THE NORUMBEGA FAULT SYSTEM IN MAINE: CAN PLUTONS PROVIDE CRUSTAL-SCALE KINEMATIC CONSTRAINTS ON LONG-LIVED SHEAR ZONES?

Can plutons provide crustal-scale kinematic constraints on long-lived shear zones? We propose that, if plutons crystallize quickly with respect to long-lived shear zones and if magmatic fabrics in plutons have a tectonic significance, these magmatic fabrics record instantaneous strain on the crustal scale. Combined with regional finite strain data from wall rocks, pluton fabrics allow the quantification of crustal scale vorticity if the relative ages of pluton crystallization and fabric formation are constrained.

Here we offer an example of such a crustal scale vorticity calculation from the Norumbega Fault System (NFS) in mid-coastal Maine, just south of Penobscot Bay. NFS fault rocks range from sillimanite-bearing gneiss to sub-greenschist-grade phyllite. Through all metamorphic grades, horizontal lineations trend NE-SW, and vertical foliations strike NE-SW. In concert with excellent isotopic age constraints on the timing of cooling and deformation, these fabrics are consistent with the interpretation of previous workers that the NFS evolved from a mid-crustal zone of transpression (during the middle Devonian) to an upper crustal zone of strike-slip faulting (during the late Carboniferous and early Permian). Adjacent to the NFS, the late Devonian Mount Waldo pluton cuts across high-grade wall rock fabrics. Magmatic fabric, determined by examining alignment of K-feldspar phenocrysts and anisotropy of magnetic susceptibility, defines a steeply dipping foliation that strikes N-S and a sub-horizontal lineation that trends N-S. We argue that magmatic foliation and lineation in the Mount Waldo pluton record an instantaneous strain on the crustal scale; metamorphic foliation and lineation in the wall rocks record finite strains on the crustal scale. A 45 degree angle between the principal axes of instantaneous and finite strain ellipsoids suggests a crustal-scale strike-slip kinematic regime (Wk=1) during crystallization of the Mount Waldo pluton. Therefore, the NFS appears to have evolved from a transpressional to a strike-slip shear zone by the late Devonian, somewhat earlier than previously hypothesized.