Geological development of the southern Long Range Mountains, southwest Newfoundland: a regional synthesis

Chorlton, Lesley B. (1983) Geological development of the southern Long Range Mountains, southwest Newfoundland: a regional synthesis. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The southern Long Range Mountains in southwest Newfoundland are traversed by a major, northeast-trending fault, the Cape Ray Fault. The fault zone is overlain by Emsian-Eifelian terrestrial sedimentary and bimodal volcanic rocks of the Windsor Point Group. Terranes juxtaposed along the fault began their development as an Early Paleozoic(?) ocean basin (Terrane I-NW) and an Ordovician arc volcano-plutonic and sedimentary basin complex at least partly resting on oceanic crust (Terrane II-SE). These terranes were rapidly converted to continental crust through thrusting, production of synkinematic granitoid rocks, and subsequent convergent wrenching (transpression). -- The Cape Ray Fault was formerly called a cryptic suture because pre-Windsor Point Group rocks along its northwest side were considered Grenvillian, and along its southeast side Hadrynian(?). The original criteria for calling the fault a cryptic suture are therefore incorrect (above). Nevertheless, the Cape Ray Fault is considered the most significant late tectonic boundary in the area. -- A low pressure granulite facies foliation predating regional deformation in the cumulate metagabbro (MOHO) layer of the oceanic crust northwest of the fault is interpreted tentatively as the result of rapid ocean floor spreading. Thrusting, involving all levels of the lithosphere from depleted upper mantle to sedimentary and volcanic cover rocks, may have occurred before this cumulate sequence had substantially cooled. Voluminous tonalite was apparently produced and emplaced during thrusting. Thrust-stacked ophiolite and synkinematic granitoid rocks were later intruded by mafic to felsic plutons. -- The volcano-plutonic centre southeast of the fault contains a high proportion of dacitic and rhyolitic volcaniclastic rocks. After its maturation, recumbent folding tectonically buried distal parts of the volcanic sequence and the adjacent sedimentary basin. Amphibolite facies metamorphism and partial melting to produce voluminous tonalite-granodiorite followed, after which megacrystic monzogranites were emplaced. -- Timing considerations suggest that the thrusting in the northwest was an early Taconic event, and the recumbent folding in the southeast was a late Taconic event. -- Later 'transpression'-related deformation events were Acadian to Hercynian. They resulted in locally intense, penetrative deformation, the emplacement of leucogranites, the strike-slip, repetition of pre-Acadian sequences, fault-controlled deposition of sedimentary and volcanic rocks, and high angle reverse faulting.

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