CENOZOIC OROGENIC GROWTH IN THE CENTRAL ANDES: EVIDENCE FROM SEDIMENTARY ROCK PROVENANCE AND APATITE FISSION TRACK THERMOCHRONOLOGY ALONG THE SOUTHERNMOST PUNA PLATEAU MARGIN (NW ARGENTINA)

Intramontane sedimentary basins along the margin of continental plateaus often preserve strata that contain fundamental information regarding the pattern of orogenic growth. Sedimentology of the clastic Miocene-Pliocene sequence deposited in the Fiambalá Basin, at the southern margin of the Puna Plateau (NW Argentina), documents the late Miocene paleodrainage evolution from headwaters in the Precordillera, to the west, towards headwaters in the Puna Plateau to the north. Apatite Fission track (AFT) thermochronology of detrital and basement rocks show that the southern Puna Plateau was the source for the youngest middle Miocene detrital population detected in late Miocene rocks and that the margin of the plateau had a relief similar to or higher than at present by ~6 Ma. Cooling ages at the southern Puna margin suggest that exhumation started in the Oligocene and continued until the middle Miocene. If a genetic link between exhumation, shortening leading to crustal thickening and uplift exists, then a high elevation region might already have been in place by middle Miocene time. Although not recorded by AFT, there certainly there has been some amount of uplift since then as suggested by independent paleoelevation data to the north and as documented by a regional paleodrainage reorganization observed in the Fiambalá Basin and other intramontane sedimentary basins along the plateau margin. Thus, while reverse-fault bounded ranges and intervening, internally drained basins are a typical feature of the present-day plateau morphology, the foundation for this setting may already have been attained in Oligo-Miocene time. The pronounced changes at the immediate eastern plateau margin in late Miocene-early Pliocene time suggest instead that wholesale plateau uplift may have affected this region subsequent to the earlier period of distributed shortening and crustal thickening. However, due to the arid climate and associated reduced erosion rates in the Puna region, this younger event is not reflected by AFT data documenting a decoupling between late Cenozoic uplift and exhumation and implying that the region has experienced less than 3 km exhumation and erosion since the middle Miocene.