TECTONICS BLOG Rev. 2022-05-19

Gregory Charles Herman, PhD Flemington, New Jersey, USA

Click on an image to enlarge it Figure 1. Overview of the study area on the mid-Atlantic margin of the North American continent and tectonic plate showing historical earthquake epicenters in the region greater than magnitude 4.0, crustal seismogenic zones (Herman, 2006), a CORS vertical-velocity (Zvel) derivative map detailed below, and scaled white vectors summarizing horizontal, plate drift. Select drift values are shown using velocity (mm/yr.) and azimuth format (ex. 14/283). Yellow regions are rising and blue regions sinking at rates of a few mm/yr. The zero, vertical-velocity break lines are traced in bold white. The straight, yellow line trending toward azimuth 350o from the Chesapeake impact crater denotes the primary stress axis of crustal compression arising from the mid-Tertiary bolide impact, interpreted as an oblique strike from the SSE. Circumferential rings are included for both the Chesapeake (~35Ma- white) and Chicxulub  (~66Ma light green) impact craters. Figure 2. Chronostratigraphic groups of the NAP mid-Atlantic region used in a recent neotectonic study of the area (Herman, 2015). References and abbreviations used for the tectonic and stratigraphic aspects are footnoted. Era and stage boundary ages from www.stratigraphy.org. Figure 3. The hydrocarbon window, crustal metamorphic faces, and tectonite classifications with respect to the number of earthquakes by depth in the Sykes and others, 2006 earthquake catalog for the metropolitan Philadelphia to New York City region. Figure 4. A screen capture from a QGIS project display of the vertical-velocity-derivative map (Zvel raster theme) showing the 296 CORS GPS station locations with theme parameters noted  and spot velocities labeled. Figure 5. The CORS Zvel derivative theme with interpreted seismic zones (Herman, 2005), NEIC earthquake epicenters colored by magnitude ranges (Herman, 2015), and US Geological Survey faults and terminal moraine. Figure 6. GE display of the Ramapo seismic zone displaying the Sykes and others (2006) earthquake catalog for the Philadelphia-New York, NY region and the Zvel derivative base map summarizing vertical crustal movements. Earthquake-derived compression (P) axes shown as thick, gray arrows. The region is faulted and wrinkled along the N-S direction in the vicinity of the Ordovician igneous plutons rooted in the Hudson Highlands (Cortland) and New Jersey Ridge and Valley thrust system (Beemerville). Figure 7.  Top view of a SketchUp Pro (SUP; rev. 2020) 3D CAD model of seismicity in the New York Recess based on Sykes and others (2006) earthquake catalog and 243 historical earthquakes by depth and magnitude. The Ramapo seismic zone is cored by Ordovician plutons cropping out near Cortland, NY . Nodal-plane solutions for three, NY earthquakes are included. The model shows how the crust is locally pinned to the mantle by the plutons, which focuses crustal seismicity. Neotectonic movements in many places utilize inherited tectonic structures. Figure 8.  North, cross-section of the SUP 3D earth model showing crustal plutons, crustal thickness, seismicity, and an interpreted tectonic sequence. The Cortland (EAST) and Beemerville (WEST) plutons (Latest Ordovician) widening ad depth into the mantle beneath the crust at about 30 km depth. The plutons were likely 1) thrust Westward during the Alleghanian Orogeny, 2) extended Eastward during Mesozoic Newark rifting, but now, 3) serve as brittle, tectonic buttresses that pin the crust to the mantle and selectively resist westward crustal drift. The Ramapo seismic zone with focused historical seismicity around the Cortland pluton includes both NW dipping (antithetic) and SE dipping (synthetic ) faults in the pluton-intruded basement with associated, rising and sinking regions. WF1974, AV1977, and AV1980 are events with published focal-mechanism solutions depicted using oval planes with a 2:1 strike-to-dip aspect. Green ovals are synthetic faults. Figure 10. Oblique NNE view of the 2022 SUP earthquake model for the New York Recess showing vertical drop lines from mapped epicenters to each earthquake hypocenter as reported in Sykes and others (2006). The Ramapo seismic zone is mapped as a pink polygon on the GE base map. Figure 11. The contents of the GE KMZ file including the SUP 3D model of Ramapo seismicity. See the text for more explanation.

