Lithospheric Architecture Research Articles

Mantle conduits of the K-Pg Reunion mantle plume rise beneath the Indian subcontinent revealed by 3D magnetotelluric imaging

The central-western region of the Indian subcontinent hosts the vast geological records of its evolution from the Archean to the Recent, including the youngest (∼65 Ma) episode of the Réunion mantle plume activity that produced a large igneous province, the Deccan Volcanic Province (DVP). A three-dimensional lithospheric resistivity image of central-western India is obtained to understand the lithospheric architecture and map any major eruption channels of the Deccan volcanism as no explicit geophysical revelation of such pathways of magma ascend has yet been made. Two high conductivity (< 30 Ωm) pipe-like geometric features originating from a common deep mantle conductive zone under the Malwa plateau (northernmost lobe of the DVP) and its proximity are detected in the resistivity model. These are interpreted to be remnants of the hitherto unknown primary lithospheric pathways of magma ascent from the deep mantle melt-rich zone related to the Reunion mantle plume upwelling under central-western India. This study gives first compelling geophysical evidence of key eruptive centers of the massive Deccan volcanism in central-western India at a locale not anticipated earlier. High to moderate conductivity crustal zones and weak to moderate lithospheric mantle resistivity in most parts of the study region represent an intense and multiphase tectono-magmatic evolution of the region spanning from the Neoproterozoic to the Cretaceous-Paleogene boundary.

Introduction to the magnetotellurics special issue of exploration geophysics

The geophysical technique of magnetotellurics (MT) was first described in 1953 and the associated technique of geomagnetic depth sounding (GDS) in 1959. Both methodologies have had a long history in Australia both for academic research and resource exploration. Since the early 1990s with advances in digital data storage, 2D inversion codes and 3D inversion codes from the 2010s, there has been a rapid increase in the number of MT surveys across Australia. Regional MT programs around Australia and since 2013 the Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP) have steadily mapped the 3D resistivity of the Australian continent. To complement the regional AusLAMP MT sites numerous additional transects, and broadband MT grids have been deployed providing remarkable new insights into the tectonic evolution of the Australian lithosphere and provide the link to crustal mineral resources. In this special edition, ten papers explore different aspects of MT research in Australia over the last decade.

Lithospheric influence on metallogenesis in the East Kunlun Orogen: Insights from isotopic and geochemical mapping

This study examines the influence of lithospheric architecture on regional metallogenesis, using geological and geochemical data from the East Kunlun Orogen (EKO). We explore how specific lithospheric components and structures affect the formation of orogenic gold and various polymetallic mineral deposits. The study reveals that metallogenic processes in the EKO initially targeted sources within the metasomatized-enriched subcontinental lithospheric mantle (SCLM) and reactivated lower crust during the Paleozoic era.Lithospheric architecture, shaped by crust-mantle interactions occurred in the Early Mesozoic led to variations in the sizes of orogenic gold deposits between the eastern and western EKO. In the west, smaller gold deposits are linked to moderate mantle underplating and slightly enriched SCLM. Gold-bearing fluids released by mantle devolatilization mixed with metamorphism fluid in collision stage, contributing to the formation of these deposits. In contrast, the eastern EKO hosts larger gold deposits, resulting from significantly enriched SCLM. Extensive lithospheric thinning process in post-collision stage lead to the mixing of gold-bearing mantle-derived and magmatic-hydrothermal fluids. Fertilized magma differentiation potentially increases gold concentrations in this region.Additionally, Triassic-era melting of the lower crust led to diverse mineralization types in porphyry-skarn-epithermal deposit systems. Key findings include the role of juvenile fertile lower crustal materials in forming copper polymetallic deposits, the importance of metamorphic basement reworking for molybdenum and tungsten deposits, and the influence of lead and zinc leaching from old sedimentary rocks on the formation of silver‑lead‑zinc deposits. This study broadens our understanding of the geological factors influencing metallogenesis and provides a framework for future exploration and research in regions with similar lithospheric architectures.

