POST-IMBRICATION, N-S SHORTENING IN THE PHILIPSBURG SLICE, SOUTHERN QUÉBEC APPALACHIANS

Detailed examination of key outcrops within the allochthonous Philipsburg slice and the nearby parautochthonous domain revealed a N-S shortening post-dating imbrication of allochthonous and parautochthonous rocks.

Most structures developed within the Philipsburg slice during the Taconian Orogeny display a mean NNE strike consistent with a WNW shortening. This strike is roughly the same throughout the entire parautochthonous and allochthonous domains in the vicinity of the slice in Quebec and Vermont. However, WNW thrusting of Philipsurg slice along Logan’s Line/Champlain Thrust cannot explain the ENE strike of Morgan's Corner thrust fault. This major fault crosscuts the regional, NNE-trending Saint-Armand Syncline in its northern area and is associated with major folds of similar strike, developed in the footwall and hanging wall of the fault. A possible dextral strike-slip movement along the Morgan's Corner Fault would be consistent with a regional WNW shortening and might explain the orientation of the fault. This hypothesis, however, is ruled out because folds are ENE-striking and parallel to the fault instead of being distributed en échelon as expected for folds associated with a strike-slip fault. Moreover, striae on the fault plane display NNE and NNW orientations that clearly indicate an average N-S shortening. Crosscutting relationships indicate that Morgan's Corner Fault post-dates an E-W strike slip fault. These strike-slip faults are believed to be characteristic of the latest WNW shortening steps of the slice.

Other indications of thrusting towards the north were recognized further south in Logan's Line hanging wall (an E-W reverse fault with NNW-trending striae) and footwall (ESE-WNW minor thrusts and ENE-trending folds). The major regional shortening direction along Logan's Line is towards WNW in the study area, and shortening towards north, therefore, must postdate imbrication of the allochthonous Philipsburg slice with rocks of the parautochthonous domain.

The late N-S shortening regcognized in Québec, north of the international border, casts new light on the NNE-directed bedding parallel slip movements documented in the hanging wall of Highgate Springs Thrust (Vermont).