Search

To better understand the geothermal system of the Gonghe Basin, we deployed 471 magnetotelluric survey points with an average distance of 2~3 km, covering the eastern and southern areas of the Basin. We used ModEM inversion software to carry out 3D inversion of 431 survey points and established a 3D-electrical model at a depth of 50 km in the area. The resistivity model shows that the low resistivity in the shallow part of the basin is related to the Cenozoic loose sedimentary cover, while the resistivity values of the mountains around the basin and the magmatic rock uplift zone are higher. The electrical model also shows that the high-conductivity layer is widely distributed in the middle and lower crust (15~35 km) of the basin, and direction of the high-conductivity layer is consistent with that of NW–SE fault in the basin. These high-conductivity layers may be the principal reason for the high heat flow values in the Gonghe Basin. Our resistivity model also shows that there is an obvious discontinuity between high- and low-resistivity blocks at different depths in the middle and upper crust. These discontinuities are consistent with the faults observed on the surface, which are related to the strong topographic relief. Our electrical model shows that these faults in the middle and upper crust are connected with the high-conductivity layer as the channel of heat transfer to the shallow part. Finally, the heat energy is enriched in the Triassic granite to form dry hot rock (HDR). The 3D-magnetotelluric imaging results depict the 3D-distribution characteristics of the geothermal system in the eastern and southern parts of the Gonghe Basin. [ABSTRACT FROM AUTHOR]

The Beiya gold polymetallic ore deposit is one of the largest gold deposit in the world. Previous studies have suggested the formation of the Beiya deposit is closely related to the porphyritic alkaline intrusions. Different tectonic models have been proposed by geologists to explain the alkaline intrusions. However, no consensus has been reached and data for imaging of the deep crustal structure is lacking. In this study, we use magnetotelluric data to image the 3‐D crustal structure beneath the Beiya deposit and adjacent areas. The 3‐D electrical resistivity model is characterized by two conductors and three resistors, which depicts the magmatic system and fluids migration path related to the Beiya deposit. One shallow conductor beneath the Beiya deposit is suggested to represent a cooled magma chamber, which provided essential heat and metal‐rich fluids to the Beiya deposit. The shallow conductor is connected to a deep conductor located beneath the Ailaoshan‐Red River shear zone. Thus, magmas originated from mantle upwelling in an extensional tectonic background is believed to provide the heat that melted the crust and generated intermediate melts, the Ailaoshan‐Red River shear zone is proposed to facilitate ascending of partial melts from the base of the crust. Moreover, two resistors encircles the shallow conductor at upper crustal depths could have acted as an impermeable barrier that helped further subtraction of metal‐rich fluids. The Beiya gold polymetallic deposit is believed to be a typical porphyry‐skarn deposit formed closely related to the Ailaoshan‐Red River shear zone in an extensional tectonic setting. Plain Language Summary: The Beiya gold deposit is one of the largest in the world. Previous studies have suggested the formation of it is closely related with one kind of igneous rocks enriched with Na‐ and K‐ bearing minerals. Different geological models have been proposed to explain the formation of these igneous rocks. However, no agreement has been reached. In this study, we use one kind of geophysical data (magnetotelluric) to image the 3‐D crustal structure beneath the Beiya deposit. The data was used to map the electrical resistivity of rocks within the Earth from the surface to a depth of 50 km. One shallow conductor is located beneath the Beiya deposit and is suggested to be the geological body which provided heat and metal‐rich fluids to the Beiya deposit. It is connected to a deep conductor underneath the Ailaoshan‐Red River shear zone, which is a deep and long deformation zone caused by shear strength. Thus, melted materials from the mantle is believed to rise and provide heat for further melting of the crust in a tectonic background where the subsurface rocks are under extensional forces. The mantle and crust melts mixed in a complex way and generated melts of intermediate composition. The Ailaoshan‐Red River shear zone is proposed to facilitate ascending of the intermediate melts from the base of the crust. Key Points: The magmatic system related to the formation of the Beiya deposit is imaged by a 3‐D magnetotelluric surveyA deep magmatic source is imaged beneath the Ailaoshan‐Red River shear zoneThe result supports the Beiya deposit was formed in an extensional tectonic setting and associated with the Ailaoshan‐Red River shear zone [ABSTRACT FROM AUTHOR]

