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The Eastern Alps were affected by a profound post‐collisional tectonic reorganisation in Neogene time, featuring indentation by the Adriatic upper plate, rapid uplift and filling of the eastern Molasse Basin, exhumation and eastward orogen‐parallel transport of Paleogene metamorphic units in the orogenic core, and a shift from northward thrust propagation in the European plate to southward propagation in the Adriatic plate. We test the idea that these events were triggered by slab detachment by reconstructing the indentation process. This involves sequentially restoring N‐S and E‐W cross‐sections of the orogenic wedge and correcting for out‐of‐section orogen‐parallel transport with a map‐view reconstruction. We propose two phases of indentation: Initially (23 and 14 Ma), the whole Adriatic crust acted as an indenter. Its northward motion was accommodated by upright folding and orogen‐parallel extensional exhumation in the Tauern Window. This phase was followed (14 Ma to Present) by continued orogen‐parallel transport of the orogenic wedge into the Pannonian Basin and deformation of the leading edge of the Adriatic indenter, forming the Southern Alps fold‐thrust belt. The lower crust of the Southern Alps indented the base of the Venediger Nappes in the Tauern Window, forming a high‐velocity (6.8–7.25 km/s) ridge in map view at 30–45 km depth. By correlating the post‐23 Ma orogenic evolution with presently imaged European slab segments in P‐wave teleseismic tomography, we discern two possible Neogene slab removal events: One from 23 to 19 Ma triggering tectonic reorganisation of the Eastern Alps and its foreland basin, and potentially a second event after 14 Ma. Key Points: The Eastern Alps were first indented by Adriatic lithosphere (23 and 14 Ma), then by Adriatic lower crust as the indenter deformed (14 and 0 Ma)Shortening of the orogenic wedge since 23 Ma requires 135 km of subduction and 90 km of eastward extrusion of orogenic lithosphereSlab detachment at 23 and 20 Ma and possibly after 14 Ma is constrained by areal balancing of crust and mantle [ABSTRACT FROM AUTHOR]

The detachment (i.e., break‐off) of down‐going subducting oceanic slabs is a major geodynamic event with far‐reaching consequences, one of which is the reduction of the slab pull force acting on the trailing plate. We investigate the motion of the Sinai Microplate where a recent (∼1 Myr ago) slab break‐off occurred along its sole converging plate boundary (Cyprian Arc) with the overriding Anatolia Microplate. Based on new bathymetric mapping, high‐resolution seismic reflection imaging, geodetic and earthquake data, we show that Sinai is actively moving in a northwest direction with respect to Nubia. Our results indicate that despite the recent slab break‐off, Sinai has and is still being pulled (or pushed) toward the overriding Anatolia Microplate. The continued convergence possibly occurs because of a persistent slab pull force, a suction force induced by the down‐going detached slab and/or by the upper mantle flow induced by the Afar Plume. Plain Language Summary: Subduction of the oceanic lithosphere into the mantle exerts the prime force that drives plate tectonic motions. The termination of subduction due to the tearing of the down‐going oceanic slab is, therefore, an important geodynamic phenomenon with far‐reaching kinematic consequences. Here we show that despite the recent tearing of the leading oceanic slab of the Sinai Microplate, convergence motion between Sinai and Anatolia Microplates continued without substantial changes. The continued convergence possibly occurs because of a persistent slab pull force, a suction force induced by the down‐going detached slab and/or by the motion of the upper mantle flow induced by the Afar Plume. Key Points: Within the eastern Mediterranean, the Sinai Microplate is moving in a northwest direction with respect to the Nubian PlateThe Sinai Microplate is still converging with the overriding Anatolia Plate, despite the recent break‐off of the leading subducting slab [ABSTRACT FROM AUTHOR]

The Zagros orogen in the Iranian Plateau serves as a natural laboratory for studying the tectonic evolution of the transition from oceanic subduction to continental collision. Various observations from both geology and geophysics show that deep oceanic slabs may have detached one after another from the Arabian continental margin, forming a complex plate front beneath the Zagros orogen. However, where slab detachment occurred and how slab detachment affects shallower continental underthrusting remain poorly understood. The mechanism behind the complex post‐collision magmatism remains elusive. While the uppermost mantle is often considered the heart of the mantle lid, its rheological variations can provide insights into the underlying thermal structure. Here, we construct a high‐resolution Pn‐wave attenuation model for the uppermost mantle beneath the Iranian Plateau and surrounding areas using a newly compiled data set and constrain the lithospheric structure by combining the Lg‐wave attenuation within the crust. Weak Pn‐wave attenuation outlines the boundary of the Arabian Plate near the Moho discontinuity, extending further in the direction of underthrusting in the northwestern and southeastern regions. This is likely due to the up‐bending and underplating of the Arabian lithosphere following the cessation of slab pull. The overlying crusts in northwestern and southeastern Zagros thickened under the compressive force from plate underplating. The correlations among the surface Miocene‐Quaternary volcanism, strong Lg attenuation in the crust and strong Pn attenuation in the uppermost mantle suggest that asthenospheric materials escaped from slab windows and further intruded into the upper mantle and crust to feed the post‐collision volcanoes. Plain Language Summary: Various geological and geophysical observations show that deep oceanic slabs may have detached one after another from the Arabian continental margin, forming a complex plate front beneath the Iranian Plateau. However, the processes of continental underthrusting and collision remain poorly understood. The formation mechanism of the complex magmatism in the Iranian Plateau is also unclear. The uppermost mantle, often considered the core of the mantle lid, exhibits rheological variations that can provide insights into the underlying thermal structure. In this study, we conducted high‐resolution Pn‐wave attenuation tomography for the uppermost mantle beneath the Iranian Plateau, using 23,251 vertical‐component waveforms recorded at 291 previous stations and 197 new temporary stations. The resulting images show that the cold Arabian plate is characterized by weak Pn attenuation beneath the Zagros orogen and the strong attenuation anomalies are related to younger magmatism. Combined with previous geological and geochemical observations, our results suggest a successive evolution process under the Iranian Plateau. The ancient Neotethys oceanic plate partially detached from the Arabian continental margin. After the slab break‐off at depth, the Arabian continental plate rose back and flattened below the overriding plate. Meanwhile, the detachment‐related asthenosphere upwelling fed the younger volcanism at the surface. Key Points: A high‐resolution broadband Pn attenuation model is obtained beneath the Iranian PlateauWeak Pn attenuation delineates the underthrusting front of the Arabian Plate and reveals the segmented plate up‐bendingPost‐collision magmatism is likely related to the detachment‐related mantle upwelling characterized by strong Pn attenuation [ABSTRACT FROM AUTHOR]

Abstract The detachment (i.e., break‐off) of down‐going subducting oceanic slabs is a major geodynamic event with far‐reaching consequences, one of which is the reduction of the slab pull force acting on the trailing plate. We investigate the motion of the Sinai Microplate where a recent (∼1 Myr ago) slab break‐off occurred along its sole converging plate boundary (Cyprian Arc) with the overriding Anatolia Microplate. Based on new bathymetric mapping, high‐resolution seismic reflection imaging, geodetic and earthquake data, we show that Sinai is actively moving in a northwest direction with respect to Nubia. Our results indicate that despite the recent slab break‐off, Sinai has and is still being pulled (or pushed) toward the overriding Anatolia Microplate. The continued convergence possibly occurs because of a persistent slab pull force, a suction force induced by the down‐going detached slab and/or by the upper mantle flow induced by the Afar Plume.

