EVOLUTION OF AN OROGEN-PARALLEL, STRIKE-SLIP FAULT SYSTEM: A CASE STUDY OF THE KELLYLAND FAULT ZONE, NORTHERN APPALACHIAN OROGEN, U.S.A

In most transpressional orogenic belts, crustal contraction is widely distributed while transcurrent motion is localized in discrete orogen-parallel fault systems. Formation of these orogen-parallel fault systems requires strain weakening in the strongest parts of the lithosphere, and understanding this crustal-scale strain partitioning depends on understanding the strain weakening mechanisms. To this end, we present an overview of strain weakening in part of the Norrumbega fault system in the northern Appalachian orogen—the brittle-ductile Kellyland fault zone in eastern Maine—that nucleated between homogeneous granite and metasedimentary rocks.

Granite cut by the Kellyland fault zone exhibits three distinct strain facies: (1) a 2–3-km-wide belt of foliated granite, (2) a 100–300-m-wide belt of small localized shear zones, and (3) a 200–400-m-wide belt of ultramylonite. Bulk compositions of deformed granites are identical to the protolith. Microstructures in these domains reveal a three-phase evolution. Phase 1 is recorded by the foliated granite, and the rheology was governed by dislocation creep of quartz. Phase 2 is recorded by the localized shear zones, and it was a transient period of brittle deformation that represents a temporal strength maxima. Phase 3 is recorded by the ultramylonite. It was a long-lived period of ductile deformation, and the rheology was governed by granular flow of ultra-fine-grained, polyphase mixtures that recrystallized from brittle fault rocks. Therefore, strain localization in granite was a direct result of grain size reduction and phase mixing during the transient brittle phase.

Metasedimentary rocks cut by the Kellyland fault zone exhibit progressive gradients in phase mixing and fabric intensity from the margins to the interior of the zone. Abundant syndeformational veins, precipitation of minerals in dilational sites, and alteration of phase abundances all indicate large fluid fluxes during deformation. The rheology of metasedimentary rocks was also governed by granular flow of ultra-fine-grained, polyphase mixtures. However, initial weakening was probably driven by progressive phase mixing, growth of micas, and enhanced fluid movement. Late-stage hardening in metasedimentary rocks is recorded by significant layer-parallel slip.