Anomalous melt production after continental break-up in the southern Iberia Abyssal Plain
T. A. Minshull, S. M. Dean, R. S. White, R. B. Whitmarsh, 2001. "Anomalous melt production after continental break-up in the southern Iberia Abyssal Plain", Non-Volcanic Rifting of Continental Margins: A Comparison of Evidence from Land and Sea, R. C. L. Wilson, R. B. Whitmarsh, B. Taylor, N. Froitzheim
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Recent geophysical work and Ocean Drilling Program drilling in the southern Iberia Abyssal Plain have indicated that, in a transition zone up to 170 km wide between thinned continental crust and oceanic crust, the basement consists of serpentinized peridotite mantle with sparse mafic intrusive or extrusive rocks. There is no evidence for the addition of significant magmatic material to the stretched continental crust landward of this zone during the last phase of rifting, whereas seaward of this zone, where the halfspreading rate is about 10 mm a-1, the crust rapidly reaches a thickness of c. 6 km, which is normal for Atlantic oceanic crust. Models of melt generation during pure shear, finite-duration continental rifting can successfully reproduce the observed absence of significant syn-rift magmatism on, within and beneath the thinned continental crust if the rifting episode is longer than 10–20 Ma. However, for normal mantle potential temperatures, such models predict significant melt generation in the transition zone seaward of the thinned continental crust even for rift durations longer than 20 Ma. Restricted melting beneath the transition zone might be explained partly by lateral heat loss to the adjacent continental lithosphere, by anomalously low mantle potential temperatures at break-up time, or by depth-dependent stretching such that the observedinfinite stretching factor for the crust is not representative of the lithosphere as a whole. An additional mechanism for restricted melt production involves a transitional state between the end of continental extension and the onset of steady-state sea-floor spreading, during which mantle upwelling is less focused than at normal oceanic spreading centres.
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Non-volcanic continental margins may form up to 30% all present-day passive margins, and remnants of them are preserved in mountain belts. The papers in this volume demonstrate the benefits of integrating offshore and onshore studies, and illustrate the range of information obtained at different scales when comparing evidence from land and sea. Data sets collected across a range of spatial scales are evaluated: thin sections, cores, outcrops, seismic reflection profiles, and other geophysical data. The outcrop scale is crucial because it enables the spatial gulf to be bridged between DSDP and ODP cores and marine seismic data. There is also the problem that basins on land and beneath the sea inevitably have had different post-rift histories resulting in their contrasting present-day elevation. In mountain belts, portions of continental margins and oceanic crust are superbly exposed, but dismembered by subsequent compressional tectonics. Off present-day passive margins, extensional features have only been slightly deformed, if at all, by compressional movements, but are buried beneath significant thicknesses of post-rift sediments and so can only be sampled by ocean drilling at a small number of points.
The first paper reviews the synergies that have occurred between investigations of the eastern North Atlantic non-volcanic margins and remnants of similar Mesozoic margins preserved in the Alps, and some later papers return to this theme. However, papers describing margins from other parts of the world show that it may be premature to use models based on the Atlantic and the Alps as the paradigm for all non-volcanic margins. The following 25 papers in the book are grouped under the following headings: (1) Margin overviews; (2) Exhumed crust and mantle; (3) Tectonics and stratigraphy; (4)Numerical models of extension and magmatism.