The Caspian Sea basins of central Eurasia constitute one of the major petroleum provinces of the world. The tectonic setting and geological evolution of the South Caspian Basin remain enigmatic, particularly with respect to the generation of such significant hydrocarbon resources. Situated in the Alpine–Himalayan collisional zone, the Caspian Sea separates the locus of young continental collision in the Caucasus to the west from large-scale strike-slip faulting in the Kopeh-Dagh system to the east. Although the basin is thought to have originated in the Mesozoic, as much as 8–10 km (4.8–6 mi) of Pliocene–Pleistocene strata have accumulated, representing average depositional rates of greater than 1.5 km/m.y. (0.9 mi/m.y.) for the last 5 m.y. The presence of numerous gas-driven mud volcanoes and active oil and gas seeps suggests that hydrocarbons areforming and migrating within the basin today. Furthermore, active seismicity in the region attests that structures and associated hydrocarbon traps in the shallow section are forming now.

New deep normal-incidence seismic reflection data from the South Caspian Basin provide the firstfull crustal and upper mantle image of this basin. Collected as part of a 1998 acquisition program directed by Chevron Overseas Petroleum Inc., two roughly perpendicular deep seismic reflection profiles were acquired offshore Azerbaijan (Figure 1). These profiles, in the vicinity of the Apsheron Ridge, are each about 70 km (43 mi) in length and are recorded to 20 s in a key area for understanding the regional tectonics of the enigmatic Caspian Basin system. The main aim of collecting deep seismic data in this important petroliferous basin was to (1) reveal the deep structure and tectonics of the Alpine–Himalanyan continent– continent collisional zone beneath the South to Central Caspian Sea; (2) portray the full Mesozoic(?)– Quaternary section of the sedimentarybasin that is inferred to be, in places, thicker than 20 km (12 mi); and (3) elucidate the thickness and nature of the crust, providing critical information for subsidence and thermal modeling, with implication for source rock maturation. These new reflection data provide a pseudo-three-dimensionalarchitecture of the South Caspian Basin in the vicinity of the Apsheron Ridge. Although recorded with standard industry parameters (airgun source of 52,111 cm³ [3180 in.³] at 1900 psi [13 MPa], 25 m [82 ft] hydrophone spacing, 5100 m [17,716 ft] streamer length, 4 ms sample rate), the two deep seismic lines provide an image of the basin down to the upper mantle depths.