Tectonic Implications Research Articles

Paleozoic Granitic Rocks from the Telmen Complex in the Tarvagatai Block, Central Mongolia: Petrogenesis, U-Pb geochronology, and its tectonic implications

The Tarvagatai Block is located in the northern part of Central Mongolia, which is a widespread occurrence and occupies roughly 60% of the whole exposure along the Khangai fault and the Tarvagatai uplift. Granitic magmatism was emplacement during the Middle Paleozoic, which is predominantly composed of granite-granodiorite and gabbro-diorite and rarely gabbro. This article represents petrographical, geochemical, and U-Pb zircon age data from the Telmen Complex of the Tarvagatai Block, Central Mongolia. The U-Pb dating of zircon yields a Late Silurian emplacement age (419±3 Ma) for the Telmen Complex. Geochemically, the Telmen Complex is an I-type intrusion of metaluminous nature with a SiO2 content ranging from 53.06 to 72.25 wt.% and mainly of medium to high K calc-alkaline series. Telmen Complex granites show enrichments in light rare earth elements, depletion in heavy rare earth elements, with a ratio of 4.053, (La/Yb)N =9.15, and weak positive or normal Eu anomalies. A spider diagram indicates that these rocks are enriched in Ba, K, Pb, and Sr and depleted in Nb, Ta, and Ti. The Early Paleozoic Telmen Complex granitics have trace element features, for example, Nb-Ta depletions, which indicate that these rock units were emplaced in a convergent-margin setting and typical of the lower continental crust. In addition, the geochemical data show that the volcanic arc tectonic setting and, moreover, the continental arc array setting display on the Nb/Yb versus TiO2/Yb diagrams. Therefore, we suggest that they were probably positioned in an active continental setting and in a Silurian ~419 Ma.

Zircon SHRIMP U-Pb Ages, Geochemistry and Nd-Hf Isotopes of ∼1.0 Ga A-Type Felsic Rocks in the Southwestern Yangtze Block, South China: Petrogenesis and Tectonic Implications

Zircon SHRIMP U-Pb Ages, Geochemistry and Nd-Hf Isotopes of ∼1.0 Ga A-Type Felsic Rocks in the Southwestern Yangtze Block, South China: Petrogenesis and Tectonic Implications

The early Permian high-temperature felsic magmatism induced by slab breakoff in Southern Mongolia, Central Asian Orogenic Belt and its tectonic implications

The early Permian high-temperature felsic magmatism induced by slab breakoff in Southern Mongolia, Central Asian Orogenic Belt and its tectonic implications

3D Geophysical and Geological Modeling of the South Orkney Microcontinent (Antarctica): Tectonic Implications for the Scotia Arc Development

AbstractThe opening of the Scotia Arc resulted in the final breakup of the land bridge between South America and the Antarctic Peninsula. The South Orkney Microcontinent (SOM) constituted part of this former connection and it is now the largest continental block in the Southern Scotia Arc. We present the first 3D model of the SOM that, given its strategic position and characteristics, allows us to advance the knowledge of the tectonic processes involved in the development of the Scotia Arc. Due to the scarcity of reliable geological data, the initial approximation of the deep structure of the SOM was supported by the calculation of three main geological boundaries from geophysical data: the acoustic basement, the boundary of the magnetic anomaly source and the Moho depth. The 3D model was built, refined and validated by forward modeling and joint inversion of gravity and magnetic data. We have accurately defined the geometry of the sedimentary cover, determined the geometry of the intrusive igneous body causing the Pacific Margin Anomaly (PMA) and mapped the heterogeneity of the crustal thickness. These structural features show a clear relationship to each other and are consistent with an important E‐W extension to the east of the SOM during early stages of the Scotia Arc formation, prior to the opening of the Powell Basin.

