11 Structural Geology and Tectonics (Posters)

Sunday, 22 March 2026
8:00 AM - 12:00 PM
CCC, Ballroom C

ACROSS-STRIKE MICRO- TO METER-SCALE TRANSECT WITHIN THE NORUMBEGA SHEAR ZONE SYSTEM, HARPSWELL NECK, SOUTHERN COASTAL MAINE

Paper number: 11-0
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

Structures within the stratified metamorphic rocks of Harpswell Neck, southern coastal Maine, have recorded lower Paleozoic ductile deformation in the NNE-SSW-trending Norumbega shear zone system where a complex strain history resulted in deformation up to and including mylonite formation. Affected rocks are layered at the cm scale, and stratified at the meter scale, resulting in varied recorded strain. We recorded meter-by-meter data across structural grain to document differences in meter- and micro-structures in these rocks across strike.

 

Exposure is nearly 100 percent at Basin Point. We collected field data along the coast at approximately 1 m spacing across strike for a total across-strike transect of 700 m. At each station we recorded layer orientation, mineral composition, and outcrop structural data including foliation, lineation, fold axis, and porphyroblast-matrix relations. We collected representative hand specimens along the transect for further laboratory study. We selected five priority specimens for microanalysis based on their stratigraphic position and rock composition. Paired thin sections (perpendicular) from each specimen were cut according to their fabrics, one parallel to mineral lineation and perpendicular to foliation and the other perpendicular to both (X-Z and Y-Z planes, respectively; 10 sections). We compiled all data to construct a geological map and WNW-ESE structure section of the Basin Point study area.

 

Field results show all rocks have recorded significant strain. The dominant rock is meter-thick quartzo-feldspathic gneiss layers with minor, meter-scale ferromagnesian gneiss layers, and schist layers of residual compositions that were formerly migmatite. The quartzo-feldspathic gneiss is Qtz with Bt and Ms, and is has local cm-thick horizons defined by cm-scale clusters of mm-diameter inequant garnet with dextral sigma tails. Locally, pyrite porphyroclasts have similar Qtz strain shadow tails. The former migmatite layers are generally thinner than 1 m thick, and are interlayered at the cm scale where darker layers are mm-diameter garnet porphyroblasts within biotite cluster matrices with minor plagioclase. Lighter colored layers are quartzo-feldspathic gneiss. Structures are consistent across strike with strong L-S fabrics with subhorizontal NE-SW-trending lineations and steeply SE-dipping foliation. All folds of layers in outcrop are isoclinal to tight with shallowly NE-plunging axes. Microfolds are found in the Y-Z plane thin sections, and comparison with complementary X-Z plane sections indicate shallowly NE-plunging microfold axes.

Geological Society of America Abstracts with Programs. Vol. 58, No. 2

doi: 10.1130/abs/2026NE-411445

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