern side of the Andes. The Fitzcarrald arch is the long-wavelength topography response of the Nazca Ridge flat subduction, 750 km inboard of the trench. This uplift is responsible for the atypical three-dimensional shape of the Amazonian foreland basin. The Fitzcarrald arch uplift is no older than Pliocene as constrained by the study of Neogene sediments and geomorphic markers, according to the kinematics of the Nazca Ridge subduction. [ABSTRACT FROM AUTHOR]

Abstract: We present a model of the subducting Cocos slab beneath Central Mexico, that provides an explanation for stresses causing the occurrence of the majority of the intraslab earthquakes which are concentrated in a long flat segment. Based on the recently developed thermal models for the Central Mexico subduction zone, the thermal stresses due to non-uniform temperature contrast in the subducting slab are calculated using a finite element approach. The slab is considered purely elastic but due to high temperature at its bottom the behavior is considered as ductile creep. The calculation results show a ∼ 20 km slab core characterized by a tensional state of stress with stresses up to 70 MPa. On the other hand, the top of the slab experiences high compressive thermal stresses up to 110 MPa, depending on the elastic constants used and location along the flat part of the subducting plate. These compressive stresses at the top of the slab are not consistent with the exclusive normal fault intraslab earthquakes, and two different sources of stress are proposed. The trenchward migration of the Mexican volcanic arc for the last 7 Ma indicates an increase of the slab dip through time. This observation suggests that the gravity torque might exceed the suction torque. Considering the flat slab as an embedded plate subject to an applied clockwise net torque of 0.5×1016–1.5×1016 N m, the upper half would exhibit tensional stresses of 40–110 MPa that can actually balance the compressive thermally induced stresses. An alternative stress source might come from the slab pull force caused by the slab positive density anomaly. Based on our density anomaly estimations (75±20 kg/m3), a 350 km slab length, dipping at 20° into the asthenosphere, induces a slab pull force of 1.7×1012–4.6×1012 N m. This force produces a tensional stress of 41–114 MPa, sufficient to balance the compressive thermal stresses at the top of the flat slab. The linear superposition of the thermally and torque or slab pull induced stresses shows tensile stresses up to 60–180 MPa inside the flat slab core. Also, our results suggest that the majority of the intraslab earthquakes inside the flat slab are situated where the resultant stresses are larger than 40–80 MPa. This study provides a reasonable explanation for the existence of exclusively normal fault intraslab earthquakes in the flat slab beneath Central Mexico, and also it shows that thermal stresses due to non-uniform reheating of subducting slabs play a considerable role in the total stress field. [Copyright &y& Elsevier]