FOREARC BASIN INVERSION BETWEEN THE CHILE RISE AND JUAN FERNÁNDEZ RIDGE CAUSED BY GLACIAL AGE TRENCH FILL

Trench fill and plate convergence rate are major parameters controlling an erosive or accretionary margin. To gain inside into the influence of climate-controlled material flux to the trench, we investigated forearc basins in south-central Chile using coastal exposures and seismic and borehole data from ENAP. Forearc depocenters along the shelf and coast consist of syn-extensional Eocene-Miocene near-shore facies, syn-extensional Tortonian-Zanclean lower bathyal siltstones, and syn-contractional Gelasian-Quaternary near-shore deposits. Growth strata adjacent to seismically-active reverse fault evidence ongoing shortening since ~2.5 Ma. Faunal assemblages indicate ~1.5 km rapid subsidence between ~11-3.6 Ma, and ~1.5 km uplift between ~3.6-2.5 Ma at ~1.3 mm/a–rate that has been fairly steady since. This structural-stratigraphic inversion occurred in the 34-45°S segment. We interpret forearc subsidence as caused by material removal from the based of the continent by subduction erosion. The low relief and slow exhumation during development of Oligocene-early Miocene extensional marine and continental intra-arc basins led to a sediment-starved trench, which in addition to the high convergence rates resulted in an erosive margin. Uplift during the mid Miocene orogenic phase and onset of major global cooling at ~6 Ma triggered glacial denudation and consequently a dramatic increase in sediment flux to the trench. We interpret forearc basin inversion and onset of ongoing compression as the response of the wedge to slope and basal friction decrease by frontal accretion and subduction of water-rich material, respectively, in order to reach a critical taper. The buoyancy gradient caused by northward increasing age of Nazca plate controls trench sediment transport. Margin-parallel currents spread glacial-derived material northward until the colliding Juan Fernández Ridge, which forms a bathymetric barrier. Sediment blocking by this stationary ridge and northward migration of the Chile Rise in the south confined and enhanced >2-km-thick trench accumulation between 34-45°S, limiting accretion and consequently forearc basin inversion. Glacial trench fill and the steady decrease in plate convergence rate shifted this segment of the margin from erosive to accretionary during the Pliocene.