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.

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

Brittle-ductile transition: semi-brittle behavior

New U–Pb zircon geochronology from the Riggins region of west-central Idaho refines the timing of contractional deformation across the Salmon River suture zone (SRSZ), a broad north- to northeast-s...

Age and structure of the Crevice pluton: overlapping orogens in west-central Idaho?

Granitic orthogneiss forms an important component of the Barkerville terrane of southeastern British Columbia. Rb–Sr whole-rock ages for the orthogneisses are ambiguous and range from Late Proterozoic to mid-Paleozoic, with large associated errors. U–Pb dating of zircon, monazite, and sphene has been employed in an attempt to establish precise crystallization ages for two of the orthogneiss bodies. U–Pb systematics for zircons from both bodies show the combined effects of inheritance of zircon cores and postcrystallization Pb loss. This complexity precludes a precise estimate of the age of emplacement of the granitic protoliths of the gneiss. The data do, however, constrain possible emplacement ages for the bodies to between 335 and 375 Ma (Late Devonian – mid-Mississippian).A U–Pb age of 174 ± 4 Ma for metamorphic sphene from one of the orthogneiss bodies is interpreted as dating the end of the second phase of deformation in the area. Two nearly concordant U–Pb ages of 114 and 117 Ma for monazite from th...

U-Pb zircon, monazite, and sphene ages for granitic orthogneiss of the Barkerville terrane, east-central British Columbia

Centrifuge analogue modelling illustrates the progressive development of active folds in multilayers upon a ductile substrate during layer-parallel shortening. Models simulate folding of a mechanically stratified sedimentary sequence upon migmatitic gneisses in a large hot orogen, or upon a thick basal evaporite ± shale sequence in deeper levels of fold belts. The absence of a weak low-viscosity and low-density layer at the interface promotes infolding of the cover sequence and ductile substrate, whereas a planar upper surface to the basal ductile substrate is preserved when it is present. Whilst fold style, wavelength, and deformation of the interface with the ductile substrate differ depending on whether a low-viscosity and low-density layer is present at the base of the cover sequence, there is no marked systematic curvature of fold axes as seen in previous sandbox models for fault-bend or fault propagation folding during bulk shortening. Bulk shortening of a layered sequence with relatively thick individual layers above a ductile substrate promotes a regular and upright train of buckle folds, whereas thinner layers promote a more irregular distribution of buckle folds with variable vergence, style, and amplitude. Buckle folds above a ductile substrate progressively develop during bulk shortening from open and upright, to angular and tight, and may further develop into cuspate structures above relatively weak horizons. Relatively thick weak horizons within the layered sequence during bulk shortening interrupt regular fold patterns up structural section and allow out-of-phase folds to develop above and below the weak horizon.

Centrifuge modelling of deformation of a multi-layered sequence over a ductile substrate: 1. Style and 4D geometry of active cover folds during layer-parallel shortening

M. Bloodgood

Papers

Geometry of a convergent zone, Central British Columbia, Canada