EXHUMATION, SUBSIDENCE, SEDIMENTATION AND EVACUATION: LINKING SURFACE PROCESSES TO MANTLE GEODYNAMICS IN THE ANDEAN PLATEAU

Uniform high elevation and broad low relief topography from southern Peru to Argentina strongly suggest uniformity in processes that created the ~4 km high, 400 km wide and 1500 km long Andean plateau. Variations in shortening, modern day mantle structure and paleoelevation estimates that differ in magnitude, space and time suggest that the uplift history may not be as uniform as the modern topography. We examine the exhumation, sedimentation and evacuation history of the eastern edge of the plateau in northern Bolivia. The eastern edge of the plateau houses a 100 km wide west-verging portion of the Andean fold-thrust belt (FTB). Cooling ages from Ar/Ar, ZFT and AFT suggest that this segment was active and exhuming from 45-25 Ma. ~60% of total Andean shortening was completed by ~28 Ma as the FTB 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, with strong correlation between thermochronologic and subcrop map exhumation estimates. Sedimentology from the 2 km thick synorogenic Salla basin which directly overlays the exhumed FTB 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. This value is twice that recorded in young 8-6 Ma strata on the Altiplano that have been used to support a plateau wide rapid change in elevation at ~6.8 Ma. We propose initial shortening of the FTB created topography and facilitated exhumation. The mantle lithospheric root, which grew during this early shortening period pulled surface elevations down and created the Salla basin. Delamination and uplift are recorded by the more negative δO18 values recorded in the overlap basin and 26-23 Ma magmatism, such as the proximal 26-25 Ma Quimsa Cruz intrusion.

Multiple uplift events suggest either amount and timing of uplift varied spatially and/or the magnitude of individual events is less than previously suggested. In particular, a localized loss of mantle lithosphere at the eastern edge of the plateau, beneath the Salla basin supports a more varying, piecemeal delamination as suggested by geophysical studies in southern Bolivia.