ALONG-STRIKE VARIATIONS IN THE APPALACHIANS PARTIALLY EXPLAINED BY A MODEL OF MID-PALEOZOIC OCEANIC RIDGE SUBDUCTION IN THE NORTHERN APPALACHIANS

Mid-Paleozoic crustal-scale dextral northeasterly trending ductile-brittle fault systems and large volumes of igneous rocks are characteristic of the northern Appalachians in eastern New England and Maritime Canada. These can be interpreted as having resulted from a subducted oceanic ridge – transform fault system, in which the fault systems formed as a result of subduction of transform faults, and the igneous rocks formed above a slab window. The model is similar to that of formation of the San Andreas fault system. In the model, a ridge-transform system existed in the Rheic Ocean, and was subducted below parts of Ganderia, Avalonia and perhaps Meguma, in Maine, New Brunswick and Nova Scotia. The westward subduction zone jumped from the Laurentian side of Avalonia and Meguma to their eastern margins, as a result of their accretion.

A latest Silurian transition from arc- to within-plate- magmatism in the Coastal Volcanic Belt in eastern Maine is interpreted as the onset of ridge subduction. Examples of increased latest Silurian to Devonian within-plate magmatism include the Cranberry Island volcanic series and Coastal Maine Magmatic Province in Maine, and the South Mountain Batholith in Nova Scotia. Widespread Devonian to earliest Carboniferous granitic to intermediate plutons, beyond the Coastal Volcanic Belt towards southern Maine and central New Hampshire, may outline the shape of a subsurface slab window.

Where the ridge-transform system was subducted, plate motions changed from predominantly convergent between the northern Rheic Ocean and Laurentian plates to predominantly dextral between the southern Rheic Ocean and Laurentian plates. This dextral motion resulted in crustal-scale dextral strike-slip fault systems within the continental crust. Examples include the Norumbega fault system in Maine and the Kennebecasis – Belle Isle – Caledonia – Clover Hill fault system in New Brunswick. While similar dextral strike-slip fault systems exist to the north in Newfoundland and to the south in the southern Appalachians, there is no evidence for extensive mid-Paleozoic igneous rocks, suggesting the model probably does not apply in those areas, potentially providing a partial explanation for along-strike variations in the Appalachians.