THICK-SKINNED STYLE GLACIOTECTONICS AT AN ICE-CORED MORAINE, MATANUSKA GLACIER, ALASKA

Glaciotectonic sediment deformation is found at actively glaciated sites as well as within the rock record at previously glaciated areas. Many different mechanisms for deformation have been proposed; however, none of them appropriately describe the deformational processes currently occurring at Matanuska Glacier, Alaska. A form of “thick-skinned” glaciotectonics (in which thrusting and deformation includes a rigid block of stagnant buried ice as well as the “thin skin” of unconsolidated overlying sediments) is hypothesized to occur at this site based on geophysical data, test pits, and surface observations collected between July 2001 and July 2003. This thick-skinned style of glaciotectonic deformation occurs when re-advancing active ice impacts stagnant buried ice within an ice-cored moraine complex. The deforming buried ice subsequently affects the overlying sediments. By this mechanism, stress from an advancing glacier--and the associated deformation--is able to propagate much further from the terminus than would otherwise be expected in relatively low shear- and compressive-strength unfrozen water-saturated sediments. Ground-penetrating radar (GPR) is used to generate 3-D sub-surface images at the site to supplement other data, such as the surface expression of thrust features. Thrust features have been observed as far as 30 meters from the active ice, and a coherent thrust plane within sediments overlying buried ice was observed in a 2-m-deep test pit. Along with GPR data, topographic surveys were conducted at the site to document the evolution of the moraine surface and make inferences about the subsurface changes taking place. The integration of indirect observations through the use of GPR with direct observations made at the surface supports that the thick-skinned style of glaciotectonics occurs and is an ongoing process in the development of structurally deformed glacial sediments at Matanuska Glacier.