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Kimberlite magmas are volatile-rich, potassic, and ultramafic, and they are host to most of the world's diamond deposits. A continental-scale kimberlite magmatic belt (the central Cretaceous kimberlite corridor [CCKC]) is found in the interior of the North American continent. Parallel to and coeval with the CCKC, the Cretaceous Omineca magmatic belt (OMB) is located in the Cordilleran orogen. Cordilleran magmatism, including the OMB, is commonly explained through long-lived east-dipping subduction beneath the western margin of the continent. However, this does not explain the temporal and spatial relationships between the OMB and CCKC. We suggest that west-dipping subduction of North American lithosphere beneath the eastern side of the Cordillera explains both. In this model, subduction resulted in arc magmatism of the OMB. The contemporaneous CCKC was formed by extensional stress acting on the continent as it flexed upon entry into the trench. Using a semi-infinite elastic beam model, we show that flexure of a subducting continental plate (elastic thickness = 120 km) produces tensile stresses in the deep continental lithosphere, coincident with the location of the CCKC. [ABSTRACT FROM AUTHOR]

The concave-inboard (concave toward the overriding plate) geometry of most convergent margins is considered a natural consequence of the depression of the edge of a thin spherical cap, whereas concave-outboard margin segments commonly form around indenters on the subducting plate. At the Cascadia subduction zone, the apex of a >500-km-long concave-outboard bend in the trench presently shows no obvious subduction of an indenter, but does coincide with the axis of an outboard-facing concavity in upper-plate rocks arched around the Olympic Peninsula in northwestern Washington State, USA. Here we synthesize paleomagnetic and structural data together with new analyses of Global Navigation Satellite System data to show that the upper plate at Cascadia has been folded from the Miocene to the present into an orocline with an axial trace that bisects the Olympic Peninsula. The processes that accommodate bending, which we suggest include (1) folding by flexural slip on the orocline limbs and (2) shortening, uplift, and escape within the core of the fold at the Olympic Mountains, have the combined result of relative motion of the forearc towards the arc at the core of the orocline, and sustained opposing rotations of the upper plate on the orocline limbs. We propose that oroclinal bending is promoted and maintained by along-strike variations in plate-boundary tractions resulting from the geometry of the plate interface at depth and suggest that these processes can contribute to the development of concave-outboard margins without the need for a subducting indenter. [ABSTRACT FROM AUTHOR]

Models of orogens identify subduction of oceanic crust as the key mechanism leading to continental collision. Such models, based inter alia on thermobarometric and geochronological evidence preserved in high-pressure metamorphic rocks and subduction-related magmatism, have been used to explain the convergence of Europe and Adria in the Cretaceous-Cenozoic and the subsequent Alpine orogen. Here, we review the metamorphic, igneous, and sedimentary record of the past 300 Ma of the Alpine orogen to show that there is no evidence of igneous activity during subduction initiation and prograde high-pressure metamorphism, leading to an ~50 Ma hiatus in magmatism, or "arc gap." The closure of rift basins forming the Piemont-Liguria ocean did not follow a classical Wadati-Benioff-type subduction. Instead, subduction initiation at passive margins allowed for the accretion of the hydrated portion of the subducting plate within an orogenic wedge as subduction of dry subcontinental lithosphere inhibited magmatism during subduction initiation and ocean closure. [ABSTRACT FROM AUTHOR]

Hydrothermal quartz veins are ubiquitous in exhumed accretionary complexes, including the Namibian Damara belt. Here, subduction-related deformation occurred at temperatures σ550 °C, and vein geometry is consistent with plate interface shear, low effective normal stresses, and mixed-mode deformation. Quartz vein δ18O values relative to Standard Mean Ocean Water (SMOW) range from 9.4‰ to 17.9‰ (n = 30), consistent with precipitation from metamorphic fluids. A dominant subset of quartz veins away from long-lived high-strain zones and basaltic slivers have δ18O values in a smaller range of 14.9‰ ± 1‰, requiring precipitation from a fluid with δ18O of 12‰ ± 1‰ at 470-550 °C. This uniform fluid isotope value is consistent with progressive local breakdown of chlorite allowing extensive hydrofracture at temperatures typical of the plastic regime. In active subduction zones, brittle deformation within the plastic regime is inferred from observations of tectonic tremor, a noise-like seismic signal including overlapping low- and very low-frequency earthquakes, which occurs below the seismogenic zone. Both tremor and hydrothermal veins correlate with zones of inferred high fluid pressure, could represent a mixture of shear and dilatant failure, and may therefore be controlled by episodic hydrofracturing within a dominantly plastic and aseismic regime. [ABSTRACT FROM AUTHOR]

