PALEOMAGNETIC EVIDENCE FOR EN ECHELON CRUSTAL EXTENSION AND CRUSTAL ROTATIONS IN WESTERN MONTANA AND IDAHO

The Bitterroot metamorphic core complex is one of many Eocene core complexes in the northern Cordillera. A middle Eocene dike swarm cuts the Bitterroot metamorphic core complex and its hanging wall, the Skalkaho slab. A 26-degrees change in strike of the dikes across the core complex suggests either a refracted stress field or rotation during Eocene or younger extension. The nearby Bitterroot Valley and Big Hole Basin also indicate tectonic rotation. To distinguish between the hypotheses of refracted stress and tectonic rotation, we collected paleomagnetic data from 28 dikes. Nineteen sites provide reliable results. Eight sites from the hanging wall east of the mylonite zone give Dec = 46-degrees, Inc = 69-degrees, alpha-95 = 13-degrees; 11 dikes to the west yield Dec = 334-degrees, Inc = 64-degrees, alpha-95 = 6-degrees. The declinations diverge from the expected direction; the inclinations do not. The hanging wall rocks are rotated 58-degrees +/- 32-degrees clockwise at the 95 % confidence level. The footwall shows 14-degrees +/- 11-degrees of counterclockwise rotation. Both normal and reversed polarity dikes are present in each area, indicating that the dike swarms may adequately average paleosecular variation. Vertical contacts reveal that the dikes have not tilted since emplacement. These paleomagnetic results and available geochronologic data show that the Bitterroot dome was pulled from beneath the Skalkaho slab in the Eocene. As the dome slid toward the northwest, it experienced a slight counterclockwise rotation, while the upper plate moved clockwise around a vertical axis near its northern edge. Rotation of the Skalkaho slab occurs because the slab lies between en echelon extensional detachments of the core complex and the next basin to the east, the Big Hole Basin. The Boehls Butte area, north central Idaho, occupies a step in the Lewis and Clark fault zone and is probably a core complex related to right-lateral motion on the fault system. Similarities in both the style and timing of extension suggest that Eocene ductile/brittle extension occupies a swath from British Columbia, past the Bitterroot metamorphic core complex, to the Pioneer core complex in south central Idaho.