Initiation of gravitational collapse of an evaporite basin margin:The Messinian saline giant,Levant Basin,eastern Mediterranean

摘要

Regional 2D and 3D seismic data from the Levant Basin reveal the updip extensional component of thin-skinned gravity tectonics on this continental margin.Because of its youth(5-7 Ma),the earliest stages of deformation of this salt basin are preserved without the severe structural overprinting common on more mature,giant salt-tectonic systems.Extension detaches onto and within Messinian evaporites up to 1800 m thick.By structural restoration,we reconstruct the tectonic evolution of the Messinian evaporites using paleobathymetric constraints from wells and seismic profiles.This analysis suggests that the Mediterranean drawdown during the Messinian Salinity Crisis was ~800 m.The 10-to 15-km-wide extensional domain tracks the landward pinch-out of the mobile Messinian evaporites against an older Late Miocene scarp.Diachronous extension began in the center of the margin in the mid-Pliocene,then spread northward in the late Pliocene,then finally southward in the early Pleistocene.Extension continues today on many of the most landward faults.Extensional strain varies greatly along strike from<1 km to as much as 12-15 km.Comparing observations with four end-member conceptual models,we infer that both extension and seaward salt flow thinned the evaporite margin and its overburden.Both processes were triggered by a combination of uplift of the continental shoulder of the Dead Sea Rift and subsidence in the Mediterranean Basin.At least three factors controlled variations in extension:(1)degree of tilting of the salt wedge,related to the interplay of coastal uplift and basin subsidence;(2)presence of pre-Messinian canyons overlain by landward salients of saand (3)variations in evaporite facies and the proportion of siliciclastic admixture.