Effects of paleogene faults on the reconstruction of the metamorphic history of the northwestern Thor-Odin culmination of the Monashee complex, southeastern British Columbia

Abstract

Differences in U-Pb metamorphic monazite ages in the northwestern Thor-Odin culmination of the Monashee complex, southern Canadian Cordillera, are explained in the context of the NNW-trending subvertical transcurrent Paleogene Victor Creek fault. Similar faults are present throughout the Canadian Cordillera. We demonstrate their potential importance in the interpretation of the history of Cordilleran deformation and metamorphism. A pervasive transposition foliation (ST) is present throughout the Thor-Odin culmination as a result of Cordilleran and possibly earlier deformation. A pre-ST (or early ST) foliation is preserved as aligned inclusion trails in porphyroblasts such as garnet and kyanite. Monazite U-(Th-)Pb isotope dilution-thermal ionization and secondary ion microprobe mass spectrometry (ID-TIMS, SIMS) ages are used to relate monazite growth to pre-and syn-ST fabrics and associated metamorphism. The ages of both pre-and syn-ST fabrics, and the gap between pre-and syn-ST ages decrease toward the east, in an apparently continuous manner. While monazite west of the Victor Creek fault is latest Cretaceous to earliest Eocene in age, monazite east of the Victor Creek fault is exclusively Eocene. Correlation of rock types across the faults is difficult because the same rock units are repeated many times on either side. However, distinctly different retained ages of metamorphism, and previously recognized differences in structures and detrital zircon signatures across the fault indicate 5-60 km offset along the fault. Similar trends occur across other faults along the western Monashee complex, and faults here and elsewhere in the Canadian Cordillera may have similar geological significance.