Eocene extensional tectonics and geochronology of the Southern Omineca Belt, British Columbia and Washington

Abstract

Eocene extension contributed significantly to the present crustal architecture of the southern Omineca Belt in British Columbia and Washington. High grade gneiss complexes (Valhalla, Okanagan, Kettle‐Grand Forks, Monashee, and Priest River) preserve Cretaceous to Eocene deformation superimposed on older structures and have Eocene biotite and muscovite cooling ages. They are juxtaposed by regionally extensive, low‐ and moderate‐angle, ductile and/or brittle normal faults (Valkyr‐Slocan Lake, Okanagan Valley, Kettle River, Granby‐Greenwood, Columbia River, Standfast Creek (in part), and Newport and Purcell Trench faults) against metamorphosed rocks with a late Paleozoic to Middle Jurassic compressional tectonic history. Some upper plate rocks are overlain by Middle Eocene strata. Upper plate rocks preserve middle Cretaceous and older mica cooling dates indicating that they were less than 300°C in the Eocene, in contrast to lower plate rocks. The complexes have features in common with metamorphic core complexes of extensional origin elsewhere. U‐Pb zircon and monazite dates on mylonitic granitic rocks in the footwalls of the Okanagan and Valkyr‐Slocan Lake shear zones prove that a significant part of their ductile fabric is related to displacement on Eocene extensional faults. On the eastern side of the Monashee complex, 55 Ma U‐Pb zircon and circa 54 Ma Rb‐Sr synkinematic muscovite ages demonstrate that the ductile‐brittle Columbia River fault is a predominantly Early Eocene normal fault. Contrary to previous interpretations, the circa 162 Ma Galena Bay stock does not intrude footwall mylonites, and therefore the interpretation that at least some of the mylonites are related to Eocene extension is permissible. The distribution of Eocene cooling ages implies that part of the Standfast Creek fault on the eastern boundary of the Clachnacudainn complex is a ductile (+/−brittle) normal fault. Analogous interpretations are made for the Kettle‐Grand Forks and Priest River complexes where similar isotopic cooling age patterns prevail. Normal fault systems which bound the metamorphic complexes are fundamental crustal breaks, with displacements of 10–20 and in some cases 40 km, and probably accommodated about 30% extension across the 300 km width of the southern Omineca Belt. Most of the east dipping fault systems were active mainly between 58 and 52 Ma, in contrast to west dipping systems which are 52–45 Ma old, although both systems may have had some younger brittle displacement. Comparison of east–west cross sections with palinspastic restorations implies that the crust was more than 50 km thick prior to extension, that the high grade core complexes were not exposed to erosion prior to the Eocene, and that they were technically denuded and exhumed on Eocene normal fault systems. This extensional model is consistent with known geology, helps to explain several enigmatic geologic relationships, and has important implications for interpreting the pre‐Eocene, compressional deformation in the region.