Slab Remnants Research Articles

Paleolatitudinal Drift and Major Rotation of the Wrangellia Superterrane in the Mesozoic: A Signal of East‐Panthalassa Plate Motion?

AbstractThe allochthonous origin of the Wrangellia superterrane relative to North America (NAM) has been established in the early days of plate tectonics using paleomagnetic and geologic data. However, orogenic deformation of the Wrangellia superterrane after its Cretaceous accretion to NAM has complicated the reconstruction of its pre‐accretionary tectonic evolution. In particular, vertical‐axis rotations linked to strike‐slip faulting have cast doubt on the usefulness of paleomagnetic declinations in deciphering its pre‐accretionary motions. Here, we compile paleomagnetic data from the Wrangellia superterrane and present new results from uppermost Triassic limestones and lowermost Jurassic lavas of the Bonanza arc, which confirm that it was at those times at a much lower latitude than today (∼50°–60°N), at a latitude of either ∼25°–30°N or ∼25°–30°S. Moreover, declinations reveal a coherent, major clockwise or counterclockwise rotation, depending on hemispheric origin. When correcting for previously documented true polar wander at the approximate longitude of the Wrangellia superterrane at ∼190 Ma, new and existing paleomagnetic data allow for two possible scenarios of Mesozoic kinematic evolution: from 190 to 80 Ma, in the southern hemisphere scenario, the Wrangellia superterrane was transported ∼5,000 km northward while rotating ∼110° clockwise with an associated north‐dipping subduction zone, while in the northern hemisphere scenario it remained at northern middle latitudes while rotating ∼70° counterclockwise at a south‐dipping subduction zone. The southern hemisphere scenario appears more plausible when tested against recent reconstructions of minimum oblique convergence beneath NAM and mantle tomography images of slab remnants.

The Role of Slab Remnants in Modulating Free Subduction Dynamics: A 3‐D Spherical Numerical Study

AbstractSeismic tomography of Earth's mantle images abundant slab remnants, often located in close proximity to active subduction systems. The impact of such remnants on the dynamics of subduction remains underexplored. Here, we use simulations of multi‐material free subduction in a 3‐D spherical shell geometry to examine the interaction between visco‐plastic slabs and remnants that are positioned above, within and below the mantle transition zone. Depending on their size, negatively buoyant remnants can set up mantle flow of similar strength and length scales as that due to active subduction. As such, we find that remnants located within a few hundred km from a slab tip can locally enhance sinking by up to a factor 2. Remnant location influences trench motion: the trench advances toward a remnant positioned in the mantle wedge region, whereas remnants in the sub‐slab region enhance trench retreat. These motions aid in rotating the subducting slab and remnant toward each other, reducing the distance between them, and further enhancing the positive interaction of their mantle flow fields. In this process, the trench develops along‐strike variations in shape that are dependent on the remnant's location. Slab‐remnant interactions may explain the poor correlation between subducting plate velocities and subducting plate age found in recent plate tectonic reconstructions. Our results imply that slab‐remnant interactions affect the evolution of subducting slabs and trench geometry. Remnant‐induced downwelling may also anchor and sustain subduction systems, facilitate subduction initiation, and contribute to plate reorganization events.

Cause of Enigmatic Upper-Mantle Earthquakes in Central Wyoming

Abstract Earthquakes deeper than 60 km generally occur in subducting slabs. However, on 21 September 2013 two earthquakes (M 4.8 and 3.0) occurred at ∼71 to 75 km depths in the upper mantle beneath central Wyoming in the stable North American continent, where there is no actively subducting slab at present. The cause of the two events is still unclear. Here, we present detailed 3D P-wave isotropic and anisotropic tomography down to 750 km depth under Wyoming and adjacent areas. Our result shows that the two Wyoming events took place within a high-velocity (high-V) body at 0–160 km depths, which may be part of dense continental lithosphere. Another high-V body exists at ∼300 to 500 km depths, which may reflect a remnant of the subducted Farallon slab. A significant low-velocity (low-V) zone appears at ∼200 to 300 km depths between the two high-V bodies, and the low-V zone exhibits seismic anisotropy that VP is greater in the vertical direction than that in the horizontal direction. The low-V zone may include ascending fluids from dehydration of the subducted slab remnant, which was promoted by the nearby hot Yellowstone plume. It is highly possible that the ascending fluids induced the 2013 Wyoming upper-mantle earthquakes.

