Cooperative Institute for Research in Environmental Sciences

Geodynamic Studies of Continental Deformation

My geodynamic studies of continental deformation follow three broad directions: (a) the kinematics of continental deformation, for which I rely largely on both geophysical and geological methods; (b) the structure of the crust and upper mantle, based largely on geophysical measurements: and (c) numerical experimentation on simple systems to understand how mass and heat are transferred to, through, and by the continental lithosphere.

Examples of the questions that motivate this work are:

1. Where does the strength of continental lithosphere reside? Is it more in the upper crust where deformation is brittle (perhaps also in the lower crust) than in the upper mantle, of conversely, does the uppermost mantle, despite being ductile (and perhaps including the lowermost crust) offer the most resistance to deformation of the continental lithosphere. (Northern Tibet, NE Tibet; Asia kinematics; New Zealand anisotropy, New Zealand-Marlborough)
2. To what extent do faults in the upper crust of continental regions pass directly through the lower crust and mantle lithosphere as narrow shear zones, or does deformation in the lower crust spread out into a broad zone of deformation? (New Zealand-Marlborough, New Zealand anisotropy)
3. Under what conditions does flow in the lower crust redistribute mass without much surface deformation? (Northern Tibet, NE Tibet)
4. Is the mantle lithosphere convectively unstable, so that if artificially thickened large portions of it sink into the asthenosphere, and in doing so thin the remaining lithosphere? (Rayleigh-Taylor, Sierra Nevada, Paleoaltimetry, West Tibet, Tibet Seismology)
5. What are typical magnitudes of deviatoric stress in the lithosphere that cause deformation? (Tibet)
   1. Kinematics of deformation. Concurrent with the recognition of plate tectonics, seismologists realized that the study of earthquakes could be used to determine the nature of faulting and the orientations of faults that ruptured in major earthquakes. This opened the question of how fast does slip occur on individual faults. Two obvious approaches offer estimates of rates of slip and rates of deformation: (a) the measurement of displacements of features offset by faults and dating of the features (Asia kinematics, Northern Tibet, NE Tibet), and (b) GPS measurements of velocities of control points (Asia kinematics, New Zealand, India-Pakistan, New Zealand GPS).
   2. Deep structure. The tightest constraints on deep structure come from seismology, and I am involved in studies using both surface waves and body waves to study both the crustal and upper mantle structure of selected continental regions. We seek tests of whether mantle lithosphere beneath continents participates in vigorous convection beneath such regions (West Tibet, Tibet Seismology). We are particularly motivated by anisotropy in seismic wave propagation, which we see as a potential strain gauge for measuring finite strain in the mantle or crust beneath actively deforming regions. Such straining and resulting anisotropy occur both within the crust (Tibet Seismology, New Zealand-Marlborough) and either in mantle lithosphere or below it (New Zealand anisotropy).
   3. Numerical experimentation. We have analyzing the conditions that determine how fast instabilities in density structures can grow. If a heavy layer overlies a lighter one, and the interface between them is perturbed from an initially flat surface, the perturbations will grow; this is Rayleigh-Taylor instability. If perturbations in density are due to temperature differences, then diffusion of will tend to reduce perturbations and if diffusion is sufficiently rapid, it can suppress convective instability. If the top layer is intrinsically less dense than the layer below it, because of, for instance, chemical differences, if sufficiently cold the top layer can be unstable, but in some such parameter ranges, the instability is oscillatory. We have been studying how such instabilities depend on the constitutive relationship (rheology), on boundary conditions, on external forcing of perturbations, and on density structure (Rayleigh-Taylor).