Deep structures of the east Dabie ultrahigh-pressure metamorphic belt, East China

The Dabie Mountain is one of the best places for geologists to study the ultrahigh-pressure metamorphism (UHPM) because coesite-bearing eclogites and other UHPM rocks are well ex-posed on the surface. The Dabie UHPM belt has been studied by many geoscientists with re-markable results1—9]. Recent researches show that the host rocks of the coesite-bearing eclogites, such as gneiss and marble, also contain coesites10—14], thus undergoing ultrahigh-pressure meta-morphism. The idea of con…

Deep structures of the east Dabie ultrahigh-pressure metamorphic belt,East China

The geophysical investigations with seismic and MT methods were carried out in the east Dabie Mountain area in 1997, producing detailed results about crustal structures with good compatibility between different geophysical methods. After integrated interpretation of both avail-able geophysical and geological data, the author compiles a crustal tectonic section across the east Dabie Mountain (Plate II), which provides much more structural details with improved reliabil-ity due to geophysical constraints applied to the deep structures. The east Dabie orogenic belt can be divided into 4 geologic units from north to south: the north Huaiyang, the north Dabie, the south Dabie and the Susong. The Mesozoic northward subduction of the Yangtze craton caused the Yangtze crust to insert into the middle and lower crust of the Susong high-pressure metamorphic zone, while the middle and lower crust below the north Huaiyang and the Hefei basin contains the basement of the Sino-Korean craton. The middle and lower crust of the south Dabie is rather dif-ferent from that of the north Dabie, showing that the north and the south Dabie had different evolu-tional trajectories and should not belong to a single tectonic unit. The current crustal pattern has resulted mainly from deformation caused by the post-collisional intracontinental subduction of both the Yangtze and Sino-Korean cratons before the late Jurassic, and deformation caused by later crustal extension including doming and unroofing around the north Dabie. It can be inferred that the suture zone of the Triassic collision between the Sino-Korea and the Yangtze was located along the Xiaotian-Mozitan fault zone, which contains a group of normal faults dipping north in the upper crust, but becomes steep thrusts of dipping south in the middle crust, accompanied by ex-tensive deformation between this fault zone and the Hefei basin. The middle crust below the north Huaiyang unit is connected to the basement of the Sino-Korean craton, showing the plate com-pressional convergence between the Yangtze and Sino-Korean cratons in the post-collisional stage. The influence of this convergent event reached as far as to Huainan, the northern boundary of the Hefei basin. As a clear reflection appears at 22s TWT in the stack reflection sections, the lithospheric thickness of the east Dabie is about 78 km. The newly obtained geophysical data in-dicate that the thickness of the east Dabie UHPM rock slices is no more than 8 km, therefore they do not present evidences to support the hypothesis involved in whole-plate exhumation of the UHPM terranes.