MIOCENE WEDGE-TOP SEDIMENTATION ADJACENT TO OUT-OF-SEQUENCE THRUST FAULTS IN THE MANANTIALES BASIN, SOUTHERN CENTRAL ANDES (~32 °S)

The Manantiales basin contains > 4 km of late Eocene–Miocene sedimentary strata that accumulated during construction of the highest topography in the western hemisphere, located in the southern Central Andes. A well-preserved Early–Middle Miocene section provides information about the relative timing of initial uplift of the Frontal and Principal cordilleras—a question framed centrally in the debate over the vergence of the Andean orogenic wedge.

We report field and analytical results from the underexplored northern portion of the basin. Geochronology data limit deposition to ca. 39–11 Ma, with ~3 km of sediment accumulating ca. 18–14 Ma. Sedimentological observations indicate this portion of the basin was characterized by eolian dune fields in the late Eocene, followed by a likely Oligocene hiatus. Between ca. 18.5 Ma and ca. 14 Ma, depositional environments in the basin evolved from rapidly aggrading axial river systems to playas, perennial lakes, and finally progradational alluvial fans. Syndepositional unconformities indicate portions of this sediment accumulated on top of growing thrust-belt structures. Detrital zircons and conglomerate clasts indicate sediment sources evolved from the western portion of the orogen to sources near the basin margin. Volcanic glass from interbedded tuffs shows a marked decrease in hydrogen stable isotopic ratios across the lower portion of the stratigraphy, indicating ca. 18 Ma surface uplift elevated the basin to ~1900 m ASL.

We find that facies and drainage patterns, sediment accumulation rates, and surface elevations in the basin diverged from the regional foredeep by ca. 18 Ma. We propose a model in which the Manantiales basin originated as part of the regional foredeep depozone and was transferred onto the top of the orogenic wedge, occupying a depression created by uplift of the Cordillera del Tigre to the east. While this model implies westward-younging Early Miocene deformation, regional foreland basin provenance and subsidence trends are inconsistent with west-vergent orogenic wedge models. Rather, these data are parsimoniously explained by fold-thrust belt propagation followed by out-of-sequence thrusting within an east-vergent orogenic wedge, consistent with predictions of Coulomb wedge models incorporating syntectonic wedge-top sedimentation.