Residual depth anomalies on the Iberian, Newfoundland, Labrador and Nova Scotian margins; implications for their lithosphere mass and density distribution

Details Residual depth anomalies on the Iberian, Newfoundland, Labrador and Nova Scotian margins; implications for their lithosphere mass and density distribution Residual depth anomalies on the Iberian, Newfoundland, Labrador and Nova Scotian margins; implications for their lithosphere mass and density distribution

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.t41a0365c&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #T41A-0365

Physics

8105 Continental Margins: Divergent (1212, 8124), 8138 Lithospheric Flexure

Scientific paper

The break-up of the Iberian and Newfoundland margins has resulted in large scale asymmetry between these conjugate margins. One documented asymmetry is that the oldest oceanic crust on the Newfoundland margin is elevated with respect to that of the Iberian margin. This work employs residual depth anomalies (RDA) to investigate this asymmetry. Observed bathymetry has been compared to the global oceanic bathymetry-age models of Parsons and Sclater (1977) and Stein and Stein (1992) in order to calculate RDA. The oceanic crust on the Nova Scotia and Labrador margins have also been included in this study in order to explore the lithosphere mass and density distribution across the wider region. Seismically derived cross-sections have been flexurally backstripped to correct the RDA for sediments. A correction has also been made to the RDA for seismically observed variation in oceanic crustal thickness, about the global mean thickness, using local isostasy. Key observations of the corrected RDA are: (1) oceanic crust on the Labrador and the Newfoundland margins are over a kilometre shallower than predicted by the global bathymetry-age models; (2) oceanic crust on the Newfoundland margin is elevated by 700 to 900m relative to its conjugate Iberian margin; (3) the RDA decrease oceanwards; (4) the RDA decrease southwards on both sides of the North Atlantic. Potential sources of the observed RDA have been investigated, including flexural coupling of oceanic and continental lithosphere, mantle plume uplift, serpentinization, magmatic intrusions and lithospheric mantle geochemical heterogeneity. Modelling shows flexural coupling of oceanic and continental lithosphere, during thermal subsidence of the oceanic lithosphere, may be the sole source of the ocean-wards decrease in RDA. However, the elevation of the Newfoundland margin relative to the Iberian margin can not be explained by flexural isostatic coupling. The Iceland plume might be the source of the southerly decrease in RDA; however, this is unlikely, due to its large distance from the margins. Moreover, the Iceland and Azores plumes are unlikely to be the source of the elevation of the Newfoundland margin relative to the Iberian margin, since the plumes are approximately the same distance from the margins. A possible explanation for the elevation of the Newfoundland margin relative to the Iberian margin, is a compositional mass deficiency within the ocean-continent transition (OCT) of the Newfoundland margin. Possible compositional deficiencies include gabbroic intrusions, serpentinized lithospheric mantle and lithospheric mantle depletion. The quantities of these compositional deficiencies, required to generate the observed positive RDA, has been estimated using local isostasy. If all the observed positive RDA is caused by a compositional mass deficiency then the required thicknesses of gabbroic intrusions, fully serpentinized mantle or partially serpentinized (50 to 60%) mantle are 4 - 10 km; 2 - 5 km or 3.5 - 9 km respectively. Partial depletion of the entire lithospheric mantle, within the OCT of the Newfoundland margin is an alternative explanation for its elevation relative to the Iberian margin. If mantle depletion is the sole source of the positive RDA of the Galicia Bank (northern Iberia) and Flemish Cap (northern Newfoundland) margins, then the entire lithospheric mantle within their OCT must be depleted by 11 and 23% respectively. These depletion levels are consistent with the measured depletions of mantle rocks from the two margins (Muntener and Manatschal, 2006).

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