Structural evolution of the Gander-Avalon terrane boundary: a reactivated transpression zone in the NE Newfoundland Appalachians

Abstract

In the Appalachian Orogen of NE Newfoundland, the Gander and Avalon zones are separated by the 1–2 km wide, dextral, Dover Fault Shear Zone. This structure overprints sinistral transpressive deformations that affect broad regions either side of the Gander-Avalon boundary and is post-dated by ?Alleghenian (or younger) brittle dextral faults that are not thought to record large regional displacements. In ?Lower Palaeozoic sedimentary rocks of the Gander Lake Subzone, a steeply dipping sinistral shear zone (up to 20 km wide) was the focus of syn-tectonic sillimanite grade metamorphism, migmatization and granitic plutonism. In lower greenschist facies late Precambrian sedimentary and volcanic rocks of the Avalon Zone, early sinistral displacements were focused into a high strain zone ( c . 2km wide) along the boundary. Although developed at different metamorphic grades, these early sinistral transpressive shear zones can be correlated and are thought to record the main phase of sinistral-oblique accretion and regional orogenesis during the mid-Silurian ( c. 415-435 Ma). The switch to dextral displacements forming the Dover Fault Shear Zone appears to have initiated during later stages of syntectonic plutonism in the Gander Lake Subzone, suggesting that it may also be Silurian in age. No evidence links the Gander and Avalon zones prior to sinistral transpression. Sinistral fabrics overprint flat-lying structures in the Gander Lake Subzone that are thought to be related to mid-Ordovician accretion of the superjacent Exploits (Dunnage) arc-ophiolite terrane to the west. The Gander and Dunnage allochthons were probably accreted to the Laurentian margin prior to the sinistral-oblique collision of Western Avalonia during the Silurian and they overlie the suture that separates Laurentia from the Central Lower Crustal Block of Avalonian affinities. The Gander-Avalon boundary in NE Newfoundland would then form the steeply-dipping surface expression of a collision-linked sinistral shear zone that allowed lateral decoupling of the Avalon and Central Lower Crustal Blocks during or following oblique collision. Along the margin of the Gander Lake Subzone, the shear zone-hosted belt of sheeted granites and melting increasingly became the focus of deformation during the later stages of sinistral transpression. This suggests significant rheological softening of the crust that would account for the ability of transpressive terrane boundaries to accommodate large displacements. Furthermore, the estab-lishment of a thermal anomaly would have a long-lived weakening effect that could explain why such terrane boundaries are prone to reactivation.