ABSTRACT
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, H.A., 1999. The geology of the
Liberty-Orrington-Passagassawakeag/Fredericton Trough terrane
boundary in the Bucksport-Orland area, coastal Maine.
Unpublished MSc. thesis, State University of New York at Albany.
114 pp., +xii; 1 folded plate (map)
University at Albany Science Library call number: SCIENCE
Oversize (*) QE 40 Z899 1999 S56
MS thesis digital
text pdf (11.4 MB)
Plate
1 - Geologic
map
of the Bucksport-Orland field area, central coastal Maine
(coloured
geological outcrop map; scale 1:12,000)
Return to MS Theses completed in the
Geological Sciences Program, University at Albany