87SR/86SR IN GYPSIC RELICT SOILS AND PALEOSOLS OF HYPERARID SETTINGS AS A PALEOALTITUDE PROXY: RESULTS FOR NORTHERN CHILE (19.5-21.7°S)

We have developed a new altimeter proxy based on the ⁸⁷Sr/⁸⁶Sr ratio of surficial accumulations of salts in hyperarid settings. Under hyperarid conditions like in the Andean forearc in northern Chile at altitudes below ~3000 m.a.s.l., gypsum and other soluble salts form saline soils. The altimeter is based on the first-order topographic control on the extent of inland incursion of fog-transported marine aerosols derived from the Pacific Ocean. Once this fog is evaporated it deposits calcium sulfates with a marine ⁸⁷Sr/⁸⁶Sr signal. At positions in the landscape not reached by marine aerosols, calcium sulfates have no marine source, except by eolian reworking, and have a lower ⁸⁷Sr/⁸⁶Sr. ⁸⁷Sr/⁸⁶Sr values for Holocene accumulations of salts show a bimodal distribution: high values between 225–1075 m.a.s.l. (0.70807 ± 0.00004) and low values below and above those altitude thresholds (0.70746 ± 0.00010). We sampled dated gypsic relict soils and Gypsisols to study post-5 Ma surface height evolution using this method, after correcting for changes in the altitudinal structure of the paleo-fog zone with time.

Locations spanning ~250 km strike-parallel distance within the forearc and at elevations between 450–1650 m.a.s.l. display moderate amounts of altitudinal change during the Pliocene, Pleistocene and Quaternary. Site results include two in the northern zone (19.5ºS and 19.65ºS) that experienced maximum net surface uplifts of 350 m and 600 m since < 2.5 Ma and < 2.6 Ma, respectively. Locations to the south at 21.4°S display an estimated minimum net decline in altitude by 250 m since < 4.5 Ma and 200 m since < 3.6 Ma. Finally, the southernmost locations in the study area resolve a maximum net surface uplift of 250 m since < 6.8 Ma and a minimum net surface subsidence of 50 m since < 5.5 Ma.

These constraints will shed light on the geodynamic processes responsible for surface elevation change, by acting as boundary conditions to numerical models of the Andean forearc.