Calcite-twinning constraints on stress-strain fields along the Mid-Atlantic Ridge, Iceland

Abstract

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.