Slab Dip Angle Research Articles

Asthenospheric buoyancy and the origin of high-relief topography along the Cascadia forearc

The forearc topography of the Cascadia subduction zone varies systematically along-strike, with high-relief in the north and south (the Olympic and Klamath ranges) separated by relatively low relief in its central region. This systematic topographic variability reflects the long-term pattern of uplift and erosion, however, the underlying cause of uplift and the mechanisms by which current topography is supported are unclear. Here, we synthesize results from seismic imaging, geodetic (decadal) and geomorphic (millennial) uplift rates, erosion rates, topographic analysis, and characteristics of the megathrust interface, with a mechanical model to infer that buoyancy in the subslab asthenosphere influences the development and longevity of Cascadia's forearc topography. The Cascadia margin can be divided into three broad segments, with the northern and southern segments characterized by rapid geodetic and geomorphic uplift rates, rapid erosion rates, high coseismic subsidence of great megathrust ruptures, shallower slab dip angles, and increased plate locking compared to the central segment. Tomographic images show low-velocity anomalies beneath the slab in the northern and southern segments, which are inferred to be regions of partial melt, resulting from localized upwellings, and regions of positive mantle buoyancy. Modeling suggests that these buoyant regions can locally increase the total shear force at the megathrust by either shallowing the slab dip — thereby increasing the area of the seismogenic zone — or increasing mechanical plate coupling along the megathrust, by increasing normal stress and/or the effective coefficient of friction. We propose that: 1) sub-slab buoyancy influences topographic development by modulating along-strike patterns of strain within the over-riding plate during the seismic cycle, 2) Permanent development of surface topography occurs due to unrecovered strain over thousands of seismic cycles, and 3) variations in Cascadia's forearc topography are laterally supported by changes in the total shear force at the megathrust interface. Using independent estimates for slab dip and plate locking, we predict the first-order variations in Cascadia forearc topography, with local maxima in the north and south as observed. Given our results, variations in subslab buoyancy may be critical to explaining forearc topography in other subduction systems.

Filling the gap in a double seismic zone: Intraslab seismicity in Northern Chile

Double seismic zones (DSZs) of intermediate-depth intraslab seismicity are observed in many subduction zones around the globe, and have been related to dehydration reactions in the downgoing crust and mantle lithosphere. These reactions occur at, to first order, constant temperatures, which explains the observed linear arrangements of seismicity that appear to follow isotherms of thermal models.Intermediate-depth seismicity in Northern Chile, however, exhibits a pattern of intraslab seismicity that substantially deviates from a classical DSZ. Whereas two parallel seismicity planes are present in the updip part of the slab, these abruptly change into a 25–30 km thick, homogeneously seismogenic volume at a depth of ∼80–100 km. Seismicity rate and moment release significantly increase in this depth interval. In order to understand which processes evoke this configuration and what distinguishes the Northern Chile subduction zone from more conventional subduction zone settings (e.g. Japan), we performed a detailed seismological investigation of slab seismicity in Northern Chile using data from the IPOC permanent network. We determined >600 moment tensors of intraslab earthquakes, processed and evaluated location uncertainties for 8 years of high-resolution earthquake hypocenter data, and performed statistical analysis of the different seismicity populations.We observe that earthquakes both in the highly active cluster and the DSZ above exhibit consistently downdip extensive source mechanisms that align with the dip angle and direction of the slab. This implies strong slab pull, which is also evident from slab steepening outlined by hypocenters towards the downdip termination of the highly active cluster. Moreover, events in the cluster show a very weak aftershock productivity and a high background event rate, which leads to a temporal distribution of seismicity that is close to a purely random process. We find that the position of the highly seismogenic volume spatially coincides with: 1) the disappearance of the velocity contrast between oceanic crust and the underlying mantle in receiver function images, 2) the transition from the “cold nose” (i.e. the stagnant part) to the hot part of the mantle wedge, as evidenced by seismic attenuation images, and 3) with an increase of the slab dip angle. Based on these different pieces of evidence, we speculate that high tensile stresses and heat input from above could lead to a sudden burst of kinetically delayed metamorphic reactions there that then enables the observed increased seismicity rates. Since these reactions have overall a negative volume change that leads to slab densification and hence further increases slab pull, the spatial pattern of seismicity we observe could result from a runaway-type process, which would explain its abrupt start and high moment release rates.

