Robert B. Hawman

Deep seismic reflection data of EDGE U.S. mid-Atlantic continental-margin experiment: Implications for Appalachian sutures and Mesozoic rifting and magmatic underplating

The EDGE seismic experiment across the Virginia continental margin delineated a Paleozoic suture, buried Appalachian terranes, and Mesozoic rifting and magmatic events. The seismic grid revealed that the Mesozoic Norfolk rift basin exists only in the northern one-third of the previously mapped area. The north-striking listric border fault of the Norfolk basin half-graben parallels seismic laminations in the basement. The Jurassic volcanic wedge pinches out just landward of the Baltimore Canyon trough hinge zone and downlaps on the hummocky oceanic basement under the continental rise. Under the continental slope, the volcanic wedgereaches depths >9 s (20 km). Two distinct intracrustal reflections at 4.0-5.0 s and at 7.0 s TWIT (two-way traveltime) dip southeastward at low angles (∼15°). The Moho reflection is disrupted where it is intersected by the 7.0 s reflection. Northwest of this point the Moho dips landward; seaward it is horizontal. Seaward of this point, the lower-crustal boundary laminations exist in a narrow interval (10.5-11.0 s) and are of strong amplitude. These changes in the Moho and lower crust represent the seaward edge of the Grenville-age North American crust and the landward edge of Jurassic magmatic underplating. A northwest-dipping reflection observed for the first time on the U.S. Atlantic margin may be the top of the Jurassic magmatic- underplating layer; the northwest-dipping reflection truncates the southeast- dipping 7.0 s TWTT reflection. Landward projection of the 7.0 s reflection yields a north-south trace on the postrift unconformity under the center of lower Chesapeake Bay. This trace is near a basement fault between low-grade metamorphic rocks (Carolina slate-Avalonia) on the east and high-grade rocks (Goochland terrane) on the west. This fault boundary and the southeastdipping 7.0 s reflection probably represent the Taconic suture.

Isostatic compensation for a portion of the Southern Appalachians: Evidence from a reconnaissance study using wide-angle, three-component seismic soundings

Wide-angle reflections generated by five controlled blasts and over 110 timed quarry blasts in the Southern Appalachians were used to test models for isostatic compensation of topography. The profiles cross the Appalachian gravity gradient and gravity low and sample the highest elevations within the orogen. Migration of P, SV, and SH reflections suggests that crustal thickness varies from 35 to 39 km within the coastal plain and 37–39 km within the Carolina terrane. It increases northwestward from 40 to 45 km across the Inner Piedmont, and then thickens to 50–52 km along the southeastern flank of the Blue Ridge Mountains. Crustal thickness within the Blue Ridge Mountains ranges from 47 to 56 km. Receiver functions for broadband stations GOGA and MYNC show a similar pattern in crustal thickness. Assuming Airy compensation, the correlation between elevation and Moho depth suggests a range of 50–150 kg/m 3 for the density contrast between the crustal root and mantle. The greatest Moho depths are associated not with the tallest peaks, but rather with the broadest portions of the mountain chain. This observation is consistent with regional bending of the lithosphere. However, the planar basement surface suggests that the root either predates Alleghanian thrusting, and therefore is unrelated to the present topography, or formed in response to some other mechanism. Bounds on curvature of the basement surface suggest a lower bound of 30–40 km for the effective elastic thickness of the lithosphere. This is consistent with previous estimates for the southern Appalachians based on analysis of gravity data.

