PLIO-PLEISTOCENE PLATE-BOUNDARY COUPLING, MEJILLONES REGION, NORTHERN CHILE: LOCAL SUPPORT FOR A CLIMATE-TECTONIC FEEDBACK MECHANISM FOR CENTRAL ANDEAN UPLIFT

The area of the Mejillones Peninsula and adjacent onshore sedimentary basins (~23°S) lies at the coastal margin of the hyperarid Atacama desert region. Along an axis at this latitude, precipitation and sediment fluxes reach minima, whereas depths in the Peru-Chile trench and peak elevations in the Central Cordillera reach maxima. Recent research has suggested that: 1) late Cenozoic hyperaridity in the Atacama region has led to sediment starvation in the trench, 2) sediment starvation enhanced mechanical coupling across the Nazca-South American plate interface, and 3) that this enhanced coupling generated the anomalous elevations and crustal thicknesses of the Central Andes.

The Mejillones Peninsula is a salient of the northern Chilean coastline. Uplifted coastal terraces within the peninsula and marine sediments exposed in the surrounding basins record vertical motions, fault activity, and paleo-oceanographic changes through the Miocene, Pliocene, and Quaternary. Miocene deposits and Pliocene deposits older than about 3.2 Ma show no evidence of large-scale vertical motions or tectonic deformation in the region. Rapid shallowing first is recorded in the Late Pliocene Tiburon basin, along with the onset of displacement on the Mejillones fault, one of a number of regional normal faults that are the dominant structures through the coastal margin and the Coastal Cordillera at this latitude. The latest Pliocene and Pleistocene in the Mejillones is characterized by rapid emergence of the peninsula and internal block faulting.

The onset of tectonic activity in the study area between roughly 4 and 3 Ma coincides with current estimates of the onset of hyperaridity in the Atacama region. Extensive research throughout the Central Andes focuses on refining this climatic history and testing its link to the regional morphotectonics. We interpret the combined evidence of vertical displacements, normal faulting, and a pervasive pattern of differential tectonics at this trench-proximal location and elsewhere to be consistent with strong mechanical coupling along the plate boundary. The onset of this tectonic pattern between about 3-4 Ma provides further evidence suggesting that the climatic-geomorphic system – through sediment starvation in the trench – may have contributed to rapid uplift of the Central Andes.