FROM ISOCLINAL FOLDS TO SHEATH FOLDS: THE PRESERVATION OF INTERMEDIATE-STAGE STRUCTURES ALONG AN ORDOVICIAN THRUST ZONE, WATERBURY RESERVOIR, CENTRAL VERMONT

The Prospect Rock Fault (PRF) and correlatives are east-dipping thrust faults that juxtaposed upper plate Late Proterozoic - Ordovician continental margin, oceanic, and suprasubduction zone rocks of the Rowe-Hawley Belt with lower plate Late Proterozoic – Cambrian rift clastic rocks of the Green Mountain Belt during the Ordovician Taconian Orogeny. These lithotectonic slices were also deformed and metamorphosed during the Devonian Acadian Orogeny. In the field area, black phyllites and phyllitic quartzites of the Ottauqueechee Fm (Co) structurally overlie black albitic schists and laminated quartzites of the Hazens Notch Fm (CZhn). At least four episodes of greenschist-facies deformation are recognizable in these lithologies; two generations are Taconian and two Acadian. Whereas the competence of the intercalated quartzites preferentially preserved a spectrum of early structures, the ductility of the pelitic units favored the destruction of previously-existing structures.

During 2006, the water level in the Waterbury reservoir was drawn down ~50’ for dam repairs, enabling us to examine a continuum of structures in the pristine newly exposed outcrops of both plates of the PRF. In thick (10’+) Co phyllitic quartzites, we observed the following sequence with increasing strain: 1) reclined isoclinal fold sets (RIF) with downdip stretching lineations that parallel fold axes, 2) intrafolial RIF where both limbs are intact, 3) highly dismembered RIF where one limb is destroyed, 4) development of incipient double closures on dismembered RIF, 5) sheath folds forming eye structures, and 6) asymmetric shear bands consuming RIF vestiges. We also hypothesize a direct pathway from step 3 to step 5. Within the thick (`3-10’) laminated quartzites of CZhn, stages 1-3 were visible, however, thin (~0.5’) quartzites within the albitic schist displayed all stages. The 1:24K bedrock geologic map of this area also shows stages 1-3 in upper plate rocks, suggesting a kilometer scale process.

Oriented thin sections on surfaces cut parallel to stretching lineations/fold axes show evidence for dynamic recrystallization, but kinematics are mostly unclear because of annealing. Ongoing research goals are to exploit strain variability to obtain more detailed kinematics on the deformation stages and to date this Taconian deformation.