Magmatic-hydrothermal evolution and tectonic implications of granitic pegmatites from the Ailao Shan-Red River shear zone in Southeast Asia

Magmatic-hydrothermal evolution and tectonic implications of granitic pegmatites from the Ailao Shan-Red River shear zone in Southeast Asia

Petrogenesis and Tectonic Implications of the Late Silurian–Early Devonian Bimodal Intrusive Rocks in the Central Beishan Orogenic Belt, NW China: Constraints by Petrology, Geochemistry and Hf Isotope

Petrogenesis and Tectonic Implications of the Late Silurian–Early Devonian Bimodal Intrusive Rocks in the Central Beishan Orogenic Belt, NW China: Constraints by Petrology, Geochemistry and Hf Isotope

The Antler orogeny reconsidered and implications for late Paleozoic tectonics of western Laurentia

Abstract Mississippian rocks of the Antler foreland basin in Nevada (western United States) record its evolution in detail, clarifying the timing and tectonic significance of the Antler orogeny. Synorogenic clastic sediments mark the initiation of tectonism; they arrived in Nevada from the north or northwest in late Early Mississippian (Osagean). An orogenic highland formed in northern Nevada in late Middle Mississippian (Meramecian), producing a regional angular unconformity and terminating deposition in the Antler foreland basin. Unconformably overlying Upper Mississippian (Chesterian) rocks provide tight age control for the redefined “Antler orogeny” sensu stricto. Palinspastic restoration of Sr isotopic data reveals a projecting step in the Laurentian margin in northern Nevada. The Antler orogeny resulted from late Middle Mississippian sinistral-oblique convergence at this step. Continuing oblique convergence produced the subsequent late Paleozoic basins, structures, and unconformities in northern Nevada. The Roberts Mountains allochthon is translated material from farther north emplaced onto the margin. Its basal bounding fault, long mapped as the “Roberts Mountains thrust,” is in fact not a single continuous fault, motion on it was not synchronous, and displacement on many fault segments mapped as the Roberts Mountains thrust postdate the late Middle Mississippian. We therefore propose that the term “Roberts Mountains thrust” be abandoned. This new paradigm not only explains anomalous characteristics of the Antler orogeny but also illustrates criteria for recognizing oblique convergence at other ancient plate margins.

The provenance of Avalonia and its tectonic implications: a critical reappraisal

Abstract The late Neoproterozoic–Cambrian interval is characterized by global-scale orogenesis, rapid continental growth and profound changes in Earth systems. Orogenic activity involved continental collisions spanning more than 100 myr, culminating in Gondwana amalgamation. Avalonia is an example of arc magmatism and accretionary tectonics as subduction zones re-located to Gondwana's periphery in the aftermath of those collisions, and its evolution provides significant constraints for global reconstructions. Comprising late Neoproterozoic ( c. 650–570 Ma) arc-related magmatic and metasedimentary rocks, Avalonia is defined as a composite terrane by its latest Ediacaran–Ordovician overstep sequence: a distinctive, siliciclastic-dominated cover bearing ‘Acado-Baltic’ fauna. This definition implies that Neoproterozoic Avalonia may consist of several terranes, and so precise palaeomagnetic or provenance determination in one locality need not apply to all. On the basis of detrital zircon and Nd isotopic data, Avalonia and other lithotectonically related terranes, such as Cadomia, have long been thought to have resided along the Amazonian–West African margin of Gondwana between c. 650 and 500 Ma – Avalonia connected to Amazonia, and Cadomia to West Africa. These interpretations have constrained Paleozoic reconstructions, many of which imply that the departure of several peri-Gondwanan terranes led to the Early Paleozoic development of the Rheic Ocean whose subsequent demise in the Late Paleozoic led to Pangaea's amalgamation. Since these ideas were proposed, several new lines of evidence have challenged the Amazonian affinity of Avalonia. First, there is evidence that some Avalonian terranes may have been ‘peri-Baltican’ during the Neoproterozoic. Baltica was originally excluded as a potential source for Avalonia because, unlike Amazonia, arc-related Neoproterozoic rocks were not documented. However, subsequent recognition of Ediacaran arc-related sequences in the Timanides of northeastern Baltica invalidates this assumption. Second, detailed palaeontological and lithostratigraphic studies have been interpreted to reflect an insular Avalonia, well removed from either Gondwana or Baltica during the Ediacaran and early Cambrian. Third, recent palaeomagnetic data have raised the possibility of an ocean (Clymene Ocean) between Amazonia and West Africa in the late Neoproterozoic, thereby challenging conventional reconstructions that show the ‘peri-Gondwanan’ terranes as a contiguous belt straddling the suture zone between these cratons. In this contribution, we critically re-evaluate the provenance of the so-called ‘peri-Gondwanan’ terranes, the contiguity of the so-called ‘Avalonian–Cadomian’ belt and the validity of the various plate tectonic models based on the traditional interpretation of these terranes. In addition, we draw attention to critical uncertainties and the challenges that lie ahead.

