High Andean deformation, exhumation, and foreland basin deposition during Neogene changes in subduction zone geometry (32°S)

STOCKLI, D.,; ALVARADO, P.; CHELSEA MACKAMAN-LOFLAND; HORTON, B.; CAPALDI, T.; OROZCO, P.,; FUENTES, F; CONSTENIUS, K.,

Texas

Simposio; 7th Annual Jackson School of Geosciences Student Research Symposium; 2018

The southern Central Andes at 27°00?S to 33°30?S represent a premier modern example of flat-slab subduction, high-magnitude thrust belt shortening, and basement-involved deformation in the foreland. Thin-and thick-skinned shortening and uplift of the Andean Cordillera at ~32°S, which locally reaches elevations >6000 m, has been variably associated with Miocene flattening of the subducting Nazca plate, maintenance of retroarc thrust-belt critical wedge taper, and/or reactivation of inherited structures. We present new detrital zircon U-Pb age distributions for synorogenic clastic fill of the Manantiales Basin and apatite (U-Th)/He (AHe) cooling ages for Andean fault blocks to pinpoint the retroarc time-space deformation record and resolve the contrasting kinematic histories and mechanical models proposed for the region. Detrital zircon U-Pb age distributions for the Manantiales deposits constrain foreland basin accumulation from at least 18.6 ± 1.1 to 14.9 ± 3.2 Ma, and show early inversion and sediment contributions from the thin-skinned western Andean cordillera followed by unroofing of the fold-thrust belt in the central and eastern cordillera, consistent with predicted thrust-belt critical wedge deformation patterns. AHe cooling ages constrain thick-skinned uplift and exhumation along the eastern Andean front to ~5-14 Ma. These cooling ages overlap the timing of wholesale shortening and exhumation in the Precordillera structural province to the east, suggesting a regional mid- to late Miocene pulse of thin- and thick-skinned deformation. At 32°S, the deformation pulse coincides temporally with Juan Fernandez Ridge collision, slab flattening, and the resulting inboard sweep of volcanism ca. 9-12 Ma, and may reflect either increased plate coupling or thermal weakening of the upper plate associated with the change in subduction geometry and arc volcanism. Finally, we interpret late Miocene?Pliocene (~2-5 Ma) AHe cooling ages in the hinterland cordillera to reflect passive uplift along reactivated west-vergent structures during the most recent stage of Andean orogenesis.