The high values of La/Yb (>15-20) obtained from late Eocene to Pliocene volcanic rocks of central Tibet indicate that source regions were in a thickened (>35 km) crust. The higher Neogene La/Yb values compared to lower Paleogene La/Yb values required continuous crustal thickening, as La/Yb values are proportional to crustal thickness. Although the Paleogene high La/Yb values can be explained by coeval thrusting, this mechanism is unable explain the Neogene higher La/Yb values because central Tibet is covered by undeformed flat-lying early Miocene strata. Here we correlate the temporal variation of Cenozoic volcanic geochemistry with the regional geology of central Tibet, which was part of a larger Eocene-Oligocene thrust belt and the related foreland basin. The Paleogene foreland basin was partitioned into the southern (Hoh Xil) and northern (Qaidam) subbasins by the Kunlun fault ca. 25-20 Ma. The two subbasins were at similar elevations at the time of their formation, but the southern basin, at ~5 km, is >2 km higher than the northern basin at ~2.8 km. Because the southern basin and the Paleogene thrust belt have been associated with Neogene volcanism and high La/Yb values, we suggest that the elevation difference between the two basins was induced by >15 km of magmatic inflation sourced from mantle melting. The magnitude of magmatic inflation is comparable to that of tectonic thickening in central Tibet, highlighting the important and often neglected role of syncollisional mantle melting and its induced magmatic inflation in constructing the thickened Tibetan crust. [ABSTRACT FROM AUTHOR]

The tectonic origin of the Pennsylvanian-Permian Ancestral Rocky Mountain system is debated. Most models invoke stress from collision along the Ouachita-Marathon belt as the primary driver for Ancestral Rocky Mountain (ARM) (western USA) tectonism, but no existing model has convincingly explained the timing, northeastsouthwest contractional kinematics, and amagmatic nature of this system. We propose that stress generated by transpressional deformation along the southwestern (Sonora) margin of Laurentia combined with that from collision along the Ouachita-Marathon and Nevada margins drove uplift and basin development of the ARM. This model explains ARM timing, uplift kinematics, and lack of magmatism, and is supported by a new generation of late Paleozoic global plate reconstructions. [ABSTRACT FROM AUTHOR]

We suggest a new explanation for the presence of crustally derived zircons in the upper-mantle rocks of ophiolitic complexes, as an alternative to subduction-related models. Integrated isotopic (U-Pb, Hf, and O isotopes) and trace-element data for zircons from the Tumut ophiolitic complex (southeast Australia) indicate that these grains are related to granitic magmatism and were introduced into the mantle rocks after their emplacement into the crust. These observations emphasize that a clear understanding of the origin of individual zircon populations and their relationship to the host rock is essential to interpretations of the tectonic history of upper-mantle rocks and the dynamics of crust-mantle interactions. [ABSTRACT FROM AUTHOR]

Intraplate alkaline lavas typically exhibit isotopic characteristics that require a source with long-term isolation from the convecting asthenosphere, such as in the sub-continental lithosphere mantle or a mantle boundary layer. Melting of metasomatically enriched domains, or metasomes, within the lithospheric mantle provides a viable mechanism for generating the geochemical characteristics of intraplate alkaline basalts. The origins and distribution of these metasomes have been attributed to recent enrichment of the lithosphere by a mantle plume or ancient events that occurred during the early evolution of the sub-continental lithosphere mantle. Here, we present a geochemical study of Ethiopian Miocene intraplate alkaline lavas: melts of a lithospheric mantle that was enriched metasomatically during lithospheric stabilization and by recent plume-lithosphere interaction. We find that these lavas have geochemical characteristics consistent with melting of an amphibole-bearing lithospheric-mantle metasome. New Pb and Hf isotope data for these lavas require a HIMU-like source component, similar to other alkaline lavas erupted through the Horn of Africa, Sudan, and Egypt, and adjacent Arabian plate lithospheres. The isotopic characteristics of this component are distinct from the Afar plume mantle source and instead are consistent with the long-term evolution of a lithospheric metasome created during a Neoproterozoic subduction event associated with the Pan-African orogeny. The widespread distribution of easily fusible lithospheric metasomes within the continental lithosphere mantle may facilitate magma generation without the need for substantial lithospheric thinning or elevated mantle potential temperatures. Mantle heterogeneity of this nature has implications for the source origin of HIMU magmas associated with continental lithosphere. [ABSTRACT FROM AUTHOR]

