Using paleolake shorelines to estimate lithospheric strength and response rates: Field observations and flexural isostatic modeling of Lake Khyargas, western Mongolia.

Neogene tectonics and geomorphic processes in response to Quaternary climate change drive landscape evolution in western Mongolia's Basin of Great Lakes (BGL). The endorheic Khyargas Nuur (Nuur=lake) in the BGL is the ultimate sink of a sequential water and sediment cascade from the adjacent Mongolian Altai and Khangai Mountains. Several intercalated lakes repeatedly joined as one major paleolake controlled by changes in atmospheric moisture supply and glacial meltwater influx throughout the Late Quaternary. Relict shoreline features up to +188 m above the modern lake (aml) provide direct geomorphic evidence of a mega (>13x modern area) paleolake Khyargas. Terrestrial cosmogenic nuclide (TCN) exposure dating of the highest observed shoreline of the Khyargas Nuur at +188 m aml, using a ¹⁰Be depth profile, provides a maximum temporal framework for the investigated paleoshoreline sequence. Notably, local offsets exist between expected and mapped/measured absolute beach ridge elevations. For example, absolute elevations for beach ridges associated with a +15 m aml lake level vary by up to 6.8 m with a standard deviation of 0.9 m (n=2760). Reconstructed beach ridge elevations vary by 10.9 ± 1.6 m for the prominent +118 m aml paleoshoreline (n = 2962). Luminescence dating of associated shoreline features yielded ages of ~2.1 ka and ~14 ka for the +15 and +118 m shorelines, respectively. Comparative statistical differences in the offset values of these two shorelines and concurrent spatial similarities of displacement hotspots suggest a time-dependent, cumulative paleoshoreline displacement mechanism. We hypothesize that observed shoreline offsets are induced by either (a) local tectonically active fault displacements, (b) hydro-isostatic adjustments similar to those observed around the margins of paleo-lake Bonneville in the U.S. Great Basin (¹) and paleo Lake Chad (²) in north-central Africa, or (3) reactivation of zones of crustal weakness (old faults) caused by water loading and unloading. To evaluate the potential driving mechanisms on these displacements, we constructed a three-dimensional flexural isostatic model that utilizes paleoshoreline observations to determine rates and magnitudes of deflection and to test for robust constraints of the lithospheric effective elastic thickness (T_e).

(¹) Adams, K. D. & Bills, B. G. Isostatic Rebound and Palinspastic Restoration of the Bonneville and Provo Shorelines in the Bonneville Basin, UT, NV, and ID. in Developments in Earth Surface Processes vol. 20 145–164 (Elsevier, 2016).

(²) Mémin, A., Ghienne, J.-F., Hinderer, J., Roquin, C. & Schuster, M. The Hydro-Isostatic Rebound Related to Megalake Chad (Holocene, Africa): First Numerical Modelling and Significance for Paleo-Shorelines Elevation. Water 12, 3180 (2020).