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The role of lower crust and continental upper mantle during formation of non-volcanic passive margins: evidence from the Alps

By
Othmar Müntener
Othmar Müntener
Institut für Mineralogie und Petrographie, ETH Zürich, CH 8092 Zürich, SwitzerlandPresent address: Geology Institute, University of Neuchâtel, CH-2007 Neuchâtel, Switzerland (e-mail:othmar.muentener@unine.ch)
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Jörg Hermann
Jörg Hermann
Institut für Mineralogie und Petrographie, ETH Zürich, CH 8092 Zürich, SwitzerlandPresent address: Geology Institute, University of Neuchâtel, CH-2007 Neuchâtel, Switzerland (e-mail:othmar.muentener@unine.ch)
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Published:
January 01, 2001

Abstract

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. 1015 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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Contents

Geological Society, London, Special Publications

Non-Volcanic Rifting of Continental Margins: A Comparison of Evidence from Land and Sea

R. C. L. Wilson
R. C. L. Wilson
The Open University, UK
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R. B. Whitmarsh
R. B. Whitmarsh
Southampton Oceanography Centre, UK
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B. Taylor
B. Taylor
University of Hawaii, Hawaii
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N. Froitzheim
N. Froitzheim
University of Bonn, Germany
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Geological Society of London
Volume
187
ISBN electronic:
9781862394353
Publication date:
January 01, 2001

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