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.