EXPLORING THE IMPACT OF SHALLOW SUBDUCTION ON THE OVERRIDING PLATE IN THE CENTRAL ANDES OF PERU (Invited Presentation)

Shallow and flat subduction is observed in modern ocean–continent subduction zones, such as Mexico and South America, and proposed for analogous ancient margins, such as during Laramide deformation and Ancestral Rocky Mountains uplift. However, not all modern zones of shallow or flat subduction result in Laramide-style inboard and basement-involved deformation. The geologic and geophysical effects on the overriding plate from ancient shallow or flat subduction remain unclear. The southern Peruvian Andes host the site of a proposed Eocene–Oligocene shallow subduction event. New thermochronologic data, and a synthesis of basin and structural data, test the geologic impacts of this proposed shallow subduction event. Previous workers provide conflicting constraints on the timing and cause of shallow subduction, as well as the impact on the volcanic arc. Eocene exhumation previously proposed in the Eastern Cordillera has been temporally linked to the shallow subduction event. This zone preserves pre-Andean extensional and compressional deformation events. New Paleocene thermochronologic cooling data from the Eastern Cordillera pre-date shallow subduction, highlighting the complicated role of structural inheritance on subsequent deformation. The Cusco and Ayaviri basins in the Altiplano accumulated sediment during the proposed shallow subduction event, and reveal contrasting sediment accumulation styles and magnitudes during shallow subduction. Both experienced near-synchronous, yet temporary, cessation of sediment accumulation at the proposed end of shallow subduction. Finally, the magnitude of observed tectonic subsidence in the Ayaviri basin is approximately half of the magnitude expected from flexural modeling, suggesting that shallow subduction may modify basin subsidence behavior in the overriding plate. This segment of the Andes highlights that a spectrum of structural and basin responses to changing subduction zone geometry is to be expected.