Toward quantifying geomorphic rates of crustal displacement, landscape development, and the age of glaciation in the Venezuelan Andes

Abstract

We present the results of dating glacial landforms in Venezuela using 10Be terrestrial cosmogenic nuclide (TCN) analysis and optical stimulated luminescence (OSL). Boulders on the La Victoria and Los Zerpa moraines of the Sierra Nevada that mark the extent of the local last glacial maximum (LLGM) yield 10Be TCN surface exposure ages of 16.7±1.4ka (8 samples). About 25km to the west in the drainage basin of the Río Mucujún, 10Be TCN dates for boulders on moraines at La Culata in the Sierra Nevada Norte yield a younger average age of 15.2±0.9ka (8 samples). The data suggest that glaciation across the Venezuelan Andes during the LLGM was asynchronous. The LLGM in Venezuela may be broadly concurrent with Heinrich Event 1 at ~16.8ka, implying that glaciation here is dominantly temperature driven. A moraine inset into the older laterofrontal moraines of La Culata has an age of 14.1±1.0ka (5 samples); it may have been deposited by a small Late Glacial readvance. Right-lateral offsets of the La Victoria and Los Zerpa moraines by the Boconó fault are each ~100m. The 10Be TCN based Boconó fault slip rate is about <~5.5 to 6.5mma−1, notably less than the total right-lateral slip of 12±2mma−1 of shear documented across the Andes from geodesy. The 10Be TCN dating of boulders on a faulted alluvial fan along the northwestern range front at Tucanízón yields a late Pleistocene uplift rate of the Andes at between ~1.7±0.7mma−1. Glacial outwash has produced valley-fill sequences within the central Andean valley along the trace of the Boconó fault and Río Chama. The valley-fill has been incised to produce the ‘meseta’, a terrace surface that sits >100m above the Río Chama and on which the major city of Mérida is built. Geomorphic observations indicate that the meseta deposits were largely derived from the glaciers of La Culata. The OSL dating suggests that the final aggradation of the valley-fill deposits occurred rapidly over a period of about 5 to 6ka and that the surface was abandoned and initially incised at ~30ka. The result implies Venezuelan valley fills record phases of aggradation that are likely modulated by climate change on glacial/Milankovitch timescales.