Relationship between observed upper mantle structures and recent tectonic activity across the Southeastern United States

The lithospheric structure of the Southeastern United States is a product of earlier episodes of continental collision and breakup. The region is located in the interior of the North American Plate, away from active plate margins. However, there is ongoing tectonism in the region with multiple zones of seismicity, uplifting arches, and Cenozoic intraplate volcanism. The mechanisms controlling this activity and the state of stress remain enigmatic. Two important factors are plate strength and preexisting, inherited structures. Here we present new tomographic images of the upper mantle beneath the Southeastern United States, revealing large-scale structural variations in the upper mantle. Examples include the relatively thick lithospheric mantle of stable North America that abruptly thins beneath the Paleozoic Appalachian orogeny, and the slow upper mantle of the Proterozoic Reelfoot rift. Our results also indicate fast seismic velocity patterns that can be interpreted as ongoing lithospheric foundering. This provides a viable explanation for seismicity, uplifting, and young intraplate volcanism. We postulate that not only tectonic inheritance but also continuing lithospheric foundering may control the ongoing activity of the region long after it became a passive margin. Based on distinct variations in the geometry and thickness of the lithospheric mantle and foundered lithosphere, we propose that piecemeal delamination has occurred beneath the region throughout the Cenozoic, removing a significant amount of reworked/deformed mantle lithosphere. Ongoing lithospheric foundering beneath the eastern margin of stable North America explains significant variations in thickness of lithospheric mantle across the former Grenville deformation front.

Deep velocity structure of rifted continental crust, U.S. Mid‐Atlantic Margin, from wide‐angle reflection/refraction data

We present new ocean-bottom, wide-angle seismic data, collected during the 1990 EDGE Mid-Atlantic multichannel seismic experiment, which provide a measurement of the deep velocity structure of rifted Appalachian continental crust beneath the U.S. East Coast continental margin. Reflections from the entire crust and Moho are visible from offsets of zero to 100 km. One-dimensional inverse traveltime modeling reveals a 34-km-thick crust consisting of four layers beneath the post-rift sediments, with velocities of 5.9, 6.3, 6.4, and 6.85 km/s. This velocity structure is indistinguishable from that found beneath several Appalachian terranes in the northern Appalachians of New England. The average velocity of 6.85 km/s in the lower crust limits the landward extent of the high-velocity (7.2-7.4 km/s) lower crust found farther seaward, and supports the interpretation of that layer as the result of rift-related magmatic underplating, rather than as thinned, pre-existing lower continental crust.

Constraining lithologic variability along the Alleghanian detachment in the southern Appalachians using passive-source seismology

Polarities and amplitudes of intracrustal P-SV conversions (P waves converted to vertically polarized shear waves) in receiver functions from the Southeastern Suture of the Appalachian Margin Experiment array and USArray Transportable Array provide new constraints on the origin of seismic reflectivity delineating the Alleghanian detachment in the southern Appalachians(eastern United States). Forward modeling of receiver functions is consistent with a 3.5-km-thick, high shear-wave velocity (Vs = 3.9 km/s) section of deformed Paleozoic platform metasedimentary rocks beneath the Blue Ridge at 3–6.5 km depth. In the Inner Piedmont, conversions from the top and base of a low-Vs zone (3.1 km/s) at depths of 5–9 km are interpreted as a package of metasedimentary rocks or a shear zone characterized by radial anisotropy. The detachment continues to the southeast beneath the Carolina terrane, where high-amplitude negative conversions at 10–13 km depth are consistent with arc rocks (Vs = 4.0 km/s) overlying sheared rocks with lower Vs (3.2 km/s). Southeast-dipping conversions at 5–10 km depth mark the boundary between the Inner Piedmont and Carolina terrane. This study demonstrates that relatively high-frequency receiver functions (up to ∼3 Hz), though still lower in frequency than P-wave energy analyzed for reflection profiling (>20 Hz), can provide important links between surface geology and active-source experiments to better constrain models of crustal structure.

