Coastal Tectonics across the South Atlas Thrust Front and the Agadir Active Zone, Morocco

Abstract

Abstract Quaternary marine terraces of the South Atlas Thrust Front near the Agadir seismogenic region are investigated by detailed geological mapping, precise height measurement and tectonic analysis. A coastal step-like morphology with four main benches (Q 1 -Q 4 ) crosses the Ait Lamine-Kasbah anticlines, 10 km north of Agadir. U-Th dating of 12 fossil samples (mainly molluscs) yields two main groups of ages for marine terraces Q 4 and Q 3 and can be correlated with oxygen-isotope stages 5 and 7 respectively. Terrace height changes significantly near the Kasbah fold, and attains a maximum of 18–28 m for Q 4 and 35.5 m for Q 3 . Estimates and measurements of terrace elevation yield an average late Quaternary uplift rate of 0.1–0.2 mm/year. Modelling of anticlinal structures suggests that the Kasbah fold may have developed as a fault-propagation fold with a low dip angle of 25° or a listric geometry as implied by flexural slip faulting. Surface ruptures associated with the 1960 ( M s 5.9) earthquake coincide with flexural slip faulting showing 4–5 m offset of terrace Q 3 . Incremental movements and uplifted marine terraces on the Kasbah fold are likely to occur during large earthquakes related to the 25 km long flexural slip fault.