The First Andean Compressive Tectonic Phase: Sedimentologic and Structural Analysis of Mid-Cretaceous Deposits in the Coastal Cordillera, Central Chile (32°50'S)

We document the effects of major mid-Cretaceous to Late Cretaceous compression within the volcanic arc of the western Gondwanan margin. The thinned Early Cretaceous Andean margin underwent rapid thickening and shortening-related exhumation of magmatic arc rocks during compressional inversion of late Early Cretaceous intra-arc basins. Clastic sedimentary and volcanic rocks recording this phase of initial Andean shortening correspond to the Las Chilcas Formation and are interpreted to have been deposited in a proximal retro-arc position. A detailed analysis of these deposits reveals multiple periods during sustained compressional deformation throughout the latest Early and Late Cretaceous, from 105 to 83 Ma. This deformation is evidenced by the exhumation of older units in the Coastal Cordillera, together with the development of contractional structures and a strong sedimentary response involving deposition of approximately 3 km of synorogenic nonmarine clastic deposits of the Las Chilcas Formation. The structures and associated deposits suggest that the strongest uplift and deformation period occurred from 100 to 95 Ma, whereas subsequent Late Cretaceous deformation was less pronounced, possibly a result of eastward migration of deformation. This tectonic phase coincided with similar coeval synorogenic deposits in Chile and other Andean regions, which have been attributed to initial Andean shortening resulting from a major plate reorganization.