Autori: Johnston, S., Hacker B., and Ducea, M.N.

Editorial: Geological Society of America Bulletin, 119, p.1232-1248, 2007.

Rezumat:

The Nordfjord-Sogn Detachment Zone of
western Norway represents an archetype for
crustal-scale normal faults that are typically
cited as one of the primary mechanisms
responsible for the exhumation of ultrahighpressure
(UHP) terranes. In this paper, we
investigate the role of normal-sense shear
zones with respect to UHP exhumation using
structural geology, thermobarometry, and
geochronology of the Hornelen segment of
the Nordfjord-Sogn Detachment Zone. The
Hornelen segment of the zone is a 2-6 km
thick, top-W shear zone, primarily developed
within amphibolite-grade allochthonous
rocks, that juxtaposes the UHP rocks of
the Western Gneiss Complex in its footwall
with lower-grade allochthons and Carboniferous-
Devonian Basins in its hanging wall.
New thermobarometry and Sm/Nd garnet
geochronology show that these top-W fabrics
were initiated at lower crustal depths
of 30-40 km between 410 Ma and 400 Ma.
Structural geology and quartz petrofabrics
indicate that top-W shear was initially relatively
evenly distributed across the shear
zone, and then overprinted by discrete
ductile-brittle detachment faults at slower
strain rates during progressive deformation
and exhumation. These results require
a three-stage model for UHP exhumation in
which normal-sense shear zones exhu

Cuvinte cheie: ultrahigh-pressure rocks, exhumation,