Tectonic collision between South America and Panama began at 23-25 Ma. The collision is significant because it ultimately led to development of the Panamanian Isthmus, which in turn had wide-ranging oceanic, climatic, biologic, and tectonic implications. Within the Panama Canal Zone, volcanic activity transitioned from hydrous mantle-wedge-derived arc magmatism to localized extensional arc magmatism at 24 Ma, and overall marks a permanent change in arc evolution. We interpret the arc geochemical change to result from fracturing of the Panama block during initial collision with South America. Fracturing of the Panama block led to localized crustal extension, normal faulting, sedimentary basin formation, and extensional magmatism in the Canal Basin and Bocas del Toro. Synchronous with this change, both Panama and inboard South America experienced a broad episode of exhumation indicated by (U-Th)/He and fission-track thermochronology coupled with changing geographic patterns of sedimentary deposition in the Colombian Eastern Cordillera and Llanos Basin. Such observations allow for construction of a new tectonic model of the South America-Panama collision, northern Andes uplift and Panama orocline formation. Finally, synchroneity of Panama arc chemical changes and linked uplift indicates that onset of collision and Isthmus formation began earlier than commonly assumed. [ABSTRACT FROM AUTHOR]

We develop a forward model of the generation of faulted seafloor topography (visualization) to estimate the relative roles of tectonic and magmatic extension in the presence of core complexes at slow-spreading ridges. The visualization assumes flexural rotation of 60° normal faults, a constant effective elastic thickness, Te, of young lithosphere, and a continuous infill of the depressed hanging wall by lava flowing from the spreading axis. We obtain a new estimate of Te = 0.5-1 km from the shapes of the toes of 6 well-documented oceanic core complexes. We model an 80-km-long bathymetric profile in the equatorial Atlantic across a core complex and the ridge axis at 13°20'N and estimate the variation in tectonic extension, which yields the variation in the fraction of upper crust extension, M, by magmatic diking at the ridge axis. Core complex formation appears to be stable for all values of M < 0.5. The visualization shows how gabbro emplaced at the base of the lithosphere during extension by magmatic diking is partitioned to each side of the spreading axis, and predicts a high probability of finding gabbros in the domes of core complexes. [ABSTRACT FROM AUTHOR]

The Tanzawa plutonic complex (TPC), central Japan, is a suite of tonalitic-gabbroic plutons exposed in a globally unique arc-arc collision zone, where an active intraoceanic Izu-Bonin-Mariana (IBM) arc is colliding against the Honshu arc. The TPC has been widely accepted as an exposed middle crust section of the IBM arc, chiefly because of geochemical similarities between the TPC and IBM rocks and previously reported precollisional Miocene K-Ar ages. However, new zircon U-Pb ages show that the main pulse of TPC magmatism was syncollisional and that plutons were emplaced rapidly and cooled soon after Pliocene collision. Trace element compositions of TPC zircon show distinctively elevated Th/Nb ratios compared to zircon from other noncollisional IBM silicic plutonic rocks, indicating the involvement of continental sediments from the Honshu arc in their magma genesis. [ABSTRACT FROM AUTHOR]

We refine conventional continental-scale geodynamic models by including conversion of mechanical work done by deformation into heat. The intensity of the shear heating is extracted from the Brace- Goetze strength envelope without any additional model parameters or assumptions. Incorporation of this, certainly present, heating rate into a model may result in up to tenfold stress reduction, which is exceeding the effects of variation of common parameters within their uncertainty limits. Shear heating with lithospheric thickening at high integrated strength solves the puzzle of the heretofore-missing heat source recorded by metamorphism, magmatism, and heat flow in mountain building. However, the mechanism is self-limiting as the rising temperature reduces stress and thus the rate of heat production. Thus this is a self-regulating mechanism maintaining a moderate integrated lithospheric strength consistent with results of model-independent forcebalanced calculations and with surface heat flow measurements. [ABSTRACT FROM AUTHOR]

