HR: 16:30h
AN: T34A-03    [Abstracts]
TI: Mantle Exhumation at Magma-Poor Rifted Margins due to Melt Suppression During Continental Break-up and Seafloor Spreading Initiation.
AU: * Fletcher, R
EM: rjf@liv.ac.uk
AF: University of Liverpool, 4 Brownlow Street, Liverpool, L69 3GP, United Kingdom
AU: Kusznir, N
EM: n.kusznir@liv.ac.uk
AF: University of Liverpool, 4 Brownlow Street, Liverpool, L69 3GP, United Kingdom
AU: Cheadle, M
EM: Cheadle@uwyo.edu
AF: University of Wyoming, Dept. 3006, 1000 University Ave, Laramie, WY WY 82071, United States
AB: Rifted continental margins exhibit large variations in magmatic activity. Non-volcanic margins may display a broad ocean-continent transition, up to 150 km wide, of exhumed mantle separating oceanic crust from thinned continental crust, whilst voluminous volcanism accompanies break-up at volcanic margins. Previous studies have shown the importance of asthenospheric temperature (White and McKenzie, 1989), lithosphere thinning rate (Bown and White, 1995; Pérez-Gussinyé et al., 2006) and initial continental geotherm (Reston and Morgan, 2004) on melt production at rifted margins, assuming that continental break-up occurs by pure-shear stretching of the lithosphere. Pure-shear models of continental lithosphere thinning generally predict melt generation before continental break-up, unless anomalously cool asthenosphere temperatures (Minshull et al., 2001) or depleted mantle source (Pérez-Gussinyé et al., 2006) are invoked. As a consequence pure-shear models have difficulty explaining mantle exhumation at non-volcanic margins. We model the onset and development of melt production during rifting of continental margins and seafloor spreading initiation using a model of depth- dependent lithospheric thinning and extension. For a mantle potential temperature of 1333°C and a half- spreading rate of 10mm/yr we predict that approximately 100km of lower crust and mantle is exhumed prior to melt production. Melt production reaches a steady state as the model reaches thermal equilibrium and seafloor spreading proceeds. The predicted width of exhumed mantle increases if the modelled spreading rate is decreased, if melt is retained in the mantle (e.g. as gabbroic intrusions), or if the subcontinental mantle is initially cool or depleted. In this model the thinning of continental lithosphere leading to break-up and sea-floor spreading initiation occurs in response to an upwelling and divergent flow-field. The melt parameterisations of Katz et al. (2003) are used to calculate melt fractions in the model space. The ability of the model to predict mantle exhumation prior to the onset of volcanism and normal seafloor spreading suggests that an upwelling and divergent flow-field may provide a better kinematic representation of continental break-up at non-volcanic margins (e.g. Iberia) than a pure shear model.
DE: 1037 Magma genesis and partial melting (3619)
DE: 4255 Numerical modeling (0545, 0560)
DE: 8105 Continental margins: divergent (1212, 8124)
DE: 8178 Tectonics and magmatism
SC: Tectonophysics [T]
MN: 2007 Fall Meeting