COMPARING FORWARD MODEL TECTONIC BOUNDARY CONDITIONS IN AN OBLIQUELY-CONVERGENT MARGIN: A CASE STUDY FROM THE SOUTHERN ANDES

Limited geologic and geophysical data constraining crustal deformation in the southern Andes pose a challenge in understanding the fundamental controls on regional strain partitioning. We have developed 3D forward models, constrained by available surface velocity data, to gain further insight into these mechanical controls. The central and southern Chilean Andes (22^oS-47^oS) are characterized by marked latitudinal strain partitioning styles, spatially coincident with dip variations of the subducting slab. North of 33^oS, low angle subduction is accompanied by crustal thickening, whereas the southern region (33^oS to 46^oS) encompassing the Southern Volcanic Zone (SVZ) is characterized by a transition to a ~30^o dipping Nazca plate, predominant margin-parallel strike-slip faulting, and associated volcanism. The main structural feature south of 38°S is the Liquiñe-Ofqui Fault System (LOFS), one of the longest strike-slip faults in any subducting margin worldwide. The LOFS extends for ~1200 km, roughly the length of the 1965 M9.5 Chilean earthquake, terminating in the north near the southern edge of the 2010 M8.8 Maule rupture. The LOFS plays a vital role in accommodating oblique convergence, partitioning strain between E-W coseismic slip on the subducting interface, N-S shortening in the coastal cordillera, and dextral transpression within the SVZ. Mechanical interaction between the LOFS, the plate interface, and smaller faults in the South American plate strongly influence geologic hazards, yet most available GPS data cover the area north of the LOFS, yielding limited constraints on present day crustal deformation to the south. We use 3D Boundary Element Method models of the subducting plate and the LOFS to differentiate between tectonic boundary conditions driving deformation by comparing modeled surface velocities to published geodetic data. We compare models driven by a uniform remote strain field to models driven by prescribed slip on a detachment surface representing the oceanic Moho. Our results suggest that, while both methods are effective in representing fault slip style, prescribed velocities on the detachment surface more closely reproduce the observed surface velocities. The results also illuminate the role of subducting slab geometry and the LOFS on regional strain partitioning.