GPS CONSTRAINTS ON CONTINENTAL DEFORMATION IN THE EASTERN MEDITERRANEAN AND CAUCASUS REGION: EVIDENCE FOR PLATE/BLOCK-LIKE BEHAVIOR OF THE CONTINENTAL LITHOSPHERE

We use Global Positioning System (GPS) observations during the period 1988 – 2004 to constrain an, elastic block model for deformation within the zone of interaction of the Eurasian, African, and Arabian plates. We constrain present-day motions of the African (Nubian), Arabian, and Eurasian plates, regional deformation within the inter-plate zone, and slip rates for major faults. Present-day deformation is for the most part consistent in sense and magnitude with estimates based on neotectonic and paleomagnetic observations covering the past 5 Ma, suggesting that the geodetic results can help constrain dynamic processes that have operated in this region during the most recent geologic period. Kinematically, we interpret the deformation field in terms of a block-like response of the continental lithosphere including the westward motion of Anatolia away from the Arabia-Eurasia collision zone. The eastern Turkey and Lesser Caucasus region also exhibits block-like behavior, including counterclockwise rotation that results in increasing rates of convergence from west to east along the Main Caucasus thrust. While most earthquakes are confined to block boundary zones, some have occurred within blocks (e.g., 1988, M=6.9, Spitak, Armenia). These intra-plate earthquakes and other neotectonic evidence suggest some internal block deformation, but relative motions within blocks are constrained to be less than 1 – 2 mm/yr, at least an order of magnitude less than the motion between adjacent blocks. We use a Monte Carlo simulation approach to investigate locking depths on the principal block bounding faults. Although poorly constrained by the GPS data, our investigations suggest shallow locking depths (~5 km) for the Main Caucasus thrust and the Hellenic subduction zone, and normal locking depths for other faults (15-20 km). This may help account for the low level of historic seismic strain release observed in the Caucasus and along the Hellenic Arc. A particularly interesting result of our model is that the East Anatolian fault (boundary between the Anatolian and Arabian plates) is transtensional. If this result persists with improved GPS control on Arabia plate motion, it would imply that the westward motion of Anatolia is currently being driven completely by gravitational body forces, and foundering of the down-going African plate along the Hellenic trench.