The break-up of continental lithosphere in magma-poor margins is
accompanied by the exhumation of mantle and crustal rocks in the
footwall of large-scale detachment faults. Although these structures
have been described from many modern and ancient margins, little is
known about how they accommodate strain and evolve in time and space
during continental break-up. The Tasna Ocean-Continent Transition (OCT)
in southeastern Switzerland is one of the rare examples where such
detachment faults are exposed and can be observed on a kilometre scale.
In this paper we describe the deformation structures and their
evolution observed along detachment faults in the Tasna OCT. Our
results show that continental break-up was attained by a series of
detachment faults. These detachment faults accommodated extensional
strain in fault zones formed by a localized core zone that is
surrounded by a several tens to hundred metres wide damage zones. The
core zone corresponds to the zone of highest strain and is well defined
structurally by the occurrence of gouges and/or foliated cataclasites
and physically by separating a hanging wall from a footwall.
Deformation in the fault zones occurred under greenschist facies to
seafloor conditions and within the stability field of serpentine. U/Pb
ages on zircon from a garnet-bearing pegmatite cross cutting
high-temperature shear zones (upper amphibolite facies and higher)
provide Carboniferous ages and demonstrate that these shear zones are
neither kinematically nor genetically related to the detachment faults
observed in the Tasna OCT. Ar/Ar ages on phlogopite from spinel
websterite suggest that mantle exhumation occurred during Middle
Jurassic time. Our data show that the detachment faults observed in the
Tasna OCT formed during latest rifting, post-date major thinning of the
crust and onset of mantle serpentinization. These results compare well
with those from the deep Iberia margin. Our observations support the
idea that rifting leading to continental break-up is a multi-phase
process, and that serpentinization is the consequence rather than the
reason for strain localization at non-volcanic margins. Apart from the
more general implications for the tectonic evolution of continental
break-up, our results have some important consequences for the
palaeogeographic reconstruction of certain Alpine domains and question
the existence of an independent Early Cretaceous Valais ocean in the
Alpine realm. (c) 2006 Elsevier Ltd. All rights reserved.