NEOTECTONIC EVOLUTION AND SEISMOTECTONICS IN THE RIF-ATLAS SYSTEM (MOROCCO): PRELIMINARY DATA

We performed a survey along the major recent faults and local travertine deposits of the Rif-Atlas system. The systematic correspondence of thermal sources and travertine deposits with the major structures confirms the role of these structures as preferential ways for hydrothermal circulation. Pliocene travertine deposits are mostly located along NE-SW faults, and are more extended than the Quaternary ones. These latter, observed in deposition in the overlap zone between the Ride Prérifaines of Meknes and Fes, and active thermal sources as well, are mostly located along E-W trending structures. The reduction in size of the travertine deposits from Pliocene to Quaternary, and shifting of hydrothermal activity to the E-W faults, could indicate a similar shifting of tectonic activity.

Structural-geological field-work and remote sensing analysis indicate that the E-W-trending faults are the most recent ones. Also the thrusting of the Rif over the Middle Atlas, in the Taza area, and the indentation of one of the Middle Atlas blocks against the E-W trending High Atlas northern front, near Ouaouizaght, indicate a more recent activity of E-W fronts with respect to the more ancient NE-SW structures. A comparison of field data and historical reports of the 1755 ‘Meknes earthquake’ allow to recognise a possible event of coseismic surface faulting along the Rides Prérifainesin the El Zerhoun and Zalagh areas, both located within the E-W elongated epicentral area. The activation of the structures of the Rides seems therefore the most probable cause of that earthquake.

Therefore, our observations, although preliminary, argue for a shifting of tectonic activity from the NE-SW trending faults of the Middle Atlas to the E-W oriented structures of the Rides Prérifainesand High Atlas, and indicate active N-S-oriented shortening. This would also be consistent with the focal mechanisms of earthquakes in the region.