The crustal structure of the southern Caspian region

Abstract

The south Caspian basin appears to behave as a relatively rigid aseismic block within the otherwise deforming Alpine–Himalayan orogenic belt. This anomalous character suggests that there is a fundamental compositional difference between the crust of the south Caspian basin and that of the surrounding region. Five new estimates of crustal velocity structure beneath the southern Caspian region are determined by the teleseismic receiver function method. These models show that the crust in Turkmenia along the trend of the Apshceron–Balkhan Sill–Kopet Dag Mountains is 50 km thick. In the southwestern part of the Caspian basin the crust is 33 km thick and consists of a 13 km thick sedimentary section lying on a high-velocity (Vp∼7.1 km s−1) lower-crustal section. In the southeastern part of the basin the crust is 30 km thick and consists of a 10 km thick sedimentary section overlying a 20 km thick low-velocity (Vp∼5.8 km s−1) crystalline crust. The receiver function models are combined with velocity models from previous Russian Deep Seismic Sounding studies into a ∼1800 km long ESE–WNW-trending crustal cross-section across the Kura Depression, the south Caspian basin and the Kopet Dag Mountains. The most significant features of this crustal model are the 20 km variation in thickness of Cenozoic sedimentary basin deposits, the absence of a ‘granitic’ (Vp∼5.8–6.5 km s−1) crustal layer in the central part of the south Caspian basin, and 20 km of crustal thinning beneath the central part of the basin. The Moho beneath the south Caspian basin has a broad arch-like structure whose western boundary is a relatively narrow zone across which the crust thins rapidly (∼20 km thinning over a 100 km zone) and whose eastern boundary has a more gradual change in crustal thickness (∼20 km thinning over a 400 km zone). The velocity–depth profiles derived from the receiver functions are compared to laboratory velocity estimates made at pressures and temperatures appropriate for the lower crust in Turkmenia, but the seismic P-wave data alone do not permit differentiation between the various possible rock types. The crustal model developed for the south Caspian basin is consistent with the hypothesis that the crystalline crust of the basin is a section of oceanic crust that is being overthrust by the continental crust around much of its margins. However, we cannot rule out the possibility that the crystalline crust beneath the basin is a section of lower continental crust whose upper crustal section has been removed by erosion or faulting.