Cenozoic magmatism is present across the Iranian Plateau but, despite playing an important role in the geodynamic evolution of the young plateau, remains poorly understood. In this study, the crustal thermal structure beneath the Iranian Plateau is investigated by high‐resolution Lg‐wave attenuation tomography using a newly compiled data set, which consists of records from three recent temporary seismic arrays (for a total of 197 stations) and previous regional networks. Strong Lg attenuation is mainly observed beneath the Zagros orogenic belt, Urumieh‐Dokhtar magmatic arc (UDMA), Makran magmatic arc, Lut magmatic zone, and Alborz reararc magmatic belt, which covers not only tectonic boundary belts but also the active internal zones of the Iranian Plateau. Combining deep seismological observations and geochemical data, the strong crustal attenuation characteristics beneath the northwestern and southeastern UDMA suggest that Miocene–Quaternary magmatism was activated by slab detachment‐induced upwelling after the cessation of oceanic subduction. Western Makran is characterized by strong Lg attenuation with a NE trend. The age of the Paleogene–Quaternary Makran volcanism decreases toward the NE and is likely related to the change in subduction dip. Diffuse magmatism and strong crustal attenuation are present in the northern Lut Block rather than the entire Lut–Afghan collision zone, which is likely associated with the accumulation of mantle magmas driven by the potential northward asthenospheric flow. In northern Iran, a strong attenuation anomaly spans the entire E‒W Alborz magma belt, suggesting that partial crustal melting and mantle thermal sources may exist in this region. Plain Language Summary: The Iranian Plateau is located in the Middle East and is a part of the Alpine‐Himalayan orogenic system. Intensive Cenozoic volcanic activity is characteristic of the Iranian Plateau, but the origination, storage, and transport of underground magma are poorly understood. Seismic attenuation is sensitive to high temperatures and partial melting and can be an indicator of the distribution of heat sources. In this study, we investigate the attenuation features of the crust with a new data set to constrain the crustal thermal structure beneath the Iranian Plateau by using a specific type of seismic wave (Lg) that travels within the crust. Hot crustal sources were observed beneath the Zagros orogenic belt and the four magmatic zones. Therefore, the origination of magmatism can be inferred based on the crustal attenuation distribution. Typically, the younger magmatism in the northwestern and southeastern Urumieh‐Dokhtar magmatic arc was likely activated by deeper mantle upwelling, which may be related to plate kinematics at depth (slab detachment or break‐off). On the southeastern edge of Iran, the Makran region is characterized by strong attenuation with a NE trend, where the volcanism also gets younger toward the NE. It is likely related to the dip change in oceanic plate subduction. Key Points: A high‐resolution Lg attenuation model is developed beneath the Iranian PlateauStrong Lg attenuation characterizes tectonically active areas or recent magmatic zonesCrustal thermal structure revealed by Lg attenuation links Cenozoic magmatism in Iran with sub‐lithospheric mantle processes [ABSTRACT FROM AUTHOR]

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The Greater Caucasus (GC) Mountains within the central Arabia‐Eurasia collision zone are an archetypal example of a young collisional orogen. However, the mechanisms driving rock uplift and forming the topography of the range are controversial, with recent provocative suggestions that uplift of the western GC is strongly influenced by an isostatic response to slab detachment, whereas the eastern half has grown through shortening and crustal thickening. Testing this hypothesis is challenging because records of exhumation rates mostly come from the western GC, where slab detachment may have occurred. To address this data gap, we report 623 new, paired zircon U‐Pb and (U‐Th)/He ages from seven different modern river sediments, spanning a ∼400 km long gap in bedrock thermochronometer data. We synthesize these with prior bedrock thermochronometer data, recent catchment averaged 10Be cosmogenic exhumation rates, topographic analyses, structural observations, and plate reconstructions to evaluate the mechanisms growing the GC topography. We find no evidence of major differences in rates, timing of onset of cooling, or total amounts of exhumation across the possible slab edge, inconsistent with previous suggestions of heterogeneous drivers for exhumation along‐strike. Comparison of exhumation across timescales highlight a potential acceleration, but one that appears to suggest a consistent northward shift of the locus of more rapid exhumation. Integration of these new datasets with simple models of orogenic growth suggest that the gross topography of the GC is explainable with traditional models of accretion, thickening, and uplift and does not require any additional slab‐related mechanisms. Plain Language Summary: The transition from subduction to building of mountain ranges is a fundamental process shaping the rock record, but our understanding of this process is limited by few well preserved examples. One where this transition is preserved is in the Greater Caucasus Mountains, but the first order drivers of rock uplift and growth of topography remain controversial. Here, it seems the eastern half of the range grew by shortening and thickening of the crust, but uplift of the western half may be driven by removal of a subducted slab. Importantly, direct records of the rate of erosion or exhumation are largely absent in the eastern range. Here we report new data, derived from zircon grains extracted from modern sediments which span the length of the range. Integrating these with prior analyses of cooling rates derived from minerals from in‐situ bedrock samples, we find no meaningful change in the rates at which rocks have uplifted or the total magnitude of rock exhumation along the whole range. Consideration of these new data with records of millennial scale exhumation rates and total amounts of plate motion imply that the evolution of growth of the Greater Caucasus is well explained by shortening and thickening. Key Points: New detrital thermochronology fills 400 km long gap in bedrock data that hindered interpretation of exhumation mechanisms within the Greater CaucasusSynthesis of new and existing thermochronology data imply similar magnitudes and timing of exhumation along‐strike in the Greater CaucasusComparison of exhumation rates with long‐term estimates of convergence do not require isostatic uplift from slab detachment [ABSTRACT FROM AUTHOR]

Subduction zones are essential drivers of plate tectonics. However, the processes causing subduction zone initiation (SZI), involving the formation of a new plate boundary, and the forces required remain disputed. Here, we focus on horizontally forced SZI at passive margins and quantify the horizontal force required for SZI with two-dimensional petrological-thermomechanical numerical models. The initial configuration, involving two passive margins bounding a 400 km wide basin with exhumed mantle, is calculated by a numerical rifting simulation to guarantee thermomechanical feasibility and isostatic equilibrium. SZI occurs at one passive margin during convergence of the basin-margin system. SZI is caused by thermal softening due to local shear heating. A local temperature increase of only ca. 50°C is sufficient to cause SZI. Corresponding simulations without shear heating do not show SZI, showing unequivocally that thermal softening controls SZI. We systematically limit the lithospheric deviatoric stress to determine the minimum force required for SZI. A minimum force (per unit length) of ca. 14 TN m−1 is required for SZI; a force magnitude that agrees with independent estimates from mantle convection models. However, for the associated stress limit of 200 MPa the lithosphere is so weak that slab detachment (SD) already occurs at the onset of basin closure in the horizontal, non-subducted region of the subducting plate. Only for stress limits >=300 MPa (yielding a force of ca. 15 TN m−1) subduction of continental crust occurs and SD occurs in the subducted slab. The force required for SZI is a proxy for the effective compressive lithospheric strength. This strength also controls subsequent SD due to tensile slab pull, which increases during subduction, also since our models include the olivine–wadsleyite phase change. Our simulations show a causal link betweeen SZI and subsequent SD: If forces required for SZI are smaller, then the lithosphere is weaker and then SD occurs at shallower levels and corresponding slabs are shorter. Concerning the European Alps, our results imply that if there was no SD since the Europe–Adria collision some 30 Myr ago (as proposed by some studies), then the subducted lithosphere must be considerably strong.

Sedimentary successions deposited within arc‐trench gaps (forearcs) record topographic oscillations related to subduction tectonism. The collision of oceanic plateaus can inhibit normal subduction and induce detachment of the subducted oceanic lithosphere (slab), leading to subduction translation or shut‐down. Such second‐order subduction processes complicate the stratigraphic record making discrete events difficult to resolve, as preserved examples are rare and typically overprinted by deformation. We compiled and averaged (stacked) top subduction wedge (Pahau Terrane) subsidence and paleo‐water depth curves from 30 locations containing mid‐Cretaceous forearc successions in Marlborough, New Zealand. Subsidence curve stacking proved effective in removing deformation due to normal subduction, allowing for resolution of higher order plate‐margin events. The stacked curves reveal a synchronous topographic oscillation affecting over 100 km of the paleo‐coastline between 110 and 95 Ma, which penetrated almost 250 km into the forearc hinterland and created up to 3 km of accommodation space. Topographic drawdown and highstand deposition from 110 Ma were followed by rebound and volcanism by 98 Ma, a response to regional tectonism that was compared to theoretical predictions for ridge and plateau collision. We conclude that plate flexure, and slab extension and tearing reasonably account for the synchronous topographic oscillation observed, which we attribute to Hikurangi Plateau collision. We postulate that collision began east of Marlborough along the present day Chatham Rise from 110 Ma, producing topographic upheaval and disruption of adjacent sedimentary systems. Sediments deposited contemporaneously along the margin preserve a record of these events and provide a rare insight into evolving mid‐Cretaceous plate dynamics. Key Points: Subsidence quantifies topographic change in the top subduction wedge over time, corrected for paleo‐water depth, between 110 and 95 MaCurves were averaged to remove overprinting due to localized wedge deformation, and extract any underlying regional tectonic driversSynchronous drawdown and rebound across fault blocks are attributed to slab necking and detachment as a consequence of plateau collision [ABSTRACT FROM AUTHOR]

