Beryllium-10 geochronology of geomorphic features within the western central Andean margin: Tectonic and climatic implications.

摘要

The striking topography of the Andes Mountains and the Altiplano, the world's second largest high plateau, are mainly related to late Cenozoic uplift and deformation along western South America. While we know that this orogen is generally a product of long-lived subduction, there are still a number of unanswered questions regarding the amounts, timings, and styles of deformation contributing to the formation and consequent degradation of the topography. As both tectonic and climatic processes contribute to the formation and preservation of topography, we have studied processes actively modifying the topography in order to evaluate broader geophysical processes affecting the range. Based on the similar crustal thicknesses and elevations of the Western and Eastern Cordilleras, it is clear that over the past 10--20Ma both the western and eastern margins of the Altiplano have accommodated similar amounts of uplift. However, the styles of deformation and possibly the timing of uplift within these margins contrast sharply. The broad eastern flank contains the active Sub-Andean fold and thrust belt that contributes significantly to both tectonic shortening and uplift, while in the western margin there has been very little evidence of post-late Miocene landscape modification. However, well-preserved offset and abandoned geomorphic features in the Peruvian forearc suggest to us that in response to active tectonic and climatic processes, the landscape along the western flank has been recently modified. In order to quantify the rates of active landscape modification of the western Andean margin such as river incision, surface erosion, the timings of glacial advances, and timings of surface abandonment, we have used high-resolution satellite imagery and field mapping in conjunction with surface age dating using in situ produced cosmogenic 10Be. The results of this study have yielded new detailed chronologies of glacial features in the cordillera of central Peru and young low-relief surfaces (&sim30--600ka) in the forearc. Some of the major findings of this study are the documentation of: (1) a recently active west-vergent thrust system in the southern Peruvian forearc (2) consistent regional incision rates along major rivers of &sim0.3mm/yr and (3) the northern extension of regional extensive extremely low surface erosion rates 0.5m/Ma in the forearc of southern Peru. These results, specifically the existence of active deformation and extensive Quaternary landscapes within the western margin contrast sharply with presently held views of the development of the Altiplano and contributes to our understanding of the nature and evolution of Andean orogenesis.