The tectonic-dynamic mechanisms that formed N-S-directed rifts in Southern Tibet remain unknown. Their formation is believed to be related to the subduction pattern of the Indian Plate; however, this pattern remains highly debated. For example, different studies have proposed various ideas about where a slab tear might be located. We created lithospheric-scale 3-D electrical resistivity model along a line that crosses the Xainza-Dinggye rift and Yadong-Gulu rift. The results reveal crustal low-resistivity zones beneath the rifts and high-resistivity zones adjacent to them. The low-resistivity zones are interpreted to indicate a weak middle-lower crust caused by the upwelling of mantle-derived materials, and leading to deformation of the crust, thereby contributing to the development of the rifts. High-resistivity zones between the rifts may be ancient crust in the central Lhasa terrane. In the southern region of the high-resistivity zones it is speculated that, due to break-off of the frontier of the eclogitized Indian lower crust, the Indian lithospheric mantle delaminated from the Indian crust and rolled back, causing steep-angled subduction. The results are interpreted to indicate that, because of the roll-back of the Indian Plate at depth in the regions between the two adjacent rifts, the subduction angle in these regions is steeper than that on both sides. The findings of our study offer an alternative to the traditional view of the subduction pattern of the Indian Plate in southern Tibet, and are in contrast to previous models that suggested smooth and monotonic variations from west to east. ... Read more

Since the Cenozoic, a series of extensional south-north normal faults and gneiss-granite domes evolved in the southern Tibet Plateau, the formation mechanism of which is of scientific interest and which has implications for the tectonic dynamics of the plateau. Typical of such features are the Xainza-Dinggye rift and the Mabja gneiss dome, which are located in the Xainza-Xietongmen-Dinggye region in central Tibet. In this study, Magnetotelluric measurements across this region are used to generate a high-resolution 3-D electrical resistivity model of the subsurface and to analyze the cause of the conductive zones. The large-scale conductive zones identified in the middle-lower crust may result from aqueous melt partial melting, whereas the smaller-scale conductive zones in the upper-middle crust may result from saline fluids, possibly with varying minor volumes of melts. Subsequently, based on the electrical resistivity model, and combined with the spatiotemporal coupling of the geological, geochemical and geophysical data, the state and migration features of crustal materials are discussed. The results show that the upwelling of mantle materials along subduction channels and slab-windows related to the tearing of the Indian lithospheric plate contributed to the partial melting of the middle-lower crust in the Lhasa terrane. Furthermore, partial melting of the upper-middle crust in the Tethys-Himalaya terrane resulted from southern extrusion of crustal materials in the Lhasa terrane. These two mechanisms can significantly reduce the effective viscosity. We speculate that the deformation of the brittle upper crust that is controlled by large-scale ductile layers characterized by weak rheology is the main dynamic mechanism of rift evolution. Meanwhile, the metamorphism and anatexis in the upper-middle crust of the Tethys-Himalaya terrane related to the southern extrusion of materials contributed to the evolution of the Mabja gneiss dome. During the middle Miocene, the southern extrusion of crustal materials may have been influenced by the cooling events beneath the Mabja gneiss dome, which can explain why the deep areas beneath the Mabja gneiss dome have middle-high resistivity. In addition, our study region is located in the Mediterranean-Himalayan seismic belt, and mainly includes shallow-focus earthquakes and intermediate-depth earthquakes. In the north, shallow-focus earthquakes are mainly controlled by the accumulation of stress in the brittle layer of the overlying crust related to the ductile layer of the middle and lower crust. In the south, shallow-focus earthquakes (e.g., Dingri Ms 6.8 earthquake) mainly occur in the rigid, resistive block, which is surrounded by conductive zones, possibly because fluid migration may be hindered by these resistive blocks. The intermediate-depth earthquakes are mainly related to the detachment of the Indian lithospheric mantle from the Indian crust. ... Read more