Large variations in the lithospheric thickness of northwestern India: Imprints of collisional and thermal reworking

The fold belts, which border the cratons, are the building blocks for understanding the origin and modification of older continents. The Aravalli-Delhi Fold Belt (ADFB) in northwestern India provides evidence of crustal block accretion due to continental collision, whereas Malani Igneous Suite (MIS) modified with widespread magmatism. Imaging of upper mantle structure and lithospheric modifications, if any, of such regions has been intricate owing to reworking by subsequent superimposed tectono-magmatic processes. We applied a 2D modelling approach to model the lithospheric architecture along a 1000-km long WNW-SSE geotransect across northwestern India, which has been deformed in the past. Our modelling technique combines terrestrial gravity anomaly, heat flow data, satellite-based geoid, and topographic datasets using the basic premise of thermal steady-state and local isostasy. The overall 38 to 40 km thick crustal geometry underneath the Marwar Block had the maximum lithological heterogeneity. The region surrounding the MIS is characterised by 8–10 km thick high-density (2.78 g/cm3) sills deposited in the upper crust down to 9 km depth and another 10–15 km thick high-density (3.05 g/cm3) mafic mantle material near the Moho. About 42 km thick crust, including an 8 to 10-km thick high-density (3.05 g/cm3) underplated layer at its bottom, characterises the high-relief ADFB. The Vindhyan region of Bundelkhand craton is defined by a ∼ 1 km thick trap, having Moho extending at a depth of ∼40 km. The lithospheric thickness varies substantially from ∼143–168 km underneath the Marwar block, which thins to ∼135 km under the ADFB and thickens gradually to ∼150–165 km beneath the Vindhyan region. Substantial crustal density differences in distinct crustal domains, when integrated with the thin lithosphere, reinforce the concept that tectono-magmatic processes might have modified the lithosphere in NW India.

Major types, spatio-temporal distribution, and metallogenesis of magmatism-related polymetallic deposits in the Bangonghu–Nujiang metallogenic belt, Tibet

The Bangong–Nujiang metallogenic belt (BNMB) is one of the vital metallogenic belts in the Tibetan Plateau. At present, the metallogenic regularity on the scale of the whole metallogenic belt is still poorly understood. In this contribution, the geological, geochronological, geochemical, and isotopic data of 87 magmatism-related polymetallic deposits in the BNMB have been systematically collected to summarize the metallogenesis of polymetallic deposits in this belt. Our research shows that different segments of the BNMB have distinct metallogenic differences. The western, central, and eastern segments of the BNMB are characterized by the development of abundant Cu–Au, Fe–Cu–Cr–Ni, and W–Pb–Zn deposits, respectively. These various types of magmatism-related polymetallic deposits in the BNMB were formed between 188 Ma and 76 Ma, and the mineralization has two peak periods of 125–110 Ma and 90–75 Ma. The spatio-temporal variation of different types of magmatism-related polymetallic deposits in the BNMB is likely to be constrained by the lithospheric architecture and deep geodynamic processes. The elemental and Sr–Nd–Hf–S–Pb isotopic results indicate that various kinds of mineralization are closely related to relevant magmatism. The Cu–Au mineralization is related to dominantly mantle-derived, highly-oxidized, and less-evolved granitic intrusions; the Pb–Zn–W mineralization is genetically associated with dominantly ancient middle–upper crust-derived, relatively reduced, and highly-evolved granitic intrusions; the characteristics of the ore-causative intrusions related to Fe–Cu–Mo polymetallic mineralization are between those of the above two end members. In combination with previous studies, we propose a comprehensive model that includes four stages (>145 Ma, ∼145–125 Ma, ∼125–105 Ma, and ∼105–74 Ma) to illustrate the Jurassic–Cretaceous tectonic–magmatic–metallogenic evolution history in the whole BNMB.

