Yuan et al. (2003) published a deep seismic reflection profile of the Dabie Shan ultrahigh-pressure (UHP) orogen, acquired by the Chinese Geological Survey. The seismic profile provides new insight into the crust structures of the orogen and gives us a better understanding of this specific UHP collisional belt. We welcome the authors' interpretation that the Moho discontinuity at km 150 represents a relic subduction feature and that the Yangtze plate was subducted down to the north beneath the North China plate. They also interpreted a crustal-scale dome structure at the middle-upper crust level across the orogen (km 20–170) and related the doming to the Early Cretaceous extension tectonics. We question the interpretation of an orogen-scale dome. The observed surface geology does not support this interpretation. They interpreted a major top-to-the-south normal fault at km 20–40 (Yuan et al., 2003, their Fig. 2B), which is in conflict with the existing field data. Detailed structural studies by many workers have shown that the high-pressure (HP) belt and the UHP belt are dominated by north-verging structures with south-southeast–dipping foliation and northwest-southeast–stretching lineation, which developed at amphibolite facies metamorphic conditions, and that kinematic indicators suggest top-to-the-northwest sense of motion (e.g., Xu et al., 1996; Hacker et al., 1995, 2000; Faure et al., 1999). The top-to-the-south normal fault is not observed by surface geology work. The interpretation of an Early Cretaceous deformation of the structures is also problematic. The structures were clearly developed at amphibolite facies conditions (see references herein), and 40Ar/39Ar dating on the muscovite/phengite of various groups from the HP-UHP belts suggests that the white mica cooled down to closure temperature at 180–227 Ma (Hacker et al., 2000), implying that amphibolite facies metamorphism took place in the Late Triassic to Early Jurassic and hence the amphibolite facies deformation took place at the same time, not the Early Cretaceous.
We suggest an alternative interpretation of the seismic profile, which may better fit the surface geology data (Fig. 1). We suggest that the south-dipping seismic reflections at 0–5 s two-way traveltime (TWTT) interval at km 20–50 (Yuan et al., 2003, their Fig. 2A) represent the blueschist-amphibolite-cold eclogite belt exposed at the surface. The south-dipping seismic reflection is underlain by moderately north-dipping multicycle and high-energy reflections at 5–8 s TWTT. These mid-crust reflections look similar in character to the gently north-dipping reflections under the UHP belt at mid-crust level (Yuan et al., 2003, their Fig. 2A). We interpret the multicycle and high-energy reflections to be stacked lower-crust rocks of the Yangtze plate, which are not exposed on the surface but are overlain by the HP-UHP sheets partially exposed on the surface. The south-dipping fault (Yuan et al., 2003, their Fig. 2B) is a top-to-the-north thrust that juxtaposes the HP and the UHP rocks against the stacked lower crust. Two phases of deformation are envisaged: (1) an early south-directed thrusting of the lower crust of the Yangtze plate and the HP-UHP sheets, which are probably related to the early stage of exhumation, seen as gently north-dipping reflections at mid-crust level (4–8 s TWTT) between km 30 and 100 (Yuan et al. 2003, their Fig. 2A); and (2) a later north-directed thrust of the HP-UHP belts over the stacked lower crust during amphibolite facies metamorphism (as observed by surface geology), which is probably related to the continued northward motion of the Yangtze plate in the Early Jurassic. It is this later phase of deformation that has produced the dominant structures and fabrics exposed at the surface today.