Along-strike variation in slab geometry at the southern Mariana subduction zone revealed by seismicity through ocean bottom seismic experiments

SUMMARYWe have conducted the first passive Ocean Bottom Seismograph (OBS) experiment near the Challenger Deep at the southernmost Mariana subduction zone by deploying and recovering an array of 6 broad-band OBSs during December 2016–June 2017. The obtained passive-source seismic records provide the first-ever near-field seismic observations in the southernmost Mariana subduction zone. We first correct clock errors of the OBS recordings based on both teleseismic waveforms and ambient noise cross-correlation. We then perform matched filter earthquake detection using 53 template events in the catalogue of the US Geological Survey and find >7000 local earthquakes during the 6-month OBS deployment period. Results of the two independent approaches show that the maximum clock drifting was ∼2 s on one instrument (OBS PA01), while the rest of OBS waveforms had negligible time drifting. After timing correction, we locate the detected earthquakes using a newly refined local velocity model that was derived from a companion active source experiment in the same region. In total, 2004 earthquakes are located with relatively high resolution. Furthermore, we calibrate the magnitudes of the detected earthquakes by measuring the relative amplitudes to their nearest relocated templates on all OBSs and acquire a high-resolution local earthquake catalogue. The magnitudes of earthquakes in our new catalogue range from 1.1 to 5.6. The earthquakes span over the Southwest Mariana rift, the megathrust interface, forearc and outer-rise regions. While most earthquakes are shallow, depths of the slab earthquakes increase from ∼100 to ∼240 km from west to east towards Guam. We also delineate the subducting interface from seismicity distribution and find an increasing trend in dip angles from west to east. The observed along-strike variation in slab dip angles and its downdip extents provide new constraints on geodynamic processes of the southernmost Mariana subduction zone.

Impact of Aseismic Ridges on Subduction Systems: Insights From Analog Modeling

AbstractThe influence of aseismic ridges on subduction kinematics and dynamics, and on the deformation of the overriding plate, remains a topic of debate. This study presents laboratory‐based geodynamic models that simulate the process of aseismic ridge subduction below an overriding plate. The analog experiments show that, depending on its size, subduction of an aseismic ridge can impact on the kinematics of subduction, overriding plate deformation and, to a lower extent, on the geometry of the slab and that of the trench. Specifically, it can cause a reduction of the subducting plate and trench retreat velocities, decreases overriding plate extension and even drives local forearc shortening for the widest ridge, increases the slab dip angle next to the aseismic ridge, but decreases the dip angle at the aseismic ridge in case the ridge is thick and wide, and produces a local perturbation (indentation) of the trench geometry. The magnitude of the described modifications relies to a comparable extent on aseismic ridge width and thickness. The effects of aseismic ridge subduction observed in our models compare to those of the Louisville Seamount Chain, the Cocos Ridge, and the d'Entrecasteaux Ridge system in nature and provide an explanation for the local deformation observed at these subduction zones.

Detailed lithospheric structure of an arc-continent collision beneath Taiwan revealed by joint inversion of seismological and gravity data

The largest compilation of seismological and gravimetry data are combined in a sequential inversion to shed light on unresolved crustal structures in Northern Taiwan and Southernmost Ryukyu area. Our density and seismic velocity models reveal a NW–SE trending Transfer Zone (TZ) between 23.25°N and 24°N. This primary TZ delimits major changes in the Eurasian Plate (EP) slab geometry and dynamics, as well as first order crustal structure variations. North of this TZ, the EP slab is breaking off from north to south as a result of (1) the extreme EP bending favouring plastic deformation, (2) the stretching of the plate caused by the downward pull of the oceanic part of the slab versus the buoyancy of continental part of the plate still deforming in the Taiwan orogen and (3) the indentation of the EP slab by the western slab edge of the Philippine Sea Plate (PSP). The change of EP slab dip angle is clearly observed in the density profiles. The subvertical northernmost tip of EP slab allows asthenospheric upwelling through slab tear and subsequent invasion of the region beneath the Longitudinal Valley. This process is migrating southward up to the TZ, which marks the transition to a different rheological behaviour. The combination of deep arc-continent collision, clockwise asthenospheric toroidal flow and westward propagation of the southern Okinawa rift results in a 100° clockwise rotation of the northernmost tip of the Central Range. The eastward extrusion of this crustal block (∼90 km in diameter) overlays and deflects the PSP slab. The indentation of the EP by the PSP (Luzon Arc) thus results in complex lithospheric accommodation processes and structures.

