Velocities, strain rates, stresses, crevassing and faulting on Glacier de Saint-Sorlin, French Alps, 1957–76

Abstract

AbstractStake surveys on Glacier de Saint-Sorlin, French Alps, during the period 1957–76 show that annual surface velocities fit a linear vectorial model, with a term depending on the site, another on the year, and an important random component. Strain rates, viscosities and stresses at shallow depth are computed using strain triangles of hectometric (102 m) size. Between 1961/62 and 1972/73 the isotropic point, where streamlines cease to converge, moved downstream about 200 m. This trend may explain increasing velocities. Crevasses appear where annual strain is > 1.2%. Faults limit the effective shear stress at the surface to about 0.38 bar. Ten modes of flow are distinguished, instead of only two for the two-dimensional model (compressive and extensive). The gradients of shallow stresses, which ensure extra driving forces, are computed with another mosaic of triangles of similar size (stress triangles). There are also important extra driving forces at the bottom, which force the flow to deviate from the direction of the steepest surface slope. Two criteria allow elimination of stress triangles where these unknown basal extra driving forces are important. Even so, no sliding law in terms of mean annual values can be obtained. This study shows that the classical perturbation theory, which explains advances and retreats by the arrival of kinematic waves, is unsuitable for glaciers of kilometric size.