| Popis: |
The structural analysis of the Precambrian III (Infracambrian) volcanic formations that outcrop in the Bou Azzer-El Graara anticlinal has revealed two complementary phases: i) a NW-SE distension, ii) N100° to N120°E left-lateral strike-slip to normal faults, parallel to the Pan-African suture. Microtectonic and stratigraphic criteria establish that the tectonic activity is syn-volcanic, while the determination of the principal stress directions prove the genetic relationship between the two phases. Evidences of NW-SE oriented distension in the PIII volcanics of other areas of south Morocco show the regional extension of the tectonic setting. Our model suggests a left-lateral strike-slip between two cratonic blocks that collided during a previous B2 phase (around 600 M.a.). During this strike-slip movement, the principal stress axis sl tilted from horizontal to 50°N while the vertical plane s1 - s2 slightly rotated from N30° to N40°E. The PIII volcanic production was triggered by the extensive wrenching-distending movement and accompanied the reactivation of WNW-ESE faults of the Panafrican basement. Coeval extension along NE-SW faults is marked by the emplacement of numerous andesitic porphyritic dykes along this direction. This rift evolution is part of the global framework that includes two Proto-Atlantic or Iapetus, (2) to the east, in the Trans-Saharian range, the E-W late-Panafrican compression with N-S left-lateral strike-slip. The extension leads to thinning of the crust, existence of a gulf during the Adoudounien transgression and emplacement of alkali magmatism at the base of these transgressive series. |
