Strain and rotation rate from GPS in Tibet, Anatolia, and the Altiplano

Abstract

Deformation measured by regional GPS networks in continental plateaus reflects the geologic and tectonic variability of the plateaus. For two collisional plateaus (Tibet and Anatolia) and one noncollisional (the Altiplano), we analyze the regional strain and rotation rate by inverting GPS velocities to calculate the full two‐dimensional velocity gradient tensor. To test the method, we use gridded velocities determined from an elastic block model for the eastern Mediterranean/Middle East region and show that to a first order, the deformation calculated directly from the GPS vectors provides an accurate description of regional deformation patterns. Principal shortening and extension rate axes, vertical axis rotation, and two‐dimensional (2‐D) volume strain (dilatation) are very consistent with long‐term geological features over large areas, indicating that the GPS velocity fields reflect processes responsible for the recent geologic evolution of the plateaus. Differences between geological and GPS descriptions of deformation can be attributed either to GPS networks that are too sparse to capture local interseismic deformation, or to permanent deformation that accrues during strong earthquakes. The Altiplano has higher internal shortening magnitudes than the other two plateaus and negative 2‐D dilatation everywhere. Vertical axis rotation changes sign across the topographic symmetry axis and is due to distributed deformation throughout the plateau. In contrast, the collisional plateaus have large regions of quasi‐rigid body rotation bounded by strike‐slip faults with the opposite rotation sense from the rotating blocks. Tibet and Anatolia are the mirror images of each other; both have regions of positive dilatation on the outboard sides of the rotating blocks. Positive dilatation in the Aegean correlates with a region of crustal thinning, whereas that in eastern Tibet and Yunnan province in China is associated with an area of vertical uplift. Rollback of the Hellenic trench clearly facilitates the rotation of Anatolia; rollback of the Sumatra–Burma trench probably also enables rotation about the eastern syntaxis of Tibet.