The Ray Corner High Strain Zone of the Norumbega Fault System in Maine: A Complex History of Ductile Shear, Brittle Deformation, and Paleoseismicity

Abstract:

Recent detailed bedrock mapping in south-central Maine in the vicinity of the regionally extensive Norumbega fault system has revealed the presence of a previously unrecognized zone of overprinting ductile and brittle deformation herein referred to as the Ray Corner high strain zone. The nearly one km wide zone trends approximately 045^o and can be mapped continuously for a distance of over 20 kms. Like nearly all previously mapped high strain zones of the Norumbega fault system, the Ray Corner high strain zone is contained within a single lithotectonic belt (Casco Bay Group) and thus while recording a polyphase history of high strain displacement at different structural levels, it does not represent a terrain boundary at the present erosional surface.

The Ray Corner high strain zone is characterized by an older (likely Devonian) pervasive steeply dipping mylonitic foliation. Lineations, where observable, are sub-horizontal and a variety of kinematic indicators indicate dextral shear. Quartz microstructures are consistent with grain boundary migration and sub-grain rotation recrystallization processes, whereas feldspars are fractured. EBSD analysis of recrystallized quartz ribbons reveal strong CPO patterns in c-axis pole figures (both Y maxima and bipolar maxima between Y and Z) that are consistent with prism <a> + romb <a> slip. These finding are consistent with mylonitic deformation occurring at temperatures in the 400-500^oC range.

Superimposed on these earlier mylonitic fabrics in zones several 10s of meters across are a wide range of lower temperature brittle deformational features. These include spectacular pseudotachylyte fault and injection veins up to 3 cm thick, cataclastite breccia cored brittle faults up to 10 cm thick, and silicified fault breccia zones up to several meters across. While most of these brittle features cross-cut the earlier ductile fabrics, locally there is microstructural evidence of deformed pseudotachylyte layers within the mylonites suggesting a time of overlap between brittle and ductile deformational processes during the evolution of the Ray Corner high strain zone. Detailed studies of these relationships are in progress and should reveal information on what appears to be a polyphase history of displacement that occurred at different ambient conditions.