Geologic Evidence for Rate of Plate Convergence during the Taconic Arc-Continent Collision

The rate of plate convergence during arc-continent collision can be estimated from the rate at which the secondary effects of subduction move across the underriding plate in advance of the plate boundary. The following sequence of events is typical: (1) shoaling and/or emergence of the continental shelf, presumably caused by lithospheric flexure; (2) rapid subsidence, by a combination of normal faulting and trenchward tilting; and (3) a change from platformal to flysch sedimentation. Such a sequence has been recognized in the Taconic foreland basin in eastern New York and interpreted as being the result of collision between the ancient passive margin of North America and an island arc terrane at an east-dipping subduction zone during Medial Ordovician times. A plot of age versus distance across strike shows that the diachronous migration of these phenomena across the foreland proceeded at rates of 2 to 3 cm/yr; we regard this as the plate convergence rate during the latter part of the Taconic Orogeny. Our result is comparable with modern rates of plate motion and also agrees with an earlier estimate for the Taconic, which was based on the rate at which a series of locations on the outer trench slope passed through fossil-defined isobaths.