IT iconb TECTONICS BLOG Rev. 2020-02-15

Gregory Charles Herman, PhD
Flemington, New Jersey, USA

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Figure 1
Figure 1. Google Earth (GE) map of the the Central Appalchian continetal margin showing the surface projection of the Chesapeake impact crater relative to Bouger gravity anomalies mapped in Virginia showing 100- and 150-km radius rings around the crater, locations and ages of Late Eocence, epithermal, sulfide-mineralzied faults in Pennsylvania and New Jersey (Mathur and others, 2015), late Eocene igneous rocks in West Virginia and Virginia (Southworth and others, 1993; Tso and others, 2004), and the Tom’s Canyon impact structure (Poag and Pope, 1988). Also shown to the north of the crater is an integrated geological theme adapted from the USGS (Herman, 2015. The interpreted bolide trajectory was from SSE to NNW and is highlighted with a yellow line drawn up the axis of Chesapeake Bay indicating the direction of lithospheric axial compression. The light gray lines project from the crater outward into the surrounding region like wheel spokes, one which symmetrical bisects the Tom’s Canyon impact structure.


Figure 2
Figure 2. Chronostratigraphic groups and subgroups used for tectonic structural analyses of the region (Herman, 2015). Two large-bolide impacts on the North American Plate during the Cenozoic are shown relative to time and stratigraphic aspects. References are footnoted after group names and abbreviations. Era and stage boundary ages from www.stratigraphy.org


Figure 3
Figure 3. GE display capture summarizing structural elements of the U.S. Appalachian margin with respect to the Chesapeake impact crater (Poag, 1999). The crater resulted from a hypervelocity bolide several kilometers in diameter impacting the continental margin along a moderately inclined flight path descending from the SSE that pushed and compacted Applachian crust in a NNW direction within a crustal wedge that displaced Jurassic dolerite dikes and older crust. Some Appalachian anticlines (red) and synclines (blue) are compiled to emphasize the link between positive epeirogenic structures lying circumferential to the crater at the same radii like the Cincinnati Arch and the Adirondack Mountains. Note the locations of Nickelsen’s Bear Valley (1987) and Cove Valley (1996) structural studies in the Pennsylvania Salient, and the Tom’s River impact that’s also of Eocene age (Poag, 1999). Circumferential rings of 150, 300, and 760 km are shown that emphasize hypothetical lithospheric welts.  




Figure 4
Figure 4. Re-Os isochron plots of sulfide-mineral analytics for eight locations in Pennsylvania (fig. 1 and Mathur and thers, 2015).  Isotopic results from Lafayette Meadows (LM) also reflect the ~35 Ma isochron but are not plotted because the Re/Os ratios are significantly larger and the trends become difficult to view.

 

 

Figure 5

Figure 5. GE map showing the Chesapeake impact crater with respect to a crustal blast pattern, regional structure contours (feet) atop Precambrian basement (Rickard , 1973; Baranoski, 2013), CAMP dolerite dikes (Herman and others, 2015), deep basement faults of the Rome trough (deWitt, 1993), and linear traces of strong aeromagnetic positive amomolies seen in  global coverage (Maus and others, 2017). Crustal-compaction values compiled from calcite-strain-guage work by Engelder (1979), Spang and Groshong (1981), Lomando and Engelder (1984), Craddock and others (1993), and Ong and others (2007).  Green circles trace approximate crestlines of lithospheric arches lying circumferentially to the crater at 300 and 760 km radius bracketing circumfential troughs depicted in profile below (figure 6). The Westminster arch of Pennsylvania (Campbell, 1929) defines where pre-impact-age Cenozoic sediment is structurally warped just northward of Cheasapeake Bay. Blast-sector abbrevations: C - compressed , E - extended crust, M - marginal.