Trans‐Lithospheric Ascent Processes of the Deep‐Rooted Magma Plumbing System Underneath the Ultraslow‐Spreading SW Indian Ridge

AbstractProcesses of magma generation and transportation in global mid‐ocean ridges are key to understanding lithospheric architecture at divergent plate boundaries. These magma dynamics are dependent on spreading rate and melt flux, where the SW Indian Ridge represents an end‐member. The vertical extent of ridge magmatic systems and the depth of axial magma chambers (AMCs) are greatly debated, in particular at ultraslow‐spreading ridges. Here we present detailed mineralogical studies of high‐Mg and low‐Mg basalts from a single dredge on Southwest Indian Ridge (SWIR) at 45°E. High‐Mg basalts (MgO = ∼7.1 wt.%) contain high Mg# olivine (Ol, Fo = 85–89) and high‐An plagioclase (Pl, An = 66–83) as phenocrysts, whereas low‐Mg basalts contain low‐Mg# Ol and low‐An Pl (Fo = 75–78, An = 50–62) as phenocrysts or glomerocrysts. One low‐Mg basalt also contains normally zoned Ol and Pl, the core and rim of which are compositionally similar to those in high‐Mg and low‐Mg basalts, respectively. Mineral barometers and MELTS simulation indicate that the high‐Mg melts started to crystallize at ∼32 ± 7.8 km, close to the base of the lithosphere. The low‐Mg melts may have evolved from the high‐Mg melts in an AMC at a depth of ∼13 ± 7.8 km. Such great depths of magma crystallization and the AMC are likely the result of enhanced conductive cooling at ultraslow‐spreading ridges. Combined with diffusion chronometers, the basaltic melts could have ascended from the AMC to seafloor within 2 weeks to 3 months at average rates of ∼0.002–0.01 m/s, which are the slowest reported to date among global ridge systems and may characterize mantle melt transport at the slow end of the ridge spreading spectrum.

Geochemical mapping of lithospheric architecture disproves Archean terrane accretion in the Yilgarn craton

Abstract The basement of Mesoarchean to Neoarchean greenstone basins in the Yilgarn craton is composed of fragments of evolved crust up to 3.7 Ga old. New cratonwide geochemical and isotopic data with unparalleled spatial resolution image a NE- to ENE-trending architecture in pre–2.73 Ga crust. These trends cannot be reconciled with plate-tectonic models, as they persist across younger NNW-striking structural fabrics, including a proposed suture previously interpreted to result from exotic terrane accretion. Our results suggest that, in spite of their substantial strike length, the NNW-trending structures have limited horizontal displacement and, although important for understanding regional geology, may be a geodynamically insignificant overprint of the primary ENE-trending architecture. We propose that these greenstone provinces or belts include individual basins formed in rifts with location, size, and orientation influenced by the interaction between basement fragments and regional crustal extension.

AusLAMP shines a light on space weather hazards in the Australian high-voltage power grid