Passive drift, wrinkling crust, and cracking plutons of the New York Recess, USA

Introduction * Geologic setting * Data and Methods  * Interpretation * Google Earth KMZ model * References * impacttectonics.org

Abstract

A deeper geological understanding of the manner in which the New York, USA region is drifting and being slowly wrinkled as part of the North American lithospheric plate is gained from a neotectonic analysis using regional geospatial data including geological maps, historical earthquake epicenters, and ground-fixed, global-positioning-system stations (GPS) that monitor the manner in which our crust moves.  A review of the prior crustal interpretations with interpreted seismogenic zones are compared and contrasted with the latest GPS vertical-velocity maps to illustrate how and why the New York recess is drifting, rising, and sinking as part of the North American plate (NAP). Focused earthquake seismicity occurs where Ordovician plutons preferentially resist crustal drift, contract, and rise.

Introduction

Geospatial computing, digital mapping, and computer-aided drafting (CAD) software provide the means to gather, analyze and communicate the myriad data sets needed to characterize the subtle neotectonic aspects of a geologically complex region like the New York Recess of the eastern, passive, continental margin of the North American tectonic plate (NAP; fig. 1). As is demonstrated herein, geographic information systems (GIS) and computer aided drafting software (CAD), including QGIS freeware, the Google Earth (GE) virtual globe, and Trimble Inc. SketchUp Pro (3D CAD software) are effective tools for generating and integrating complex, spatially georeferenced themes of crustal seismicity, geological faulting, and three-dimensional (3D) crustal movements to visually explore subtle links between our current states of crustal stress and the strain responses, or neotectonic setting.  

This work uses the most recent GPS-based, crustal-motion data together with detailed earthquake-epicenters catalogs covering the New York and Philadelphia metropolitan area (Sykes and others, 2006; US National Earthquake Information Center (NEIC, 2015), to generate maps that portray how our crust is evolving in three-dimensional (3D) detail. These maps are then combined with regional geology maps including exposed fault traces, Early Jurassic dike swarms, and glacial moraines to visually inspect the link between plate drift of a passive tectonic margin and elastodynamic crustal-strain responses down to 20 km depth that have occurred in the past century.

Geological Setting

This article represents an update to a prior, regional neotectonic study of the New York Recess as summarized in the annual proceeding and contributions to the Geological Association of New Jersey (GANJ; Herman, 2015). That work also covered the outcropping structural details observed within different chronostratigraphic units (fig. 2). But the focus here is primarily on neotectonic movements that have occurred over the past century and includes an updated geospatial-data analyses showing where and why historical seismicity occurs within our region . The results show remarkable congruity between seismogenic zones and on-going crustal uplift and subsidence involving deep-seated igneous plutons that serve as tectonic buttresses that resist plate drift and focus low-magnitude brittle seismicity around them in compensatory movements within the upper 20 km of the Earth as the crust is cracked and wrinkled within the brittle, structural regime (figs. 3 - 5) . As a result, the pluton cores are contracted and wedged upward in our current, compressive stress regime.  Moreover, current, vertical movements of our crust include rising and subsiding areas that are inherited, pull-apart structures that are now being squeezed under a compressive stress regime as the North American tectonic plate (NAP) rotates about a pole located in the Southeast Pacific.  Our geological provinces and watershed physiography reflects these dynamics, including far-field strains from the Chesapeake impact of Eocene age (~35.5 Ma).

The late Cenozoic geological evolution of the middle Atlantic passive margin is detailed 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 coastal plain. Increased sediment flux into the Baltimore Canyon trough (BCT), coupled with erosional unloading caused flexure of the hinged, continental margin (Pazzaglia and Gardner, 2004).  Continued Middle Tertiary flexural warping of the margin arched early Miocene terraces and contributes to the continued down cutting of 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 drift on a passive margin.  Erosion rates in Susquehanna River basin reportedly doubled from prior amounts immediately after the Chesapeake impact at ~ 35.5 Ma 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 that is a secondary geological structure of Early Tertiary age lying immediately foreland of Chesapeake Bay (fig. 1).

