RAPID TECTONIC EXHUMATION OF THE CORDILLERA BLANCA, NORTHERN PERÚ

The northern Peruvian Andes are part of the high, elevated Andean orogenic belt that sit above a flab slab and are non-magmatic. This part of the chain has average elevations between 3-4 km, but unlike most other areas in the Andes, local topography is in excess of 6 km, and relief as high as 4 km. This high topography is largely driven by orogen-perpendicular extension. This extension occurs simultaneously with contraction in the Sub-Andean fold and thrust belt to the east at low elevations. Most of the exhumation of the Cordillera Blanca in the crest of the high Andes has been accomplished in the last 5 Ma following intrusion in the Middle Miocene. Forty new fission track (FT) ages and published 40Ar/39Ar indicate rapid cooling from the Pliocene to the Recent. The similarity of cooling ages between the apatite FT, zircon FT, and 40Ar/39Ar, systems indicate the most rapid cooling and exhumation between c. 5 and 2 Ma, and much of this exhumation was accomplished by slip on the range-bounding Cordillera Blanca normal fault. Maximum cooling rates and fastest exhumation (c. 2-5 km/myr) occurred between c. 3.0 and 2.3 Ma; since ~2 Ma, exhumation has slowed (c. 1 km/myr). Exhumation rates appear to correspond to high average elevations, with the most dramatic exhumation appearing to have occurred near Nevados Huascaran, the highest peak in Peru (c. 6800 m), which occurs in the center of the range. The Callejon de Huaylas basin, which sits piggyback on the hanging wall, contains Pliocene sedimentary rocks that record initiation of erosional exhumation and eventually the onset of glaciation. A deceleration in cooling rates since c. 2.0 Ma may be linked to a change in regional tectonic regime from extension to contraction between 2.0-2.5 Ma. We suspect that exhumation of the Cordillera Blanca resulted from gravitational instability driven by regional uplift of the Andean highlands and that exhumation, driven by normal faulting, began at about 5 Ma and has continued to the present.