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Synoptic View of Lithospheric S-Wave Velocity Structure in the Southern United States: A Comparison of 3D Seismic Tomographic Models
Table of Contents
- Tectonic Setting
- Velocity Models
- Crustal Thickness Models
- Data and Methods
- Results and Interpretations
- Discussion and Conclusions
- References Cited
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The southern U.S. continental margin records a history spanning ca. 1.2 Ga, including two Wilson cycles. However, due to a thick sediment cover, the paucity of significant local seismicity, and, until recently, sparse instrumentation, details of this passive margin’s tectonomagmatic evolution remain disputed. This paper compares recent S-wave tomography and crustal thickness models based on USArray data to help establish a framework for geodynamic interpretation. Large-scale patterns of crustal velocity anomalies, corresponding to major regional features such as the Ouachita orogenic front and the Precambrian margin, are generally consistent between the models. The spatial extent of smaller-scale tectonic features, such as the Sabine Uplift and Wiggins block, remains poorly resolved. An inverse relationship between crustal thickness and Bouguer gravity across the continental margin is observed. This model comparison highlights the need for additional P-wave tomography studies and targeted, higher density station deployments to better constrain tectonic features.
Manuscript received 19 Sept. 2018. Revised manuscript received 10 Jan. 2019. Manuscript accepted 14 Jan. 2019. Posted 13 Feb. 2019.
© The Geological Society of America, 2019. CC-BY-NC.
The southern U.S. margin (Fig. 1) ranges from the stable Laurentia craton beneath Oklahoma to a stretched and thinned passive margin to oceanic lithosphere in the deep Gulf of Mexico, recording within it a geologic history that includes two complete Wilson cycles (Thomas, 2006). Due to its extensive hydrocarbon reserves, the southern U.S. has been the focus of intensive seismic exploration. However, until recently, studies of its deep structure trailed those of other U.S. continental margins. The result is that the tectonomagmatic evolution of the southern U.S. margin remains poorly understood. The primary contributing factors to this status quo are (1) the presence of a thick sediment cover that obscures crustal structure through most of the region, (2) the paucity of significant local seismicity, and, until recently, (3) sparse seismic instrumentation in the region. Earthscope’s USArray temporarily densified the set of broadband seismographs available for studies of the region’s lithosphere (http://www.usarray.org/researchers/obs/transportable). Approximately 435 stations occupied a total of 1830 locations in the continental U.S., for two years each, at a nominal spacing of 70 km. In USArray’s wake, there has been a surge in the number of continental-scale tomographic studies presenting snapshots of the compressional and shear wave velocities of the region’s crust and upper mantle. Although the volume of seismic data available for studies of the region has increased dramatically and sampling of the subsurface has improved as well, the presence of a thick layer of sediments and relatively low levels of seismicity (with the exception of Oklahoma) continue to challenge efforts to image the lithosphere.
The collection of models for the southern U.S. region represents the state-of-the-art of seismic tomography: a broad range of approaches, the inclusion of various types of data, and different choices of solution schemes. These seismic velocity models can be used to study the mineralogical, compositional, and thermal state of the current crust and upper mantle, and thereby provide critical constraints on geodynamic models, as well as serving as a foundation to launch further investigations. They also showcase the various techniques and innovations of seismic tomography. But, first, robust tectonic features must be identified. Well-constrained features should appear consistently across models. Differences between models could be due to (1) types of data incorporated, such as body wave arrival times, surface wave dispersion, receiver functions, or combinations of two or more data types; (2) measurement techniques employed; (3) the theoretical basis of the forward calculation, such as ray theory versus finite difference versus finite frequency; (4) the initial model and parameterization used; (5) regularization choices (“damping” and “smoothing” schemes and parameter values); and (6) inversion methods, such as gradient-based local minimization versus global optimization techniques.
The purpose of this study is to provide a systematic analysis of similarities and differences between recent shear wave tomographic models with respect to the lithospheric structure of the southern U.S. continental margin. Similar comparisons have been conducted for the western U.S. by Becker (2012) and Pavlis et al. (2012).
The region that now comprises the southern U.S. has witnessed two complete Wilson cycles of orogeny and rifting (Fig. 1). These cycles can be chronologically split into four major tectonic events, beginning with the closing of an ocean and assembly of the Rodinia supercontinent.
