NATURE OF CRUSTAL DEFORMATION ARISING FROM OBLIQUE SUBDUCTION: STRAIN PARTITIONING, STRIKE-SLIP FAULTING AND SLIVER MOTION IN THE SOUTHERN ANDES

The Chilean margin is characterized by the oblique subduction of the Nazca plate beneath the South American plate at a rate of 66 mm/yr. Most of the deformation arising from this tectonic regime is taken up by slip at the plate’s interface during great megathrust earthquakes. However, seismic and structural neotectonic data show contrasting tectonic styles within the overriding plate, particularly, along and across the latitudinal range between 33°and 47°S. Between 33° and 35°S, directly south of the flat-slab segment of subduction, a compressive, non-partitioned setting is evidenced by structural and shallow seismic data. The intra-arc region between latitudes 35°-47° is structurally dominated by the 1200 km long, margin-parallel, intra-arc, Liquiñe Ofqui Fault System (LOFS) which displays transpressive tectonics. The fore-arc region all along this latitudinal range (33°-47°) displays evidence of NS-trending shortening. From this context arises the fundamental question, what are the mechanical controls on the observed strain partitioning patterns?

Evidence of dextral slip along the LOFS suggests that this structure accounts for strain partitioning taking up part of the margin-parallel component of convergence. As a result, it is hypothesized that the block west of the fault becomes a decoupled fore-arc sliver with northward motion, which encounters a buttress in the thickened crust at 33°S and in the flat-slab region. The deformation gradient arising from northward displacement should result in north-south compression in the fore-arc region and in the intra-arc at 33°-34°S.

To further test this hypothesis we have implemented an elastic numerical model of subduction and related crustal deformation at the Chilean margin using the Boundary Elements Method. By doing so, calculated surface displacements are compared to modern day GPS and long term geologic data. Results show that models including the LOFS fit better geodetic data, which suggests that the LOFS plays a key role in accommodating margin-parallel deformation. We also show dextral-reverse kinematics of the LOFS, which results in a pop-up structure in its southern, duplex-style end. Our model provides and independent constraint of a slip rate of 7 mm/yr for the LOFS localized in its southern end, consistent with the hypothesis of a fore-arc sliver.