HR: 0830h
AN: T51F-0218    [PDF]
TI: Insights into along-strike passive continental margin variability from seismic reflection, refraction and gravity data, Northwest Australia
AU: * Tischer, M
EM: mtischer@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Columbia University P.O. Box 1000 61 Route 9W, Palisades, NY 10964 United States
AU: Ten Brink, U
AF: USGS - Woods Hole Field Center, 384 Woods Hole Road Quissett Campus, Woods Hole, MA 02543-1598 United States
AU: Karner, G
AF: Lamont-Doherty Earth Observatory, Columbia University P.O. Box 1000 61 Route 9W, Palisades, NY 10964 United States
AU: Goodliffe, A
AF: SOEST, University of Hawaii 1680 East-West Rd., Honolulu, HI 96822 United States
AU: Robb, M
AF: SOEST, University of Hawaii 1680 East-West Rd., Honolulu, HI 96822 United States
AU: Taylor, B
AF: SOEST, University of Hawaii 1680 East-West Rd., Honolulu, HI 96822 United States
AU: Driscoll, N
AF: Scripps Institution of Oceanography, UCSD 9500 Gilman Drive, La Jolla, CA 92093 United States
AU: Ryan, D
AF: Geoscience Australia, GPO Box 378, Canberra, ACT 2601 Australia
AB: We image for the first time the complete continent-ocean transition zones for the Northwest Australian passive margin, using recently collected multi-channel seismic reflection, ocean bottom seismometer wide-angle, and gravity data. Our results help to classify the margin, provide crustal models that shed light on margin formation and architecture variability, and bear insight into the evolution of adjacent margin segments. The marginal Exmouth Plateau, which forms the northern portion of the Northwest Australia margin, has been previously characterized as a wide, non-volcanic passive margin. Recently, several studies have questioned this classification: it has been suggested that its formation was due to rifting along an eastward dipping detachment that caused large lower crustal extension and was accompanied by little upper crustal brittle deformation on the plateau. Our crustal model supports this formation model, indicating a very wide continent-ocean transition zone (200 km) and a highly thinned lower crust. Besides several seaward-dipping reflector groups visible in the seismic reflection data, seismic refraction and gravity data also indicate a zone of high p-wave velocities of significant lateral extent at the bottom of the lower crustal layer beneath the plateau. This is possibly caused by underplating or magmatic intrusions, which supports the idea of the Exmouth Plateau being a volcanic margin. In contrast, the Cuvier margin, located south of Exmouth Plateau and separated from it by the Cape Range fracture zone is thought to represent a more typical volcanic margin due to its narrow width, the presence of seaward-dipping reflectors and the anomalously thick oceanic crust. Our crustal and gravity modeling confirms this by showing a relatively narrow continent-ocean transition zone (70 km) bounded oceanward by oceanic crust that thickens from 7 to 10 km towards the transition zone. Below oceanic crust, there is a zone of unusually high mantle velocity (8.5 km/s) of large lateral extend (100 km) that we believe can be attributed to mantle anisotropy with its fast velocity in the margin-normal direction. If our interpretations hold true, small-scale convection across the Cape Range fracture zone (margin-parallel), invoked in this area to explain the excess volcanism visible along the Cuvier margin is not supported by our data and other mechanisms which can explain margin variability along the Northwest Australian margin need to be sought.
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3025 Marine seismics (0935)
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 8015 Local crustal structure
SC: Tectonophysics [T]
MN: 2003 Fall Meeting