Data and methods

Three of the geospatial data themes used in this study were published on line as part of the aforementioned 2015 GANJ meeting. The earthquake themes are available as customized Google Earth (GE) files (KMZ files) with the methodology and data sources for the geospatial themes summarized by Herman (2015).  At that time, a NEIC query returned 3532 earthquake events located between longitudes 55^oW to 95^oW and latitudes 32^oN to 55^oN on the North American tectonic plate (NAP). The events were parsed into folders having integer-based magnitude ranges, then displayed using variously colored- and sized symbols in GE (fig. 5).  A second, more detailed and localized earthquake-epicenter catalog covering the New York City and Philadelphia metropolitan areas (Sykes and others, 2006) was used to generate two geospatial themes from 383 event records that range in magnitudes from 0.2 to 5.2 and depths from 0 to 21 km (fig. 6). 

A third GIS theme is a polygon theme representing crustal seismic zones showing where earthquake occurrences are most prevalent on the North American tectonic plate (NAP) between latitudes 20°N to 55°N and longitudes 25°W to 105°W for the period of 1973 to 2001. This theme was built at the onset of a neotectonic study using a custom, regional, epicenter database built using different catalogs maintained by the NEIC, 3 US states (NJ OH, IN), and the Weston Observatory at Boston University (Herman, 2006). That database had 28,139 epicenter records for the period 1900 to 2005 after combining and parsing to eliminate duplicate events.  The resulting point theme included 27,852 epicenters having magnitudes 2.0 to 7.3, with 26,625 of those also having recorded depth values. This point theme was used to generate the seismic zones included here. The process then involved using ArcView Spatial Analyst software to rasterize the point theme and populate 0.1-degree cells with calculated earthquake-epicenter density values. The densities were calculated by sequentially querying the number of events lying within a 50 km search radius from the center of each cell. A set of polygons representing crustal seismogenic zones were generated as isolines from the raster theme by enclosing areas having epicenter densities greater than 0.001 earthquakes per sq. km. These values were derived and symbolized using trial and error densities, search radii, and grid-cell sizes. This work was completed shortly before the work of Sykes and others (2006) was published.

A fourth geospatial theme used here is GPS data known as CORS; continuously operating receiving stations, that are coordinated and maintained by the U.S. National Oceanic and Atmospheric Association. Each monitoring station is grounded and reports derivative crustal motions along directions parallel to longitude (X) and latitude (Y), and normal to Earth's surface (Z). The CORS Zvel theme included here was generated using  396 of the 2405 total CORS stations in the conterminous United States.  Station data were downloaded and processed from degree, minute, and second coordinate format of the 1984 world geographic system (WGS1984) to decimal-degree coordinates. QGIS (rev. 3.16.14) computer software was used to generate a raster theme covering a region from longitudes 82.2^o to 69.2^oW and latitude 34.5^o to 44.7^o N (fig. 1).  The vertical-derivative values of plate motions were interpolated between adjacent points (GPS stations) to map the differences in vertical ground motion in the area using ~.05-degree cells and a yellow (positive or up) to blue (negative or down) color scheme (figs. 1 and 4 to 6). The raster image was therefore generated using QGIS, captured as a visual display, saved as a PNG graphics file, and georeferenced in GE in order to assess the spatial nature of the different rising and subsiding regions with respect to the seismogenic zones and geological features mapped at Earth's surface. The colored image is a single band, raster, linear-interpolated theme that displays the vertical-velocity (Zvel) field by highlighting areas that are rising and sinking according to the GPS data. A set of velocity contours at 0.5 mm/yr. intervals was also generated using the raster theme. The resulting subset of isolines equaling zero (0.0) velocity were highlighted in order to show the boundary between currently rising and subsiding areas within the region.

Interpretation

A SketchUp Pro (SUP) 3D crustal model was built using both magnitude- and depth-based GE displays of Sykes and others (2006) earthquake catalog and the Zvel raster image as base maps.  The igneous plutons are modeled in three dimensions (3D)together with 243 earthquakes having both depth and magnitude parameters in the catalog (figs. 6 to 9). The model includes points, lines, and polygons components that were manually added into the model in order to gain a visual perspective on the rheological and spatial controls associated with the earthquakes.  In the SUP model, I portray both the Cortland (NY) and Beemerville (NJ) pluton complexes as inflating beneath the base of the crust, assumed to be about 30 km thickness based on nearby, deep,  seismic-reflection profiling of the New Jersey Coastal Plain Atlantic shelf (Sheridan and others, 1991; Herman, 1992).

