Uplift rates accelerations along 23°S Chilean coast in the Quaternary: preliminary results from the case of Mejillones Peninsula

Abstract: Most of the coastal areas along the South Pacific are mainly uplifting due to subduction processes. The geomorphology of the Mejillones Peninsula, located in one of the seismic gaps of northern Chile at 23°S, is characterized by Quaternary alluvial fans, marine terraces, coastal cliffs, and fault scarps, among others. These features are very well preserved due to hyper-aridity conditions recognized in the area from the Plio-Pleistocene and represent the evidence of the uplift during that time. Quaternary marine terraces (QMT) have been studied to understand the permanent deformation of the forearc, in particular the differences in the uplift rates along the coast. A morpho-metric analysis using ALOS-PALSAR remote sensors and local differential GPS data, besides the use of software, as well as fieldwork, allows us to define the best-preserved QMT sequences and the height at which they are found with respect to the current mean sea level. From this, we correlate the platforms of each marine terrace with the corresponding Marine Isotope Stage (MIS) during the Quaternary, and we estimate associated uplift rates in order to study the role of the Quaternary faults in the differential uplift along the coastal area. From our morpho-metric analysis we determined 3 representative areas with well-preserved marine terraces: Punta Angamos (~12x10 km²), Hornitos (8x4 km²) and Punta Chacaya (4x4 km²). Hornitos and Punta Chacaya are both located in the continent, while Punta Angamos is located in the north part of the peninsula. The results show significant differences both in the morpho-structural features and in the estimates of the uplift rates. We have identified at least 13 QMT in Punta Angamos that can be separated into 2 groups: the last 9 platforms would be associated to the last 570 ka, with uplift rates between 0.42 to 0.55 m/ka; and the highest 4 platforms, that would be associated with Early Pleistocene and Pliocene, where it is not possible to obtain reliable uplift rates for the moment. In Hornitos, we have identified 3 QMT, with uplift rates between 0.24 to 0.31 m/ka for the last 225 ka, and in Punta Chacaya, we identified 4 QMT, with uplift rates between 0.14 and 0.29 m/ka for the last 321 ka. We also identified a platform that could be correlated to the last interglacial (MIS 1) in Hornitos and Punta Angamos, with an estimated uplift rate of 0.92 m/ka and 1.7 m/ka respectively. These preliminary results suggest that, for the last ~20 ka, there has been an acceleration in the uplift rates. That change can be interpreted as the result of the distance to the trench – the closer to the trench, the subduction process affects the most –, which could indicate a change in the subduction regime, as well as the Quaternary activity of the Morro and Mejillones faults, among other faults, that allows differential uplift.

Keywords: Morro fault, Mejillones fault, MATLAB, TerraceM, differential GPS.