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Geology; January 2004; v. 32; no. 1; p. 49-52; DOI: 10.1130/G19905.1
© 2004 Geological Society of America
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Calcite-twinning constraints on stress-strain fields along the Mid-Atlantic Ridge, Iceland

John P. Craddock1, David W. Farris*,1 and Aimee Roberson*,1

1 Department of Geology, Macalester College, St. Paul, Minnesota 55105, USA

Calcite veins and amygdule fillings within basalts (older than 0.7 Ma) are mechanically twinned and preserve a subhorizontal shortening strain that resulted from compression and shortening normal to the Mid-Atlantic Ridge on both sides of the plate boundary. Our sample suite includes 19 specimens, 7 from the North American plate (4 veins, 3 amygdule fillings) and 12 from the European plate (9 veins, 3 amygdule fillings), 18 of which record ridge-normal subhorizontal shortening. Five of the strain analyses, two from the North American plate and three from the European plate, have a high percentage of negative expected values, and these secondary strain results record a ridge-parallel shortening strain with plunges that vary parallel to the ridge axis. Averaged shortening strain magnitudes for the twinned calcite (–2.5%, European plate; –6.02%, North American plate) and inferred differential stresses (–48 MPa) that caused the twinning are modest and are thought to represent regional tectonic conditions (i.e., ridge push), not local (e.g., hotspot or glacial loading) phenomena.

Key Words: divergent plate boundary • calcite twinning • stress-strain field • ridge push




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