Flow Rheology of the Lithosphere

Effective elastic thickness, a measure of lithospheric strength, in the western US Cordillera [Lowry & Pérez-Gussinyé, 2011]. This is essentially an integral of bending strength derived from multi-windowed, multitaper spectral analysis of the coherence of Bouguer gravity and topography. The solid line approximates the (pre-Oligocene) eastern limit of Sevier thin-skin thrusting; the dashed line delimits Laramide thick-skin tectonism. Thin-skin thrusts are limited to regions where the mantle lithosphere contributes negligibly to total strength.

Interest in characterizing the flow rheology of the crust and uppermost mantle is growing rapidly within the geodesy and geodynamics communities. We have been investigating methods for estimating depth-dependent lithospheric viscosity for some time now, using data that include observations of effective elastic thickness Te from the isostatic relationship between gravity and topography [Lowry and Smith, 1994; 1995; Lowry et al., 2000; Pérez-Gussinyé et al., 2004; 2007, 2008, 2009a, 2009b], from geodetic measurements of postseismic rebound [ Paul et al., 2007; 2011], and most recently from modeling shoreline deformation of Pleistocene Lake Bonneville.

In the past several years, we've added seismic imaging and new approaches to geotherm modeling to our tools for estimation of physical properties. These new tools have led to the serendipitous discovery that the bulk crustal abundance of quartz may control continental deformation.

Investigative directions include:

  • Is lithospheric flow strength compositionally controlled?
  • Structural geologists have long noted phenomena such as "basin inversion" and "reactivation" which suggest that deformational strength can be inherited. This is sometimes ascribed to weakening by faults, but faults respond passively to near-surface strain moments originating from flow in the ductile regime, and must inevitably form wherever those strain moments arise. (This is readily observed, for example, at evolving plate boundaries such as the Marlborough region of South Island New Zealand: New major faults will develop when the locus of deeper deformation shifts by as little as 15 to 20 km). As inheritance sometimes occurs across time scales exceeding that for thermal re-equilibration of the lithosphere, flow strength should be partially controlled by a less transient field than temperature. The relationship of elastic thickness Te to heat flow Qs in the western U.S. strongly suggested that a compositional control on ductile flow rheology is required to explain the differences in strength of stable versus deforming lithosphere [ Lowry et al., 2000]. This was suggested previously based on large variations in Te over short distances [Lowry & Smith, 1994; 1995], but these early studies inferred a compositional variation in the mantle was responsible. New seismic imaging of the crust in the western US Cordillera indicates that quartz abundance in the crust is a more likely candidate compositional mechanism for inheritance [ Lowry & Pérez-Gussinyé, 2011], and that the weakness of quartz-rich crust is amplified throughout the lithosphere by deformation-induced processes that warm and wet the lithospheric column.

  • Why is Te from spectral models different than from rebound studies?
  • Geodetic studies of western U.S. postseismic rebound and lake loading rebound, which model the Earth as an elastic layer over a viscoelastic halfspace, typically yield elastic layer thicknesses approximately equal to the crustal thickness (30–40 km) and low (<5X1019 Pa s) uppermost mantle viscosity. Coherence analysis of gravity and topography at the same locations give Te a factor of 4 to 6 smaller, however. Part, but not all, of the Te difference can be explained by the difference in timescale of loading (100–104 years for rebound versus 106–108 years for gravity and topography). The rest of the discrepancy suggests a problem with one or both modeling techniques.
    Recent efforts to carefully examine the sensitivities and assumptions of the spectral isostatic analysis methodology have focused on recovery of Te from simulated data. Efforts by our collaborators, Marta Pérez-Gussinyé at the Royal Holloway, University of London, and Jon Kirby & Chris Swain at Curtin University of Technology (Perth, Australia) have demonstrated that many long-held concerns about methodological assumptions regarding the nature of loading and possible effects of dynamical processes are either unfounded or can be recognized in the course of analysis. USU MSc candidate Lisa Seunarine is currently taking the method a step further by incorporating seismic data as an independent constraint on mass load structure, thus obviating the need to make assumptions about correlation properties of loading processes.
    USU MSc candidate Eric Beard is simultaneously examining the limitations and assumptions of Pleistocene Lake Bonneville rebound models. He is currently adding to the (ca. 1980 Curry) estimates of lakeshore uplift using modern geodetic and topographic data (and with the aid of new signal analysis tools). Once that phase of the project is completed, he will examine the effects of a more realistic (i.e., temperature dependent, compositionally layered, and power-law stress-dependent) viscosity structure. MSc candidate Anamitra De is also examining the effects of more realistic rheology and pre-existing state of stress on postseismic rebound, and assessing whether the combination of long-term isostatic models and geodetic data can be used to separate effects of viscoelastic, poroelastic and rate-state-dependent frictional response in postseismic rebound models. Our earlier results from the Andaman Islands indicated that most of the near-field postseismic deformation in the first two years following the 2004 Great Sumatra/Andaman earthquake results from fault slip [Paul et al., 2007]. More recent results indicate that slip still dominates the near-field signature six years after the event, suggesting that near-field postseismic rebound will be far more useful for constraining fault frictional rheology than for deeper flow rheology.

    This material is based upon work supported by the National Science Foundation's Geophysics and EarthScope Science programs under grants 0454541 and 0955909, and a Utah State University New Faculty Research Grant. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the NSF or of Utah State University.

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