Transcurrent ductile shear zones and the dispersal of the Avalon superterrane

Abstract

Abstract A mosaic of steep, often sinistral, transcurrent shear zones cuts late Precambrian to earliest Cambrian rocks in eastern Canada, southern Britain, and northwest France. Examples include mylonite belts in Saint John (New Brunswick), Cobequid Highlands (Nova Scotia), Burin Peninsula (Newfoundland), Malverns (England), and N Brittany (France). These shear zones often separate geologically distinct areas, each broadly of Avalonian or Cadomian character, that together are grouped into an Avalon superterrane. The shear zones occur as: (1) major fault systems juxtaposing unrelated geological units, e.g. the Dover-Rosslare-Menai Strait mylonite belt, a well defined transcurrent terrane boundary that locally contains exotic slivers of blueschist; (2) shear zones separating high grade gneisses from low grade metasediments which may represent tectonized basement/cover sequences (e.g. Brookville Gneiss/Green Head Group of New Brunswick; Bass River Complex/Gamble Brook Formation of Nova Scotia; Rosslare Complex/Cullenstown Formation in SE Ireland); (3) shear zones (locally with syntectonic granites) separating migmatitic gneisses from low grade rocks of similar protolith, e.g. the Cancale shear zone affecting the Brioverian of northern Brittany; such boundaries may form due to transtensional shearing within an arc system above an obliquely subducting oceanic plate; (4) shear zones separating rocks of similar character so that no case exists for defining the ductile fault as a suspect terrane boundary (e.g. Malverns); (5) long-active steep fault systems, the earliest movements along which have been obscured by Phanerozoic reactivation (e.g. Welsh Borderlands Fault System). Constraints on the timing of the ductile fault movements remain poor, although in Britain and France an early Cambrian age is implied. The transcurrent faulting dismembered and dispersed a long-active late Precambrian arc system and provided a structural template that localized subsequent fault movements and so exerted a major controlling influence on the Phanerozoic geology of the region. Use of the term ‘Avalonian/Cadomian Orogeny’ refers to subduction and terrane dispersal events during the early history of the SE side of the Caledonian/Appalachian Orogen.