The Tibetan Plateau is undergoing east-west extension manifested by north-trending rifts. Rift dynamics have been attributed to both mantle convection, which induces vertical motion causing general extension, and plate convergence, with northward motion causing along-strike extension, driven by the subducted Indian slab. However, the cause of lithospheric extension remains debated. We carried out electrical resistivity modeling of the entire Tibetan Plateau and present a quantitative interpretation of low-resistivity structures in terms of high fluid fraction and low viscosity. The model reveals low-resistivity features intruding and overlying the resistive lithosphere of Lhasa and Qiangtang. The low-resistivity features show a transition from vertically oriented to horizontally oriented positions at ∼50−70 km depth and appear to be oriented north-south below the Himalaya and Lhasa and east-west below Qiangtang. The anomalies can be explained by partial melts and fluids and may represent the signatures of material migration and locally weakened lithosphere. This material migration must have been significant enough to sustain rifting and drive the rift tips northward, despite the complex tectonic setting of the Tibetan Plateau, which is composed of a number of independent blocks. The results suggest that north-trending rifts were formed in response to fluid flow, after or during lithospheric foundering below Lhasa. Furthermore, fluid flow can explain the surface distribution of rifts in bands and the variations in rift formation and development between Qiangtang and Lhasa, which are attributed to the local rheological differences and specific regimes of vertical and/or horizontal stresses that are induced by fluid migration. ... Read more

Ophiolite mélanges are commonly found in ancient accretionary orogenic belts. However, they may have diverse origins because they can form in various tectonic settings, including fore-arc, back-arc, active continental margins, mid-ocean ridges, and continental rifts. Identifying and characterizing suture zones in accretionary orogenic belts is crucial for understanding their tectonic evolution. The Beishan Orogenic Collage (BOC) is located in the southernmost part of the Central Asian Orogenic Belt (CAOB), one of the world's largest accretionary orogens, and formed through ongoing subduction and consumption of the Paleo-Asian Ocean and its branch ocean basins. It contains four east-west trending ophiolite mélange belts: (1) Hongshishan; (2) Shibanjing-Xiaohuangshan; (3) Hongliuhe-Niujuanzi-Xichangjing; and (4) Liuyuan, from north to south. Despite abundant geochemical, structural, and geochronological data, no consensus on the settings in which these ophiolite mélange belts formed and their subduction polarities remains elusive. This uncertainty has resulted in several contrasting models hypothesized for the tectonic evolution of the Beishan region.

In this study, we used 60 broadband magnetotelluric measurements and 16 long-period magnetotelluric measurements sites recently acquired across the Beishan region in northwest China to obtain a three-dimensional electrical resistivity model. The model reveals a generally high-resistivity upper crust with several low-resistivity features aligning with suture zones and tectonic boundaries. The high-resistivity lithosphere beneath Niujuanzi is compatible with northward and southward subduction of the Niujuanzi Ocean, potentially revealing remnants of a cold fossil oceanic lithosphere. In contrast, the deep lithosphere beneath the other three ophiolite belts is characterized by low-resistivity features. Since MORB-type rocks have lower iron, hydrogen, and carbon content, they tend to exhibit high resistivity characteristics, compared to a back-arc basin or rift. The model suggest that the Njiujuanzi Ocean was possibly the major ocean of the southern branch of the Palaeo-Asian Ocean, and that it had bi-directional subduction polarity. ... Read more

New evidence worldwide has linked the surface locations of mineral deposits and their crustal-scale electrical conductivity footprint. We examine the relationship between the Gangdese Miocene porphyry copper deposits, Tibetan Plateau, and the electrical conductivity signature from a three-dimensional model generated from 311 magnetotelluric measurements. The distribution of electrical resistivity throughout the crust and the conductance within the mid-lower crust (depth range of 25–70 km) is analyzed. The results clearly show that the large and ultra-large Miocene porphyry copper deposits coincide spatially with conductive zones and areas of very-high conductance (>10,000S) in the mid-lower crust. Computations are undertaken to determine the influence of water-bearing silicate melts and alkali-bearing (Na+ and K+) fluids on conductivity. Based on this, the bulk conductivity is interpretated to be caused by a system of alkali-rich volatile-rich partial melt. The alkali-rich volatile-rich magmatic-hydrothermal fluids facilitate the migration and concentration of metal ions originating in deep areas. The volumes necessary are much less than partial melt alone and can thus help to reconcile large conductivity variations with small seismic velocity variations. The electrical structure indicates the magma source area of anatexis in the lower crust, a multi-stage magmatic system with large mid-crustal and small upper-crustal magma reservoirs, and complex pathways related to rift zones. We determine that the conductive zones in the mid-lower crust have an influence on the development of the mineralization and the location of the mineral belt. ... Read more

