Accretionary unit formats in subduction complexes: examples from the Franciscan and Miura-Boso complexes

ABSTRACT

Subduction accretionary complexes are composed of major and minor structural, stratigraphic, and tectonostratigraphic units. The major architectural units, Accretionary Units (AUs), are bounded by major faults, but differ from terranes in that they may contain units stratigraphically correlative with units in other AUs and they are smaller than the largest terranes. AUs occur in three basic formats – (1) singular sheets or blocks of stratigraphic layers, dismembered formation, or mélange, (2) folded units composed of one or more stratigraphic or block-in-matrix units, and (3) faulted stratigraphic masses (i.e., broken formations). Composite AUs with multiple units and multiple attributes are common. Multiple suites of structures may arise in AUs from progressive early deformation or later superimposed deformational events. AUs may be subdivided through detailed mapping into sub-units such as fault blocks, mélanges, dismembered formations, broken formations, intact formations, and members. Each AU should be defined on the basis of unique characters that derive from a thorough description of the AU, including its distinct rock types and character; and where possible, lithofacies, metamorphic facies, structures, and unit history. Descriptions of partially described AUs from the Franciscan Complex of California and the Miura-Boso area of Japan provide examples of the character of AUs. Ideally, the architecture and history of a subduction complex can be reconstructed by assembling detailed map and text descriptions of the constituent AUs, their 3D-positions, their relationships, and their histories.

KEYWORDS:

• Accretionary complex
• subduction
• Franciscan
• Miura-Boso

Acknowledgments

Support for this project was provided by the Geology departments at University of California, Davis; Appalachian State University (ASU); Southern Oregon State College; and Sonoma State University (SSU). This research was supported, in part, by the National Science Foundation [grant EAR 76-06062]. David Bero (Sonoma State University) and John Wakabayashi (California State University at Fresno) had ongoing discussions on various aspects of the ideas presented herein with the first author, periodically, over the past three years. Nick Hayman (University of Texas, Austin), Gary Ernst (Stanford University), and two anonymous reviewers reviewed earlier versions of the manuscript and provided constructive criticisms. Anthony Love (ASU) provided technical assistance. We thank all of the above for their support or assistance.

Disclosure statement

No potential conflict of interest was reported by the authors.

Correction Statement

This article has been republished with minor changes. These changes do not impact the academic content of the article.

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Funding

This work was supported by the National Science Foundation, Division of Earth Sciences [EAR 76-06062].