HR: 1330h
AN: T12A-0433    [PDF]
TI: Numerical Modelling of the Transition from Continental Rifting to Mantle Exhumation at the West Iberia Margin.
AU: Reston, T J
EM: treston@geomar.de
AF: GEOMAR, Wischhofstr. 1-3, Kiel, 24148 Germany
AU: * Perez-Gussinye, M
EM: martap@earth.ox.ac.uk
AF: Dept. Earth Sciences, Parks Road, Oxford, OX1 3PR United Kingdom
AU: Phipps Morgan, J
EM: jmorgan@geomar.de
AF: GEOMAR, Wischhofstr. 1-3, Kiel, 24148 Germany
AB: The continental margin of West Iberia lacks significant synrift magmatism and exhibits a zone up to 100 km wide thought to consist mainly of serpentinized and exhumed mantle between the thinned continental and the oceanic crust. However, the existence of linear magnetic anomalies pose an ambiguity regarding the exact amount of synrift magmatism produced during mantle exhumation at the surface. We investigate how the thinned continental crust gives way to a broad zone of exhumed and serpentinized mantle with little synrift magmatism. For this we use a finite element code that includes brittle and ductile deformation in both crust and mantle, production of serpentine and melt. Serpentinisation is only allowed to occur when the entire crust has become brittle so that large amounts of water can reach the mantle through brittle faults. The increase in temperature due to the exothermic nature of serpentinisation and the decrease in the coefficient of friction where serpentinisation occurs is also taken into account. Melt production includes the effect of increased depletion in mantle temperatures. In a first approximation, melt is assumed to migrate instantaneously upwards and accumulate at crustal levels. We present tests with a range of extension velocities and asthenospheric temperatures. Preliminary model runs shows how the entire crust becomes brittle after it is has reached a thickness of less than 10 km. For slow rifting velocities ($<$ 5 mm/yr), serpentinisation occurs prior to melting, whereas for faster rifting velocities the opposite is true. In all models, crustal separation and the exposure of mantle at the continent-ocean transition (COT) occurs after the entire crust has become brittle. The relative amount of serpentinite and melt in the COT depends on the rifting velocity, with slower velocities promoting the production of more serpentinite than melt. However, for a normal mantle temperature (1300 C), even for the slow extension rate of 5 mm/yr, 3-4 km of melt should be found at shallow levels in the COT, mixed with serpentinised mantle. In order, to shut off the production of melt a lower asthenospheric temperature is required, e.g. 1200 C.
DE: 8109 Continental tectonics--extensional (0905)
DE: 8159 Rheology--crust and lithosphere
SC: Tectonophysics [T]
MN: 2003 Fall Meeting