Bendick and Flesch (2007) model a band of high topography in northern Tibet (Qiangtang Bulge, Fig. 1) as caused by a high-viscosity (rigid) lower-crustal indenter present below 45 km depth (their Fig. DR4) that extends 120 km north of the Banggong-Nujiang suture (their Fig. DR3). They explicitly describe this rigid indenter as Indian crust (their Fig. 2A). I do not argue with their theoretical model, but I disagree with their geologic interpretation of the rigid indenter because of the ~350 km discrepancy between their claimed northern limit of Indian crust and all published interpretations based on seismic imaging. Although Bendick and Flesch state that “the exact indenter shape has very little effect on the topography”, the northern limit of their indenter is a key parameter in matching the topography of the Qiangtang Bulge (their Fig. DR3). Thus the “prow” shape (their Fig. 2; “BF07” in my Fig. 1) is not required, but the northerly limit of rigid crust (0 km in Fig. 1) is a prediction and a test of their model.
Figure 1 locates seismic images of the top of subducting, rigid Indian basement (Main Himalayan thrust) from seismic reflection (Zhao et al., 1993), refraction (Makovsky et al., 1999), and receiver-function data (Kind et al., 2002; Schulte-Pelkum et al., 2005; Hetényi et al., 2007). All these data indicate a >300 km discrepancy with Bendick and Flesch's northern limit of a rigid Indian indenter. Nor can the Bendick and Flesch indenter be the northern limit of Indian mantle lithosphere, even if the indenter top was twice as deep as the 45 km they model; receiver-function images (Kosarev et al., 1999) place the northern limit of Indian lithosphere >400 km south of the Bendick and Flesch indenter tip (Fig. 1).
Data in Figure 1 span 85.5°–91.5°E and, though most data are from the eastern part of the swath modeled by Bendick and Flesch (84°–91°E), the remarkably good agreement between seismic images spanning 5° of longitude suggests the northern limit of rigid India trends west-east, parallel to the Indus Tsangpo and Banggong-Nujiang sutures. Certainly “the front of the Indian indenter may have a complicated shape, probably varying in position both with depth and along strike” (Bendick and Flesch, 2007), but currently no seismic images claim to show Indian basement north of the Banggong-Nujiang suture at mid-crustal levels.
Possibly seismologists have misinterpreted their images, though the collinearity of the Main Himalayan thrust identified by modern seis micity (top of subducting Indian basement) (Zhao et al., 1993) with the various images of a seismic reflector/refractor/converter at depths increasing northward from 20 km to the Moho makes this a difficult argument to sustain. Or possibly the Qiangtang Bulge is simply inherited topography from an older thrust belt or magmatic arc (P. Kapp, personal comm., 2007). More likely, the viscosity contrast inferred by Bendick and Flesch is not the northern limit of Indian basement, but a different boundary, possibly the northern limit of the Lhasa terrane that is sutured to northern Tibet along the Banggong-Nujiang suture. Such an interpretation in no way negates the Bendick and Flesch paradigm of Tibet (following many others, cf. Klemperer, 2006) as divided into a southern part underlain by rigid lower crust/upper mantle, and a northern part underlain by low-viscosity lower crust/upper mantle, but is important in any discussion of the degree of underthrusting and convergence that has taken place in the Himalayan orogen.