The geological nature of the Cortland igneous plutonic complex is discussed in detail by Merguerian (2008). It's a complex amalgamation of intrusive plutons of mantle origin with mafic to granitic phases that outcrops within a circular area of about 8 km radius (figs. 6 and 7). It's portrayed in the model as a cylindrical plutonic complex of constant diameter within the crust that expands outward beneath it in the uppermost mantle to portray ponding of magma beneath the crustal base. This is a simple form, and does not reflect the upward-expanding conical shape of some of the individual plutons and any unknown complexities like plume coalescence and cross-cutting in the deep subsurface, or their relationship with crustal faults.  The Beemerville intrusion is also represented as a vertical, cylindrical body but with a comparatively smaller-diameter (~6 km).  Its subsurface geometry is modeled using gravity and magnetics by Ghatge and others (1992), and its structural profile by Drake and others (1996) and Herman (1997).  It's also of Latest Ordovician age and is located about 57 km to the WSW of the Cortland plutons in the Valley and Ridge province of northern New Jersey.  

Earthquake P-axis solutions summarized by Herman (2015) are included in figure 6. Also, three other earthquakes having focal-plane solutions in the Ramapo seismic zone are reported by Seborowski and others (1982) for the period 1974 to 1980.  Two are located near Annville, NY and the other, Wappinger Falls, NY, with both near the Hudson River and within 60 miles of New York City. They are all low-magnitude events (1.5 and 2.8 m_blg) with reported depths of 1 km. Their respective focal mechanism solutions include the following fault planes:

1) Wappingers Falls (1974) N40^oW/30^oE and N40^oW/60^oW,
2) Annsville 1 (1977) N34^oW/26^oNE and N58^o/66^oW, and
3) Annsville 2 (1980) 2^oW/29^o E and N16^oW/62^oW.

All three solutions have gentle, east-dipping fault planes, and steep, west-dipping ones as summarized in cross section in figure 8.

As mapped, the Cortland plutons outcrop in the center of the northern half of the Ramapo seismic zone (fig. 5) but the structural form of the plutonic complex at depth is unknown. A primitive, cylindrical pluton form is assumed for this model (fig. 8). The crust is most wrinkled from recent strains immediately to the south of the plutons as it drifts toward azimuth 290^o (figs. 5 and 6).

The Cortland plutons together with sister plutons of the New Jersey Beemerville intrusive suite form the cores of neotectonic crustal arches that have an intervening trough striking about north-south between them, here designated the Watching trough, because its lateral boundaries are defined by the Watchung syncline in the Newark Basin where Jurassic lava flows and sedimentary beds sag downward along the Ramapo border fault (fig. 6). N-S trends of seismicity are focused on the western side of this structure where inherited, late Mesozoic pull-apart structures developed under tension but are now being compressed and inverted in our contemporary, compressive stress regime (Herman, 2009). The Ordovician plutons appear to locally weld the crust to the underlying mantle, thereby focusing brittle and elastoviscous crustal adjustments around them. The plutons impart a local rheological contrast in the crust signaled by the earthquake swarms surrounding it and thereby causing the Ramapo seismic zone.

The chronologic sequence of geological events involving the Cortland and Beemerville intrusions after emplacement near the end of the Taconic Orogeny therefore includes at least three, subsequent tectonic episodes:

1) Late Paleozoic contraction and thrusting of the New York recess bedrock northwestward during the Alleghanian orogeny and final suturing and assembly of Pangaea,

2) Newark extension and normal faulting of the continental margin during the Mesozoic era that stretched and offset all earlier structures, and finally

3) Contemporary, post-inversion drift involving compressive brittle cracking of the intruded foreland, and lithospheric drift towards azimuth 290^o. 

