PALEOMAGNETISM AND GEOCHRONOLOGY OF THE CARBONIFEROUS BAIRD FORMATION, EASTERN KLAMATH MOUNTAINS: CRATONIC PALEOLATITUDE AND VARIABLE ROTATION

The Klamath orogen is variably featured in the diverse accretional histories of western North America. Permian-Jurassic age rocks in the Eastern Klamath terrane (Redding Section; RS) of northern California have yielded paleomagnetic data that are interpreted to represent either terrane-scale rigid body rotation or differential vertical-axis rotation between structural domains. Our ongoing study of paleomagnetism and U-Pb geochronology of the Carboniferous Baird Fm. aims to refine the aforementioned interpretations of paleomagnetic data. The Baird Fm. is composed of ~1000m of well-bedded shallow marine to emergent island arc volcanic and volcaniclastic strata, interfingering with the overlying late Pennsylvanian to early Permian McCloud Limestone. Current age constraints for the Baird Fm. are based solely on biostratigraphy. Here we report a CA-TIMS ²⁰⁶Pb/²³⁸U zircon weighted mean age of 325.31 ± 0.28 Ma from a normally magnetized silicic ignimbrite in the Baird Fm. An andesitic tuff ~ 20m structurally below this also yields normal polarity, whereas 5 sites inferred to be stratigraphically younger have reverse polarity, suggesting that the Baird Fm. records the onset of the Kiaman reverse polarity superchron, although additional constraints are needed for a robust chronostratigraphic placement. Tilt-corrected directions from the lower, normal polarity sites yield antipodal directions with virtual geomagnetic poles consistent with Carboniferous cratonic poles. Sites from other structural domains with variable strike, believed to overlie these units, have reverse polarity as expected for Kiaman-aged magnetization but declinations varying by ~90°, consistent with the range of bedding strike. These data suggest that the RS originated on the North American plate, possibly as a fringing offshore island arc, and was internally deformed subsequent to the deposition of the conformable mid-Jurassic strata at the top of the Redding Section.