Seismic evidence for slab detachment beneath the Taiwan Orogen

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.

Sources of Neoproterozoic post-collisional granites from the Porto Alegre region, eastern Dom Feliciano Belt, Brazil: Sr-Nd-Pb isotopic constraints

The Dom Feliciano Belt, southernmost Brazil and Uruguay, is a Neoproterozoic to Cambrian mobile belt that resulted from the convergence of the Kalahari, Congo, and Río de la Plata cratons. The present paper focuses on post-collisional granites located in the eastern sector of this belt, in the Porto Alegre region, RS, Brazil. The studied rocks yield crystallization ages of ca. 600 Ma and intrude subduction-related, calc-alkaline gneisses from the host basement. This work presents novel isotopic data (Sr-Nd-Pb) for representative samples of the Viamão, Porto Alegre, and Itapuã intrusive suites. For the Viamão Intrusive Suite, the results (87Sr/86Srt = 0.7089–0.7168; εNdt=−6.0 to −6.7; TDM = 1.4–1.5 Ga; 206Pb/204Pb = 18.20–19.71) suggest mixing of mantle (DM) and Paleoproterozoic crust-derived sources (Arroio dos Ratos Complex, a remnant of the basement). For the Porto Alegre Intrusive Suite, the results (87Sr/86Srt = 0.7090–0.7437; εNdt=−4.2 to −8.3; TDM = 1.3–1.8 Ga; 206Pb/204Pb = 18.89–22.19) suggest mixing of mantle (HIMU) and Tonian-Cryogenian crust-derived components (Chácara das Pedras Gneiss, a remnant of the basement). In turn, the results for the Itapuã Intrusive Suite (87Sr/86Srt = 0.7116–0.8733; εNdt=−1.6 to −6.0; TDM = 1.2–1.6 Ga; 206Pb/204Pb = 18.44–22.20) suggest mixing of the mantle (possibly HIMU) and a Nd-depleted, crust-derived component (in comparison to the source of the Porto Alegre Intrusive Suite). All the studied suites are interpreted as being formed in a Neoproterozoic post-collisional setting. The mantle-derived signatures (DM and HIMU) are interpreted as the result of the partial melting of remnants of former oceanic slabs related to previous subduction events. In turn, the crust-derived signatures are interpreted as the result of assimilation of basement rocks. We suggest that the post-collisional magmatism of this sector of the Dom Feliciano Belt is associated with a deep hot zone.

Whole‐Mantle Tomography of Southeast Asia: New Insight Into Plumes and Slabs

AbstractWe present detailed 3‐D images of the whole mantle P wave velocity structure beneath Southeast Asia and surrounding regions. The results are obtained by applying an improved method of global seismic tomography to invert ∼8 million P, pP, PP, PcP, and Pdiff arrival times from 23,587 earthquakes recorded at 14,136 stations distributed all over the world. Our tomographic model reveals a continuous, thin low‐velocity (low‐V) zone from the surface to the core‐mantle boundary beneath the Hainan hotspot, which may reflect the Hainan plume that exists in the whole mantle. Beneath the Australian slab that has subducted into the lower mantle, a strong low‐V anomaly is detected, which may reflect subslab hot mantle upwelling (SHMU) due to return flow of the slab subduction. Our model also clearly reveals the subducted slabs in the upper mantle and slab remnants in the lower mantle. In particular, a hole in the subducting Australian slab is revealed at depths of 280–460 km beneath eastern Java. The low‐V anomaly in the mantle wedge above the Australian slab is connected with the SHMU through the slab hole, suggesting that mixture of the island arc magma and the SHMU may have caused huge eruptions of the Tambora and Rinjani volcanoes in eastern Java.