Andean mountain building and magmatic arc migration driven by subduction-induced whole mantle flow

Subduction along the western margin of South America has been active since the Jurassic, but Andean orogeny started in the middle Cretaceous and was preceded by backarc extension in the Jurassic-Early Cretaceous. The timing and sequence of these events has remained unexplained. Here I present a four-dimensional buoyancy-driven whole-mantle subduction model implying that the ~200 Myr geological evolution can be attributed to sinking of a wide slab into a layered mantle, where upper-mantle wide-slab subduction causes backarc extension, while whole-mantle (upper+lower) wide-slab subduction drives Andean orogeny. The model reproduces the maximum shortening and crustal thickness observed in the Central Andes and their progressive northward and southward decrease. The subduction evolution coincides with a 29° decrease in slab dip angle, explaining ~200 km of Jurassic-present eastward migration of the Central Andean magmatic arc. Such arc migration negates proposed long-term subduction erosion and continental destruction, but is consistent with long-term crustal growth.

Horizontal mantle flow controls subduction dynamics

It is generally accepted that subduction is driven by downgoing-plate negative buoyancy. Yet plate age –the main control on buoyancy– exhibits little correlation with most of the present-day subduction velocities and slab dips. “West”-directed subduction zones are on average steeper (~65°) than “East”-directed (~27°). Also, a “westerly”-directed net rotation of the lithosphere relative to the mantle has been detected in the hotspot reference frame. Thus, the existence of an “easterly”-directed horizontal mantle wind could explain this subduction asymmetry, favouring steepening or lifting of slab dip angles. Here we test this hypothesis using high-resolution two-dimensional numerical thermomechanical models of oceanic plate subduction interacting with a mantle flow. Results show that when subduction polarity is opposite to that of the mantle flow, the descending slab dips subvertically and the hinge retreats, thus leading to the development of a back-arc basin. In contrast, concordance between mantle flow and subduction polarity results in shallow dipping subduction, hinge advance and pronounced topography of the overriding plate, regardless of their age-dependent negative buoyancy. Our results are consistent with seismicity data and tomographic images of subduction zones. Thus, our models may explain why subduction asymmetry is a common feature of convergent margins on Earth.

Trench motion‐controlled slab morphology and stress variations: Implications for the isolated 2015 Bonin Islands deep earthquake

AbstractThe subducted old and cold Pacific Plate beneath the young Philippine Sea Plate at the Izu‐Bonin trench over the Cenozoic hosts regional deep earthquakes. We investigate slab morphology and stress regimes under different trench motion histories with mantle convection models. Viscosity, temperature, and deviatoric stress are inherently heterogeneous within the slab, which we link to the occurrence of isolated earthquakes. Models expand on previous suggestions that observed slab morphology variations along the Izu‐Bonin subduction zone, exhibited as shallow slab dip angles in the north and steeper dip angles in the south, are mainly due to variations in the rate of trench retreat from the north (where it is fast) to the south (where it is slow). Geodynamic models consistent with the regional plate tectonics, including oceanic plate age, plate convergence rate, and trench motion history, reproduce the seismologically observed principal stress direction and slab morphology. We suggest that the isolated ~680 km deep, 30 May 2015 Mw 7.9 Bonin Islands earthquake, which lies east of the well‐defined Benioff zone and has its principal compressional stress direction oriented toward the tip of the previously defined Benioff zone, can be explained by Pacific slab buckling in response to the slow trench retreat.