We investigate the impact of three coupled weakening mechanisms on the viscous detachment of a stalled lithospheric slab: structural weakening due to necking, material weakening due to grain size reduction, using a two-phase grain damage model, and thermal weakening due to shear heating (thermal damage). We consider a combined flow law of dislocation and diffusion creep. To understand and quantify the coupling of these three nonlinear weakening processes, we derive a mathematical model, which consists of three coupled nonlinear ordinary differential equations describing the evolution of slab thickness, grain size, and temperature. With dimensional analysis, we determine the dimensionless parameters which control the relative importance of the three weakening processes and the two creep mechanisms. We derive several analytical solutions for end-member scenarios that predict the detachment time, that is the duration of slab detachment until slab thickness becomes zero. These analytical solutions are then tested against numerical solutions for intermediate cases. The analytical solutions are accurate for end-member scenarios where one of the weakening mechanisms and one of the creep mechanisms is dominant. Furthermore, we use numerical solutions of the system of equations to systematically explore the parameter space with a Monte Carlo approach. The numerical approach shows that the analytical solutions typically never deviate by more than 50% from the numerical ones, even for scenarios where all three weakening and both creep mechanisms are important. When both grain and thermal damage are important, the two damage processes generate a positive feedback loop resulting in the fastest detachment times. For Earth conditions, we find that the onset of slab detachment is controlled by grain damage and that during later stages of slab detachment thermal weakening becomes increasingly important and can become the dominating weakening process. We argue that both grain and thermal damage are important for slab detachment and that both damage processes could also be important for lithosphere necking during continental rifting leading to break-up and ocean formation.

Subducting slabs are the primary drivers of plate tectonics and mantle circulation, but can also undergo various instabilities that cause dramatic adjustments in tectonic evolution and motion. Slab rollback or trench retreat is possibly a dominant form of time dependence in the plate-mantle system, causing plates to shrink and the mantle to undergo complex flow patterns. Likewise, slab detachment can induce abrupt adjustments in both plate motions and vertical displacement of continents. The arrival or accumulation of continental crust over a subduction zone induces high stresses on the plate and slab that can trigger either rollback or detachment or both. However, these processes necessarily interact because of how stress is relieved and plate motions altered. Here we present a simple boundary-layer like model of coupled trench retreat and slab detachment, induced by continent accumulation, and with slab necking augmented by grain-damage self-weakening (to allow for abrupt necking). With this model we find that, with continental accumulation, initial rollback is at first modest. However, as the stress from continental accumulation peaks, it triggers abrupt slab detachment. The subsequent slab loss causes the plate to lose its primary motive force and to thus undergo a more dramatic and rapid rollback event. After the larger rollback episode, the contracted continental mass re-expands partially. Plausible grain-damage parameters and 40 km thick crust cause abrupt detachment and major rollback to occur after a few hundred million years, which means the plates remain stable for that long, in agreement with the typical age for most large plates. While the complexity of some field areas with a well documented history of detachment and rollback, such as the Mediterranean, taxes the sophistication of our toy model, other simpler geological examples, such as on the western North American plate, show that episodes of rollback can follow detachment. [ABSTRACT FROM AUTHOR]

The Late Jurassic Springbok unconformity in the Surat Basin remains to date at odds with our understanding of basin-forming processes. A previously proposed cause for the unconformity is uplift related to the rifting of the Argo landmass from Western Australia, but the great distance between the Surat Basin and the Argo rift-zone argues against this interpretation. A more likely scenario is that the uplift was related to a west-dipping subduction zone along the east Australian margin. Numerous examples around the world link subduction processes, such as tears and detaching segments of the slab, to variations in surface topography. We used geodynamic forward modelling to generate uplift in the overriding plate of a subduction zone, as well as topographic changes that occur as subduction is disrupted by slab breakoff or tears. Our findings suggest that slab tearing can produce distribution and magnitude of uplift consistent with the Jurassic uplift in the Surat Basin, which potentially adds new insights into the tectonic evolution of eastern Australia. [ABSTRACT FROM AUTHOR]

The idea that prominent geological phenomena observed in the crust and at the surface may be caused by the detachment of subducting dense lithosphere has been anticipated by tectonic models in the Sixties and Seventies, and imaged with improved geophysical techniques in the Eighties. In the mid-Nineties, the model of slab breakoff was defined formally by Davies and von Blanckenburg. Initially proposed as a thought-provoking working hypothesis, the theory rapidly received wide a priori acceptance and was applied to virtually every mountain range around the world, and even in orogens as old as Paleoproterozoic and Archean, to explain a range of different phenomena. These include magmatic flare-ups, rapid topographic uplift, increasing sediment supply, fast exhumation of metamorphic rocks, ore mineralizations, anomalous distribution of seismicity, changes in stress regimes, and inversion of subduction polarity. Even multiple breakoff events were assumed to have occurred at different times in the same orogenic belt or subduction zone. In the last 20 years, slab breakoff has been invoked in so many settings and time frames that it could have hardly taken place in each and every case in which it was called upon. This article does not critically examine the theory, but critically examines its use. The extensive review of the vast literature presented here on the subject reveals how the model has been often applied to provide ad hoc explanations for a range of poorly understood observations based on incomplete evidence of deep-seated processes. Our aim is to illustrate a paradigmatic example of how earth scientists, in the face of evidence that challenges our capacity of understanding, often recur to hypotheses based on other hypotheses. Such an approach may induce researchers to look for confirmation in the absence of compelling constraints, or even in the face of conflicting evidence. The faith in models should not lead us to confuse speculative theories with axiomatic truths, and to build upon them theoretical edifices that are vulnerable and exposed to the risk of circular reasoning. [ABSTRACT FROM AUTHOR]

On 8 September 2017, a great ( M w 8.2) normal faulting earthquake ruptured within the subducting Cocos Plate ~70 km landward from the Middle American Trench beneath the Tehuantepec gap. Iterative inversion and modeling of teleseismic and tsunami data and prediction of GPS displacements indicate that the steeply dipping rupture extended ~180 km to the northwest along strike toward the Oaxaca coast and from ~30 to 70 km in depth, with peak slip of ~13 m. The rupture likely broke through the entire lithosphere of the young subducted slab in response to downdip slab pull. The plate boundary region between the trench and the fault intersection with the megathrust appears to be frictionally coupled, influencing location of the detachment. Comparisons of the broadband body wave magnitude ( m B) and moment-scaled radiated energy ( E R /M0) establish that intraslab earthquakes tend to be more energetic than interplate events, accounting for strong ground shaking observed for the 2017 event. [ABSTRACT FROM AUTHOR]

Complete subduction of an oceanic plate results in slab-window opening. A key uncertainty in this process is whether the higher heat flux and asthenospheric upwelling conventionally associated with slab-window opening generate a detectable topographic signature in the overriding plate. We focus on the Baja California Peninsula, which incorporates the western margin of the Gulf of California rift. The topography and tectonics of the rift flank along the peninsula are strongly bimodal. North of the Puertecitos accommodation zone, the primary drainage divide attains a mean elevation of ca. 1600 m above sea level (asl), above an asthenospheric slab-window opened by Pacific-Farallon spreading ridge subduction along this section of the trench at ca. 17–15 Ma. To the south, mean topography decreases abruptly to ca. 800 m asl (excluding the structurally distinct Los Cabos block at the southern tip of the peninsula), above fragments of the oceanic Farallon slab which stalled following slab tear-off at ca. 15–14 Ma. Along the peninsula, a low-relief surface established atop Miocene subduction-related volcaniclastic units has been incised by a west-draining canyon network in response to uplift. These canyons exhibit cut-and-fill relationships with widespread post-subduction lavas. Here, we utilise LANDSAT and digital elevation model (DEM) data, integrated with previously published K-Ar and 40 Ar/ 39 Ar lava crystallisation ages, to constrain the onset of rift flank uplift to ca. 9–5 Ma later than slab-window formation in the north and ca. 11–10 Ma later in the south. These greatly exceed response time estimates of ca. 2 Ma or less for uplift triggered by slab-window opening. Instead, uplift timing of the high-elevation northern region is consistent with lower-lithospheric erosion driven by rift-related convective upwelling. To the south, stalled slab fragments likely inhibited convective return flow, preventing lithospheric erosion and limiting uplift to the isostatic response to crustal unloading during rifting. [ABSTRACT FROM AUTHOR]