Slab-driven flow at the base of the mantle beneath the northeastern Pacific Ocean

Flow in the mantle's bottom boundary layer plays an important role in shaping structures and processes in the deep mantle; however, knowledge of lowermost mantle flow patterns remains elusive. In particular, the influence of remnant slabs on lowermost mantle flow is poorly known, although it is likely that slabs play an important role in driving flow and thus in controlling key aspects of lowermost mantle behavior. Measurements of seismic anisotropy can yield relatively direct constraints on slab-induced lowermost mantle flow; however, such observations are challenging to make. We take advantage of the excellent raypath coverage beneath the northeastern Pacific Ocean provided by the USArray deployment in North America to provide detailed sampling of a region that has a long subduction history, with remnant slabs likely impinging on the core-mantle boundary. We present observations of coherent, strong shear wave splitting of SKKS and Sdiff phases across USArray stations and show through global wavefield modeling that the splitting is due to lowermost mantle anisotropy. A stacking approach enables us to make robust estimates of lowermost mantle splitting parameters, which we model by considering realistic mineral physics scenarios. Under the assumption of simple horizontal shear deformation, our observations are consistent with generally north-south flow directions for either a post-perovskite or a bridgmanite mineralogy; ferropericlase cannot explain observations. We speculate that this flow is driven by subducting slab remnants impinging on the core-mantle boundary.

Triple-stage India-Asia collision involving arc-continent collision and subsequent two-stage continent-continent collision

The enigmatic geodynamic processes involved in the India-Asia collision shape our understanding of uplift and deformation of the Tibetan Plateau and the subsequent changes in the land-sea distribution pattern, monsoon-arid environments and biotic evolution in Central-Eastern Asia. However, the current geodynamic models dealing with the India-Asia collision history exhibit significant discrepancies in how, when and where exactly the collision of India with Asia occurred. Recently, we proposed a new hypothesis that favored the opening and closure of a North India Sea during the late Cretaceous to Paleogene. Here we present new additional paleomagnetic data from the red siliceous shales and cherts of the Sangdanlin Formation, which were deposited on the most distal continental margin of the Tethyan Himalaya terrane, i.e. the southern margin of the Neo-Tethys Ocean, during the middle Paleocene. The combination of our new and previously reported paleomagnetic data shows that the Tethyan Himalaya was at a paleolatitude of 14.1° ± 1.9°N during the interval 62.5–59.2 Ma and refines the maximum size of the North India Sea to about 2200 ± 500 km. Combining our geodynamic model with recently published paleomagnetic data from the Kohistan-Ladakh arc, we conclude that the India-Asia collision was a triple-stage process, which involved an arc-continent collision followed by a two-stage continent-continent collision. The arc-continent collision occurred at ca. 64 Ma between the Tibetan Himalaya (Tethyan Himalaya plus Greater Himalaya) and the Trans-Tethyan subduction zone at a paleolatitude of ~8°N. The initial continent-continent collision took place at ca. 61 Ma between the Tibetan Himalaya (plus the accreted Trans-Tethyan subduction zone) and Lhasa terranes at a paleolatitude of ~14°N. The final continent-continent collision occurred during 53–47 Ma between India and the Tibetan Himalaya, diachronously suturing the North India Sea from west to east. The triple-stage collision scenario is in agreement with the history of India-Asia convergence rates, and is reflected by the presence of three slab remnants below Tibet and India as documented by seismic tomographic imaging. Our updated collision scenario reconciles multidisciplinary evidence for the India-Asia collision and provides better constraints on the resulting paleoclimatic and paleoenvironmental changes in Central-Eastern Asia. • The Tethyan Himalaya was paleomagnetically constrained to a paleolatitude of 14.1° ± 1.9°N at ca. 61 Ma. • The arc-continent collision between the Tibetan Himalaya and the Trans-Tethyan subduction zone was estimated to be ca. 64 Ma. • The India-Asia collision was a triple-stage process.

Focal Mechanism Analysis of the Earthquakes Beneath the Sunda-Banda Arc Transition, Indonesia, Using the BMKG Data

Structural complexity in the Sunda-Banda arc transition is a topic of much debate amongst Earth scientists. We have processed focal mechanism study using moment tensor inversion for 20 events in the region using the Agency for Meteorology, Climatology, and Geophysics (BMKG) data from 2014-2016 for earthquakes of magnitude Mw ≥ 5.0. Our result shows different solutions that depend on the source region of the earthquakes that include subduction zones and collision zones, which host active faults and the back-arc thrusts. Earthquakes that occurred in the subduction zone appear to rupture on thrust faults for shallow and intermediate events, while the deep events have normal fault mechanisms. The shallow events in the collision zone occur on thrust faults with differing strike directions, but generally, there are largely parallel to the Timor trough. We also found normal fault mechanisms at play for deep events below the collision zone. The occurrence of deep earthquakes in this area is consistent with remnant slab activity that persists to the present day. Focal mechanism solutions for shallow events in the north of Sumbawa island indicate a thrust fault with a strike direction that is almost parallel to the back-arc thrust in this area. We also found evidence of strike-slip motion along local-scale active faults in the area.

