COMPARISON OF DUCTILE STRUCTURES ACROSS THE HINESBURG AND CHAMPLAIN THRUST FAULTS IN NW VERMONT

From structurally lowest to highest levels, the bedrock geology of our field area consists of 3 domains: 1) L. Cambrian-M. Ordovician sedimentary rocks of the parautochthon, 2) M. Cambrian – M. Ordovician sedimentary rocks of the upper plate of the Champlain Thrust (CT), and 3) L. Proterozoic – E. Cambrian chlorite-sericite grade metamorphic rocks of the upper plate of the Hinesburg Thrust (HT). These lithotectonic slices were imbricated during the Ordovician Taconian Orogeny along the east dipping CT and HT. Subsequent deformation, including folding and renewed faulting may be related to the Devonian Acadian Orogeny. In general terms, the HT separates the foreland (west) from the hinterland (east).

The oldest structures in the field area are related to thrusting along the HT and CT. Near the HT, these include mylonites and F1 isoclinal folds of bedding that are deformed by F2 folds with asymmetric shear bands. Directly below the CT, within Stony Point Fm shales, early structures include intraformational minor thrusts that rotate an early pressure solution cleavage and tight folds (F1) with axial surfaces and an east-dipping axial planar slaty cleavage. The isoclinal F1 folds range from upright to reclined. Along Lake Champlain, intraformational thrusts within limestones and shales have E- and SE-trending, bedding parallel calcite slickenlines.

All early thrust-related folds and faults in each of the 3 domains are folded by N-S-trending, asymmetric open- tight folds (F3) and E-W-trending open folds (F4) that create an elongate dome and basin pattern across the field area. Although this Ramsey Type 1 pattern is best seen at the outcrop scale, it also is visible at the map scale where lithologic markers are present. Above the HT, these fold sets were confirmed through field mapping, slope maps derived from LIDAR, and domestic well logs. Depending on the rheology of the rock units in which they occur, S3 and S4 varied from axial planar fracture to slaty cleavages. On average, the S3 cleavage strikes northerly, the S4 strikes easterly.

Ongoing analysis seeks to: 1) further resolve the relative ages of thrust related structures in the parauthochonous and upper plate of CT domains and 2) resolve whether the orthogonal F3 and F4 folds sets formed during discrete deformational events or evolved in a constrictional stress field.