The role of syn-rift magmatism in the rift-to-drift evolution of the West Iberia continental margin: geophysical observations
R. B. Whitmarsh, T. A. Minshull, S. M. Russell, S. M. Dean, K. E. Louden, D. Chian, 2001. "The role of syn-rift magmatism in the rift-to-drift evolution of the West Iberia continental margin: geophysical observations", 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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The presence of a well-defined ocean-continent transition (OCT) and the absence of large volumes of extrusive or intrusive rocks on the West Iberia margin make it a good place to investigate how the largely amagmatic rifting and break-up of continental lithosphere evolves into oceanic crust produced by magmatic sea-floor spreading. In the southern Iberia Abyssal Plain there is a broad OCT with a characteristic seismic and magnetic character, distinct from both thinned continental crust and normal oceanic crust, which supports the notion that it consists predominantly of exhumed and serpentinized mantle. Interpretations of magnetic and seismic data indicate that on average only small amounts of syn-rift melt exist within the OCT. Isolated, probably margin-parallel, intrusive melt bodies are scattered within the eastern part of the OCT well beneath the top of acoustic basement. Within the western part of the OCT, closer to unambiguous sea-floor spreading magnetic anomalies, such bodies were later(?) emplaced at higher levels and more closely together in the basement until eventually sea-floor spreading began. The evidence does not support the hypothesis that ultraslow sea-floor spreading can explain the magnetic anomalies observed within the wider parts of the West Iberia OCT, where the OCT evolution is best resolved.
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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.