Abnormally low heat flow in Southeast China resulted from remnant slab subducted beneath the east Asian lithosphere

AbstractSubducted remnant slabs play an important role in the geodynamics and evolution of the Earth, but their behaviour in the mantle is not well understood. Terrestrial heat flow and thermal lithosphere structure provide crucial constraints on lithospheric dynamics. Based on whole‐well steady‐state temperature logs and conductivity measurements of core samples from a scientific borehole, we obtained a new high‐quality terrestrial heat flow values for East Asia. The heat flow was calculated at 60.6 ± 16.0 mW/m2 with a high crust/mantle heat flow ratio of 1.41, suggesting a hot‐crust‐cold‐mantle lithospheric structure. The thermal lithospheric thickness of eastern Fujian is estimated to be 152–166 km, indicating a thick and cold lithosphere. We demonstrate that the cooling of the lithosphere is caused by a remnant palaeo‐slab of the Palaeo‐Pacific plate subducted beneath SE China. The remnant slab blocked heat convection into the bottom of the lithosphere of the Eurasian Plate.

Complex Patterns of Mantle Flow in Eastern SE Asian Subduction Zones Inferred From P‐Wave Anisotropic Tomography

AbstractWe determine 3‐D isotropic and anisotropic P‐wave velocity models beneath eastern SE Asia by inverting a large number of P‐wave arrival times selected from the ISC‐EHB database. Our results reveal detailed structures of the subducting South China Sea (SCS), Negros, Molucca Sea, Philippine Sea, and Banda slabs, the previously subducted Proto‐SCS slab and remnants of other paleo slabs, showing long‐lived subductions of these oceanic slabs in eastern SE Asia. There is an obvious change in the subduction angle of the SCS slab at 18°N along the Manila trench. We suggest that the relatively low dip angle of the SCS slab to the south of 18°N was caused by subduction of an extinct mid‐ocean ridge, and a slab tear is possibly formed at 18°N. This feature is supported by the resolved fast velocity directions (FVDs) of P‐wave azimuthal anisotropy in the mantle wedge, which show that 3‐D toroidal mantle flow may develop around the southern part of the SCS slab. Trench‐normal FVDs are revealed in the deeper mantle wedge of the Sangihe subduction zone, which are associated with the deep subduction and stagnancy of the Molucca Sea slab. A nearly trench‐parallel FVD is observed beneath the Molucca Sea slab, which may be caused by trench‐parallel extension due to the retreating slab or reflect subslab mantle flow associated with the double‐sided subduction. The subducting Banda slab exhibits a curved feature, which greatly affects the flow pattern in the mantle wedge.

Mapping the edge of subducted slabs in the lower mantle beneath southern Asia

SUMMARYWe investigate the presence of seismic structures in the Earth's mantle by searching for seismic signals that travel off the great circle path direction and are reflected or scattered off structures in the lower mantle. We focus on areas of current and past subduction beneath Eurasia by using events from Indonesia and Japan recorded at the broad-band stations in Germany, Morocco and Namibia. Applying seismic array techniques, we measure the direction and traveltime of the out-of-plane arrivals and backtrace them to their location of reflection/scattering. We backtrace the signals as P-to-P and S-to-P waves and extend the methodology to P-to-S waves. There seems to be a low number of reflection points in the regions beneath Eurasia in our study. Investigating possible causes, we find that the focal mechanism influences the presence of out-of-plane reflected waves. However, the potential coverage with our data set is large and should allow detection, but there may potentially be few seismically visible structures in the region. Most of our backtraced reflectors are located beneath southern Asia and are found shallower than 1500 km depth. They correlate well with the edges of prominent high velocity anomalies in tomographic inversions beneath southern Asia, which have been interpreted as remnants of fossil slabs of the subduction of the Tethys Oceans. We also observe few reflectors deeper than 1600 km that are located away from subducting regions and their positions coincide with the eastern edge of the African low velocity anomaly. These observations suggest that the presence of reflectors in the mid-lower mantle is not exclusively related to current or past subducting regions, but widespread throughout the mantle.