Figure 6

Figure 6. Ouput from an AutoCAD model showing the upper half of sliced Earth detailing profile depths to interior phase boundaries and the hypothetical ITFF strains.  Shear fractures locally thicken the lithosphere by compressional wedging downrange of the crater owing to the obliquety of impact. Primary seismic reflections off interior phase boundares to produce long-wavelength surface ripples cresting at roughtly 660 and 2900 km radius from the crater with estimated peak amplitude of 8 km. Profiles a. and b. depict a 20-km diameter sphere descending at an angle of 60o toward Earth’s surface. Profile c. details the geometry of the thickened lithospheric wedge. The wedge is modelled as conforming to seismic p-wave transmisson geometry that may vary in depth of penetration with impact angle and size of the bolide (Tectonics Blog Chapter 1). Shear fractures likely develop at acute angle to refractory head waves that disspate absorbed ground energy within the wedge with the remainder returned to Earth’s surface roughly 2900-km distance from impact.  Therefore, compression, excavation, and indentation immediately downrange of the crater corresponds to foreland uplift at great distance from the crater.

 

Figure 7Figure 7. Oblique, SW view of the compressed crustal sector and lithoispherice wedge that is the foreland part of the radial blast pattern around the Chesapaeake crater. The foreland wedge is portayed here together with a 5-km diameter bolide descending along a moderate angle from south to north. The primary path of crustal compression coincides with the azimuth of Chesapeake Bay and the maximim measured value of crustal compaction (%) in the foreland based on mechanically twinnned calcite (see refs in fig. 5). The compression axis bisects the Juniata Culmination and a foreland wedge of thickned crust through parts of the New York recess and Pennsylvania Salient. Also shown are locations of the Re-Os isotope data (Mathur and others, 2015), the Tom’s River impact, and Eocene magmatism.  Nickelsen’s Bear Valley (1987) and Cove Valley (1996) structural studies of deformation kinematics straddle the culmination.  C – compression, M – marginal, and E – extensional crustal sectors surroiunding the crater (white circular lines).

Figure 8Figure 8 Photographs of complex transtentional structures of probable Mesozoic age with later (Cenozoic?) wrenching in upper Paleozoic strata at a coal mine from Bear Valley, Pa. (Nickelsen, 1963; location noted in fig. 1). Late-stage structural grabens (A) of probable Newark group (fig. 2; Herman, 2009) dip steeply west and have two sets of slickenlines (B) on graben-bounding faults. The earlier slickenlines indicate normal, dip-slip shearing during graben development, and later ones indicate compressional wrenching that is congruent to regional joints sets of sub-parallel strike (Engelder and others, 2001). Photo A from www.princeton.edu.


Figure 9Figure 9. Optical BTV imagery collected in Early Mesozoic bedrock in New Jersey reveal late-stage, ~E-W-striking, reverse shear fractures that dip gently SE (circled), less so to the NW, that are among the most open and permeable conduits in these fractured aquifers. The schematic diagrams on the left illustrates a gently dipping plane cut by a borehole in wrapped and unwrapped versions of an optical-borehole image. The interpretation is conducted on the unwapped image. Note how early, mineralized  fractures of steeper dip are sheared and offset in the image to the right along one of the moderatly dipping reverse shear planes. Well locations and details from Herman (2015). Depth units for the BTV imagery are feet below land surface.

 

Figure 10Figure 10. Mesozoic Bedrock in the Trenton, NJ is structurally inverted by reverse shear fractures and faults striking about E-W that overprint and reactivate earlier tensile-transional fractures and faults developed from Newark rifting (Herman, 2009). The complex structural geometry in the region results in positive, transpressional flower structres that are deeply weathered and commonly masked at land surface.  Note the location of core MW-20C at RACER-TRENTON where transtentional fault breccias healed with spar cailte are overprinted and twinned by tectonic compression (fig. 11).

 

 

Figure 11Figure 11. Calcite-cemented autoclastic breccia in the Stockton Fromation retrieved at MW-20C by Haley-Aldrich, Inc. at he RACER-TRENTON site (fig. 10 and Herman, 2016).

 

Figure 12Figure 12. GE display of the mid-Atlantic , continental and oceanic margin of the North American Plate (NAP), centered near 40o latitude and -75o longitude showing recent crustal motions determined using ground-fixed GPS receiving stations, crustal seismogenic zones, principal axes of crustal compression reported from focal-mechanism solutions from well-constrained earthquakes, and some key structural analyses used for interpretations (Herman, 2016). Note the parabolic line connecting the seismic zones wrapping around the rising Adirondack Mountains, which appear to act as a buttress in resisting the slow, northwestward plate drift with rates that slowly increase NE up the spine of the Appalachian Mountains. Also note the pronounced GPS-velocity break on the western side of the Pennsylvania Salient.

