DEGLACIAL STYLE AND PATTERN OF THE LAST CORDILLERAN ICE SHEET NEAR ITS GEOGRAPHIC CENTRE

Decay of the last Cordilleran Ice Sheet (CIS) near its geographic centre has been conceptualized as being dominated by stagnation (passive downwasting due to a rapid rise in equilibrium-line altitude), in part because of the lack of large recessional moraines. Yet multiple lines of evidence from interior British Columbia (BC), including trends in glacioisostatic adjustment, and recent inferences of the lateglacial reconfiguration of the CIS based on the pattern of ice-marginal channels, suggest a more systematic pattern of ice margin retreat back toward the Coast Mountains. We integrate digital terrain and aerial photograph analysis, sediment logging and geophysical surveys to enhance lateglacial reconstructions near the geographic centre of the last CIS: the southern Fraser and northern Thompson plateaus of BC. Here, we identify and map glaciotectonic and grounding-line moraines, glacial lake outburst flood (GLOF) eskers and ice-marginal channels, and reconstruct ice-marginal lake evolution. Together, these observations support contiguous, frequently lake-terminating, ice margin retreat northwestward, back toward the Coast Mountains, similar in style and pattern to that proposed for the lateglacial Fennoscandian Ice Sheet. Ice-marginal lakes would have enhanced sliding and intensified thermo-mechanical melting of the ice front; they likely amplified melt rates beyond those expected based on climate warming alone. Glaciotectonic moraines suggest active retreat, and are consistent with enhanced sliding. Nested ice-marginal channels and flat-topped esker segments support downwasting (ice thinning), and GLOF eskers suggest the formation and drainage of supraglacial lakes, both consistent with rapid climate-induced melting. Patches of hummocky terrain record local stagnation. This deglacial style is analogous to parts of the Greenland Ice Sheet today.