Histoire volcano-tectonique du massif permien de Khénifra (Sud-Est du Maroc central)

Abstract

The volcano-tectonic history of the Khenifra permian massif is subdivided into three very contrasted periods. The first period is characterized by an extrusive andeffusive andesitic volcanic activity. It isrepresented, to the west of the volcanic complex, by pyroclastic breccias deposits with predominating rhyodacitic composition followed by formation of composite domes withhybrid lavas and effusion of flows with olivine and pyroxene-andesite composition. Thesecond period is essentially explosive and rhyolitic. It began by air fall pyroclasticdeposits followed by rhyolitic explosion alloclastic breccias and/or dacitic flows andassociated pyroclastic rocks. The third period is marked by the late intrusion of somepyroxene-doleritic dykes. The first and second periods bear characteristics of an orogenic calc-alkaline and bimodal volcanism. The last period is a magmatic event with an anorogenic and hybrid character.The analysis of fracturation and it’s relationships with volcanic activity, shows that thedistribution of a large number of volcanic emission centers is tectonically controlled. Their localization, parallel to the sinistral strike-slip faults oriented NE-SW suggests that the volcanic emissions occurred along tension gashes developed by submeridian compression wich is responsible for the opening of an Autunian basin. This fact, associated with others such as evidence of Autunian silicified trunk wood and interstratified red sedimentary rocks in the volcanic pile, confirm the permian age of complex. The volca-nological characteristicsof the Khénifra permian volcanism are similar to the continental strato-volcano model facies with a mutiple fissurai vents. The geodynamic context of the Permian volcanismof the study area and, by extension, of the hercynian late-orogenic volcanism of Northern Morocco, is discussed. A relationships with an active or fossile oceanic subduction is not possible; the hypothesis of an intracontinental subduction (type A) is proposed. © 1955 Masson, Paris.