REFINEMENT OF TECTONOSTRATIGRAPHY OF THE NORTHEAST BIG MARIA MOUNTAINS, RIVERSIDE COUNTY, CALIFORNIA

The Big Maria Mountains, in the eastern Mojave Desert, north of Blythe, California, contain the Paleozoic stratigraphy of the Grand Canyon, but it has been overturned and, in some places, attenuated to a tenth of its original thickness. Several structural geology studies over the past 30 years have identified multiple late Jurassic to early Cretaceous deformation events. Attempts have been made to relate the geology of the range to Cordilleran Mesozoic tectonics, but they are beset by age uncertainty of three related metamorphic units: an augen gneiss, a quartz-epidote schist, and a fine-grained quartzofeldspathic gneiss. They are either Proterozoic or Mesozoic. Our investigation focused on the northeast part of the range, where all three units are concentrated. A geologic map based on field data and satellite imagery was created with GIS software. Samples were analyzed using X-ray fluorescence and optical petrography. LA-ICP-MS U/Pb zircon geochronology was applied to the metamorphic units. Mapping reveals a detachment fault and confirms a nappe in the field area. Although previous research indicated a single overturned syncline, it has become evident that there are at least two. Preliminary radiometric dating has provided indefinite results, but the units could still be differentiated. Zircons from the augen gneiss produced Mesoproterozoic ages between 1.6 and 1.0 Ga. The quartz-epidote schist and fine-grained quartzofeldspathic gneiss provided Mesozoic ages between 260 and 110 Ma. The Mesozoic units appear to be genetically similar and differ only in their fabric; the fine-grained unit being associated with a detachment-related shear zone in the northeast of the range. Petrographic analysis reveals that the Mesozoic rocks contain biotite, chlorite, and epidote; whereas the Proterozoic rocks contain white mica, and have little biotite, chlorite, or epidote. CIPW norms indicate that the Proterozoic rocks have 25 to 30 % quartz and Mesozoic rocks have lower proportions. Similar schists and gneisses are found throughout the Big Maria Mountains. It will now be possible to correlate the age of these rocks geochemically and petrographically, providing an opportunity for more complete geologic interpretation of the range and incorporation of its tectonic history into the mosaic of Mesozoic Cordilleran tectonics.