Quaternary landscape evolution in the Western Argentine Precordillera constrained by 10Be cosmogenic dating

Abstract

Alluvial fans of the western piedmont of the Western Argentine Precordillera developed during the Early-Middle Pleistocene are excellently preserved markers of Quaternary-active structures, such as the N-S strike-slip El Tigre Fault and minor piedmont faults and folds. A temporal model was obtained to evaluate the Quaternary tectonic and climatic control on the landscape evolution by integrating alluvial fan exposure ages (10Be), topographic scarp measurements, fluvial morphometry data, and slip rates. Geomorphic features and cosmogenic dating (10Be) allowed identification of five alluvial abandonment events: 1) Q5: 926 ± 38 ka; 2) Q4: 767 ± 29 ka; 3) Q3: 592 ± 54 ka; 4) Q2: 352 ± 27 ka; and 5) Q1: 258 ± 23 ka. This chronology suggests that alluvial fan systems typically became inactive at the beginning of or during interglacial periods. Thus, aggradation seems to have occurred during glacial stages and dissection during interglacial ones. Inset alluvial fan surfaces suggest a sustained base-level lowering linked to low-rate regional uplift superimposed onto the climatic control. Topographic scarp measurements and 10Be exposure ages allowed estimation of intermediate-term (105–106 y) slip rates linked to six neotectonic events during the Pliocene and Early-Middle Pleistocene. Longitudinal river profiles and drainage pattern analysis constrained by 10Be exposure ages confirmed that the El Tigre Fault has tectonically controlled the fluvial network since the Middle-Pleistocene. A Holocene tectonic event was recorded by the presence of a current rectangular drainage pattern, numerous knickpoints and highest channel steepness index (Ksn), spatially associated and controlled by the minor piedmont scarps. The current drainage pattern and main river profiles show a stable and mature configuration. However, the landscape is still developing, being rejuvenated by piedmont structures and by the Quaternary activity of the Sierra del Tigre backthrust. Uplifts and shortenings for the El Tigre Fault scarp ranged from 0.454 ± 0.088 to 0.661 ± 0.130 and 0.122 ± 0.024 to 0.241 ± 0.047 mm/a, respectively, in agreement with estimates from similar structures of the Precordillera. Uplifts and shortenings for minor piedmont fault and fold scarps ranged from 0.009 ± 0.001 to 0.049 ± 0.005 and from 0.004 to 0.028 ± 0.003 mm/a, respectively. Our results confirm that the alluvial fan surfaces and the current fluvial landscape are driven by both tectonic and climatic controls. Finally, we propose that Quaternary-active structures accommodate part of the crustal shortening in the Pampean flat-slab segment, although the study area represents a backthrust zone of the current deformation front.