Carving mountains : aeolian erosion as a major factor controlling the topographic evolution in the Gobi region (Mongolia).
Résumé
The Tsagaan Els Basin in SE Mongolia is a 31 000 km2 endoreic depression submitted to active erosion and surface lowering. The present-day depression is superimposed to a complex pattern of Cretaceous grabens and semi-grabens separated by basement highs composed of Proterozoic to Paleozoic metamorphic and magmatic rocks (Johnson et al., 2001). The summits of those basement highs correspond to fragments of a very low relief planation surface that formed prior to the Cretaceous extension phase, most probably during the Early to middle Jurassic (Jolivet et al., 2007). The Late Cretaceous sediments largely seal the extension phase. Paleogene to Neogene sedimentation is extremely condensed, marked by numerous aeolian dune deposits and carbonated paleosols, both indicating a long-lasting arid to semi-arid climate. The Quaternary sediments are limited, mainly composed of aeolian dunes, thin alluvial deposits, patchy outcrops of shale corresponding to small ephemeral lakes, and grain-thick sheets of gravels to pebbles covering strongly winnowed surfaces. Mapping of the drainage system shows a first-order trend towards the lowest point of the basin, associated to a second-order highly complex river network associated to local base levels. Numerous river capture events are observed indicating a poorly stabilized, locally constrained drainage system. In the center of the Tsagaan Els Basin, the c.a. 55 km long, 20 km wide Dulan Ul hill corresponds to one of the basement horsts, cropping out of the Cretaceous series. This flat-topped mountain, culminating around 950 m a.s.l. and 200 m above the basin is bordered to the north by a large fan-like morphological structure carved within the Cretaceous series. Similar fan-shaped morphologies are observed on several locations within the Tsagaan Els Basin, that cannot be explained through alluvial deposits or river incisions.
In this study, we performed a detailed mapping of the displacement of the sand (that largely corresponds to the mode of wind direction) over the central part of the Tsagaan Els Basin. We show that the fan-like morphology upwind of the Dulan Ul hill is effectively related to wind-erosion carving into the Cretaceous sediments in front of the basement high. This strong aeolian erosion, associated to the occurrence of large lithological contrasts between the basement and the sedimentary series explains the complex and extremely versatile drainage network.