HR: 1330h
AN: T12A-0436    [PDF]
TI: North Atlantic Rifted Margin Crustal Thickness and Crustal Thinning from Satellite Gravity Data
AU: * Hurst, N W
EM: N.Hurst@liverpool.ac.uk
AF: Department of Earth Sciences, Liverpool University, Liverpool, L69 3BX United Kingdom
AU: Kusznir, N J
EM: sr11@liverpool.ac.uk
AF: Department of Earth Sciences, Liverpool University, Liverpool, L69 3BX United Kingdom
AU: White, R S
EM: rwhite@esc.cam.ac.uk
AF: Bullard Laboratories, Cambridge University Madingley Rd, Cambridge, CB3 0EZ United Kingdom
AB: Crustal thickness has been derived from satellite gravity data for the North Atlantic between 50 and 70 degrees N in order to determine rifted margin crustal thinning. Satellite derived gravity anomaly data (Smith \& Sandwell 1997) and bathymetry data (Gebco 2003) are used to derive the mantle residual gravity anomaly which is then inverted to give Moho depth. The gravity anomaly inversion to determine Moho depth is carried out in 3D in the spectral domain. Oceanic lithosphere and stretched continental margin lithosphere contain a large negative residual thermal gravity anomaly (up to - 380 mgal) which must be corrected for in order to determine Moho depth. This thermal gravity correction may be determined for oceanic lithosphere using oceanic isochron data, and for the thinned continental margin lithosphere using margin rift age and beta stretching estimates iteratively derived from crustal basement thickness determined from the gravity inversion. Beta stretching estimates and the thermal correction rapidly converge within a few iterations. The use of the thermal correction within the gravity inversion has been tested by producing profiles of derived crustal thickness running across the mid-Atlantic Ridge from Hatton and Edoras Banks to their East Greenland conjugate margins. The gravity inversion using the thermal gravity correction predicts oceanic crustal thicknesses consistent with seismic observations, while that without the thermal correction predicts much too great oceanic crustal thicknesses. Because of errors in the location of the ocean-continent transition within the oceanic isochron data set, two approaches may be used to define the thermal gravity correction for the determination of rifted margin crustal thickness. In option 1, oceanic isochron data is used to define the location of the COB, the oceanic lithosphere beta stretching factor is assumed to be infinity, and ocean isochron data is used to define oceanic age for the calculation of the thermal gravity anomaly correction. Option 1 has the advantage that it correctly predicts increasing thermal gravity anomaly correction towards the ocean ridge, but the disadvantage that it unrealistically assumes that oceanic isochrons (and therefore the COB) are accurately known. In option 2, the oceanic isochron data is ignored and is not used to define the COB, and the thermal gravity correction is determined in a similar way to that for continental lithosphere by using the rift age specified for continental breakup and lithosphere beta stretching factors derived from crustal basement thickness from gravity inversion. Option 2 has the advantage that it does not assume the location of the COB, but the disadvantage that it fails to predict increasing thermal gravity correction towards the ocean ridge, and as a consequence incorrectly predicts thickening of oceanic crust with decreasing oceanic age. In the absence of reliable sediment thickness data, sediment thickness is assumed to be zero giving an upper bound of Moho depth, crustal basement thickness and continental beta stretching factor. In the absence of seismic estimates of volcanic addition thickness (magmatic underplating) for continental margin crust, volcanic addition is assumed to be zero also giving a lower bound of continental beta stretching factor.
DE: 1234 Regional and global gravity anomalies and Earth structure
DE: 4263 Ocean prediction
DE: 8105 Continental margins and sedimentary basins
DE: 8109 Continental tectonics--extensional (0905)
DE: 8122 Dynamics, gravity and tectonics
SC: Tectonophysics [T]
MN: 2003 Fall Meeting