Weakening mechanisms in reactivated crustal-scale
faults: the Dover Fault Shear Zone, Newfoundland

Abstract

Crustal-scale faults have been argued to represent relative and absolute zones of weakness in
comparison to the intact continental crust due to their preferential reactivation and accumulation of
strain. In order to understand the long term deformation behaviour of crustal-scale faults, it is
important to study how deformation processes in the mid-crustal frictional-viscous transition zone
(FVTZ) can alter the frictional strength of the crust. Exhumed fault rocks from the Dover Fault
Shear Zone (DFSZ) record evidence of long-term weakening mechanisms. The DFSZ in north
eastern Newfoundland represents a major Appalachian terrane boundary that separates highly
metamorphosed gneisses in the Gander Zone from deformed volcanics in the Avalon Zone.
Analysis of field data, hand specimens and microstructures revealed a series of progressively lower
temperature, overprinting deformation phases in increasingly narrower, localised shear zones. The
fault rocks show increasing strain towards the boundary as grain size reduces, fabric intensifies and
folds tighten and become progressively curvilinear. Evidence of fluid influx during deformation
includes microstructures that are indicative of fluid assisted diffusive mass transfer (DMT) and a
high degree of phyllonitization of the fault rocks.
Increasing strain and structural overprinting towards the centre of the shear zone is indicative of
strain weakening and the later brittle faulting that has reactivated the DFZ is evidence of this long
term weakening. The most important weakening mechanisms to have affected the DFSZ arose from
the syn-tectonic influx of fluids, including both hydrous fluids and magmas, as this led to
production of phyllosilicates in reaction softening, the development of interconnected weak layers
and thermal perturbations in the fault zone. These processes produced a highly localised network of
shear zones whose frictional strengths were permanently reduced, thus impacting the long-term
strength and behaviour of the fault in the upper crust.