Field tests of rolling hinges: Existence, mechanical types, and implications for extensional tectonics

In rolling-hinge models, a locus of isostatic footwall uplift migrates together with the withdrawing hanging wall of a large-displacement (tens of kilometers) normal fault. A fault-bend fold that migrates through the footwall results, causing progressive deactivation of the updip part of the fault as it is tilted to a lower dip. Rolling-hinge migration should leave a discernible geologic record, but compelling tests are difficult to design because predictions of rolling-hinge models are nonunique and alternative models commonly can explain individual data sets. Field tests based on thermochronology, paleomagnetism, and structural analysis are typically applied to footwall rocks, whereas sedimentology, geochronology of volcanic rocks, and structural analysis are typically applied to upper plate rocks. We review existing theoretical models and case studies in 14 detachment terranes. Some negative results are reasonably compelling, but positive tests as yet tend to permit, rather than demand, a rolling hinge. Locally, both positive and negative conclusions have been reached about the passage of a hinge within the same terrane; such differences in interpretation may depend more on the scale of observation or on the specific model considered than on the geological development of the area. Subvertical simple shear and flexural failure mechanisms, with or without significant induced flow of lower crustal material, are thought to dominate hinge-related footwall deformation in individual sites. Although rolling-hinge models offer an alternative to low initial fault dips, hinge-related deformation does not demand a steep initial fault dip: few areas in which rolling-hinge tectonics are permissible had initially steep faults (>35 degrees). Rather, where the evidence most strongly favors a rolling hinge, detachment faults had initial dips <35 degrees. In the future, conclusive results are most likely to be obtained if diverse tests are applied to both the upper and lower plates of detachment terranes where other geological sources of ambiguity are minimal. At best, the rolling-hinge mechanism does not universally explain the observation that earthquakes are rare on low-angle normal faults nor solve the mechanical problems of frictional slip on such faults.