K2O contents have long been recognized as a potential indicator of tectonic processes, and in the Sierra Nevada, California, high-K2O volcanism has been attributed to lithosphere root delamination. However, new data from the central Sierra suggest a very different control: K2O concentrations can be explained by variations in the degree of partial melting in the mantle, where high-K2O volcanics are derived from low-degree partial melts of mantle lithosphere. Field evidence in the central Sierra further suggests that the pulse of high-K2O volcanism there was synchronous with the development of a pull-apart structure along a series of right-stepping dextral transtensional faults at the onset of Walker Lane transtensional faulting. In our alternative interpretation, high-K, low-degree partial melts were tapped by the inception of transtensional stresses, recording the birth of a plate boundary. We speculate that high-K2O lavas in the southern Sierra are similarly related to the onset of transtensional stresses, not delamination. A regional southward increase in incompatible element contents and decrease in erupted volumes are also consistent with a model for crustal thickness controls on magmatism. Depth-integrated density models show that dry mafic magmas beneath thick crust have insufficient buoyancy to erupt, but low-degree partial melts carry sufficient volatiles to allow eruption; as with K2O, degree of partial melting, not source-region heterogeneity, controls water contents and buoyancy. [ABSTRACT FROM AUTHOR]

The Mesozoic boundary separating accreted oceanic rocks and continental North America in west-central Idaho forms a prominent right-angle bend, the origin and kinematic significance of which have had a great influence on models for the evolution of the North American Cordilleran orogen. Based on structural and geochronologic relations, we conclude that the bend in the steep crustal boundary is formed by the intersection of two shear zones with different ages. Structural fabrics of the north-striking western Idaho shear zone are overprinted by penetrative structures of the west-northwest—striking Orofino shear zone. U-Pb sensitive high-resolution ion microprobe ages on metaplutonic rocks and late synkinematic leucocratic dikes constrain the age of deformation in the Orofino shear zone to ca. 90-70 Ma, distinctly younger than ca. 120-90 Ma magmatism and deformation within the western Idaho shear zone. The Orofino shear zone is interpreted as a southwest-vergent contractional relay structure that linked basement-cored foreland thrusts of the Middle and Southern Rocky Mountains with the deep crustal duplex of the Shuswap complex in the Canadian Rockies. Truncation ca. 90 Ma terminated Late Cretaceous margin-parallel translation of terranes along the western Idaho shear zone and required displacement to step outboard to poorly understood structures to the west. [ABSTRACT FROM AUTHOR]

The 40Ar/39Ar hornblende crystallization ages from the Las Pirquitas thrust of the Sierra de Pie de Palo, Argentina, indicate that imbrication of the proto-Andean margin had initiated by Ca. 515 Ma. The ages do not permit the Pie de Palo block to represent basement of the adjacent Argentine Precordillera terrane, but rather require it to be a separate fragment not juxtaposed with the Precordillera prior to the Ordovician. The Pie de Palo block was likely autochthonous to the Famatina arc of the proto-Andean margin in the Early Cambrian. When combined with existing regional ages of magmatism, metamorphism, and deformation, the ages suggest that east-dipping subduction beneath the Famatina arc initiated by the Early to Middle Cambrian. The newly established convergent margin resulted from a westward shift of the plate boundary at the end of the Pampean orogeny in the eastern Sierras Pampeanas. The Famatina arc persisted as an Andean-type active margin until the collision and accretion of the Precordillera terrane in the Middle Ordovician. [ABSTRACT FROM AUTHOR]

