Geometry and Style of Post-Obduction Thrusting in a Paleozoic Orogen: The Taconic Frontal Thrust System

Abstract

The Taconic Frontal Thrust System, associated with the later stages of the medial Ordovician Taconic orogeny, a continental margin-volcanic arc collision, defines the present western boundary of the Taconic Allochthon in eastern New York and west-central Vermont. The Frontal Thrust emplaces regionally folded and foliated Taconic rocks over deformed, but essentially unmetamorphosed, flysch. The Frontal Thrust is temporally and kinematically related to the Champlain and Orwell faults to the north, that emplace variably deformed sections of Early Paleozoic shelf rocks over partially coeval sections of the Early Paleozoic platform. Major movement on these faults occurred after initial obduction of the Allochthon onto the Early Paleozoic North American continental shelf and prior to being unconformably overlapped by the Early Devonian. This fault system is considered to be part of the final phase of a continuum of thrusts that evolved during the Taconic Orogeny, and therefore records only the final emplacement geometry of the Allochthon. Detailed mapping has revealed a complex, fine-scale imbricate structure at and near the front of the Allochthon. The imbricate structure in front of the Allochthon is defined by slivers and small thrust sheets of Lower and Middle Ordovician shelf carbonate and overlying Middle Ordovician shales, flysch and wildflysch (debris flow deposits), and melange. These imbricates have dimensions of hundreds of meters to kilometers parallel with strike and a few to several hundred meters structural thickness across strike. Ramps and tear faults within the imbricate stack control the final geometry of the otherwise gently east-dipping Frontal Thrust surface. Complex superposed fold patterns occur locally within the overlying regionally folded Taconic sequence at these frontal structures. At Bald Mtn. and other localities to the north, the carbonate slivers have an internal duplex structure, with the Frontal Thrust forming the roof fault to the duplex. In some areas the Frontal Thrust appears to follow the earlier, pre-cleavage thrusts (e.g., Basal Taconic Thrust) along which initial obduction occurred. Microscopic and mesoscopic fault-related deformation associated with the Frontal Thrust appear to have developed in fully lithified sedimentary sequences. Deformation involving partially lithified rock is limited to the local incorporation of olistostromes into tectonic melange at the leading edge of the Allochthon. The pattern of essentially coeval thrusting, lithification, cleavage development, low-grade metamorphism deduced here may be generally representative of the final stages of continental margin-island arc collision.