Paleomagnetic Record of Crustal Tilt During Uplift of the Cretaceous Ecstall Pluton in Western British Columbia
Butler, R F (1)
(1)Department of Geosciences, University of Arizona Tucson, AZ 85721 United States
Based on U/Pb and K/Ar geochronologic and petrologic data, the Ecstall pluton crystallized at $\geq$20 km depth at 95 Ma, then uplifted and cooled to argon (biotite) retention temperatures at $\sim$64 Ma. Paleomagnetic directions have been determined from 27 sites (11 reversed polarity, 16 normal polarity) in a SW to NE transect across the Ecstall pluton north of the Skeena River. Sites within 4 km of the southwestern margin of the pluton have a mean direction (inclination [I] = $14\deg$; Declination [D] = $58\deg$) which is discordant from the expected mid-Cretaceous direction (I = $78\deg$; D = $331\deg$) by $\sim75\deg$. Sites 12 to 13 km from the southwestern margin have a mean direction (I = $81\deg$; D = $343\deg$) which is concordant with the expected mid-Cretaceous direction. Sites located 4 to 12 km from the southwestern margin have intermediate directions distributed along a small circle. This pattern of paleomagnetic directions indicates that the Ecstall pluton experienced $\geq70\deg$ of northeast-side-up tilt synchronous with uplift, cooling, and magnetization between 95 and 64 Ma. Sites on the west side of the pluton were cooled and magnetized first and recorded the full amount of tilt; sites progressively towards the east were uplifted from deeper levels and were cooled and magnetized later in the tilting and uplift history, therefore recording a smaller amount of tilt. As described by Beck [1992], this pattern is predicted for a large pluton which experienced tilt during uplift. Documentation of tilt in the Ecstall pluton suggests that the discordant directions in other Cretaceous plutons in western B.C. may be the result of northeast-side-up tilt rather than 3000 km of post-mid-Cretaceous northwards transport coupled with clockwise vertical-axis rotation.
Reference:
Beck, M. E., Jr., Some thermal and paleomagnetic consequences of tilting a batholith, Tectonics, 11, 297--302, 1992.
