UC Santa Barbara

Intermontane basins are illuminating stratigraphic archives of deformation, denudation, and environmental conditions within the heart of actively growing mountain ranges. Commonly, however, it is difficult to determine from the sedimentary record of an individual basin whether basin formation, aggradation, and dissection were controlled primarily by climatic, tectonic, or lithological changes and whether these drivers were local or regional in nature. By comparing the onset of deposition, sediment-accumulation rates, incision, deformation, changes in fluvial connectivity, and sediment provenance in two interrelated intermontane basins, we can identify diverse controls on basin evolution. This work focuses on the Humahuaca basin and the Casa Grande basin, two adjacent intermontane basins currently connected by a bedrock gorge through the Sierra Alta, in the Eastern Cordillera of NW Argentina at ~23-24°S. We combine detailed geologic mapping, stratigraphic analysis of measured sections, provenance data, and geochronology to reconstruct the history of deformation, deposition, basin isolation, and incision in these basins. The exceptional time control provided by U-Pb geochronology of numerous volcanic ashes contained within the Neogene-Quaternary basin fill combined with an unambiguous magnetostratigraphic record in the Humahuaca basin enables the comparison of multiple types of datasets, e.g., sediment-accumulation rates, timing of deformation on individual faults, detrital zircon provenance, paleocurrents ,and sedimentary facies, from both basins to discriminate between potential controls on specific events in each basin’s history.

In both basins, sediment accumulation occurred ~4 – 0.8 Ma in response to renewed uplift of ranges that had already experienced an earlier phase of deformation. Both basins experienced temporary channel defeat at the outlet of the basin ~2.5 Ma as a result of increased aridity in the rain shadow of the growing ranges to the east. In the Humahuaca basin, channel defeat resulted in ponding at the outlet, deposition of fine-grained fluvial and lacustrine strata over a larger area, and an increase in sediment-accumulation rates from 2.5 Ma until 2.1 Ma. This event apparently contributed to the integration of the northern Humahuaca subbasin with the southern Humahuaca subbasin, such that rivers previously flowing east into the Andean foreland were diverted south. In the Casa Grande basin, its isolation is recognized from the loss of a distinctive, Casa Grande-specific detrital zircon age peak in the downstream Humahuaca basin. Casa Grande’s isolation lasted from ~2.4-2.1 Ma until <1.7 Ma. Despite the similar timing of deposition, sediment-accumulation rates in the Humahuaca basin were an order of magnitude higher than in the Casa Grande basin. Additionally, the Casa Grande basin strata are relatively undeformed, whereas faults in the Humahuaca basin were active from ~4.4-5 Ma until <1.6 Ma. Segmentation of faults on the western side of the Humahuaca basin results from the presence of E-W-striking Cretaceous normal faults that bounded Mesozoic grabens. The timing of deformation in the Humahuaca basin and bounding ranges is similar to the main phases of deformation in the Eastern Cordillera and broken foreland ~200 km to the south, and large-scale differences in the style and spatial distribution of deformation in these two regions probably reflect differences in the position of each region relative to the Cretaceous Salta rift and the orientation of rift-related normal faults.