Isotope paleotopography evidence for formation of the Sierras Pampeanas basement uplifts before 55 Ma

MARK BRANDON; LUCAS FENNELL; MICHAEL HREN

Congreso; AGU 2023; 2023

A long-standing idea is that the foreland basement uplifts in Sierras Pampeanas, which lie east of the South Central Andes in Argentina, were formed during the present “flat slab” subduction of the Nazca plate beneath this region. This idea is appealing, but there remains little information about when the basement highs were formed. There is some evidence from thermochronology and 10Be erosion rates that the highs were formed during the Late Cretaceous or early Cenozoic. A recent paper by our group, Fennell et al. (2023, EPSL), reports on the Cenozoic evolution of the 𝛿2H composition of precipitation that fell on the east flank of the South Central Andes. These results support the idea of an old origin for the Sierras Pampeanas basement uplifts. This topic received little discussion in our paper, so we use this opportunity to explain the evidence and conclusion.Our study reported 𝛿2H measurements from hydrated volcanic glass from 107 samples collected from a 55 to 10 Ma stratigraphic section in the Malargüe basin. The results are supported by a large dataset of modern meteoric water samples (n = 197), which were used to characterize the modern relationship between orographic lifting and precipitation isotopes. Numerical modeling of the modern water isotopes shows that precipitation at the Malargüe study area is sourced solely by moist northeasterly winds. The Malargüe section accumulated prior to 10 Ma in the foreland basin east of the Andes and would have had an elevation of about 400 m (like the modern foreland), which is too low to explain the observed isotopic fractionation. We argue that the prolonged record of fractionated precipitation observed at Malargüe is due to the lifting of moist winds coming out of the northeast and flowing over the Sierras Pampeanas basement uplifts. The modern wind path crosses the Córdoba and San Luis basement highs with elevations of 2500 m and 1600 m, respectively. In other words, our Malargüe study provides a record of the topographic evolution of the Andean foreland, and not the elevation of the Andes. This accounts for why we prefer the term “isotope paleotopography”, rather than “isotope paleoaltimetry”.