Geochronology of pediments and marine terraces in north-central Chile and their implications for Quaternary uplift in the Western Andes,

Description

In north-central Chile, a wide shore platform is morphologically connected with a high fluvial terrace and a pediment. The eastward extension of Quaternary coastal uplift in the Southern Central Andes is poorly constrained since no age correlation between marine and continental landforms has been reported. We use 26Al and 10Be concentrations to constrain the geomorphic evolution of these marine and continental landforms near the Choapa valley (31.6° S). 10Be ages for the shore platform indicate that this surface was repeatedly reoccupied during sea-level highstands between ~ 800 and 500 ka and uplifted after 500 ka. While 'zero erosion' ages for the pediment between ~ 600 and 300 ka only partly overlap the shore platform age range, more realistic exposure ages calculated for an erosion rate of 1 m/Ma are between ~ 945 and 475 ka, fitting the age range of the correlated shore platform. 10Be concentrations of the high fluvial terrace are highly scattered evidencing vertical mixing of clasts probably due to slow lowering of the surface. Although it is not possible to determine an age for this landform, the scattering among its 10Be concentrations implies that this marker is several hundreds of thousands of years old and that the high fluvial terrace began to form at ~ 1200 ka or after. Finally, 10Be concentrations of the high fluvial terrace, the pediment and the shore platform are of the same order of magnitude, which is consistent with the clear morphologic correlation between these three types of landforms. These data suggest that the marine and continental landforms studied formed synchronously, with some local differences, during a long period of relative tectonic stability between ~(1200?) 800 and 500 ka and uplifted after 500 ka. Our results confirm recent studies showing a post-400 ± 100 ka renewal of uplift along the Pacific coast after a Lower to Middle Pleistocene period of slow uplift. Moreover, the extension of the surfaces suggests that a broad region of ~ 40 km has been uplifted ca. 150 m during the Quaternary.

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