STRUCTURAL ASPECTS OF THE MIDDLE AND THE HIGH ATLAS (MOROCCO) - PHENOMENA AND CAUSALITIES

Since the beginning of the Mesozoic the structural development of the Middle Atlas and the central High Atlas was controlled by a pre-existe nt fault pattern, which was reactivated repeatedly in various manners. The fault pattern is characterized by two main directions. The first runs 35-45-degrees and dominates in the Middle Atlas, the second runs 70-degrees and is common in the southern part of the central High Atla s; between these regions both directions overlap. In the Atlas of Demn at, Beni Mellal and El Ksiba at the northern border of the central Hig h Atlas a further direction, the one of 120-degrees, locally gains in significance. Into the gores of the pattern commonly magmatites intrud ed during the Mesozoic, in the course of the following compressive def ormation often a cleavage was formed there. The compressive deformatio n started in the Oligocene; the direction of the main compressional st ress sigma(1) lay at 160-degrees. Dependant on their orientation to si gma(1) the pre-existent faults reacted differently on this stress: The 70-degrees faults were reactivated as upthrusts, the 35-degrees ones as sinistral oblique slip reverse faults and those of 120-degrees as d extral oblique slip reverse faults. At the northwestern border of the hinge area between the Middle and the High Atlas this scheme of moveme nts was complicated by an interference with movements along the Transa lboran fault system. Because of changes in the style of deformation th e so-called B-B fault zone, running ENE from El Ksiba to Aghbala, can be defined as the structural border between the High and the Middle At las.