HR: 16:45h
AN: T34B-04 INVITED     [Abstracts]
TI: Detachment Faults and Grooved Footwalls in Diverse Magmatic and Tectonic Settings
AU: * Spencer, J E
EM: jon.spencer@azgs.az.gov
AF: Arizona Geological Survey, 416 W. Congress St., #100, Tucson, AZ 85701 United States
AB: Grooved slip surfaces produced in diverse geologic settings over an enormous range of scales possibly result from the same basic physical process. Subaerial extrusion of viscous magma in 'squeeze-ups' has produced striated surfaces, as has submarine flow of basaltic magma from beneath seafloor lava-lake crusts. Striations range from mm to cm in wavelength, and decimeters to meters in length. The geologic process of producing linear forms in cooling and solidifying magmas adjacent to a slip surface is imitated in the widely used industrial process of continuous casting. Similar but much larger grooves were possibly produced by continuous casting at tectonic rates. Grooves in continental metamorphic core complexes and their bounding low-angle normal faults have wavelengths of 1-15 km and groove lengths of up to 50 km. Levels of crustal exposure change greatly along some groove axes, with shallow crustal levels indicated by non-mylonitic rocks and pre-exhumation K-Ar mica dates grading laterally into intermediate crustal levels indicated by syn-exhumation mylonitic petrofabrics and K-Ar mica dates. Grooves developed in upper crustal crystalline rocks reflect the irregular shape of the normal fault along which extension began, and are the tilted and exhumed footwalls of such faults. However, where grooves are tens of kilometers long, as in some complexes, either the fault and its down-dip continuation as a mid-crustal mylonitic shear zone formed simultaneously with a grooved form, or the most deeply exhumed rocks were molded into grooves as they flowed laterally and upward into contact with the colder and stronger, grooved underside of the hanging-wall block. In the former option, it remains unexplained why grooved shear zones would develop with identical form to a brittle fault tens of kilometers up dip. In the latter case, the grooves were produced by continuous casting at tectonic rates and scales. Deep-sea metamorphic core complexes show remarkably grooved forms, with wavelengths of hundreds of meters to kilometers and lengths of many kilometers. The grooves parallel adjacent transform faults, and thus are parallel to plate-divergence direction. Grooves on what appears to be an enormous metamorphic core complex at the center of Artemis Corona, Venus, are up to 170 km long, and parallel an adjacent set of en echelon scarps that are interpreted as normal faults resulting from groove-parallel transform faulting. If analogous to continental metamorphic core complexes, the earliest exhumed parts of deep sea and Venutian grooved complexes are simply tilted normal-fault footwalls, whereas the contiguous, later-exhumed grooves were molded by the grooved underside of a colder, stronger hanging wall.
DE: 3035 Midocean ridge processes
DE: 3045 Seafloor morphology, geology, and geophysics
DE: 8004 Dynamics and mechanics of faulting (8118)
DE: 8109 Continental tectonics: extensional (0905)
SC: Tectonophysics [T]
MN: Fall Meeting 2005