Volume loss during cleavage formation in the Meguma group, Nova Scotia, Canada

Abstract

Diffusive mass transfer (pressure solution) is a widely recognized cleavage-forming mechanism under low-grade conditions. The amount of accompanying volume change during cleavage development, however, is difficult to quantify and remains controversial. The distance between sites of mineral dissolution and reprecipitation and transport mechanisms also are controversial. The structural history of the Cambrian-Ordovician Meguma Group, Nova Scotia, Canada, is well known and abundant strain markers provide an opportunity to assess geometrically volume changes accompanying cleavage development. During diagenesis about 70% compaction in muds and 15% in silts and sands occurred. The pelitic-to-psammitic Meguma Group was cleaved prior to being folded. Pressure solution was the dominant cleavage-forming deformation mechanism under chlorite-grade metamorphic conditions during the early Devonian Acadian orogeny. Measurement of tectonic strain in principal directions ( X > Y > Z) shows that bedding-parallel coaxial shortening ( Z) is 60%, and there is no evidence of fold-hinge parallel extension ( Y). Estimation of tectonic vertical extension ( X) in the cleavage plane is complicated by diagenetic compaction corrections to primary strain markers, but is insufficient to compensate for the observed 60% shortening in Z, indicating 40–60% volume loss during cleavage formation. Because cleavage formed at least 50 Ma after deposition and under approximately 6 km of Silurian to early Devonian strata, porosity of the sediments upon onset of cleavage was likely less than 15%. Pore-space reduction therefore cannot account for the apparent volume loss during cleavage formation. Cleavage-related volume loss applies regionally and to a stratigraphic thickness of several thousand meters, implying that a large amount of material was removed and transported large distances. When combined with 50% shortening by folding, a bulk tectonic shortening of 80% is estimated for the Meguma Group during the Acadian orogeny.