The role of lower crust and continental upper mantle during formation of non-volcanic passive margins: evidence from the Alps
Othmar Müntener, Jörg Hermann, 2001. "The role of lower crust and continental upper mantle during formation of non-volcanic passive margins: evidence from the Alps", 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 remnants of a Mesozoic passive continental margin and of the Tethyan ocean floor are preserved in the Austroalpine and Upper Penninic nappes in eastern Switzerland and northern Italy. Reconstructions of the continent-ocean transition indicate that large areas of subcontinental mantle rocks, but only limited areas of lower-crustal rocks were exposed on the Tethyan sea floor. Microstructures, large shear zones, and the retrograde metamorphic evolution of peridotite and gabbro from Malenco (northern Italy) are investigated to evaluate the role of lower crust and upper mantle during formation of non-volcanic passive continental margins. The combination of petrological constraints and microstructures suggests two contrasting stages: (1) high-temperature (> 650°C) shearing and annealing of microstructures are attributed to pre-rift tectonics; (2) localized mylonitic shear zones cut the high-temperature structures and developed during nearly isothermal decompression (T < 600°C), followed by cooling and hydration of the rocks. These shear zones formed during exhumation of the lower crust and upper mantle and are related to early rifting of the Adriatic passive continental margin. The microstructures of the hydrous mylonites display drastic grain-size reduction, which results from a combination of dynamic recrystallization and metamorphic hydration reactions at temperatures <650°C. Strain softening facilitated the formation of crustal-scale shear zones along which the lower crust and upper mantle were exhumed to shallow crustal levels of c. 10–15 km. Such large shear zones excised 10−20 km of mostly intermediate and lower crust, and are linked to and contemporaneous with the formation of rift-related basins in the upper crust. Boudinage of the lower crust during early rifting is proposed as a major process to explain the scarcity of exposed lower crust along non-volcanic passive margins. The compilation of pressure-temperature data and rift-related structures in the deep crust and upper continental mantle from the Alps suggests that most peridotites preserve a high-temperature evolution that is not related to Mesozoic rifting. Granulite-facies rocks occur in pre-rift lower and middle continental crust. Exhumed granulites along passive continental margins preserve much of a history that is not related to the exhumation itself, but to tectonic processes predating rifting.
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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.