Wide‐angle, three‐component seismic reflection profiling of the crust along the East Coast Gravity High, southern Appalachians, using quarry blasts

Inversion of refraction/wide-angle reflection travel times for two-dimensional velocity structure along the axis of the East Coast Gravity High in the Carolina Terrane of Georgia indicates much higher crustal P wave velocities than previously modeled. Thirty-nine timed quarry blasts recorded with a 20-element array of three-component recorders at 330-m spacings provide reversed and overlapping coverage along the profile which crosses Consortium for Continental Reflection Profiling Georgia line 5. Ray paths are concentrated in high-grade rocks of the Charlotte and Kiokee Belts. Shot gathers show strong first arrivals, Moho reflections, and numerous reflections from within the crust. Shear wave arrivals are particularly strong, with prominent direct arrivals (Sg) and reflections from the Moho (SmS) observed for all quarries. Three alternative models show average crustal P wave velocities of 6.5-6.6 km/s and Moho depths between 37 and 39 km. P wave velocities increase from 6.0 - 6.2 km/s at 1 km depth to 6.7 - 6.8 km/s at a first-order discontinuity at a depth of 24-26 km. Alternative models for the lower crust feature both a slight increase in velocity and velocity reversals at the 25-km discontinuity, with velocities steadily increasing to between 6.9 and 7.5 km/s at the base of the crust. Low average V p /V s ratios for the crust (1.70 - 1.75) estimated from travel time ratios for SmS and PmP reflections suggest that the high compressional-wave velocities derived by inversion are associated with rocks of high metamorphic grade and granitic to intermediate, not mafic, average composition over most of the crust. The low average ratios can also be explained in part by the presence of a layer of low P wave velocity in the midcrust, possibly associated with a package of quartz-rich platform and rift-basin sediments overthrust late in the Alleghanian orogeny by the Carolina Terrane allochthon, as suggested by earlier workers. Crustal models incorporating the new estimate for crustal thickness beneath the Carolina Terrane show that the long-wavelength component of the gravity gradient in Georgia can be explained in terms of crustal thickness variations ; aeromagnetic data indicate that density variations responsible for local steepening of the gradient are confined largely to the lower half of the crust.

Shallow mantle velocities beneath the southern Appalachians from Pn phases

To constrain mantle structure that might contribute to the topography of the southern Appalachian Mountains, Pn phases from regional earthquakes recorded in northern Georgia by EarthScope Southeastern Suture of the Appalachian Margin Experiment and Transportable Array stations were used to solve for shallow mantle P wave velocities. Mantle velocities vary laterally, with values of 7.6–7.8 km/s beneath the higher elevations of the Blue Ridge terrane and northwestern flank of the Inner Piedmont terranes and values of 8.3–8.5 km/s farther south where elevation is lower. The zone of low-velocity mantle could represent a source of buoyancy that helps to support the higher elevations, in addition to the root of thickened crust that also exists beneath the mountains.

Reconstructing the end of the Appalachian orogeny

In contrast to crustal deformation observed in the actively forming Himalayas, where shallowly dipping crustal detachments extend over hundreds of kilometers, prior work on the Paleozoic southern Appalachian orogeny inferred that the final continental collision occurred on a steeply dipping crustal suture, permitting collision models that are dominated by strike-slip motion. Here, we use scattered seismic phases to instead reveal the Appalachian (Alleghanian) crustal suture as a low-angle (<∼15°) southward-dipping interface that soles into a flat-lying mid-crustal detachment. The observed suture geometry implies more than 300 km of head-on shortening across a plate boundary structure similar to the Himalayan mid-crustal detachment, indicating that this mode of deformation has been fundamental in continental collisions over hundreds of millions of years.

Structure of the crust and upper mantle beneath the Great Valley and Allegheny Plateau of eastern Pennsylvania 1. Comparison of linear inversion methods for sparse wide‐angle reflection data

