Alluvial fans in an extreme continental-interior basin: Interplay of climate-cyclicity, tectonics, and watershed dynamics in western Mongolia

Quaternary landscape evolution in western Mongolia's Basin of Great Lakes, one of the most continental regions on Earth, is controlled by a Neogene tectonic setting and orbitally-forced climate cyclicity. Lake Khyargas forms the terminal water and sediment sink of a cascading hydrological system draining the adjacent Altai and Khangai Mountains. Well-preserved sequences of relict shorelines provide evidence of repeated lake transgressions since the mid-Pleistocene in response to increased atmospheric moisture supply and widespread glacier melt in the broader catchment following the local last glacial maximum. The Khankhukhii Range, uplifted along the westernmost expression of the active Bulnay fault system to peak elevations of 2928 m asl, separates the endorheic catchments of Lakes Khyargas and Uvs. The southern slopes of the Khankhukhii Range are inclined towards Lake Khyargas, where mesoscale alluvial fans form the interface between the mountain system and the terminal lake basin. Morphostratigraphic mapping combined with 10 Be depth-profile exposure dating and feldspar luminescence ages reveals two preserved aggradational fan generations formed during glacial-interglacial transitions: The Marine Isotope Stage (MIS) 6 to MIS 5 transition (∼130 ka) and the commencing MIS 3 (∼47 ka). From the interplay with previously dated shoreline features we infer ongoing fan incision since the Pleistocene to Holocene transition (14 ka), coinciding with a general wetting trend in Arid Central Asia following the local last glacial maximum. We explore the sensitivity of this alluvial fan record to climate cyclicity under nonlinear coupling with coeval controls imposed by tectonic uplift, dynamic drainage adjustments, and base level changes. We argue that transient increases in transport capacity induce the pulsed evacuation of stored sediment, ultimately superimposing the sedimentary signal of background tectonic forcing. However, tectonic uplift provides a continuous sediment supply on 10 6 ka timescales, the required accommodation space and the topographic precondition for the alluvial fan-catchment systems.