Some remarks on rheology and fluid migration in the Paleozoic eastern Midcontinent of North America from regional calcite twinning patterns

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 transmission that is associated with late Paleozoic (“Alleghenian”) activity at the Laurentian plate margin. Calcite twinning analysis permits the determination of regional differential paleostress (σD) and finite strain (e) patterns that are combined to obtain rheologic 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=1 0 ‐5 . σD 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 carbonate cover changes its permeability, perhaps permitting fluids to migrate along evolving grain boundaries in rocks that otherwise have low porosities.