We use τ(p) data extracted from a small number of wide-angle recordings of quarry blasts to construct averaged, one-dimensional velocity models of the crust beneath portions of the Great Valley, Newark Basin, Valley and Ridge, and Allegheny Plateau of the central Appalachians of Pennsylvania. Using the linear form of the equations for τ(p) in terms of slowness-depth structure, we compare inversion results for the extremal, Backus-Gilbert, and generalized least squares methods. Although the uncertainties are large, the models do show a well-constrained increase in midcrustal velocities and crustal thickness beneath the Allegheny Plateau. Inversion of precritical reflections from the Moho suggests bounds of 7.1–7.6 km/s on P wave velocity and 3.9–4.2 km/s on S wave velocity near the base of the crust. P wave velocity models for the Great Valley show a crustal thickness between 40 and 45 km, with an average velocity between 6.4 and 6.6 km/s. Models for the Allegheny Plateau show a larger crustal thickness (47–52 km) and a much higher average velocity (6.8–6.9 km/s). Estimates of average shear wave velocities for the Great Valley range from 3.6 to 3.8 km/s, with crustal thickness estimates between 37 and 44 km. For the relatively small number of singular values retained, standard errors in depth for models derived by generalized least squares range from ±1 to 2 km for the P wave slowness models and from ±2 to 4 km for the S wave models; extremal depth bounds in general are 2 to 3 times as wide. Corresponding uncertainties in interval velocity, estimated from resolving kernels in slowness, range from 0.15 to 0.45 km/s for P wave models and from 0.15 to 0.40 km/s for S wave models. T2–X2 inversion of PmP data gives similar estimates for total crustal thickness and average velocity after correction for refraction effects. T2–X2 inversion of PmP data for a fourth profile suggests the possibility of a slight thickening of the crust (47–48 km) directly beneath the axis of the Great Valley gravity low. Estimates of average VP/VS for the crust based on average velocities for models derived by generalized least squares inversion range from 1.73 to 1.77. Estimates based on travel time ratios for events interpreted as P wave and S wave reflections from the Moho lie between 1.75 and 1.79. For a crust in which the effects of velocity anisotropy can be neglected, these estimates correspond to crustal averages between 0.25 and 0.27 for Poissons ratio. The one-dimensional velocity models derived here provide estimates of the long-wavelength component of velocity structure. Besides demonstrating the maximum resolving power inherent in a limited τ(p) data set, these averaged models can be used for migrating reflection data and as starting models for determining two-dimensional velocity structure.

Using Delay-Fired Quarry Blasts to Image the Crust: A Comparison of Methods for Deconvolving Mixed-Delay Source Wavelets

Quarry blasts can be effective, low-cost energy sources for seismic imaging, provided that one can deconvolve the extended source signatures produced by ripple firing. This study uses real and synthetic quarry-blast data to compare the performance of several methods for deconvolving mixed-delay signals. The problem considered here is the recovery of the effective source function along a given azimuth from traces in a shot gather. The tests focus particularly on design criteria for minimum-entropy filters and optimization of source-wavelet estimates derived by inversion of minimum-entropy filter coefficients. Predictive deconvolution reduces ringing but can generate coherent artifacts when the source wavelet is not minimum delay. Wiener filtering using source-wavelet estimates derived by least-squares inversion of minimum-entropy filter coefficients preserves relative amplitudes, allows the user to specify the degree of spiking, and avoids delays in the output. Of 47 blasts recorded in Georgia and Tennessee with 15- to 19-channel arrays, 37 yielded a localized wavelet estimate in which the duration of the most energetic portion of the wavelet estimate trace was close to the reported duration of the blast. In general, extraction of the source wavelet directly from quarry-site recordings is complicated by nonlinear effects, interference from S and Rayleigh waves, and the variation of the source wavelet with azimuth. In spite of those complications, waveforms observed at quarry sites were similar to wavelets derived from field traces for about 10 of the blasts studied. Wavelet estimates derived from minimum-entropy filter coefficients are not affected by static shifts between traces. Where statics have been removed and where recording arrays are long enough to resolve differences in ray parameter for overlapping events, localized slant stacks can be a useful alternative for estimating the source wavelet. Methods, such as Wiener filtering, that collapse energy to the onset of each event in the gather ensure that the events will migrate with the correct depths and apparent dips.