Alpine landscape evolution dominated by cirque retreat

Despite the abundance in alpine terrain of glacially dissected landscapes, the magnitude and geometry of glacial erosion can rarely be defined. In the eastern Kyrgyz Range of central Asia, a widespread unconformity exhumed as a geomorphic surface provides a regional datum with which to calibrate erosion. As tectonically driven surface uplift has progressively pushed this surface into the zone of ice accumulation, glacial erosion has overprinted the landscape. With as little as 500 m of incision into rocks underlying the unconformity, distinctive glacial valleys display their deepest incision adjacent to cirque headwalls. The expansion of north-facing glacial cirques at the expense of south-facing valleys has driven the drainage divide southward at rates of as much as two to three times the rate of valley incision. Glacial erosion rules based on ice flux incompletely explain this dominance of cirque retreat over valley incision. Local processes that either directly sap cirque headwalls or inhibit erosion down-glacier appear to control, at least initially, alpine landscape evolution. Keywords: glacial erosion, cirque retreat, Kyrgyz Range, Tien Shan.