Figure 13Figure 13. Photographs of strained Tuscarora quartzite in profile (ac tectonic plane normal to regional fold axes) from the Juniata Culimnation, Pennsylvania Salient (left; Herman, 1984) and shocked quartz crystals retrieved from core in Virginia (right - Horton and Izett, 2005). 

 

Figure 14

Figure 14. A structural palinspastic reconstruction of the Pennsylvania Salient was an unpublished aspect of my Masters thesis at the University of Connecticut (Herman, 1984) that was included in Hatcher and others (1989) showing that the curved mountain chain fans outward from the head of Chesapeake Bay.

Chapter 2. Mid-Cenozoic tectonic disruption of the
Central Appalachians by the Chesapeake Invader

Introduction * Geological evidence * Discussion * References * Impacttectonics.org

Abstract

Geological, geochronological, and geophysical data mapped across the Pennsylvania Salient and New York Recess show that this region was uplifted from tectonic compression in the Cenozoic period.  One phase of regional lithospheric compression is proposed here to have occurred in the Tertiary period stemming from the oblique bolide impact that produced the large crater now buried beneath the mouth of Chesapeake Bay, Maryland, USA (Poag and others, 1992; 1994, Koerbel  and others, 1996; Poag, 1996; 1997, and 1999; Horton and others, 2005). The hypervelocity impactor is modelled to have obliquely struck the Chesapeake Bay area along a decent trajectory aligned along the bay axis (azimuth 347o). The impact compressed and thickened the Appalachian crust in a foreland sector situated downrange where brittle shear fracturing and faulting was focused as part of a circumferential blast pattern and far-reaching lithospheric welts formed at great distances from the crater. The foreland compressional sector fans northward away and outward from the impact crater into the central Appalachian foreland where bedrock strains of Cenozoic age affect all older rocks and measurements of bulk crustal compaction may reach 10%.  Prior to the recent discovery of the concealed crater, most of the tectonic effects interpreted in this region are thought to solely result from the suturing of tectonic plates during Paleozoic orogenesis.   In contrast, it is shown how this bolide impact contributed to the geological expression of the central Appalachian Mountains by overprinting the Paleozoic orogenic belt, a transtentional strain field imparted by Mesozoic continental rifting, and basal Tertiary strata of the Cenozoic coastal plain.  The Chesapeake impact is therefore one of the latest of a series of causative tectonic agents contributing to the physiographic expression of the scalloped, passive margin. 

Introduction

The cratering effects from large, hypervelocity asteroid, comet and meteorite (bolide) impacts on terrestrial planet and moon surfaces are visually apparent and well known. Numerous studies have shown that large, episodic bolide collisions with host bodies disrupt the atmosphere, obliterate the impactor, and leave telltale scars behind on the host’s crust including not only complex craters, but near-field fracture fields lying close to the point of impact (Sharpton and Ward, 1990; Koerbel and Henkel, 2005). But there has been little consideration for, or accounting of impact-tectonic, far-field (ITFF) crustal strains stemming from large-bolide impacts lying outboard of the crater at distances of hundreds to thousands of kilometers (Chapter 1). Moreover, there are only abstract concepts of any long-lasting geodynamic consequences to periodic, large-bolide impacts (Herman, 2006). 

