Deep seismic studies of the central Eurasian lithosphere: Southern Uralian orogen and south Caspian basin.

摘要

Deep seismic studies of the Southern Urahan orogen and South Caspian basin indicate that (1) the Uralian foreland preserves a Precambrian Moho; in contrast, eclogitization of the crustal root possibly served to stabilize the Southern Urals in Mesozoic time, and (2) the South Caspian Basin is possibly the thickest sedimentary basin in Earth history.; Seismic character of the Moho boundary in the Late Paleozoic Southern Urals forms the basis for a model of geodynamic evolution of this orogen. While the Moho is a sharp and highly reflective interface of inferred Precambrian age (∼1.6 Ga) beneath the Southern Uralian foreland (at 40–42 km depth), it dies out in near-vertical reflectivity toward the main axis of the orogen, where it appears as a sub-horizontal boundary (at ∼50–53 km depth), based on wide-angle seismic reflection/refraction data. These observations, together with a crustal scale restoration of the Uralian foreland fold and thrust belt, suggest that the Moho (1) was superimposed by a metamorphic phasechange to eclogite facies rocks, developed across the structural fabrics produced during the Urahan orogeny, and (2) is comprised of ∼70% European crust. Moreover, the post-collisional eclogitization of the Urahan root produced an isostatically-balanced system, and may have served to stabilize and preserve the orogenic architecture.; The first crustal scale seismic reflection study of the Caspian Sea reveals Cenozoic deformation at the southern margin of the Eurasian continent. A bright reflection at 26–28 km depth is interpreted as the basement/cover contact, making this possibly the deepest sedimentary basin in Earth history. The sedimentary section displays large, south-vergent fault-propagation folds that root into an intra-sedimentary detachment at 14–20 km depth. An apparent ∼10 km thick reflective crystalline crust, with a gentle northward dip, is interpreted to be of oceanic affinity, and suggests an incipient northward subduction of the South Caspian lithosphere beneath the Eurasian continent.; The shallow seismic section of the South Caspian basin indicates accumulations of gas hydrates, which (1) are buried well beneath the seafloor, (2) accumulate free gas within the underlying sediment, and (3) may have caused recent slope failure in the overlying strata.