The detachment or breakoff of subducted lithosphere is investigated using scaled three-dimensional thermo-mechanical analogue experiments in which forces are measured and deformation is monitored using high-speed particle imaging velocimetry (PIV). The experiments demonstrate that the convergence rate in a subduction zone determine if and when slab detachment occurs. Slow subduction experiments (with scaled convergence rates ∼1 cm yr −1 ) have lower Peclet numbers and are characterized by lower tensile strength subducted lithosphere, causing detachment to occur when the downward pull force exerted by a relatively short subducted slab is relatively low. Therefore when continental collision is preceded by slow oceanic subduction, the hot and weak subducted lithosphere need not be very long or extremely negatively buoyant to cause detachment. Under such conditions, detachment may occur sooner after the onset of continental subduction than previously predicted. In contrast, if collision is preceded by rapid subduction (∼10 cm yr −1 ), breakoff will be delayed and occur only when the convergence rate has slowed sufficiently to thermally weaken the slab and cause its eventual failure. The analogue experiments further confirm that slab detachment occurs diachronously as it propagates along the plate boundary. Stereoscopic PIV reveals a characteristic strain pattern that accompanies the detachment. Horizontal contraction and subsidence (with scaled values up to 1200 m) in the trench and forearc area precedes the passage of the detachment, and is followed by horizontal extension and uplift (up to 900 m). High-frequency monitoring captures rapid propagation of the detachment along the plate boundary at scaled rates of up to 100 cm yr −1 . However this rate is not constant and interaction between the slab and lower mantle or opening of a backarc basin in the upper plate can reduce or stop slab breakoff propagation altogether. [ABSTRACT FROM AUTHOR]

To clarify the deep structure of the Taiwan Orogen formed by the collision between the Eurasian and the Philippine Sea plates, we determine a robust 3-D shear-wave velocity (Vs) model of the crust and upper mantle beneath the Taiwan Island and its surrounding regions by applying a novel joint inversion method to local and teleseismic P and S wave travel times, Rayleigh-wave phase velocities and S-wave receiver functions. A notable new feature revealed by our model is the morphologic variation of the subducting Eurasian slab along the orogen belt. Beneath South Taiwan, the subducting Eurasian slab is imaged as a continuous high-Vs zone down to ∼400 km depth with an obvious dip angle change at ∼70 km depth. Beneath Central Taiwan, the Eurasian slab is imaged as intermittent high-Vs anomalies in the upper mantle, whereas the Eurasian slab exhibits as a sheet-like high-Vs zone with a low dip angle down to only ∼100 km depth beneath North Taiwan. These results suggest that parts of the subducting Eurasian slab have been detached beneath the Philippine Sea east off the Taiwan Island, which may result in the remnant slab rebounding to underplate the upper plate and so promote mountain building on the Taiwan Island. • A 3-D Vs model of the Taiwan Orogen is derived from 4 kinds of seismic data. • Shape variations of the Eurasian slab are revealed along the orogen belt. • Parts of the Eurasian slab are detached beneath the Taiwan Orogen. [ABSTRACT FROM AUTHOR]

The northern Sanandaj–Sirjan Zone (SaSZ) in northwestern Iran contains several granitoid and gabbroic bodies that are parallel to the Zagros Suture zone in a northwest–southeast direction. The fine-grained Taghiabad gabbro and coarse-grained Kangareh gabbroic bodies southwest of the Ghorveh in the northern SaSZ were crystallized at 158.0 ± 10.0 Ma and 148.3 ± 3.6 Ma, respectively, based on U–Pb zircon dating. The SiO 2 contents of the Kangareh and Taghiabad rocks are similar and range from 45.5 to 51.5 wt.%, and the Al 2 O 3 contents vary from 14 to 24 wt.%, with high Mg number (Mg# = 61–72% and 46–63%, respectively). The Taghiabad body has higher contents of TiO 2 , Fe 2 O 3 , and P 2 O 5 than the Kangareh rocks. The Fe 2 O 3 contents of the Taghiabad rocks vary from 6 to 15 wt.%, whereas those of the Kangareh rocks range from 5 to 9 wt.%. The TiO 2 concentrations of the Taghiabad samples are high (1–3 wt.%), and these rocks can be classified as high Ti and Fe rocks. The chemical compositions of the two bodies show some clear differences. Positive εNd(t) values (+ 3 to + 8) and low initial 87 Sr/ 86 Sr ratios (0.7034 to 0.7054) indicate a depleted mantle source for both bodies. The variation and distribution of trace elements, REEs patterns, and initial 87 Sr/ 86 Sr– 143 Nd/ 144 Nd ratios show that the Kangareh and Taghiabad bodies have high affinities to tholeiitic and alkaline magmatic series. These results suggest an arc and back-arc system regime in an intra-oceanic system than the Andean magmatic type regime for the origin of these bodies in the Middle to Late Jurassic period. These complexes merged with the northern SaSZ during accretion-type continental growth in the Late Jurassic. [ABSTRACT FROM AUTHOR]

A simple model for necking and detachment of subducting slabs is developed to include the coupling between grain-sensitive rheology and grain-size evolution with damage. Necking is triggered by thickened buoyant crust entrained into a subduction zone, in which case grain damage accelerates necking and allows for relatively rapid slab detachment, i.e., within 1 My, depending on the size of the crustal plug. Thick continental crustal plugs can cause rapid necking while smaller plugs characteristic of ocean plateaux cause slower necking; oceanic lithosphere with normal or slightly thickened crust subducts without necking. The model potentially explains how large plateaux or continental crust drawn into subduction zones can cause slab loss and rapid changes in plate motion and/or induce abrupt continental rebound. [ABSTRACT FROM AUTHOR]

Abstract: The Vrancea region of the southeastern Carpathians is one of the most active seismic zones in Europe and it is well-known for its strong intermediate depth earthquakes. Seismic tomography had revealed a high-velocity body beneath Vrancea and the Moesian platform that extends to a depth of at least 350 km and can be interpreted as descending lithosphere. The strong earthquakes occur within the northeastern part of this high-velocity body, in a very limited seismogenic volume at intermediate depth (70–180 km). Several geodynamic models have been proposed for this area. They can be split into two main categories, in terms of the nature of the high-velocity anomaly, which may (a) be associated with descending relic oceanic lithosphere beneath the bending zone of the SE-Carpathians, either attached or already detached from the continental crust; or (b) it may represent continental lithosphere that has been delaminated, after continental collision and orogenic thickening. Based on currently available information, it appears difficult to distinguish between these two types of models. In this paper we attempt to shed more light on the nature of the seismic anomaly, as well as that of the origin of the intermediate depth seismicity in the Vrancea zone, by investigating the waveform character of P-waves excited by local earthquakes beneath this area, and in particular the dependence of group arrival times on frequency. We present observations of such a dispersion from stations situated at the bending zone of the SE-Carpathians. On the other hand, signals from the same earthquakes, but observed at reference stations outside of the anomalous zone do not show that frequency dependence. A natural explanation for these observations is that it is caused by the presence of a low-velocity channel at the top of the seismic anomaly, which is too thin to be resolved by classical seismic tomographic techniques. Similar observations of dispersed first-arriving P-waves have been made above subduction zones around the world, in which low-velocity layers with a thickness of several kilometers are known to exist. This suggests that a tabular slab of subducted oceanic crust is present within the seismic anomaly under the Vrancea region, and that the anomaly consists of subducted oceanic lithosphere rather than continental lithosphere, at least at depths shallower than the seismically active zone. [Copyright &y& Elsevier]

Abstract: Collision between continents can lead to the subduction of continental material. If the crust remains coupled to the downgoing slab, a large buoyancy force is generated. This force slows down convergence and promotes slab detachment. If the crust resists to subduction, it may decouple from the downgoing slab and be subjected to buoyant extrusion. We employ two-dimensional thermo-mechanical modelling to study the importance of crustal rheology on the evolution of subduction–collision systems. We propose simple quantifications of the mechanical decoupling between lithospheric levels (σ ⁎) and the potential for buoyant extrusion of the crust (ξ ⁎). The modelling results indicate that a variable crustal rheological structure results in slab detachment, delamination, or the combination of both mechanisms. A strong crust provides coupling at the Moho (low σ ⁎) and remains coherent during subduction (low ξ). It promotes deep subduction of the crust (180km) and slab detachment. Exhumation occurs in coherent manners via eduction and thrusting. Slab detachment triggers the development of topography (>4.5km) close to the suture. A contrasting style of collision occurs using a weak crustal rheology. Mechanical decoupling at the Moho (high σ ⁎) promotes the extrusion of the crust (high ξ), disabling slab detachment. Ongoing shortening leads to buckling of the crust and development of topography on the lower plate. Collisions involving rheologically layered crust allow decoupling at mid-crustal depths. This structure favours both the extrusion of upper crust and the subduction of the lower crust. Such collisions are successively affected by delamination and slab detachment. Topography develops together with the buoyant extrusion of crust onto the foreland and is further amplified by slab detachment. Our results suggest that the occurrence of both delamination (Apennines) and slab detachment (Himalayas) in orogens may indicate differences in the initial crustal structure of subducting continental plates in these regions. [Copyright &y& Elsevier]

