Patterns of extension and magmatism along the continent–ocean boundary, South China margin
Peter D. Clift, Jian Lin, 2001. "Patterns of extension and magmatism along the continent–ocean boundary, South China margin", 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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Early Oligocene sea-floor spreading in the South China Sea was preceded by at least two episodes (in Maastrichtian and Mid-Eocene time) of continental extension that generated a series of rift basins on the South China margin, which are separated from the continent–ocean boundary (COB) by an outer structural high. Regional multichannel seismic profiles showing faulting of the pre-rift basement allow the amount of extension in the upper crust to be measured. The total subsidence across the South China margin is far in excess of that predicted using a forward flexural-cantilever model of extension and the degree of faulting measured seismically in the upper crust. This mismatch suggests preferential extension of the lower crust, increasing towards the COB to account for the subsidence. The same feature is seen in the Nam Con Som Basin, which is located close to the southwest end of an extinct propagating spreading ridge offshore from Vietnam. However, in the Beibu Gulf Basin, which is not adjacent to the COB, subsidence is approximately compatible with uniform extension in the upper and lower crust across the entire basin, if not at all locations. We predict that extension of the lower crust exceeds that in the lithospheric mantle along the COB. Heat-flow measurements at Ocean Drilling Program (ODP) sites on the Chinese continental slope and on the conjugate Dangerous Grounds margin yield values consistent with, or slightly higher than, those predicted by models of uniform extension in the lithosphere. Although there is no magmatism comparable with the seaward-dipping volcanic rocks of rifted volcanic margins, there is seismic evidence of rift-related volcanic rocks spanning a width of c. 25 km landward of the COB. Simple adiabatic melting models do not predict magmatism, and we suggest that the presence of water in the mantle lithosphere, together with residual pre-rift heat, may instead be responsible for increasing melting here. Deep-water syn-rift sediments recovered by the ODP near the COB indicate that volcanism was submarine and that rifting culminated in a mass wasting event that marks a break-up unconformity. The average extension in South China Sea is much less than that seen in the extreme ‘non-volcanic’ Iberian margin. The South China margin may represent an intermediary form of continental extension between the end member extremes of the Iberia-type non-volcanic and the Greenland-type volcanic margin.
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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.