TRACKING THE COMPETITION BETWEEN BOTH MANTLE AND CRUSTAL GEODYNAMICS AND SURFACE PROCESSES IN CREATING THE UNIFORM LANDSCAPE OF THE ANDEAN PLATEAU

Tracking temporal changes in surface elevation has become an important proxy for understanding feedbacks between crustal deformation, mantle accommodation and surface processes. Uniform high elevation and broad low relief topography from southern Peru to Argentina strongly suggest a uniform process for plateau formation. However, shortening estimates, modern day mantle structure and paleoelevation estimates differ in magnitude, space and time. The eastern edge of the Andean plateau in Bolivia was actively shortening and exhuming from 45-25 Ma. By ~28 Ma, 60% of total Andean shortening was completed and the fold-thrust belt began to be buried by its own synorogenic sediments. The magnitude of exhumation that predates burial at 28 Ma ranges from 4.5 to 7 km. Sedimentology from the 2 km thick synorogenic Salla basin which directly overlays the exhumed region indicates the sediment source was from local topographic highs, while oxygen isotope data suggest the basin was at low

elevations through 25 Ma. A thin ~100m thick overlap basin deposited between 22 and 8 Ma records a 4-6 ppm shift in δO18 that may reflect an abrupt change in elevation. At 24 Ma, the Eastern Cordillera became the dominant source area for sediment deposited in the eastern Altiplano region. We propose crustal shortening created topography and facilitated exhumation. This shortening was likely accommodated by the development of a metastable package of eclogite in the lowermost crust and thickened mantle lithosphere at depth The mantle downwelling associated with removal of this material produced a regional downwarping at the surface creating the Salla basin. Similar histories of basin formation and inversion are present in other locations in the Andean Plateau, such as the proposed rapid uplift of the Altiplano at ~6.8 Ma. Multiple uplift events require either the amount and timing of uplifts to vary spatially and/or the magnitude of uplift derived from modern lapse rates to be less than previously suggested. While the sedimentary record on the Andean Plateau, combined with δO18w and D47-derived temperature data make a strong argument for regionally-variable, and diachronous episodes of surface uplift, as well as provide a spatial scale for geodynamic processes, the magnitude of elevation change is uncertain.