Abstracts
Abstract
The Ganderian Cambrian–Ordovician Miramichi terrane narrows in east-central Maine and terminates at the junction of faults that separate it from the mostly Silurian Central Maine/Aroostook–Matapedia basin (CMAM) to the northwest and Fredericton trough to the southeast. The terrane was emergent after Middle Ordovician recumbent folding and shed sediment to both adjacent depocenters. Its boundary faults are the youngest deformation events and play important roles in its termination, but do not by themselves explain it. The presence of distinctive CMAM strata southeast of the northwest boundary fault indicates that the first step in developing the current relationships was an episode of hitherto unrecognized late Silurian eastward thrusting. In the northern (Danforth) segment of the terrane, intermediate facies CMAM strata were thrust onto their Miramichi source rocks. The thrust sheet was deformed by Acadian upright folds, then dissected by dip-slip offset along boundary and internal faults prior to intrusion of the 409 ± 2 Ma Skiff Lake pluton. Subsequent erosion isolated a remnant of the thrust sheet as the Dill Hill klippe, its allochthonous CMAM strata isolated among Miramichi rocks. The southern (Greenfield) segment experienced similar events, but current relationships are different and timing of the late-stage faults is not well constrained. Allochthonous CMAM strata may have overridden the Miramichi terrane completely, so that a remnant of distinctive CMAM strata is now exposed east of the Miramichi terrane in fault contact with rocks of the Fredericton trough. The entire Greenfield segment is interpreted as a fault block exposed within the thrust sheet.
Résumé
Le terrane cambro-ordovicien gandérien de Miramichi se rétrécit dans le centre-est du Maine, et il prend fin à la jonction des failles qui le sépare du bassin principalement silurien du centre du Maine/Aroostook-Matapédia (CMAM) au nord-ouest et de la cuvette de Fredericton au sud-est. Le terrane a émergé après la formation de plis couchés de l’Ordovicien moyen et la diffusion de sédiments dans deux zones de dépôt adjacentes. Ses failles limitrophes constituent les épisodes de déformation les plus récents, et ils jouent des rôles importants dans sa terminaison, sans toutefois expliquer celle-ci par eux-mêmes. La présence de strates distinctives du bassin CMAM au sud-est de la faille limitrophe nord-ouest révèle que le premier stade de l’apparition des liens existants a été un épisode de chevauchement vers l’est du Silurien tardif qui n’avait pas été reconnu jusqu’ici. Dans le segment septentrional (Danforth) du terrane, des strates du faciès intermédiaire du bassin CMAM ont été charriées sur leurs roches mères de Miramichi. La nappe de charriage a été déformée par des plis droits acadiens, puis disséquée par un déplacement horizontal de rejet-pendage le long des failles limitrophes et internes avant l’intrusion du pluton de 409 ± 2 Ma du lac Skiff. Une érosion subséquente a isolé un vestige de la nappe de charriage pour créer la klippe de la colline Dill, ses strates allochtones du bassin CMAM isolées parmi les roches de Miramichi. Le segment méridional (Greenfield) a été touché par des phénomènes similaires, mais ses liens actuels sont différents et le moment de la formation des failles tardives n’est pas bien délimité. Les strates allochtones du bassin CMAM pourraient avoir complètement recouvert le terrane de Miramichi, de sorte qu’un vestige des strates distinctives du bassin affleure aujourd’hui à l’est du terrane de Miramichi en contact faillé avec les roches de la cuvette de Fredericton. L’ensemble du segment de Greenfield est interprété comme un bloc faillé découvert à l’intérieur de la nappe de charriage.
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