SCALE INDEPENDENCE OF DÉCOLLEMENT THRUSTING: OROGEN TO METER SCALE

Orogen-scale décollements (detachment surfaces) are an enduring subject of investigation by geoscientists. Uncertainties remain as to the detailed mechanisms of how crustal convergence processes maintain the stresses necessary for the development of low-angle fault surfaces above which huge slabs of rock are transported horizontally for 10s to 100s of km. Seismic reflection profiles from the southern Appalachian crystalline core and foreland provide useful comparisons with high-resolution shallow-penetration seismic reflection profiles recently acquired over the frontal zone of the Michigan lobe of the Wisconsin ice sheet, northwest of Chicago, Illinois. These high-quality profiles provide images of sub-horizontal and overlapping dipping reflections that reveal a ramp-and-flat thrust system developed in poorly consolidated glacial till. The system is rooted in a décollement at the top of bedrock. These 2-3 km-long images contain analogs of images in seismic reflection profiles from the Appalachians and other orogens, except the scales of observation in the profiles of the glacial materials are two orders of magnitude less. Whereas the décollement beneath the ice lobe example lies at about 70 m depth below thrusted anticlines having wavelengths of 10s of meters driven by a southward-advancing ice sheet, seismic images from overthrust terranes are related to lithospheric convergence that produce décollements traceable for 1000s of sq. km at depths to over 10 km. Dual vergence or reversals in vergence (retrocharriage) developed over abrupt changes in depth to the décollement can be observed at all scales. The strikingly similar images, despite the contrast in scales and driving mechanisms, suggest a scale- and driving mechanism-independent (self-similar) behavior for décollement thrust systems, but initially having the mechanical properties needed to produce very similar geometries with a compressional driving mechanism directed subparallel to the Earth’s surface. Subduction-related accretionary complexes, such as the Barbados Ridge complex, also produce thrust systems with similar geometries in semi- to unconsolidated materials.