Geophysical Survey in Iquique and Alto Hospicio, northern Chile Cities: Tectonic and Geologic Implications.

Abstract:

The crustal deformation nature during the Cenozoic in northern Chile forearc (Coastal Cordillera; CC) is complex due to the several tectonics process involved, such as plate convergence, a continental orocline (Arica bend) and great degree of coupling between Nazca and South America plates. Such deformation could be regionally explained by an EW-extension generally accommodated by trench parallel faults system (TPFS). Also between ca. 19°-22° S, a small but significant NS-shortening produce folds and faults scarps striking orthogonal to plate boundary denominated trench-orthogonal faults system (TOFS).

The main goal of this study is to understand the recent deformation history of the rocks units making up this part of the CC and the structural controls exerted on them by this fault systems. To achieve this, geophysical surveys have been conducted in Iquique and Alto Hospicio cities, both located in northern Chile.

The results obtained by Gravity and Electromagnetics Transient (TEM) surveys are consistent with the nature and geometry of geological units previously identified in the zone. Sedimentary and intrusive rocks shows high values of density and resistivity, whereas volcanic rocks are associated with high values of density but low values of resistivity. This difference could be explained by water hosted by the fractured volcanic unit. Moreover, the sedimentary units are related to low values of density and resistivity.

A first-order feature shown by the gravity anomaly is the existence of a NS sedimentary basin filled with gravels. However, this basin presents several discrete depocentres coinciding with EW faults. This could be interpreted as Quaternary deformation of the CC. TEM data shows changes in resistivity of the subsurface suggesting different relative levels for the same lithological units. These variations can be explained by normal or inverse movements of NS and EW faults that cut and displace all the units.

In conclusion, results obtained from geophysical surveys and their interpretation based on the superficial geology, has allowed us to understand the deformation nature and style and the structural control that TPFS and TPOS exert on the geological units during the Cenozoic.