Sr isotopic composition of gypsic paleosols as a proxy for Neogene forearc uplift in the South America ? Nazca plate system at latitudes 19 to 22ºS.

COSENTINO, N.J.; JORDAN, T.E.

San Francisco

Conferencia; 2011 AGU Fall Meeting; 2011

AGU

Quantification of uplift of a continental surface relative to sea levelis challenging. The study area comprises the forearc associated with theSouth America - Nazca plate margin in northern Chile. The CoastalCordillera (CdlC) is a mountain range 800 to 2300 m in elevation thatends abruptly to the west at the Pacific Ocean, terminated at a 400 m to1700 m high scarp. The CdlC is formed mainly of Jurassic - EarlyCretaceous igneous rocks, the remnants of a Mesozoic magmatic arc. Tothe east, the Central Depression (CD) is a forearc basin filled withCenozoic sediments derived from the arc to the east. Arid to hyperaridclimate conditions dominated throughout the Neogene. Profiles fromrivers draining the CD and cutting through the CdlC are stronglysuggestive that at least 1 km of relative surface uplift occurred since10 Ma. Paleogeographic reconstructions of continental deposits, marineterraces and tilted originally horizontal depositional surfaces in theCdlC constrain surface uplift histories. However, we seek quantitativeinformation about the magnitude as well as ages of uplift, so thatnumerical models of forearc geodynamics can be tested against uplifthistory. We are testing a new paleoaltimetry proxy based on the87Sr/86Sr ratio of gypsum-rich paleosols. Published studies show thatmodern pedogenic gypsum in the study area is derived from two sources ofdistinguishable Sr isotopic values; salts precipitate from aerosols inpersistent winter marine fogs and dust comes from the weathering ofAndean rocks. It has been shown for modern soils that a transect fromthe coast to the Andes reveals a progressive decline in 87Sr/86Sr,corresponding to the mixing of marine aerosols and weathered dust. Below1.5 km altitude, the marine signal diminishes as altitude rises. The lowmass difference between 87Sr and 86Sr leads to little fractionation byenvironmental processes, which is ideal for studying the primary marinevs. Andean signal. Because the Neogene strata and landforms conservesets of paleosols that formed during long-lasting hyperarid intervalsbetween ~ 1 Ma and 10 Ma, we hypothesize that we may be able to extractthe elevations at which the gypsic paleosols formed from their87Sr/86Sr, for multiple time intervals. With a grid of additionalsamples of modern gypsisols, we are investigating the variations in theelevation-87Sr/86Sr profiles that may relate to the positions of samplelocations relative to major canyons and salt pans. This pilot study willlead to estimations of the altitude uncertainty associated with given87Sr/86Sr values, and to understanding of systematic errors. 87Sr/86Srratios for a suite of samples from paleosols of varying known agesprovide the first application of this potential paleoaltimeter.Corrections are needed to isolate the paleoaltitudes, including temporalchanges in seawater Sr isotopic composition, eustatic sea level, and theheight of the temperature inversion due both to changes in sea surfacetemperature and regional climate change. If the paleoaltimeter works, weexpect to obtain a better-resolved history of uplift/subsidence for theforearc in northern Chile.