Contributions of plate strength and dip geometry on the localization of deformation in Central Andes: a data-driven modelling approach

The southern Central Andes (29°S-39°S) is a key area for understanding the interplay between the oceanic plate and the continental plate and its resulting surface expressions in a subduction zone.  In this area, the dip of the oceanic plate changes from normal subduction (~30° between 33°S and 35°S) in the south to flat subduction (< 5° between 29° and 33°S) in the north. This region displays remarkable along- and across- strike variations in both tectonic and seismic deformation patterns. In this context, the relative contributions of each plate on the localization of the long- and short-term deformation along the mountain belt and its neighbouring regions have been a matter of long-standing debate. To address this issue, we investigated the relative contribution of various key factors to strain localization in the Southern Central Andes, including compositional and thickness variations in the upper plate, sedimentary basins, surface topography, frictional strength of the subduction interface and changes in the dip geometry of the lower plate. Using multiple geophysical approaches and data sources, we have built a series of structural, density, thermal, rheological and integrated them in a thermomechanical geodynamic model to quantify the relative importance of these key factors to strain localization at tectonic and seismic timescales. This forward data-driven modelling approach allows us to reconcile long- and short-term deformation as close as possible with geophysical and geological measurements.

We found that the compositional and thickness configuration of the upper plate, weak inherited faults associated with weak sediments, topography and thickness of the radiogenic crust plays a prominent role in modulating strain location between the flat and steep subduction segments. The flat slab in the northern part of the region, cools and further strengthens the upper plate, preventing the plate from pronounced deformation and propagating the deformation far inland to the eastern edge of the broken foreland. A complex broad shear zone developed at the transition between flat to steep subduction which is associated to the development of a thick to thin skinned foreland deformation style transition at the surface. In addition, the strength of the upper plate ultimately controls the spatial distribution of the short-term deformation occurs above the modelled transition from brittle to ductile conditions and seismicity is localised in regions at the transition between rigid and weak lithospheric blocks, such as the front of the forearc, which acts as a rigid indenter. These results highlight the importance of considering the interactions between the upper and lower plate to better understand multiscale scale deformation processes in subduction zones and their resulting surface expression.