I welcome the Comment by Flesch and Bendick (2007) regarding my model and interpretation of global positioning system (GPS) velocities at the India-Asia collision zone (Meade, 2007), as well as the opportunity to further discuss the interpretation of GPS velocities in general. To be clear, it appears that this Comment represents an emerging discussion. With regard to my block model, Flesch and Bendick agree that this approach is “required” for “understanding the temporal and spatial distribution of strain at the scale of individual crustal structures…” Further, they do not state any disagreement nor note errors with any quantitative estimate presented in my paper. Instead, the Comment from Flesch and Bendick focuses on “an ongoing controversy about the dynamics of the Tibetan Plateau,” and, in particular, the role of a “bulk mean rheology of Tibetan lithosphere.”
My work (Meade, 2007) does not attempt to contribute to parameterizations of the bulk rheology of lithosphere such as that mentioned in the Flesch and Bendick Comment. The focus of my work is the development of a block model of the elastic upper crust that is constrained by geodetic data and compared with geologic estimates of fault slip rates and seismic moment release rates. This model can be interpreted in the context of layered visco-elastic (Maxwell body) models (Savage and Prescott, 1978; Savage, 2000) if the viscosity of the lower crust or upper mantle exceeds 1019 Pa·s, and if the characteristic earthquake recurrence intervals are less than 500 yr (e.g., Savage, 2000). Also, I do incorporate a simple earthquake cycle model, and consider the short-term moment balance; there is no explicit consideration of the “energy cycle in the uppermost crust.” However, this is still a vertically layered treatment of the upper lithosphere rather than the “very high bulk viscosity” parameterization discussed by Flesch and Bendick.
As pointed out in my paper, several theoretical studies have demon strated that steady-state surface velocities provide no diagnostic information about the distribution of viscous deformation throughout the lithosphere below the elastic layer (Hetland and Hager, 2004; Li and Rice, 1987; Savage, 2000; Zatman, 2000). These results preclude the determination of steady-state lithosphere kinematics in viscous or visco-elastic layers below the elastic upper crust from observations of steady-state surface motions (e.g., GPS velocities). Thus, as has been discussed by previous authors, steady-state geodetic observations primarily provide information about the behavior of the upper crust and, in particular, the effects of interseismic elastic strain accumulation. However, transient GPS velocities following large earthquakes have been used to constrain the rheology of the lower crust and upper mantle (e.g., Hetland and Hager, 2003).