MORAINE PRESERVATION AND BOULDER EROSION IN THE PERUVIAN ANDES

Deglaciated valleys bordering the Junin Plain in the Peruvian Andes display extraordinary preservation of moraines from multiple glaciations dating back more than a million years. Using cosmogenic dating (¹⁰Be) of boulders on moraines in Alcacocha Valley (S 11° 03', W 75° 58', elev. ~4100-4800 m), we have identified deposits ranging in age from the last glaciation (~12-31 ka) to >1.0 Ma. Moraines of the last glacial maximum are farther upvalley and smaller than large outer moraines of older glaciations. Preservation of the older moraines and of polished surfaces on some old boulders argues for extremely low boulder erosion rates. We estimate a maximum erosion rate of ~0.3 m/Myr, which approaches published rates for Antarctica and suggests that aridity is important in slowing boulder erosion. Our findings are consistent with results from other Andean locations where moraines of the last glacial maximum are less extensive than those of older glaciations. The apparent decrease in glacial extent and ice volume in the Junin Plain during the Quaternary may have been controlled by tectonics, climate, or a combination of both. A late Tertiary pulse of tectonism may have resulted in an increase in high-elevation topography in the Junin region, allowing growth of large ice masses. The subsequent trend in decreasing ice volume could be a result of either a decrease in the area of high-elevation topography through glacial erosion or a decrease in the amplitude of climate variability. These hypotheses could be tested by analysis of the sediment record preserved in the Junin basin.