FAR-FLUNG MORAINES AND RIDGE-CAPPING TILLS: EXPLORING THE FEEDBACKS OF LONG-TERM ALPINE GLACIAL LANDSCAPE MODIFICATION

Modification of alpine landscapes by glaciers early in the Plio-Pleistocene ice ages can be sufficient to degrade the health of subsequent glaciers. We seek to document field cases and to model the signatures of such feedbacks. The features we target include i) far-flung moraines that are well beyond the last glacial maximum (LGM) limit, features that either remain unexplained or have been interpreted to record much deeper glaciations in the past, ii) ridge-capping enigmatic diamictons that are well above the LGM glacial limit, and iii) beheaded doubly-hung glacial valleys. Specific examples that serve as model targets are drawn from mountain ranges in the western US. We employ a 1D glacial flowline model in which winter and summer mass balance profiles are prescribed, stresses are longitudinally coupled, and the thermal state of the ice mass is calculated (via advection, production and diffusion of heat). Glacial sliding rate is governed by a spatially variable flotation fraction that honors the expected spatial pattern of a seasonal cycle. The mass balance and thermal components of the model allow assessment of the thermal state at the bed; sliding is disallowed, and hence erosion is prevented, when temperatures are below the pressure melting point. Glacial erosion is tied to sliding rate and to spatially variable rock erodibility. Climate is controlled by time series of temperature and of snowfall that mimic the long-term climate drivers either through generic double-sine cycles, or through scaled versions of the marine isotope record.

As shown in earlier modeling studies, the key effect of glaciation is to flatten valley floors and steepen headwalls, effectively cutting a notch in the initial fluvial profile. Model results demonstrate that it is plausible for glacial valleys in tectonically inactive regions to evolve such that early glacials leave moraines that are many kilometers beyond those left by the latest glaciations, and trimlines with associated deposits that are well above the present glacial limit, despite identical glacier climates. This suggests that a different landscape, rather than a different climate, is required to reproduce the early glacier extents. As a corollary, the long term drift toward smaller glaciers favors survival of early moraines in the face of later advances on a more eroded landscape.