UPLIFT AND EXHUMATION OF THE CORDILLERA HUAYHUASH, NORTHERN PERU, BASED ON U+TH/HE AND FISSION-TRACK DATING OF ZIRCON

The Cordillera Huayhuash is a 30-km-long, north-south oriented mountain range that sits along the crest of the Northern Peruvian Andes. The range has distinctive high topography with the highest peak, Nevados Yerupaja, with an elevation of 6,617 m, but many peaks in excess of 5500 m, well above the surrounding areas that have an average elevation of c. 4000 m. The bedrock of the area is dominated by a folded and thrusted sequence of Mesozoic quartzites and limestones that is unconformably overlain by Mid-Tertiary volcanic rocks. This entire sequence is intruded by late Tertiary granitic rocks and their possible extrusive equivalents. A zircon fission track (ZFT) transect beginning on the west side of the Cordillera Huayhuash and ending in the core of the range was completed to assess the thermal evolution of exhumed and uplifted rocks. Fourteen granite, quartzite and volcanic rock samples were collected along the 50 km transect. These samples will elucidate the cooling ages of the rocks through ~240° C for ZFT and c. 180° C for zircon U+Th/He (helium) dating. The ZFT ages from quartzites and granites from the core of the range (5 samples) yield single peak ages ranging between 10-14 Ma. To the west, away from the core uplift, variable resetting shows an interesting thermal history with cooling ages from the Cretaceous to the late Tertiary. Principle cooling ages, which must represent thermally reset ages occur at c. 26 Ma, 42 Ma, and 62 Ma. Older populations of grain ages of 139 Ma and 262 Ma are of limited utility because the amount of resetting cannot be determined. Several of the younger age peaks including ages from the core correspond with known tectonic and magmatic events that occurred in the Northern Peruvian Andes. Cooling ages in the core of the range correspond to a well-known Miocene compressional tectonic pulse at c. 10 Ma, and it is likely that the range has formed since this cooling. Two other compressional tectonic events are well recognized in this part of the Andes: 1) Late Oligocene, c (26-28 Ma) and 2) Late Eocene (c. 42 Ma). Both ages correspond with two of the prominent age peaks dated, suggesting that contraction across the range results in exhumation and rock cooling at depth. The earliest Tertiary cooling age (62 Ma), presumable corresponds to cooling a relaxation after Late Cretaceous thrusting.