Emplacement ages, geochemical and Sr–Nd–Hf isotopic characteristics of Cenozoic granites in the Phan Si Pan uplift, Northwestern Vietnam: petrogenesis and tectonic implication for the adjacent structure of the Red River shear zone

Emplacement ages, geochemical and Sr–Nd–Hf isotopic characteristics of Cenozoic granites in the Phan Si Pan uplift, Northwestern Vietnam: petrogenesis and tectonic implication for the adjacent structure of the Red River shear zone

Metamorphic P-T paths and zircon U-Pb ages of Paleoproterozoic metabasic dykes in the Jiapigou area, South Jilin Province: Implications for the tectonic evolution of the North China Craton

The distribution and tectonic evolution of the Paleoproterozoic orogenic belts in the North China Craton (NCC) remain controversial. Especially, the extent of the Paleoproterozoic magmatic and metamorphic influences on the Archean Longgang Block needs further demonstration. This paper presents detailed petrographic and mineralogical analyses, phase equilibria modeling and zircon dating for three metabasic dyke samples in the Jiapigou area, South Jilin Province, in order to discuss their metamorphic evolution and tectonic implications. Petrographic observation and phase modeling using THERMOCALC suggest that these metabasic dykes have experienced the metamorphism with clockwise P-T paths under low-amphibolite facies, high-amphibolite facies and high-pressure granulite facies. Their peak P-T conditions were constrained to be 0.6–0.9 GPa/580–650 °C (JN1929), ∼0.95 GPa/757 °C (JN1918) and ∼1.15 GPa / 820 °C (JN1932), according to the stability fields of mineral assemblage and isopleths of Ti content in amphibole, anorthite component (XAn) in plagioclase, pyrope content (XPy) and grossular content (XGrs) in garnet. The peak conditions correspond to apparent geothermal gradients of 19–22℃/km, being medium-pressure type. Zircon U-Pb dating shows that the metabasic dykes have a protolith crystallization age of 2199 ± 5 Ma (MSWD = 0.43), and a metamorphic age of 1855 ± 41 Ma (MSWD = 0.21). The protolith ages, metamorphic characteristics and ages of the metabasic dykes in South Jilin can be well compared with those for metabasic dykes in East Hebei and North Liaoning, indicating that the northern part of the Longgang Block was extensively affected by the mid Paleoproterozoic magmatism, and was involved in an orogen to be metamorphically reworked during the late Paleoproterozoic (1.82–1.85 Ga). In addition, this orogeny mainly occurred along the northern margin of the craton, which is different from the ∼1.95 Ga orogeny in the NCC.

Late Triassic tectonic stress field of the southwestern Ordos Basin and its tectonic implications: Insights from finite-element numerical simulations

Abstract The tectonic stress field of the southwestern Ordos Basin during the Late Triassic is controversial. The major controversy is whether the southwestern Ordos Basin was a compressional basin throughout the Late Triassic or whether it transformed from an extensional into a compressional basin during this period. We divided the Late Triassic into the early to middle and late to terminal periods. Two paleotectonic stress field simulation models of the southwestern Ordos Basin were constructed using finite-element software (ANSYS 10). Our results showed high consistency with regional geologic correlations, suggesting the credibility of the models. We found that the southwestern Ordos Basin was dominated by NE-SW extensional stress and strain during the early to middle Late Triassic, associated with strike-slip faulting along the western margin of the Ordos block. This is consistent with the development of syndepositional normal faults and was probably induced by the scissor collision from east to west between the North China craton and Yangtze block. The tectonic stress field of the southwestern Ordos Basin during the late to terminal Late Triassic mainly manifested as NE-SW compressive stress and strain. The dominant tectonic dynamics for the Ordos block during this period may have changed to northward compression of the Songpan-Ganzi and Qiangtang terranes. The southwestern Ordos Basin was characterized by compressional deformation and northeastward migration of the depocenter. The southwestern Ordos Basin transformed from an extensional basin associated with strike-slip faulting during the early to middle Late Triassic into a compressional depression basin during the late to terminal Late Triassic.

