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.
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