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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,

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.

4 GSA Today | November 2019

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