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Axial projections and modes of crustal thickening, eastern Wopmay orogen, northwest Canadian shield

By
Paul F. Hoffman
Paul F. Hoffman
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Rein Tirrul
Rein Tirrul
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Janet E. King
Janet E. King
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Marc R. St-Onge
Marc R. St-Onge
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Stephen B. Lucas
Stephen B. Lucas
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Published:
January 01, 1988

Eastern Wopmay orogen comprises an external thin-skinned thrust-fold belt and a contiguous metamorphic-plutonic internal zone produced by the closing of a short-lived (1.90–1.88 Ga) ensialic(?) back-arc(?) basin on the west margin of Slave craton, an Archean granite-greenstone terrane in the northwest Canadian shield. Sets of high-amplitude, basement-involved folds subsequently developed parallel (D2) and oblique (D3) to the strike of the thin-skinned (Dl) thrust-fold belt. The D3 folds permit the construction, by means of axial projections, of composite cross sections of the Dl and D2 structures through a depth interval of 30 km. The cross sections show that the sole thrust of the externides and the underlying autochthonous cover continue beneath the metamorphic-plutonic internal zone, which was thrust as a hot allochthon onto the relatively cold Archean craton.

In the externides, thrusting produced a minimum of 40 percent east-west shortening and thickening of the allochthonous cover. This is manifested by about 25 mappable imbrications of a lower cratonic shelf sequence (stratigraphic thickness of 1.5 to 2.5 km), and upright chevron-type folds in overlying foredeep flysch and molasse (minimum, 5 km of stratigraphic thickness). Thrust propagation was eastward (toward the craton), and an active accretionary wedge of relatively low taper is indicated by high basal “step-up” angles, common back-thrusts, and lack of overall change in exposed stratigraphic level across the belt. The sole thrust is consistently positioned 100 to 300 m above the unfaulted basement surface, independent of footwall lithology, for at least 100 km west of the frontal thrusts.

In the internides, a basal allochthonous assemblage of rift-facies clastics and bimodal volcanics is characterized by recumbent isoclinal folds, most of which are east-vergent. Allochthonous syn-rift deposits occur in both the eastern and western internides, but the overlying slope-rise facies of the cratonic margin sequence and succeeding synorogenic foredeep deposits are limited to the east. The slope-rise and foredeep strata are deformed by east-vergent thrusts and folds with steeply dipping axial surfaces. The medial and western internides are occupied by a salic-through-mafic plutonic suite (Hepburn Intrusive Suite), emplaced as the basin closed. North-trending folds and thrusts both predate and postdate prograde metamorphic isograds that envelope the composite Hepburn batholith of the medial internides. Eastward translation of the hot batholith on the sole thrust caused a minimum 250°C inversion of the contemporaneous geothermal gradient beneath the allochthon. This is evident from inverted metamorphic isograds that transect the sole thrust and the underlying, upward-facing, autochthonous cover.

Following thin-skinned deformation, the allochthon and autochthon were congruently shortened to form large-scale north-trending folds (D2) having a ca. 35 km wave-length and an apparent structural relief of 13 and 6 km on the basement surface in the internides and externides, respectively. Thrusting of the basement-cover contact is insignificant, and the folds, which are completely exposed in oblique cross section, are clearly not the result of basement involvement in D1 thrusting. However, the existence of a blind thrust at depth is not ruled out.

The cross folds (D3) are similar in style to the D2 folds but trend northeast and have first-order wavelengths of 80 to 140 km and amplitudes at the basement-cover interface of up to 15 km. Locally, D3 produced belts of well-developed higher order folds, typically having cuspate-lobate basement-cover profiles, and inhomogeneous but locally strong northeast-striking foliations. In common with D2 folds, the D3 folds have no significant associated thrusts, and are well developed in areas where the contemporary basement temperature exceeded a threshold of ca. 350°C. Zones of strong D3 deformation occur east of the frontal D1 thrusts, far beyond the limit of tectonically transported heat.

The observed evolution in modes of crustal thickening—early thin-skinned thrusting followed by coaxial, and later transverse, thick-skinned folding—appears to have occurred in several other well-exposed, broadly coeval, orogenic belts in the Canadian shield. Detailed mapping of the steeply plunging segments of these orogens will allow their three-dimensional structural and metamorphic configurations to be elucidated by the use of axial projections, as successfully employed in Wopmay orogen.

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Contents

GSA Special Papers

Processes in Continental Lithospheric Deformation

Sydney P. Clark, Jr.
Sydney P. Clark, Jr.
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B. Clark Burchfiel
B. Clark Burchfiel
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John Suppe
John Suppe
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Geological Society of America
Volume
218
ISBN print:
9780813722184
Publication date:
January 01, 1988

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