Early obduction-related deformation features of the Taconic Allochthon: Analogy with structures observed in modern trench environments

Abstract

An early generation of probably recumbent isoclinal folds and veined and slickensided faults has now been documented at several localities in the western Taconic Allochthon. These folds and thrust faults predate development of the “regional” slaty cleavage and associated folds. The early generation of structures either predates or developed synchronously with obduction of the Taconic Allochthon on the coeval early Paleozoic shelf of eastern North America during early stages of the Taconic Orogeny. Evidence for this early phase of deformation is threefold: (1) recognition of small-scale, downward-facing folds at single outcrops; (2) local occurrences of downward-younging directions (structural facing directions) projected on slaty-cleavage surfaces; and (3) large-scale inversion of stratigraphic succession, subsequently refolded by the slaty-cleavage generation of structures. Clear examples of case 3 are seen at Willard Mountain and near Granville, New York. The amplitude and wavelength of the early folds are measured in kilometres, but structural and stratigraphic observations are inconsistent with a regionally recumbent structure of the entire Giddings Brook Slice of the Taconic Allochthon. These examples provide convincing evidence for complex, polyphase deformation of the Taconic Allochthon prior to thrusting into its present structural position. These observations accord well with recent large-scale models that suggest emplacement of the Taconics in a collision-related subduction-accretion environment. Deep Sea Drilling Project (DSDP) and geophysical data from modern trenches suggest that similar scale and style of deformation may occur during incorporation of thick, already lithified sedimentary sequences into the trenchward side of accretionary-prism complexes.