The Early Cretaceous tectonic setting of southeastern Arizona was one of northeast-southwest extensiomal deformation characterized by differential vertical movements along northwest-trending normal faults. Local fault-block uplifts created a basin-and-range–type paleogeography that included local clastic-filled basins and mountain ranges rimmed by alluvial fans. The Glance Conglomerate, the basal formation of the Lower Cretaceous Bisbee Group, represents the proximal parts of these syntectonic alluvial fans. This extensional back-arc tectonic setting is strikingly different from the compressional setting found behind the continental margin magmatic arc farther to the north (Sevier orogeny). Possible explanations for this profound difference in back-arc tectonic style include propagation of an “aulacogen” from the Gulf of Mexico to southeastern Arizona, variable-dip subduction along the continental-margin magmatic arc, and an apparent anticlockwise rotation of the North American plate relative to the continental margin arc.

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