A geomagnetic storm, also known as a geomagnetic disturbance (GMD), is a major disturbance of the Earth’s magnetic field caused by solar activity. A geomagnetic storm induces electric currents in the Earth that feed into power lines through substation neutral earthing, causing instabilities and even blackouts in electricity transmission systems. The intensity of geomagnetically induced currents (GICs) is closely associated with the electrical conductivity of the surrounding geology. In this paper, we analyse one of the most well-known geomagnetic storms, the 1989 “Québec storm” and 688 magnetotelluric (MT) survey sites from the Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP) to gain insight into the space weather hazard posed for Australia's modern-day power grids. Transmission lines may exhibit local maxima at differing times depending on their spatial orientation and length with respect to the time-varying magnetic field. Localised peak voltages over 100 V can be observed on some individual lines. This assessment identifies the distribution of GICs in south-eastern Australia for the 1989 Québec storm and transmission lines that are more vulnerable to GICs. It is relevant to national strategies and risk assessment procedures to mitigate space weather hazards in the Australian high-voltage power grid and the design of a more resilient power transmission system. We also analyse the 2015 “St Patrick’s Day storm” to study under-estimation of the space weather hazard associated with the band-limited geomagnetic data and MT data sets. Key points The subsurface geology has a great influence on the intensity of geomagnetically induced electric fields, potentially causing up to three orders of magnitude difference between conductive basins and resistive cratonic regions in south-eastern Australia. Analysis using the 1989 “Québec geomagnetic storm” and AusLAMP magnetotelluric data shows the intensity of the geoelectric fields in south-eastern Australia could reach up to 5 [V/km]. Geomagnetically induced voltages in the Australian high-voltage power grid could be in excess of 100 V in some transmission lines for a geomagnetic storm with intensity comparable with the 1989 Québec geomagnetic storm.

Lithospheric Structure and Extensional Style of the Red Sea Rift Segments

Abstract The Red Sea provides an opportunity to study the processes during the transition from continental rifting to early-stage seafloor spreading during ocean initiation. We delineate variations of lithospheric architecture and the nature of extension along the Red Sea region through joint interpretation of gravity and geoid anomalies and gravity-topography transfer functions. We use lithospheric-scale models to compare stretching factors with upper mantle gravity anomaly, residual mantle Bouguer anomaly, and effective elastic thickness. Based on our observations, the Red Sea is divided into four segments; each having distinct lithospheric characteristics and stretching styles. These are: (i) southernmost Red Sea and Danakil having regionally weak and stretched lithosphere, (ii) southern Red Sea with fully developed seafloor spreading and asymmetric lithospheric architecture, (iii) central Red Sea having discontinuous magma accretion with newly formed seafloor spreading, and (iv) northern Red sea with a stronger lithosphere and limited stretching revealing a stage of continental rifting. In these segments, lithospheric stretching correlates with regions of weak lithosphere, including a regime of sublithospheric plume channel beneath the southern Red Sea. The Zabargad fracture zone between the central and northern segments is revealed as a major lithosphere-scale boundary that may act as a barrier to the propagation of seafloor spreading into the northern Red Sea. The weak and highly stretched lithosphere in this region may indicate the onset of a new spreading cell. Our results conclude that the evolution of the Red Sea is more complex than the previously suggested kinematic models of simple “unzipping” and illustrate that several extensional styles can exist within different segments during the initial stages of ocean formation.

Zircon xenocryst geochronology and implications for the Lhasa terrane evolution: Insights from Cenozoic volcanic rocks (Coqen, Tibet)

The Cenozoic volcanic rocks of Coqen are located in the western central Lhasa subterrane, where knowledge of the regional lithospheric architecture and related magmatic processes is still lacking. To address this issue, we used in situ U–Pb dating, Lu–Hf isotopic and mineral inclusion analysis in zircon xenocrysts among trachyandesite. These zircons from volcanic samples yielded wide U–Pb ages from 18 Ma to 2424 Ma, showing three significant age peaks at ∼246 Ma, 193 Ma, and the most intense one at 128 Ma, which represent three major magmatic processes that developed in the area. The earliest zircon dates suggest the age of volcanic eruption in the Coqen area was not earlier than 18 Ma. zircon xenocrysts trace elements data point to the central Lhasa subterrane continental crust origin. We provide evidence for evolutive magmatism of the Lhasa terrane by calculating ΔFMQ, and show that the oxygen fugacity goes from reduced to oxidized (Cambrian to Miocene). The very low εHf(t) indicate a long-term magmatic event sequence, with the Zenong Group having the most vigorous magmatic activity. The old U–Pb ages and TDMC confirmed that an ancient basement of the Precambrian exist in the middle Lhasa terrane. Our work also restores the tectonic and crustal thickness evolutionary process of Coqen area even entire Lhasa terrane. In the back of a series of thickening and thinning processes, the Tibetan crust drastically thickened at ∼54 Ma, and gradually created what is now believed to be the thickest crust in the world.

