EVIDENCE FOR A LATE MIOCENE-PLIOCENE PULSE OF EXHUMATION FROM LOW-T THERMOCHRONOLOGY IN THE PERUVIAN ANDES

Identifying the relative contributions of tectonic and climatic processes to orogenic evolution is critical to our understanding of how and why topography forms. The South American Andes represent the classic orogenic setting of oceanic-continental collision, yet the most fundamental processes that control the generation of high topography are still poorly understood. Quantifying exhumation rates in orogenic settings is an important way to assess the nature of rock uplift and when combined with techniques that measure the degree of surface modification (i.e. in situ produced radionuclides) can be used to develop records of surface uplift. In this study, we present three new zircon and apatite (U-Th)/He age-elevation transects from the northern Peruvian Andes. By using both apatite and zircon we are able to constrain temporal changes in the rates of exhumation at three different locations spanning two degrees of latitude (~6.5ºS to ~8.5ºS). In the most northern field site, located on the Rio Marañon, our results indicate very slow exhumation from the Mesozoic to the Miocene. Since the mid-Miocene, exhumation has increased to a constant rate of ~0.3mm/yr. The southern location is within the Western Cordillera and our data show an exhumation rate of ~0.05mm/yr during the Miocene and a recent, rapid increase to ~2mm/yr since the Pliocene. This study presents the first data of this kind from this region and supports the notion of a significant acceleration in exhumation from Miocene time to the present. Prior studies in the central and northern Peruvian Andes have suggested a tectonic history consisting of five or more short, compressive pulses of activity separated by longer periods of quiescence or extensional stress (Megard et al., 1984; Noble et al., 1985, 1990; McKee and Noble, 1982; Wise et al., 2008). However, it is unclear if and/or how the most recent tectonic pulse (~8.7Ma; Wise et al., 2008) that has been identified is related to the increase in exhumation our data show. Instead, an increase in climate-driven erosion during the late Miocene-Pliocene may be responsible for the acceleration in exhumation rate we document here.