When the polarity reversal of the regional state of crustal stress flipped from principal tension during Newark rifting to subsequent compression that continues today is speculative. One interpretation is that compression began during the Mid Jurassic during the 'rift-to-drift transition' when the mid-Atlantic Ridge was born. Upwelling, active magmatism beneath opening segments of oceanic ridges thermally welted the crust and provided both gravitational and tractive pushes (Schlische and others, 2003). But another option is that the polarity switch came after the Nemesis bolide impact at the Cretaceous-Tertiary boundary (~66 Ma) when the Chicxulub crater was excavated on the south rim of the Gulf of Mexico. Systematic, tensile fracturing of Mesozoic bedrock in the Newark Basin reflects clockwise rotation of the North American tectonic plate during the breakup of Pangaea (Herman, 2009). But now, the plate is rotating counterclockwise about a pole located west of Ecuador in a location within the East Pacific Nazca plate (Herman, 2005). A 'stable-craton' rotation pole for the NAP is located more northward around Panama, closer to the Chicxulub crater. In total, the South, Central, and North American lithospheric plates drift in concert around a Chicxulub hub even though today's plate-rotation pole for the NAP isn't near the crater. Nevertheless, oceanic transform faults in the floor of the northwestern Atlantic Ocean basin show a geometry change from concave to convex at about the ~66 Ma magnetic isochron (Herman, 2009) that reflects a wholesale rotation change of the plate shortly after impact, as if the event and associated astrobleme pinned the crust to the mantle, thereby setting up a new geodynamic system that most to dispel the newly introduced energy flux that is also marked by a key bed coinciding with a major biological extinction event at the Era boundary.

We also don't know if crustal drift differs from mantle drift. That is, does the entire lithosphere moves as a lithic block, or is it subject to vertical differences in drift speeds with simple-shear profiles determined by vertical rheological contrasts? Lateral variations of plate drift of the NAP reflect increasing angular velocities with increasing distances from the rotation pole (Herman, 2006). I suspect that shearing a plate from drift would reflect the opposite trend having minimal velocities occurring at the surface that increase with depth. It's difficult to imagine an opposite scenario where the crust moves faster than the mantle, because the crust is relatively thin and brittle in comparison with the stiffest material lying closest to Earth's surface where the maximum resistance to lateral drift occurs. Beneath the crust, things strain viscously (fig. 3). Historical seismicity shows brittle failure occurs principally above 20 km depth, but lithospheric plates beneath continents can vary from 100 to 200 km. Therefore, the nature of drift in the elastoviscous and viscoelastic layers below the crust are unknown owing to the lack of measurable seismic responses to flow. If a vertical-velocity gradient exists, then two possibilities are that the mantle would either move forward (NW) or backward (SE) relative to the crust. Lacking a vertical gradient, the third option is that the bulk lithosphere drifts laterally at the same speed. But this latter option seems unlikely because temperature, pressure, and geochemical gradients occur at depth, and to assume a constant velocity therefore is anomalous.

Pondering these options is what drove me to build the SketchUpPro model of the mantle plumes. After having built it, it now appears that the primary motivating factor behind plate drift is the lateral push exerted on the base of the lithosphere by upwelling, very large mantle plumes rising off Earth's outer core, the largest of which under Africa is tilted in the direction of plate drift. But I still don't know if their is a varying drift profile of the NAP in the New York recess. It's reasonable to assume that in some places there will be a vertical drift gradient within the lithosphere, particularly in areas of focused seismicity, but in other places the crust and associated lithosphere may drift as a single, rigid block. But having focused, brittle seismicity associated with the Ordovician plutons in the mid-Atlantic Appalachian region spurs me to think that the crust is dragged along the top of a drifting lithospheric plate that's getting pushed about, but with local pockets of resistance.  After all, the Earth is cooling from the inside out and associated kinematic movements should likewise, generally decrease outward.

Google Earth KMZ file

A GE KMZ file including the Zvel raster theme, the SUP seismicity model, the Cortland and Beemerville plutonic complexes, and an E-W cross section summarizing neotectonic structures is available as a free download. The contents are outlined in figure 10.

Most of the contents of this file are detailed above, but it also contains two additional folders holding interpretations of seismic lineaments. These are polyline interpretations of the strike of secondary structures indicated by the systematic alignment of earthquake epicenters with neotectonic crustal arches and troughs. These features are highly speculative and are intended to highlight linear trends along which the crust is failing with brittle strains occurring above 20 km depth.

