Orogen-Parallel Transition From a Decoupled Fore-Arc Sliver to Andean-Type Mountain Chain: Paleomagnetic and Geologic Evidence From Southern Chile (37-39°S)

The Chiloe fore-arc sliver is an approximately N-S elongated crust block detached from South America along the dextral intra-arc Liquine-Ofqui fault zone (LOFZ). The sliver is internally dissected by active NW-SE sinistral faults whose relations with the LOFZ are speculative, also due to widespread fluvioglacial and volcanic blanket hiding the substratum. We focus on the northern LOFZ end and on the Biobio fault, supposedly the northernmost of the sinistral fault set, reporting on the results from field investigation and paleomagnetism of 48 (mostly Oligo-Miocene) volcanic sites. We find that the Biobio fault is an old inherited crust discontinuity that did not yield significant block rotation and deformation during the Cenozoic, thus testifying the end of sinistral shear at about 38 degrees S. At the same latitudes, a northward transition from pure strike-slip to transpressive LOFZ deformation occurs. Intense tectonic deformation and >90 degrees clockwise rotations characterize the main LOFZ strand. Conversely, a supposedly western LOFZ strand displays counterclockwise rotations, similar to the pattern previously documented in the forearc; thus, it does not represent a LOFZ segment. LOFZ and sinistral fault kinematics must be related, and we suggest that crust pushed northward west of the LOFZ escapes laterally toward the trench along the sinistral faults. We also speculate that the northward increasing age of the subducting Nazca plate implies a concomitant decrease of heat transfer on the upper plate, thus an increasing crust rigidity that eventually inhibits strain partitioning and sliver decoupling from 38 degrees S.