Evidence for a massive, catastrophic disruption of this region's crust and lithosphere is summarized herein that includes post-Meosozoic brittle strains found throughout the central Appalachians stemming from dissipated ground strains imparted by the Chesapeake impact event. Poag (1999) named the asteroid that struck the Mid-Atlantic margin of the North American tectonic plate (NAP) during the late Eocene epoch (35.5 Ma) 'The Chespeake Invader'. The resulting Chesapeake impact crater is about 85-kilometer diameter with circumferential, outboard gravity anomalies ringing the structure with radii exceeding 150 km (fig. 1). It was discovered lying beneath a blanket of Cenozoic sediment shed off the flanking Appalachian Mountains and is correlated here to regional, penetrative lithopsheric and crustal strain fields including circumferential low-amplitude lithospheric welts situated hundreds to thousands of kilometers distance away from the crater (figures 1, 3, 5, 6 and Chapter 1).  Potential-field geophysical data and structural-kinematics mapped in Paleozoic and Mesozoic bedrock in Pennsylvania and New Jersey, together with radiometric age dates of mineralized faults (Mathur and others, 2008; 2015) signal the far-field, Cenozoic-aged tectonic disruption of the central Appalachians.  ITFF strains stemming from this impact also include structurally compressed, thickened and elevated crust downrange of the crater indicating an oblique descent rajectory along the bay axis of azimuth 347o (fig. 1). The bedrock structural kinematics are consistent with a Cenozoic-age tectonic northwest-directed push from the head of Chesapeake Bay that tightened the central Appalachian foreland fold-and-thrust belt, accentuated the scalloped Appalachian margin, and left Cenozoic igneous intrusions systematically aligned in its wake (figs. 3 and 5). These tectonic aspects overprint strain fields stemming from earlier Paleozoic orogenesis and Mesozoic rifting, the latter also having localized epeirogenic movements (Herman, 2015) associated with intrusive emplacement of the Central Atlantic Magmatic Province (CAMP) into the crust during breakup of Pangaea (Marzolli and others, 1999; Herman and others, 2013).  

Stratigraphic, structural, geophysical, and radiometric evidence

The nature of the regional stratigraphic succession, structural observations, and geophysical evidence leading to this impact-tectonic, far-field (ITFF) strain hypotheses are deailed below. Figure 2 summarizes the tectono- stratigraphic groups for the New Jersey area in the central Appalachians covering Proterozoic through Cenozoic crustal rocks (Herman, 2015).  An expanded Cenozoic section includes subunits separated by major unconformities developed on the central Appalachian margin of the North American tectonic plate (NAP) beginning with a prominent one at the Cretaceous-Tertiary boundary that correpsonds with the timing of Chicxulub impact event.  Figures 1 and 4 summarize radiometric ages obtained from outcropping or cored sulfide-mineralized fault breccia retrived from both Paleozoic and Mesozoic bedrock that plot near an isochron of ~35 Ma (figure 1 and Mathur and others, 2015).  This geochronology was obtained from sulfide-mineralized fault breccia having Os isotopic concentrations indicating upper-mantle to lower crustal depths of origin (Mathur and others, 2008).  Early sulfide mineralization is also seen in extension veins of Mesozoic age like those commonly associated with calcite vein fill and hydrocarbon expulsion from the Newark rift basins (Parnell and Monson, 1995; Herman, 2009). Early veins in foreland carbonate and siliciclastic rocks also have sulfides with calcite and quartz intergrowths respectively that may stem from older Alleghanian orogenesis, but the earliest forms and varieties of vein sulfides including pyrite (with lesser chalco- and arenopyrite) are superseded by sulfide-cemented fault breccia (Mathur, 2008) returning a 29-point isochron and indicating Eocene ages that closely match the impact age when considering measurement errors (table 1 and figure 4). Earlier vein sulfides do not return reliable Re-Os isochron because Re may not have reached closure temperature in pyrite (~300o C; Brennan and others, 2000) during either the Mesozoic thermal pulse or any Mississippi-valley type, late-Paleozoic sulfide mineralization (Roden and Miller, 1989; Kohn and others, 1993; Blackmer and others, 1994; Smith and Faill, 1994; Smith, 2003).  Fluid-inclusion analysis of sphalerite mineralization in Mesozoic rocks provides age estimates of a possible 155-167 Ma (Cretaceous) hydrothermal event in this region (Smith, 2003), however fluid inclusion temperatures from the Eocene fault breccia indicate a higher-temperature thermal event that exceeded  300o C (Mathur and others, 2008). This Re-Os isotopic data clearly reflect an impact-driven epigenetic mineralization event in the central Appalachians having a deep lithospheric origin.

