Kalin T. McDannell , Peter K. Zeitler, Bruce D. Idleman

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 200 Ma, with a mean of 122.7 ± 24.0 Ma (2σ). Detrital apatite samples from the Selenga and Orkhon Rivers, north of the mountains, yield dominant (U-Th)/He age populations of ~115 to 130 Ma, 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 <1-2 km since the Jurassic-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 and spatially variable exhumation driven by isostasy in concert with relief evolution to produce the observed cooling ages in the Hangay region. Alpine cirques and intact moraines are indicative of more recent, climate-driven erosion in the higher peaks of the western Hangay. Regionally, modeling suggests topographic "planation" in the Jurassic followed by rapid relief growth that was completed by the mid-Cretaceous. These results support Mesozoic topograhic evolution and relative stability of the landscape enduring throughout the Cenozoic with very little subsequent exhumation. These data support the notion that in the absence of strong tectonic or climate forcing, erosion is limited and remnant landscapes can persist over 10s-100s of millions of years in a state of disequilibrium.