Styles of detachment faulting at the Kane Fracture Zone oceanic core complex, 23°N Mid-Atlantic Ridge.

摘要

In December 2004, the RV Knorr sailed to the Kane Fracture Zone at 23°N on the Mid-Atlantic Ridge to collect samples, bathymetric, and geophysical data from the Kane Fracture Zone Oceanic Core Complex (OCC). Eleven hundred samples were collected by submersible and by dredging from both the surface of the complex and from high-angle fault scarps that cut the complex. Examination of the deformed samples by hand sample analysis, petrography, electron backscatter diffraction, and geothermometry confirms that the Kane OCC is bounded by a detachment fault system that initiated at high temperatures (>700°C) below the brittle-plastic transition. Fault rocks reveal a history of deformation from granulite and amphibolite through subgreenschist facies including brittle cataclasis. We present two cross sections through the detachment fault based on samples collected from the secondary high-angle normal fault scarps. One section, through Cain Dome, is dominated by peridotite and shows an ∼450-m zone of discrete ductile shear zones overprinted by a 200-m zone of semi-brittle and brittle deformation. The other section, through Adam Dome, is dominated by gabbroic rocks, which show a paucity of deformation, and is inferred to have a zone of ductile shear and brittle faulting less than 120 m thick. A rheologic analysis, using LPO-deduced deformation mechanisms and geothermometry to construct deformation mechanism maps, gives strain rates for the amphibolite-bearing gabbros, the gabbronorites, and the peridotites of 10-10s -1, 10-12s-1, and 10-13s -1 respectively. These differing rates are consistent with the observation that high-temperature strain may be distributed over thicker zones in peridotites than in amphibole-bearing gabbros. An alternate and perhaps additional explanation for the thin zone of deformation in the gabbro section through Adam Dome is that it is only 2 km from the breakaway and therefore does not reveal the deepest part of the detachment fault. Finally, the presence of deformed and undeformed diabase dikes, peridotite mylonites that have been intruded by a gabbroic melt, and Fe-Ti oxide microstructures indicative of deformation with a melt present suggest that detachment faulting was coeval with magmatism.