High-Resolution Tomography Of The South-Central Tan-Lu Fault Zone And Its Geodynamic Implications

In central-eastern China,many studies suggest that subduction-related mantle-crustal interactions have resulted in the modification of the lithosphere.In the process,the collision of the North China craton and the South China Craton resulted in the largescale left-lateral translation of the Qinling-Dabie orogens and the southern boundary of the North China craton,producing among other structures,the Tan-Lu Fault Zone.The presence of the continental fault zone in Eastern China added more complexity to the geodynamics of the region with many unresolved research controversies.The Tan-Lu Fault is often associated with regional geodynamic processes such as lithospheric thinning,seismicity,magmatism,deformation among others.To provide more constraints for the understanding of the fault system in the central-southern segments where the fault,North,and the South China Cratons are in geodynamic contact,we embarked on this thesis.We took advantage of dense seismic arrays deployed in the region for the high-resolution seismic tomography of the crust and uppermost mantle using both ambient noise and teleseismic P-waves arrival times.Three independent studies were carried out using different datasets and applying different advanced tomography methods.Firstly,we performed ambient noise tomography using the phase velocity dispersion data obtained from Empirical Green's Functions derived from cross-correlation functions.We adopted the direct tomography method for surface waves to invert all phase velocity dispersions for the 3-D shear-wave velocity model in the crust and uppermost mantle.The results show velocity segmentations revealing high-velocity structures beneath the metamorphic belts and uplifts in the first 10 km depth,which correspond to the regions where High pressure/ultra-high pressure metamorphic rocks have been reported in other studies.The results also revealed low-velocity structures that correspond to Cretaceous basins and depressions in the upper crust and the middle crust beneath deep basins and along the Tan-Lu fault zone.The lower part of the upper crust and the middle crust of the Tan-Lu fault zone is characterized by high velocity which was interpreted as the brittle upper-middle crust that constitutes the seismogenic zone.The lower crust is characterized by widespread low shear-wave velocity,just below the seismogenic zone,and could serve as a stress accumulator for the reactivation of faults and enhance the initiation of earthquakes in the seismogenic zone.In the seismogenic zone,the spatial distribution of earthquakes in the Tan-Lu fault zone reveals two seismicity segments:the southern segment and the Suqian-Tancheng segment.The earthquake segmentation of the fault zone suggests the different responses to stresses by the fault,with the low-velocity zone at Suqian,identified to be the transition zone between the southern and middle segments.We observed clusters of earthquakes around regions of the sharp shear-wave velocity gradients,which seem to favor earthquake nucleation and initiation.Secondly,depth-dependent shear-wave(isotropic)and azimuthal anisotropy were jointly inverted using the direct inversion method that has the advantage of skipping the intermediate step of inverting for group and/or phase velocity maps before obtaining 3-D models.The results revealed the following findings.Mainly the intense faulting in the upper crust and the shearing in the Tan-Lu fault zone in the crust and the uppermost mantle control the deformations.Intense thrust and fold faults exist on the surface that could be characterized by cracks that strike in the same direction as the fast axes in the upper crust.The lateral variation of anisotropy is observed in the upper crust is high but becomes diminished with depth.Beyond the upper crust,the Tan-Lu Fault has a strong control on the anisotropy that is characterized by NNE-SSW to N-S polarized fast axes.The anisotropy in the southeastern North China Craton is weak in the lower crust and uppermost mantle.We associate the sources to likely the thrusting,tight trending folds,and dipping thrusts that are observed to be parallel to the trend of the Dabie orogenic belt.Meanwhile,the Zhangbaling uplift Group shows a strong NNE-SSW striking anisotropy which is interpreted as the effect of the Tan-Lu fault ductile shearing and the dragging of the South China craton in the crust and the uppermost mantle;signifying the presence of the fault movement in the uppermost mantle.The drag of marginal structures by the Tan-Lu fault is minimal on the North China craton compare to the extended large-scale drag observed in the South China craton,which supports the assumption that the South China craton is less rigid than the North China craton.In addition,we observed a wide discrepancy between shear-wave splitting results and the azimuthal anisotropy model,suggesting that observed anisotropy are purely lithospheric while the splitting results are related to deep asthenospheric sources.Finally,Chao Lake is a Geoheritage site that is strategically seated on the Tan-Lu fault at the conference point between the Yangtze craton,the North China cratons,and the Dabie orogenic belt in the southwest.It presents a good location for the study of the southern segment of the Tan-Lu Fault at a local scale to reveal insight into the behavior of the fault and its affinity to marginal structures.A high-resolution 3-D Vp velocity model was obtained from teleseismic earthquake arrival times tomography using the station-pair double-difference method.The results show that the velocity structure of the upper crust is segmented across the Tan-Lu fault with structures trending in the strike of the Tan-Lu fault.Sedimentary basins exist on both sides of the fault with slow velocities in the upper crust,indicating thick sediment deposition with their evolution controlled by the Zhangbaling intrusive materials in the Tan-Lu zone.A strong high-velocity structure characterized by an uplifting of the Moho exists in the fault zone and the far eastern edge of the Chao Lake in the crust and the uppermost mantle.This signifies that the crust and the mantle are coupled in the Zhangbaling segment of the fault.Two tectonic processes control the velocity pattern in Chao Lake.The upper crust is controlled by the extensional tectonics of the Tan-Lu fault and the Meso-Cenozoic magnetism in the central segment of the fault.The trend of the distributed marginal structures such as the Hefei basin and the intrusive rocks of the southern Zhangbaling,which show high affinity to the Tan-Lu fault system signifies the fault influence on their evolution.In the middle crust to the uppermost mantle,the E-W trends of velocity structures signify the compressional and extensional controls of the Dabie Orogenic belt during the Meso-Cenozoic collision of North China and the South China cratons as well as the exhumation at the Dabie Orogenic belt.This injected mantle materials into the crust via weak zones created by the deep-seated Tan-Lu Fault.