Reactivation of prethrusting, synconvergence normal faults as ramps within the Ordovician Champlain-Taconic thrust system

Abstract

We present new geologic maps of the Lake Champlain region of west-central Vermont and east-central New York State. This region contains a shallow-crustal section of an Ordovician foreland basin and a far-traveled thrust system that transported the Upper Cambrian-Lower Ordovician platform-sequence, allochthonous rise-facies pelites and arenites and Middle Ordovician basinal shales and flysch. Early foreland and shelf sections are cut by post-depositional cross faults within the thrust system, and the required stratigraphic throw across the cross faults is not matched by the amount they offset the thrust planes. As thrust structures, these cross faults control structural style and kinematics by laterally bounding regions that contrast sharply in amount of thrust imbrication, duplexing, and net transport. The dominant set of normal faults in the autochthon, which had time-correlative slip with the thrust-system cross faults, is oriented parallel to the paleotrench, though there is also a subordinate transverse, linking set. The normal faults developed in response to the migrating flexural forebulge during convergence between the Laurentian margin and the accretionary prism above an outboard-facing subduction zone. Within the thrust system, any prethrust normal faults that were parallel to the paleotrench are largely masked by thrust structures. However, an out-of-sequence thrust, which emplaced shelf rocks above the westernmost allochthon and surrounding parautochthonous shale, may have localized on a paleotrenchparallel normal fault in the outer shelf. We also identify a postthrust normal fault that significantly obscures the original thrust map pattern. Reactivation of synconvergence, prethrust, flexure-induced normal faults adequately explains many otherwise puzzling stratigraphic and structural relationships in the Taconic foreland.