The Hangay Dome, central Mongolia: A relict Mesozoic landscape

Abstract:

The Hangay Dome is a broad upland in central Mongolia characterized by a high elevation (>3000-4000 m), low relief landscape within the greater Mongolian Plateau (~2000 m avg. elevation) of central Asia. We have assessed the long-term, large-scale landscape evolution of the region using thermochronologic analysis. Detrital apatite (U-Th-Sm)/He samples from the Selenga River (n = 55) and Orkhon River (n = 15) basins north of the Hangay Dome yield central ages of 134.2 ± 6 and 131.3 ± 9.8 (1σ) Ma, respectively. The regional granitic bedrock apatite (U-Th-Sm)/He single grain age distribution is approximately 95 to 200 Ma, with a homogenized grain central age of 131.2 ± 6.1 Ma. These low-temperature data, in conjunction with K-feldspar MDD ⁴⁰Ar/³⁹Ar ages of ~200-230 Ma, suggest regional exhumation in the Mesozoic. HeFTy (Ketcham, 2005) modeling corroborates these data and suggests cooling rates of ~3°C/Ma from 220-185 Ma, and applying a geothermal gradient of 21 ± 3°C/km for central Mongolia (Lysak and Dorofeeva, 2003), rock uplift rates from Late Triassic to Mid-Late Jurassic are approximately 100 m/My and from the Early Cretaceous (130 Ma) to the present approximately ≤ 30 m/My. Regional bedrock age patterns, detrital age populations, and thermal modeling suggest that significant recent, rapid rock uplift in central Mongolia is unlikely. Pecube thermo-kinematic models (Braun, 2003) indicate that any rapid (> 500 m/My) event in the Late Miocene-Pliocene would produce Early-Mid Cenozoic cooling ages in lower elevations of the Selenga River drainage basin, which is not supported by the detrital age signal. Pecube modeling of slow rock uplift rates of <50 m/My since the Early Triassic produce regional ages in agreement with geomorphic and geochronologic data. Regional apatite helium age-elevation patterns suggest long-term thermal stability of the upper crust and possible lowering of relief since Mesozoic exhumation. Basalt total fusion ⁴⁰Ar/³⁹Ar ages demonstrate the existence of Hangay Dome paleotopography since the Early Miocene and subsequent Miocene-Pliocene base level rearrangement and landscape incision. Alpine cirques and preserved moraines are indicative of a more recent, climate-driven erosional signal in the higher peaks of the western Hangay, possibly augmented by modest surface uplift over the past 10 m.y.