Thermal studies on the lower Carboniferous basins of Khenifra and Qasbat-Tadla, Morocco: What do they teach us about the pre-Variscan stages in NW Africa?

Abstract

Within the Variscan Belt of Morocco, the Central Massif preserves Early Carboniferous rift basins. The Lower Carboniferous Khenifra and Qasbat-Tadla basins are aborted rifts, developed just before the Variscan orogenesis in Morocco that occurred during the Pennsylvanian-Cisuralian in NW Africa. Due to both weak inversion of these basins during the Variscan orogeny and limited burial afterwards, these basins offer the opportunity to study the Early Carboniferous pre-orogenic thermal regimes. In the Khenifra basin, 77 samples collected across the basin and its basement's boundaries, allowed the determination of the maximal temperature reached during the rocks burial by means of Raman spectroscopic analyses. The Ordovician basement shows mean temperatures between 230 and 300 °C whereas the upper Visean/Serpukhovian infill has a wide range, from temperatures <160 °C to 250–260 °C. This thermal variation within the basinal series has been evidenced from west to east and cannot have been acquired during the Variscan events. The acquisition of these maximal temperatures occurred between Late Devonian to Upper Visean/Serpukhovian and is thought to result from the formation of an extended rift. In the Qasbat-Tadla basin, Rock-Eval data from Ordovician to Devonian source-rocks indicate significantly lower maximal temperatures reached by pre-Carboniferous samples that are within the oil window. Our results are in favor of a heterodox model for the Variscan belt in Morocco and NW African in general, suggesting that no pre-Variscan compressional events are needed. Instead, the development of the intraplate Variscan belt in NW Africa was permitted through the development of hot and weak Lower Carboniferous basins, subsequently inverted in a far-field stress field. The striking thermal differences between the Khenifra and Qasbat-Tadla basins suggest that important tectonic segmentation must have shaped the area during the Early Carboniferous extensional phase.