FLAT VERSUS UPRIGHT STRUCTURES IN SOUTH-CENTRAL MAINE: EVIDENCE FOR DISTINCT DEFORMATIONAL EVENTS RATHER THAN STRAIN PARTITIONING

Osberg et al. (1989) called attention to an area of central New England in which “flat” or recumbent structural features are dominant, in contrast to surrounding parts of the orogen in which upright structures dominate (see their Plate 4, Map B). These authors suggested several testable hypotheses which might account for this difference in structural style between the two regions, including: (1) a decrease in intensity of upright folds in the flat region where they would still be present but not dominant; (2) a reorientation of upright folds into flat orientations by a younger deformation affecting only the flat region; or (3) a fundamentally different stress orientation at different crustal levels, simultaneously producing upright and flat structures in lower and higher temperature regimes, respectively. The critical data, lacking in 1989, is the age of the deformational fabrics in question. Here we report on new structural and geochronologic data from south-central Maine that bears on the question of timing.

Detailed mapping along the southeastern margin of the central Maine sequence in the Bowdoinham 7.5' quadrangle (in the zone of “flat structures”) has revealed the presence of a previously unrecognized, strongly deformed and recrystallized plutonic complex – the Hornbeam Hill gneiss. The plutonic gneiss, along with the surrounding country rocks, contains a north-northwest-trending relatively shallow dipping regional foliation that is consistent with fabrics in the zone of flat structures. A U-Pb (SHRIMP) zircon crystallization age of 393 ± 4 Ma dates the intrusion of the Hornbeam Hill gneiss, and since this plutonic gneiss contains the regional fabric, the “flat foliation” must have formed after the ca. 393 Ma intrusion of the pluton.

Along strike to the northeast, in the Waterville area within the zone of upright structures, Tucker et al. (2001) have established that upright folding in the Central Maine sequence must be older than 399 ± 1 Ma dikes that cut them. Thus, in the area of our study, the upright structures are distinctly older than the flat structures. This suggests the two distinctly different structural styles do not represent the partitioning of strain (either vertically or horizontally) during the same deformational episode, but rather they represent two temporally and kinematically distinct deformational events.