QUATERNARY BEDROCK INCISION HISTORY OF THE RIO DIAMANTE, MENDOZA PROVINCE, ARGENTINA

The Rio Diamante traverses the foreland of the Andes in a latitudinal zone across which there are significant changes in morphology, including relief, peak height and mountain range width (~33-35°S). This transition has been attributed both to a major change in tectonic regime (due to along-strike variations in the angle of the subducting slab and pre-existing crustal structures), and to latitudinal climate variations. The neotectonic activity in the foreland of this zone has not been well characterised even though it may help in the determination of which of these controls is most important. We are making use of the link between tectonic processes and the process of fluvial incision to yield information about the rates and spatial distribution of rock uplift along a ~40 km reach of the Rio Diamante, the upstream end of which crosses the mountain front Carrizalito fault. This will help to establish if fold and thrust belt deformation has continued to propagate eastwards in this zone during the late Cenozoic.

The bedrock incision history of the river is being determined by establishing the longitudinal profiles of the straths preserved in the valley. This involves correlating fill and strath terrace fragments along the reach and measuring strath height above the modern river. By obtaining chronological constraints for each paleo-long profile, incision rates will be estimated. These will relate to rock uplift rates if it can be shown that rock uplift, rather than base-level fall or climate change, has been the overriding control on the incision rate on timescales greater than one glacial-interglacial cycle.

Preliminary terrace correlations are based on elevation, relative soil development, and terrace alluvium stratigraphy. Terraces are being dated using cosmogenic ¹⁰Be dating of fill terrace surfaces and ⁴⁰Ar/³⁹Ar dating of volcanic ash units interlayered with alluvium. Geochemical correlation of ash units using trace and RE elements will further facilitate terrace correlation.

Preliminary work indicates that there are at least four fill terraces and two straths which have long profiles that converge slightly downstream. Tentative correlation of the fill terraces to dated glacial deposits in adjacent valleys suggests bedrock incision rates of ~0.4 m/ky in the upper part of the reach and ~0.2 m/ky in the lower part.