Abstract: In a recent paper Booden et al. (2011) present new geochemical and petrological data of Early Miocene volcanics from the Northland region (Northland volcanic belt) in New Zealand, and interpret these data to support a particular regional tectonic model. This tectonic model involves Early Miocene westward subduction of Cretaceous Pacific oceanic lithosphere below the Northland volcanic belt and the authors interpret the volcanic belt as a continental magmatic arc. Although the new data are not in disagreement with such a tectonic model, they provide more support for an alternative interpretation that involves a northeast-dipping subduction zone. Furthermore, geometric and plate kinematic data show that the west-dipping subduction model is unviable, geological and geophysical data contradict the model, while geodynamic arguments indicate that the model is implausible. Here it will be shown that a subduction model, involving a northeast-dipping southwestward retreating slab (made of the Late Cretaceous–Paleocene South Loyalty backarc basin lithosphere) that subsequently detaches, is in agreement with the local geology, geophysics and geochemistry, is geometrically, kinematically and geodynamically viable, and fits within the regional Southwest Pacific tectonic framework. [Copyright &y& Elsevier]

Abstract: Studies have documented several rapid changes along the Pacific–North American, Pacific–Antarctic and Pacific–Australian plate boundaries in latest Miocene to earliest Pliocene time consistent with a sudden clockwise rotation of Pacific plate velocity relative to hotspots during this period. We test the hypothesis that this change in plate motion was initiated by cessation of subduction along the northern Melanesian arc due to the collision between the arc and the Ontong Java plateau. This hypothesis has long been formulated but never tested quantitatively. We use a geomechanical model of the lithosphere to determine the changes in plate boundary forces that are necessary to obtain the observed change in the Pacific plate motion. Our model results show that the change in motion can be explained by a clockwise rotation of the slab-related (basal-strength) component of plate driving force. The change of slab-related force from a post-6Ma to a pre-6Ma setting is perpendicular to the arc and points towards the Australian plate. The force per unit length is in the range of currently accepted values for subduction zones. Since there have been no other relevant changes at subduction zones along the Pacific plate boundary during the latest Miocene, we relate this change in slab-related force to the former southward-dipping Pacific plate slab along the northern Melanesian arc system which is now detached. Our model results suggest that rapid changes in plate motion can be triggered by slab detachment, with consequences for plate boundary processes even at great distances from the event. [Copyright &y& Elsevier]

Abstract: The Hercynian Köse composite pluton (KCP) is located in the Eastern Pontides, Turkey, and consists of two units of high-K calc-alkaline, primarily peraluminous granites: (i) the internal body, and (ii) the external body. The internal body, which was emplaced at 322–318Ma (40Ar/39Ar ages on biotite and hornblende, respectively), displays a wide compositional range (49–71wt.% SiO2) and contains several lithologies: hybrid equigranular rocks, microgranular magmatic enclaves, mafic dikes, porphyry dikes and mylonites. The external body, which was emplaced at 306.7Ma (40Ar/39Ar age on K-feldspar), consists exclusively of monzogranite (>71wt.% SiO2). Field relationships, mineralogy, major- and trace element geochemistry, and initial Sr–Nd isotope values (I Sr =0.70821 to 0.71002, eNd(t)=−6.6 to −8.0) show that the internal body was differentiated and evolved by crystal fractionation and magma mixing processes. The end-members of the mixing process were a mafic rock and a felsic rock. Mafic magma was derived from a relatively deep-seated (25–30km) crustal storage reservoir, not directly from the mantle, and underwent significant differentiation by fractional crystallization and crustal contamination before mixing. In addition, these magma storages probably supplied the additional heat necessary to initiate crustal melting. Some of the additional heat may have also been released by the radiogenic decay of heat producing elements. Eventually, the existing felsic magma from the melting of K-bearing meta-greywackes was raised to its emplacement level at a depth of ~10–16km. After partial crystallization, it was sporadically intruded by modified mafic magma from the deeper crustal reservoir to generate hybrid rocks. The hybrid rocks were then elevated to a shallower depth by normal faults during the collapse of the orogen and erosion. Mylonites that were later overprinted by pseudotachylites are typically constrained to the internal body and are regarded as markers of this event. The external body is characterized by a significantly less radiogenic and limited range of Sr–Nd isotope values (I Sr =0.70639 to 0. 70792, eNd(t)=−4.4 to −6.5) than those of the internal body and a lack of rocks documenting the open system differentiation processes. Fractional crystallization is the exclusive process responsible for the elemental range within the body. The rocks also contain less biotite relative to those of the internal body. All these involve less K-bearing mid-crustal rocks (orthogneisses) in their source, which was probably located at depths near the lower crust. The absence of purely lower crustal-derived melts can be explained by the removal of this type of material during the formation of the parental melt. This melt later ascended to its emplacement level at a depth of around ~5–10km and cut the hybrid rocks of the internal body and regional metamorphic rocks that had been raised previously due to ongoing erosion. The melt that injected into the cracks of the internal body crystallized into porphyries because there was not enough time for the entire crystallization of magma. The data presented here indicate that late Early Carboniferous and Late Carboniferous magmatism occurred in a collisional setting. Slab detachment and subsequent delamination seem to be the most plausible mechanisms for the generation of the Hercynian high-K calc-alkaline magmatism in the Eastern Pontides, Turkey. [Copyright &y& Elsevier]

Abstract: We conducted a set of numerical experiments to study the evolution of a subduction–collision system subject to spontaneous slab breakoff. The study takes into account complex rheological behaviour including plasticity, viscous creep and Peierls creep. By varying the oceanic slab age and initial plate convergence rate, four different end-members were observed. In this parameter space, breakoff depth can range from 40 to 400km. Each of those breakoff modes displays complex rheological behaviour during breakoff. Peierls creep in olivine turns out to be a key mechanism for slab breakoff, generally causing slabs to break earlier and at shallower depths. Models involving different depths of breakoff are subject to different topographic evolution, but always display a sharp breakoff signal. Post breakoff uplift rates in foreland and hinterland basins range between 0.1km/My for deep detachment and 0.8km/My for shallow detachment. Our systematic study indicates an approximately linear relationship between the depth of breakoff and the rate of uplift. Continental crust subduction was observed in breakoff experiments involving oceanic lithosphere older than 30My. Different exhumation processes such as slab retreat and eduction occur according to the depth of breakoff. These models are likely to undergo large rebound following breakoff and plate decoupling if the subducted oceanic slab is old enough. [Copyright &y& Elsevier]

Abstract: An analytical solution is presented for the nonlinear dynamics of high amplitude necking in a free layer of power-law fluid extended in layer-parallel direction due to buoyancy stress. The solution is one-dimensional (1-D) and contains three dimensionless parameters: the thinning factor (i.e. ratio of current to initial layer thickness), the power-law stress exponent, n, and the ratio of time to the characteristic deformation time of a viscous layer under buoyancy stress, t/t c . t c is the ratio of the layer''s effective viscosity to the applied buoyancy stress. The value of t c /n specifies the time for detachment, i.e. the time it takes until the layer thickness has thinned to zero. The first-order accuracy of the 1-D solution is confirmed with 2-D finite element simulations of buoyancy-driven necking in a layer of power-law fluid embedded in a linear or power-law viscous medium. The analytical solution is accurate within a factor about 2 if the effective viscosity ratio between the layer and the medium is larger than about 100 and if the medium is a power-law fluid. The analytical solution is applied to slab detachment using dislocation creep laws for dry and wet olivine. Results show that one of the most important parameters controlling the dynamics of slab detachment is the strength of the slab which strongly depends on temperature and rheological parameters. The fundamental conclusions concerning slab detachment resulting from both the analytical solution and from earlier published thermo-mechanical numerical simulations agree well, indicating the usefulness of the highly simplified analytical solution for better understanding slab detachment. Slab detachment resulting from viscous necking is a combination of inhomogeneous thinning due to varying buoyancy stress within the slab and a necking instability due to the power-law viscous rheology (n >1). Application of the analytical solution to the Hindu Kush slab provides no “order-of-magnitude argument” against slab detachment and, therefore, supports existing studies suggesting a currently ongoing slab detachment in the Hindu Kush slab. [Copyright &y& Elsevier]

