CONSTRAINING THE LATE CENOZOIC TOPOGRAPHIC EVOLUTION OF CENTRAL MONGOLIA (HANGAY MTNS) USING LANDSCAPE METRICS AND SEQUENCE DIVERGENCE OF FRESHWATER FISHES

Located deep in central Asia, Mongolia is positioned along the southern margin of the Siberian craton, north of the India-Asia collision, and far to the west of the subducting Pacific plate, yet appears to owe much of its structure and evolution to its location between these plate-scale phenomena. Here, the Hangay Mountains occupy a broad upland embedded in the greater Mongolian Plateau. The interior of the range rises to ~1.5 km above regional base level, contains a high-elevation low-relief surface, and a 30 My record of intermittent volcanism. As part of a broad collaborative project, we synthesize bio-geomorphic data that collectively support a mid to late Miocene increase in elevation of the Hangay, while topographic relief remained largely unchanged.

The crest of the Hangay is the watershed divide between the Selenga River (and Arctic Ocean) and internal drainages of the Mongolian Depression of Lakes (MDL) and northern Gobi depression (NGD). DNA from > 250 individuals representing three genera of freshwater fish (Barbatula/Triplophysa, Thymallus, and Oreoleuciscus) help to constrain the timing and mechanisms of population separation. Cytochrome oxidase 1 (CO1)-based molecular clocks from Barbatula indicate that the Selenga-MDL divide was established as a gene-transfer barrier by 18.4 ± 2 Ma. This is consistent with estimates of surface uplift (~ 1 km – basalt vesicle paleoaltimetry) and paleorelief (c. 800 m) from basaltic infilling of late Miocene valleys cut into basement. In contrast to the Barbatula results, variation in both CytB and CO1 from Thymallus collected on either side of the Selenga River – MDL divide indicate population divergence as recently as 2.4 to 0.7 Ma. In addition, topographic separation between the MDL and NGD occurred between 2.1 and 1 Ma, as observed in the divergence in CO1 from Oreoleuciscus and Barbatula populations. We speculate that these younger ages are due to either: (1) drainage capture in response to significant lateral transfer of topography along the major strike-slip fault zones bounding the Hangay, or (2) Glacial reorganization of valleys and/or overtopping of divides by pluvial lakes during the Quaternary. These phylogeologic results are critical to understanding linkages between mantle dynamics, surface topography, and climate in this intracontinental setting.