GENERATION OF SUMMIT PLATEAUS BY PERIGLACIAL PROCESSES: IMPLICATIONS FOR TECTONIC INTERPRETATIONS

Low-relief summit plateaus present in many alpine environments throughout the world are characterized by low slope curvature, thin mantles of regolith, and occasional tors. The plateaus are used as geomorphic datums where these features are assumed to be remnants of uplifted surfaces. The inferred remnants are interpolated in order to derive estimates of mass removal and isostatic rebound from a previously more extensive erosion surface (peneplain). Alternatively, the origin of these plateaus has been explained by periglacial processes that bevel slowly uplifting or purely erosional mountains at the elevation where the temperature controlling the processes operates most effectively. We compare patterns in topographic relief, erosion rates, and climate to assess the dominance of periglacial processes in the formation of summit plateaus in the Gobi and Mongolian Altai, and Hangay Mountains of central Mongolia. Summit plateaus in these ranges only exist within a narrow elevation band despite differing uplift histories. These summits persist due to (1) decoupling of summit plateaus and rivers and (2) present-day occurrence of frost cracking that supplies fresh regolith. We put forth a geomorphic process model in which seasonally late-lying snowbanks initiate cryoplanation terraces that are elongated (widened) by parallel retreat of the terrace risers. Summit plateaus form as the final condition of terrace retreat and frost cracking on summits. The continuation of summit plateau formation injects complications into using these surfaces as geomorphic datums for tectonic interpretations.