TECTONIC EVOLUTION OF THE CORDILLERAN OROGENY IN THE CENTRAL SIERRA NEVADA: INTEGRATING DETRITAL ZIRCON DATING WITH MAPPING, STRUCTURAL, AND PLUTONIC STUDIES

In order to better understand the regional tectonics of the Cordilleran orogeny in the Sierra Nevada and the construction of the Mesozoic Sierran arc, we have started a regional detrital zircon study of metasedimentary units in this part of the orogeny and combined this dataset with new pendant mapping, and studies of strain, structures, and of plutonic ages and emplacement. Using G. Gehrels LA-ICPMS lab at the University of Arizona we have dated ~7000 zircons from 80 samples with samples collected and analyzed by A. Chapman (5), D. Clemens-Knott (5), G. Dunne and J. Saleeby (3), R. Cecil and M. Ducea (4), W. Cao (5), V. Memeti (9), and S. Paterson and students (49). These results support that the Sierra Nevada region is a collage of (1) oceanic terranes to the west, all of which received both Phanerozoic and Precambrian zircons, (2) displaced passive margin fragments along the central axis of the Sierra Nevada, possibly derived from northern sources such as the Idaho suture region, (3) displaced deepwater sediments in the eastern Sierra Nevada, probably related to the Golconda allochthon, and (4) locally displaced North American passive margin units in easternmost pendants. All of these units are unconformably overlain by ~250-95 Ma volcanic sequences and a widespread Jurassic westward younging (from ~200 to 150 Ma) marine sedimentary sequence. These data combined with our new mapping and structural studies reinforces previous suggestions that the two most important pre-Mesozoic arc faults are the fault between the oceanic terranes and displaced passive margin units (Foothills suture) and between the central Sierra passive margin units and North American passive margin units: the displaced deepwater (Golconda) sediments may occur as tectonic fragments along this latter fault. The widespread unconformity indicates a major period of uplift and exhumation between ~250-300 Ma followed by arc magmatism and volcanism. Although additional important faults were active during Mesozoic arc magmatism, the widespread Jurassic marine sequence uncomformably lying on top of all basement terranes suggests that major displacements did not occur after the Triassic.