- The Mesoproterozoic Grenville orogeny along the southern margin of Laurentia is a result of continent-continent and continent-arc-continent collision, a result of which is the ca. 1.2 Ga granitic core of the Llano uplift (Fig. 1) (e.g., Culotta et al., 1992; Mosher et al., 2008).
- The Grenville orogeny was followed by a Neoproterozoic rifting episode that resulted in the breakup of Rodinia and the subsequent opening of the Iapetus Ocean. The resulting passive margin, outlining the southeastern continental margin of Laurentia, is widely thought to be composed of a series of rift and transform segments (Hatcher et al., 1989; Thomas, 1991, 2011).
- The closing of the Iapetus Ocean and the consequent assembly of the Pangaea supercontinent during the late Paleozoic included a collision between Laurentia, Gondwana, and enclosed island arcs, which resulted in the Ouachita orogeny. Contemporaneous terranes that participated in the Ouachita orogeny, the composition and origin of which are widely debated, are associated with basement highs, such as the Sabine and Monroe uplifts in Louisiana and the Wiggins Arch in Alabama and Mississippi.
- Finally, Late Triassic rifting episodes, during the breakup of Pangaea, led to the opening of the Atlantic Ocean and the Gulf of Mexico (Dickinson, 2009; Thomas, 2011; Huerta and Harry, 2012). Continental extension, followed by mid-Jurassic seafloor spreading, produced the current configuration of an arcuate wedge of oceanic crust beneath the deep Gulf of Mexico surrounded by transitional continental crust of variable width (Christeson et al., 2014).
Although there is general agreement on the sequence of tectonic events that formed the present-day crustal structure of the southern U.S. continental margin, several details of its evolutionary history remain unresolved. These include, but are not limited to, the geometry of the Precambrian rift-transform margin, depth and spatial extent of the allochthonous terranes that participated in the Ouachita orogeny, and variations in continental stretching and magmatic activity across the northern Gulf of Mexico during Mesozoic rifting.
The following eight models are compared in this study: DNA13 (Porritt et al., 2014); PLH15 (Porter et al., 2016); PM15 (Pollitz and Mooney, 2016); SR16 (Shen and Ritzwoller, 2016); SLK15 (Schmandt et al., 2015); NA07 (Bedle and van der Lee, 2009); SL14 (Schmandt and Lin, 2014); and YFCR14 (Yuan et al., 2014). Our comparison is limited to shear wave velocity (Vs) models because only a few P-wave velocity models span the southern U.S. Table 1 presents pertinent details about models that include the crust: SR16, DNA13, PLH15, PM15, and SLK15. The latter four models were generated via an iterative, linearized inversion algorithm. The global optimization technique used to generate SR16 makes it unique within our set of models, allowing for more formal estimates of uncertainties. An extended review of data types and methods used to generate all eight models examined in this study is presented in the GSA Data Repository1; for further details readers are referred to the original publications.
1GSA Data Repository item 2019099, an extended review of data types and methods used to generate all eight models examined in this study, is available online at www.geosociety.org/datarepository/2019.
Crustal Thickness Models
Crustal thickness varies substantially across passive margins, including the northern Gulf of Mexico, and hence serves as an important parameter in reconstructing the tectonic evolution and pre-rift geometry of such regions (Reston and Morgan, 2004; Huismans and Beaumont, 2011; Sutra and Manatschal, 2012). To evaluate crustal thickness variations across the study region, four models were considered: SLK15, SR16, PnUS2016, and LITHO1.0. PnUS2016 (Buehler and Shearer, 2017) uses Pn arrivals, which are P waves that refract just below the Moho and are the first arrivals at regional distances, to constrain crustal thickness. PnUS2016 utilizes the seismic velocities from SR16 to map crustal thickness. LITHO1.0 (Pasyanos et al., 2014) is constructed by perturbing an initial model parameterized vertically as a series of geophysically identified layers, that is, a combination of the CRUST1.0 model (Laske et al., 2012) and the LLNL-G3D model (Simmons et al., 2012), to fit surface wave dispersion maps over the 5–40 mHz frequency band. Additionally, Moho depths from velocity models based on results from the 2010 GUMBO experiments, consisting of four long-offset seismic refraction profiles in the northern Gulf of Mexico (Fig. 1) (Eddy, 2014; Christeson et al., 2014; Eddy et al., 2014; Van Avendonk et al., 2015), were digitized and added to the comparison. With 11–12-km station spacing and the incorporation of coincident seismic reflection data, crustal thickness estimates from the GUMBO lines have the highest resolution in this study.
