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