Kinematics and Dynamics of the Pamir, Central Asia and Main Ethiopian Rift

Cassidy N Jay Seymour, Purdue University

Abstract

Plate tectonic theory predicts that deformation is localized in narrow zones along plate boundaries, but geodetic, seismic, and geological observations show that deformation of continents is diffuse and complex. Furthermore, dense, recently published GPS velocities hint at regional heterogeneities within two of the most widely studied continental deformation zones in the world: the India-Eurasia collision zone and East African Rift System. Here, we use numerical models to investigate deformation in the Pamir, a region west of the Tibetan Plateau, and the Main Ethiopian Rift, the northernmost segment of the East African Rift System. Inverse and forward numerical models are constrained by GPS velocities, fault slip rate information, earthquake and magmatism data, plate rotations, and lithospheric density information. Models quantify 1) the distribution of surface deformation, 2) lithospheric force balance, and 3) the effects of lithospheric strength heterogeneities. Models indicate that the Pamir is kinematically similar to the Himalayan arc and Tibetan Plateau despite different deformation length scales. Modeled force balance is a combination of forces due to gravitational potential energy that create east-west extension and boundary stresses from India-Eurasia collision that create north-south compression. We find evidence that subducting continental slab beneath the Pamir structurally stiffens the region compared to the rest of the collision zone. Through forward modeling of the Pamir, we demonstrate that slab pull from continental subduction creates compression along the subduction interface and shear along the eastern and western Pamir boundaries. In Ethiopia, modeled deformation associated with the Main Ethiopian Rift is distributed over a broad region in the Ethiopian Highlands, contrary to prior models that predict narrow rifting. Styles of deformation in Ethiopia can be explained by force balance dominated by gravitational potential energy. Modeled stress field boundary conditions point to a small contribution from basal drag that resists stresses associated with gravitational potential energy. This body of work demonstrates that detailed, regional models reveal small-wavelength variations in deformation and force balance that are often unresolved in continent-scale models. We conclude based on our model results that in both convergent and divergent settings, 1) continental deformation occurs over multiple length scales, 2) buoyancy forces play an important and sometimes dominant role in force balance, and 3) lateral weak zones along major tectonic features play an important role in controlling deformation.

Degree

Ph.D.

Advisors

Flesch, Purdue University.

Subject Area

Geophysics

COinS