Strain Localization Mechanisms and Damage Zone Microstructures Across the Brittle-Ductile Transition: A Case Study of the Kellyland Fault Zone

Walter (Bill) Sullivan, Colby College
Emma O'Hara, Colby College
Emily Peterman, Bowdoin College

Abstract

Granite cut by and adjacent to the Kellyland fault zone (KFZ) exhibits a spectacular suite of fault rocks and damage zone microstructures recording a range of fault processes and deformation mechanisms across the brittle-ductile transition. The northeast-striking KFZ is one of three strands of the crustal-scale Norumbega fault system in eastern Maine, USA. In the area of this study, the KFZ juxtaposes low-grade metasedimentary rocks with coarse-grained granite. Solid-state deformation began shortly after granite crystallization, and strain was localized across a series of mappable domains as the granite cooled. From southeast to northwest across strike these domains include: (1) a 2–3-km-wide band of variably foliated granite, (2) a 50–300-m-wide band of foliated granite cut by numerous 0.5–100-cm-wide mylonitic shear zones and coeval brittle shear fractures, (3) a ~2–3-m-wide breccia zone hosting coeval shear zones and local recrystallized pseudotachylyte, and (4) a >100-m-wide zone of ultramylonite and minor mylonite derived from granite. Foliated granite distal from the core of the KFZ preserves high-temperature microstructures that record the earliest stage of deformation. Quartz dislocation creep was likely the rate controlling mechanism at this stage. Centimeter-scale shear zones cutting foliated granite formed after brittle fault rocks, and wall rocks of these shear zones host pervasive networks of healed microfractures that likely record dynamic pulverization over a 50–300-m-wide domain. Minimum microfracture densities measured in pulverized microcline grains are 96–127 mm/mm2. Pulverization along with local recrystallized pseudotachylyte record the onset of seismogenic deformation when flow stresses recorded by dynamically recrystallized quartz reached 96–104 MPa at temperatures of 400–450 °C. Interseismic viscous creep at similar flow stresses is recorded by mutual cross cutting relationships between breccia-hosted shear zones, brittle fractures, and pseudotachylyte. Field and microstructural observations indicate that breccia-hosted shear zones are low-strain equivalents of the >100-m-wide ultramylonite zone, and seismogenic deformation abated as the ultramylonite formed. The rheology of ultramylonites was governed by grain-size-sensitive (GSS) creep at 112–124-MPa flow stresses. Hence, from the onset of seismogenesis, the KFZ was likely a constant stress system wherein the rate controlling mechanism shifted from episodic seismogenic slip and interseismic viscous creep to steady state GSS creep in ultramylonites. Flow stresses recorded by these rocks also imply that the whole zone was relatively weak if the brittle-viscous transition and uppermost viscous zone are the strongest region of the crust.

Session

Session 1: Fault Zones from Top to Bottom