Effect of pre-existing faults on the distribution of lower crust exhumation under extension: numerical modelling and implications for NW Ghana

Abstract

3D thermo-mechanical models have been constructed to explore the influence of pre-existing faults on the temporal-spatial distribution of high-grade amphibolite-granulite facies metamorphic rocks during extension. The different dip amounts of three pre-existing faults (refer to the Bole-Nangodi shear zones, the Jirapa shear zone and the Bulenga shear zone in the study area) are studied in models. The results show the lower crust exhumes occur at a relatively low rate prior to attaining a Stretching Factor = 4.2%. The partially molten lower crustal rocks tend to move (up to 20 km) towards the center of the model and focus exhumation in regions where pre-existing faults intersect. The high-strain corridors in models are used to understand the loci of exhumation in the Bole-Bulenga domain of NW Ghana. Accordingly, in the eastern and western parts of the high-grade rock corridors in NW Ghana, partially molten rocks exhumed from the lower into middle-upper crustal levels are interpreted to have been dominantly facilitated by the km-scale high-strain corridors. In the central part of the Bole-Bulenga domain, the high-grade rocks are interpreted to have been exhumed because of a coupling between two mechanisms: (1) The exhumation of partially molten rocks between the Jirapa and Bole-Nangodi faults increases in spatially due to the reduction in space from north to south. (2) The exhumation of lower partially molten rocks in the central part, as a result of inherited orthogonal (E-W) structures.