4 MA UPLIFT CYCLES OF THE SOUTHERN PERUVIAN FOREARC SINCE THE LATE MIOCENE

We explore coastal morphology along a 500 km long uplifted coastal segment in the central Andes, between the cities of Chala (Peru) and Arica (Chile). We use accurate DEM and field studies to extract uplifted shoreline sequences along the study area. In addition, we consider continental pediment surfaces that constrain both the geographic and vertical extent of marine landforms. We establish a chronology based on published dates for marine landforms and pediment surfaces. We extend this corpus with new 10Be data on uplifted shoreline platforms. We find that the last 12 Ma is marked by three periods of coastal stability or subsidence dated at ~12-11 Ma, ~8-7 Ma, and ~5-2.5 Ma. Uplift that accumulated between these periods of stability has been ~1000 m since 11 Ma; its rate is up to 0.25 mm/a (m/ka). For the last period of uplift only, during the last 800 ka, the forearc uplift has been accurately recorded by the cutting of numerous coastal sequences. Within these sequences, we correlated marine terraces with high sea levels (interglacial stages and substages) up to MIS 19 (790 ka), i.e., with a resolution of ~100 ka. The uplift rate for this latter uplift period increases westward from 0.18 mm/a at the Peru-Chile border to ~0.25 mm/a in the center of the study area. It further increases northwestward to 0.45 mm/a due to the influence of the Nazca Ridge. In this study, we document unusual cyclic forearc uplift with cycles of ~4 Ma in duration. This periodicity is consistent with predictions made by Menant et al. (2020) based on numerical models, and may be related to episodic tectonic uplift (subduction slab detachment) beneath the coastal forearc zone.