The Grenville Front foreland fold-and-thrust belt in the Southwestern Labrador: mid-crustal structural and metamorphic configuration of a Proterozoic orogenic thrust wedge

Gool, Jeroen Antonius Maria van (1992) The Grenville Front foreland fold-and-thrust belt in the Southwestern Labrador: mid-crustal structural and metamorphic configuration of a Proterozoic orogenic thrust wedge. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Gagnon terrane in the Parautochthonous Belt of the Grenville Province in southwestern Labrador is a metamorphic foreland fold-and-thrust belt which carried Lower Proterozoic metasediments and their Archean crystalline basement in a north-northwest-directed thrust movement onto the Superior Province foreland. It is separated from the foreland in the northwest by the Grenville Front and overlain by the Parautochthonous Molson Late terrane to the southeast. -- Metasediments in Gagnon terrane are part of the Knob Late Group and were deposited in the Early Proterozoic on the passive continental margin of the Superior Craton. They form the southern extension of the Labrador Trough into the Grenville Province. The basement rocks are predominantly granulite-facies gneisses, migmatites and amphibolites, which form part of the Ashuanipi Metamorphic Complex of the Superior Province. A system of linked extensional faults presumably existed in the basement at the time of sediment donation as a result of crustal thinning. -- Deformation and metamorphism in Gagnon terrane were a result of the thrust emplacement of Molson Lake terrane over Gagnon terrane along a major ductile shear zone, during the Grenvillian orogeny. Molson Lake terrane formed a thrust wedge which tapered to the northwest and which progressively incorporated thrust sheets of the underlying metasediments of Gagnon terrane. The angle between the Superior continental margin and the front of the advancing Molson Late terrane placed the belt in a transpressive setting, and resulted in an oblique orientation of the structures and the metamorphic zonation in the area, with respect to the trend of the belt in southwestern Labrador. -- Gagnon terrane in the map area consists of two thrust systems, a lower, basement-dominated system and an upper, metasediment-dominated system. Thrust sheets in the lower system, are separated by steeply east-southeast-dipping, narrow, ductile shear zones, are several kilometers thick and weakly deformed internally. In contrast, the upper, thin-skinned thrust system consists of an imbricate stack of pervasively, ductilely deformed thrust sheets which vary in thickness from about 200 m in the northwest, to several kilometers in the southeast. Thrusting in Gagnon terrane was initiated in the upper thrust system and progressed in a regular sequence towards the foreland. Thrusting in the lower system occurred in a similar foreland-directed sequence, but postdated the thrust activity in the upper system and caused out-of-sequence thrusting in the latter, where basement thrusts breached the metasediments. The thrusts in the basement formed by reactivation under oblique compression of the pre-exising extensional faults. -- A pervasive southeast-dipping simple-shear foliation (S₁) and south-southeast-plunging elongation lineation (L₂) characterize the fabric in the upper thrust system and in the shear zones of the lower thrust system. The thrusts in both systems are formed by southeast-dipping ductile shear zones. D₁ deformation occurred in the footwall of the thrust wedge and during accretion of the rocks into the wedge. During progressive thrusting, D₁ structures inside the thrust wedge were folded into northwest-verging F₂ folds and were affected by northwest-directed out-of-sequence thrusts. D₂ structures reflect changes in the shape of the thrust wedge during the thrust movement. The southeastern part of the map area is dominated by late, kilometer-size F₃ cross-folds that developed as a result of the oblique convergence of the belt. Although the geometry of the stacked thrust sheets resembles that of thrust belts at high crustal levels, all Grenvillian deformation was ductile, typical for rocks of mid-crustal levels. -- Grenvillian metamorphic grade of the rocks increases towards the structurally higher thrust sheets in the southeast, and also along the strike of the belt towards the southwest. It ranges from sub-greenschist facies in the north, at the Grenville Front, to upper amphibolite facies (700°C and 11 kbar) in the southeast, near Molson Lake terrane. Thrusting in Gagnon terrane caused a telescoping and inversion of the metamorphic gradient, with discontinuities in the metamorphic field gradient across thrust faults. P-T paths derived from zoned garnets in metapelites have a “hairpin shape” and suggest rapid cooling and decompression of the rocks after attainment of the “peak” metamorphic conditions. Incorporation of thrust sheets into the thrust wedge caused the switch from prograde to retrograde metamorphism and from predominantly mylonitic deformation (S₁), near the base of the thrust wedge, to folding and out-of-sequence thrusting (D₂), in the remainder of the thrust wedge. -- Metasedimentary thrust sheets were incorporated in the thrust wedge by underplating, rather than by accretion at the toe, after they were buried underneath Molson Lake terrane down to a depth of about 30 km. Underplating as mode of accretion of material to the thrust wedge may be the mechanism that causes inversion of the metamorphic gradient in this and other thrust belts.

Item Type: Thesis (Doctoral (PhD))
URI: http://research.library.mun.ca/id/eprint/6627
Item ID: 6627
Additional Information: Bibliography: leaves 288-303.
Department(s): Science, Faculty of > Earth Sciences
Date: 1992
Date Type: Submission
Geographic Location: Canada--Newfoundland and Labrador--Labrador
Library of Congress Subject Heading: Folds (Geology)--Newfoundland and Labrador--Labrador; Thrust faults (Geology)--Newfoundland and Labrador--Labrador; Geology--Newfoundland and Labrador--Labrador

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