Tectonic fracturing and paleostress reconstruction in the Tazekka Variscan massif (Morocco): implications for magmatic processes

Abstract

The Tazekka Variscan massif is formed by Meso-Cenozoic cover unconformably overlying the Paleozoic rocks recording polyphase tectonic stages contemporary with the emplacement of the magmatic system such as rhyolitic lavas and microdioritic dyke swarms attributed to the Upper Paleozoic tectonic phases and alkaline basaltic lavas attributed to Plio-Quaternary age. The mesostructural and tectonic analyses associated with paleo stress reconstruction are mainly based on the processing of tectonic measurement of slickensides fault planes with striation pitch determined by inversion of fault-slip data and allowed us to highlight the following tectono-magmatic events: (i) the Visean–Namurian phase is divided into two tectonic episodes. The first episode is characterized by NW–SE trending extension, while the second is marked by a relatively minor subhorizontal σ1 compressive axis oriented NE–SW (σ1–σ2 permutation). This latter episode controls the tectonic dislocation of Paleozoic rocks and the emplacement of a few rhyolitic lava flows along submeridian strike-slip faults; (ii) in the Stephanian–Permian age, the tectonic regime changes to a compressive stress state with a horizontal NE–SW trending σ1 axis and a horizontal NW–SE trending σ3 axis, which is what causes the development of NW–SE and NE–SW to ENE–WSW faults filled with microdioritic dyke swarms; (iii) the Permian–Lower-Middle Triassic period was marked by a compressive regime with a horizontal NNW–SSE to submeridian trending σ1 axis and a horizontal ENE–WSW to subequatorial trending σ3 axis, as well as NNE–SSW, WNW–ESE faults, and submeridian microdioritic tension gashes. The compression changes to WNW–ESE during the Vallesian or Lower Pliocene age. Early-Middle Quaternary is divided into two tectonic episodes with a σ1–σ2 axis permutation. Its axis changes from NNW–SSE associated with the ENE–WSW extension and is characterized by NE–SW faults affecting basaltic lavas. Subequatorial strike-slip faults, submeridian normal faults, and Late-Variscan brittle tectonic structures are mostly filled by basaltic lavas.