DEFORMATION MECHANISMS IN THREE PROTOLITHS ACROSS THE KELLYLAND FAULT ZONE, WASHINGTON COUNTY, MAINE

We compare deformation mechanisms and strain gradients in different rock types cut by a crustal-scale fault zone, the Kellyland Fault Zone (KFZ), near the brittle-ductile transition. The KFZ is a strike-slip shear zone that cuts the Deblois granite and metasedimentary rocks of the Flume Ridge Formation (FRF). The Deblois granite contains three distinct strain facies. With increasing proximity to the fault boundary these are: 1) foliated granite, 2) foliated granite cut by localized shear zones, and 3) a main ultramylonite domain. Formation of ultramylonites and localized shear zones was catalyzed by brittle grain-size reduction. Here we compose deformation of the granite with differed rock types across the zone.

The FRF is a variably calcareous, fine- to medium-grained wacke with pelitic layers. Outside the KFZ, pelitic layers are foliated while psamitic layers appear relatively undeformed. Foliation becomes more homogenized and boudinaged quartz veins are prevalent in pelitic layers in the transition zone between undeformed FRF and the shear zone. Deformed and undeformed pseudotachylyte veins also exist in this transition zone. Original bedding is visible throughout the main shear zone. Thus, the FRF is not homogenized as a result of brittle deformation, as seen in the granite. FRF rocks also show no indication of localized shear zones nucleating along brittle faults.

In one locality in the main ultramylonite domain, a deformed diorite body separates the granite and the FRF. Deformed granite next to the diorite is mylonite while all other deformed granite in the main ultramylonite domain is ultramylonite. Quartz grains in mylonites are 6-60 μm in diameter with an average of 22.5 μm. Quartz grains elsewhere in the main ultramylonite domain are 3-22 μm in diameter with an average of 10.1 μm. These observations suggest increased deformation temperatures and/or decreased strain rate near the diorite. Deformed leucogranite dikes and quartz veins occur in all rock types across the granite-diorite contact in this locality. These dikes and veins are only found in mylonites, and they suggest increased fluid activity during deformation.