HR: 16:00h
AN: T34B-01    [Abstracts]
TI: Are Oceanic Detachment Faults Really Analogous to Those on the Continents?
AU: * John, B E
EM: bjohn@uwyo.edu
AF: Dept of Geology and Geophysics, University of Wyoming, Laramie, WY 82071 United States
AU: Cheadle, M J
EM: cheadle@uwyo.edu
AF: Dept of Geology and Geophysics, University of Wyoming, Laramie, WY 82071 United States
AB: Oceanic core complexes were first noted along the mid-Atlantic Ridge based on their corrugated, domal bathymetry. Analogies were made with continental core complexes and their associated detachment fault systems. Drilling, submersible and ROV data from oceanic systems suggest that the two types of detachment fault may be fundamentally different. Here we discuss the characteristics of detachments faults in both environments. Both continental and oceanic detachment fault systems are characterized by corrugated, domal topography; exposures of the doubly plunging fault surfaces vary up to tens of km in the down-dip (ridge-normal) direction, with dips < 20°. The fault systems comprise a network of anastomosing fault zones, with associated fault rocks (mylonite - cataclasite and gouge) 1- >200m thick, typically exhibiting a progressive down-temperature continuum in deformation. Both form at strain rates ~10^-12/sec -10^-14/sec, and accommodate asymmetric extension. Despite these similarities, the two systems are different: i) Oceanic detachment faults are initiated at slow spreading ridges and often show an intimate association with magmatic accretion; magmatism is more commonly associated with detachment faults in the oceans than the continents. ii) Oceanic detachment fault systems are likely influenced by a serpentine, olivine and/or plagioclase dominated rheology compared to a quartz and feldspar rheology in continental settings. Oceanic detachment faults are `new' faults, which do not interact with pre-existing weaknesses/older faults. iii) Fault rocks associated with the evolution of oceanic detachment faults follow a down-temperature path from magmatic to very low temperature deformation; variations in thickness and the intensity of ductile and/or brittle deformation depends on the magnitude and longevity of slip, and proximity to the fault breakaway. In continental settings, syntectonic magmatic fabrics are rarely recognized, and high-temperature fault rocks rarely noted. iv) Paleomagnetic data from the footwall to several oceanic detachment faults suggest rotations up to 50° since the rocks passed through the Curie temperature, consistent with a rolling hinge model. In contrast, many continental detachment faults were demonstrably initiated at a low angle. v) Detachment faults cutting oceanic lithosphere are clearly non-conservative; in many cases the footwall is formed by gabbro emplaced at the same time spreading was accommodated at the ridge axis. Like continental detachment faults, the active part of an oceanic detachment may be a few hundred meters to a few kilometers long; the `extruded' footwall may be tens of kilometers in the slip direction. We propose a model in which the footwall is continuously accreted as the detachment system moves. If magmatism dominates, the footwall exposes a large tract of mid-lower crustal gabbro, if magmatism is reduced or episodic, the footwall may comprise a mixed suite of interleaved screens of peridotite and gabbro. If oceanic detachment faults are so non-conservative, the use of the term `detachment fault' may be a misnomer.
DE: 3035 Midocean ridge processes
DE: 8010 Fractures and faults
DE: 8031 Rheology: crust and lithosphere (8159)
DE: 8109 Continental tectonics: extensional (0905)
SC: Tectonophysics [T]
MN: Fall Meeting 2005