CRUSTAL-SCALE STRUCTURE OF THE SOUTH CASPIAN BASIN REVEALED BY DEEP SEISMIC REFLECTION PROFILING

Abstract

Two deep seismic reflection profiles from the South Caspian basin provide the first normal-incidence image of possibly the thickest sedimentary basin in the world. Significant features imaged with these seismic data are: (1) a series of high-amplitude folds, developed within a thick (>10 km) Neogene sedimentary section, bounded discordantly below by (2) a gently N-dipping, intermittent reflection at 14–20 km (8.0+ s), (3) a prominent deep reflector at 26–28 km depth (12.8–13.0 s) with a gentle northward dip, (4) an underlying layered interval with discernibly lower-frequency reflections down to ∼36–40 km (16.0–16.5 s), and (5) a noticeable decrease in reflectivity below ∼36–40 km. Based on the downward termination of fault-cored folds and their discordance with subhorizontal underlying reflections, the reflection at 14–20 km is interpreted to be the basal detachment for structures associated with the Absheron Ridge. The bright reflection at ∼26–28 km depth is interpreted as the basement/cover contact, and the underlying ∼10 km portion of the section is interpreted as crystalline basement. Downward diminution of reflectivity at ∼36–40 km is thought to represent the Moho despite the absence of a clearly reflective horizon. These interpretations are consistent with previous velocity models from both seismic refraction and teleseismic studies in the South Caspian region that suggested a minimum sedimentary thickness of 20 km and a crustal thickness of 35–40 km. Both the apparent lack of local structural relief on the basement reflector, and seismic velocities (Vp≈6.8 km/s) obtained from earlier studies suggest this crust is not thinned continental material. Although thicker than typical oceanic crust (6–8 km), the apparent ∼10 km thickness of the South Caspian crystalline crust is interpreted to be of oceanic affinity for this part of the basin. Active seismicity down to mantle depths (80 km) and gentle deepening of the crust from south to north suggest northward subduction of the South Caspian oceanic lithosphere beneath the continental lithosphere of the Central Caspian basin.