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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).



          GSA Today, v. 29, https://doi.org/10.1130/GSATG387A.1. Copyright 2019, The Geological Society of America. CC-BY-NC.

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