Electrical Structure And Fluid Distribution Of The Lithosphere Beneath The Northeastern Boundary Of The North China Craton

The Northeastern boundary of the North China Craton (NCC) consists of the Central Asian Orogen Belt (CAOB) to the north and part of the NCC to the south, which is the conjunction zone of the Palo-Asian ocean and Pacific ocean domains. The CAOB and NCC experienced a series of tectonic events during the Phanerozoic, dominated by lithospheric thinning of the eastern NCC in the late Mesozoic and Cenozoic. In order to better understand the tectonic evolution of the NCC and the CAOB, a magnetotelluric (MT) study is performed along a 1200km-long NW-SE trending profile that extends from the CAOB across the Xar Moron fault, the North-South gravity gradient belt, the Yanshan fold belt, the Tanlu fault zone to the Liaodong Peninsula. This work establishes the subsurface two-dimensional (2D) electrical resistivity model along the profile by MT data collection, processing and inversion. The electrical resistivity features are analyzed and interpreted in combination with geological data and other geophysical data. Based on the attained electrical resistivity model, the fluids contents in the upper mantle are estimated in terms of laboratory studies of rock conductivity. In combination with the electrical resistivity model, the fluids contents, other geophysical observations and petrological studies, this thesis attempts to explore the dynamic process and mechanism of the NCC destruction. The main research content and results of this thesis are summarized below.1. MT data collection, processing and analysisThe broadband magnetotelluric (BBMT) data were collected at 48 sites along the profile. In order to perform deeper imaging, long period data (LMT) were also recorded at 11 stations. The BBMT and LMT time series data are processed using the Robust algorithm and remote reference method to produce high-quality responses. At stations where both BBM and LMT data were recorded, the data are merged to obtain responses of longer periods. The GB tensor decomposition, phase tensor and induction vectors are used to investigate dimensionality and geoelectric strike direction. MT data along the profile are characterized by two dimensions and a reasonable geoelectric strike direction is determined together with the geological strike. MT data can be decomposed into the transverse electric (TE) mode and transverse magnetic (TM) mode by rotating the impedance tensor into the regional geo-electric strike direction. To know the variations of the apparent resistivity and phase along the profile, the qualitative analysis is made for the rotated MT data.2.2D MT inversionBefore MT data are converted to a 2D electrical resistivity model, data quality is assessed using one-dimensional Rhoplus algorithm and bad data points induced by strong culture noise are removed. The 2D NLCG isotropic inversion is implemented for the edited data. During the inversion, a significant number of inversions are performed by choosing different control parameters (data mode, rotation angle, regularization factor, data error floor). After multiple inversions and model validation, the final model is chosen with the regularization factor of 3. The error floors are set to 20% and 10% for TE and TM apparent resistivity,5% for TE and TM phase and an absolute value of 0.05 for Tipper, respectively. The model has a final RMS misfit of 2.57 after 200 iterations. In addition, the 2D anisotropic inversions are performed for the MT data using 2D NLCG anisotropic code. The electrical resistivity features inferred by the anisotropic resistivity model are almost consistent with that in isotropic resistivity model. However, the electrical resistivity anisotropy paralleling to the profile is observed beneath the Tanlu fault.3. Interpretation of the electrical resistivity modelCoupled with geological and other geophysical data, the 2D electrical resistivity model and its tectonic implications are analyzed in detail. The 2D model shows a northwest dipping low resistivity zone beneath the Solonker suture that is identified as the suture zone formed by the collision between the Siberian and North China Craton. The low resistivity zone could be explained by a layer of sulfide-bearing graphite. The upper mantle resistivity exhibits the distinct difference from northwest to southeast along the profile. In the northwestern part of the profile beneath the CAOB, the resistivity values (300-1000Ω.m) are relativity high compared to the values (10-100Ω.m) in NCC. This diversity could be attributed to the high temperature and fluids in the upper mantle of NCC. The three low resistivity bodies are imaged below the Xar Moron fault, the north-south gravity gradient belt and the Tanlu fault, and can be explained as regions of partial melts and water, perhaps caused by asthenospheric upwelling.4. Estimation of fluid content in upper mantleBased on the electrical resistivity model derived from MT data, the water content and melt fractions in range of 40-100km depth for two stations, located at the CAOB and the NCC, respectively are estimated using the laboratory studies of rock conductivity, a modified Archie’s equation and geotherm structure. For the stations in the CAOB, the observed resistivity could be explained by a combination of water and partial melt at depth shallower than 55-65km. However, water alone can explain the observed resistivity below a depth of 55-65km and the water content is up to 800-1000 ppm, which indicates the lithosphere mantle at depth of 55-65km has been hydrated beneath the CAOB. It can be seen that the required water content to explain the observed resistivity generally decrease with depth. In contrast, the observed resistivity at depth of 40-100km needs to be explained by the combination of water and around 1% partial melt. The value of 500 ppm at depth of 100km is higher than that in stable cratons. By comparison of the water contents and melt fractions in upper mantle beneath the CAOB and the NCC along the profile, the water contents and melt fractions in the NCC are higher than that in the CAOB. This thesis suggests that the lithospheric mantle in the NCC has been strongly hydrated and hydrous melting has occurred.5. The dynamic process and mechanism of the NCC destructionIntegrating the 2D electrical resistivity model, the fluids estimated, the xenolith studies of water contents, seismic imaging results and lithospheric geotherm data, it is inferred that the thinning of the lithosphere associated with craton destruction has occurred in a spatially non-uniform manner in the region of the NCC investigated in this study. The mechanism of the lithospheric thinning varies with different places. Beneath the Yanshan Orogen, the lithospheric thinning was caused by delamination, whereas thermal erosion dominated below close to the Tanlu fault. The high water contents and melt fractions could be associated with the subduction of the Pacific plate to east. Since the dehydration of the subduction plate released lot of water into the lithospheric mantle in the NCC and resulted in the hydration of the lithosphere, the lithospheric strength was lowered. Meanwhile, the high water contents would induce the hydrous melt of the mantle rocks and further reduce the strength the lithosphere by decreasing the viscosity, resulting in the destruction of the NCC due to the delamination and thermal erosion. From this view of point, the westward subduction of the Pacific plate could contribute more to the destruction of the NCC compared with other dynamic factors.