Preliminary investigation of a major high-strain zone in the Caledonian Highlands, southern New Brunswick.

The Caledonian Highlands consist mainly of volcanic, sedimentary, and plutonic rocks formed in a continental margin magmatic arc complex in combination with later extensional tectonic events over a span of at least 70 million years in the late Neoproterozoic. The southern and eastern parts of the highlands are dominated by the Broad River Group, an assemblage of ca. 620 Ma rocks, including intermediate and felsic tuff, chloritic phyllite, and arkosic sandstone and conglomerate. Plutonic rocks of inferred ca. 620 Ma age occur in spatial association with the Broad River Group, and show a wide range in composition from gabbro and diorite to tonalite, granodiorite, and granite. The ca. 560-550 Ma Coldbrook Group forms most of the western part of the highlands, but also extends into the eastern part, where it is inferred to originally have had an unconformable relationship with the underlying Broad River Group. Typical Coldbrook Group rocks include intermediate to felsic lapilli tuff, dacitic to rhyolitic flows and plugs, laminated tuffaceous siltstone, amygdaloidal basalt flows, and coarse clastic sedimentary rocks. Circa 560-550 Ma plutons are widespread throughout the central and western parts of the Caledonian Highlands intruded into the lower units of the Coldbrook Group. Most consist of syenogranite with less abundant diorite and gabbro. A major high-strain zone up to 5 km in width can be traced for at least 70 km diagonally across the highlands from at least the Big Salmon River area in the southwest to the Caledonia Mountain area in the northeast. In this zone, both the Broad River Group and associated plutons and the Coldbrook Group contain similar structural elements, related to a largely shared deformation history. Some of this history is apparent also in the 560-550 Ma plutonic rocks. A pervasive foliation (S1) lies parallel to bedding (S0), and although evidently composite (S0-1) in the Broad River Group, this fabric is very heterogeneous in the younger Coldbrook Group, where low strain enclaves are widespread. No folds have been seen of an F1 generation, and no reversals of facing or vergence are apparent. A mineral lineation (L1m) is locally prominent. The plutonic rocks have early fabrics, including a foliation (S1) producing augen-gneiss with a prominent L-tectonite (L1m). S1 also includes a schistosity associated with the growth of white mica and breakdown of feldspar. Geometry suggests that S1 in the granites is related to S0-1 in the supracrustal rocks, and L1m in both units shares a common orienta tion. S1 and S0-1 are crenulated by a strong second cleavage (S2) axial planar to folds (F2), the large-scale expression of which is an asymmetric synform containing a belt of Coldbrook Group rocks lying between Stuart Mountain and Point Wolfe River. Kinematic indicators suggest an overall top-to-the-southeast motion along thrusts that stack units of Broad River Group, Coldbrook Group, and plutonic rocks. Fabric development in the plutonic rocks implies a history of exhumation beginning under hot, anhydrous conditions, followed by hydration during retrogression as plutonic rocks were tectonically emplaced into this crustal stack. The youngest deformation features are brittle fractures filled with pseudotachylite. The age of these tectonic events is not yet well constrained, but could be as young as Carboniferous. [ABSTRACT FROM AUTHOR]