MULTIPLE DEFORMATION OF CHLORITE-GRADE LATE ORDOVICIAN TO EARLY DEVONIAN STRATA IN EASTERN MAINE

Several lines of evidence indicate that chlorite-grade Late Ordovician to Early Devonian “cover rocks” in eastern Maine were deformed more complexly than previously thought:

1. Refolded folds recognized in several formations
2. Layering in pelitic and carbonate units is in many cases an axial planar transposition fabric rather than bedding as originally thought, indicating at least two folding events
3. Regional distribution of steeply to vertically plunging folds
4. Blind thrusts imaged in a seismic reflection transect
5. Lithofacies relationships suggesting cover rocks were thrust over their sources prior to upright folding

The number of ductile and brittle events and precision with which they can be dated vary geographically, generally in agreement with the diachronous model of Acadian tectonism proposed by Bradley et al (2000). Upright folding east of the Miramichi terrane is bracketed between 423 Ma intrusion of the Pocomoonshine pluton and the 430 Ma youngest detrital zircons in the folded rocks. Timing of west-directed shallow crustal thrusting is uncertain. Dextral shearing in the broad Norumbega fault system (NFS) began at 380 Ma, with dip-slip reactivations throughout the Carboniferous (Wang and Ludman, 2003) and at least as recently as 80 Ma (West and Rhoden-Tice, 2003); one such (undated) event on the NSF Waite strand offset the Moho vertically by >1 km. High-angle dip-slip faults SE of the NFS post-dated Pocomoonshine intrusion and 400 Ma emplacement of bimodal plutons.

Earliest deformation west of the Miramichi terrane involved both eastward (at the west edge of the Miramichi terrane) and westward thrusting (near the Penobscot River). This was followed by two foliation-producing events -- first upright folding and then shearing in the Chester, Kingman, and Passadumkeag shear zones -- both of which produced foliations defined by small white micas. Ar/Ar dating of micas and dates of the cross-cutting Center Pond (377) and Mattamiscontis (407) plutons constrain the folding to ~410 Ma and the shearing to ~380 Ma (Ghanem et al., 2016). NNE-trending zones of high angle dip-slip faulting post-date thrusting and upright folding but predate emplacement of the 380 Ma Bottle Lake pluton.