TRANSTENSIONAL FOUNDERING OF THE LAURENTIAN (NEW ENGLAND) MARGIN AT ~453-451 MA

During the early Late Ordovician (Sandbian), the Laurentian passive margin was overridden by the Taconic allochthons concomitant with the destruction of the Taconic Seaway. An episode of renewed compressive stresses along the Laurentian margin initiated a peripheral bulge and retroarc basin that migrated across the Laurentian passive margin, accompanied by extensive normal faulting and changes in accommodation space that formed the Taconic foreland basin. The sedimentary record of these late orogenic events, as preserved in central New York (Mohawk Valley region), begins with carbonates of the Trenton and Black River (TBR) groups. These rocks are overlain by laminated black mudstones of the Utica Shale. Volcanic ash beds and marine phytoplankton (graptolites) preserved within the Taconic foreland succession are particularly useful in constructing a series of high-resolution time slices within which we examined the distribution of sedimentary facies.

In drill cores and outcrops between Schenectady and Utica, NY, the TBR and lower Utica Shale thicken dramatically across the Little Falls and the Hoffmans faults, suggesting that local deposition was heavily influenced by fault movement. The TBR and lower Utica Shale packages thicken to the WNW, toward the bounding fault systems and away from the orogen within the proximal sub-basin, which is bounded on the NW by the Hoffmans Fault, as well as in the distal sub-basin, which is bounded on the NW by the Little Falls Fault. Although kinematic indicators suggest mainly dip-slip motion, 3D seismic to the south, map patterns, and local accommodation space changes also suggest rhombochasms associated with oblique slip. The age of the lower Utica shales and the contact at the top of the underlying TBR sequence display no measurable diachroneity across the full width (~100 km) of the sub-basins. Taken together, these results suggest that the Laurentian passive margin in New York foundered in association with extensional to transtensional forces that affected the retroarc domain during a brief interval from ~ 453 to 451 Ma. Sinistral oblique faulting is also associated with the Taconic 3 events in Quebec and Newfoundland, suggesting similar dynamics affected the Laurentian margin broadly at this time.