A natural example of brittle-to-viscous strain localization under constant-stress conditions: a case study of the Kellyland fault zone, Maine, USA

This article integrates field, powder X-ray diffraction and microstructural data to constrain deformation mechanisms in and the rheology of granite-derived fault rocks exposed along the SE side of the crustal-scale, strike-slip Kellyland fault zone. Deformation in this area of the Kellyland fault zone localized during cooling and is marked by (1) foliated granite, (2) a similar to 50 m wide band of pulverized foliated granite, (3) a similar to 2.8 m wide breccia zone hosting coeval shear zones, and (4) a >100 m wide ultramylonite zone. The earliest fabric in the foliated granite is defined by elongated quartz grains, and quartz dislocation creep was the rate-controlling deformation mechanism. Seismogenic deformation initiated when recorded flow stresses reached 96-104 MPa at temperatures of 400-450 degrees C and is marked by coeval pulverization and formation of breccia. 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 creep at 112-124 MPa flow stresses. Hence, from the onset of seismogenesis, the Kellyland fault zone was likely a constant-stress system wherein the rate-controlling mechanism shifted from episodic seismogenic slip and interseismic viscous creep to steady state grain-size-sensitive creep in ultramylonites derived from brittle fault rocks. 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 part of the crust.

Automated summary

This study uses various data to determine the deformation mechanisms and rheology of granite-derived fault rocks in the Kellyland fault zone. Deformation in this zone occurred during cooling and can be characterized by different types of rocks and their corresponding deformation mechanisms.