| The South China Block(SCB),lying in the connection of the Eurasian Plate,Indian Plate and the Philippines Plate,is a major tectonic unit belonging to the continental margin of eastern Asia.The SCB is located to the east of the Tibetan Plateau,to the south of the Qinling-Dabie Orogen,to the west of the Pacific Ocean,and exhibits a transition of tectonic architecture from intra-continent to continental margin,oceanic trench,island arcs and sea basins.Due to the westward subduction of the Pacific Plate and the eastward extrusion of the Tibetan Plateau,this continental block has been repeated deformed and modified to different extents.Since the assembly of the Yangtze and Cathaysia Block along the Jiangshan-Shaoxing Fault in the Neoproterozoic,the SCB has undergone continental reworking and rejuvenation in the Phanerozoic,which mainly include two episodes of intracontinental orogeny in the Paleozoic and Mesozoic and widespread magmatism in the late Mesozoic.Thus,it is an excellent location to study how the modification of lithospheric properties may influence the longevity of the continents.Several issues have been hotly debated in the SCB,such as the way of assembly between the Yangtze and Cathaysia Blocks in the Neoproterozoic,the boundary of the Yangtze and Cathaysia Blocks,the geometry of the Jiangshan-Shaoxing Fault,and the driving force of the Late Mesozoic lithospheric extension and thinning.The Cathaysia Block has a higher surface heat flow(73 mW/m~2),as well as high sublithospheric mantle heat flow,with relatively low seismic velocity in both the lower crust and the upper mantle.In contrast,the Yangtze lithosphere is more resistive and cratonic,possessing higher seismic velocity and lower surface and sublithospheric mantle heat flow.Continental crust typically has a crustal thickness of 30-45 km,with an average value of 40 km.The crustal thickness of the Yangtze Block is in the range of 36-42 km;however,it is in the range of 33-35 km in the Cathaysia Block.Therefore,the thickness of the Cathaysian crust is much thinner than the global continental average value.While the present-day Yangtze lithosphere extends to a depth of~170 km,the Cathaysian lithosphere has been dramatically thinned to a current thickness of~70-80 km.The upper-mantle S-wave velocity in southeastern China is anomalously low for a stable continental platform,which was found to be 4.45 km/s or less in the uppermost mantle.On the other hand,the S-wave velocity at the base of the lithosphere is above 4.50 km/s in the Yangtze Block,while much lower in the Cathaysia Block,which ranges from 4.25-4.40 km/s.All these features suggest distinct tectonic evolution of the two continental blocks and motivate us the discover the driving forces responsible for the evolution of the lithosphere.The paucity of geophysical data in South China inhibits our ability to examine the lithospheric structure and understand how this region has evolved.Resolving these questions is essential for the understanding of the tectonic evolution and the fate of the Eurasian continent.The magnetotelluric(MT)method provides images of electrical resistivity,a physical parameter that is sensitive to temperature and composition,particularly the presence of highly conductive phases such as water,melt,graphite and sulfide minerals.MT data can detect both free water,or water in the form of hydrated minerals.Thus,it can be used to define(a)the locations of mechanical defects,such as sutures and fault zones and(b)the presence of fluids,melts or hydrated minerals that control the lithospheric rheology.MT has been an important part to complement seismic,gravity and magnetic methods in the studies of tectonic evolution and lithosphere structure.This thesis addresses the three-dimensional(3-D)lithospheric structure of South China using the MT method.The study covers an area of~180000 km~2 in the SCB,in which MT data at 225 stations were collected on seven profiles across the eastern Yangtze Block,the Neoproterozoic suture(the Jiangshan-Shaoxing Fault)and the Cathaysia Block.The time series data were processed with a Robust method using remote reference.Dimensionality analysis and phase tensor maps led to the suggestion of 3-D resistivity structure in this region.Topography and coast effect were assessed and the final preferred resistivity model has an overall RMS misfit of 1.46,indicating a good fit with the observed data.The final preferred resistivity model shows that the upper crust at depths above 2 km is characterized by low resistivities of 10-50?m,with dipping conductors being explained by fluids