Continental Fragments in the South China Block: Constraints From Crustal Radial Anisotropy

AbstractThe lithospheric architecture of the South China Block (SCB) is crucial to understanding the formation and evolution of this distinctive and highly reworked continental lithosphere with over 3 billion years of tectonic history. However, due to a lack of high‐resolution geophysical datasets, a detailed picture of the SCB lithosphere is absent, and fundamental questions regarding its formation, assembly, and subsequent reworking processes are actively debated. Assuming that unique deformation patterns due to such tectonic processes can be mapped by seismic anisotropy, we present a new crustal radially anisotropic shear‐wave velocity model along a 1500‐km seismic transect that spans the major tectonic domains of the SCB to characterize the past deformation processes. The new seismic models show significant lateral variations in seismic anisotropy and velocity, suggesting that the SCB consists of several separated (micro)continental blocks or terranes that likely have different origins and have survived the prolonged deformation history since the early formation of these continental fragments. Combining available geophysical datasets, we link individual crustal domains of distinct anisotropy to constrain the multiphase deformation processes of the SCB, including the early formation of the Proto‐Yangtze and Cathaysia Blocks, the assembly of the SCB, and the subsequent reactivation of the interior and extensive deformation that have formed the Basin‐and‐Range style tectonics in the Cathaysia Block. We suggest that relict continental fragments have played critical roles in the formation and reactivation of the SCB lithosphere.

A national-scale mineral potential assessment for carbonatite-related rare earth element mineral systems in Australia

The production of rare earth elements is critical for the transition to a low carbon economy. Carbonatites (>50% carbonate minerals) are one of the most significant sources of rare earth elements (REEs), both domestically within Australia, as well as globally. Given the strategic importance of critical minerals, including REEs, for the Australian national economy, a mineral potential assessment has been undertaken to evaluate the prospectivity for carbonatite-related REE (CREE) mineralisation in Australia. CREE deposits form as the result of lithospheric- to deposit-scale processes that are spatially and temporally coincident.Building on previous research into the formation of carbonatites and their related REE mineralisation, a mineral system model has been developed that incorporates four components: (1) source of metals, fluids, and ligands, (2) energy sources and fluid flow drivers, (3) fluid flow pathways and lithospheric architecture, and (4) ore deposition. This study demonstrates how national-scale datasets and a mineral systems-based approach can be used to map the mineral potential for CREE mineral systems in Australia.Using statistical analysis to guide the feature engineering and map weightings, a weighted index overlay method has been used to generate national-scale mineral potential maps that reduce the exploration search space for CREE mineral systems by up to ∼90%. In addition to highlighting regions with known carbonatites and CREE mineralisation, the mineral potential assessment also indicates high potential in parts of Australia that have no previously identified carbonatites or CREE deposits.

Mapping the Structure and Metasomatic Enrichment of the Lithospheric Mantle Beneath the Kimberley Craton, Western Australia

