Original paper
From extension to compression: high geothermal gradient during the earliest Variscan phase of the Moroccan Meseta; a first structural and RSCM thermometric study
Lahfid, Abdeltif; Baidder, Lahssen; Ouanaimi, Hassan; Soulaimani, Abderrahmane; Hoepffner, Christian; Farah, Aboubaker; Saddiqi, Omar; Michard, André
European Journal of Mineralogy Volume 31 Number 4 (2019), p. 695 - 713
98 references
published: Dec 12, 2019
published online: Sep 5, 2019
manuscript accepted: Aug 8, 2019
manuscript revision received: Aug 7, 2019
manuscript received: Apr 8, 2019
DOI: 10.1127/ejm/2019/0031-2882
Open Access (paper may be downloaded free of charge)
Abstract
The thermal structure inherited from the high-temperature rifting phase may control the deformation regime during the first phase of compression. The Zaian Mountains and adjoining Jebel Hadid constitute a demonstrative case study in the southern branch of the Variscan belt in Central Meseta, Morocco. There the thick, Cambrian-Ordovician quartzite series form a pre-Upper Visean, Eo-Variscan belt of inclined or recumbent folds detached from their Precambrian basement. The post-Serpukhovian, Variscan reworking of the Zaian early fold belt occurred through re-folding and reactivation of previous, reverse or wrench faults. We applied the Raman Spectroscopy of Carbonaceous Material method to obtain the maximum temperature (T RSCM) reached by the rock units from both sides of the Upper Visean unconformity. Results range between 290 ± 9 and 365 ± 7 °C (M = 323 ± 23 °C) in the Cambrian-Ordovician formations, and from ~160 ± 20 °C to 260 ± 10 °C (M = 209 ± 37 °C) in the Upper Visean samples (respectively 11 and 9 samples out of 25). Regional stratigraphy suggests that folding of the Zaian belt occurred during the Famennian-Tournaisian transition beneath a ~2 km-thick Silurian-Devonian sedimentary pile. The T RSCM were probably reached after the ~50% shortening of the Eo-Variscan belt was achieved. A linear geothermal gradient of ~60 °C/km can be estimated. We assume that this gradient was inherited from the Devonian rifting evolution heralded by scattered magmatic activity in a back-arc setting above the subducting lithosphere of the Rheic Ocean.
Keywords
polyphase orogeny • sub-greenschist metamorphism • thermometry • Raman Spectroscopy of Carbonaceous Material (RSCM) • Variscan belt • Zaian Mountains • Morocco