Extension in high-grade terranes of the southern Omineca belt : evidence from paleomagnetism
Date
1992
Authors
Wingate, Michael Thomas David
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Abstract
Previous paleomagnetic studies have shown that significant tilts have occurred in plutonic and high-grade metamorphic terranes within the zone of Eocene extension in the southern Omineca Belt (Bardoux and Irving 1989; Marquis and Irving 1990). The previous extent of sampling in the area (34 sites) has been augmented, during the current study, by an additional 78 sites, and the data used to produce a map of paleomagnetically-observed structural tilts across the extended region. The data are now of sufficient density that it is possible to test specific ideas and hypotheses regarding the kinematics of Eocene extension in the region. The following are the main conclusions of the present study:
1) A previously undiscovered, major west-dipping Eocene extensional fault, named here the Arrow Lake Fault (AF), has been defined paleomagnetically. Eastward apparent tilts are observed in the hanging wall of the east-dipping Kettle River Fault (KF), in the opposite sense to that expected in the hanging wall of the fault. These are juxtaposed, to the east, against westward apparent tilts north of Arrow Lake, and against apparently untilted syenites in the eastern parts of the 52 Ma Coryell Pluton. The AF is inferred to extend along the eastern limit of the eastward tilts, and to bisect the Coryell Pluton.
2) Apparent tilt data, combined with correlation of the polarities of the sampled rocks with the geomagnetic time scale, are employed to constrain the timing and duration of extensional events. The eastern parts of the study area were apparently tilted about 35° down-to-the-west, commencing at approximately 53 Ma, as a result of displacement on the east-dipping KF and Slocan Lake (SLF) Faults. The absence of apparent tilt in the eastern parts of the Coryell Pluton confirms that displacement on the SLF ended before cooling and magnetization of the Coryell Pluton at about 52 Ma. Normal displacement on the west-dipping AF and Granby Faults produced down-to-the-east apparent tilts averaging 38° in the hanging walls of these faults, but left the eastern parts of the Coryell Pluton untilted. Cooling of the Coryell Pluton is inferred to have occurred very rapidly, and therefore motion on the AF may have commenced very soon after 52 Ma.
3) Apparent tilt data allow inferences to be made regarding the subsurface geometry of the major Eocene extensional faults. The similarity of apparent tilt in the hanging wall and footwall of the SLF suggests it is a planar rotational fault. Conversely, the absence of apparent tilt in the footwall of the AF suggests the fault is listric at depth.
4) Re-evaluation of older structures is possible after they are restored to their pre-extensional attitudes by removing the effects of Eocene deformation. Restoration of about 40° degrees of westward apparent tilt observed in the footwall of the SLF indicates that, during the Early Eocene, the Valkyr Shear Zone (VSZ) dipped about 40° to the east, and is indeed an east-rooting extensional fault, thus making unlikely the possibility (raised by Carr and others 1987) that the VSZ is a west-rooting thrust fault. Similarly, eastward apparent tilts observed in syenites of the Granby Pluton confirm the interpretation of Carr and Parkinson (1989) that the Jones Fault, which presently is flat-lying, dipped originally to the west. The Mount Verde normal fault, and the Waneta, Tillicum Creek, and other thrust faults, all of Jurassic age, are believed to have been overturned to the west (Hoy and Andrew 1990; Einarson 1991 ); the paleomagnetic data provides independent confirmation of these interpretations.
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UN SDG 14: Life Below Water