Structural analysis of the Lower Paleozoic rocks of Western Gaspe, Quebec.
Creators: name: Sikander, Abdul Hakim.
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 1967
Subjects: subject: Geology.

description: The Shickshock Group of Cambrian-Ordovician metavolcanic, and metasedimentary rocks outcropsalong mountains of the same name bounded to the south by the Shickshock Fault. Within these rocks a schistosity S1 and layering are parallel, and there is a lineation L1 marked by a mineral elongation and fine striation. These structures suggest the presence of isoclinal, F 1 folds, and the Shickshock rocks are considered to form the core of a major recumbent F1 fold. The absence of F1 structures from Lower Ordovician rocks to the north suggests a pre-Lower Ordovician period of folding. F2 folds, the dominant structures in the area, deform all Paleozoic rocks older than upper Middle Devonian and form the stratigraphically defined Gaspe Synclinorium, an Acadian structure. On the basis of the inclination and symmetry of the small scale F 2 folds, the Gaspe synclinorium is divided into three major domains. (I) The northern limb of the synclinorium, north of the Shickshock Fault, where folds in Cambrian to Ordovician rocks are overturned northwest; anticlines have short northwest limbs. (II) The northern limb, south of the fault to the axial area of the synclinorium, where folds in SiluroDevonian.rocks are upright; folds are both asymmetric as in domain I and symmetric. (III) The southern limb of the synclinorium where axial planes of folds in Ordovician to Devonian rocks are inclined northwest; anticlines have short southeast limbs. The folds in domains I and II are gradational in inclination and symmetry; but are separated by vertical displacement across the Shickshock Fault. In the study area, the F2 folds affect the S1 schistosity and trend northeast, co-axially with F1 structures. However, to the northeast, near Mount Albert, F1 and F2 structures are divergent. The F2 folds vary in shape with lithology, from open in competent, to isoclinal in incompetent rocks. They are dominantly flattened concentric in competent limestone and sandstone, and similar in style in argillite. The folds are doubiy plunging, and the axes of variable plunge lie on planar axial surfaces. Slickensides on axial-plane calcite veins dominantly plunge down the dip. This is regarded as the movement direction during the F2 folding. Variation in the amount of movement resulted in the curved fold axes. It is suggested that the folds progressed from an initial concentric mechanism determined, by the competent beds to a flattening and extension along the axial planes determined by flow of the incompetent beds. An S2 cleavage, a close fracture set parallel to the F 2 axial planes is present in all incompetent rocks except those of the Shickshock Group foliated by S1. A classification of the cleavage is suggested by means of the ratio of slip across individual fractures and the average fracture spacing. The cleavage is termed fracture if the ratio is less than .25, and fracture slip if it is .25 or more. F3 folds, restricted to the northern coastal sections are mostly S-shaped in plan, and plunge steeply to the southeast. A rough S 3 fracture lies parallel to the steep axial planes striking E-W. The folds may have formed by N-S compression and sinistral shear along the northeast striking F2 structures. F4 folds, restricted to the belt of Cambrian Ordovician rocks with S1 schistosity, are mesoscopic to microscopic, open with sharp rectilinear hinges, and resemble kink bands. The folds trending NW-SE are S and Z shaped and are considered as a conjugate system developed in response to a northeast-southwest compression parallel to the general strike of the country rock. A low plunging fine kink lineation and larger kink bands with low dipping axial planes that deform S2 occur along the axial area of the synclinorium. The kink bands consistently displace the upper rocks away from-the axis. They are considered as late stage structures. Joint patterns in competent rocks (sandstone, siltstone and limestone) consist of dip, strike, and oblique joints which are normal to bedding. In slates there is a predominance of dip sets accompanied by oblique sets, both of which are steeply inclined. Strike joints in slates are rare and form normal to cleavage. The steep dip joints in slates bear no consistent angular relationship to meso-F2 fold axes. Thus it is suggested that joint planes-in folded rocks are independent of attitudes of local folds and are post-kinematic in origin.