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White Mountain is centrally located in the bedding-plane portion of the Eocene Heart Mountain detachment and contains the only upper plate Mississippian Madison Group rocks that have been metamorphosed into marble. The marble rests upon the thickest (1 m) part of a carbonate ultracataclasite that marks the detachment. Thermodynamic and mechanical calculations based on possible frictional melting of calcite and other minerals, geochemical data, the characteristics of the carbonate ultracataclasite, and the geometrical characteristics of White Mountain suggest a possible initial upper plate emplacement rate of 126-340 m/sec and that the duration of the emplacement event was less than 4 min, too brief a time to develop an emplacement-related calcite twinning strain overprint in upper or lower plate carbonates. While the detachment-related carbonate ultracataclasite did not form by melting, it does preserve a magnetic fabric where [K.sub.max] is parallel to the detachment slip direction and records a westward and down paleopole (287[degrees] and 27[degrees]), where magnetite is the carrier mineral. The Eocene (49.6 Ma) paleopole for this latitude in North America was southerly and upward (0[degrees] and 45[degrees]). This brief and catastrophic detachment event produced a significant amount of C[O.sub.2] by flash heating. This report is the first to quantify the emplacement rate of the upper plate of the Heart Mountain detachment based on physical and geochemical parameters. Keywords: Heart Mountain detachment, calcite strains, paleomagnetism, thermodynamics, flash heating.

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. Keywords: divergent plate boundary, calcite twinning, stress-strain field, ridge push.

Bedding-parallel, subhorizontal shortening strains, as preserved by twinned calcite in Paleozoic carbonates that cover cratonic eastern North America are perpendicular to the orogenic sites of the Appalachian and Ouachita mountains. Moreover, the shortening strain magnitudes and the calcite twinning differential stress magnitudes decrease exponentially away from the orogenic front.

Paleozoic carbonates that cover the eastern half of the North American cration preserved a bedding-parallel shortening fabric up to 800 km away from the Appalachian-Ouachita orogenic front as recorded by mechanical twins in calcite. A strain fabric is present in 29 of 31 samples from three traverses within the flat-lying cratonic cover sequence: Jasper, Arkansas, to Minneapolis, Minnesota; Bluefield, West Virginia, to Madison, Wisconsin; and Albany, New York, to Hamilton, Ontario. Twinning is found only in Paleozoic carbonates and is absent in Cretaceous limestones. In the two southernmost traverses the orientation of the principal shortening axis (e1) is perpendicular to the thrust front and parallel to the thrusting direction of the southern Appalachians and the Ouachitas. The magnitude of calcite shortening strain (

Abstract: Mafic dykes of different ages were collected from three different tectonic settings and analyzed using anisotropy of magnetic susceptibility (AMS) as a proxy for magmatic flow during intrusion. In Iceland, ridge-parallel basaltic dykes were sampled on each side of the active tectonic boundary. The dykes are <10 m wide along a 1–2 km strike, and are the result of a single intrusion from 1–2 km deep magma chambers in oceanic crust. Thirteen samples were collected (7 N. American plate; 6 European) and 153 cores were analyzed by AMS and preserve a vertical K max orientation indicating vertical emplacement. The Eocene Aishihik dyke swarm intrudes the Yukon–Tanana terrane in the Yukon province, Canada over an area ~200 by 60 km. These dykes were intruded normal to the accretionary margin, are porphyritic andesites, and have an intermediate geochemical signature based on major and trace element analyses. Ten dykes were sampled and 111 cores analyzed using AMS, and the dykes preserve a vertical K max orientation, indicating intrusion was vertical through ~30 km of continental crust. The 2.06 Ga Kenora–Kabetogama dykes in northern Minnesota and western Ontario crosscut a variety of Archean terranes (thickness~50 km) in a radiating pattern. The unmetamorphosed basaltic dykes are 1–120 m wide, 10–110 km in length, are vertical in orientation and can be grouped as either being single intrusion or multiple intrusion (composite) dykes. AMS data preserve a vertical K max orientation for the southerly locations (2 dykes, n =53) and horizontal K max for the remainder to the northwest (15 dykes, n =194). Maximum magnetic susceptibility axes (4 dykes, n =92) for composite dykes are scattered and yield inconsistent flow directions with regard to the dyke margin. Almost all of our results are “normal” in that, the magnetic foliation (the plane containing K max and K int, normal to K min) is parallel to the dyke planes, which gives us confidence that the magnetic lineations (i.e., K max orientations) are parallel to magmatic flow. [Copyright &y& Elsevier]