FISSION-TRACK EVIDENCE FOR RAPID TECTONIC EXHUMATION OF THE CORDILLERA BLANCA, PERU

The arc-parallel, west-dipping Cordillera Blanca Normal Fault (CBNF) extends for ~210 km along the crest of the Andes in the non-magmatic part of the chain above a nearly flat slab. The fault clearly marks the abrupt transition into the high topography of the Cordillera Blanca, which is the exhumed footwall with elevations >6500 m and typical relief >2500m. Footwall rocks consist of the Late Miocene Cordillera Blanca Batholith (CBB) and Mesozoic metasediments. Apatite fission-track (AFT) analysis of samples from the range yield cooling ages from 1.8 to 4.2 Ma, and therefore indicate rapid cooling of footwall rocks. In the southern end of the range, which has lower average elevations, a quartzite from Pastoruri (4960 m) gives an age of 4.2±3.5 Ma. Our youngest ages come from the center part of the range, which is the widest segment. Granites from Cojup (4150 m) give an age of 1.8±0.7 Ma, and 2.7±0.5 Ma from Ishinca (4930 m). To the north, at the base of Nevados Huascarán (highest peak in Peru), ages are slightly older than the central section (but within error): apatite from granites at Llanganuco (both c. 3855 m) yield FT ages of 2.9±0.5 and 3.0±0.6 Ma and a mylonitic granite from the fault zone yields an age of 2.3±0.9 Ma. Assuming closure occurred at 3.2 km, and an average AFT cooling age of ~2.8 Ma, these data indicate that the overall exhumation rate for the last several million years would have been ~1.1 km/Myr. Our lowest sample gives an exhumation rate of c. 1.5 to 2.0 km/Myr, which might be more typical of the central segment of the range which has the largest area over 5000 m. FT ages from zircon from the same samples indicate similar rates of exhumation extended into the Early Pliocene and Late Miocene. Assuming a 35° dip on the fault, and 3.2 km of exhumation, the total slip on the CBNF has been ~5500 m in the last 2.8 Ma. This slip integrated over the average cooling age gives a slip rate on the CBNF of approximately 1.9 mm/a, which is similar to estimates based on offset moraines. From these data we cannot determine paleoelevations, but the first debris clearly related to erosional exhumation of the CBB is in Late Pleistocene moraines, which may suggest that topographic development and large-scale glacial incision was very late in the development of this range (<1 Ma).