This study contributes the first palcomagnetic data from dated Cretaceous units of the Coast Belt (51.3øN, 123.8øW) in which palcohorizontal bedding surfaces can be identified with certainty. We sampled 38 sites through a 1.5-km stratigraphic interval of Cenomanian Silverquick sedimentary strata and Cenomanian to Campanian Powell Creek volcanics in the Tatlow syncline in the Tyaughton-Methow Basin. Magnetizations at 21 sites have high unblocking temperatures (TUB) and pass the tilt test (precision k = 5 before untilting, maximum k = 48 after 100% untilting). Magnetic minerals of volcanics show original textures, mean declinations in volcanic and sedimentary rocks are statisticall•lelltlCal, and---'•: ........ ' •-• : ^,: •,.• • •o •.4 o pmanty is consistent + , 1101111•1,1. 111• mean lil•11natlon ..• _ •, probability P -0.05) of this pretilting magnetization is 20.1 ø _+ 4.0 ø shallower than that expected if the rocks were deposited in their present position relative to cratonic North America. Field and laboratory observations rule out other causes for these anomalous shallow inclinations. These rocks therefore have been displaced northward by 3000 +_ 500 km relative to cratonic North America since mid-Cretaceous time. This displacement is almost 3 times greater than that estimated from mid-Cretaceous volcanics in the nowadjacent Intermontane Belt. Evidently the two belts were not amalgamated until latest Cretaceous or Early Tertiary time. These results are inconsistent with all previous geologically based tectonic reconstructions of the Cordillera. Magnetizations at eight sites in Silverquick and Powell Creek beds have low Tus and directional characteristics similar to those in three younger diorite intrusions. Reversed and normal polarities occur, the rocks are commonly highly altered, and maximum precision occurs after 40% untilting. The magnetizations therefore are interpreted as overprints of latest Cretaceous age acquired during folding. magnetization direction to be corrected to the palcohorizontal plane, and (2) by obtaining observations from Cretaceous stratified rocks in which accurate bedding attitudes can be determined. The first test has already been carried out for the 96-93 Ma Mount Stuart batholith (Figure 1); Ague and Brandon [1992], on the basis of hornblende geobarometry, have corrected the palcomagnetic observations [Beck et al., 1981] for tilt and obtained an estimate of motion from the south of 2900 + 700 km. We describe the first palcomagnetic results from Cretaceous bedded rocks of the Coast Belt, which allow us to carry out the second test. The sedimentary and volcanic sequence studied is particularly suitable for six reasons. They contain thinly bedded, planar-laminated siltstones and fine-grained sandstones of fluvial overbank origin from which paleohorizontal surfaces can be determined with precision. The nøAr/39Ar data provide the age of the Powell Creek volcanics. The strata occur in a syncline, the Tatlow syncline (Figure 2), with an interlimb 6073 6074 WYNNE ET AL.' MOUNT TATLOW PA...
The Mesozoic Tyaughton–Methow Basin straddles the Fraser–Yalakom–Pasayten – Straight Creek (FYPSC) strike-slip fault zone between six tectono-stratigraphic terranes in southwestern British Columbia. Data from Hauterivian–Cenomanian basin fill provide constraints for reconstruction of fault displacement and paleogeography.The Early Cretaceous eastern margin of the basin was a region of uplifted Jurassic plutons and active intermediate volcanism. Detritus shed southwestward from that margin was deposited as the marine Jackass Mountain Group. Albian inner to mid-fan facies of the Jackass Mountain Group can be correlated across the Yalakom Fault, suggesting 150 ± 25 km of post- Albian dextral offset. Deposits of the Jackass Mountain Group overlap the major strike- slip zone (FYPSC). If that zone represents the eastern boundary of the tectono-stratigraphic terrane, Wrangellia, then accretion of Wrangellia to terranes to the east occurred before late Early Cretaceous time.The western margin of the basin first became prominent with Cenomanian uplift of the Coast Mountain suprastructure. Uplift is recorded by dispersal patterns of the volcaniclastic Kingsvale Group southwest of the Yalakom Fault.Reversing 110 km of Late Cretaceous – early Tertiary dextral motion on the Fraser – Straight Creek Fault followed by 150 km of Cenomanian – Turonian motion on the Yalakom – Ross Lake Fault restores the basin to a reasonable depositional configuration.
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