Lithological nature of the subduction channel: Insights from the Karabakh suture zone (Lesser Caucasus) and general comparisons,

Description

The lithological nature of major interplate boundaries is estimated by a field analysis of a well preserved exhumed subduction channel in the Caucasus Karabakh region. From this field example the subduction channel is a narrow geological object of about 500 m width formed at approximate depth of 10 km along an Andean-type subduction zone. It is comprised by an upper 'sedimentary' channel formed by an upper section of detrital and volcanic rocks thrusted on top of pelagic sediments scrapped off the oceanic floor. This sedimentary mélange is thrusted on top of an intensely deformed tectonic mélange. The tectonic mélange comprises blocks of basalt from the oceanic floor and a focussed deformation zone 50–100 m in width. This zone is mainly formed by mud-supported conglomerates exhibiting a chlorite + carbonate matrix with blocks of basalt, cross-cut by numerous chlorite-carbonate-epidote-albite veins. It overlies an undeformed ocean floor section. Superposed chlorite- and calcite-bearing veins in the mélange evidence high fluid:rock ratios of 0.3–2.3, with varied δ18O and δ13C isotopic ratios (+17 < δ18O < +25‰; −7 < δ13C < +4‰), which agrees with fluid mixing between pelagic sediments and a hydrothermal component at temperatures ranging from 120 to 400 °C, and thus mixing between deep and shallow reservoirs along the subduction interface. These data show that the several fluid reservoirs situated along the interplate boundary could have been connected by high-magnitude co-seismic displacements along the subduction zone. These subduction channel features are confronted to other similar fossil examples and current settings, such as the Andes accretionary prism to propose a reconstructed geometry of the interplate contact zone from the surface to the base of the crust.

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