We welcome the reinterpretation of our seismic profile (Yuan et al., 2003) as a reminder that any interpretation is just that: an attempt to infer the origins of seismic reflections, based on their geometries and relationships to surface geology. But it is important to recognize the relative importance of different aspects of our interpretation.
We are glad that Zhao and Fang accept our most fundamental interpretation, that the Yangtze plate was subducted down to the north beneath the North China (Sino-Korean) plate, since these authors have previously argued (Zhang et al., 2002) for the opposite polarity (southward subduction of the North China plate) to create the Dabie Shan ultrahigh-pressure (UHP) belt. We are glad they accept our recognition of the Moho offset (“mantle suture”) defining the subduction polarity that leads to our most important conclusion, that this archetypal UHP orogen is geometrically indistinguishable from non-UHP orogens, hence that many, if not all, collisional belts may contain cryptic UHP provinces.
Zhao and Fang take issue with a less-important aspect of our paper, our interpretation of south-dipping reflection fabrics in the southern Dabie Shan as representing a south-dipping normal fault or shear zone. In contrast, they interpret these and other reflections as a south-dipping, north-directed thrust fault. Zhao and Fang state that kinematic indicators in the high-pressure (HP) and UHP belt (km 20–80 in their Fig. 1) show top-to-northwest sense of motion (Hacker et al., 1995, 2000), and they infer this to represent northwest-directed thrusting, in contrast to Hacker et al. (2000) and Ratschbacher et al. (2000) who map these top-to-northwest structures as extensional faults rotated into their present geometry by later up-doming. Zhao and Fang then extrapolate the surface top-to-northwest structures to depth and interpret the origin of reflections as deep as 15 km as northwest-directed thrust zones. This aspect of their interpretation may lack internal consistency, because Zhao and Fang show the northward-directed thrust as cutting off a series of south-directed thrusts and shears in the middle and lower crust. In their interpretation, the northward-directed thrust sheet must carry northward rocks that contain top-to-the-southeast indicators, so equally at odds with the exposed geology, unless this early phase has been entirely obliterated and overprinted by younger structures. Thus the Zhao and Fang interpretation shows structures of two different ages, lower-crustal Triassic south-directed thrusts that are overprinted in the upper crust by Early Jurassic north-directed thrusts.
Readers of our paper will recall that our interpretation (Yuan et al., 2003, Fig. 2B) similarly includes elements of different ages. It is important to note that we suggest the south-dipping reflections may represent a structure active during Cretaceous exhumation of the UHP belt, following Ratschbacher et al. (2000) in recognizing that a major part of the exhumation of the Dabie Shan took place in the Cretaceous (see also Xu et al., 2002). We also note that there is outcrop evidence of top-to-south normal shear zones along the boundary between the Northern orthogneiss unit (Yuan et al., 2003, Fig. 2) and the UHP belt (Hacker et al., 1995; Ratschbacher et al., 2000; Suo et al., 2000), though there is remaining disagreement on the magnitude and importance of these shear zones.
In our paper (Yuan et al., 2003) we took no position on whether the south-dipping reflective structures might have had an earlier contractional history. Thus the “alternative interpretation” of Zhao and Fang is not in conflict with our interpretation; it merely suggests a richer and more complex history for some of the shallow reflections along our profile.
Brad Hacker and Lothar Ratschbacher commented on a preliminary draft of this reply.