Geology of the Allan Creek area, Cariboo Mountains, British Columbia

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The Allan Creek area is divided into two tectonic domains by the steep north-west/southeast trending, southwest-side-down, post-metamorphic Allan Creek Normal Fault (ACF). Late Proterozoic Horsethief Creek and lower Kaza Group strata on the overturned limb of a phase one nappe, north of the ACF, are juxtaposed against lower and middle Kaza Group strata to the south, which belong to a higher level, upright limb of a phase one nappe. Fabrics on both sides of the fault preserve evidence for two major phases of folding, and north of the ACF there is evidence for a third phase of deformation. The second phase structures control the map pattern. They change in style, regardless of lithology, from isoclinal folds south of the ACF to more open folds north of the ACF. North of the ACF the Camp Creek Fault, which pre-dates the second phase of deformation, separates Horsethief Creek strata from younger, lower Kaza strata. South of the ACF, metamorphic grade increases northeastward from garnet grade to kyanite grade. The metamorphic grade north of the ACF, although still entirely in kyanite zone, is greater than in the south: leucosome is found only north of the ACF and the first appearance of leucosome corresponds with the trace of the ACF. Metamorphic histories differ across the ACF. South of the fault there is evidence for two distinct metamorphic events, whereas strata north of the fault may have experienced one continuous metamorphic event. On both sides of the ACF the peak of metamorphism post-dates the second phase of deformation. Retrogressive minerals are present only south of the ACF. North of the fault peak metamorphic mineral assemblages have been quenched and preserved in textural equilibrium. Pressures and temperatures determined by geobarometry and geothermometry from both sides of the fault (570 and 540 MPa, and 510°c and 540°c south and north of the ACF, respectively) are within error limits of one another, indicating that throw on the ACF may be less than 4 km. Preliminary U-Pb zircon and monazite age determinations of 154+/-6 Ma from a pegmatite that pre-dates phase two deformation, and 125+/-7 Ma from one that post-dates the second phase of deformation, constrain the age of phase two deformation to between 160 and 118 Ma (Late Jurassic to Early Cretaceous). The age of the metamorphic peak is inferred from a 135+/-4 Ma (Early Cretaceous) U-Pb monazite age from a pelite from north of the ACF. Since peak metamorphism post-dated phase two deformation, the age of phase two deformation in the Allan Creek area, north of the ACF, is further constrained to between 165 and 131 Ma. The timing of movement on the ACF post-dates the peak of metamorphism and is therefore later than 140 Ma. It is likely that the ACF formed to accommodate the northward decrease in offset on the Middle -Eocene North Thompson River-Albreda River Normal Fault, and therefore is also of Middle Eocene age.
Bibliography: p. 125-139.
Currie, L. D. (1988). Geology of the Allan Creek area, Cariboo Mountains, British Columbia (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from doi:10.11575/PRISM/19067