Exhumation of oceanic blueschists and eclogites in subduction zones :Timing and mechanisms

Direct observation and extensive sampling in ancient margins exposed in the Alps, combined with drill-hole and geophysical data from the present-day Iberia margin, result in new concepts for the strain evolution and near-surface response to lithospheric rupturing at magma-poor rifted margins. This paper reviews data and tectonic concepts derived from these two margins and proposes that extension, leading to thinning and final rupturing of the continental lithosphere, is accommodated by three fault systems, each of them characterized by a specific temporal and spatial evolution during rifting of the margin, by its fault geometry, and its surface response. The data presented in this paper suggest that margin architecture and distribution of rift structures within the future margin are controlled first by inherited heterogeneities within the lithosphere leading to a contrasting behaviour of the future distal and proximal margins during an initial stage of rifting. The place of final break-up appears to be determined early in the evolution of the margin and occurs where the crust has been thinned during a first stage to less than 10 kilometres. During final break-up, the rheology of the extending lithosphere is controlled by the thermal structure related to the rise of the asthenosphere and by serpentinization and magmatic processes.

http://www.geosci.usyd.edu.au/users/prey/Teaching/Geos-3003/geos3003papers/Extension/Manatschal_magma_poor_rifted_margins_IJES2004.pdf

New models for evolution of magma-poor rifted margins based on a review of data and concepts from West Iberia and the Alps

The continental margin of the Mesozoic Tethys in the Western Alps

From rifting to drifting: tectonic evolution of the South-Alpine upper crust from the Triassic to the Early Cretaceous

Summary This overview summarizes aspects of 150 years of research in Alpine tectonics and in particular introduces the tectonic setting for the more detailed papers in this volume. The Alpine Mesozoic ocean, Tethys, formed as a large elongate pull-apart basin in the Jurassic, as a consequence of the opening of the Atlantic and of the movement of Africa towards the east relative to a fixed Europe. The NNE trending Tethys was bounded by WNW trending transforms, by the European/Iberian margin in the W and by the Adriatic promontory of Africa in the E, and its shape determined the present day configuration of the arcs of the Alpine chain. The closing of this ocean and the collision tectonics began during the Cretaceous, as Africa moved to the NE relative to Europe and as the N Atlantic gradually opened, to drive Iberia and the southern part of the European plate to the E. Subduction of oceanic crust and adjacent continental crust led to high pressure metamorphism of Cretaceous age. Ophiolites were obducted over the southern continental margin, but after collision the shear sense reversed so that the Austro-Alpine nappes of the African Adriatic promontory overthrust Europe in a WNW direction. During the main Tertiary deformation the overall anticlockwise rotation of Africa led to a change-over from N to NW and WNW-directed collisional structures. The E-W striking sector of the Alps in Switzerland and Austria is therefore a diffuse transpressive dextral shear belt, approximately reworking the northern transform boundary of Tethys, modifying it by compression related to the rotation of the African Adriatic promontory. Approximately 250 km of European lithosphere were involved in the building of the western Alps. As Alpine nappes consist largely of rock material confined to the upper crust, a large amount of lower crust and lithospheric mantle of the two continental blocks must be duplicated and/or subducted during the Alpine collision history.

Hubert Arnaud

Papers

The continental margin of the Mesozoic Tethys in the Western Alps

Paléogéographie des calcaires urgoniensdu sud de la France

Salt tectonics, Tethyan rifting and Alpine folding in the French Alps

Interpretation of gamma-ray logs: The distribution of uranium in carbonate platform

Urgonian deposits and Barremian-Early Aptian sequence stratigraphy in the Vercors Massif.