AN INTEGRATED STUDY OF FILL AND DEFORMATION IN THE ANDEAN INTERMONTANE BASIN OF NABON (LATE MIOCENE), SOUTHERN ECUADOR

The Nabon basin is part of several Miocene intermontane basins related to Andean uplift, continental are formation and coeval deformation du ring subduction of the Nazca plate under the South American continent. The basin sequence consists of fluvial, alluvial fan, lacustrine, and pyroclastic mass-flow deposits corresponding with intereruptive and s yneruptive stages of basin-fill history. Combined zircon fission-track and palaeomagnetic measurements suggest that the series lies within t he 4r chron (ca. 8.5-7.9 Ma). It overlies unconformably the ignimbrite basement dated at ca. 26-19 Ma. Synsedimentary tectonic deformation i n the basin is manifested by a master reverse fault along the western margin, differential basement block uplift and related sedimentary bed ding geometries such as cumulative sedimentary wedges. The spatial ori entation of the tectonic and sedimentary features indicates that the b asin underwent WNW-ESE maximal shortening during the fill stage. The s hort-lived sediment-fill history may be explained, either by subductio n-related contractional pulses also recognized in the Peruvian and Col ombian Andes or, by the particular position of the basin in the centra l axial part near the weak fault of a regional translational system.