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

A 20-km-long seismic line characterises the crustal reflection pattern of t he easternmost Dabie Shan, the archetypal ultrahigh-pressure (UHP) orogen o f eastern China. The weak- to non-reflective upper crust (5 s two-way trave l 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 r ather abrupt change to diffuse but strong reflectivity within the mid to lo wer 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 cra tonal Yangtze basement, unaffected by the UHP metamorphism. The prominent l owermost reflectors (10-12 s TWT; similar to 33-40 km depth) are interprete d to trace the Moho, excluding the presence of a crustal root inherited fro m the UHP orogeny. A tomographic P-wave velocity model for the uppermost cr ust (< 700 m) traces shallowly W-dipping sedimentary rocks east and UHP gne isses west of the Cenozoic Tan Lu fault which is imaged to dip steeply east ward. 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 structu res, e.g. folds, known from outcrops and maps. (C) 2001 Elsevier Science B. V. All rights reserved.