EVIDENCE OF MULTIPLE SLIP EVENTS DURING FORMATION OF THE CHAMPLAIN THRUST FAULT ZONE, BURLINGTON, VERMONT

Near Burlington, Vermont, the Champlain thrust fault (CT) placed massive Cambrian dolostones over calcareous shales of Ordovician age during the Ordovician Taconian Orogeny. Although the CT has been studied previously throughout the Champlain Valley, its history of motion has never been systematically defined. To document this history, we completed detailed structural analyses of multiple outcrops located along a 51 km transect between South Hero and Ferrisburgh, Vermont.

Evidence for multiple slip events is well-preserved within the ~180 m thick foot-wall damage zone of the CT. Two sets of subsidiary faults form thrust duplexes that truncate older recumbent folds of bedding planes and early bedding-parallel thrusts. Slickenlines along the duplexes and those on the older thrusts record a change in slip direction from top-to-the-W to top-to-the-NW. The transition from the top of the damage zone into the base of the fault zone core, where most of the deformation was accommodated, is marked by the appearance of mylonite, ultramylonite, phyllite, and cataclasite. In addition, tight folds of duplex faults occur within the core and normal faults both cut and merge with shear zones that approximately parallel the CT surface. At the top of the core, which is up to 8 m thick, all normal faults are cut by the CT surface, indicating that they formed synchronously with thrusts in the core and are older than the last phase of motion on the CT. Core structures record slip directions that differ from those in the damage zone: motion in the core evolved from S-N slip, to SSW-NNW, to SW-NE slip. The CT surface records top-to-the-W motion followed by SSW-NNW and SE-NW slip. Axes of mullions on this surface trend to the SE and do not parallel slickenlines.

Collectively, there are at least five distinctive episodes of slip recorded within the CT fault zone. The earliest pulse of motion is preserved on both the CT surface and deep within the damage zone. Later motion within the damage zone evolved towards, but does not mimic, the direction of the youngest motion which is only preserved in the core and on the CT surface. This study highlights the variability of slip directions recorded by structures within a fault zone and documents the importance of comparing the fault zone core to the damage zone to determine the comprehensive history of motion.