We thank Chung et al. (2003) for their comment, which permits further discussion of Indo-Asian tectonics. In our paper (Kohn and Parkinson, 2002), we presented three petrologic lines of evidence for slab breakoff, including (1) Himalayan eclogites, (2) partial melting of the Greater Himalayan Sequence, and (3) putative Eocene K-rich volcanic rocks in southeastern Tibet (Chung et al., 1998). We argued that the first two observations are necessary consequences of continental subduction and rapid exhumation, and that insofar as K-rich volcanic rocks are associated with several ultrahigh-pressure (UHP) terranes (Parkinson and Kohn, 2002), their reported occurrence in southeastern Tibet was consistent with slab breakoff. Chung et al. (2003) agree with us regarding the first two observations, but correct Chung et al. (1998) by noting that the “Eocene K-rich volcanic rocks” in southeastern Tibet are simply miscorrelated bodies of the Gangdese Batholith. If so, then we agree that the occurrence of these rocks is not good evidence for slab breakoff.
We believe there may yet be igneous evidence in the hinterland for Eocene slab breakoff, but on further review it has become unclear to us which rocks are truly best suited for testing this hypothesis. Compositional changes ca. 40–45 Ma within the Gangdese Batholith could reflect slab breakoff and asthenospheric upwelling (Chung et al., 2003). A. Yin (2002, personal commun.) also kindly suggested to us that evidence for Eocene slab breakoff may lie ~300 km due north of Nepal in the Paleogene Linzizong volcanic rocks, which are associated with the Gangdese Batholith. Another possibility is the 24–42 Ma, southeastern China and Indochina high-K intrusive and volcanic suite (Wang et al., 2001). Both Yin and Harrison (2000) and Wang et al. (2001) have argued that the Linzizong volcanic rocks and southeastern China and Indochina high-K rocks were influenced geochemically by continental subduction. The southeastern China and Indochina high-K rocks may, however, correlate with rocks of similar age and chemistry in the Qiangtang block, which are over 500 km north of the Greater Himalayan Sequence, much farther distant than ordinarily found in other UHP terranes. Finally, it is worth reiterating that Davies and von Blanckenburg (1995) predict that initiation of K-rich magmatism will post-date slab breakoff by 0–25 m.y. Thus, although we focused on Eocene rocks, the widespread initiation of K-rich igneous activity ca. 25 Ma in southern Tibet (e.g., Miller et al., 1999) may well be the delayed consequence of Eocene slab breakoff. In sum, Chung et al. (2003) agree with us that slab breakoff likely occurred in the late Eocene, with clear implications for eclogites and migmatites in the Himalaya, and for igneous activity in Tibet, but at least four igneous suites—the Gangdese Batholith, the Linzizong volcanic rocks, Eocene-Oligocene southeastern China and Indochina high-K rocks, and Miocene high-K rocks of southern Tibet—all have ages, chemistry, and proximities to the Greater Himalayan Sequence explainable in part by Eocene slab breakoff.
In the remainder of their comment, Chung et al. argue for two geodynamic events—slab breakoff ca. 45 Ma and lithospheric delamination ca. 25 Ma—both causing significant topographic uplift in southern Tibet. Tibetan geodynamics and topography were not major points of our paper, but are worth discussing. We agree this interpretation has logical consistency and may be correct. However, our original point was that occurrence of K-rich rocks may be consistent with delamination, but is insufficient to prove it, and hence is topographically inconclusive. There are certainly instances where K-rich volcanic rocks do not occur in high plateaus, e.g., in K-rich igneous provinces proximal to the Appalachian and Alpine orogens. Conversely, there are more definitive indicators of high topography and plateaus, such as oxygen isotope depletions in rainwater (e.g., Rowley et al., 2001) and monsoons (Dettman et al., 2001). Thus, if paleoelevations are to be linked to geodynamics, we favor stable isotope studies (e.g., Garzione et al., 2000; Dettman et al., 2001; Rowley and Currie, 2002) that have documented monsoons by 11 Ma, modern elevations (5 km) in the southern plateau as early as 15 Ma, and moderate elevations (up to 2 km) in the northern plateau by the Eocene.
Overall, Chung et al. (2003) agree with us on most of the main tectonic points, particularly the occurrence and timing of slab breakoff. Our disagreements center on Tibetan geodynamics, and the igneous and topographic consequences in Tibet. These issues fortunately are the focus of much ongoing research by several research groups, and presumably will be resolved shortly.