Early Andean tectonomagmatic stages in north Patagonia: insights from field and geochemical data

The Andes in northern Patagonia are mainly formed by Mesozoic magmatic units: the mostly Jurassic–Cretaceous North Patagonian Batholith and volcanism of the Jurassic Lago La Plata (Ibáñez) Formation as well as the mid-Cretaceous Divisadero Group. These rocks represent the development of a magmatic belt through Jurassic–mid-Cretaceous time, during a switch of the tectonic regime from extension to compression. To study arc evolution during this transition, we carried out fieldwork and geochemical sampling at c. 43°S, clarifying structural relationships and characterizing the magmatic sources. Multi-element diagrams for both volcanic units suggest a slab-derived signature, whereas isotopic ratios (Sr–Nd–Pb) indicate parental melts sourced from the subduction-modified asthenospheric mantle interacting with crustal sources during their emplacement. An angular unconformity is identified between the synextensional Jurassic volcanic rocks and Lower Cretaceous sedimentary rocks beneath the mid-Cretaceous sequences. Although this deformational event was simultaneous with generalized overriding plate compression, geochemical ratios indicate an immature Aptian–Albian arc with no associated crustal thickening. Late Jurassic to mid-Cretaceous arc settlement after a trenchward retraction of magmatism from the foreland between c. 41 and 45°S, with an associated increase in slab dip angle, may have provoked crustal softening facilitating the subsequent initial fold–thrust belt growth. Supplementary material: Petrographic descriptions and geochemical–isotopic data are available at https://doi.org/10.6084/m9.figshare.c.3677974

Compression-extension transition of continental crust in a subduction zone: A parametric numerical modeling study with implications on Mesozoic-Cenozoic tectonic evolution of the Cathaysia Block.

The Cathaysia Block is located in southeastern part of South China, which situates in the west Pacific subduction zone. It is thought to have undergone a compression-extension transition of the continental crust during Mesozoic-Cenozoic during the subduction of Pacific Plate beneath Eurasia-Pacific Plate, resulting in extensive magmatism, extensional basins and reactivation of fault systems. Although some mechanisms such as the trench roll-back have been generally proposed for the compression-extension transition, the timing and progress of the transition under a convergence setting remain ambiguous due to lack of suitable geological records and overprinting by later tectonic events. In this study, a numerical thermo-dynamical program was employed to evaluate how variable slab angles, thermal gradients of the lithospheres and convergence velocities would give rise to the change of crustal stress in a convergent subduction zone. Model results show that higher slab dip angle, lower convergence velocity and higher lithospheric thermal gradient facilitate the subduction process. The modeling results reveal the continental crust stress is dominated by horizontal compression during the early stage of the subduction, which could revert to a horizontal extension in the back-arc region, combing with the roll-back of the subducting slab and development of mantle upwelling. The parameters facilitating the subduction process also favor the compression-extension transition in the upper plate of the subduction zone. Such results corroborate the geology of the Cathaysia Block: the initiation of the extensional regime in the Cathaysia Block occurring was probably triggered by roll-back of the slowly subducting slab.

Effects of crustal eclogitization on plate subduction/collision dynamics: Implications for India-Asia collision

2D thermo-mechanical models are constructed to investigate the effects of oceanic and continental crustal eclogitization on plate dynamics at three successive stages of oceanic subduction, slab breakoff, and continental subduction. Crustal eclogitization directly increases the average slab density and accordingly the slab pull force, which makes the slab subduct deeply and steeply. Numerical results demonstrate that the duration time from initial continental collision to slab breakoff largely depends on the slab pull force. Specifically, eclogitization of subducted crust can greatly decrease the duration time, but increase the breakoff depth. The detachment of oceanic slab from the pro-continental lithosphere is accompanied with obvious exhumation of the subducted continental crust and a sharp uplift of the collision zone in response to the disappearance of downward drag force and the induced asthenospheric upwelling, especially under the condition of no or incomplete crustal eclogitization. During continental subduction, the slab dip angle is strongly correlated with eclogitization of subducted continental lower crust, which regulates the slab buoyancy nature. Our model results can provide several important implications for the Himalayan-Tibetan collision zone. For example, it is possible that the lateral variations in the degree of eclogitization of the subducted Indian crust might to some extent contribute to the lateral variations of subduction angle along the Himalayan orogenic belt. Moreover, the accumulation of highly radiogenic sediments and upper continental crustal materials at the active margin in combination with the strong shear heating due to continuous continental subduction together cause rising of isotherms in the accretionary wedge, which facilitate the development of crustal partial melting and metamorphism.

