Localization of shear along a lithospheric strength discontinuity: Application of a continuous deformation model to the boundary between Tibet and the Tarim Basin

Abstract

A marked contrast in strength (or viscosity) within a continuously deforming zone can lead to concentration of shear strain in the weaker material adjacent to the boundary between them, but localization comparable to the width of the Altyn Tagh shear zone requires an additional weakening process. During numerical experiments on a thin viscous sheet indented by a rigid object, a shear zone develops adjacent to a strong region mimicking the Tarim Basin, when the boundary between the weak and strong regions is oblique to the orientation of convergence. The width of this shear zone narrows with increased strain, and for comparable penetration by the indenter, the strain is more concentrated for larger values of n, the exponent that relates strain rate to a power of stress, and for smaller values of the Argand number Ar, a measure of buoyancy‐induced stress relative to viscous stress. Increasing concentration of shear occurs as the indentation develops without weakening because of a change in material properties. Additional localization develops with the inclusion of strain‐dependent weakening associated with, for instance, a temperature increase due to shear heating. For such localization to scale to the width of the Altyn Tagh fault zone of Tibet, the initial temperature near the Moho must be relatively low (∼600°C), and a large value of n (∼10) is required. This suggests that deformation there is described by a high‐strength flow law, such as that proposed by Evans and Goetze (1979), in which the lithosphere would deform approximately plastically.

The Pingding segment of the Altyn Tagh Fault (91°E): Holocene slip‐rate determination from cosmogenic radionuclide dating of offset fluvial terraces

“…), the Altyn Tagh Fault (ATF) is usually considered to be the largest active strike‐slip fault of Eurasia. Slip‐rate and seismic behavior of the fault are keys to the present‐day kinematics of collision tectonics in Asia, and viewed as an important factor in evaluating whether continental deformation can be approximated by block tectonics or not, a topic that remains the subject of debate [ Avouac et al , 1993; Calais et al , 2006; Dayem et al , 2009; England and Houseman , 1986; England and Molnar , 1997; Hilley et al , 2009; Holt et al , 2000; Loveless and Meade , 2011; Meade , 2007; Peltzer and Saucier , 1996; Thatcher , 2007; Vergnolle et al , 2007]. Recent slip‐rate estimates based on complementary and/or similar techniques currently yield disparate results along the Altyn Tagh Fault.…”

Folding and necking across the scales: a review of theoretical and experimental results and their applications

“…Assuming n = 5, a total horizontal stress 4η D xx = 350 MPa, a density difference between mantle lithosphere and water ρ = 2300 kg m −3 and H = 25 km yields α d ≈ 9, which is not sufficient for significant folding. However, if we assume that the mechanically competent (upper) region of the mantle lithosphere is deforming by low-temperature plasticity (Peierls creep), then the effective (or apparent) value of n is in the range 10-25 (Dayem et al, 2009;Schmalholz and Fletcher, 2011) and we get α d ≈ 19-47, a value sufficient for significant folding. Furthermore, if we assume that the competent level of the oceanic lithosphere is not overlain by water, but by unconsolidated sediments, then the density difference reduces to ρ = ∼ 1000 kg m −3 (Martinod and Molnar, 1995) and α d ≈ 43-107.…”

Deformation of Indian Ocean lithosphere: Evidence for a highly nonlinear rheological law

“…More recently, however, Dayem et al . [] found that the vertically integrated rheology of the lithosphere beneath the Altyn Tagh Fault zone on the Tibetan Plateau may be better described with a power‐law exponent even greater than 10. An inferred value of ≈ 3 suggests that most of the strength in deforming continental lithosphere lies in the part of the lithospheric mantle that deforms by creeping flow [ Sonder and England , ].…”

“…Because oceanic lithosphere has a much thinner crust and the mantle is generally more resistant to creeping flow, fracturing and faulting typically extend to greater depth in oceanic lithosphere and the relevant exponent may then be greater than 3. The vertically integrated constitutive law for the lithosphere combines the properties of the upper lithosphere, which deforms by faulting and fracturing and for which the appropriate power‐law exponent is n → ∞, and the lower lithosphere, which deforms by dislocation glide [ Goetze , ; Evans and Goetze , ; Raterron et al ., ; Dayem et al ., ; Mei et al ., ] and obeys an exponential law, and by dislocation creep, which obeys a power‐law with n ≈ 3 [ Kohlstedt et al ., ]. Although only the lowest layer obeys a power‐law, the integrated behavior of deforming lithosphere can be well approximated by a power‐law [ Sonder and England , ].…”