Juxtaposition of contrasting structural regimes across a portion of the Norumbega fault system in the northern Casco Bay region of Maine

Abstract:

It has long been recognized that Paleozoic stratified rocks in some regions of central New England are dominated by relatively flat structural features (e.g., recumbent folds, shallow dipping foliation) while other areas are dominated by near vertical upright structures. The northern Casco Bay region of coastal Maine (Brunswick 7.5' quadrangle and adjacent areas) provides an excellent venue for studying the relationships between these two structural regimes as they are in close proximity due to juxtaposition by high angle faulting associated with the Norumbega fault system.

Stratified rocks exposed west of the Flying Point fault in northern Casco Bay are dominated by moderately east dipping foliation (ave. = 025^o, 37^o), moderate northeast plunging mineral lineations, and recumbent to gently inclined minor folds. In stark contrast, immediately east of the Flying Point fault, stratified rocks are dominated by steep east dipping foliation (ave. = 014^o, 73^o), subhorizontal mineral lineations, and upright to steeply inclined minor folds. The structural differences correspond directly to differences in the thermal histories preserved in these rocks as revealed by earlier thermochronological studies. Rocks in the zone of upright structures east of the Flying Point fault were last subjected to high grade metamorphic conditions and granitic plutonism in the Late Devonian and were relatively cold (<300^oC) by Late Carboniferous time. In contrast, flat lying rocks west of the Flying Point fault were over 500^oC in the Early Permian and Permian pegmatites are common.

Geochronological studies north of the study area have revealed that the two distinctly different structural styles are not the product of strain partitioning during the same deformational episode, but rather they represent two temporally and kinematically distinct deformational events. Swanson (1999), originally suggested flat structures west of the Flying Point fault are consistent with an episode of northwest directed thrusting and our findings are consistent with this interpretation. However, this flat phase of deformation significantly post-dates the older upright structures preserved to the east and thus models for the structural evolution of the region must integrate both the kinematic and temporal differences in this deformation.