Active tectonism in the intracontinental Middle Atlas Mountains of Morocco: synchronous crustal shortening and extension

Abstract

Geological field observations are integrated with digital topography, LANDSAT imagery, and earthquake focal mechanisms to investigate the Middle and Late Quaternary tectonism in the intracontinental Middle Atlas mountain belt in northern Morocco. The NE-SW-trending Middle Atlas Mountains, approximately 80 km in width and about 200 km long, are part of the Atlas system of northwestern Africa and represent an inverted rift that developed into an intracratonic mountain system in the foreland of the Alpine collisional zone. The Middle Atlas is composed of two provinces, the Folded and Tabular Middle Atlas, representing the palaeo-rift and a flank of the palaeo-rift, respectively. Evidence for Late Quaternary tectonism is provided by the analysis of stream morphology in addition to geological relations. Kinematic analysis of fault-slip data and earthquake focal mechanisms demonstrate the coexistence of both extensional and compressional deformation in different areas of the Middle Atlas with a common sinistral component of slip along NE-SW-striking fault zones. Compressional features dominate the Folded Middle Atlas, whereas extension predominates in the Tabular Middle Atlas. Extension is also manifested by widespread Middle to Late Quaternary alkali volcanism. The observed kinematic variations appear to correlate with the Mesozoic palaeogeography; one possible model may involve tectonic escape. This suggests that differences in the structures inherited from the Mesozoic and Palaeozoic may influence the responses of the different regions to the Cenozoic Alpine collision between Europe and northwest Africa.

Geoelectrical Tomography Investigating and Modeling of Fractures Network around Bittit Spring (Middle Atlas, Morocco)

“…All these geological trends are seen on the electrical sections. These two tectonic phases were previously described in Morocco and belong to a main lower Triassic event [16][17][18][19].…”

The Canary Islands origin: a unifying model

“…Cherkaoui et al (1991) present an analysis of the focal mechanism of the great 1960 Agadir earthquake (see location in Fig. 7) compatible with the dextral-reverse movement of a N49E fault, though Harmand and Moukadiri (1986) and Gomez et al (1996) propose a left-lateral strike-slip fault as the cause; the last two interpretations are coincident with that of Mezcua et al (1992) for the Canary Islands' quake of 1989. Giese and Jacobshagen (1992) and Beauchamp et al (1999) have proposed that the Atlas chain is the result of an important (>30 km) shortening, during which the Jurassic rift faults became thrusts, newly formed thin-skinned thrusts added to that thick-skin tectonics, and the crust under the High Atlas thick ened to 38-39 km.…”

“…This discontinuous magmatism has given rise to different tectonogenetic hypotheses: Harmand and Cantagrel (1984), Berrahma (1989) and Ait Brahim and Chotin (1990) tried to connect the active periods with tectonic phases; for instance, Harmand and Cantagrel (1984) claim that volcanism and compression are coeval, but the loose tirne-strati graphic control limits the validity of this and similar hypotheses. Gomez et al (1996) proposed that the whole Meseta block (the lithospheric subplate north of the Atlas Mountains) is escaping towards the Atlantic along the Trans-Alboran Fault as a consequence of the compression from the north, a scheme already advanced by Olsen and Schlische (1990). Another tectonic synthesis (Froitzheim et aI., 1988) takes into aCCOlll1t both compression and lateral response, and on the basis of the frequent flower structures found puts forward the hypothesis that the Atlas Mountains has been subjected during the whole Ceno zoic to transpressive and transtensive movements.…”

“…(1) The escape hypothesis of Gomez et al (1996) could explain why the Meseta Atlantic shelf shows signs of instability, such as fold ing, transtensive faulting and a possible Oligocene angular unconformity (Summerhayes et aI., 1971;Pique et aI., 1998). (2) Transpression is the stress field indicated by the fault plane solution of the last earthquake in the Canary Islands (Mezcua et aI., 1992).…”

Geodetic constraints on active tectonics of the Western Mediterranean: Implications for the kinematics and dynamics of the Nubia-Eurasia plate boundary zone

“…Geomorphologic studies of active faulting demonstrate recent tectonic activity in the Rif (Morel and Meghraoui, 1996) and the Atlas Mountains (Gomez et al, 1996;Meghraoui et al, 1998;Gomez et al, 2000), and suggest that most of the present-day convergence is 85 accommodated in the Rif-Betic-Alboran region.…”