The role of changing geodynamics in the progressive contamination of Late Cretaceous to Late Miocene arc magmas in the southern Central Andes

The tectonic and geodynamic setting of the southern Central Andean convergent margin changed significantly between the Late Cretaceous and the Late Miocene, influencing magmatic activity and its geochemical composition. Here we investigate how these changes, which include changing slab-dip angle and convergence angles and rates, have influenced the contamination of the arc magmas with crustal material. Whole rock geochemical data for a suite of Late Cretaceous to Late Miocene arc rocks from the Pampean flat-slab segment (29–31 °S) of the southern Central Andes is presented alongside petrographic observations and high resolution age dating. In-situ U–Pb dating of magmatic zircon, combined with Ar–Ar dating of plagioclase, has led to an improved regional stratigraphy and provides an accurate temporal constraint for the geochemical data.A generally higher content of incompatible trace elements (e.g. Nb/Zr ratios from 0.019 to 0.083 and Nb/Yb from 1.5 to 16.4) is observed between the Late Cretaceous (~72Ma), when the southern Central Andean margin is suggested to have been in extension, and the Miocene when the thickness of the continental crust increased and the angle of the subducting Nazca plate shallowed. Trace and rare earth element compositions obtained for the Late Cretaceous to Late Eocene arc magmatic rocks from the Principal Cordillera of Chile, combined with a lack of zircon inheritance, suggest limited assimilation of the overlying continental crust by arc magmas derived from the mantle wedge. A general increase in incompatible, fluid-mobile/immobile (e.g., Ba/Nb) and fluid-immobile/immobile (e.g., Nb/Zr) trace element ratios is attributed to the influence of the subducting slab on the melt source region and/or the influx of asthenospheric mantle.The Late Oligocene (~26Ma) to Early Miocene (~17Ma), and Late Miocene (~6Ma) arc magmatic rocks present in the Frontal Cordillera show evidence for the bulk assimilation of the Permian–Triassic (P–T) basement, both on the basis of their trace and rare earth element compositions and the presence of P–T inherited zircon cores. Crustal reworking is also identified in the Argentinean Precordillera; Late Miocene (12–9Ma) arc magmatic rocks display distinct trace element signatures (specifically low Th, U and REE concentrations) and contain inherited zircon cores with Proterozoic and P–T ages, suggesting the assimilation of both the P–T basement and a Grenville-aged basement. We conclude that changing geodynamics play an important role in determining the geochemical evolution of magmatic rocks at convergent margins and should be given due consideration when evaluating the petrogenesis of arc magmas.

Simulation of late Cenozoic South American flat-slab subduction using geodynamic models with data assimilation

The formation mechanisms of flat slabs in South America remain unclear. To quantitatively evaluate the earlier proposed mechanisms, we simulate the post-100 Ma subduction history below South America using 4-D geodynamic models by progressively incorporating plate kinematics, seafloor ages and key tectonic features including the buoyant oceanic crust, continental cratons, oceanic plateaus (i.e. the inferred Inca plateau, subducting Nazca Ridge and Juan Fernandez Ridge), as well as deformable trench profiles according to recent geological reconstructions. We find that, in the absence of an overriding plate and subducting buoyancy features, the seafloor age affects slab dip angle by controlling the slab's mechanical strength (i.e., the resistance to bending) and negative buoyancy (integrated positive density anomaly that enhances bending). Our models show that slab strength dominates its buoyancy at age >30 Ma and the opposite for younger ages. The existence of a thick overriding plate reduces the slab dip by increasing dynamic suction, and individual cratonic roots further lead to along-trench variations of dip angle reduction. While dynamic suction from the overriding plate generates a permanent reduction of the long-wavelength slab dip angle, it is the final addition of subducting oceanic plateau and aseismic ridges that produces the transient and localized flat-slabs as observed. These results suggest that all mechanisms except the buoyancy features affect the slab dip only at large spatial scales. Our best-fit model with all the above tectonic features included provides a good match to both the upper mantle Benioff zones and the temporal evolution of volcanic arcs since the mid-Miocene. The imperfect match of the Peruvian flat-slab is likely associated with the uncertain 3-D configuration of the Amazonian craton.