Outcrop-based finite-strain studies of Paleozoic rocks of the Appalachian Plateau through Mesozoic-aged bedrock of the Newark Supergroup also show, penetrative bulk compaction of rock-cementing calcite grains approaching 10% within in a finite-strain field fanning northward out in front of Chesapeake Bay (figs 3 and 5). Bedrock in this region was subject to secondary crustal compaction that overprints both Alleghanian orogenic and Newark rift structures. The Tertiary disruption would likely have been a sudden push that translated and thickened the crust as part of a lithospheric wedge downrange of the crater. We portray the event in Google Earth (figs. 3 and 7) using an impactor of 10-km radius and a 45o incidence angle to give this tectonic event geospatial perspective (see also Chapter 1). Most reports of the Chesapeake Invader cite a 3-5 km bolide based on the crater dimensions but this multi-ringed impact structure was probably more energetic than prior estimates.  Circumferential, positive Bouger gravity anomalies clearly lie at 150-km radius from the point of impact (fig. 1), and although the oblique incidence angle is poorly constrained, it may become more clear after a thorough compilation and review of other post-impact tectonic structures mapped in the central and southern Appalachians that must fall into the other, marginal and extensional strain sectors arranged circuferantailly about the crater (figs. 3 and 5).

The regional stratigraphic succession and structural kinematics of Paleozoic rocks throughout the Pennsylvania Salient and New York Recess (fig. 2) indicate at least two episodes of post-CAMP tectonic inversion in the region (Herman, 2015; Merguerian, 2015).  The earliest hypothetically stems from the Mesozic-ending Chicxulub impact in the Gulf of Mexico (~65 mya) and is overprinted by a later event having cross-cutting kinematic indicators in the Pennsylvania Valley & Ridge Province showing sinistral-oblique slip slip congruent with a NNW-directed tectonic push directed up Chesapeake Bay. The resultant structures fan outward downrange of the crater into the Appalachian foreland (figs. 1, 3, 5, 7 and 8).  Mesozoic (Newark) transtentional structures affecting CAMP rocks are also reactivated and structurally inverted by this late compression that opened suitably oriented fractures (fig. 9) and imparted mechanical twins in calcite cemented tensile-transitional fractures mapped as joints and autoclastic fault breccia in Triassic rocks (figs. 10 and 11).  

Steckler and others (1993) estimate a minimum of 3 km of denudation over the Newark basin and surrounding region based on fission-track analysis of zircons from both Proterozoic basement and Jurassic basalts.  More recent estimates of up to 6 km of erosion in the piedmont are reportedly associated with late-stage intra-basin faulting (Withjack and others, 2013). Basal sections of the NJ Coastal Plain of Mesozoic age younger than the impact are likewise compressed, fractured, and locally folded (Herman and others, 2013; Herman, 2015; 2016). It is likely that this crustal disturbance also produced a pronounced, post-impact, mid-Tertiary unconformity in the region (fig.  2).  Miocene and younger strata mostly lack any secondary tectonic structures except in areas in the Delaware coastal plain (Andres and Howard, 1998) in an area having some of the fastest rates of crustal subsidence (~3-4 mm/yr) based on ground-fixed GPS monitoring and reporting (Herman, 2016). The relatively rapid rate of current subsidence in the Pennsylvania culmination may reflect an inherited tectonic response to the Early Tertiary uplift (fig. 12). The pronounced, linear break in the historical GPS vertical-velocity field on the west side of the Pennsylvania salient (fig. 12) therefore probably signals current, continued and prolonged far-field crustal-strains that continue to operate today as inherited from this ancient event.

The late-stage penetrative tectonic compaction and wrenching reported in the Appalachian is modeled here as an fanned-shaped envelope whose basal geometry follows P-wave refraction paths that penetrate most deeply directly in front of the oblique impact, and diminish in intensity laterally (figs. 6 and 7). We hypothesize the return of a considerable amount of impact-generated ground energy at radial distance of about 2900 km from the crater, the same approximate radial distance to the core-mantle boundary, and therefore perhaps, a significant distance away from the crater where epeirogenic welting is pronounced as a result of constructively interfering far-field lithospheric strain responses, one of shear fracturing and rippling of Earth’s crust at distances reflective of internal mineral-phase boundaries.  

