KINEMATIC DEVELOPMENT AND STRUCTURAL ARCHITECTURE OF THE SOUTHERN CENTRAL ANDEAN FOLD-THRUST BELT (31−33°S): IMPLICATIONS FOR ANDEAN DEFORMATION MODES AND DRIVING MECHANISMS

Andean deformation above the Argentina-Chile flat-slab segment (31−33°S) has been variably attributed to flattening of the subducted Nazca plate, maintenance of thrust belt critical wedge taper, or inversion of inherited rift structures. Conflicting interpretations of deep crustal geometries—including detachment depths for basement-involved structures, location and connectivity of footwall ramps and flats, and even orogenic vergence—have further been proposed. We present new structural interpretations and thermochronological data for the hinterland Frontal Cordillera and flanking Precordillera thrust belt and foreland basin system that provide insights into the timing, geometry, and kinematics of deformation and facilitate interrogation of the potential driving mechanisms. Neogene shortening in the Frontal Cordillera was largely influenced by the inversion of Permian-Triassic extensional structures, which produced a series of broad wavelength (~10−20 km), mechanical basement-cored anticlines exhumed along west-dipping reverse faults. Reactivated basement structures transferred slip to the thin-skinned Precordillera imbricate system along a non-emergent footwall ramp beneath the easternmost basement-cored range. Hinterland structural relationships, foreland basin provenance, and apatite (U-Th)/He thermochronology results indicate sequential reactivation along Frontal Cordillera and potentially western Precordillera faults from ~24 to 14 Ma, followed by regionally synchronous thin- and thick-skinned shortening by ~12−9 Ma. Preliminary apatite (U-Th)/He cooling ages from a strike-parallel transect along the eastern Frontal Cordillera confirm mid-Miocene rapid exhumation spanned 29−33°S, refuting interpretations that deformation advanced southward during progressive flattening of the oceanic plate. Finally, late Miocene-Pliocene (~2−8 Ma) cooling ages along the eastern Frontal Cordillera and Chile-Argentina border reflect (1) late-stage reactivation of basement-involved structures, and (2) passive hinterland exhumation, potentially associated with coeval east-vergent basement reactivation.