Paleoproterozoic high-pressure granulite facies metamorphism in the Yinshan Block, North China craton

The Yinshan Block located in the northern margin of the North China Craton (NCC) has been regarded as a typical Archean cratonic block, but it is argued whether the block had been affected by Paleoproterozoic tectonic and metamorphic events. This paper reports the late Paleoproterozoic high-pressure (HP) granulites for the first time from the Xiwulanbulang area in the Yinshan Block, and presents a systematic study on their petrography, mineral chemistry, phase equilibria modelling and zircon dating to confirm their metamorphic evolution and tectonic implications. Two representative samples contain the typical HP basic granulite assemblages involving garnet, clinopyroxene, plagioclase, amphibole and quartz, with garnet commonly occurring as coronae around clinopyroxene and plagioclase. Both samples show clockwise P–T paths with their peak conditions of ∼930 °C/∼15kbar (20NM01) and 970–980 °C/∼13.5 kbar (21NM05) respectively, constrained using the maximum Ti contents in amphibole and minimum anorthite (XAn) in the mantle of plagioclase on P–T pseudosections. Zircon dating yields a magmatic age of ∼ 2.3 Ga and metamorphic ages of ∼2.5 Ga and ∼1.8 Ga. The ∼2.3 Ga magmatic age represents the time of protolith gabbro crystallization, which can be comparative to the extension-related bimodal magmatism in the Khondalite Belt. The ∼ 1.8 Ga age was determined from metamorphic zircons that have flat HREE patterns with the presence of garnet, interpreted as the time of the post-peak cooling and uplifting of the HP granulite facies metamorphism. The ∼2.5 Ga is defined from the captured zircon grains by gabbroic intrusions from the country Neoarchean rocks. The HP granulite facies metamorphism registers a geothermal gradient of 19–20 °C/km, indicating a collision-related orogenic event that may have occurred at ∼1.85 Ga along the northern margin of the NCC. Therefore, the Yinshan Block may represent a weak deformation domain in the orogenic belt on the northern margin of the NCC.

Paleomagnetism and geochronology of Early Cretaceous volcanic rocks in the eastern segment of the Lhasa terrane, Tibetan Plateau, and their tectonic implications

Paleogeographic reconstructions of the Lhasa terrane for the Cretaceous provide important constraints on the evolution of the Neo-Tethys Ocean and crustal shortening within Asia. However, the Cretaceous paleogeography of the Lhasa terrane remains contentious. A direct way to study this issue is to conduct paleomagnetic investigations of the Cretaceous rocks of the Lhasa terrane; however, most previous Cretaceous paleomagnetic investigations of the Lhasa terrane were conducted in the middle and western segments of this terrane. Different vertical-axis rotations affected different parts of the Lhasa terrane following the India-Asia collision, and therefore paleomagnetic data from the western and middle segments of the Lhasa terrane cannot necessarily be used to directly constrain the paleolatitudes of its eastern segment. This study presents paleomagnetic data from the Early Cretaceous volcanic rocks with well-constrained ages from the Luolong area in the eastern segment of the Lhasa terrane. Isotope geochronology reveals that these rocks formed at 127−124 Ma. The mean high-temperature direction obtained from 17 paleomagnetic sites is declination/inclination (Ds/Is) = 21.8°/19.0° with κs = 65.8 and α95 = 4.4° (κ—best estimate of the precision parameter; α95—radius of the 95% probability ellipse around the mean direction; s—stratigraphic coordinates). Petrographic investigations, a positive fold test, a reversal test, and a paleosecular variation test indicate the primary origin of this characteristic remanence. A paleomagnetic pole of 60.9°N, 227.2°E with dp/dm = 2.4°/4.6° (dp/dm—semi-axes of the 95% probability ellipse around the mean pole) yields a paleolatitude of 9.2 ± 2.4°N for the eastern segment of the Lhasa terrane. Combined with reliable results from previous paleomagnetic studies, we draw the following conclusions. (1) During the Early Cretaceous, the Lhasa terrane was oriented WNW-ESE as a whole, and the eastern-middle segments may have been oriented nearly E-W. (2) Asia has accommodated 2050 ± 230 km of N-S crustal shortening along 96°E longitude since the Early Cretaceous. (3) The minimum N-S width of the Neo-Tethys Ocean at ca. 125 Ma was 4185 ± 300 km.

