Continuum Deformation Explains the Kinematics of Iranian Continental Convergence

Abstract:

Iran is one of the most rapidly deforming zones of active continental convergence, and a large and growing population within the country is exposed to high seismic hazard. However, despite the region’s tectonic importance, the force balance responsible for Iran’s large-scale deformation field is not well understood. Previous continuum deformation models for Iran suggest that lateral variations in lithospheric strength are necessary to account for the non-deforming region of Central Iran, but these studies were undertaken before the availability of a GPS velocity field against which to compare such models.

Here we use a physical model that treats the Iranian lithosphere as a thin sheet of viscous material overlying an inviscid substrate. We find that the GPS velocity field is well described by such a continuum model with homogeneous properties. Contrary to the suggestions of previous studies, we find that an anomalously strong Central Iran is not required to match the strain-rate field. Instead, this distribution of deformation can be replicated by considering buoyancy forces acting in the lithosphere. We also find that overthrusting of South Caspian oceanic lithosphere by Iranian continental lithosphere in the Talesh mountains plays an important role in determining local kinematics in NW Iran. Finally, we develop a novel method for estimating seismic hazard where velocity measurements are sparse. We assume that the motion of upper crustal blocks conforms to the velocity field derived from our dynamical calculations, and allow the geometry of blocks to be specified from geological considerations. We then solve for the Euler rotation vector for each block that best fits our model velocities. We use these rotation vectors to derive fault slip rates along block boundaries, and find that our predicted rates agree well with independent Quaternary and geological estimates.