Data and Methods
Model comparisons with original figures are complicated by the different choices authors make with respect to presentation: color scales and ranges, color palettes, and perturbations with respect to a model average or a global standard, etc. Here we plot all models on the same scale, in terms of perturbations with respect to the average of all models, using a consistent color scale. Most models used in this study were downloaded from the IRIS Earth Model Collaboration (http://ds.iris.edu/ds/products/emc); others were received via private correspondence. Using MATLAB, each velocity model was linearly interpolated onto a three-dimensional (3D) grid with 0.2° × 0.2° uniform lateral spacing and 0.5-km depth spacing. For the southern U.S., the domain of interest was bounded latitudinally between 26°–37° N and longitudinally between 78°–102° W. Using the interpolated shear velocity models, the lateral root-mean-square velocity, Vrms, for each model was calculated and compiled to create average one-dimensional (1D) velocity individual models, and an average 1D model for the study area (SUSavg; Fig. 2A). This procedure was repeated to create similar 1D shear velocity models for the western U.S. (WUS), 28°–50° N by 102°–130° W; the central U.S. (CUS), 36°–50° N by 90°–102° W; and the eastern U.S. (EUS), 36°–50° N by 70°–90° W (Fig. 2B). In addition to the original data set, models by Schmandt and Humphreys (2010), James et al. (2011), Obrebski et al. (2011), and Chai et al. (2015) were used for the western U.S.; results from Chen et al. (2016) were used for the central U.S.; and the model by Savage et al. (2016) was used for the eastern U.S. Figure 3 displays model perturbations with respect to SUSavg at four depths: 5 km and 15 km plus 5 km above and 5 km below the Moho, with cooler colors (blue and green) representing faster regions and warmer colors (red and yellow) representing slower regions. Authors of each model indicate their best guess of the Moho depth throughout their model, so the last two panels represent different absolute depths for each model. Perturbations of models at depths of 75 km, 150 km, 400 km, and 415 km are presented in the GSA Data Repository (see footnote 1).
Results and Interpretations
Velocity Models for the Southern U.S. and Comparison to Other Regions
Root-mean-square velocities (Vrms) as a function of depth for the 3D models introduced above are shown in Figure 2A. The models fall into two distinct groupings: (1) PLH15, SLK15, SR16, and SL14 and (2) PM15, NA07, YFCR14, and DNA13, with as much as ~0.75 km/s difference between the two groups at upper mantle depths. The average of all models is shown in red (SUSavg). Comparable regional averages for the eastern, western, and central U.S. are shown in Figure 2B, along with the SUSavg, and the 1D reference models, ak135, and PREM.
All four average regional models (Fig. 2B) show the Moho at deeper depths than the reference 1D models. CUS has the highest velocities in the crust and upper mantle, which is consistent with the fact that it largely represents the cratonic core of Laurentia. Due to the presence of thick sedimentary basins in the southern U.S., SUSavg has the lowest velocities in the uppermost crust. In the upper mantle, however, the western U.S. (WUS) is the slowest of all models. Despite the lack of recent tectonism in the southern U.S. margin, the SUSavg model is more similar to the WUS model for the tectonically active western U.S. than any of the other models. Interestingly, Gulf Coast Q0-values (Q at 1 Hz) are also considerably lower than mid-continent values but similar to those in the western U.S., with boundaries that correspond to the Oklahoma-Alabama Transform and Ouachita thrust (Cramer, 2017). The SUSavg model is also distinctly slower than the EUS model at all depths.