Discussion

Syntheses of late Cenozoic geological evolution of the middle Atlantic passive margin are provided by Poag and Sevon (1989) and Pazzaglia and others (2006). They report a deeply eroded early Tertiary Appalachian landscape of lower relief than today. Climate change, epeirogenic uplift, or rapid increase in the size of the Atlantic slope drainage basin, or some combination of all three factors, initiated the stripping of mature regolith in the middle Miocene and delivery to the Fall Zone. Increased sediment flux into the Baltimore Canyon trough (BCT), coupled with erosional unloading caused flexure of the margin with the Fall Zone located at the flexural hinge (Pazzaglia and Gardner, 2004).  Continued Middle Tertiary flexural warping of the margin arched early Miocene terraces and contributes to the continued incision by the Susquehanna River channel. The incised Appalachian landscape now delivers an immature, heterolithic load to the Coastal Plain and shelf region that reflect both periodic, positive and negative, isostatic adjustment to the loading and removal of Quaternary continental glaciers and slow continental convergence on a passive margin.  Erosion rates in Susquehanna River basin reportedly doubled from prior amounts immediately after the Chesapeake impact at ~ 35.5 Mya  based on cosmogenic dating of the oldest river terraces and associated upland gravel at 36.1 + 7.3 Ma (Pazzaglia and others, 2006). Younger terraces yield dates of 19.8 +2.7 Ma and 14.4 +2.7 ka respectively.  Campbell (1929) mapped the oldest gravels as mantling a doubly-plunging basement arch referred to as the Westchester anticline lying immediately foreland of Chesapeake Bay (fig. 5).

The inclusion of far-field, impact generated strains as part of the tectonic expression of the Appalachian Mountains addresses many, puzzling aspects of central Appalachia. For example, Herman (1984; 1985) reported complex tectonic structures in the Pennsylvania culmination that don’t fit the model foreland fold-and-thrust belt paradigm following a ‘break-forward’ advance of stacked thrust sheets during orogenesis. Paleozoic beds are structurally tightened and elevated in the Juniata Culmination relative to flanking areas (fig. 3), and the manner in which the Pennsylvania crust was crumpled and compounded from thrust faulting is uncharacteristic and contains ‘out of sequence’ sections where beds subsequently pushed and raised along very steep reverse faults.  Another noteworthy aspect of this work are conjugate planar microfractures (PMs) seen in strained Silurian quartzite occupying fold limbs across the width of the Pennsylvania culmination (fig. 14). These rocks are the basal section of the blind-thrusted ‘roof’ section that was under thrusted and mostly compacted by wedge faulting and folding during penetrative cleavage development in the associated finer-grained clastic and carbonate strata. The PMs in the Tuscarora resemble planar deformation features (PDFs) in quartz grains subject to shock metamorphism at pressures of ~10 to 30 Gpa (French, 1998; Lee and Leroux, 2015) but occur with less density (fig. 7).  Because PDFs are shock barometers, more work is needed in order to understand if any of the conjugate PMs in the Juniata culmination result from tectonic orogeny, shock geodynamics, or both. Nevertheless, the out-of-sequence, tightened fault slices in the Juniata part of culmination (Herman, 1984; Sak and others, 2014) are sympathetic with other late-stage brittle shear strains reported at Cove Valley and Bear Valley (Nickelsen, 1987; 1996), where late wrench faulting on steep faults that are congruent with a late, N-S push centered directed from the head of Chesapeake Bay (fig. 5).

In the early 1980s when the Appalachian Tectonics Study Group were interpreting root causes of salient curvature and crustal strains stemming from orogenesis, there was no knowledge of the Chesapeake impact crater, or for that matter, actual plate motions in the region that are now available from global positioning systems (Herman, 2015). Early attempts to reconcile curvature of secondary geological structures in the Pennsylvania Salient were only considered with respect to Alleghanian orogenesis and contradicted the simplest reconstruction of serial palinspastic sections that merged to a point at the head of Chesapeake Bay (Geiser, 1988; Hatcher and others, 1989). Attempts at deciphering the kinematic evolution of the Pennsylvania Salient using remnant thermal magnetization of beds involved in regional folding have proven uncertain (Cederquist and other, 2006).  However, the aforementioned Re-Os radiometric evidence of widely distributed Lower Tertiary, brittle faulting associated with late-stage compaction in the Pennsylvania Salient and New York Recess substantiates the widespread, regional, far-field brittle strain field fanning outward in front the Chesapeake Bay impact crater for distances greater than 500 km through into foreland areas (Mathur and others, 2015; Herman 2015).

