Collaborative Research: Mesoproterozoic Tectonics of Inboard Southwestern Laurentia: Insight Into Assembly and Configuration of Rodinia From Study of Sedimentary Successions

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This project is investigating Mesoproterozoic (1300-1000 Ma) sedimentary rocks in the western U.S. to test aspects of the controversial Rodinia hypotheses. The hypothesis being tested is that the inboard record of sedimentation and tectonics in southwestern Laurentia, preserved in well-exposed sedimentary successions, can be used for understanding stresses at the surrounding plate margins, nature of drainage patterns and basins in southwestern Laurentia, and the age of possible outboard sources of detritus. The research involves structural, petrologic, and geochronologic study aimed at correlation of, and characterizing provenance for, several key areas and units. These areas represent preserved remnants of once more extensive sedimentary successions; they are well exposed, but widely separated, and correlations have been hampered by a lack of reliable geochronology. The study concentrates on the Apache Group (Arizona), Unkar Group (Grand Canyon), Crystal Spring Formation (Death Valley), and Lanora Formation (Texas). The researchers are reconstructing the regional intracratonic basinal and tectonic patterns and searching for a "fingerprint" of possible outboard sources of detritus. The geochronological studies involve: dating ash beds using precise U-Pb geochronology, U-Pb dating of detrital zircons, new in situ U-Pb microprobe dating of monazite grains, and Ar-Ar laser dating of mica grains. Sedimentary-tectonic studies are evaluating which faults were active during deposition by looking for facies changes across faults and relationships to syntectonic magmatism. Petrologic studies are emphasizing provenance of siliciclastic rocks using standard petrologic techniques. Detailed regional tests involving geochronology linked to sedimentary tectonics are providing better understanding of the tectonic setting of western Laurentia and hence the most likely adjacent outboard continental masses that collided with Laurentia from 1.3-1.0 Ga.