Crustal structure of the eastern Dabie Shan interpreted from deep reflection and shallow tomographic data

摘要

A 20-km-long seismic line characterises the crustal reflection pattern of the easternmost Dabie Shan, the archetypal ultrahigh-pressure (UHP) orogen of eastern China. The weak- to non-reflective upper crust (5 s two-way travel time (TWT), similar to 15 km depth) is interpreted to comprise UHP rocks thrust over lithologically similar bur non-UHP crust. The tectonic contact, although not imaged as a distinct reflector, is probably outlined by the rather abrupt change to diffuse but strong reflectivity within the mid to lower crust. Thus, the seismic pattern of the upper crust implies that mafic, oceanic crust does not constitute a significant proportion. The middle to lower crust (5-10 s TWT, similar to 15-33 km depth) probably represents cratonal Yangtze basement, unaffected by the UHP metamorphism. The prominent lowermost reflectors (10-12 s TWT; similar to 33-40 km depth) are interpreted to trace the Moho, excluding the presence of a crustal root inherited from the UHP orogeny. A tomographic P-wave velocity model for the uppermost crust (< 700 m) traces shallowly W-dipping sedimentary rocks east and UHP gneisses west of the Cenozoic Tan Lu fault which is imaged to dip steeply eastward. The UHP rocks exhibit little lateral and vertical velocity variations (< 10%). reflecting grossly homogeneous, gneissic lithology. Hundred-metre-scale velocity variations, however, may trace distinct large-scale structures, e.g. folds, known from outcrops and maps. (C) 2001 Elsevier Science B.V. All rights reserved. [References: 42]