Intramontane basin formation during oblique convergence in the Eastern Desert of Egypt: magmatically versus tectonically induced subsidence

Abstract

The Neoproterozoic Kareim molasse basin in the Central Eastern Desert occupies an intramontane position and is bound by faults of the crustal-scale Najd Fault System. Basin subsidence and sediment delivery rates were linked with exhumation of adjacent core complexes. Sedimentation dynamics evolved in two steps. (1) Early basin infill was magmatically induced by extraction of magmatic material from the lower crust, which was balanced by downward movement of cold and dense material. Small subsidence rates, high heat flux and sediment delivery from rising diapirs and comagmatic volcanic rocks characterise this stage. (2) Late-stage sedimentation and modification of the basin geometry were controlled by strike-slip deformation and isostatic compensation of the orogen. Subsidence rates increased and coarse clastics were delivered. From distribution of sedimentary facies and structural studies we propose a model for the formation of intramontane basins within obliquely convergent island arc orogens. Minor crustal thickening, but with intense magmatic activity, may characterise these orogens in their early history. Erosion and deposition of sediments within intramontane denudation zones does not require high relief or significant mountain topography when basin formation is largely controlled by uprise of magmatic rocks during plate convergence. Ongoing transcurrent motion in the collisional stage may release in major strike-slip faults that trigger final exhumation of core complexes. Enhanced relief energy and high sediment delivery rates suggest formation of significant mountain topography during this evolutionary stage.