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

Abstract

Recent syntheses of Cordillera tectonics contain contradictory views of subduction polarity in the late Mesozoic, and this contradiction has implications for whole-earth processes. The long-held view of eastdipping subduction throughout the Late Jurassic–Early Cretaceous Cordillera is challenged by tectonic models calling on a west-dipping subduction system that led to the collision of oceanic arcs, ribboncontinents, or both, with North America. Evidence in support of these models are seismic anomalies in the deep mantle inferred to represent subducted lithosphere from a west-dipping slab. We argue that this “bottom-up” approach to tectonic synthesis carries assumptions that are as great as or greater than ambiguities from the “topdown” approach of surface geology. Geologic evidence from the northern Cordillera is inconsistent with west-dipping subduction in Jura-Cretaceous time and requires long-lived east-dipping subduction along much of the Cordilleran margin. West-dipping subduction in Triassic–Early Jurassic time has been documented and may be the source of the seismic anomalies. We encourage the broader community to come to consensus on integration of these deep images with surface geology. INTRODUCTION Regional syntheses of Cordilleran tectonics were central to the plate tectonic revolution with a series of papers that placed the geology of the continental United States into the new paradigm (e.g., Burchfiel and Davis, 1972) in which the Sierra Nevada, Great Valley, and Franciscan triad formed above a late Mesozoic, east-dipping subduction zone. Similar relations have since been used to reconstruct arc polarity in many other orogens. Cordilleran tectonics saw a paradigm shift in the late 1970s when paleomagnetic data (e.g., Hillhouse, 1977) together with geologic syntheses led to the terrane concept (e.g., Coney et al., 1980). These insights led to the recognition that both collision and strike-slip juxtaposition must have occurred along the Cordillera margin, and multiple terranes comprising different arc elements were scrambled to make the terrane collage. There has been a recent resurgence in Cordilleran-wide syntheses based in large part on three new data sources: (1) developments in geochronology; (2) Earthscope geophysical data; and (3) geodetic data that reveal active deformation in the Cordillera. The integration of these data provides new opportunities for understanding the longterm evolution of the Cordillera. Challenges arise from a disconnect between two approaches: (1) geological studies that use a top-down approach, in which surface geology is projected to infer relations at depth and back in time; and (2) geophysical studies that use a bottom-up approach that pro‐ jects features imaged in the lower crust and mantle to the surface and back in time. Although these approaches should converge on a similar solution, they are often diametrically opposed because of different underlying assumptions. We here consider an example of where geologic and geophysical interpretations lead to fundamentally different conclusions regarding the polarity of subduction along the Cordilleran margin during late Mesozoic time. We argue from a northern Cordilleran perspective that some recent syntheses (e.g., Johnston, 2008; Hildebrand, 2009; Sigloch and Mihalynuk, 2017) ignored or dismissed a fundamental observation; namely, that there is compelling geologic evidence that subduction along the northern Cordilleran margin has been east-dipping for at least the last ~125 m.y., and likely can be traced ~75 m.y. further back into the Late Triassic. The objective of this article is to compare these approaches for evaluating subduction polarity in ancient margins. Successful integration of the two approaches will be required to fully understand the configuration of ancient subduction zones. FUNDAMENTAL CONTROVERSY OF SUBDUCTION POLARITY Uncertainties regarding the late Mesozoic evolution of the Cordilleran margin focus primarily on (1) the size of the ocean basin separating the Wrangellia composite terrane (WCT) or Insular superterrane from the continental margin; and (2) the location, polarity, and age of subduction zones that closed this basin (Fig. 1). One set of models, mainly based on geologic observations, shares an interpretation that this basin closed during Jura-Cretaceous time along an east-dipping1 subduction zone built along the continental margin, 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 GSA Today, v. 29, https://doi.org/10.1130/GSATG402A.1. Copyright 2019, The Geological Society of America. CC-BY-NC. 1Note 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. and that a second east-dipping subduction zone existed along the outboard margin of the WCT (Figs. 1A and 1B [see footnote 1]). A second group of models emphasizes collision along a west-dipping subduction zone on the inboard margin of the WCT (Sigloch and Mihalynuk, 2017; Fig. 1C) or between the entire terrane collage and North America (Johnston, 2008; Hildebrand, 2009). The top-down interpretation of subduction polarity is based on (1) structural vergence in accretionary prisms; (2) the presence and position of high-P/T mineral assemblages; (3) the location of forearc versus backarc strata; and (4) age and geochemical patterns within the magmatic arc. These features have been used to infer subduction polarity since the advent of plate tectonics (e.g., Miyashiro, 1972; Ernst, 1973; Dickinson, 1974). These interpretations are complicated by the potential for large-scale displacement along strike-slip faults within and between the various convergent margin assemblages, and by removal of elements by subduction erosion or exhumation during collision. These complications are the reasons for discrepancies among existing models based on geology (Fig. 1). For example, a minimum of 700–1500 km of post-latest Cretaceous dextral strike-slip is known from geologic relationships alone in the northern Cordillera (Stamatakos et al., 2001), and the total dextral slip could be far larger (e.g., Garver and Davidson, 2015). Similarly, the boundary between the WCT and the continent records closure of an ocean basin, a relationship first established by Richter and Jones (1973), but along much of the suture zone2, exhumation along this contact reaches lower-crustal depths in the hanging-wall (e.g., Hollister, 1982), confounding any attempts to reconstruct the eroded material. The bottom-up interpretation of polarity is based on tomographic images of large, near-vertical features in the mantle interpreted as subducted slabs (Sigloch and Mihalynuk, 2017). These slabs are now in the mantle more than 3000 km from their presumed paleotrench. To restore the pathway over this distance requires multiple assumptions, including the nature of the mantle anomaly, uncertainties in slab sinking rates, and models of absolute plate motion. Problems with absolute plate motion models based on hot spots have been known since the first plate reconstructions that used them (Engebretson A Large ocean basin: Two parallel subduction zones B Back arc basin +/oblique opening and closure C West-dipping subduction