AbstractThe lithology, geochemistry, and architecture of the continental lithospheric mantle (CLM) underlying the Kimberley Craton of north‐western Australia has been constrained using pressure‐temperature estimates and mineral compositions for &gt;5,000 newly analyzed and published garnet and chrome (Cr) diopside mantle xenocrysts from 25 kimberlites and lamproites of Mesoproterozoic to Miocene age. Single‐grain Cr diopside paleogeotherms define lithospheric thicknesses of 200–250 km and fall along conductive geotherms corresponding to a surface heat flow of 37–40 mW/m2. Similar geotherms derived from Miocene and Mesoproterozoic intrusions indicate that the lithospheric architecture and thermal state of the CLM has remained stable since at least 1,000 Ma. The chemistry of xenocrysts defines a layered lithosphere with lithological and geochemical domains in the shallow (&lt;100 km) and deep (&gt;150 km) CLM, separated by a diopside‐depleted and seismically slow mid‐lithosphere discontinuity (100–150 km). The shallow CLM is comprised of Cr diopsides derived from depleted garnet‐poor and spinel‐bearing lherzolite that has been weakly metasomatized. This layer may represent an early (Meso to Neoarchean?) nucleus of the craton. The deep CLM is comprised of high Cr2O3garnet lherzolite with lesser harzburgite, and eclogite. The peridotite components are inferred to have formed as residues of polybaric partial mantle melting in the Archean, whereas eclogite likely represents former oceanic crust accreted during Paleoproterozoic subduction. This deep CLM was metasomatized by H2O‐rich melts derived from subducted sediments and high‐temperature FeO‐TiO2melts from the asthenosphere.

Refertilized continental root controls the formation of the Mianning–Dechang carbonatite-associated rare-earth-element ore system

Rare earth element ore deposits associated with carbonatite derived from Earth’s mantle supply half of the world’s rare earth element. However, the formation of carbonatite and initial enrichment and transport of rare earth element in the mantle, is unclear. Here, we image the lithospheric architecture of a Cenozoic rare earth element ore belt in southwestern China by integrating seismic tomography with geochemical data. The subduction of the Indian continent caused vertical upwelling and lateral flow of the asthenosphere, which triggered the melting of the overlying subcontinental lithospheric mantle to generate carbonatites. Such a mantle source that previously metasomatized by fluids from recycled marine sediments is a precursor process critical for forming a giant rare earth element system. For the studied ore belt, three key factors are prerequisites to generating ore-forming carbonatites: thick lithosphere with a continental root; prior fertilization of the subcontinental lithospheric mantle; and trans-lithospheric weakness for magma ascent.

A Model for the Lithospheric Architecture of the Central Andes and the Localization of Giant Porphyry Copper Deposit Clusters

Abstract In the central Andes, giant porphyry copper deposits of similar ages group into discrete geographic clusters that are regularly spaced and aligned within orogen-parallel belts. This clustering highlights how exceptional geologic processes affected localized regions of the lithosphere during mineralization and that the spatial and temporal distribution of giant porphyry deposits is nonrandom. Development of favorable regions of lithosphere for significant metal concentration are linked to the overlap of structural pathways that focus fluid and magma flow from the mantle to upper crust during high-horizontal-compressive-strain events. These structural pathways are notoriously difficult to identify in the field due to their often-subtle surficial manifestations and continental scale. Field mapping at multiple scales in northwest Argentina and southern Peru, as well as regional structural traverses throughout the central Andes, indicates the presence of regional-scale structural corridors 5 to 25 km wide and hundreds of km long that consist of myriad fault planes. The variable width and diffuse surface expression of these corridors is interpreted to reflect the upward propagation of underlying zones of basement weakness through younger supracrustal sequences in the overriding plate. Such structural corridors are (1) apparent at multiple scales of investigation, (2) long-lived, (3) preferentially reactivated though time, and (4) evident in geophysical data sets. This structural architecture formed in response to the interplay of pre-Cenozoic tectonics and the orientation of inherited structural weaknesses. These fault systems persist in the upper crust as steep zones of enhanced permeability that can preferentially reactivate as pathways for ascending hydrous magmas and fluids during major deformation events. Linear orogen-parallel structural belts cogenetic with the magmatic arc provide the first-order control to giant porphyry copper deposit distribution. The second-order control is the intersection of orogen-oblique structural corridors with the orogen-parallel belts, localizing deposit clusters at these intersections. Such regions are inferred to have been zones of deep permeability, with vertical translithospheric pathways activated during high-strain tectonic events that affected the intra-arc stress field.

