Formation of the superdeep South Caspian basin: subsidence driven by phase change in continental crust

Abstract

The large hydrocarbon basin of South Caspian is filled with sediments reaching a thickness of 20–25 km. The sediments overlie a 10–18 km thick high-velocity basement which is often interpreted as oceanic crust. This interpretation is, however, inconsistent with rapid major subsidence in Pliocene-Pleistocene time and deposition of 10 km of sediments because the subsidence of crust produced in spreading ridges normally occurs at decreasing rates. Furthermore, filling a basin upon a 10–18 km thick oceanic crust would require twice less sediments. Subsidence as in the South Caspian, of ≥20 km, can be provided by phase change of gabbro to dense eclogite in a 25–30 km thick lower crust. Eclogites which are denser than the mantle and have nearly mantle P velocities but a chemistry of continental crust may occur beneath the Moho in the South Caspian where consolidated crust totals a thickness of 40–50 km. The high subsidence rates in the Pliocene-Pleistocene may be attributed to the effect of active fluids infiltrated from the asthenosphere to catalyze the gabbro-eclogite transition. Subsidence of this kind is typical of large petroleum provinces. According to some interpretations, historic seismicity with 30–70 km focal depths in a 100 km wide zone (beneath the Apsheron-Balkhan sill and north of it) has been associated with the initiation of subduction under the Middle Caspian. The consolidated lithosphere of deep continental sedimentary basins being denser than the asthenosphere, can, in principle, subduct into the latter, while the overlying sediments can be delaminated and folded. Yet, subduction in the South Caspian basin is incompatible with the only 5–10 km shortening of sediments in the Apsheron-Balkhan sill and south of it and with the patterns of earthquake foci that show no alignment like in a Benioff zone and have mostly extension mechanisms.

Lithospheric folding and sedimentary basin evolution: a review and analysis of formation mechanisms

“…Attempts to explain it as an EB or as a FB have been unsuccessful.There is a lack of evidence for basin extension and both stretching models and topographic loads and slab pull operating on down £exed lithosphere in FB systems cannot explain di¡erential vertical motions of this order of magnitude. As a result, phase changes in continental crust have been proposed as a mechanism for the formation of this superdeep basin (Artyushkov, 2007). A limitation in conclusively resolving the basin formation mechanism is the lack of multichannel seismic re£ection data capable of imaging crustal structure below the thick sedimentary sequences.…”

“…Stel et al (1993) propose that non-extensional crustal thinning is induced by basaltic underplating, which causes thermal uplift and erosional thinning. Tectonic loading at plate boundaries is also proposed as a cause of or contributor to large-scale sag-type subsidence in plate interiors (Quinlan, 1987;Cloetingh, 1988;Leighton & Kolata, 1990) as well as phase transformations (Artyushkov, 2007). Ritzmann & Faleide (2009) point out that intraplate stresses and crustal inhomogeneities coupled with loading scenarios provide the best explanation for the Barents Sea Basin and other ICB evolution.…”

Thermochemical Heterogeneity and Density of Continental and Oceanic Upper Mantle in the European‐North Atlantic Region

“…We explain these LM density anomalies by partial eclogitization of the lithosphere mantle which may require the presence of ~10–20% of eclogitic material in the LM of the sedimentary basins (Figures a and ). In particular, thinning of the crystalline crust locally to 15–20 km in the South Caspian basin is sometimes interpreted as an evidence of oceanization (Allen, Jones, et al, ) and its subsidence is attributed to phase transitions in the lower crust (Artyushkov, , ). Consequent sinking of a high‐density material into the mantle may create high‐density LM anomalies as observed in our model.…”

Relocation of aftershocks of the 2001 Bhuj earthquake: A new insight into seismotectonics of the Kachchh seismic zone, Gujarat, India

“…2d) suggest a dominant reverse movement with a minor strike-slip component along a south-dipping plane characterizing the NWF and reverse motion (with a strike-slip component) along a north-dipping plane dominating the transverse feature (Mandal and Horton, 2007). The reverse motion along the north-dipping transverse feature might be attributed to the thrust movement of a basin generated by subsidence resulting from the effect of active fluids infiltrated from the sub-crustal mantle lithosphere to catalyze the gabbro-eclogite transition (Artyushkov, 2007). Similarly, focal mechanisms of the events or SWF 1 and SWF 2 reveal a reverse motion with a strike-slip component along steeply south-dipping planes.…”