REGIONAL COMPENSATION OF THE GREATER CAUCASUS MOUNTAINS BASED ON AN ANALYSIS OF BOUGUER GRAVITY DATA

Abstract

Teleseismic and geological observations of the continent-continent collision zone in the Greater Caucasus mountains of the southwestern Soviet Union indicate that the range is presently being underthrust primarily from the south. Since the onset of Alpide deformation, tectonic activity has been concentrated along the southern slope of the mountains, but ongoing deformation along the northern slope in the eastern part of the range may be associated with thrusting of the mountains northward over the Scythian platform in this area. To understand the present-day plate interactions in the Caucasian region and the mode of compensation of the Greater Caucasus mountains, we analyze Bouguer gravity anomaly data using both spectral (coherence and admittance calculations) and traditional spatial domain forward modeling techniques. As a whole, the range appears to be in regional isostatic equilibrium with compensation by flexure of an elastic plate of approximately 40 km thickness (D = 6×1023Nm) loaded strongly from the bottom as well as the top. Subsurface loads corresponding to stacking of thrust sheets, uplifts of basement rocks, active volcanic zones, and thick sequences of basin sediments play an important role in improving the fit between observed and calculated Bouguer values. Closer examination of the individual tectonic provinces which make up the range yields results consistent with support by a 40–50 km thick plate in the eastern and central parts of the Greater Caucasus mountains and by a plate with no flexural rigidity in the western part of the range. Coupled with the active volcanism and the relative lack of seismicity in the western part of the Greater Caucasus region, these results indicate the presence of hot, thermally-altered lithosphere in this area contrasting to the colder, more brittle, and more elastic lithosphere of the seismically active eastern region.