TECTONIC IMPLICATIONS OF A REVISED BEDROCK GEOLOGIC MAP OF EAST-CENTRAL MAINE

Mapping in E- and E-central Maine (Lincoln, Mattawamkeag, Danforth, Calais 100,000 quadrangles) requires major changes in the Maine Bedrock Geologic Map (Osberg et al, 1985). The revised map solves long-standing stratigraphic problems and suggests that faulting played a more important role in Acadian and post-Acadian tectonism than shown on the 1985 map.

NW limbs of upright Acadian (~400 Ma) anticlines defined by an expanded Aroostook-Matapedia (AM) suite are thinned tectonically relative to the SW limbs, probably by mesoscale high-angle reverse faults similar to outcrop-scale structures observed south of Houlton. The Central Maine Boundary Fault (CMBF) juxtaposes several units of the (intermediate facies) Central Maine basin against pelites of the Waterville=Smyrna Mills formations. Late-stage CMBF motion, best observed in the Chester shear zone, was dominantly dextral strike-slip but the absence of offset Central Maine rocks east of the fault suggests a multi-phase history for the boundary in which early westward thrusting and high-angle dip-slip faulting preceded dextral motion in shear zones trending generally NE or ENE..

Boulder to granule Cambro-Ordovician clasts in proximal AM strata prove that the Miramichi terrane shed debris westward to form intermediate and distal AM clastic units. AM distal strata were thrust eastward onto their Miramichi source rocks and locally isolated as klippen by high-angle dip-slip faults. A comparable proximal facies is absent on the east flank of the Miramichi in the eastern part of the study area where the contact with the Fredericton belt was modified by late to post-Acadian thrusting and subsequent dextral strike-slip faulting.

Post-Acadian dextral strike-slip faulting in the Norumbega fault system (NFS) is well documented in the Fredericton belt, but the northernmost NFS strand is now known to separate the Fredericton and Miramichi terranes southwest of the Bottle Lake pluton. Dextral shearing in the CMBF may be coeval with that in the NFS; Ar/Ar dating of white micas in CMBF phyllonites may provide evidence soon. There appears to be a sharp (fault) boundary between rocks deformed just prior to 420 Ma (Salinic) and those deformed ~400 Ma.