Submarine Basaltic Magmatism in the Subbetic Basin (Southern Spain): Insights into Melt-Weakening Processes during Mesozoic Continental Rifting

Abstract Mantle-derived volcanic rocks from the Subbetic hyperextended basin in SE Spain provide new insights into the composition and mechanical behavior of the mantle during continental rifting. The present study describes a sequential restored cross-section along with geochemical characteristics of the basaltic rocks interbedded within the Mesozoic succession of the basin. Sedimentary stacking patterns of minibasins above the mobilized salt reflect the relationships with coeval basaltic volcanism. We recognize two type localities on the basis of volcanic facies, the presence of shallow intrusive bodies, and age of the associated sedimentary formations. The first type corresponds to subaqueous pillow-lava flows and subvolcanic sills and dikes associated with Lower Jurassic marly limestones and Middle Jurassic oolitic limestones. The Jurassic basalts present enriched MORB compositions with moderate La/Sm and low Sm/Yb ratios. Interestingly, a significant group of this Jurassic basaltic magmatism departs from the typical MORB-OIB array, showing deep Nb-Ta negative anomalies and high Th/Nb ratios. The second type comprises subaqueous lava flows, also including pillow-shaped basalts interlayered with hyaloclastite deposits and Upper Cretaceous clays, radiolarites, and marly limestones. The Cretaceous magmatism is characterized by highly enriched MORB compositions. Furthermore, the moderate Sm/Yb values and the positive correlation between LREE/HREE and Zr point to the involvement of deep (Grt-present) mantle sources in the origin of the Cretaceous basaltic melts. We interpret the Lower-Middle Jurassic calc-alkaline signal as due to the partial melting of recycled crustal rocks within the upper mantle, i.e., associated with remnants of pre-Mesozoic subducted slabs. These characteristics are similar to those described in Triassic basaltic rocks widespread throughout the External Zone of the Betic Cordillera. Mantle-derived basalts interlayered within the Lower Jurassic syn-rift deposits indicate that melting and deformation within the lithospheric mantle was initiated early during continental rifting. Accordingly, we suggest that Early to Middle Jurassic mantle melts promoted failure within the upper mantle, thus contributing to the inception of lithospheric-scale shear zones, which, in turn, controlled the evolution of this magma-poor hyperextended margin. Subsequently, rift evolution gave way to the activation of deeper melt sources in the mantle and an increase of the alkaline signature at the Cretaceous time.

Modeling of Seismic Anisotropy Observations Reveals Plausible Lowermost Mantle Flow Directions Beneath Siberia

AbstractObservations of seismic anisotropy just above the core‐mantle boundary are a powerful way to understand the dynamics of the lowermost mantle. Here we present models of seismic anisotropy in the lowermost mantle beneath Siberia based on new and previously published body wave observations. We compile a set of measurements based on a variety of data types, including the differential splitting of SK(K)S‐PKS and S‐ScS phases and polarities of PdP and SdS reflections off the D″ discontinuity. We carry out ray theoretical forward modeling of these data sets in combination, using a novel approach that allows for tighter constraints on the geometry of seismic anisotropy than would be possible using a single data type. Observations for each seismic phase alone only provide limited information; by combining different data types into one modeling approach, we can constrain D″ seismic anisotropy more tightly. We test a range of plausible mechanisms for seismic anisotropy, including a variety of candidate minerals (bridgmanite, post‐perovskite, and ferropericlase), dominant slip systems, and orientations, and consider both single‐crystal elasticity and elastic tensors based on polycrystalline texture modeling. In general, we find that post‐perovskite (with either a [100](010) or [100](001) dominant slip system) or bridgmanite models provide the best fit to the observations. The best‐fitting models suggest a dominant shear direction to the southwest or northeast direction at the base of the mantle. This is consistent with flow directed toward the Perm anomaly, potentially driven by slab remnants impinging on the core‐mantle boundary.