Necking, tearing, slab detachment and subsequently slab loss complicate the subduction zone processes and slab architecture. Based on evidences which include patterns of seismicity, seismic tomography and geochemistry of arc volcanoes, we have identified a horizontal slab tear in the subducted Indo-Australian slab beneath the Sunda arc. It strongly reflects on trench migration, and causes along-strike variations in vertical motion and geochemically distinct subduction-related arc magmatism. We also propose a model for the geodynamic evolution of slab detachment. [ABSTRACT FROM AUTHOR]

Abstract: Conditions and dynamics of subduction–collision and subsequent 3-D slab break-off and slab tear propagation are quantified, for the first time, using fully dynamic numerical models. Model results indicate that collision after the subduction of old, strong subducting oceanic slab leads to slab break-off at 20–25Myr after the onset of continental collision, and subsequently a slab tear migrates more or less horizontally through the slab with a propagation speed of 100–150mm/yr. In contrast, young, weak oceanic slabs show the first break-off already 10Myr after continental collision, and can experience tear migration rates up to 800mm/yr. Slab strength plays a more important role in the timing of slab break-off and the speed of a propagating slab tear than (negative) slab buoyancy does. Slab break-off is viable even for slabs that are supported by the viscosity jump and phase change between the upper and lower mantle. The density of the oceanic slab and the subducting continental block is important for the amount of continental subduction and the depth of slab break-off. A 40-km thick continental crust can be buried to depths greater than 200km, although this maximum depth is significantly less for younger or very weak slabs, or thicker continental crust. Slab break-off typically starts at a depth of 300km, mostly independent of mantle rheology, but, like continental crustal burial, can be shallower for young or buoyant plates. Our 3-D models illustrate how, due to the difference in necking in 2-D and 3-D, break-off has an intrinsic small preference to start as a slab window within the slab''s interior, rather than as a slab tear at the slab edge. However, any significant asymmetry in the collision setting, e.g. earlier collision at one end of the subduction zone, would override this, and leads to slab tearing starting near one edge of the slab. These results put important new constraints on the dynamics of the collision and subsequent slab break-off for modern collision belts. For a proposed timing of the initial Arabia–Eurasia collision at 35Ma, break-off of the ~200-Myr-old Neo-Tethys slab is unlikely to have occurred before 15–10Ma. Furthermore, our results illustrate that shallow, early break-off of weak slabs provides a viable explanation for the absence of blueschists and ultra-high pressure metamorphism in the Precambrian geological record. [Copyright &y& Elsevier]

Abstract: We analyzed receiver-function data recorded by a temporary broadband array deployed as part of the BOLIVAR project and the permanent seismic network of Venezuela to study the mantle transition zone structure beneath the Caribbean-South American plate boundary and Venezuela. Significant topography on both the 410-km and the 660-km discontinuities was clearly imaged in the CCP (common-conversion-point) stacked images. Beneath the southeastern Caribbean, the 410-km is featured by a narrow (∼200km EW) ∼25-km uplift extending in the NS direction around 63° west, while the 660-km is depressed by ∼20km in a narrow region slightly west to the uplift, a scenario that is more consistent with westward descent of the oceanic South American plate rather than a break-off of NNW dipping proto-Caribbean oceanic lithosphere along the El Pilar Fault. We also found a thick transition zone beneath the Falcon region in northwestern Venezuela, possibly associated with the subducted Nazca plate. A flat 410-km was observed beneath the Guayana shield, suggesting that the shield has a stable and moderately deep keel, which has little effect on the underlying transition zone structure. [Copyright &y& Elsevier]

Abstract: The Southwest Pacific region is tectonically complex and is home to numerous fossil and active subduction zones. At the Earth''s surface, there remains a geological controversy regarding the polarity and continuity of fossil subduction zones in New Zealand and New Caledonia, the origin of obducted ophiolites, the presence of high-pressure metamorphism, the occurrence of widespread Cenozoic magmatism, and the potential disappearance of one or more ocean basins. This controversy can be solved by looking at the lower mantle rather then at the Earth''s surface. New P-wave and S-wave mantle tomography models from the Southwest Pacific are presented, which identify a previously unrecognized lower-mantle high-velocity anomaly that cannot be linked to Pacific subduction. The anomaly is located below the Tasman Sea at ~1100 km depth, strikes NW–SE and is ~2200 km by 600–900 km in lateral extent. By combining relative and absolute plate motions it is demonstrated that when the geological structures at the surface are reinterpreted as a single northeast-dipping 2500-km middle Cenozoic subduction zone (the so-called New Caledonia subduction zone) the lower mantle anomaly can be accounted for, as it is found at the predicted location and depth. Discovery of the lower mantle slab anomaly thereby solves a long-standing geological controversy in the New Zealand–New Caledonia region. Finally, reconstructions and analytical calculations predict a lower mantle slab sinking velocity of ~1.5 cm/yr and a lower mantle viscosity of ~1022 Pa·s. [Copyright &y& Elsevier]

Abstract: The Tonga Wadati–Benioff zone is characterized by a large seismicity gap beneath the Lau Basin that raises the question of the slab continuity between the shallow to intermediate part (60–300 km) and the deep part (400–700 km). To address this problem, we investigated the Wadati–Benioff Zone geometry and stress regime through a detailed analysis of the spatial distribution of moment tensors and variation of the stress tensor, using the global seismicity [Engdahl, E., Van der Hilst, R., Buland, R., 1998. Global teleseismic earthquake relocation with improved travel times and procedures for depth determination. Bull. Seism. Soc. Am. 88, 722–743.] and the Centroid Moment Tensor solutions (CMT) catalogs [Dziewonski, A.M., Chou, T., Woodhouse, J.H., 1981. Determination of earthquake source parameters from waveform data for studies of global and regional seismicity. J. Geophys. Res. 86, 2825–2852.]. The stress tensors were computed using the Gephart''s program [Gephart, J.W., Forsyth, D., 1984. An improved method for determining the regional stress tensor using earthquake focal mechanism data: application to the San Fernando earthquake sequence. J. Geophys. Res. 89, 9305–9320.]. The stress inversion results indicate that between 21°S and 27°S, and depths down to 700 km the slab is under homogeneous down dip compressional stress regime, while north of 21°S we found strong variations of the stress orientations between the intermediate and deep portions of the slab. We also show that between 14° and 19°S, the stresses at intermediate depth (60–300 km) can be resolved into two slab parallel domains, a thin upper part of the slab that is under downdip compression and the lower part that is under downdip extension. This pattern of two zones with opposite mechanical behavior is characteristic of a subducted plate with a free lower limit that does not interact with the 670-km depth boundary. These results together with the large seismicity gap within the slab argue for a slab detachment. [Copyright &y& Elsevier]

Abstract: The detachment of subducted lithosphere (slabs) has been proposed in a number of locations as the cause for observed volcanism and regional tectonic evolution. For example, along-strike progression of uplift in the Adriatic Sea and west-to-east progression of volcanism in the Trans-Mexican Volcanic Belt may be caused by propagating detachment of subducted slabs. An understanding of the dynamics of slab detachment is necessary to relate the timing of detachment to surface observations. We demonstrate using two-dimensional (2D) numerical models that the timing, occurrence, and surface effects resulting from the detachment of subducted lithosphere are strongly dependent upon the stiffness (i.e. stress supported viscously) of the lithosphere and rheology of the upper mantle. In particular, in models with a Newtonian upper mantle where plastic yielding is the only weakening process within the slab, detachment does not occur. Models accounting for non-Newtonian upper mantle deformation demonstrate two modes of detachment: (1) slow and deep detachment of strong slabs (500 MPa or greater maximum yield strength) controlled by heating of the slab, and (2) fast and shallow detachment of weaker slabs (300 MPa) controlled by yielding within the slab interior. For slabs with a maximum yield strength of 500 MPa, the time to detachment increases with slab age (for ages ranging from 40 to 120 My), indicating that detachment time is controlled by the integrated stiffness of the slab, which limits the sinking rate. However, the detachment time for weaker slabs (300 MPa) is independent of age because weaker slabs sink rapidly, and the time to detachment is limited by the rate of flow of the surrounding mantle. [Copyright &y& Elsevier]

