One-Km of Subduction-Induced Subsidence in the Southern Andes at 10 Ma, as Measured Using Hydrogen Isotopes in Hydrated Volcanic Glass

BRANDON, MARK; HU, JIASHUN; FENNELL, LUCAS; LIU, LIJUN; HREN, MICHAEL

Congreso; AGU Fall Meeting 2019; 2019

We report new results for the Cenozoic evolution of topography in the South-Central Andes at ~35 S latitude. Our work is based on a piggy-back basin near Malargüe, Argentina, which provides a continuous record from 55 to 10 Ma. We have separated volcanic glass and measured hydrogen isotopes from 107 samples. Studies over the last several decades have shown that volcanic glass will take up precipitation water by hydration on a 1 to 10 ka time scale. This reaction is irreversible, and later diffusive exchange is too slow to alter the initial isotopic composition. Thus, we conclude that our data provide a record of the isotopic composition of precipitation for most of the Cenozoic. Empirical and theoretical work indicate that the isotopic composition of precipitation decreases in a linear fashion with increasing orographic lifting. We have calibrated this relationship by isotopic modeling of modern water isotopes (152 samples) at 35 S across Chile and Argentina, and that work indicates a lifting relationship of ~20 per mil/km. For reference, the isotopic response due to a typical climate event is comparable to that due to a 0.5 km change in elevation.Our hydrogen isotopic record shows a steady decline with time that matches the isotopic response due to cooling during the Cenozoic. This record otherwise indicates orographic fractionation similar to the modern, except for a ~23 per mil increase over the time interval from 20 to 0 Ma. We interpret these results to indicate that the topography in this region of the South-Central Andes has been steady from ~55 Ma to present, except for an event at about 10 Ma, which involved a reduction and then rebound of about 1 km of topography.Ongoing geodynamic modeling provides independent evidence of a large subsidence event in this region of South America as determined from the history of slab age and subduction velocity, both constrained by plate kinematics. This dynamic subsidence would have affected both the Andes and the eastern ?retroarc? basin. Previous workers have viewed the subsidence history of the retroarc basin as providing a diagnostic record of the growth and decay of orogenic topography, but subduction-induced dynamic topography can produce effects of similar magnitude. In addition, our work clearly refutes the prevailing view that the Andes are a young range, formed during the Miocene.