PROGRESSIVE STRAIN LOCALIZATION AND FLUID FOCUSING IN MANTLE SHEAR ZONES DURING RIFTING: PETROSTRUCTURAL CONSTRAINTS FROM THE ZABARGAD PERIDOTITES, RED SEA

Abstract

This article documents the evolution of pressure and temperature conditions and the successive influence of hydrous melts and aqueous fluids on the operation of extensional shear zones, which exhumed mantle slivers from deep lithospheric or asthenospheric depths, in a rift-to-drift setting. These results are based on a re-analysis of 40 samples from three peridotite massifs of Zabargad island in the northern Red Sea. By integrating high-resolution mapping of the microstructure by electron backscattered diffraction with recent developments in barometry for plagioclase-bearing peridotites and thermodynamic modelling of peridotitic compositions, this study: (1) constrains the temporal and spatial evolution of petrological and tectonic processes in the shallow mantle during rifting and (2) documents the presence of melts or aqueous fluids throughout the activity of the shear zones, unravelling substantial feedback between petrological and tectonic processes. Thermobarometry and thermodynamic modelling, constrained by the microstructural observations, document progressive strain localization associated with shearing under decreasing pressure and temperature, from near solidus conditions at >1 GPa (in the north and central peridotite massifs) or ~0.7 GPa (in the southern massif) to < 600°C and <0.3 GPa in all three massifs. The data substantiate local aqueous fluid saturation in the shear zones. This together with higher contents of hydrous minerals in ultramylonites indicate fluid focusing in the shear zones, with seawater ingress extending to >10 km depth. The presence of melts or fluids enabled concurrent dislocation and dissolution-precipitation creep, resulting in weakening of the shear zones. However, fluid supply was spatially heterogeneous and likely intermittent, with equilibrium achieved only locally in the ultramylonites. The present study documents therefore how the feedback between progressive strain localization and fluid-focusing in extensional shear zones contributes to thinning and exhumation of the mantle during continental rifting and the rift-to-drift transition.