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The Steinmann Trinity revisited: mantle exhumation and magmatism along an ocean-continent transition: the Platta nappe, eastern Switzerland

By
Laurent Desmurs
Laurent Desmurs
Geology Institute, ETH-Zentrum, CH-8092 Zürich, Switzerland (e-mail: desmurs@erdw.ethz.ch)
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Gianreto Manatschal
Gianreto Manatschal
Geology Institute, ETH-Zentrum, CH-8092 Zürich, Switzerland (e-mail: desmurs@erdw.ethz.ch)EOST-UMR 7517 CNRS, Université Louis Pasteur, F-67084 Strasbourg, France
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Daniel Bernoulli
Daniel Bernoulli
Geology Institute, ETH-Zentrum, CH-8092 Zürich, Switzerland (e-mail: desmurs@erdw.ethz.ch)
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Published:
January 01, 2001

Abstract

The close association of serpentinites, basalts and radiolarites, later known as the Steinmann Trinity, was clearly described by Steinmann from the south Pennine Arosa zone and its southern prolongation, the Platta nappe of the eastern Swiss Alps. This classical ‘ophiolite’ is distinctly different from typical fast-spreading ridge associations and can be compared with the transitional crust occurring along non-volcanic passive continental margins in present-day oceans. It includes serpentinized peridotites that we interpret as subcontinental mantle rocks, which were exhumed along low-angle detachment faults and locally overlain by extensional allochthons of continental crust, minor gabbroic intrusions, tholeiitic pillow lavas and flows and a succession of oceanic sediments. The serpentinized peridotites record deformation at falling temperatures during extension leading to final exposure of the mantle rocks at the sea floor and their inclusion in tectonosedimentary breccias (ophicalcites). Field relationships, and mineral-chemical and radiometric data show that the gabbros intruded already serpentinized mantle rocks at shallow depth 161 Ma ago. They are Mg gabbros, Fe gabbros and Fe-Ti gabbros, cut by dioritic pegmatoid veins and albitite dykes, which originated by differentiation from the same parental magma. All gabbros show the same metamorphic evolution, i.e. intrusion at relatively low pressure, oceanic hydration at elevated temperature and a subsequent static oceanic alteration. The pillow lavas stratigraphically overlie the exhumed mantle rocks and the tectono-sedimentary breccias related to the exhumation of both mantle rocks and gabbros. However, both gabbros and pillow basalts are characterized by eNd values typical for an asthenospheric mid-ocean ridge-type source of the melts. They may be the products of a steady process that combined extensional deformation with magma generation and emplacement. They appear to document the onset of sea-floor spreading across an exhumed subcontinental mantle during the earliest phases of a slow-spreading ridge.

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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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