EVIDENCE FOR POST-CALEDONIAN OROGENESIS IN THE SOUTHERN AND CENTRAL CHAMPLAIN VALLEY

The primary deformational episodes in the Champlain Valley have long been attributed to Taconian orogenesis, but recent work has shown that the emplacement of the major thrust sheets (including the Taconic allochthons and the Champlain thrust sheet) happened later, probably as part of Caledonian orogenesis. Subsequent Paleozoic deformation includes an extensional event followed by a contractional event; unfortunately, there are no known post-Ordovician Paleozoic strata in this area with which to date these later events. The extensional event has been attributed to Taconian deformation, but the associated block faulting affects late Ordovician strata and subsequent (Caledonian?) thrust structures. The following contractional event caused some reactivation of earlier thrust surfaces, but is largely marked by breach thrusts which dissect the Taconics, Champlain thrust sheet, and many of the other thrust systems in the region, and create the many structural anomalies that have confounded regional geologists for generations. In addition, the normal faults are deformed by the later deformation and locally transected by the associated thrust surfaces. The frequency of the breach thrusts is attributed to the thinness of the remnant earlier (Caledonian?) orogenic material following post-orogenic erosion and the need to reestablish a critical taper during this later (Acadian?) orogenic event.