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Crustal flow is an important tectonic process active in continent‐continent collisions and which may be significant in the development of convergent plate boundaries. In this study, the results from multidimensional electrical conductivity modeling have been combined with laboratory studies of the rheology of partially molten rocks to characterize the rheological behavior of the middle‐to‐lower crust of both the Songpan‐Ganzi and Kunlun terranes in the northern Tibetan Plateau. Two different methods are adopted to develop constraints on melt fraction, temperature, and crustal flow velocity in the study area. The estimates of these parameters are then used to evaluate whether crustal flow can occur on the northern margin of the Tibetan plateau. In the Songpan‐Ganzi crust, all conditions are satisfied for topography‐driven channel flow to be dominant, with partial melt not being required for flow at temperature above 1000°C. Further north, the Kunlun fault defines the southern boundary of a transition zone between the Tibetan plateau and the Qaidam basin. Constrained by the estimated melt fractions, it is shown that channel injection across the fault requires temperatures close to 900°C. The composition of igneous rocks found at the surface confirm those conditions are met for the southern Kunlun ranges. To the north, the Qaidam basin is characterized by colder crust that may reflect an earlier stage in the channel injection process. In the study area, at least 10% of the eastward directed Tibetan crustal flow could be deflected northward across the Kunlun Fault and injected into the transition zone defining the northern margin of the Tibetan plateau.

Long-period magnetotelluric (MT) data from project SINOPROBE were acquired and modeled, using three-dimensional (3D) MT inversion, to study the electrical structure of Ordos Block, a component of the North China Craton. For the first time, a high-resolution 3D resistivity model of the lithosphere is defined for the region. Contrary to what would be expected for a stable cratonic block, a prominent lithospheric conductive complex is revealed extending from the upper mantle to the mid-to-lower crust beneath the northern part of Ordos. Correlating well with results of seismic studies, the evidence from our independent magnetotelluric data supports regional modification of the lithosphere under the north Ordos and lithosphere thinning beneath Hetao Graben. The abnormally conductive structure may result from upwelling of mantle material in mid-to-late Mesozoic beneath the northern margin of the Ordos block.

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The ongoing India-Asia collision since the Paleogene created the Tibetan Plateau, the most prominent elevated plateau on our planet. This convergence also contributed to the formation of two distinct types of active surface deformation of the plateau, namely, north-south trending normal fault systems and "conjugate" strike-slip fault systems. The tectonics and geodynamic mechanism(s) behind this curious combination are still unclear, despite numerous theories proposed over past decades. Here we present a new three-dimensional, lithospheric-scale, electrical conductivity model with unprecedented resolution of the central part of Tibetan Plateau derived from the SINOPROBE magnetotelluric array data set and discuss its inferences related to this question. Our model reveals contrasting conductivity structures corresponding to the surface deformation patterns, namely, highly conductive lower crustal anomalies beneath the graben systems in the Lhasa and Qiangtang terranes and moderately resistive crustal features in the strike-slip region near Bangong-Nujiang Suture Zone. With the help of experimentally calibrated constraints between conductivity and melt fraction, the conductivity model and the inferred lateral viscosity distribution together suggest a weak lower crust beneath the graben regions, compared to a stronger crustal rheology associated with the strike-slip zone. Here we expand the previously proposed "extensional extrusion" tectonic model in central Tibet to interpret our conductivity model and other geophysical/geodesic observations. The weak rheology under a N-S directed primary stress may have caused the east-west extension of the graben regions, which further aides the eastward extrusion of the conjugate strike-slip zone and eventually shapes the surface deformation of central Tibetan Plateau into its current, complex pattern. Plain Language Summary The collision between the Indian and Asian continents built the Tibetan Plateau, the highest plateau in the world. This active collision also formed distinctly different types of large-scale geological structures on the plateau surface. Among the most important features on the plateau, the origin of the N-S directed normal (or spreading) fault zones in two regions named Lhasa and Qiangtang, and the NW/SE directed slip fault zones between these two regions are still unclear. In order to understand the generation of those structures, we use a geophysical imaging method called magnetotellurics to measure the naturally occurring electromagnetic waves in the earth. We model and analyze these magnetotellurics data to obtain the deep structure beneath those surface structures, using sophisticated computer codes. We find that the two dominant but different types of those structures are most likely due to the different strengths of the deep crust. This strength difference can lead to a mechanical stress contrast, which further builds the distinctly different surface structures. These kinds of deep physical structure differences have not been detected for the area bef...

A detailed study of the upper Ordovician Wufeng shale and lower Silurian Longmaxi shale in northwestern Guizhou province of the upper Yangtze block was conducted on the basis of systematic analyses of a series of experimental measurements on core samples. Trace element ratios V/Cr, U/Th, and V/(V + Ni) reveal that variable paleoredox conditions existed during the deposition of the shale and the degree of anoxia decreased upward. The shale has high thermal maturity with an average R o value of 3.38% (ranging from 2.94 to 3.65%) and high amounts of organic matter with an average total organic carbon (TOC) content of 2.02% (ranging from 0.36 to 6.73%), the type of which is sapropelic (I) to humic−sapropelic (II 1 ). This indicates that the shale is good gas-producing shale in terms of geochemistry. The mineralogy of the shale is composed of 44.7% quartz and 32.6% clay minerals on average. The shale is characterized by low porosity ranging from 0.6 to 4.4% (averaging 1.8%) and low permeability varying from 0.0066 × 10 −3 to 0.1098 × 10 −3 μm 2 (averaging 0.0378 × 10 −3 μm 2 ). The correlations of porosity with the TOC content, quartz content, and clay mineral content demonstrate that the shale interval rich in quartz and organic matter has higher porosity than the clay-rich interval. The gas adsorption capacity increases with the increasing of the TOC content, indicating that organic matter is responsible for adsorbing gas in this shale. Comprehensive analyses through experimental data show that the studied Wufeng−Longmaxi shale succession, especially the lower section, holds the potential for shale gas exploration and development because of TOC enrichment, high thermal maturity, good fracability, and potentially large gas content.

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