PRELIMINARY RESULTS FROM VOLCANIC GLASS PALEOALTIMETRY AND MIOCENE STRATIGRAPHY OF THE CENTRAL ANDES, SOUTHERN PERU

In southern Peru, widespread felsic volcanic ashes are well-preserved at multiple stratigraphic levels, providing the opportunity to constrain the uplift history of the Central Andes across the northern Altiplano using hydrogen isotope ratios of hydrated volcanic glass (δD_glass). Conflicting uplift histories of the Central Andes stem from existing paleoelevation studies, many of which use data from a single basin to extrapolate surface uplift across the width of the orogen. Spatially and temporally diverse datasets are needed to quantify surface uplift, which is critical for distinguishing between Andean tectonic models. Here we present preliminary δD_glassvalues from samples collected along a 250-km-long transect across the Peruvian Andes. In addition, we correlate this record to a stratigraphic record of the Moquegua Basin to provide a context for interpreting drainage system dynamics during orogenesis. This is part of an ongoing effort to constrain the timing and rates of surface uplift and associated drainage system evolution of the Peruvian Central Andes in the Eocene-Pliocene.

Hydrogen isotope ratios of 16 hydrated volcanic glass samples from 26-2 Ma show a progressive decrease from nearshore to depleted δD_glass values for samples collected across the high Western Cordillera and northern Altiplano. 20-10 Ma samples from the Altiplano show significantly lower δD_glass values than 20-10 Ma samples from the Western Cordillera. Detailed stratigraphy of Miocene sediments deposited in the Moquegua Basin show the following lithofacies progression: 1) oxidized bioturbated siltstone and cross-laminated sandstone with thinly-bedded evaporite, to 2) intensively soft-sediment-deformed sandstone, topped by 3) a 51-m-thick traction-structured cobble-boulder conglomerate. We interpret this succession to represent a shift from deposition in a mudflat and endorheic lacustrine system fed by rivers with relatively low sediment supply to a fluvial system with high sediment supply, dominated by volcaniclastic material. These stratigraphic observations provide context for paleotopographic changes that will be predicted from future δD_glass values and 3D isotope-enabled climate modeling, which will be placed within an improved chronology to distinguish among the geodynamic drivers for Andean uplift.