AGE, PROVENANCE, AND LATERAL VARIATION OF DETRITAL ZIRCON POPULATIONS IN THE PENINSULAR RANGES FOREARC BASIN, ORANGE COUNTY, CA

U-Pb detrital zircon (DZ) data from Late Cretaceous sediments of the Peninsular Ranges (PR) forearc basin display lateral and outcrop-scale variation in the proportions of zircons derived from the arc and its metamorphic framework, as well as extra-regional sources. Sandstone samples were collected along two traverses across the Late Cretaceous Ladd and Williams Formations, exposed along the western flank of the Santa Ana Mountains: (1) a northern traverse along Silverado and Williams Canyons; and (2) a southern, along the Ortega Highway, ~23 km to the south. The Arizona LaserChron Center’s LA-ICP-MS was used to measure the U-Pb dates of 87 to 99 DZs from six samples. Maximum depositional ages (MDA) estimated from the DZ data are compared to biostratigraphic estimates of sediment age. MDAs of the stratigraphically lowest sample (Baker Canyon mbr, Ladd Fm) from the north and south transects are indistinguishable (97.9+3.0 Ma (N); 97.0+1.6 Ma (S)), but are significantly older than the stratigraphic age of 91-89 Ma. Samples from the overlying Williams Fm (Schulz Ranch mbr) also exhibit differences between the estimated MDAs and stratigraphic age (83-75 Ma), as well as differences in MDA along strike: 97.3+1.4 Ma (N) and 88.5+1.4 Ma (S). Two samples of the stratigraphically highest unit, (Pleasants mbr, Williams Fm), also display lateral variations in MDA: 75.3+1.8 Ma (N) and 84.0­+1.2 Ma (S), though both MDAs fall within the unit’s stratigraphic age of 83-75 Ma. The stratigraphically lowest samples have the greatest amount of arc-derived DZs, while the stratigraphically highest sample contains a significant population of extra-regional DZs. Possible origins of these extra-regional DZs include ≈83 Ma plutons exposed in the northeastern PR batholith and ≈75 Ma plutons exposed in western Arizona. These data suggest that the northern and southern drainage basins were isolated, with differing amounts of metamorphic framework contributing to the detritus. These mismatches between MDAs and stratigraphic ages suggest that the arc may have been dormant between ≈95 and 91 Ma, or possibly later (≈83 Ma) in the vicinity of Silverado Canyon. The appearance of extra-regional DZs in the Pleasants mbr indicates that the arc crest was breached by ≤83 Ma fluvial systems that transported zircons from Laramide-aged uplifts east of the PRB.