Tectonic Implication of the Permo-Triassic Khao Yai Mafic Volcanic Rocks in Nakhon Nayok Province, Thailand: Evidence from Geochemistry and Rare Earth Elements (REEs)

The Permo-Triassic Khao Yai Volcanics in Nakhon Nayok Province, Thailand, is a part of the Loei-Phetchabun-Nakhon Nayok Volcanic Belt. The purpose of this study is to clarify the geochemistry and REEs characteristic of Khao Yai Volcanics that is useful for identifying the paleo-environment or tectonic setting eruption in this area. The least-altered, mafic volcanic rocks from the Permo-Triassic Khao Yai volcanics, are seriate-textured to porphyritic, with variable amounts of phenocrysts/microphenocrysts. The mineral compositions include plagioclase, clinopyroxene, orthopyroxene, olivine, minor Fe-Ti oxide mineral, amphibole, apatite, biotite/phlogopite, monazite/zircon, glassy, and quartz. The geochemical characteristic of the Khao Yai Volcanics informed that the rocks had the same parental magma with different degrees of crystal fractionation. Most of the mafic volcanic rocks are subalkalic andesite on the basis of their Zr/TiO2 and Nb/Y ratios and calc-alkalic on diagrams Ti-Zr, Ti-Zr-Y, Hf-Th-Ta and Y-La-Nb. The least-altered mafic volcanic rocks have (La/Sm)cn and (Sm/Yb)cn ranging from 2.41 to 2.71 and 1.86 to 2.22, respectively. The studied calc-alkalic andesite are analogous to the Quaternary calc-alkalic dacite and andesite from Maca Volcano, Patagonian Andes and the Middle Eocene andesite from Shimokoh cauldron, SW Japan in terms of chondrite-normalized REEs and N-MORB normalized patterns. Accordingly, the studied Khao Yai volcanics have been developed in an active continental margin, formed by eastward underthrusting Paleo-Tethys, the leading edge of Shan-Thai, beneath Indochina in the Late Permian to Early Triassic. HIGHLIGHTS New tectonic model of the Permo-Triassic Khao Yai mafic volcanic rocks in Nakhon Nayok Province, Thailand The geochemistry and rare earth elements (REEs) analysis by XRF and ICP-MS The REEs analog to modern volcanic rock The classification of pre-Jurassic volcanic belt in Thailand GRAPHICAL ABSTRACT

New palaeomagnetic results of Permian tectonic blocks in the western segment of the Yarlung Zangbo Suture Zone and their tectonic implications

The composition and structure of the Yarlung Zangbo Suture Zone (YZSZ) provide important constraints on long‐standing disputes about the evolution of the Neo‐Tethys Ocean and continental collision. The Permian tectonic blocks (mainly composed of limestones and volcanic rocks) are widely distributed in the YZSZ. There are still different understandings about the origin of these blocks and their relationship with the evolution of the Neo‐Tethys Ocean. Here, we report the first reliable palaeomagnetic results of the Permian tectonic blocks in the Gyanyima and Mayum areas, the western segment of the YZSZ. Fossil records and our new zircon U–Pb results provide 254–252 Ma and 249 ± 2 Ma age constraints for the strata in the Gyanyima and Mayum sections, respectively. A total of 109 specimens were subjected to stepwise thermal demagnetizations. Among them, 83 specimens yielded interpretable results. After removing a recent geomagnetic field direction at low to intermediate temperature ranges, characteristic remanent magnetization (ChRM) directions can be isolated from the upper Permian–lower Triassic Gyanyima Formation (Fm). Unblocking temperature spectra of the ChRM indicate magnetite and haematite as the main carriers in the basalt and limestone/andesitic‐basalt samples, respectively. The ChRM passed the fold tests and a reversal test at 95% confidence level, indicating that it is most likely a primary remanence. The tilt‐corrected mean direction for 14 sites is D = 18.0°, I = −14.4°, k = 20.5, and α95 = 9.0°, which corresponds to a palaeolatitude of 7.3°S and a palaeopole at 48.3°N, 233.3°E with A95 = 8.1°. Combining the rock assemblage, palaeomagnetic, palaeontological, and geochronologic data, we conclude that these blocks were most likely seamounts formed in the low‐latitude area of the Neo‐Tethys Ocean. The palaeolatitude and origin of these blocks indicate that the evolution of the Neo‐Tethys Ocean was a complex process interspersed with the development of a series of seamounts and microblocks.