Geologic and Tectonic Patterns
In the upper crust, large-scale patterns of anomalies are consistent between all the models, matching the geometry of major features in the region; i.e., the Ouachita orogenic front and the Precambrian margin (Fig. 3 with locations in Fig. 1). Areas to the north of the Precambrian margin, which comprise cratonic continental crust, are faster at shallower depths than in the region enclosed between the Alabama-Oklahoma transform and Texas Rift segments, which is covered by thick sediments. This latter region displays a reversal in anomalies in three of the five models (PLH15, SR16, and SLK15) at depths around the Moho. This fast velocity zone could correspond to the base of the Sabine block, as proposed by Clift et al. (2018). The Southern Oklahoma Aulacogen is consistently represented in the models by a slow anomaly, although with varying size, geometry, and location. Conversely, the Llano Uplift is represented by a fast anomaly that is especially prominent at shallow depths. A large proportion of the seismic data used to generate the models derives from the USArray’s Transportable Array (TA), which has a nominal station spacing of ~70 km. The resulting relatively low horizontal resolution of these models makes it difficult to constrain effectively the exact geometry of small-scale geologic features in this region.
Amplitudes of anomalies vary significantly between models (note the different ranges in the color bars). There are at least two reasons to expect such variations. First, constraints imposed by data on model parameters usually range from overdetermined to underdetermined in tomography, so additional regularization is needed to stabilize the inversion numerically. Choices of values for regularization parameters are largely subjective and will therefore differ between authors. Second, only a portion of the travel time variance is explained by the 3D structure to be resolved. Other components of the variance include random and systematic errors in the data, inaccuracies in the model parameterization’s representation of Earth, and oversimplifications in the physical theory that relates Earth’s structure to travel time observations. Again, differences between individual choices will map into differences in anomaly amplitudes, although patterns should be robust between techniques and parameterizations.
PM15 shows the least change in anomaly pattern from 5 km depth to 5 km below the Moho, reflecting its decreasing resolution with depth. DNA13 has a relatively small range of anomaly amplitudes at depths of 5 km and 15 km, with less consistent anomaly patterns compared to the other models (Fig. 2). This difference, with respect to other models, is likely due to the lack of surface wave data in DNA13.
Crustal Thickness Variations
Figure 4 shows the Bouguer gravity anomaly, topography, and crustal thickness along the L1-GUMBO1 and L3-GUMBO3 profiles (onshore extensions of GUMBO1 and GUMBO3; see locations in Fig. 1) based on the models discussed above. Similar profiles for L2, GUMBO2, and GUMBO4 are shown in GSA Data Repository Figure DR2 (see footnote 1). A general trend exhibiting crustal thinning toward the Gulf of Mexico basin, corresponding to a steady increase in Bouguer gravity anomalies, is consistent among the models. SLK15 and SR16 are consistent along the L1-GUMBO1 profile, while a crossover with PnUS2016 is observed around the 400-km profile distance, in the vicinity of the San Marcos Arch. The LITHO1.0 model has the largest deviations from the other models; due to its sparse parameterization, LITHO1.0 is not a reliable benchmark in regional studies.
There is a lack of general agreement between models concerning the landward limit of oceanic crust in the Gulf of Mexico (arrows in Fig. 4). Along GUMBO3, the majority of the proposed locations are coincident with a sharp increase in Bouguer gravity, which is not the case along GUMBO1 in the western Gulf of Mexico, where the large Louann salt province complicates geophysical interpretation.
Discussion and Conclusions
The general consistency of large-scale anomaly patterns between models within the upper crust suggests that data selection is more important than model parameterization, forward modeling and inversion methods, and other methodological differences. However, disparities in anomaly amplitudes and the fact that the velocity models fall into two distinct groups (which is discussed in the GSA Data Repository [see footnote 1]) present a challenge for geodynamic interpretations of the margin, for understanding the margin’s magmatic evolution, and for reconstructions of its pre-rift crustal and lithospheric thickness. It is clear that current 3D velocity models are unable to resolve the geometry of some smaller-scale tectonic features of this region, such as the spatial extent of the Wiggins block, or smaller arches (e.g., Luling, Waco, San Marcos), and to test the extent to which magmatic input facilitated rifting in the northern Gulf of Mexico.
This comparison serves as a baseline for future geological and geophysical investigations in the southern U.S. by providing a comprehensive assessment of currently available S-wave tomographic models. There is a need for additional P-wave tomography studies within this region which, given the lack of regional seismicity, is difficult but essential.
We thank two anonymous reviewers and editor Mihai Ducea for their thoughtful and constructive comments that helped improve the manuscript. Thanks to Fred Pollitz and Cemal Biryol for providing their velocity models to us directly and to the authors of other models for contributing them to the IRIS Earth Model Collaboration (http://ds.iris.edu/ds/products/emc).
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