Abstract: We report the results of an isotopic study of the large gabbro-dioritic Beja Igneous Complex (BIC) in the boundary between the highly contrasting Ossa–Morena (OM) and South Portuguese (SP) Zones of the Southern Iberian Variscan orogen. This boundary is interpreted as a major suture zone materialized by discontinuous, scattered strips of mafic/ultramafic rocks (the so-called Beja–Acebuches ophiolite complex, BAOC), and by mélange deposits of Middle to Late Devonian age in the Pulo do Lobo accretionary prism (PLAP). The Beja gabbro was interpreted either as part of the ophiolite-like units, or as a broadly arc-related massif reflecting the northward subduction of oceanic lithosphere. U–Pb zircon (ID-TIMS) dating of two diorites and a granodiorite points to igneous emplacement ages of 350±2 Ma (Serpa), 352±2 Ma (Torrão), and 353±4 Ma (São Pedro), respectively, whereas a felsic dyke yields a slightly younger age of 345±2 Ma. These results show that published Ar/Ar dates do not represent igneous crystallization ages, but merely reflect regional cooling below ca. 500 °C, at least 10 Ma after the major intrusive event, probably as a result of uplift of the OMZ side of the suture zone relative to the subsiding SPZ. 87Sr/86Sr350 and εNd350 display a large range of values (from 0.7041 to 0.7093 and from +4.0 to −6.1, respectively) which documents a rather complex petrogenetic history, with an important role played by crustal contamination processes. The more primitive Sr and Nd isotope signatures are measured in the mafic cumulates, while radiogenic Sr and unradiogenic Nd isotope compositions occur in the more evolved rock-types. The broad trend of decreasing εNd350 with decreasing Sm/Nd and increasing SiO2 concentration is reminiscent of crustal assimilation combined with fractional assimilation (AFC). εNd values of flasergabbros and associated cumulates ascribed to the ophiolite-like unit in the Guadiana valley are close to zero or even slightly negative, irrespective of the age chosen for the correction for in situ radioactive decay of 147Sm in the range 350–500 Ma, assumed to encompass the true geological age. This observation conflicts with a truly oceanic derivation of these rocks, which could rather represent early stage, deformed and metamorphosed equivalents of the Beja gabbro. Most metabasalts of the same area have positive, albeit variable εNd, consistent with mildly depleted mantle sources and/or significant crustal assimilation. Only a few samples reach highly positive values (εNd350–500 ca. +8) similar to those typical for N-MORBs, as do the Acebuches amphibolites near Aracena (εNd350–500 ca. +9). These data suggest that the BAOC is not only disrupted but also composite, since the gabbroic rocks are not cogenetic with the basalts. Based on the Late Devonian age of the collision between the OMZ and SPZ, the emplacement of the Beja gabbro cannot be related to an active subduction of oceanic lithosphere. Instead, the generation in the mantle, and emplacement in the overlying continental crust of copious volumes of mafic magmas with concomitant LP-HT metamorphism is tentatively ascribed to a process of transcurrent slab break-off that followed oblique subduction and collision. [Copyright &y& Elsevier]

Abstract: Imaging of seismic velocity discontinuities with receiver functions suggests a complex mantle structure beneath the East Anatolian Plateau. By stacking all the receiver functions as a function of ray parameter, we examined the moveout characteristics of the data set and tested different velocity models to fit the arrival times of converted phases from the Moho and the lithosphere–asthenosphere boundary and their multiples. The best fitting model shows anomalously thin crust (40 km) and lithosphere (65 km), and low uppermost mantle velocities causing systematic delays on later discontinuity arrivals. These findings are consistent with the high plateau supported by partially molten and buoyant asthenosphere. The upper mantle discontinuities are all visible in different frequencies (0.15 and 0.4 Hz). Especially, the 410 km discontinuity does not show frequency dependence in amplitude and suggests a sharpness of 10 km or less. Spatial variations of the 410 and 660 km discontinuities reveal anticorrelated topography and distinct zones of diminished amplitude associated with a detached slab and delaminated fragments of lithospheric mantle. In the south, the slab becomes deeper and horizontally deflected towards the east suggesting westward migration of slab detachment and resistance to slab penetration at the 660 km discontinuity. In the north, the mantle transition zone is characterized by two spatially limited lithospheric fragments with complex geometry. At the center of the study area, the transition zone is thin (230 km) suggesting excess temperature of 120 °C. This warm mantle anomaly most likely is due to hot material trapped within the upper mantle between slab in the south and delaminated fragments in the north. [Copyright &y& Elsevier]

Abstract: The Southern Italy volcanism is characterized by the unusual occurrence of volcanic rocks with ocean-island basalt (OIB)-like characteristics, in particular at Etna and Iblean Mts in Sicily. The geochemical properties of the source of the Italian magmatism are usually explained by a north–south binary mixing between a mantle- and a crustally-derived end-members. The nature of the mantle end-member is, however, not agreed upon. One type of interpretation invokes a mixture of depleted mantle (DMM) and high U/Pb (HIMU) end-members [Gasperini, D., Blichert-Toft, J., Bosch, D., Del Moro, A., Macera, P., Albarède, F., 2002. Upwelling of deep mantle material through a plate window: Evidence from the geochemistry of Italian basaltic volcanics, J. Geophys. Res. 107, 2367–2386], whereas an alternative view holds that the mantle end-member is unique and homogeneous, and similar to the FOZO- or C-type end-member identified in oceanic basalts [Bell, K., Castorina, F., Lavecchia, G., Rosatelli, G., Stoppa, F., 2004. Is there a mantle plume below Italy? EOS 85, 541–547]. Because mixing does not produce linear relationships between the isotopic compositions of different elements, we applied Principal Component Analysis (PCA) to the Pb isotope compositions of the Italian volcanics inclusive 36 of Sicily volcanoes. We demonstrate that HIMU cannot be an end-member of the Italian volcanics, but rather that the common component C (∼FOZO), which we interpret as reflecting the lower mantle, best represents the mantle source of the Italian magmatism. Our PCA calculation shows that the first principal component alone, which we take to be a mixture of two geochemical end-members, C and a crustally-derived component, explains 99.4% of the whole data variability. In contrast, the DMM end-member (the second principal component) is only present in the volcanics from the Tyrrhenian Sea floor. The C-like end-member, well represented by the Etna and Iblean Mts (Sicily), has relatively low 3He/4He ratios suggesting upwellings of lower mantle material from the 670 km transition zone. A slab detachment beneath the central-Southern Italy and probably the Sicily could account for the particular character of Italian magmatism. [Copyright &y& Elsevier]

Abstract: Strong compositional variations are observed in the late-Miocene to Quaternary volcanic rocks of the eastern Trans-Mexican Volcanic Belt. Geochemical and isotopic analyses of samples well constrained in age indicate an abrupt change in magma composition in the late-Miocene (∼ 7.5 Ma), when calc-alkaline, subduction-related magmatism was replaced by mafic, alkaline, OIB-like volcanism. Afterwards, volcanism migrated toward the trench and the erupted lavas showed increasing contributions of subduction components reflected in higher Th/Nb, La/Sm(n), Ba/Nb, and Ba/Th ratios. Lavas from volcanic fields located closer to the trench show clearer, although strongly variable, arc signatures as well as evidence of subducted sediment contributions. Farther from the trench, only lavas emplaced in late-Pliocene time appear to be slightly modified by subduction components, whereas the youngest Quaternary lavas can be regarded as intraplate lavas modified by crustal assimilation. The sudden change in magma composition in the late-Miocene is related to detachment of the subducting slab, which allowed the infiltration of enriched asthenospheric mantle into the mantle wedge. After detachment, the subducting plate started to increase its dip because of the loss of slab pull. This caused (1) the migration of the arc toward the trench, (2) convection of enriched asthenosphere into the mantle wedge, and (3) an increasing contribution of slab components to the melts, in a process that resulted in a highly heterogeneous source mantle. The variable contribution of subduction-related components to the magmas is controlled by the heterogeneous character of the source, the depth of the subducting plate, and the previous magmatic history of the areas. [Copyright &y& Elsevier]

