INTRODUCTION In his landmark paper, Oliver (1986) proposed that Appala- chian tectonics at the margin of eastern North America was responsible for a variety of geologic features that occur well inboard from that active margin. In particular, Oliver emphasized the role of fluids that were expelled from the evolving thrust belt by loading of sedimentary cover (Fig. 1). Much of the subsequent work has focused on the evidence for the presence of this fluid and the nature of its migration (e.g., Bethke and Marshak, 1990, and references therein). Some have argued, based in part on the basis of thermal considerations, that a fluid must have migrated in essentially a single pulse, while others favor more long-lasting fluid migration. Major fluid migration was proposed during the late Paleozoic Alleghenian orogenic event that is prominent in the southern and central portion of the Appalachians as well as in the Ouachitas of the southern United States. In addition, physical evi- dence that includes large wavelength folds, cleavage, deformed fossils, and jointing indicates that large tracts of the cratonic cover sequence have undergone penetrative deformation that is related to late Paleozoic activity at the plate margin (e.g., Rodgers, 1963; Nickelsen, 1966; Engelder and Engelder, 1977; Geiser and Engelder, 1983; Anderson, 1988). Craddock and van der Pluijm (1989) and Craddock et al. (1993) collected samples across the eastern interior of North America (Fig. 2) and showed that calcite of the cratonic cover preserves a record of regionally consistent differential paleostress and strain patterns associated with late Appalachian convergent activity. They concluded that lateral transmission of orogenic stresses away from the evolving margin produced these twins in Geological Society of America Special Paper 308 1996 Some remarks on rheology and fluid migration in the Paleozoic eastern Midcontinent of North America from regional calcite twinning patterns Ben A. van der Pluijm Department of Geological Sciences, University of Michigan, 1006 C. C. Little Building, Ann Arbor, Michigan 48109 John P. Craddock Geology Department, Macalester College, St. Paul, Minnesota 55105 ABSTRACT Mechanical twins in calcite from carbonates of the Paleozoic cover sequence of eastern North America preserve a record of far-field paleostress and strain transmis- sion that is associated with late Paleozoic (“Alleghenian”) activity at the Laurentian plate margin. Calcite twinning analysis permits the determination of regional differ- ential paleostress (σ D ) and finite strain (e) patterns that are combined to obtain rheo- logic properties of the cover rocks. The combination of strain vs. distance and stress vs. distance relationships produces a stress-strain relationship of the form: e = 10 –5 . σ D 2 . Modeling of linear viscous rheologies by others supports the observation of far-field effects transmitted from this convergent margin if the convergence vector between the Laurentian and Gondwanan plates is at a high angle to the margin. Using differential paleostress data from calcite twinning at the orogenic front, high fluid pressure is calculated (λ = 0.65). Moreover, the occurrence of twinning in the carbon- ate cover changes its permeability, perhaps permitting fluids to migrate along evolving grain boundaries in rocks that otherwise have low porosities. Van der Pluijm, B. A., and Craddock, J. P., 1996, Some remarks on rheology and fluid migration in the Paleozoic eastern Midcontinent of North America from regional calcite twinning patterns, in van der Pluijm, B. A., and Catacosinos, P. A., eds., Basement and Basins of Eastern North America: Boulder, Colorado, Geological Society of America Special Paper 308. 181 on February 11, 2013 specialpapers.gsapubs.org Downloaded from