Fault rheology in an aseismic fold-thrust belt (Shahdad, eastern Iran)

Abstract:

This presentation describes geodetic observations of aseismic deformation in a thrust belt near Shahdad in eastern Iran, which have been used to place constraints on the rheology of creeping faults in a thin-skinned thrust belt. Creep on shallow and high-angle thrust ramps at the range-front occurs at a steady rate, in response to the topographic gradient across the thrust belt. Parts of these thrust ramps, and the low-angle basal thrust they connect to at depth in a ramp-and-flat geometry, underwent accelerated creep following the nearby Mw 6.6 Fandoqa earthquake in 1998. Estimates of the rate of fault slip and the driving stresses in these two contrasting times reveal a non-linear relationship between the stresses and sliding velocity. The degree of non-linearity rules out bulk shear of a weak layer in the sedimentary section (e.g. evaporites) as the deformation mechanism, and suggests that the motions are accommodated by slip on faults governed by a friction law with a rate or state dependence, or possibly both. The high-angle thrust ramps are responsible for building aspects of the geological and geomorphological signs of active shortening visible at the surface, but the folding preserved in the geology must be accomplished by other methods, possibly during the rapid transient postseismic deformation following nearby earthquakes.