Beach ridges U–Th dating in Tongoy bay and tectonic implications for a peninsula–bay system, Chile

Along the Chilean coast, peninsulas associated with bays seem to behave as a complex system. They act as barrier to propagation of megathrust earthquakes along strike. To better understand how such a system works from ocean side to more inland, we investigated the area between the Tongoy bay and the Altos de Talinay in northern Chile (∼30°S). It represents a forearc peninsula–bay system in which a normal fault (Puerto Aldea fault) has been described as accommodating a relative vertical motion between the two parts, the peninsula being uplifted with respect to the bay. We dated shells from beach ridges by U–Th disequilibria in order to compare the bay area to the peninsula area for which 10Be dating of wave-cut platforms are available (Saillard et al., 2009). These indicate: (1) the Puerto Aldea fault activity probably ceased since at least ∼230 ka, implying the bay and peninsula parts are evolving together since then; (2) the uplift rate is variable and has decreased from ∼0.8 m/ka to ∼0.2 m/ka between ∼300 and 100 ka.