The Dinggye region, in the central part of the Himalayan orogenic belt, includes the southern part of the Xainza-Dinggye rift and the Mabja Gneiss Dome with leucogranite cores. Previous studies of gneiss domes in this region report the existence of channel flow processes or tectonic exhumation, in addition to partial melting of orogenic mid-crust. However, the relationship between the crustal migration of materials and the north-south-trending normal rifts remains largely unexplored. In this work, we generate a new 3-D electrical resistivity model from an array of magnetotelluric data in the Dinggye region and examine it in addition to other electrical resistivity models to the north and east from previous works. By comparing the geophysical models with available geological and geochemical evidence, we find a clear relationship between the electrical resistivity structure, the presence of gneiss domes, north-south-trending normal rifting, and deep plunging subduction which is related to the source of Helium isotopes (crustal or mantle origin). Overall, the results suggest that the southern migration of lithospheric materials likely contributed to the evolution of the rifts in the Tethys-Himalaya terrane, which also may have been influenced by uplifting and cooling of gneiss domes. The models are consistent with tearing of the Indian lithosphere beneath the Xainza-Dinggye rift and other adjacent rifts. Additionally, the difference in the electrical structure related to the Indian crust along the east-west direction likely results from the exhumation of the continental slab, metamorphism in the Tethys-Himalaya terrane, and southern extrusion of materials in the Lhasa terrane. ... Read more

Some of the largest and most significant Miocene porphyry copper systems in China are within the Gangdese metallogenic belt on the southern Tibetan Plateau. It has been recognized that the crustal architecture and rheology, derived from regional tectonic events, has direct implications for the evolution and transport of fluids and magmas, and thus for the metallogenesis and prospectivity. Using data from a magnetetolluric array, which intersects the Lhasa–Mozugongka district of the Gangdese metal belt, a 3-D electrical resistivity model was generated, with the goal of investigating the tectonic and rheological controls on the magmatic mineral system. The lithospheric temperature distribution was estimated by applying the Arrhenius equation to conductivity profiles generated from 1-D Monte-Carlo models of long-period magnetotelluric data. The conductivity of partial melts in the lower and middle crust (30–60 km depth) was estimated for local conditions by applying the experimentally-derived equation of X. Guo et al. (2018). Subsequently, we estimated the melt fraction required to explain the observed bulk resistivity in each part of the study area. Variations in the effective viscosity of the lower and middle crust were constrained by the electrical resistivity model by applying the empirical relation of Liu and Hasterock (2016). Beneath the Miocene Cu–Mo deposits in the Lhasa terrane, conductive features in the lower and middle crust are attributed to partial melt fractions of more than 5% and viscosity reductions of 1–2 orders of magnitude. These conductive features may represent the signatures of (ore-controlling) melt/fluid migration channels and deeper melt/fluid source regions in the form of extensive crustal reservoirs of partial melt. Based on the interpretations of the geophysical model, and other available geological and geochemical evidence, a model of the metallogenic dynamics of the Miocene porphyry Cu–Mo deposits is proposed. Overall, the study highlights the applicability of electromagnetic geophysical methods to reliably link resistivity structures to melt/fluid transport channels and sources within a mineral system and supports the hypothesis that crustal rheology exerts a major control on the distribution of ore deposits. ... Read more

The Xainza-Dinggye rift, an approximately north-south trending Cenozoic fault zone across the Lhasa Terrane, is an ideal location to investigate extensional mechanisms in the upper crust and lithospheric deformation caused by the subducting Indian Plate beneath the central-southern Tibetan Plateau. The 3-D electrical resistivity structure was obtained by modeling magnetotelluric data from an array across the rift zone. Using the temperature distribution throughout the crust combined with the pressure and water content, we compute the pure melt conductivity. This enables estimation of the partial melt fraction of large-area conductive zones imaged throughout the crust, and thus allows their rheology and strength to be evaluated. The heterogeneous distribution of high melt fraction areas in the crust has implications for the local continuous migration of fluids in the east-west direction. The electrical structure also reveals a dipping resistive zone beneath the Tethys-Himalaya terrane that represents the subducted Indian Plate. Significantly, its depth is observed to vary substantially from east (deeper) to west (shallower), which may indicate tearing of the plate. We suggest that the cause of extension and the formation of the crustal rift zone is related to tearing of the plate directly below this location, and the subsequent partial melting of the mid-lower crust. Furthermore, the ascent of deep hydrothermal fluids, and heating of the shallow subsurface, likely led to the formation of the congruent Xainza-Dinggye Thermal Belt. Additionally, the emplacement of numerous adjacent magmatic-hydrothermal ore deposits is also likely related to partial melting and hydrothermal fluid migration in the crust. ... Read more