Landscape response to tectonic deformation and cyclic climate change since ca. 800 ka in the southern central Andes

Abstract

Theory suggests that the response time of alluvial channel long profiles to perturbations in climate is related to the magnitude of the forcing and the length of the system. Shorter systems may record a higher frequency of forcing compared to longer systems. Empirical field evidence that system length plays a role in the climate periodicity preserved within the sedimentary record is, however, sparse. The Toro Basin in the Eastern Cordillera of NW Argentina provides an opportunity to test these theoretical relationships, as this single source-to-sink system contains a range of sediment deposits, located at varying distances from the source. A suite of eight alluvial fan deposits is preserved along the western flanks of the Sierra de Pascha. Farther downstream, a flight of cut-and-fill terraces has been linked to eccentricity-driven (100 kyr) climate cycles since ca. 500 ka. We applied cosmogenic radionuclide ( 10Be) exposure dating to the fan surfaces to explore (1) how channel responses to external perturbations may or may not propagate downstream and (2) the differences in landscape response to forcing frequency as a function of channel length. We identified two generations of fan surfaces: the first (G1) records surface activity and abandonment between ca. 800 and 500 ka, and the second (G2) does so within the last 100 kyr. G1 fans record a prolonged phase of net incision, which has been recognized throughout the central Andes and was likely triggered by enhanced 100 kyrglobal glacial cycles following the Mid-Pleistocene Transition (MPT). Relative fan surface stability followed, while 100 kyrcut-and-fill cycles occurred downstream, suggesting a disconnect in behavior between the two channel reaches. G2 fans record higher-frequency climate forcing, possibly the result of precessional forcing of climate (ca. 21/40 kyrtimescales). The lack of a high-frequency signal farther downstream provides field support for theoretical predictions of a filtering of high-frequency climate forcing with increasing channel length. We show that multiple climate periodicities can be preserved within the sedimentary record of a single basin. Differences in the timing of alluvial fan and fluvial terrace development in the Toro Basin appear to be associated with how channel length affects fluvial response times to climate forcing and local controls on net incision, such as tectonic deformation.