A number of geodynamic models have been proposed for the southeastern margin of the Tibetan plateau and include a range of deformation processes. One unresolved issue is whether crustal and mantle flow occurs, and if it does, how flow contributes to the mass balance of the India‐Asia collision. To address this question, new magnetotelluric data were collected and used to derive a three‐dimensional electrical resistivity model of the crust and upper mantle beneath the Red River Fault (RRF) zone and adjacent areas. The most prominent features of the model are (1) a resistor in the upper‐middle crust directly beneath the trace of the RRF; (2) a major change in upper mantle resistivity across the RRF; and (3) a significant conductor in the upper mantle northeast of the RRF, which extends upward into the crust, and which requires a melt fraction of up to 3%. The model suggests that the lower crustal conductors may be due to melt/fluids derived from the mantle, rather than from outward flow from Tibet. The most likely source of fluids and melts could be upwelling mantle flow related to the Hainan mantle plume. The change in resistivity across the RRF implies a change in lithospheric strength may explain the present‐day localization of deformation and uplift at this location. The resistivity model may also give insights into the distribution of ore deposits in Ailao Shan, since many mineral deposits are derived from magmatic fluids generated in the mantle at the edges of regions of thick lithosphere. Plain Language Summary: Continent‐continent collisions are an important tectonics process that have formed mountain ranges such as the Himalaya and features such as the Tibetan Plateau. Some recent models have proposed that deformation in such regions is partly accommodated by ductile deformation processes such as crustal flow. They proposed that flow occurs locally overs tens of kilometers beneath the Himalaya and are supported by geological and geophysical evidence. Much larger crustal flow systems have been proposed to occur beneath the eastern margin of the Tibetan Plateau, where the collision has extruded large regions of the crust with upper crustal deformation focused on a system of major transform faults. This study investigates these models by using geophysical data to image the properties of rocks beneath the Red River Fault area of southeastern Tibet. The magnetotelluric method was used to map the resistivity of the rocks, which is a parameter sensitive to the presence of fluids, which in turn control the strength. Regions of low resistivity were mapped and inferred to be due regions of partial melt. These appear to originate in the mantle, and not be connected to similar features to the North. This implies that large‐scale crustal flow may not occur in this region. Key Points: A 3‐D MT survey imaged a significant conductor in the upper mantle northeast of the Red River Fault, which extends upward into the crustThis implies that lower crustal conductors may not be due to crustal flow from beneath the Tibetan Plateau but have a mantle originThe change in resistivity across the Red River Fault implies a change in mantle strength may explain the deformation at this location [ABSTRACT FROM AUTHOR]

[Display omitted] • An electrical resistivity model was obtained across the western Yunnan Tethyan belt. • Conductors underneath the Gaoligong shear zone indicate a crustal channel flow. • The Changning-Menglian belt represents the remnant of a main branch of Paleo-Tethys. The Tethyan tectonic belt in western Yunnan, SW China, has witnessed a complex, multistage Tethyan evolution since the Paleozoic, including a late stage of continental collision between the Indian and Eurasian plates in the Cenozoic. Consequently, this belt has become a collage of Gondwana-derived microcontinental blocks and arc terranes wedged between two continents, including the Tengchong block, Baoshan block, Lincang block, and Simao block from west to east. To better understand the structure of these ancient continental blocks and the dynamic mechanism of the Indian-Eurasian continental collision, we investigate the electrical resistivity structure of the crust and uppermost mantle in the region. Magnetotelluric data were collected along a NW-SE striking transect crossing multiple tectonic terranes and were then modeled with a 3-D approach. The final resistivity model shows conductors beneath the Tengchong volcanic area, which may represent magma chambers that feed the active volcanism in the area. The electrical structure between the Tengchong area and the Wanding fault consists of layer-like lower crustal conductors along the transect, which may indicate mechanically weak layers where the tectonic escape of the Tibetan Plateau may occur via lower crustal flow. The Lincang block shows higher resistivity characteristics than the surrounding crust and demonstrates a trend of eastwardly napping resistors onto the Simao block, making up a crustal boundary of high-resistivity and low-resistivity bodies. This observation suggests that the Changning-Menglian suture between the Baoshan block and the Lincang block may be the boundary between Gondwana and Cathaysia. [ABSTRACT FROM AUTHOR]

