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Synoptic View of Lithospheric S-Wave Velocity

Structure in the Southern United States:

A Comparison of 3D Seismic Tomographic Models

Alden Netto, Jay Pulliam, Dept. of Geosciences, Baylor University, Waco, Texas 76706, USA; and Patricia Persaud,
Dept. of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803, USA

ABSTRACT evolution of the southern U.S. margin tomography. But, first, robust tectonic fea-
The southern U.S. continental margin remains poorly understood. The primary tures must be identified. Well-constrained
records a history spanning ca. 1.2 Ga, contributing factors to this status quo are features should appear consistently across
including two Wilson cycles. However, (1) the presence of a thick sediment cover models. Differences between models could
due to a thick sediment cover, the paucity that obscures crustal structure through be due to (1) types of data incorporated,
of significant local seismicity, and, until most of the region, (2) the paucity of sig- such as body wave arrival times, surface
recently, sparse instrumentation, details nificant local seismicity, and, until recently, wave dispersion, receiver functions, or
of this passive margin’s tectonomagmatic (3) sparse seismic instrumentation in the combinations of two or more data types;
evolution remain disputed. This paper region. Earthscope’s USArray temporarily (2) measurement techniques employed;
compares recent S-wave tomography and densified the set of broadband seismo- (3) the theoretical basis of the forward
crustal thickness models based on USArray graphs available for studies of the region’s calculation, such as ray theory versus
data to help establish a framework for lithosphere (http://www.usarray.org/ finite difference versus finite frequency;
geodynamic interpretation. Large-scale researchers/obs/transportable). Approx- (4) the initial model and parameterization
patterns of crustal velocity anomalies, imately 435 stations occupied a total of used; (5) regularization choices (“damp-
corresponding to major regional features 1830 locations in the continental U.S., for ing” and “smoothing” schemes and param-
such as the Ouachita orogenic front and the two years each, at a nominal spacing of eter values); and (6) inversion methods,
Precambrian margin, are generally consis- 70 km. In USArray’s wake, there has been such as gradient-based local minimization
tent between the models. The spatial extent a surge in the number of continental-scale versus global optimization techniques.
of smaller-scale tectonic features, such as tomographic studies presenting snapshots The purpose of this study is to provide a
the Sabine Uplift and Wiggins block, of the compressional and shear wave veloc- systematic analysis of similarities and dif-
remains poorly resolved. An inverse rela- ities of the region’s crust and upper mantle. ferences between recent shear wave tomo-
tionship between crustal thickness and Although the volume of seismic data avail- graphic models with respect to the litho-
Bouguer gravity across the continental able for studies of the region has increased
margin is observed. This model compari- dramatically and sampling of the sub- spheric structure of the southern U.S.
son highlights the need for additional surface has improved as well, the presence continental margin. Similar comparisons
P-wave tomography studies and targeted, of a thick layer of sediments and relatively have been conducted for the western U.S.
higher density station deployments to low levels of seismicity (with the exception by Becker (2012) and Pavlis et al. (2012).
better constrain tectonic features. of Oklahoma) continue to challenge efforts
to image the lithosphere. TECTONIC SETTING
INTRODUCTION The collection of models for the south- The region that now comprises the
The southern U.S. margin (Fig. 1) ranges ern U.S. region represents the state-of-the- southern U.S. has witnessed two complete
from the stable Laurentia craton beneath art of seismic tomography: a broad range Wilson cycles of orogeny and rifting
Oklahoma to a stretched and thinned pas- of approaches, the inclusion of various (Fig. 1). These cycles can be chronologi-
sive margin to oceanic lithosphere in the types of data, and different choices of solu- cally split into four major tectonic events,
deep Gulf of Mexico, recording within it a tion schemes. These seismic velocity mod- beginning with the closing of an ocean and
geologic history that includes two complete els can be used to study the mineralogical, assembly of the Rodinia supercontinent.
Wilson cycles (Thomas, 2006). Due to its compositional, and thermal state of the 1. The Mesoproterozoic Grenville orogeny
extensive hydrocarbon reserves, the south- current crust and upper mantle, and along the southern margin of Laurentia
ern U.S. has been the focus of intensive thereby provide critical constraints on is a result of continent-continent and
seismic exploration. However, until geodynamic models, as well as serving as continent-arc-continent collision, a result
recently, studies of its deep structure trailed a foundation to launch further investiga- of which is the ca. 1.2 Ga granitic core of
those of other U.S. continental margins. tions. They also showcase the various the Llano uplift (Fig. 1) (e.g., Culotta et
The result is that the tectonomagmatic techniques and innovations of seismic al., 1992; Mosher et al., 2008).

4 GSA Today | July 2019

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