HR: 13:40h
AN: T23F-01 INVITED     [Abstracts]
TI: Tectonic Windows Throw Light on the Processes That Create the Upper Ocean Crust
AU: * Cann, J
EM: j.cann@earth.leeds.ac.uk
AF: School of Earth Sciences, University of Leeds, Leeds, LS2 9JT United Kingdom
AB: Several of the most critical problems of the creation of oceanic crust at mid-ocean ridges cannot be answered by observations of the ocean floor alone because of the lack of a third dimension. Models abound, but need testing. Examples of the problems are: What is the width of the zone of crustal construction, including the distribution of dike intrusion about the spreading axis and the role of off-axis eruptions in creating the volcanic section? What is the nature of the hydrothermal reaction zones below the mid-ocean ridges in which seawater is transformed to black smoker fluid? Are rotations of the upper crust produced by loading during the construction of the lava pile, or in the hanging wall of near-axis faults? What proportion of the crustal extension by spreading takes place by faulting as opposed to dyke intrusion? Results from ocean drilling act as a partial constraint, but generalisation from these one-dimensional samples is not easy. Ophiolites can begin to bridge the gap, but both their tectonic environment of formation and their spreading rate remain controversial. In addition no intact ophiolite is known that contains a magnetic reversal, and that turns out to be an important limitation. Tectonic windows allow integration of research on a section through mature upper ocean crust with studies of the adjacent sea floor, and can provide sections tens of kilometres long that comfortably span a magnetic reversal. Especially fruitful have been wide-ranging projects that include petrology, palaeomagnetism, geochemistry and fluid inclusion studies as well as seafloor imagery at visual and sidescan scales and magnetic field surveys. Results from studies of tectonic windows published to date show significant progress in answering some of the questions posed above. For example, rotations in the upper crust have been shown to be towards the spreading centre rather than away from it as one of the faulting models would predict. Other questions will be harder to tackle, especially the last, for which individual faults must be tracked down into the crust to determine their geometry and the extent to which they have been active during volcanism and hence have a throw that increases with depth. Further observations through tectonic windows will help clarify and generalise the results of those already made, and will significantly improve our understanding of the processes that create the ocean crust.
DE: 3035 Midocean ridge processes
DE: 3045 Seafloor morphology, geology, and geophysics
DE: 8424 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8135)
SC: Tectonophysics [T]
MN: Fall Meeting 2005