Lithospheric architecture beneath the Amsterdam-St. Paul plateau, Southern Indian Ocean using the integrated gravity, magnetic and seismological study

Evolution of the Amsterdam-St. Paul (ASP) plateau is debated as a consequence of the Kerguelen mantle plume interaction with the South-East Indian Ridge (SEIR) or related with a weaker ASP plume origin. We performed an integrated geophysical approach using gravity and magnetic modeling along with the joint inversion of Ps receiver function and Rayleigh wave group velocity dispersion data to determine the crustal architecture, and upper mantle structure below the plateau. The gravity-magnetic model revealed that the plateau is associated with three crustal layers as basaltic layer, gabbroic layer and underplated material underlain by sedimentary strata. Our models further suggests that beneath the ASP plateau the Moho depth varies from ∼6.6 to 18.0 km. Joint inversion of Ps receiver function and Rayleigh wave group velocity dispersion curve suggested the Lithosphere-Asthenosphere Boundary at depth of 50 km below the Amsterdam Island. Further seismological result suggests evidence of a Low Velocity Zone below the Moho at depth of 20–36 km which may represent a magma chamber. Our model suggests that the evolution of the ASP plateau is guided by the Amsterdam and St. Paul fracture zones, which acted as a thermal boundary confining the horizontal movement of the ASP plume. We propose that the ASP plateau is not linked with the Kerguelen plume and emphasize origin of the ASP plateau by the interaction of SEIR with the ASP plume.

Structural inheritance controlled by olivine viscous anisotropy in fossil mantle shear zones with different past kinematics

Geophysical and geological observations hint for the presence in the lithospheric mantle of both active and fossil shear zones with varied past kinematics. Shear in the lithospheric mantle produces olivine crystallographic preferred orientations (CPO), which lead to dependence of the viscous behavior on the direction of the load. Yet, the role of anisotropic viscosity in the mantle is seldom considered in models of structural reactivation. Here, we present 3-D geodynamic finite-element models that quantify the strain and viscosity distribution in a lithosphere containing a fossil thrust (or extensional) mantle shear zone with variable orientation relatively to a new extensional or compressional tectonics. The results were compared to that of a fossil strike-slip mantle shear zone. The fossil olivine CPO in the shear zone produces an anisotropic response of the lithospheric mantle, which is modelled using a parameterized description of viscous anisotropy derived from polycrystal plasticity simulations. We found that CPO-induced reactivation of fossil extensional or thrust mantle shear zones is favored for dips 30–60°, with maximum strain localization if the load is normal to the trend of a fossil shear zone dipping by 45°-50°. This represents a broader range of reactivation potential than in fossil strike-slip mantle shear zones. Both fossil shear zones trending at 45° to the imposed load are reactivated regardless of their dip. For a given viscous anisotropy in the lithospheric mantle (i.e., fossil shear zone orientation), the associated strain localization in the crust is always higher under extensional reactivation, where strain rates may be up to ∼100 times higher than those in the (isotropic) surrounding crust. These results imply that viscous anisotropy associated with fossil mantle shear zones plays a key role in large-scale structural reactivation during successive tectonic episodes, shaping the current lithospheric architecture, such as in the Pyrenees and Norwegian margin.

Evolution of the foreland basins on either side of the Tian Shan and implications for lithospheric architecture

