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Structural sequence and tectonic significance of Mesozoic dikes in southern coastal Maine

By
Mark T. Swanson
Mark T. Swanson
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Published:
January 01, 1992

Mesozoic dikes in southern Maine occur within a 15–20-km-wide northeast-trending coast-parallel swarm ~150 km in length. The swarm is best exposed from Kittery to Ogunquit, but extends northeast into the Casco Bay area and southwest into New Hampshire. The dikes are dominantly mafic (dolerites and lamprophyres) but syenitic and granitic varieties are also present. Composite and multiple intrusive relations are common. Average dike width is 1.12 m and maximum dike width is ~25 m. Maximum extension values of 23% and dike intensities of 176 dikes/kilometer are found along the interpreted swarm axis. Dike orientations are dominantly northeast-trending and steeply dipping with apparent maxima at N60°E, N45°E, and N35°E, as well as minor northwest trends. These trends reflect a strong structural control on intrusion by the N60°E vertical bedding and a N45°E vertical cleavage in the host Kittery Formation. The horizontal component for dike dilation is dominantly N55°W-S55°E, which results in many sinistral-oblique opening directions and left-stepping en echelon offsets for the more structurally controlled dikes. Most of the mafic dikes were intruded between the syenite-alkaline granite phase and the later biotite granite phase of the Triassic Agamenticus alkaline intrusive complex. Dike intrusion was also contemporaneous with the emplacement of Triassic explosive igneous breccias at Gerrish Island, but prior to the intrusion of the Late Cretaceous Cape Neddick gabbro complex. A linear dike swarm: central intrusive complex model based on the Tertiary igneous province of northwest Scotland is adopted for this phase of early Mesozoic magmatism. The linear coast-parallel dike swarm and associated Triassic Agamenticus alkaline intrusive complex in southern Maine are part of the coastal New England igneous province of McHone and Butler; a 500-km-long, north-northeast-trending zone of crustal extension that includes the early Mesozoic dikes of eastern Massachusetts and Rhode Island to the southwest developed during Triassic rifting.

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GSA Special Papers

Eastern North American Mesozoic Magmatism

John H. Puffer
John H. Puffer
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Paul C. Ragland
Paul C. Ragland
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Geological Society of America
Volume
268
ISBN print:
9780813722689
Publication date:
January 01, 1992

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