Deep recycling of oceanic asthenosphere material during subduction

AbstractUncertainties in the origin and composition of oceanic asthenosphere lead to different views on its temporal evolution upon subduction. We investigate the evolution of asthenosphere material during subduction using high‐resolution geodynamic models. In contrast to some earlier models suggesting that limited amount of asthenosphere material can be entrained during subduction, we find that much of the subslab mantle (>100 km thick) could recycle into the deep mantle following the slab, even if this mantle layer remains buoyant and less viscous during entrainment. Our results support the hypothesis that observed trench‐parallel subslab seismic anisotropy is a downward continuation of the anisotropic asthenosphere. The entrainment of sometimes buoyant asthenosphere material provides a new mechanism for reducing slab dip angle which is consistent the shallower Pacific slab underneath Japan than that farther south. Episodic release of entrained buoyant materials during subduction can also explain enigmatic intraplate volcanism, such as the Changbaishan volcano in Northeast China.

Deflection of mantle flow beneath subducting slabs and the origin of subslab anisotropy

AbstractGlobal compilations of subslab shear wave splitting parameters show a mix of trench‐parallel and trench‐perpendicular fast directions that often directly contradict predictions from two‐dimensional models of slab‐entrained flow. Here we show that subslab anisotropy is consistent with three‐dimensional geodynamic models that feature the interaction between subducting slabs and regional mantle flow. Each model represents a specific region for which high‐quality source‐side shear wave splitting data are available. We compare the distribution of finite strain in the models with shear wave splitting observations, showing that both trench‐parallel and trench‐perpendicular fast directions can be explained by deflection of regional mantle flow around or beneath subducted slabs. Subslab maximum elongation directions calculated from our models depend on a combination of geometry factors (such as slab dip angle and maximum depth), mechanical parameters (such as decoupling between the slab and the subjacent mantle), and the orientation and magnitude of the regional mantle flow.

Crustal density structure in northwestern South America derived from analysis and 3-D modeling of gravity and seismicity data

This paper presents a three-dimensional (3-D) interpretation of new gravity and seismicity datasets for northern South America. A 3-D forward density model was constructed on the basis of deep wide-angle seismic refraction sections, Moho depth from receiver functions, and surface geology. Density values were estimated from published borehole data for sediments by using empirical velocity–density functions and considering mineralogical–chemical composition variations under typical pressure–temperature conditions for upper and lower crustal rocks. The modeled 3-D density structure was kept as simple as possible. The continental and oceanic plates were formed by two sedimentary bodies, one crustal body, and one mantle lithosphere body overlying a sub-lithospheric mantle. The Caribbean plate was modeled with an atypical crustal thickness of ~18km (including sediments). The geometry of the Caribbean plate was modeled using a combination of gravity modeling and analyses of the seismicity and focal-mechanism solutions. Intermediate seismicity and the orientation of the T-axes appeared aligned along the predicted position of the slab. As a result, the estimated slab dip angle under Maracaibo and the Mérida Andes was ~15° and increases up to ~20° after 100km depth. The model shows two orientations in the slab strike: ~N150°E±5 in western Colombia and southward underneath the Maracaibo block. The modeling results suggest that the northern South American upper and lower crusts are relatively light and the density of the Caribbean crust is typical for an oceanic crust.

Effects of slab geometry and obliquity on the interplate thermal regime associated with the subduction of three-dimensionally curved oceanic plates

We investigated the relationships among slab geometry, obliquity, and the thermal regime associated with the subduction of oceanic plates using a three-dimensional (3D) parallelepiped thermal convection model. Various models with convex and concave slab shapes were constructed in the numerical simulation, and the temperature and mantle flow distributions were calculated. The results revealed that when the slab dip angle increases, or the obliquity of subduction becomes steeper, the interplate temperature decreases remarkably. Cooler (warmer) zones on the plate interface were identified from the modeling where there was a larger (smaller) subduction angle. Consequently, the interplate temperature distribution is partly controlled by the true subduction angle (TSA), which is a function of the slab dip angle and the obliquity of subduction. The rate of change of the interface temperature for the TSA was 10–50 °C (10°< TSA < 20°) at depths ranging from (TSA – 10) × 5 km to 60 + (TSA – 10) × 5 km for a flat slab after a subduction history of 7 Myrs. The along-arc slab curvature affects the variation in TSA. The slab radius also appeared to influence the radius of induced mantle flow.

