Relict Topography Within the Hangay Mountains in Central Mongolia: Quantifying Long-Term Exhumation and Relief Change in an Old Landscape

The Hangay Mountains are a high-elevation, low-relief landscape within the greater Mongolian Plateau of central Asia. New bedrock apatite (U-Th)/He single-grain ages from the Hangay span 70 to 200Ma, with a mean of 122.724.0Ma (2 sigma). Detrital apatite samples from the Selenga and Orkhon Rivers, north of the mountains, yield dominant (U-Th)/He age populations of 115 to 130Ma, as well as an older population not seen in the Hangay granitic bedrock data. These low-temperature data record regional exhumation of central Mongolia in the Mesozoic followed by limited erosion of <1-2km since the Cretaceous, ruling out rapid exhumation of this magnitude associated with any late Cenozoic uplift. Apatite (U-Th)/He age elevation patterns suggest long-term thermal stability of the upper crust and thermal model inversions require late Mesozoic uplift, spatially variable exhumation, and/or relief evolution to produce the observed cooling ages in the Hangay. Regionally, modeling suggests reduction of topography in the Jurassic followed by relief growth that was completed by the mid-Cretaceous. These results support Mesozoic topographic evolution and relative stability of the landscape throughout the Cenozoic with very little subsequent exhumation. Alpine cirques and intact moraines are indicative of more recent, modest climate-driven erosion in the higher peaks of the western Hangay. These data reinforce the notion that in the absence of strong tectonic or climate forcing, erosion is limited and remnant landscapes can persist over tens to hundreds of millions of years in a state of disequilibrium. Plain Language Summary The Hangay Mountains in central Mongolia are a mysterious area of high topography within central Asia. The Hangay topography has typically been thought of as a more youthful feature, uplifted and created in the last 20-30 million years. The geochemical and geologic data presented here instead suggest that this is a very old mountain landscape that has persisted for nearly 100 million years. Numerical modeling performed for central Mongolia advocates for a scenario where topography was created in the Mesozoic and over the past 100 million years has undergone very minimal erosion due to a persistently arid climate and minimal plate tectonic activity. Key Points