ALLUVIAL-FAN TERRACES AS STRAIN MARKERS FOR QUATERNARY CONTRACTILE DEFORMATION IN THE INTERIOR OF THE SOUTHERN CENTRAL ANDES: INTERMONTANE ALLUVIAL BASIN FILLS IN THE CALCHAQUÍ VALLEY OF THE EASTERN CORDILLERA (CACHI-PAYOGASTA AREA), NW ARGENTINA (25°03'S

MANFRED R STRECKER; LEONARDO ESCALANTE; HONGN FERNANDO

Toulouse

Congreso; 33rd INTERNATIONAL MEETING OF SEDIMENTOLOGY; 2017

Universidad Toulouse

Intermontane basins in the Eastern Cordillera of the southern Central Andes in NW Argentina constituteoutstanding natural laboratories and their sedimentary fills are excellent archives where the competinginfluences of tectonics and climate on sedimentation and landform evolution can be evaluated. These basinsare located in the interior parts of the Andean orogen and straddle the eastern boundary of the high-elevationintra-orogenic Andean Plateau. To the east the basins are delimited by reverse-fault bounded mountain rangesthat occur in the transition to the broken foreland. One of the largest intermontane basins is the N-S orientedCalchaqui Valley. The basin is bounded by E and W-dipping reverse faults, respectively, and it is characterizedby deformed Tertiary strata that are unconformably overlain by Quaternary conglomerates that are part ofcoalesced alluvial fans. The northernmost sector of the valley narrows considerably and neotectonic activityin this sector of the basin has been thought to be associated with the range-bounding faults. However, recentremote sensing inspection and fieldwork have revealed that tectonic activity also affects the internal sectors ofthe basin. Here, we use the Quaternary sedimentary deposits and associated landforms, such as fluvial terracesand alluvial fans, as strain markers that unambiguously record protracted tectonic activity in this region thathas migrated into the interior sectors of the basin.Different Quaternary alluvial deposits in the region between Cachi and Payogasta (25°03?lat,66°07?long) at successively lower elevations shed light on the history of Quaternary deposition, landformevolution and faulting. These alluvial deposits overlie or are faulted by metamorphic rocks of the ubiquituousPuncoviscana Formation and its metamorphic equivalents: The Quaternary units also cover rocks of the lateCretaceous-Paleogene Salta Group and the Tertiary Payogastilla Group. Three alluvial-fan levels involvingdowncutting and intermittent aggradation (Q1, Q2 and Q3) can be identified, each of them with characteristicconglomeratic composition and clast provenance. Virtually all of these geomorphic strain markers are affectedby the effects Quaternary tectonic activity. Fault scarps, folds, unconformities, and cataclasized and tiltedconglomerates characterize these landforms and their deposits. In addition, changes in paleocurrents andprovenance, and convex longitudinal stream profiles unambiguously demonstrate the impact of Quaternarytectonic activity in the Calchaqui Valley. Importantly, this region broadly coincides with areas frequentlyimpacted by destructive historical seismicity.Taken together, available sedimentological and structural information as well as historical andinstrumental seismicity data show that the northern sector of the Calchaquíes Valley has been activelydeforming during the Quaternary. This emphasizes the need to perform additional systematic investigations ofthe characteristics of the Quaternary deposits and their tectonic overprint at regional scale. Both, conglomeraticgravels and their associated landforms constitute valuable strain mark-ers to achieve this goal and helpunderstand the spatiotemporal evolution of tectonic processes in light of ongoing contraction.Acknowledgments: The research was sponsored by Proyecto CIUNSa 2636, StRATEGy and CUAADAHZ.