The deformation in the Asal Rift (Djibouti) is characterized by magmatic inflation, diking, distributed extension, fissure opening, and normal faulting. An 8 yr time line of surface displacement maps covering the rift, constructed using radar interferometry data acquired by the Canadian satellite Radarsat between 1997 and 2005, reveals the aseismic behavior of faults and its relation with bursts of microseismicity. The observed ground movements show the asymmetric subsidence of the inner floor of the rift with respect to the bordering shoulders accommodated by slip on three of the main active faults. Fault slip occurs both as steady creep and during sudden slip events accompanied by an increase in the seismicity rate around the slipping fault and the Fieale volcanic center. Slip distribution along fault strike shows triangular sections, a pattern not explained by simple elastic dislocation theory. These observations suggest that the Asal Rift faults are in a critical failure state and respond instantly to small pressure changes in fluid-filled fractures connected to the faults, reducing the effective normal stress on their locked section at depth. [ABSTRACT FROM AUTHOR]

The proposed national high-level nuclear waste repository at Yucca Mountain is close to Quaternary cinder cones and faults with Quaternary slip. Volcano eruption and earth. quake frequencies are low, with indications of spatial and temporal clustering, making probabilistic assessments difficult. In an effort to identify the most likely intrusion sites, we based a three-dimensional finite-element model on the expectation that faulting and basalt intrusions are sensitive to the magnitude and orientation of the least principal stress in extensional terranes. We found that in the absence of fault slip, variation in overburden pressure caused a stress state that preferentially favored intrusions at Crater Flat. However, when we allowed central Yucca Mountain faults to slip in the model, we found that magmatic clustering was not favored at Crater Flat or in the central Yucca Mountain block. Instead, we calculated that the stress field was most encouraging to intrusions near fault terminations, consistent with the location of the most recent volcanism at Yucca Mountain, the Lathrop Wells cone. We found this linked fault and magmatic system to be mutually reinforcing in the model in that Lathrop Wells feeder dike inflation favored renewed fault slip. [ABSTRACT FROM AUTHOR]

Documents boninitic magmatism within the continental Yukon-Tanana terrane in Canada. Geologic setting and stratigraphy; Geochemical characteristics of the boninitic rocks; Evidence for boninitic magmatism linked to continental crust; Petrological models explaining the thermal and petrological requirements of the Fire Lake boninites.

Documents determinations of mafic magmatic rocks emplaced into the base of the continental crust of Europe. Thermal and mechanical impact of extensive tectonics on the lower crust; Lower crust granulite facies metamorphism; Lower crust seismic layering.

Presents strontium and neodymium isotope data for a group of mafic and ultramafic xenoliths from the Cima volcanic field in southeastern California. Similarity of the magma source to a mid-ocean ridge basalt (MORB); Late Cretaceous intrusion age; Invasion of the lower crust by asthenospheric magmas; Crustal heating by ascent of asthenosphere.

Presents data indicating that Early Jurassic arc magmatism was a characteristic of the Nisling and Stikinia terranes located near Aishihik Lake in southwest Yukon, Canada. Metamorphism resulting from intrusion of Nisling by Lower Jurassic plutons; Formation of a hot-side-up aureole from metamorphic mineral assemblage; Petrogenesis; Geothermobarometry.

Examines the possibility that syn- to postdeformational granitic magmatism in the Limpopo belt of southern Africa was coeval and that pressure-temperature (P-T) paths are similar. Indication that magmatism occurring at distinct times in different parts of the belt; Independence of juxtaposition of pressure-temperature-time (P-T-t) paths.

Proves lithospheric thickness' action as an important physical control on the composition of asthenosphere-derived melts by restricting the minimum depth at which decompression melting can occur. Example of three successive magma types; Suggestion that lithospheric erosion can be a rapid process; Implications for various starting plume models.

The article discusses the role played by volcanic eruptions and tectonic movements in the formation of caldera-forming eruptions (CFEs). It explores the study conducted by G. R. Hughes and G. A. Manhood which focuses on evaluating the formation of calderas in relation to its tectonic environment, such as volcanic eruptions. It reveals that the flow of basaltic magma may control the formation of silicic magmatic systems that contributes to the development of CFEs. It states that the work carried out by Hughes and Manhood plays essential role in understanding the relation of arc magmatism and volcanic eruption with CFEs.

Commentary. Dicusses the importance of links between tectonic and magmatic activity during the rifting of Pangea to form the central North Atlantic Ocean. Reference to R. Oyarzun et al., (1997) article in `Geology;' Discussion on magmatism and rifting; Information on dikes.