North American Plate,
Mid-Atlantic Inversion Resulting from the Chesapeake
Download the New Jersey region Google Earth KMZ file (1KB)
A hypervelocity bolide of about 3-5 km diameter indented the Chesapeake Bay about 35 million-years ago. The impact probably directed a northward tectonic push in the direction of the flight path, assumed to be moderately inclined to the Earth's surface along the NW axis of the bay as illustrated below. The strain field stemming from the impact is shown as being centered in the Pennsylvania Salient, including the Juniata Culmination (JC). The field is wedge shaped and includes three major river courses radiating outward from the point of impact toward the foreland, cutting through crustal strata along zig-zagged paths that mimic currently rising and subsiding regions. It is quite possible that a regional uplift would have resulted after the impact from crustal contraction with impact-generated sets of fractures, folds, and faults. The river courses may have developed along paths of dense fracturing with unroofing of foreland areas beginning in the Late Eocene. Nearby oceanic margin deposits now include a regional Oligocene unconformity topped by Miocene age and younger strata.
The Potomac, Susquehanna, and Delaware river systems continue to feed the continental shelf today. Could these major rivers have formed in direct response to impact stresses? Sets of fractures, folds and faults that developed at relatively late stages in the tectonic history of the Mesozoic Newark Basin, Pennsylvania and New Jersey (nb - above), indicate NE directed strain (JW above). Wise (2004) shows that late-stage extension fractures joints fanned out laterally across the culmination in Paleozoic rocks. Naiser and others (2001) report that major east-flowing Mid-Atlantic rivers did not breach the Blue Ridge until early or middle Miocene time. Impact-related fractures therefore may have formed within a contracted crustal wedge lying above deep-seated, crustal block faults that may have readjusted, or perhaps formed in direct response to impact stresses. Eocene igenous rocks occur along the southern margin of the proposed wedge near the upper reaches of the Potomac river and along a linear swarm of recorded earthquakes trend ing southeastward to the crater. The spatial association of the igneous complex with basement seismicity suggests that impact-related fractures penetrated to upper-mantle depths. A pronounced, closed gravity anomaly (yellow) occupies a congruent position on the other side of the culmination to the NE. It therefore appears that the Chesapeake Invader (Poag, 1999) helped shape today's regional landscape and current neotectonic setting of the mid-Atlantic foreland region. Current earthquake seismicity and crustal plate motions continue to reflect this ancient, catastrophic event today.