Abstract

The emplacement of ultramafic blocks in accretionary complexes poses a geodynamic problem due to their negative buoyancy. In this study, we explore plausible emplacement scenarios for peridotite bodies in the Coastal Cordillera of south-central Chile by combining geophysical observations, numerical modeling and available petrological data for ultramafic blocks exhumed along the subduction channel. The La Cabana massif is the largest serpentinized peridotite complex yet recognized in the Coastal Cordillera, however, its size and petrophysical characteristics remain unknown. The geophysical measurements were performed to determine the size of this body, involving magnetic airborne surveys and electrical resistivity tomography. Inversion data show that the largest ultramafic block in La Cabana is 3 km long, 1.5 km wide and at least 1.2 km deep. This result constrains the characteristics of the block transported by the subduction channel. In the second step, we developed a numerical model for the subduction channel assuming a viscous rheology. In this modeling effort we search for tectonic scenarios that provide adequate conditions for the exhumation of the ultramafic body in La Cabana. These scenarios included a combination of key parameters, subduction angle dip and velocity, subduction channel geometry, rheology and density contrast. Scenarios compatible with the exhumation of La Cabana body type includes channel viscosity range of 10(19-20) Pa s, fast exhumation rates at mantle depths (10-20 mm/yr; >30-40 km), steep subduction angles (30 degrees-60 degrees), subduction channel widths of 3-5 km, density contrast between -200 and -400 kg/m(3) and a body diameter of 1.5 km which is consistent with our geophysical inversions. On the contrary, slow exhumation rates (similar to 1 mm/yr), low subduction angles (15 degrees), high-density contrasts (-400 to -600 kg/m(3)) and bodies larger than 1.5 km wide, are not viable exhumation scenarios. The methodology developed provided insights to infer ancient subduction channel geometries and rheologies that include peridotite.