The Liupanshan orogenic belt (LOB) is located in the convergence zone of the NE Tibetan Plateau and the Ordos Block (OB). A crustal and upper mantle structure study of the LOB is vital for providing a deeper understanding of upper crustal shortening and orogenic uplift in the NE Tibetan Plateau. In this study, we utilized broadband magnetotelluric (MT) data at 122 stations that cross the LOB to determine an optimal electrical model by comparing the models obtained from 2D and 3D inversions. Several revealed upper crustal resistivity boundaries are connected with a conductive layer in the middle-lower crust by our model, which acted as a decollement system for the eastward thrusting of the upper crust. Our model also showed that a large-scale westward-dipping high-resistivity body developed beneath the western margin of the OB, which suggests that the thin and rigid crust in the OB was pushed into the thick and soft block in the NE Tibetan Plateau, which indicates that a double Moho might appear beneath the LOB and its adjacent areas. A coherent U-shaped medium-lower resistivity anomaly zone (UMLRZ) developed at depths of approximately 60–80 km, which is shown in our model and may be related to partial melting or a hotter mantle. Combined with previous research results, we suggest that the present crustal structure and topography in the LOB and its adjacent areas are probably controlled by two main stages of activity: the far-field effect of the oblique subduction of the Pacific plate toward the Asian continent plate and the collision between the Indian and Eurasian plates. Owing to the existence of the ductile zone UMLRZ, the LOB easily incurs gravitational collapse and cannot increase indefinitely, which is the key factor responsible for the relatively gentle topography of the transition zone between the LOB and OB. • The middle-lower crustal conductive layer acted as a decollement system for the eastward thrusting of the upper crust in the NE Tibetan. • The present crustal structure and topography in the LOB is probably controlled by two main stages of tectonic activity. • A coherent U-shaped medium-lower resistivity anomaly zone (UMLRZ) developed at depths of 60–80 km is revealed. • The UMLRZ is the key factor responsible for the gentle topography of the transition zone between the LOB and OB. [ABSTRACT FROM AUTHOR]

Thrust fault zones around the Tibetan Plateau (TP) record the tectonic evolution between the Plateau and its surrounding terranes, which is helpful for understanding the uplift mechanism and deformational processes of the TP. The Longmenshan fault (LMSF) is the tectonic boundary (TB) between the Yangtze terrane (YT) and Songpan-Garze terrane (SGT), while the TB of its western segment, either the Lijiang-Xiaojinhe fault (LXF) or Jinhe-Qinghe fault (JQF), is controversial. Therefore, we conducted magnetotelluric (MT) imaging, surface structure surveys, and petrologic analysis to further determine the deep–shallow structural relationship of the western LMSF segment. Resistors R1 and R2 revealed by MT imaging may have originated from different magmatism. Among them, R2 may have originated from plume underplating, which is consistent with previous studies, while R1 may have originated from the residue of episodic mafic magma intrusion along the JQF over a broader period. Based on regional geophysics, surface structural patterns and petrologic mineralogy, it is suggested that the JQF may have deformed deep into the lower crust or upper mantle, accommodating the southeast expansion of the TP by thrusting and acting as the TB between the YT and SGT before ∼15 Ma. After ∼15 Ma, due to the activation of the large-scale strike-slip faults, the LXF gradually replaced the JQF to dominate the structural deformation of the western LMSF segment. Our results indicate that the above tectonic transition might be associated with the geodynamic process from centralized deformation to diffuse deformation within the southeast TP during the late Cenozoic. • Magnetotelluric images of crust–mantle resistor R1 with a tapered shape beneath the Jinhe-Qinghe fault (JQF) are presented. • The JQF deformed into the lower crust, acting as NW tectonic boundary (NWTB) of Yangtze terrane (YT) before the late Cenozoic. • The Lijiang-Xiaojinhe fault replaced the JQF as NWTB of YT, adjusting the late Cenozoic deformation of the SE Tibetan Plateau. [ABSTRACT FROM AUTHOR]

Fluids play an important role in intracontinental seismicity and have become an important issue in solid earth sciences. In this study, we present evidence of fluid-related seismicity on the southeastern Tibetan Plateau. A sequence of large earthquakes (Ms > 6.0) occurred in the Dayao and Yao'an areas within the Chuxiong Basin from 2000 to 2009, where no obvious traces of active faults at the surface indicate the source fault of these earthquakes. To reveal the deep earthquake structure and the mechanism of these earthquakes, we carried out two magnetotelluric (MT) profiles that intersected in the epicentral region. The three-dimensional (3-D) inversion method was used to provide a more realistic electrical resistivity model since phase tensor analysis showed 3-D features. The electrical structures along the two profiles showed similar characteristics: lower crustal conductors underlie the upper crustal resistive zone, the resistive zone may correspond to a crystalline basement, and the lower crustal conductors were interpreted as aqueous fluid-related and water-derived partial melting. Earthquakes were distributed mainly in the upper crustal resistive zone but not in the underlying conductive zones, which indicated that fluid migration into the upper crust may have contributed to triggering earthquakes in this area. • Aqueous fluids produce anomalously high conductivities in the lower crust. • Earthquakes mainly occurred in the upper crustal resistive zones. • Vertical migration of fluids from deep crust into the upper crust may trigger earthquakes. [ABSTRACT FROM AUTHOR]