Oblique Subduction and Strain Partitioning: Tectonic Role of Margin-Parallel and Margin-Transverse Structures at the Southern Volcanic Zone of the Andes

Abstract:

Convergent margins undergoing oblique subduction develop margin-parallel structural domains, which are responsible for the accommodation of long-term crustal deformation within the overriding plate. Although the Andes are no exception, evidence of transverse-to-the-orogen regional structures has been documented along its entire length. The relevance of these structures is widely recognized; however, whether and how they participate in the accommodation of crustal deformation is not yet fully understood. The Southern Volcanic Zone of the Andes (SVZ) is an excellent area to address this problem because it displays regional transverse-to-the-arc structures which play a strong structural control on the spatial distribution and genesis of volcanoes and geothermal springs, which proves the existence of a strong interplay between tectonics and magmatic/hydrothermal fluid flow.

This work aims to determine the nature and kinematics of selected case studies of transverse structures along the SVZ during the different phases of the subduction seismic cycle and their structural interaction with margin-parallel fault systems, particularly the strike-slip, intra-arc Liquiñe-Ofqui Fault System (LOFS). By the implementation of an elastic, regional-scale, numerical model of the subduction at the Chilean margin using the boundary elements method (BEM), fault slip rates and associated displacement, strain and stress fields in the surrounding continental lithosphere are calculated for the case study sites.

Preliminary results show that the LOFS behaves as a dextral-reverse structure during the interseismic period of the subduction seismic cycle, with increasing fault slip rates towards its southern end, where the western block moves at a rate of 7 mm/yr to the north, suggesting that the LOFS accounts for strain partitioning. It is expected that transverse structures, particularly those misorientated with respect to the prevailing stress field, will display considerably lower slip rate and require fluid overpressure to activate. These conditions should be roughly the opposite during the coseismic phase, when misorientated structures become favorably oriented for reactivation and for the favoring of magma/fluid ascent.