Seismic Evidence for Thermal and Chemical Heterogeneities in D″ Region Beneath Central America From Grid Search Modeling

AbstractLarge lateral variations in the depth, sharpness, and magnitude of the D″ discontinuity are observed beneath Central America and the Caribbean using a new innovative grid search modeling method. The strong correlation between the D″ topography and the underlying seismic velocities suggests a large lateral gradient in the thermal condition in the lowermost mantle. Low‐velocity patches were identified among the high‐velocity areas, coinciding with regions showing a disruption in the D″ discontinuity. The low‐velocity zones may be attributed to the heat trapped in the middle of the Farallon slab remnant while those farther to the west are likely to be associated with the upwelling generated at the edge of the ancient slab. A broad range in the sharpness of the D″ discontinuity was also observed, suggesting the existence of different chemical components in the region, as a result of a disparate mixture of basalt, harzburgite, and pyrolite.

A high-resolution map of Hawaiian ULVZ morphology from ScS phases

We use core reflected ScS waves sensitive to a broad region of the core-mantle boundary beneath Hawaii to create the first high-resolution map of the Hawaiian ultralow-velocity zone (ULVZ). Positive ScS-S differential times are used to identify regions of strong slow velocity anomalies in the lowermost mantle, and the presence of pre/post-cursors around the main ScS phase confirms the sharp top of a basal ULVZ layer. Pre/post-cursor arrivals are mapped into a volume across their region of sensitivity to produce a detailed image of ULVZ morphology. The variability observed across the ULVZ can be interpreted in terms of varying height or velocity reduction, but the large range of velocity variations required to explain observations suggests that most variability reflects varying layer thickness.The Hawaiian ULVZ is observed to be a large-scale regional feature of varying height (∼5-25 km) extending across a wide area along the edge of the Pacific large low-velocity province (LLVP). Variability in previous models of the Hawaiian ULVZ can be explained by studies imaging different parts of this strongly variable, large-scale feature. Maximum ULVZ thicknesses (no taller than 30 km) are found in a flat-topped, steep-sided region on the order of 1000 km in diameter located west of Hawaii. This feature is coincident with the previously identified Hawaiian mega-ULVZ, and is interpreted to represent the root of the Hawaiian plume, which is offset from the volcanic surface expression. A tall, asymmetric ridge of ULVZ material is observed to the east of Hawaii. Both regions of maximum ULVZ heights are bounded by the Pacific LLVP to the south, and fast seismic velocity anomalies interpreted as slab remnants to the north. This points to a potential geodynamical scenario where dense ULVZ material is pushed by subducted slab remnants into thicker piles against LLVP margins.

Nature and (in-)coherent metamorphic evolution of subducted continental crust in the Neoproterozoic accretionary collage of SW Mongolia

In continental subduction complexes minor volumes of high-pressure mafic rocks (eclogites) often co-exist with much more abundant felsic (granitic) and metasedimentary rocks, which are vital for resolving the origin and metamorphic evolution of subducted continental crust. In SW Mongolia, the Alag Khadny eclogite-bearing accretionary complex (AKC) is assumed to represent either a remnant of oceanic slab, or a continental margin, subducted in the Early Cambrian. Here we present geochronological, geochemical and petrological evidence of subduction records for the three major types of lithologies that host mafic eclogites, including Mesoproterozoic and Neoproterozoic granitic basement and overlying Neoproterozoic continental-margin sediments. Variably deformed, ferroan and peraluminous metagranitoids compose a major part of AKC and are interlayered with eclogites in its southern and eastern margins. They have geochemical features of post-collisional/intraplate high-K calc-alkaline granites. LA-ICP-MS U-Pb zircon geochronology of three distinct metagranite samples show uniform protolith crystallization ages of ca. 0.96 Ga and uncertain re-crystallization in the Late Neoproterozoic or Early Paleozoic metamorphic event, whereas abundant zircon inheritance indicates older, Mesoproterozoic to Paleoproterozoic crustal substrate during granite generation. The existence of Mesoproterozoic crust is highlighted by finding of distinct metagranitoids with the U-Pb zircon crystallization age of ca. 1.6 Ga. Hafnium isotope signatures (TDMC 2.88–1.85 Ga) of zircons from all lithologies preserved the evidence of reworked Neoarchean to Paleoproterozoic crust, similar to that of the Baidrag block (southern Mongolia), for both Mesoproterozoic and Neoproterozoic rocks. Regardless of the specific lithology, the rocks display indicators of high-pressure metamorphic re-equilibration, including garnet (XCa up to 0.65) + epidote + phengite (Si p.f.u. up to 3.56) ± rutile assemblage in metagranitoids, garnet + phengite (Si p.f.u. up to 3.42) in quartz-rich semi-pelites and garnet + phengite (Si p.f.u. up to 3.39) + medium-Mg chloritoid (XMg up to 0.25) + kyanite + rutile in metapelites. Corresponding P-T conditions recovered from different lithologies reveal incoherent subduction of rocks, which could be shallow for granitic basement (1.1–1.4 GPa and 600–670 °C) and clastic metasediments (1.4–1.6 GPa, 570–620 °C), but deeper for metapelites (2.1–2.3 GPa, 500–570 °C). consistent with that of eclogites, The combined data show that the Alag Khadny complex represents a remnant of a rifted Mesoproterozoic to Neoproterozoic (ca. 1.6–0.96 Ga) continental margin consequently metamorphosed under HP conditions during Late Neoproterozoic–Early Cambrian evolution of the southern Central Asian Orogenic Belt. Acquired P-T estimates imply that high-pressure metagranitoids and metasedimentary rocks equilibrated at different depths, but most likely shared a common subduction-related metamorphic evolution.

