Synkinematic Intrusion of the 1.4‐Ga Boriana Canyon Pluton, Northwestern Arizona: Implications for Ca. 1.4‐Ga Regional Strain in the Western United States

The transcontinental belt of ca. 1.4‐Ga granite plutons that extends from southern California to Labrador has conventionally been interpreted as either “anorogenic” or emplaced in an extensional tectonic setting. Recent studies in the western United States, however, demonstrate that several ca. 1.4‐Ga plutons were emplaced in a broadly northwest‐southeast shortening strain field. Our study of the \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $1404+1.5$ \end{document} ‐Ma Boriana Canyon pluton, northwestern Arizona, establishes the emplacement age by U‐Pb zircon analysis and reveals synintrusive deformation consistent with northwest‐southeast shortening. The Boriana Canyon pluton contains northeast‐striking mylonite zones that record reverse‐sense, northwest‐side‐up motion. Undeformed late dikes crosscut solid‐state deformation zones in the pluton, indicating synintrusive deformation. The mylonites also contain a continuum of microstructures that record high‐temperature plastic deformation through low‐temperature brittle deformation. This study adds to the growing catalog of ca. 1.4‐Ga plutons deformed by synintrusive, broadly northwest‐southeast shortening strains. Several studies, however, document shear zones with orientations that may not be strictly compatible with northwest‐southeast shortening. Our results are consistent with models invoking a convergent or transpressive tectonic margin at the southern margin of Laurentia at ca. 1.4 Ga. Alternatively, however, we propose that the 1.4‐Ga granites may have been emplaced into an ambient, generally compressional, intracontinental stress field in the absence of an active plate margin at the southern margin of Laurentia. An intracontinental stress field coupled with a large‐scale thermal anomaly would allow local deviations from overall northwest‐southeast shortening while still explaining why the majority of studied 1.4‐Ga plutons record shortening strains.