A CENTRAL MAINE PERSPECTIVE ON THE CAMBRO-ORDOVICIAN GANDER TERRANE

The Gander terrane, most inboard of the Gondwanan terranes in the northern Appalachians, underlies Maine from Rangeley to the coast. It is defined on the basis of quartz-rich, Cambrian-Ordovician sequences exposed in the Lunksoos, Liberty-Orrington-Miramichi, and St. Croix inliers that are distinct from peri-Laurentian and Avalon sequences. Silurian turbidites of the Central Maine and Fredericton troughs blanket Ganderia, hence the structural and stratigraphic relationships of this cover must be deciphered to reveal the Cambrian-Ordovician geography.

Along the northwestern flank of the CMT, Ordovician black shales and turbidites underlie a Silurian fining-upward sequence. Ribbon limestones are prominent in the central part of the basin. A clastic sequence along the southeast flank (Vassalboro) contrasts only slightly with rocks of the FT, which both coarsen and become increasingly calcareous upward. In the CMT, upright folds with dominantly SE-younging limbs and anticlockwise cleavage typify Acadian dextral transpression; they locally overprint early structures whose vergence is unknown. The FT preserves early SE-vergent structures in the footwall of the Liberty-Orrington thrust.

Ganderia separated from Gondwana either because of its back-arc position or due to ridge jump. In the Early Ordovician, spatially heterogeneous ophiolite obduction (Penobscot orogeny) consolidated the terrane, which was then fragmented and dispersed following subduction polarity reversal. Hurricane Mountain mélange marks the site of Late Ordovician arc-arc collision (Popologan-Boil Mountain). Salinic convergence, related to a sub-Laurentian near-flat slab (short-lived Attean-Point aux Trembles arc), brought the distal, northern Gander and main southern block (St. Croix and its Ellsworth allochthon) together along the LO (Dog Bay) Line by 424 Ma. Timor-like uplift shed sediments northwestward and caused Late Silurian black shale deposition farther northwest. Finally, the Acadian foreland basin migrated northwestward behind the Coastal Volcanic Belt and Gander-Avalon suture in the Gulf of Maine. The best modern analog of the Gander terrane is the New Caledonia-Norfolk Ridge microcontinent, which locally experienced intra-oceanic ophiolite obduction and whose complex evolution has just begun.