Exhumation of the Cordillera Blanca Batholith (Perú). New insights from thermo-kinematic modeling.

The Cordillera Blanca Batholith (CBB) is a >160 km long, NW-oriented granodioritic body intruded along the western half of the Marañón fold-and-thrust belt (MFTB) in the Central Andes of northern Perú. This segment of the Andean orogen is characterized by a flat-slab, highly coupled, subduction zone controlled by the collision of the Nazca ridge since mid-Miocene times. The available U-Pb and Ar-Ar crystallization ages, in zircon and hornblende/biotite, respectively, indicate that the emplacement of the CBB took place between 4-8 Ma. Moreover, paleo-barometric studies in amphibole samples indicate emplacement depths ranging from 3.5 to 7 km, below the paleosurface of the MFTB. A major west-dipping, high-angle fault scarp that bounds the western side of the CBB has been used to support the interpretation of normal faulting (e.g., core-complex style) as the main tectonic driver for the 1.75-2.5 mm/a exhumation rates calculated from low-temperature (FT and U-Th/He in zircon and apatite) thermochronological modeling.

Recent field campaigns have demonstrated that normal faulting is confined to the boundary between the CBB and the Santa river valley, with no evidence of widespread associated extensional features throughout the region. Furthermore, the uppermost stratum of the CBB systematically corresponds to the regional décollement level of the MFTB (hornfelsed Jurassic mudstones), suggesting that the emplacement occurred at the interface between the MFTB and the regional basement. Furthermore, detailed kinematic analyses of the mylonitized contact of the granodioritic body reveal low-angle and normal sense of shearing towards SW and NE along the western and eastern margins of the CBB, respectively. New U-Th/He thermochronological dating in zircon (ZHe) of samples retrieved from complementary sectors of the CBB, but focusing on its less studied eastern sector, have been obtained showing cooling ages roughly ranging between 3-5 Ma. Remarkably, cooling of both sides of the CBB occurred almost synchronously, being just 1-1.5 Ma older on the eastern side.

Several thermo-kinematic scenarios for a segment in the central part of the CBB have been modeled with Pecube by combining the obtained ages with twelve available AFT ages from the literature. While the simplest, low-angle, normal faulting model fits better the obtained ages, it fails to explain the field observations and implies fault displacement of more than 12 km that would necessarily require development of widespread extensional features. A series of alternative models implying contractional tectonics (e.g., deep-seated, blind thrusting of the basement) aiming to explain both structural data and cooling ages are presented for discussion in this contribution.