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Subduction Polarity in Ancient Arcs: A Call to Integrate

                         Geology and Geophysics to Decipher the Mesozoic

                Tectonic History of the Northern Cordillera of North America






          Terry L. Pavlis, Dept. of Geological Sciences, University of Texas, El Paso, Texas 79968, USA; Jeffrey M. Amato, Dept. of Geological
          Sciences, New Mexico State University, Las Cruces, New Mexico 88003, USA; Jeffrey M. Trop, Dept. of Geology and Environmental
          Geosciences, Bucknell University, Lewisburg, Pennsylvania 17837, USA; Kenneth D. Ridgway, Dept. of Earth, Atmospheric and Planetary
          Sciences, Purdue University, West Lafayette, Indiana 47907, USA; Sarah M. Roeske, Earth and Planetary Sciences Dept., University of
          California, Davis, California 95616, USA; and George E. Gehrels, Dept. of Geosciences, University of Arizona, Tucson, Arizona 85721, USA


          ABSTRACT                           Davis, 1972) in which the Sierra Nevada,   We here consider an example of where
            Recent syntheses of Cordillera tectonics   Great Valley, and Franciscan triad formed   geologic and geophysical interpretations
          contain contradictory views of subduction   above a late Mesozoic, east-dipping subduc-  lead to fundamentally different conclu-
          polarity in the late Mesozoic, and this con-  tion zone. Similar relations have since been   sions regarding the polarity of subduction
          tradiction has implications for whole-earth   used to reconstruct arc polarity in many   along the Cordilleran margin during late
          processes. The long-held view of east-   other orogens.               Mesozoic time. We argue from a northern
          dipping subduction throughout the Late   Cordilleran tectonics saw a paradigm   Cordilleran perspective that some recent
          Jurassic–Early Cretaceous Cordillera is   shift in the late 1970s when paleomag-  syntheses (e.g., Johnston, 2008; Hilde-
          challenged by tectonic models calling on    netic data (e.g., Hillhouse, 1977) together   brand, 2009; Sigloch and Mihalynuk,
          a west-dipping subduction system that led   with geologic syntheses led to the terrane   2017) ignored or dismissed a fundamental
          to the collision of oceanic arcs, ribbon-  concept (e.g., Coney et al., 1980). These   observation; namely, that there is compel-
          continents, or both, with North America.   insights led to the recognition that both   ling geologic evidence that subduction
          Evidence in support of these models are   collision and strike-slip juxtaposition   along the northern Cordilleran margin
          seismic anomalies in the deep mantle   must have occurred along the Cordillera   has been east-dipping for at least the last
          inferred to represent subducted lithosphere   margin, and multiple terranes comprising   ~125 m.y., and likely can be traced ~75 m.y.
          from a west-dipping slab. We argue that this   different arc elements were scrambled to   further back into the Late Triassic. The
          “bottom-up” approach to tectonic synthesis   make the terrane collage.  objective of this article is to compare these
          carries assumptions that are as great as or   There has been a recent resurgence in   approaches for evaluating subduction
          greater than ambiguities from the “top-  Cordilleran-wide syntheses based in large   polarity in ancient margins. Successful
          down” approach of surface geology.   part on three new data sources: (1) develop-  integration of the two approaches will be
          Geologic evidence from the northern   ments in geochronology; (2) Earthscope   required to fully understand the configura-
          Cordillera is inconsistent with west-dipping   geophysical data; and (3) geodetic data that   tion of ancient subduction zones.
          subduction in Jura-Cretaceous time and   reveal active deformation in the Cordillera.
          requires long-lived east-dipping subduction   The integration of these data provides new   FUNDAMENTAL CONTROVERSY
          along much of the Cordilleran margin.   opportunities for understanding the long-  OF SUBDUCTION POLARITY
          West-dipping subduction in Triassic–Early   term evolution of the Cordillera. Challenges   Uncertainties regarding the late Mes-
          Jurassic time has been documented and   arise from a disconnect between two   ozoic evolution of the Cordilleran margin
          may be the source of the seismic anomalies.   approaches: (1) geological studies that use a   focus primarily on (1) the size of the ocean
          We encourage the broader community to   top-down approach, in which surface geol-  basin separating the Wrangellia composite
          come to consensus on integration of these   ogy is projected to infer relations at depth   terrane (WCT) or Insular superterrane
          deep images with surface geology.  and back in time; and (2) geophysical stud-  from the continental margin; and (2) the
                                             ies that use a bottom-up approach that pro‐  location, polarity, and age of subduction
          INTRODUCTION                       jects features imaged in the lower crust and   zones that closed this basin (Fig. 1). One
            Regional syntheses of Cordilleran tecton-  mantle to the surface and back in time.   set of models, mainly based on geologic
          ics were central to the plate tectonic revolu-  Although these approaches should converge   observations, shares an interpretation that
          tion with a series of papers that placed the   on a similar solution, they are often dia-  this basin closed during Jura-Cretaceous
          geology of the continental United States   metrically opposed because of different   time along an east-dipping  subduction
                                                                                                    1
          into the new paradigm (e.g., Burchfiel and   underlying assumptions.  zone built along the continental margin,

          GSA Today, v. 29, https://doi.org/10.1130/GSATG402A.1. Copyright 2019, The Geological Society of America. CC-BY-NC.

          1 Note that because the Alaskan margin is curved this terminology can be confusing. We generally use east dipping or west dipping to reflect the pre-oroclinal
          geography, but also use north or south dipping when discussing modern geometries.

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