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.

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Seismic reflection data from the south central Canadian Cordillera covering the interval from the easternmost metamorphic core complexes near Arrow Lakes to the Fraser River fault system along the Fraser River reveal a highly reflective and complex crust. The base of the crustal reflectivity, interpreted as the reflection Moho, is clearly delineated by a continuous sharp boundary that is essentially planar and slopes uniformly over a distance of 250 km from about 12.0 s in the east to about 10.5 s in the west. This virtual lack of relief at the base of the crust contrasts sharply with surface structures that involve 25 km or more of structural relief. Some of these surface structures can be readily correlated to structures that are outlined by the reflection data and that can be followed into the middle and lower crust. Even though part of this area was subjected to large amounts of Eocene extension, the crust is not divisible into transparent upper and reflective lower layers as it is in parts of the U.S. Cordillera. Three structural culminations, the Monashee complex, the Vernon antiform, and the Central Nicola horst, are interpreted on the basis of the reflection configuration and the surface geological relationships to have formed initially during Jurassic to Eocene compression and then to have been modified and exposed during early and middle Eocene extension. An example of a compressional structure observed on the profiles is the Monashee decollement, which can be traced from the surface westward into the lower crust. Extension is manifested along a variety of normal faults, including the regionally extensive low angle Okanagan Valley‐Eagle River fault system, moderately dipping faults such as the Columbia River and Slocan Lake faults, and high‐angle faults such as the Quilchena Creek and Coldwater faults. Both Jurassic to Eocene compressional shear zones and early to middle Eocene extensional shear zones are listric into the lower crust or Moho under the Intermontane belt.

The Thor‐Odin ‐ Pinnacles area is a structural culmination in the Shuswap complex of the southern Omineca Belt of the Canadian Cordillera. It comprises amphibolite‐facies rocks that were deformed during Mesozoic‐Paleocene compression and were exhumed in the footwalls of Eocene normal faults during crustal extension. The Ladybird leucogranite suite coincides with the extended terrane in the southern Omineca Belt. It is generally restricted to a midcrustal level which lies in the hanging walls of deep‐seated thrust faults and the footwalls of extensional faults. Field relationships of the leucogranites and U‐Pb geochronology place timing constraints on compressional and extensional shear zones. The last thrust motion on the Monashee décollement occurred in the latest Paleocene, and the shear zone had stopped by 58 Ma. Crustal‐scale normal faults were active in the early Eocene, indicating that crustal extension closely followed the compressional regime. Geological and geochronological data are consistent with an anatectic crustal origin for the Ladybird granite. The granites apparently postdate the thermal peak of metamorphism (Carr, 1990) and were generated during the final stages of thrusting, perhaps due to decompression melting as the midcrustal rocks were carried up a thrust ramp and unroofed and/or due to the introduction of hydrous fluids into the system. In situ magma and hot intrusions probably played an important role in the nucleation of extensional shear zones. The extensional regime then facilitated the intrusion of vast late‐synkinematic to posttectonic plutons. U‐Pb systematics reveal that zircons in high‐temperature shear zones may have suffered high‐temperature Pb loss, perhaps due to deformation‐ or fluid‐enhanced diffusion, and that monazite systematics from samples from high‐grade terranes are complex. Magmatic monazite populations contain crystals of different ages that do not coincide with zircon ages and apparently represent neither a crystallization age nor a cooling age.

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