The previous understanding of Tian Shan's lithospheric architecture can be summarized into three models: unidirectional underthrusting, bidirectional underthrusting, and mantle upwelling. These models make distinctive predictions regarding the migration and subsidence histories of the foreland basins on either side of the range. Seismic profiles crossing both foreland basins document their migration and subsidence history. We analyze a seismic profile across the Southern Junggar foreland basin (SJFB) to the north of the range, which demonstrates that the basin migrated northward at ∼3.1 mm/yr during 22–16 Ma. Combining this with previously known migration rates of the Northern Tarim foreland basin (NTFB) to the south of the range, we constrain the migration of these two foreland basins relative to their basements. We then employ an elastic plate model to simulate the subsidence profiles across these two basins at ∼20 Ma, 13–16 Ma, ∼5 Ma, and 0 Ma constrained by back stripping well log data and seismic records of the basin onlaps. Our modelling reveals the propagation rates of the southern and northern Tian Shan thrust wedges and the evolution of their topographic loads. Our analysis and modelling demonstrate that the NTFB has the characteristics of a retro-foreland basin, whereas the SJFB resembles a pro-foreland basin during early-middle Miocene times. This suggests that the Tian Shan's growth during this period was dominated by underthrusting of the Junggar lithosphere. Furthermore, our analysis and modelling reveal a convergent pattern between the Tarim lithosphere and the Tian Shan changed from lithospheric indentation to lithospheric subduction after the middle Miocene, as the plate setting of the NTFB and SJFB changed. The Tian Shan had begun to uplift before the middle Miocene, with the orogen's height reaching half of its present elevation before the late Miocene.

Spatially heterogeneous lithospheric architecture of the Cambay rift basin and adjoining Aravalli-Delhi Fold Belt, western India – A synthesis of magnetotelluric results

The Cambay rift basin (CRB) evolution has been associated with separation of the Indian continent from the African continent and subsequent interaction with the Réunion mantle plume, which resulted in significant alternations in the regional lithospheric structure of western India, more prominently in the CRB due to its proximity to the plume track. In our continued effort to delineate the lithospheric structure of the CRB and adjoining regions, we acquired magnetotelluric (MT) data at 31 sites along a profile across the CRB and analyzed these. The 2D geoelectric model derived from this dataset reveals a collage of conductive and resistive formations. The resistive blocks indicate the Archaean lithosphere on the west, the uplifted crust within the CRB, and various structural units of the Precambrian Era on the east providing evidence for continuation of the Aravalli-Delhi Fold Belt trend beneath the CRB into Saurashtra. The conductive anomalies beneath the CRB infer the magmatic underplating associated with thermal influx derived from the upper mantle as a result of the Réunion plume interaction. Integration of previous geophysical and present results infers a spatially heterogeneous lithosphere possibly representing an assemblage the Archaean and Proterozoic lithospheres beneath the CRB. The Proterozoic lithosphere is more vulnerable to the impact of the Réunion plume due to its weak nature in comparison to the Archaean lithosphere. The Archaean lithosphere on the western side of CRB indicates initial stage of rifting, whereas Proterozoic lithosphere infers second stage coeval with the Deccan volcanic eruption. The heterogeneity in sedimentary thickness within the CRB spatially correlates with the contact of Archaean and Proterozoic lithospheres, which might have played a role in the formation of the basin structure, and sediment deposition.

Three-dimensional magnetotelluric imaging of the Eastern Qinhang Belt between the Yangtze Block and Cathaysia Block: Implications for lithospheric architecture and associated metallogenesis

The Eastern Qinhang Belt represents the suture zone between the Yangtze and Cathaysia Blocks and controls the world-class Dexing porphyry copper (Cu) deposits. A three-dimensional three-dimensional structural interpretation based on magnetotelluric array data covering the Eastern Qinhang Belt was determined to provide new information on the lithospheric architecture of this mineral system. The regional electrical structure of the Eastern Qinhang Belt is consistent with the pattern of the Appalachian-style multi-terrane Wilson cycle, with the main electrical gradients corresponding to pre-existing lithospheric faults. It is possible that the central domain between the Huaiyu Terrane and Western Cathaysia Block preserved the slivers of the mafic lower crust generated by the modification of Neoproterozoic subduction. Partial melting of this juvenile mafic lower crust was triggered by the extension from the rollback of the Paleo-Pacific Plate during the Mid-Jurassic. The Northeast Jiangxi Fault represents the conduit for the rise of ore-bearing magmas capable of producing porphyry Cu mineralization.