References

Drake, A. A., Jr., Volkert, R. A., Monteverde, D. H., Herman, G. C., Houghton, H. F., Parker, R. A., and Dalton, R. F., 1996, Bedrock geological map of northern New Jersey: U.S. Geological Survey Miscellaneous Investigation Series Map I-2540-A, scale 1:100,000, 2 sheets.

Ghatge, S. L., Jagel, D. L., and Herman, G. C., 1992, Gravity investigation to delineate subsurface geology in the Beemerville intrusive complex area, Sussex County, New Jersey: N.J. Geological Survey Geologic Map 92-2. 1:100,000 scale.

Herman, G. C., 1992, Deep crustal structure and seismic expression of the central Appalachian orogenic belt: Geology, Vol. 20, No. 3, p. 275-278

Herman, G. C., Monteverde, D. H., Schlische, R. W., and Pitcher, D. M., 1997, Foreland crustal structure of the New York recess, northeastern United States: Geological Society of America Bulletin, v. 109, no. 8, p. 955-977.

Herman, G. C., 2006, Neotectonic setting of the North American Plate in relation to the Chicxulub impact: Geological Society America Abstracts with Programs, Vol. 38, No. 7, p. 415 (1.3 MB PDF file)

Herman, G. C., 2009, Steeply-dipping extension fractures in the Newark basin (5 MB PDF), Journal of Structural Geology, V. 31, p. 996-1011.

Herman, G. C., 2015, Neotectonics of the New York Recess, in Herman, G. C., and Macaoay Ferguson, S., Neotectonics of the New York Recess: 32nd Annual proceedings and field guide of the Geological Association of New Jersey, Lafayette College, Easton, Pa., p. 80-151

Merguerian, C., 2008, Cortland Igneous Complex, Buchanan, New York: Guidebook notes for Geology 133 Field Trip 21 April, 2008, Hofstra University. 42 p.

Pazzaglia, F. J., Braun, D. D., Pavich, M., Bierman, P., Potter, N., Jr., Merritts, D., Walter, R., and Germanoski, D., 2006, Rivers, glaciers, landscape evolution, and active tectonics of the central Appalachians, Pennsylvania and Maryland, in Pazzaglia, F.J., ed., Excursions in Geology and History: Field Trips in the Middle Atlantic States: Geological Society of America Field Guide 8, p. 169–197, doi: 10.1130/2006.fld008(09).

Poag, C. W. and Sevon, W. D. 1989. A record of Appalachian denudation in post rift Mesozoic and Cenozoic sedimentary deposits of the U.S. middle Atlantic continental margin: Geomorphology v. 2: p. 119–157

Schlische, R. W., Withjack, M. O. and Olsen, P. E., 2003, Relative Timing of CAMP, Rifting, Continental Breakup, and Basin Inversion: Tectonic Significance, in The Central Atlantic Magmatic Province: Insights from Fragments of Pangaea (eds. W. Hames, J. Mchone, P. Renne and C. Ruppel). https://doi.org/10.1029/136GM03

Sheridan, R. E., Olsson, R. K ., and Miller, J. J., 1991, Seismic reflection and gravity study of proposed Taconic suture under the New Jersey Coastal Plain: Implications for continental growth: Geological Society of America Bulletin, v. 103, p. 402-414.

Seborowski, K. D., Williams, G., Kelleher, J. A., and Statton, C. T., 1982, Tectonic implications of recent earthquakes near Annsville, New York,: Bulletin of the Seismological Society of America, v. 72, no. 5, p. 1601-1609

Sykes, L. R., Armbruster, John, Kim, Won-Young, and Jacob, Klaus, 2006, Earthquakes in Greater New York-Philadelphia Area: Catalog 1677 to 2005 and Tectonic Setting; Appendices to paper Earthquakes in the Greater New York City-Philadelphia Area: 1677-2004.

This blog entry is dedicated to my father, the late Donald Edward Herman, whose birthday in 1930 was on the same May day that this entry was completed.

Abstract * Introduction * Geologic setting * Data and Methods  * Interpretation * Google Earth KMZ model * References * References * impacttectonics.org