Vertical axis rotation (or lack thereof) of the eastern Mongolian Altay Mountains: implications for far-field transpressional mountain building

The Altay Mountains of Western Mongolia accommodate 10–20% of the current shortening of the India-Asia collision in a transpressive regime. Kinematic models of the Altay require faults to rotate anticlockwise about a vertical axis in order to accommodate compressional deformation on the major strike slip faults that cross the region. Such rotations should be detectable by palaeomagnetic data. Previous estimates from the one existing palaeomagnetic study from the Altay, on Oligocene and younger sediments from the Chuya Basin in the Siberian Altay, indicate that at least some parts of the Altay have experienced up to 39 ± 8° of anticlockwise rotation. Here, we present new palaeomagnetic results from samples collected in Cretaceous and younger sediments in the Zereg Basin along the Har-Us-Nuur fault in the eastern Altay Mountains, Mongolia. Our new palaeomagnetic results from the Zereg Basin provide reliable declinations, with palaeomagnetic directions from 10 sites that pass a fold test and include magnetic reversals. The declinations are not significantly rotated with respect to the directions expected from Cretaceous and younger virtual geomagnetic poles, suggesting that faults in the eastern Altay have not experienced a large degree of vertical axis rotation and cannot have rotated >7° in the past 5 m.y. The lack of rotation along the Har-Us-Nuur fault combined with a large amount of rotation in the northern Altay fits with a kinematic model for transpressional deformation in which faults in the Altay have rotated to an orientation that favours the development of flower structures and building of mountainous topography, while at the same time the range widens at the edges as strain is transferred to better oriented structures. Thus the Har-Us-Nuur fault is a relatively young fault in the Altay, and has not yet accommodated significant rotation

Combined uranium series and 10Be cosmogenic exposure dating of surface abandonment: A case study from the Ölgiy strike-slip fault in western Mongolia

Time-averaged fault slip-rates can be established by reliably dating the abandonment of an alluvial deposit that has been displaced by Quaternary movement along a cross-cutting fault. Unfortunately, many Quaternary dating techniques are hindered by uncertainties inherent to individual geochronometers. Such uncertainties can be minimised by combining multiple independent techniques. In this study, we combine 10Be exposure dating of boulder tops and U-series dating of layered pedogenic carbonate cements accumulated on the underside of clasts from two separate alluvial surfaces. These surfaces are both displaced by the active Ölgiy strike-slip fault in the Mongolian Altay Mountains. We date individual layers of pedogenic carbonate, and for the first time apply a Bayesian statistical analysis to the results to develop a history of carbonate accumulation. Our approach to the U-series dating provides an age of initiation of carbonate cement formation and avoids the problem of averaging contributions from younger layers within the carbonate. The U-series ages make it possible to distinguish 10Be samples that have anomalously young exposure ages and have hence been subject to the effects of post-depositional erosion or exhumation. The combination of 10Be and U-series dating methods provides better constrained age estimates than using either method in isolation and allows us to bracket the abandonment ages of the two surfaces as 18.0-28.1kyr and 38.4-76.4kyr. Our ages, combined with measurements of the displacement of the surfaces, yield a right-lateral slip-rate for the Ölgiy fault of 0.3-1.3mmyr-1, showing that it is a relatively important structure within the active tectonics of Mongolia and that it constitutes a substantial hazard to local populations

Links between climate, erosion, uplift, and topography during intracontinental mountain building of the Hangay Dome, Mongolia

The Hangay mountain range, a dome in central Mongolia, provides a window into understanding how climate influences the erosion and resulting geomorphic and sedimentary signatures of continental topography. Specifically, asymmetric erosion of the Hangay, associated with a distinct orographic precipitation gradient, offers a natural experiment for exploring uplift, erosion, and the isostatic response to erosional unloading. The flat-topped Hangay peaks preserve low-relief remnant surfaces that provide markers of rock uplift. This makes it possible to map the deformation of a former planar surface during doming and hence to estimate the total extent of erosion by the difference from present day topography. Erosion into the Hangay surface has been significant but incomplete; the morphology of the range indicates a nonequilibrium landscape that may have persisted for millions to tens of millions of years, implying a long response time in this semiarid climate. The extent of erosion across the range correlates with mean annual precipitation. Variability in present-day peak heights across the north-south climatic and erosional gradient provides empirical support for the generally accepted theory that climate-driven erosion will increase the height of mountain peaks by generating greater surface uplift through isostasy. Correction for this isostatic response makes it possible to reconstruct primary surface uplift of the Hangay. Results highlight the importance of considering the interplay between climate, erosion, and uplift in shaping intracontinental topography and thus when interpreting the geomorphic, sedimentary, and geodynamic signatures associated with such topography