Because such penetrative structures probably stem from the Chesapeake impact, then consideration should be extended to large impacts elsewhere and associated  far-field strains that probably include the concentric lithospheric welts lying outboard of the crater at hundred to thousands of kilometers distances. In particular, the well-known Chicxulub crater appears to have similar far-field welts  at ~660, ~1500, and ~2900 radii that are reflective of depths to major phase boundaries in Earth’s interior (Herman, 2006) and represent the radial dispersion of ground energy following  impact.  This helps explain intracratonic epeirogenic movements and not only illustrates the likelihood of far-field strain effects occurring on Earth, but also points to the need for plate-tectonic theory to include such far-field impact-generated strains on other terrestrial planets. The introduction of periodic catastrophism into modern plate-tectonic theory would include such far-field lithospheric strains and any geodynamic fluxes, plate fragmentation episodes or adjustments caused by large, hypervelocity bolide impacts on terrestrial planets.  

Historical interpretations of tectonic plate motions also lend credence to these impact-tectonic hypotheses. For example, at the dawn of the Cenozoic, shortly after the Chicxulub impact in the Gulf of Mexico, major plate reorganizations began that involved the North American, Eurasian, and African plates accompanied by major changes in the deep-ocean water circulation, permitting cold polar waters to move southward in the Atlantic Ocean basin (Klitgord and Schouten, 1986).  Now, the North American tectonic plate and border plates in the Central American region move in concert about a hub containing the Chicxulub impact crater on the south rim of the Gulf of Mexico (Herman, 2005). Moreover, shortly after the Chesapeake impact oceanic sea-floor spreading halted west of Greenland and suddenly accelerated to the east by Iceland where it’s currently focused in the North Atlantic region (Dore´ and others, 2015). Such corroborative evidence focuses consideration on how these two, known, large-bolide impacts on the NAP, at the beginning of and during the Cenozoic Era, have not only helped shape the crust, but continue to exert a dynamic neotectonic signature on our landscape that is so remarkably  remote to the causative agents.  Ribiero (2002) postulated the need for considerations of external forces on our open, dynamic tectonic system, and finally over the past two decades we have ground-fixed, GPS instrumentation available to gauge actual plate motions to gain a realistic perspective on plate dynamics.  

A plate-tectonic paradigm that includes far-field strain effects stemming directly from periodic and catastrophic bombardment by large bolides including intraplate crustal deformation and epeirogenesis simply makes sense. For now though, more work is needed to hypothesize and test models that integrate short- and long-term strain mechanisms that serve to dissipate pin-point energy fluxes, and the geometry of associated crustal and lithospheric strain fields. By definition, neotectonic strains are those that form in our current stress regime (Stewart and Hancock, 1994). In this context, do catastrophic impact strains qualify as a neotectonic features, or is that reserved to the more standard, uniformitarian viewpoints only? Regretfully, this work leaves many aspects of this neotectonic treatment and some conclusions unaddressed. For example, the scalloped, curved nature of our continental interior has been historically chronicled and debated for decades (Thomas, 1977; Marshak, 2004; Wise and Werner, 2004).  The along-strike transition from the Pennsylvania salient in to the New York recess is perhaps the most studied and reported instance of a scalloped, passive margin that has historically been treated mostly as a byproduct of differential plate convergence with irregular docking of land masses at different places, times and directions as the orogenic suture closed.   The evidence presented here supports the rather unheralded notion that much of our regional architecture, and especially the geometry of our scalloped margin, is more likely a product of continental rifting (Herman, 2014; 2015).

This work uses modern geospatial tools to help outline some new hypotheses to test both new and old ideas. We are encouraged by recent technological advances available with geographic information systems that facilitate the dynamic study and portrayal of Earth systems. Although considered tectonically passive with respect to currently active orogenic margins, the central Appalachian region has been steadily drifting as part of the NAP but punctuated with sudden tectonic upheavals that humanity has been fortunate to evade simply by our juvenile presence here.  These conditions echo observations by Gould (2007) who characterized biological punctuated equilibrium through geologic time, which leads us to think that large, catastrophic impacts are causative agents for concurrent geological and biological revolutions on this terrestrial planet occupying the Goldilocks zone.

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Introduction * Geological evidence * Discussion * References  * Impacttectonics.org