Lu, Chang

The Tibetan Plateau is growing by both vertical uplift and horizontal extension. It is a continuing debate how the Tibetan Plateau interacts with its surrounding plates and blocks. Due to intense tectonic activity, which produced catastrophic earthquakes, the tectonic zone between the northeast margin of the horizontal extending Tibetan Plateau and the stable Ordos Block has garnered considerable interest. This study investigated the spatial distribution of gas geochemical anomalies (e.g., high flux of CO2 in correspondence of the main faults) at regional scale together with the seismic tomography in correspondence of this tectonic zone with the aim to figure out the domain of convergent boundary between the Ordos block and Tibetan plateau, and trace the tectonic discontinuities which are able to transfer fluids through the crustal layers between the two main geological units. From northwest to southeast, obvious difference of spatial distributions of geochemical and geophysical features in the tectonic zone between the northeast margin of the Tibetan Plateau and the Ordos Block is inferred. The northeast area (Zone A) is dominated by thrust and strike-slip faults with clear velocity boundary underneath, where low crack density (ε), saturation rate (ξ) and Poisson’ ratio (σ) in the middle-lower crust coincided with the low values of heat flow and CO2 emissions, tectonic compression and regional locked-fault can be inducements. The southeast area (Zone C) is dominated by extensional tectonics with roughly E-W fast-velocity direction (FVD) of P-wave azimuthal anisotropy, where high permeability and porosity can be deduced from crustal high ε, ξ and relatively high σ anomalies, resulting in high heat flow, CO2 concentrations and fluxes at the surface, and predominantly crustal-derived gases. The intermediate area (Zone B) also dominated by thrust and strike-slip faults is an extraordinary zone, where intensely locked-fault were clearly revealed, while the predominant anisotropic FVDs in the middle crust changed obviously, more contribution of shallow gas component was detected, and CO2 flux, heat flow, and regional ε, ξ, and σ in the upper crust were higher, compared with those in Zone A, which indicated the regional crushing fragmentation underneath Zone B. The adopted multidisciplinary approach demonstrated that Zone B is the convergent boundary between the Tibetan Plateau and the Ordos Block.

Fluid geochemistry in active fault zones has been proven to be sensitive to tectonic activity. The North China Craton (NCC) has attracted much attention because of its complex and intense tectonic activity. In this study, fluid geochemistry in the primary active fault zones in the NCC was investigated, including inference of its tectonic activity. Stronger degassing from soil and springs has been observed in the northeastern Tibetan Plateau (NETP) and the Zhang-Bo seismic zone (ZBSZ) than in the other seismic zones. Both geological soil gas and deepderived gas (crust- or mantle-derived gas) from springs were concentrated there. Also, a comprehensive analysis has indicated that the development of new fractures might have occurred widely beneath the NETP and ZBSZ because of the strong regional tectonic activity there. The 3He/4He and 4He/20Ne of gas from the springs in the ZBSZ suggest that the low-velocity zone 20– 40 km deep might be a magmatic intrusion derived from the mantle. However, crust-derived gas accompanied with a negligible mantle-derived component has been detected in the Diebu-Bailongjiang fault (DBF), the West Qinling fault (WQLF), and the Liupanshan fault (LPSF) in the NETP. There, the occurrence of more new fractures was probable, in accordance with the obvious δ18O shift of the water from the springs in the fault zones. This suggests that a channel flow, also depicted by the low-velocity zone 20– 40 km deep, could have formed within the crust and that the probable leading front reached the LPSF.