Three‐dimensional dynamic models of subducting plate‐overriding plate‐upper mantle interaction

We present fully dynamic generic three‐dimensional laboratory models of progressive subduction with an overriding plate and a weak subduction zone interface. Overriding plate thickness (TOP) is varied systematically (in the range 0–2.5 cm scaling to 0–125 km) to investigate its effect on subduction kinematics and overriding plate deformation. The general pattern of subduction is the same for all models with slab draping on the 670 km discontinuity, comparable slab dip angles, trench retreat, trenchward subducting plate motion, and a concave trench curvature. The narrow slab models only show overriding plate extension. Subduction partitioning (vSP⊥ / (vSP⊥ + vT⊥)) increases with increasing TOP, where trenchward subducting plate motion (vSP⊥) increases at the expense of trench retreat (vT⊥). This results from an increase in trench suction force with increasing TOP, which retards trench retreat. An increase in TOP also corresponds to a decrease in overriding plate extension and curvature because a thicker overriding plate provides more resistance to deform. Overriding plate extension is maximum at a scaled distance of ~200–400 km from the trench, not at the trench, suggesting that basal shear tractions resulting from mantle flow below the overriding plate primarily drive extension rather than deviatoric tensional normal stresses at the subduction zone interface. The force that drives overriding plate extension is 5%–11% of the slab negative buoyancy force. The models show a positive correlation between vT⊥ and overriding plate extension rate, in agreement with observations. The results suggest that slab rollback and associated toroidal mantle flow drive overriding plate extension and backarc basin formation.

Small-scale convection in the subduction zone mantle wedge

Small-scale convection in the upper mantle can appreciably influence large-scale mantle dynamics and has important implications for geophysical observables such as seismic anisotropy and surface heat flow. We develop numerical models to evaluate the likelihood of small-scale convection in the mantle wedge above subducting slabs. The characteristics of small-scale convection are analyzed using the 3-D single-mode approximation, in which one characteristic wavenumber represents small-scale convective motions with rotational axes parallel to the convergence direction. Numerical simulations are run for a range of characteristic wavenumbers, Rayleigh numbers, and subduction parameters (e.g., slab dip angle, convergence velocity, and downgoing plate age). For each simulation, we quantify the excitation due to small-scale convection and determine under what conditions and characteristic wavenumbers this excitation is maximized. We find that of the parameters examined, mantle wedge viscosity plays the most significant role in dictating the occurrence and strength of small-scale convective motions in the mantle wedge. Numerical models run with subduction parameters similar to that of northeast Japan, where it has been proposed that small-scale convection may be occurring in the mantle wedge, require a mantle wedge viscosity of ∼1018Pas for significant small-scale convection to occur.

Structure of central and southern Mexico from velocity and attenuation tomography

The 3D Vp, Vp/Vs, P‐ and S‐wave attenuation structure of the Cocos subduction zone in Mexico is imaged using earthquakes recorded by two temporary seismic arrays and local stations. Direct P wave arrivals on vertical components and direct S wave arrivals on transverse components from local earthquakes are used for velocity imaging. Relative delay times for P and PKP phases from teleseismic events are also used to obtain a deeper velocity structure beneath the southern seismic array. Using a spectral‐decay method, we calculate a path attenuation operatort* for each P and S waveform from local events, and then invert for 3D spatial variations in attenuation (Qp−1 and Qs−1). Inversion results reveal a low‐attenuation and high‐velocity Cocos slab. The slab dip angle increases from almost flat in central Mexico near Mexico City to about 30° in southern Mexico near the Isthmus of Tehuantepec. High attenuation and low velocity in the crust beneath the Trans‐Mexico Volcanic Belt correlate with low resistivity, and are probably related to dehydration of the slab and melting processes. The most pronounced high‐attenuation, low‐Vpand high‐Vp/Vsanomaly is found in the crust beneath the Veracruz Basin. A high‐velocity structure dipping into the mantle from the side of Gulf of Mexico coincides with a discontinuity from a receiver functions study, and provides an evidence for the collision between the Yucatán Block and Mexico in the Miocene.