Arc‐Type Magmatism Due to Continental‐Edge Plowing Through Ancient Subduction‐Enriched Mantle

AbstractThe puzzling <7 Ma old “postsubduction” arc magmatism of New Guinea contains geochemical subduction‐type signatures yet did not occur above an active subduction zone. Here we show that these arc magmas formed at the North Australian continental lithospheric edge when it plowed northward through mantle above the detached Arafura slab remnant. This mantle preserved its subduction signature and the edge plowing process generated new melts that ascended via an active transform fault. Arafura slab subduction occurred at an intraoceanic subduction zone that ended ~30–25 Ma ago, when the Australian continental edge was still ~1,000 km to the south. Our absolute plate tectonic reconstruction of continental‐edge plowing suggests that ancient mantle wedges remain semistationary in the upper mantle and can preserve their geochemical signature for tens of Ma, explaining previously enigmatic “postsubduction” arc magmatism.

P-wave velocity anomalies atop and in the mantle transition zone beneath the northern South China Sea from triplicated waveforms

Broadband P waveforms from the China Digital Seismograph Network are used to image velocity structures of the mantle transition zone (MTZ) and lowermost upper mantle beneath the northern South China Sea. The study area is divided into five profiles from southwest to northeast with an azimuthal interval of 10°, and the best-fit 1-D velocity models are obtained from the observed and synthetic triplicated waveform fitting based on an iterative grid-search procedure. Relative to the Earth model IASP91, our results reveal that there are ubiquitous high-velocity anomalies (HVAs) in the lower part of the MTZ with the P-wave velocity increment of 1.5–3.5% and thickness of 209–219 km. The HVAs show no apparent progressive velocity increments or decrements along the southwest-northeast direction. Combining with previous seismological studies, we suggest that the HVAs provide an implication for the proto-South China Sea north slab remnants. The slight 5 km uplift of the 410 km discontinuity and 5–15 km depression of the 660 km discontinuity further support the low-temperature anomalies related to slab remnants stagnated in the MTZ. We also find a low-velocity layer (LVL) atop the MTZ with a P-wave velocity decrement of 2.0–2.5% and a thickness of 60–75 km, and the LVL can be interpreted by the partial melting induced by upwelling MTZ materials, which are hydrated by the water released from stagnant slab. We infer that small lateral variations of the LVL is relevant to the percolation of the melts under vertical pressure in the mantle.

Lateral Variations of Shear‐Wave Velocity in the D″ Layer Beneath the Indian‐Eurasian Plate Collision Zone

AbstractSeismic tomography has demonstrated that the shear‐wave velocity is relatively high over a 3,000‐km wide region in the lowermost mantle beneath southern and eastern Asia. This seismic anomaly demarcates the current position of slab remnants that may have subducted in the Cretaceous. To further characterize the seismic structure at smaller scales, we measure 929 residual travel time differences (δt) between the phases ScS and S using recordings of eight earthquakes beneath the Indian Ocean at stations from the Chinese Digital Seismic Network. We interpret variations of δt up to 10 s as due to horizontal shear‐velocity variations in D″ beneath northern India, Nepal, and southwestern China. The shear velocity can vary by as much as 7% over distances shorter than 300 km. Our observations provide additional observational evidence that compositional heterogeneity and possibly melt contribute to the seismic structure of the lower mantle characterized by long‐term subduction and mantle downwelling.