ALBANY NAPPES (NOT OUR ELECTED OFFICIALS), ARGENTINA, AND PLATE TECTONIC MODELS: IMPLICATIONS FROM SEISMIC AND OUTCROP DATA IN THE ORDOVICIAN FORELAND BASIN OF NYS

Mitchell and colleagues found that in the Utica basin the Trenton/Utica contact is synchronous from east to west across the NNE-striking “Taconic” faults. That regional synchroneity, plus the basal Utica age of ~453 MA, indicate that Utica-age events are not clearly related to a model of lithospheric flexure as Laurentia drifted easterly into the trench associated with the Shelburne Falls arc (~475). What then is the origin the Utica basin and syndepositional faults?

Jacobi and colleagues showed 3D seismic in NYS that indicates rhombochasms initiated in Trenton/Utica contact time along reactivated Iapetan-opening ENE-NE striking fault systems. NE of the 3D seismic, outsized angular dolomite clasts occur in Trenton rocks within one of a horsetail of grabens along the “Taconic” NNE-striking Hoffmans fault system, suggesting that the graben was active in Trenton times. These grabens are thus synchronous with those in the 3D seismic, but the geometries of these grabens suggest an opposite sense of transcurrent motion, similar to what might be found at a continental promontory.

Ductile deformation of Utica strata in cores and slump folding along the Thruway Discontinuity suggest a deformation pulse in the D. spiniferus Zone (~451.5 MA). New data from west-directed Taconic thrusts near Vischer Ferry (west of Albany, NY) reveal scaly cleavage, ductile deformation of sand bodies, and D. spiniferus Zone graptolites in the lower limb of a nappe in the Austin Glen Fm. Thus, this pre-consolidation thrusting is contemporaneous with the deformation that appears to have extended across the basin to Herkimer NY (~100 km).

If the model of only westward subduction at ca. 475-445 Ma under the Bronson Hill arc is correct, then the contemporaneous Utica formed in a backarc setting. In Argentina, foreland basin thrust belts such as the Agria are far removed from the trench (some 500 km) and are related to a relatively flat slab subduction under the continent coupled with variations in convergence rate. Although convergent rate variations could account for the Utica basin development and partial destruction, the short duration between basin development and contraction is inconsistent with the length of South America convergence rate cycles. In contrast, escape tectonics related to final convergence could be the source of the strike slip motion.