HR: 10:20h
AN: T22B-01 INVITED     [Abstracts]
TI: Constraints of the Style of Tibetan Lithospheric Deformation and their Consequences for Resolving the Orogeny Paradox.
AU: * Silver, P G
EM: silver@dtm.ciw.edu
AF: Carnegie Institution of Washington, DTM, 5241 Broad Branch Rd. NW, Washington, DC 20015 United States
AU: Flesch, L
EM: lmflesch@purdue.edu
AF: Purdue University, Dept. of Earth & Atmospheric Sciences, 550 Stadium Mall Drive, West Lafayette, IN 47907 United States
AB: The process of continent-continent collision is central to our understanding of continental dynamics. The Tibetan plateau, the most spectacular manifestation of this phenomenon, has long been used as a natural laboratory for studying the collisional process. While the most obvious properties of Tibet, namely its high topography and thickened crust, are easily explained by the shortening and thickening of continental lithosphere, idealized by the deformation of a thin viscous sheet, there remain basic thermomechanical properties that do not fit this simple picture. In particular, the expected advective thickening of continental geotherms predicts a strong, cool, upper mantle. In contrast, the basic morphology of the plateau, the observation of low mantle seismic velocities, the presence of N-S-oriented normal faulting, and significant volcanic activity, all argue instead for weak, high temperature lithosphere beneath the plateau. This discrepancy, which we have referred to as the "Orogeny Paradox", has led researchers to propose models that invoke more complex styles of deformation, such as mantle delamination, lower crustal flow, and continental subduction. The simple thin-viscous-sheet model, as well as these more complex models, make predictions about the relationship between crust and mantle deformation, that form the basis for a test. We have recently completed a study of the mechanical properties of Tibet (Flesch et al., 2005) and surrounding regions that places important constraints on the actual style of Tibetan lithospheric deformation. This study jointly modeled the surface deformation field estimated from geodesy and Quaternary fault slip, and the mantle deformation field inferred from shear-wave splitting observations. The results strongly support a lithosphere in which its crust and mantle components are mechanically coupled, deform coherently, and are roughly similar in strength. It is thus difficult to reconcile these observations with the more complex styles of deformation used to address the Orogeny Paradox. We consider ways in which this paradox can be resolved within this new set of observational constraints. As is traditional, we assume that the continental lithospheric strength profile is dominated by a combination of composition (contrast between quartz and olivine rheologies) and temperature (through the temperature dependence of viscosity). For a typical conductive continental geotherm this produces the well-known "Christmas-tree" strength profile characterized by a weak lower crust. We thus seek a process that can transform this into a weakened, roughly-uniform, strength profile. Assuming that temperature is the controlling parameter, the lithosphere can be weakened by increasing its average temperature, and its strength made more uniform by concentrating the temperature increase where the original lithosphere is strongest, such as at the top of the mantle. We will discuss candidate processes that might create such a modified thermal profile. One viable mechanism is viscous dissipation, produced by the strain energy of the deformation. This mechanism is attractive since it both provides a source of heat that weakens the lithosphere, and concentrates the heat where the lithosphere is strongest (i.e., where the strain energy is greatest).
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
SC: Tectonophysics [T]
MN: Fall Meeting 2005