HR: 16:00h
AN: V12J-01    [PDF]
TI: Unconformities in Slow-Spread Oceanic Crust: Implications for Spreading Processes and Dismembered Ophiolites
AU: * Karson, J A
EM: jkarson@duke.edu
AF: Division of Earth & Ocean Sciences, Duke University, Durham, NC 27708-0230 United States
AB: Parallel studies of ophiolites and oceanic lithosphere have provided progressively more sophisticated and complex views of processes at mid-ocean ridge spreading centers. Investigations of major seafloor escarpments in slow-spread lithosphere provide tectonic windows into the geology of oceanic crust that may be interpreted in terms of spreading processes. Traditional models of oceanic crust, largely based on the internal structure of ophiolite complexes, anticipate a laterally persistent, predictable layered assemblage of rock units including the familiar sequence of pillow basalts, sheeted dikes, and gabbroic rocks overlying mantle peridotites. However, direct observations in slow-spread oceanic crust of the Mid-Atlantic Ridge commonly reveal a diverse array of crustal structures generated at a slow-spreading rate and variable magma supply. One of the most common features of crustal sections found in these settings are unconformities in which relatively fresh basaltic lavas and/or sedimentary rocks directly overlie other variably deformed and metamorphosed crustal or upper mantle rock units. Crustal sections in different areas document these types of unconformities over serpentinized peridotites, metagabbros, and lava/dike units. Despite the common exposure of relatively deep crustal and even upper mantle material, sheeted dike complexes are rare in these settings. Major unconformities represent discontinuities in magmatic accretion during continued tectonic extension, which may be common along slow-spreading ridges. Given their peculiar structural settings, crustal structures exposed in these tectonic windows should not be regarded as typical of even slow-spread oceanic lithosphere, but rather as part of a spectrum of structures that reflect crustal accretion with a highly variable and inconsistent magma supply. Some ophiolite complexes that appear to have been dismembered may owe their structural complexity to processes at spreading centers rather than post-obduction processes and may provide details of spreading unconformities.
DE: 1020 Composition of the crust
DE: 1065 Trace elements (3670)
DE: 3035 Midocean ridge processes
DE: 5120 Plasticity, diffusion, and creep
DE: 8005 Folds and folding
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting