Three-Dimensional Electrical Structure Of Crust Beneath The Tanlu-Dabie Tectonic Region And Its Dynamical Significance

The Yangtze plate and the North China plate began to converge and collide in the Mesozoic,during which the strong orogeny formed the central orogenic belt composed of the Qinling,Dabie and Sulu orogen.The formation of the Tan-Lu fault zone accompanied by the orogeny.The sinistral strike-slip Tan-Lu fault zone disconnects the Dabie and Sulu orogen.During the formation process of Dabie orogen and Tan-Lu fault zone,there are some phenomena such as the exhumation of high-pressure and ultrahigh-pressure metamorphic rocks in orogen and the formation of Zhangbaling uplift.The extensional movement and magma activity after orogeny changed the structure and composition of the orogen and the Tan-Lu fault zone at the beginning of their formation to some extent.The deep structure obtained by geophysical method plays a very important role in understanding and reconstructing the evolution process of Tanlu-Dabie tectonic zone.Most of the existing geophysical deep structures are sectional data,so it is impossible to understand the structure of the tectonic zone in an all-round and three-dimensional way.A few deep three-dimensional structures in the study area have very limited resolution.Magnetotelluric method is low-cost,not shielded by resistance body and sensitive to fluid and thermal material.Three-dimensional magnetotelluric inversion can obtain three-dimensional electrical structure with high reliability.Therefore,this thesis use magnetotelluric method to study the electrical structure of crust beneath the Dabie-Tanlu tectonic region.894 existing MT sites were gathered and 159 new MT sites were surveyed in the Tanlu-Dabie tectonic region.They formed a magnetotelluric data set.Three MT profiles were extracted from the data set to study the distribution characteristics of geo-electrical strike from the Dabie orogen to the Tanlu fault zone.A dense MT array covering the earthquake area in northern margin of the Dabie orogen were extracted to study the seismogenic structure and seismogenic environment.A magnetotelluric array covering the whole Tanlu-Dabie tectonic region was obtained by rarefying the data set to study the three-dimensional electrical structure beneath the Tanlu-Dabie tectonic region.Applying the conjugate impedance transformation and magnetotelluric impedance tensor imaging technique,the distribution of geo-electric strikes at different sites and frequencies in three magnetotelluric profiles,which stretch across south section of Tan-Lu fault zone and Dabie orogeny are analyzed in detail.Distribution characteristic of geo-electric strike reflect the distribution and contact relationship of different tectonic unit.Statistical results show the Tan-Lu fault zone is displayed by the orientation of geo-electric strike and the two-dimensional effective factor in the upper part,but it may not extend to the lower part.In Yangtze plate and southeast Dabie block,geo-electric strike orientations in the upper part is inconsistent with those in lower part.This reveals the upper-lower unconformable contact between upper layer and lower layer,and indicates tectonic forces with different directions affected lower and upper media respectively.During the period of plate convergence,the north-south compressive stress influenced the lower ductile layer in the south of Dabie orogen and on the south of the orogenic,forming the east-west trending geo-electric strike.Meanwhile the Yangtze plate on the east of Tan-Lu fault zone moved along the north-north-east Tan-Lu fault zone,the lower ductile layer formed north-west-west geo-electric strike.Uniformity geo-electric strike of the upper and the lower in North Dabie unit conforms to the evolution process of the upraising and thickening of the orogen in Mesozoic.These results provide new evidences for understanding the evolution process Dabie-Tanlu tectonic zone.In the Huoshan earthquake area where has the most frequent and concentrated seismic activities in the study area,the three-dimensional electrical structure beneath the Huoshan earthquake area was obtained by fine three-dimensional inversion of the magnetotelluric array data composed of 83magnetotelluric observing sites.In this three-dimensional inversion with topography,we also used the multi-grid method,impression and iterative reconstruction inversion technique and standard minimum structure nonlinear conjugate gradients method(NLCG).The electrical structure reveals that beneath North Dabie and North Huaiyang region,mid-upper crust is a high resistance area with the resistivity above 1000?·m and the mid-lower crust is a relatively low resistance area with tens of?·m resistivity.The Xiaotian-Mozitan fault with northwest strike separates the North Huaiyang high resistivity layer from the North Dabie high resistivity layer.Fault dip is NE in the shallow and changes to SW in the deep.The northeast-strike Luoerling-Tudiling fault cuts through the high resistivity layer of North Dabie upper crust.Background seismicity relocated using the double difference earthquake location algorithm indicates:earthquakes mainly occurred in north Dabie and north Huaiyang high resistivity layer near the Luoerling-Tudiling fault,and are concentrated in the tectonic weak belt formed by early motion of Xiaotian-Mozitan fault.2014 M_s4.3 Huoshan earthquake has a deeper focal depth,and located at a big electrical gradient zone in north Dabie high resistivity layer.Based on the above results,we think the primary seismogenic fault in Huoshan earthquake area is Luoerling-Tudiling fault.Fluid from crustal source conductive body below the fault propagates along the fault.It weakens the fault strength.The kinematic and deformation of the fault makes full use of the tectonic weak belt formed by Xiaotian-Mozitan fault.Since the existence of significant high resistivity layer in north Dabie belt and north Huaiyang belt,we consider there is a deep seismogenic environment beneath Huoshan earthquake area where earthquakes above magnitude six maybe occur.Appling magnetotelluric three-dimensional inversion method,the high-resolution three-dimensional electrical structure of crust beneath Tanlu-Dabie tectonic region is obtained.Based on the results model,the characteristics of electrical structure beneath the study area are analyzed in detail and formation mechanism and tectonic evolution process of the Dabie orogen and Tan-Lu fault zone are discussed.In the inversion,the multi-grid method,impression and iterative reconstruction inversion technique are applied to reduce the influence of the initial model on the inversion results.Sensitivity tests of three high conductors and deep structures in the resulting model verified the reliability of the model.Comparation between uniform half space three-dimensional inversion result and multi-grid method three-dimensional inversion result and comparation between three-dimensional inversion with terrain and three-dimensional inversion without terrain show that adding terrain and multi-grid method is necessary to magnetotelluric three-dimensional inversion.The inversion results show there is a high resistivity anomalous body beneath the Dabie orogeny and the high conductors on the north and south sides of the orogeny converge to the lower part of the orogeny.There is a significant high conductor beneath the orogen high resistive body on the east of the Dabie orogen.The banded high resistivity anomalous body beneath the Zhangbaling uplift is along the Tan-Lu fault zone.There is high conductive layer at the depth of 10 km along the Yangtze River.During the orogeny,the crust of Yangtze block was compressed and thickened at the convergent boundary,forming the high-resistivity anomalous body beneath the Dabie orogen.The compressive folds on the eastern side of Dabie orogen were dragged by the sinistral strike-slip Tan-Lu fault zone and transformed the Zhangbaling uplift.The strong sinistral strike slip of Tanlu fault zone resulted in the Detachment of the upper crust and middle-upper crust of the lower Yangtze.Under the combined action of sinistral strike movement of Tanlu fault zone and magmatic activity in the cretaceous,the decoupling layer developed further and formed a highly conductive layer in the result model.During the orogeny,the high-pressure/ultrahigh-pressure metamorphic rocks exhumed along the boundary of the rock units.In postorogenesis,the magma upwelled along the edge of Dabie orogen and the fragile exhumed channel of high-pressure/ultrahigh-pressure metamorphic rock,and the intense magmatic activities changed the structure and composition of the deep orogen.