DIGITAL MAPPING OF PSEUDOTACHYLYTE IN THE HARBOR ISLAND FAULT ZONE, EAST MUSCONGUS BAY, MIDCOAST MAINE

The Harbor Island Fault Zone is a 6 km long NE-trending zone of left lateral strike-slip along the eastern contact of the Late Devonian Waldoboro pluton. The zone of small-scale brittle faulting has been traced from Friendship Long Island south thru Black, Harbor, and Crane Islands in layered metamorphic rocks and minor granites. Pseudotachylyte (pt) occurs in left lateral brittle faults formed by friction melting during Late Paleozoic seismic events. Field methods involved highlighting the pt-fault veins and injection veins with chalk and photographing the outcrop using a digital camera pole for low elevation aerial images. The photographs were merged into mosaics in Adobe Photoshop and georeferenced in ArcGIS using RTK GPS control points. The previously chalked pt-veins were digitized on-screen in ArcGIS to create maps delineating the geometry and extent of the pt-fault structures. Nearly 100 pt-fault veins were mapped within the study area; ranging from 1mm to 2cm wide, and continuous for up to 35 m of strike-length. The pt-fault veins trend along the steeply dipping metamorphic layering as single faults, in pairs as generation zones, or multiple sets with injection veins, complex reservoirs, step-over geometries, and strike-slip duplexes. Left lateral offsets of earlier vertical quartz and granitic veins measured at a dozen locations average 34 cm (7-90 cm) corresponding to 5-6M earthquakes. Measurements on one layer-parallel pt-fault vein, 35 m long, recorded thickness variations, the geometry of injection veins, and off-fault reservoirs and indicate an average fault vein thickness of 0.33 cm for a 28-30.5 cm displacement. Associated with the main layer-parallel pt-fault veins are 26 right and left lateral cross-layer faults with average displacements of 5.4 cm (1-22 cm). Right and left stepping geometries in paired layer-parallel fault vein sets had an average width of 0.8 m. Left-stepping geometries developed minor layer-parallel extension as conjugate cross-layer fault sets. Right-stepping geometry developed minor layer-parallel shortening as seen in minor open conjugate folds. Truncation of warped layers by pt-fault veins may reflect early asymmetric folding cut off by later brittle layer-parallel slip.