Early Cretaceous displacement on the Tanymas thrust fault, Northern Pamir, Tajikistan, and regional tectonic implications

An ongoing question in understanding the evolution of the Himalayan-Tibetan orogeny is how much of the observed upper crustal shortening and crustal thickness is related to the Cenozoic collision between India and Asia vs earlier tectonic events along the southern margin of Asia. While the Pamir Mountains located at the western end of the orogen have been proposed to have experienced significant Cenozoic shortening, recent studies have interpreted upper crustal shortening to be primarily mid- to Late Cretaceous. To further understand the timing of upper crustal deformation in the Pamir, we investigated synorogenic clastic deposits within the footwall of the north-dipping Tanymas thrust fault along the suture between the Northern and Central Pamir terranes. Sandstones from these deposits were analyzed by detrital zircon U-Pb, zircon fission track, and muscovite 40Ar/39Ar analyses to assess the age and source of the detritus. Results show the deposits were sourced from the Northern Pamir (hanging wall of the Tanymas thrust) and provide an Early Cretaceous maximum deposition age of ∼130–120 Ma, interpreted to constrain their age and date motion on the Tanymas thrust fault as Early Cretaceous. Our results, integrated with previous studies, show Cretaceous deformation in the Pamir began in the Northern Pamir (∼140–110 Ma) before sweeping into the Southern Pamir in the mid- to Late Cretaceous (∼110–75 Ma). These results are consistent with previous interpretations of an Early Cretaceous phase of shallow- or flat-slab northward subduction followed by slab rollback and southward migration of deformation and magmatism in the mid- Cretaceous.

Three-dimensional coseismic displacements and slip distribution of the 2021 Mw 7.4 maduo earthquake: Synergy of SAR, InSAR and optical images

On 21 May 2021, an earthquake of moment magnitude (Mw) 7.4 occurred near Maduo county in Qinghai Province China. This is the first major earthquake that occurred in the interior of the Bayan Har block in the Tibetan Plateau over the past 70 decades. Focusing on this event, we conducted a study on three-dimensional (3D) coseismic displacement reconstruction and its tectonic implication. We acquired both Synthetic Aperture Radar (SAR) and optical imaging satellite imagery, including SAR images from Sentinel-1 and Advanced Land Observation Satellite-2 (ALOS-2) as well as the multi-spectrum images from the Sentinel-2 (S2) satellite. We applied the Interferometric SAR (InSAR) and pixel-offset tracking (POT) techniques to coseismic SAR data pairs and reconstructed two dimensional displacements. With the constructed displacement fields in multiple viewing directions, we resolved the 3D coseismic displacements (north-south, east-west, and up components) by integrating the regional strain model with variance components estimation (SM-VCE). We recommend using the standard deviation value at each grid cell, which can be calculated in the resampling, as the initial weight. Based on the resolved 3D coseismic displacements, we further estimated the dip angles for the two segments (F4 and F5) in the east of the rupture zone. The associated moment magnitude is about Mw7.4, which corresponds to the released energy of ∼ 1.74×1020 Nm.

Uppermost Mantle Seismic Pn‐Velocity in Continental China and Its Tectonic Implications