concentration at fault zones.The lower crust is resistive due to the magmatic intrusions and crystalline basement rocks.The upper mantle shows a major change of resistivity structure across the Jiangshan-Shaoxing Fault,with relatively high resistivities in the Yangtze Block whereas low resistivities in the Cathaysia Block.Particularly,regions of low resistivity were observed beneath the Wuyi Mountains,extending from the asthenosphere to the lowermost lithosphere.The distinct variation of lithosphere structure both vertically and horizontally suggests that the present day eastern SCB may not be an intact craton.We made an interpretation of the resistivity models using a multi-disciplinary approach that includes xenolith,geothermal,geochemical,gravity and seismic data.Prominent features have been revealed in the 3-D resistivity model and helped to constrain the formation and evolution of the SCB.Major findings include:(1)Discovery of a southeast-dipping conductive zone in the Jiangshan-Shaoxing Fault that implies a southeast dip for the suture,which sheds new light on the Neoproterozoic collision and suturing of the Yangtze and Cathayisa Blocks,as well as the Phanerozoic reworking and reactivation.This boundary fault had been reactivated to different extents by the Early Paleozoic and Early Mesozoic intracontinental orogenies of the South China.Its present structural style,however,was interpreted as a lithospheric-scale detachment fault that acted during the Late Mesozoic extension and rifting.(2)Given the asymmetries of topography,electrical resistivity,Bouguer gravity anomaly and Late Mesozoic volcanism across the Gan-Hang Rift,we propose an asymmetric simple shear extension model for the South China rift system,which may also explain the distinct evolution of the Yangtze and Cathaysian lithosphere.(3)Discontinuous conductors observed at 15-20 km depths within the Cathaysia Block agree well with the seismic low velocity zone,corresponding to a brittle-ductile transition zone in the crust,possibly resulting from the concentration of aqueous fluids.(4)The 3-D resistivity model reveals regions of enhanced electrical conductivities in the upper mantle beneath the central Wuyi mountains at 70 km depth.These conductive regions may be caused by elevated water content and the presence of partial melt resulting from the hydration process of the subduction of Paleo-Pacific plate.The hydrated regions correspond to zones of low lithospheric strength and viscosity that facilitated intracontinental deformation and lithospheric thinning.(5)The hydration weakening of the mantle lithosphere promoted both the gravitational instability and convective removal of the lowermost lithosphere in South China.We suggest a convective thinning model involving gravitational(Rayleigh-Taylor)instability to interpret the evolution and thinning of the Southeast China lithosphere.(6)Therefore,the lithospheric extension and thinning of the South China were driven by both the plate boundary and intraplate processes.While the lithosphere of the eastern SCB was influenced by the westward subducted Paleo-Pacific plate,the Jiangshan-Shaoxing Fault had been acting as a lithospheric detachment and controlling the asymmetric simple shear extension.The heterogeneous electrical resistivity structure implies that the continental lithosphere of the eastern South China Block has been repeatedly reworked and reactivated in the Phanerozoic by plate boundary and intra-plate processes.Finally,based on the discoveries in this study,the evolution of the Phanerozoic lithosphere was divided into four stages:(a)The Early Paleozoic intracontinental orogeny produced a thickened crust and lithospheric mantle beneath the Wuyi Mountains.(b)Mesozoic subduction of the Paleo-Pacific plate introduced water into the overlying lithosphere,reduced its viscosity and weakened the cratonic root.The lithospheric mantle became gravitationally unstable and delaminated as mantle blobs.The descending blobs created space for asthenosphere upwelling and thermal convection.(c)The upwelling asthenosphere led to uplift centered at the Wuyi Mountains in middle Jurassic.The lower part of the lithosphere was removed by the process of delamination.(d)Extension caused by rollback of the Pacific slab stretched the crust and lithospheric mantle,accelerated the thinning rate,and produced numerous normal faults and rift basins in the Late Cretaceous.The Jiangshan-Shaoxing Fault was reactivated and acted as a lithospheric detachment to allow simple shear extension and rifting. |
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