Paper No. 2
Presentation Time: 9:15 AM
TECTONIC AND CLIMATIC PROCESSES RECORDED IN THE 10BE CHRONOLOGY OF LOW RELIEF SURFACE ABANDONMENT IN SOUTHERN PERU
Arid regions tend to preserve regional low-relief surfaces that have been recognized as key features to understanding tectonic and climatic processes and timescales. Models of low-relief surface formation involve a period of tectonic quiescence during which time the fluvial systems may carve the landscape to a uniform elevation. Fluvial incision into the low-relief landscape results from the puctuation of a tectonically quiescent period by periods of surface uplift. Low-relief surfaces are left as relects of the former uniform elevation surface as rivers continue to incise. The incision, while linked to falling base-level, may occur during periods linked to changes in sediment supply and river discharge. By determining the age of abandoned surfaces, we can identify the onset of a change in the tectonic or climatic setting. An assumption of this model is that the low-relief surfaces are in fact abandoned with no current processes acting on the surface. We have produced a chronology of low-relief surfaces in the hyperarid forearc region of southern Peru between ~14° and 18°S based on detailed mapping and absolute dating with cosmogenic 10Be. Within this hyperarid region, marine terraces and strath terraces reflect Plio-Pleistocene surface uplift. With our new chronology we address 1) the spatial and temporal correlations of the various surfaces, and 2) the correlation of the surface abandonment ages to various regional climatic and tectonic datasets. Multiple surfaces yield 10Be surface abandonment ages spanning 35 ka to >2 Ma with most of the surfaces considerably less than 1 Ma. Surfaces >2 Ma yield erosion rates of <0.1m/Ma. With such vanishingly low erosion rates, it is hard to imagine that these surfaces have been modified much at all during the past ~2 Ma, however incision (uplift) rates of ~0.3mm/yr and multiple surfaces ~35 ka suggest that the hyperarid forearc landscape has been recently modified through surface uplift and climate events. Surface abandonment ages tend to correlate with cold wet periods preceding deglaciation on the Altiplano. Pleistocene surface abandonment is the product of both forearc surface uplift and high-discharge climate periods in the high Andes; while a change in base-level may require river incision, in this hyperarid region, the timing of the incision is linked to climate.