RUI: Paleomagnetic, Geochemical and Age Discrimination of ~18-19 Ma Tuffs of the Colorado River Extensional Corridor

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Regionally extensive ash flow tuffs represent important, approximately isochronous units for correlation within complexly deformed regions. This is particularly true within the highly extended ranges of the Colorado River extensional corridor of the Basin and Range Province of the western U.S., where the pre-Tertiary sedimentary section was erosionally removed prior to Miocene and younger extension. Ash flow tuffs, such as the widely distributed 18.5 million year old Peach Springs Tuff, often represent the only correlatable units between the largely volcanic sections of isolated mountain ranges. In addition to their value to regional correlation and as time horizons within extensional growth fault basins, the paleomagnetic signatures of ash flow units are also highly useful markers for assessing structural rotations due to extensional events.Recent research on 18-19 million year old ash flows within the Colorado River extensional corridor suggests that (1) the Peach Springs Tuff, widely used for correlation, may be more stratigraphically complex and have erupted over a significantly longer period than previously realized, (2) the paleomagnetic signature of the Peach Springs Tuff can deviate substantially from the widely cited Peach Springs Tuff reference direction, possibly due to protracted eruption during rapid secular variation related to a magnetic excursion, and (3) other tuffs occupy the same stratigraphic position as the Peach Springs Tuff and may be regionally important, including the previously known Cook Canyon and Rock Creek rhyodacite tuffs, a potentially new tuff discovered in the western Cerbat Mountains, and possibly others. Paleomagnetic, geochemical and geochronologic techniques are being used to study the 18-19 million year old tuffs of the Colorado River extensional corridor. Research goals are directed at the following questions: (1) Did the Peach Springs Tuff eruptions indeed occur over a much longer time interval than previously thought? (2) How many different and distinct tuffs of approximately 18-19 Ma age exist in the Colorado River extensional corridor and how do we discriminate them? (3) Which of the 18-19 Ma tuffs of the Colorado River extensional corridor, if any, have a paleomagnetic direction suitable for use as a structural marker? (4) Where are the source areas for these various tuffs? These studies will allow better resolution of stratigraphic correlation and timing within extensional growth fault basins, provide a better basis for assessing structural rotations during extension and for testing models of extensional deformation and, potentially highlight relationships between major caldera eruptive centers and regional extensional structures.