Abstract: We use measurements of crustal and upper mantle seismic attenuation in the southeastern Carpathian Arc and surroundings to test mantle responses to Tethys closure in this region. Active seismicity at the Carpathians'' Vrancea bend zone can delimit lithosphere and asthenosphere distributions through attenuation observations at overlying seismic stations. We measure frequency-independent attenuation via an iterative spectral ratio method which compares P and S spectra for evolving time windows of both arrivals, yielding 400 individual estimates of apparent differential QS and one composite measurement based on a normalized sum of the spectral ratios determined for the individual Q estimates. The procedure allows exclusion of subtle multipathed phases with attenuation different from that of the direct arrivals and yields a robust estimate of the measurement uncertainty. Measurements are retained for interpretation if the mean of the 400 individual Q estimates and the composite spectra Q estimate fall within the standard deviation of the 400 measurements. Results for 65 earthquakes recorded at the German–Romanian K2 accelerometer network during 1999 fall into clear groups: attenuation is low (high QS) at stations east and north of the Vrancea zone on the East European platform, the Scythian Platform, and in the eastern portion of the Moesian Platform. Inconsistent results at stations west of the Intramoesian fault, including those in and around Bucharest, probably reflect strong site effects at several locales. Attenuation at stations above and near the Vrancea zone, and at stations in the Transylvanian Basin, is high (low QS), most likely due to the presence of hot, shallow asthenosphere in these areas. The technique we developed may ultimately yield a strong test of slab detachment and continental lithosphere delamination hypotheses put forth to explain the unusual seismicity and volcanism of the Carpathian arc. [Copyright &y& Elsevier]

During the Alpine orogeny, the Penninic zone of the Alps was affected by Eoalpine high-pressure metamorphism. In the central and western Alps, this was followed by Lepontine medium-pressure, high-temperature metamorphism during exhumation. We compare the pressure–temperature–time (PTt) trajectories established in two key areas in the central and western Alps with 2-D numerical models of two possible causes of Lepontine metamorphism: (1) detachment or breakoff of a subducting slab, and (2) the presence of a wedge of accreted radiogenic material. Numerical models show that both mechanisms are capable of producing significant heating during orogeny. Heating by slab detachment is fast and transient (more than 100°C in up to 10 million years, depending on the location), whereas radiogenic heating requires time spans of the order of tens of millions of years and cessation of the subduction process. The combination of PTt trajectories and synthetic PT paths deduced from our thermomechanical modelling results suggests that the metamorphism observed in the central Alps has not been caused by radiogenic heating alone. Slab breakoff, on the other hand, seems a viable mechanism to account for the documented rise in metamorphic temperatures during exhumation. In view of the time constraints posed by the geological data, and acknowledging the effects of large-scale block rotations and out-of-section transport, slab detachment is also a more likely mechanism to have provided sufficient heat to cause re-heating in the western Alps. [Copyright &y& Elsevier]

Detachment of the deeper part of subducted lithosphere causes changes in a subduction zone system which may be observed on the Earth''s surface. Constraints on the expected magnitudes of these surface effects can aid in the interpretation of geological observations near convergent plate margins where detachment is expected. In this study, we quantify surface deformation caused by detachment of subducted lithosphere. We determine the range of displacement magnitudes which can be associated with slab detachment using numerical models. The lithospheric plates in our models have an effective elastic thickness, which provides an upper bound for rapid processes, like slab detachment, to the surface deformation of lithosphere with a more realistic rheology. The surface topography which develops during subduction is compared with the topography shortly after detachment is imposed. Subduction with a non-migrating trench system followed by detachment leads to a maximum surface uplift of 2–6 km, while this may be higher for the case of roll-back preceding detachment. In the latter situation, the back-arc basin may experience a phase of compression after detachment. Within the context of our elastic model, the surface uplift resulting from slab detachment is sensitive to the depth of detachment, a change in friction on the subduction fault during detachment and viscous stresses generated by sinking of the detached part of the slab. Overall, surface uplift of these magnitudes is not diagnostic of slab detachment since variations during ongoing subduction may result in similar vertical surface displacements. [Copyright &y& Elsevier]

The Sierra Madre Occidental (SMO) of western Mexico is one of the largest silicic volcanic provinces on Earth, but the mechanism for the generation of such a large volume of ignimbrites has never been clearly defined. We present new 40Ar/39Ar ages, geologic mapping, and structural data for the southern part of the SMO demonstrating that most of this volcanic province was built in two episodes of ignimbrite flare-up in Oligocene (31.5-28 Ma) and early Miocene (23.5-20 Ma) time, and that extensional deformation occurred mostly before the transfer of Baja California to the Pacific plate. Extensive ignimbrite successions, with 40Ar/39Ar ages clustering at ∼23 and ∼21 Ma, cover most of the southern SMO, thus correlating in age with ignimbrites exposed in southern Baja California and central Mexico. Grabens with a 020° to N-S orientation developed in the east almost concurrently with this volcanic episode. Half grabens and NNW striking listric normal fault systems formed at the end of middle Miocene as far as 150 km from the present coast. A belt of left-lateral transpressional structures formed along the southern boundary of the SMO during the same period. We link these magmatic and tectonic events to the evolution and dynamics of the Farallon and North America plates during the Miocene. Particularly, we propose that a first detachment of the lower part of the Farallon plate in early Miocene time produced a transient thermal event and partial melting of the crust via mafic underplating. Middle Miocene extension would be related to a second detachment event, resulting from the slowing subduction that preceded the final capture of the Magdalena microplate by the Pacific plate at 12.5 Ma. Transpression at the southernmost end of the SMO occurred along the inland projection of the Magdalena-Cocos plate boundary and may be explained by a difference in subduction rate and by a temporal convergence between the two plates in the eve of the end of subduction of the Magdalena plate. [ABSTRACT FROM AUTHOR]

We map the lateral variations of P-wave velocity beneath the Zagros collision zone in Iran down to 800 km depth by regional travel time tomography. We invert 32,293 relative travel time residuals from five temporary seismic networks across the collision zone in NW and central Zagros and northern Iran, and a large number of permanent stations distributed over the Iranian Plateau. A salient feature of our model is a ~225 km thick lithosphere beneath the Zagros, almost twice as thick as in the rest of Iran. Slab detachment at a depth range of 250–300 km from the base of the subducted continental Arabia is clearly distinguishable in central Zagros. In NW Zagros detachment appears to be at an earlier stage. Beneath the central Iran Plateau, we observe shallow low-velocity regions down to 200 km depth, and smaller patches with significantly higher velocity in the 200–400 km depth range. This pattern might suggest a post-collisional foundering of the mantle lithosphere in the upper plate. The subducted slab seems to penetrate into the lower mantle in a segmented fashion, with no evidence for a stagnant slab above the 660 km interface. We map the lateral variations of P-wave velocity beneath the Zagros collision zone in Iran down to 800 km depth by regional travel time tomography. Our tomographic study documents variations of lithospheric thickness between the Zagros and central Iran and a post-collisional slab detachment around 250 km depth. If we assume that the detachment occurred right beneath the base of the subducted Arabian continental margin, then the maximum thickness of the continental root must be no >225 km. This would imply a doubling of the Zagros lithosphere as a result of collision. The absence of high-velocity anomalies far north from the MZT at the lithospheric depth range indicates that the lithosphere thickening has not propagated into the interior of central Iran. The high-velocity anomalies at 200–300 km depth beneath central Iran suggest the presence of fragments of delaminated mantle lithosphere. They also suggest post-collisional slab break-off in central Zagros that seems to be at an earlier stage in NW Zagros compared to central Zagros. From the shallow depth of the top of the detached slab, we infer that the break-off is recent (5–10 Ma), in good agreement with geological records. Effective detachment of an oceanic slab from continental lithosphere is often followed by the upwelling of the asthenosphere through slab windows, eduction, slab retreat, and eventually volcanism and exhumation of ultra-high-pressure rocks, which are not currently observed in central Iran. On the other hand, lack of deep seismicity, especially deep extensional events in the Zagros, indicates that the subducted slab does not exert gravitational pull on the base of the continental plate to reconcile these apparently contradictory observations, we propose that slab break-off in NW Zagros is very young. [Display omitted] • Upper mantle structure of the Zagros collision zone via teleseismic tomography. • Evidence for a thicker lithosphere beneath the Zagros versus thinner lithosphere in the surrounding regions. • Slab detachment near the base of the thickened Zagros lithosphere. • Seismological evidence for mantle lithospheric delamination beneath central Iran. [ABSTRACT FROM AUTHOR]