Abstract

A seismic reflection profile crossing the Jemez lineament in north-central New Mexico images oppositely dipping zones of reflections that converge in the deep crust. We interpret these data as a Paleoproterozoic bivergent orogen, centered on the Jemez lineament, that formed during original Proterozoic crustal assembly by collision of Mazatzal island arcs with Yavapai proto–North American continent at ca. 1.68–1.65 Ga. The two major sets of reflections within the Yavapai-Mazatzal transition boundary dip at 15°–20°, and we interpret them as part of a south-dipping thrust system and as a north-dipping crustal-scale duplex that formed synchronously with the thrust system. The upper crust shows structures recording a succession of tectonic and magmatic events from the Paleoproterozoic to the Holocene. Notable among these structures is a system of nappes that formed during development of the bivergent orogen. Elements of the nappe system are exposed in Rocky Mountain uplifts and have been dated as having formed at 1.68 Ga, at depths of 10 km and at temperatures of >500 °C. We also see continuous bright reflections in the upper part of the middle crust that we associate with basaltic sills that postdate accretion. The data show that the Yavapai-Mazatzal suture is low angle (∼20°), an observation that explains why the boundary between the provinces has previously been so hard to define in the surface geology. The Jemez lineament overlies the root of this bivergent orogen that we also suggest is a Paleoproterozoic zone of weakness that has subsequently acted as a conduit for magmas and a locus of tectonism.

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