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

摘要

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 ~(10)Be 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.