Geophysical analysis of rock glacier internal structure and implications for deformation mechanics

An analysis of the internal composition and structure on the active portion of the Lone Peak Rock Glacier (LPRG), Madison range, southwest Montana revealed links between internal structure and surface topography. Seismic refraction surveys performed along transverse and longitudinal profiles corroborate borehole and excavation data by demonstrating a consistent and distinct transition from unconsolidated (unfrozen) surface debris (2-3 m thick) to a consolidated (frozen) subsurface material. Refraction velocities for the seismic survey transects were relatively consistent along their length with 400 m s-1 for the upper layer detected, and 3500 m s-1 for the lower layer detected at a depth of 2-3 m. This second velocity of 3500 m s-1 is consistent with other observed refraction velocities for ice. Ground penetrating radar (GPR) data along similar longitudinal and transverse profiles identified up-slope dipping structures to a depth of ~10 m, consistent with layering of materials with contrasting radar properties within the ice-rock unit. The GPR data is interpreted as a sequence of alternating debris-poor and debris-rich layers which dip upslope toward the rock-glacier headwall along the longitudinal profile, and which show correspondence with transverse ridges at the surface. The presence of fault bounded blocks (i.e. structural horses) detected in the longitudinal GPR data suggests passive roof duplex thrust faulting, in which the roof sequence - unconsolidated (unfrozen) debris - has not been displaced toward the foreland (down glacier), but has been underthrust by the duplex. Transverse ridges commonly characterize rock glacier surfaces in a range of locations worldwide. Approximately one third of 383 rock glaciers inventoried in southwest Montana demonstrate pronounced transverse ridges. It has previously been suggested that transverse ridges are the product of thrusting, which is caused by compressive flow in rock glaciers. Thrusting however has not been invoked to explain either a direct connection between individual transverse ridges to sub-surface structures or a specific structural regime. Our passive roof duplex faulting interpretation of GPR data at the LPRG is consistent with findings from previous studies on the internal composition and structure of rock glaciers and thus provides a testable model for improved understanding of rock glacier deformation mechanics.