Structural interpretation of the eastern Notre Dame Bay area, Newfoundland: regional post-Middle Silurian thrusting and asymmetrical folding

Abstract

The structural geometry of the eastern Notre Dame Bay area is that of an allochthonous sequence (related to F1 thrusting and folding), folded by F2 asymmetrical folds, and offset by F3-related high-angle faults. F1 folds are isoclinal and commonly intrafolial. F2 folds are asymmetrical (generally tight) with axial surfaces that dip 30–85° southeast. The prominent northeast-striking, penetrative cleavage (S2) is axial planar to F2 folds. F3 structures include chevron folds, box folds, kink bands, and local S3 crenulation cleavage.F1 folds are overprinted by F2 folds producing coaxial and mushroom interference patterns. This and the subhorizontal enveloping surface of F2 indicate that F1 folds were recumbent prior to F2. A zone of F1 folds separating Silurian turbidites from subaerial volcanics on the Port Albert Peninsula suggests F1 was related to macroscopic thrusting, as does the association of F1 with mesoscopic thrust faults.F2 folds are the most obvious structures in the area. Macroscopic examples include an anticline–syncline pair on Milleners Peninsula; an anticlinorium cored by the Dunnage mélange, and a synclinorium cored by subaerial sandstones on the Port Albert Peninsula. These folds repeat the stratigraphy across areas that have been incorrectly interpreted in terms of fault-bounded, north-younging homoclines. A shallow enveloping surface for F2 is supported by the uniformly low metamorphic grade in the area and detailed structural profiles. In view of the shallow dip of the enveloping surface, it is possible to correlate the Dunnage mélange, the Carman ville mélange, and other mélanges in the area.Contrary to prevailing interpretations of a structural setting involving fault-bounded tectonostratigraphic zones, we demonstrate a similar deformational history and a general continuity of macroscopic F2 folds throughout our area. Rectilinear faults on geologic maps, used by previous workers as the boundaries between tectonostratigraphic zones, are post-F2 features. They cross-cut F2 folds and are at a high angle to the F2 enveloping surface. These include the Chanceport, Lukes Arm, Dildo, and Reach Faults.All three fold generations affect rocks ranging in age from Ordovician to Middle Silurian. Thus, irrespective of when the earliest deformation began, it continued into Silurian times.