CLIMATIC AND GEODYNAMIC INFLUENCES ON DIACHRONOUS OROGENIC WEDGE PROPAGATION IN THE CENTRAL ANDES

Similar to the Tibetan Plateau but at three-quarters the height and extent, the Andean Plateau provides an ideal natural laboratory for understanding tectonic and surface process feedbacks during mountain growth. The outward expansion of mountain belts illuminates the dynamics of their growth. Plate tectonics drives orogenesis in general, but the space-time pattern and style of deformation is influenced by how climate, geodynamics, and basement structure modify the orogenic wedge. Late Cenozoic exhumation of the Subandean thrust belt, which lies along the boundary between the arid Central Andes and its humid foreland, figures prominently into debates of orogenic wedge propagation. We integrate 12 new (U-Th)/He thermochronometer samples with 25 previously published fission track samples from four Subandean structural cross-sections between 15° and 20°S. Cooling ages vs. structural depth indicate that accelerated exhumation of the Subandes is diachronous south (since 12 Ma) vs. north (since 7 Ma) of the Bolivian orocline. Recent work by others indicates that Altiplano surface uplift is similarly diachronous in the south (16-13 Ma) vs. north (10-7 Ma). Phases of accelerated foreland exhumation indicate forward propagation of deformation; in detail, each phase is coeval with the completion of an abrupt shift to cooler, more arid conditions in the adjacent hinterland. End-member interpretations of the hinterland paleoclimate change are either (1) 2.5 km of rapid surface uplift due to removal of dense lower lithosphere, or (2) gradual surface uplift that drove abrupt changes in climate dynamics/orography once a 3 km elevation threshold was reached. Both of these mechanisms would drive forward propagation of the orogenic wedge by augmenting the hinterland backstop, either through surface uplift or spatially variable erosion. A hybrid model incorporating both mechanisms might be more appropriate. In summary, we suggest that diachronous Subandean exhumation is driven by piecemeal hinterland uplift, orography, and the outward propagation of deformation.