Date of Award
Master of Science (MS)
The Liberty-Orrington fault separates two tectonic terranes of widely different lithologies and metamorphic grades within the Coastal Lithotectonic Belt of Maine. While the juxtaposition of the sillimanite-bearing Passagassawakeag gneiss and the chlorite grade Bucksport Formation (turbidites) requires a fault between them, field evidence for, and an understanding of, the nature of the fault has hitherto been lacking. Although the Liberty-Orrington fault has previously been interpreted as a thrust, strike-slip, and/or normal fault, the most recent debate has been centered around two models of Acadian amalgamation involving thrusting of the Passagassawakeag terrane from the southeast vs. thrusting from beneath central Maine (from the northwest) (Osberg et al., 1998; Stewart et al., 1995).
My detailed mapping shows the existence of a 250-500m wide mylonitic shear zone separating the gneiss and the turbidites in the southern portion of the study area. Foliation within the shear zone is predominately near-vertical, with near-horizontal stretching lineations and pervasive dextral (present orientation) sense-of-shear indicators. The mylonites are deformed by open Acadian folds on both map and outcrop scales, and are cut by Devonian (371 Ma) granite. Followed eastward, this northeast-striking Passagassawakeag-Bucksport terrane boundary turns north, as do highly-strained rocks and local foliation. A thin unit of alternating layers of quartz and garnet+biotite+magnetite, previously interpreted as a stratigraphic unit showing possible original bedding (Rider Bluff unit), lies along the north-south striking part of the Passagassawakeag-Bucksport boundary. Thin sections demonstrate that the layering in this unit is a tectonic fabric.
The field data suggest that the Liberty-Orrington fault is a major dextral strike-slip shear zone, with the eastern boundary as a transpressional thrust; if this is the case, the Liberty-Orrington shear zone may represent a continuum of orogen-scale dextral shear (with the Penobscot Bay and Norumbega fault zones) through the Acadian. This tectonic model is more likely than that of a folded shear zone generated by a thrust, as the sense-of-shear in the unfolded mylonites would require large-scale thrusting parallel to the orogen: This study necessitates a re-evaluation of the role of transpression in the exhumation of high-grade rocks in coastal Maine during the Acadian orogeny.
Short, Heather A., "The geology of the Liberty-Orrington-Passagassawakeag/Fredericton Trough terrane boundary in the Bucksport-Orland area, coastal Maine" (1999). Geology Theses and Dissertations. 84.