PLIOCENE TO PRESENT DEDUNDATION IN THE CORDILLERA BLANCA, PERU: INTERACTIONS OF CLIMATE AND TECTONICS IN A TECTONICALLY ACTIVE GLACIATED MOUNTAIN RANGE

The Cordillera Blanca of north-central Peru is host to the highest peaks in Peru, active extensional faulting, and the largest volume of tropical glaciers. As such, this region has been the subject of many recent studies documenting various types of chronological information spanning multiple timescales. In this study, we investigate the spatial and temporal patterns of surface denudation and range uplift by combining new ¹⁰Be derived basin-averaged erosion rates, ¹⁰Be surface exposure ages, (U-Th)/He thermochronology of apatite and zircon, and field and remote mapping, with existing regional datasets. Clear north-south trends in range elevations, relief, and hillslopes are evident through the analysis of digital elevation maps, however, ¹⁰Be basin-averaged erosion rates (~10³-10⁴ year timescale) and apparent exhumation rates (~10⁶ year timescale) derived from thermochronologic age elevation relationships do not follow the observed trends in range parallel morphology. Instead, both erosion and exhumation rates are quite similar both spatially along the length of the range, and also in magnitude; the range of exhumation rates (~0.1 to 0.5 mm yr^-1) is almost identical to the range of ¹⁰Be basin-averaged erosion rates (~0.1 to 0.4 mm yr^-1). Both the northern and southern portions of the range display an apparent exhumation rate increase ca. 4 Ma, most likely due to a change in regional tectonic uplift, normal faulting, or possibly the onset of glaciation. Differences in pre-4 Ma time-temperature paths from the northern and southern portions of the range require markedly different cooling histories, likely due primarily to conductive cooling of a shallowly emplaced batholith. ¹⁰Be surface exposure ages of fluvially carved bedrock and offset markers yield Holocene river incision rates ~1-2mm yr^-1 and fault slip rates of ~0.6-5mm/yr. Taken together, we see regionally consistent rates of denudation through erosion, rock exhumation, river incision, and active normal faulting on the order of ~0.2-2mm yr^-1 during the Pliocene to Present corresponding temporally with fault initiation, the onset of glaciation, and ongoing regional uplift since at least the mid-late Miocene.