AbstractWe construct a high‐resolution Pn‐velocity model in continental China through a new tomographic scheme simultaneously inverting event epicenter location and Pn‐velocity and accurately accounting for the Pn propagation effects of irregular crustal thickness variation. The inversion integrates a combined data set of 32,427 absolute Pn travel times and 62,431 interstation Pn differential travel times manually picked from the recordings of 2,989 stations in and around continental China, yielding a model of spatial resolutions ranging from 0.75° × 0.75°–3° × 3°. Major Pn‐velocity features revealed by the model and the tectonic processes we attribute to include: (a) high Pn‐velocities beneath major cratonic blocks and the southern Tibetan Plateau attributed to low temperatures of cratons, (b) low Pn‐velocities beneath the eastern North China craton around the Tan‐Lu fault and the central South China block attributed to high temperatures and low‐Mg# () peridotites in response to the Mesozoic‐Cenozoic subduction of the Pacific plate, and beneath the Trans‐North China orogen, most regions of the Tibetan blocks and the central Tianshan orogen attributed to high temperatures after multiple tectono‐thermal events in response to the Mesozoic‐Cenozoic subduction of the Pacific plate, the Triassic convergence of the North China, Yangtze, and northern Tibetan blocks and the Cenozoic India‐Eurasia collision, and (c) intermediate Pn‐velocities beneath the marginal regions of the Ordos block and Micang‐Daba region attributed to possible extrusion of hot asthenospheric flow from the Tibetan Plateau in response to far‐field effects of the India‐Eurasia collision.

Role of Depleted-MORB Mantle in the Genesis of Basalts from the Neoarchean Eastern Felsic Volcanic Terrane of the Sandur Greenstone belt, Dharwar Craton, India

Abstract The mafic volcanic rocks from the eastern felsic volcanic terrane (EFVT) of the Neoarchean Sandur greenstone belt of Western Dharwar Craton (WDC) are studied to evaluate their geochemical characteristics and tectonic implications. Characterised as sub-alkaline, tholeiitic basalts, they display moderate SiO2 (49.82-53.74 wt.%), Al2O3 (9.08-15.91 wt.%), TiO2 (0.25-0.66 wt.%) and total alkalies (Na2O+K2O= 0.60-3.97 wt.%) with Mg# ranging from 37-55 at low Cr (7-33 ppm), Ni (8-24 ppm) and V (252-612 ppm). Their rare earth and trace element patterns are coherent exhibiting moderate fractionation (La/Yb=0.16-1.09), slight negative Eu anomalies indicating limited plagioclase fractionation (Eu/Eu*= 0.69-0.96), negative Nb, Ta, Ti and Zr-Hf anomalies revealing their arc signatures. The Nb-Th and Zr-Y relationships of these basalts suggest their derivation from the partial melting of depleted mantle in an intra-oceanic arc setting. Trace element modelling of these basalts indicates their genesis by ~15% partial melting of depleted MORB-mantle and depletion of the source is attributed to previous melting events. These basalts are geochemically similar with the depleted arc and forearc basalts of the Phanerozoic Tonga and Izu-Bonin-Mariana arcs emphasising the operation of analogous tectonic processes during the Archean era.

Crustal azimuthal anisotropy in the lateral collision zone of the SE margin of the Tibetan Plateau and its tectonic implications

SUMMARYResolving the detail of crustal deformation of the seismically and technically active Sichuan-Yunnan block located in the southeast margin of the Tibetan Plateau is crucial for understanding the lateral extrusion of the Tibetan Plateau. Using seismic data from two regional seismograph stations of the China Earthquake Networks Center and twenty recently deployed broad-band temporary stations from the MCD Array (Middle ChuanDian block seismic array), we constrained crustal thickness and seismic anisotropy at the west boundary of the Sichuan-Yunnan block, near the east Himalayan syntaxis by receiver function techniques including CCP stacking and sinusoidal moveout of PmS conversions. Crustal thickness varies significantly across major faults inside the block which increases by >20 km from west to east. East of the Chenghai Fault, a clear positive phase lies above the Moho and is interpreted to be caused by the magma intrusion related to a late Permian Emeishan large igneous province (ELIP). The crustal anisotropy shows strong zoning features and clockwise rotation with respect to due north. To the west of the Lijiang-Xiaojinhe and the Honghe Faults, anisotropic fast orientations are near NS, indicating strong crustal deformation and movement of southward crustal flow along the Nujiang-Lancangjiang Fault; to the east of this fault, anisotropic fast orientations are near NE. The variation of the anisotropic fast orientations indicates that the southeastward crustal flow from the Tibetan Plateau may be prevented by fast velocity anomalies beneath the ELIP. These observations provided new evidence for strong crustal deformation along the tectonic boundary in the southeastern Tibetan Plateau with the expansion of the Tibetan Plateau.