LONG-TERM STRENGTH CONTRASTS BETWEEN TEXTURALLY SIMILAR FAULT ROCKS ACROSS A LARGE STRIKE-SLIP SHEAR ZONE: A CASE STUDY OF THE KELLYLAND FAULT ZONE, EASTERN MAINE

We compare deformation mechanisms and flow stresses in intensely deformed clastic metasedimentary rocks and granite-derived ultramylonites across the strike-slip Kellyland fault zone (KFZ). Fault rocks from all protoliths contain texturally similar load-bearing frameworks of very fine-grained, well-mixed sheet and network silicates with no through-going single-phase networks. Diffusion-assisted granular flow was likely the dominant deformation mechanism across the zone. The textural and mineralogical similarity of pelitic rocks in the interior and margins of the KFZ with those outside the zone indicates that they underwent little or no deformation weakening. Wacke intervals record progressive textural and reaction weakening that increased the proportions of very fine-grained, polyphase matrix and sheet silicates. Granitic rocks underwent rapid strengthening as the pluton cooled followed by rapid weakening catalyzed by grain-size reduction and phase mixing during a transient phase of brittle deformation. Bedding in the metasedimentary rocks was subparallel with the incipient KFZ. Therefore, pelitic beds presented through-going weak zones on which strike-slip motion could easily localize, and they likely controlled the long-term strength of the KFZ at this depth. Both metasedimentary rocks and granite-derived ultramylonites host isolated, nearly monomineralic quartz bodies that deformed by dislocation creep. Quartz recrystallized grain sizes indicate that granite-derived ultramylonites supported 2.1–2.5-times more flow stress than metasedimentary rocks in the interior of the KFZ. Metasedimentary rocks exhibit many quartz +/- calcite veins, pressure-shadow overgrowths, and evidence for synkinematic reactions. Granite-derived ultramylonites exhibit little mineral deposition in dilatational sites and did not interact with the calcite-buffered fluids present in the metasedimentary rocks. This indicates that fluid movement in the KFZ was largely restricted to foliation-parallel pathways within single rock units. A lack of across-strike fluid communication along with pre-existing variations in sheet-silicate concentrations help explain the formation and maintenance of a strength contrast between texturally similar fault rocks across the KFZ.