WHAT THE CRATON CAN TELL US ABOUT APPALACHIAN TECTONICS FROM IAPETAN OPENING THROUGH THE TACONIC OROGENY

Integration of seismic reflection data across NYS with surface mapping in the Mohawk Valley and the first oriented horizontal core from the Black River Formation in the Finger Lakes of NYS allow us to construct a tectonic history in the Appalachian Basin from Iapetan opening through the final paroxysms of the Taconic Orogeny. The early opening began with reactivation of northerly trending Grenvillian suture faults and easterly trending faults. The second phase (that temporally overlapped the first) was an arcuate (map-pattern) trend of Iapetan opening faults that marked the development of the Pennsylvania Salient and the NYS Recess. The Cambro-Ordovician unconformity observed in the Mohawk Valley section reflects either the initiation of subduction, or later forearc spreading events(s), depending on the age of the Cambro-Ordovician boundary (the age of the boundary has been revised significantly in the past 10 years). The Knox unconformity has long been thought to mark (at least in part) the passage of the continent over the peripheral bulge. The Thruway Unconformity marks slide scars that were caused by reactivation of the Iapetan opening faults (and development of new faults) as the continent entered the trench. Northerly trending faults in NYS were extensional at the time of the peripheral bulge, whereas the NW and NE-ENE trending faults would have experienced shear (oblique motion), with the ENE faults right lateral and the WNW-NW faults left lateral. During final collision the maximum compressive horizontal stress SH would have been oriented EW. The reorientation of SH would result in the northerly faults now “tight” or reactivated as high angle reverse faults. Northerly-trending broad anticlinal structures also were constructed. The easterly faults were now extensional. The NW and NE trending faults would have an opposite shear sense from that during the earlier, peripheral bulge times. Evidence consistent with this model of reorientation of SH is found in the outcrop slickensides, fault patterns and offsets, and vein sequences in core and outcrop. Final significant Taconic tectonics on the craton can be dated in seismic reflection data from local unconformities over broad anticlines, and in the east from an unconformity over a thrust fault.