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Paleomagnetism and geochronology of the Ecstall pluton in the Coast Mountains of British Columbia: Evidence for local deformation rather than large-scale transport

 
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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. Bl, 10.1 029/200 1JB000270, 2002 Paleomagnetism and geochronology of the Ecstall pluton in the Coast Mountains Of British Columbia: Evidence for local deformation rather than large-scale transport Robert F. Butler, George E. Gehrels, and Suzanne L. Baldwin1 Department of Geosciences, University of Arizona, Tucson, Arizona, USA Cameron Davidson Department of Geology, Beloit College, Beloit, Wisconsin, USA Received 16 August 2000; revised 27 April 2001; accepted 18 July 2001; published 18 January 2002 [i] Samples for geochronologic, geobarometric, and paleomagnetic analyses were collected across the northern portion of the Ecstall pluton southeast of Prince Rupert, British Columbia. Al-in- homblende geobarometry indicates pressures from 740 ± 10 to 840 ± 30 MPa corresponding to crystallization depths of ~25 to ~30 km. U/Pb analyses of zircons from western, central, and eastern localities within the pluton yield crystallization ages of 91.5 ± 1.0 Ma, 90.8 ± 1.0 Ma, and 90.5 ± 1 .0 Ma, respectively. Rock magnetic experiments, reflected light microscopy, and thermal demagnetization behavior suggest that natural remanent magnetism is carried by low'-Ti titanohematite. Unblocking temperatures of the characteristic remanent magnetization (ChRM) are dominantly in the 560°C to 630°C range, with age of magnetization approximated by the 40Ar/39Ar hornblende ages of 84.2 ± 0.10 Ma on the western margin and 76.4 ± 0.6 Ma in the center of the pluton. Site-mean ChRM directions were isolated for paleomagnetic samples from 23 sites and are distributed along a small circle with subhorizontal axis at ~340° azimuth. ChRM directions from the central portion of the pluton are concordant with the expected Cretaceous magnetic field direction, while ChRM directions from the western margin are discordant by >70°. Folding of the Ecstall pluton, either during Late Cretaceous west directed thrust transport above the convex upward Prince Rupert Shear Zone or during younger deformation of the pluton and underlying shear zone, can account for the paleomagnetic data and is consistent with the geochronologic, geobarometric, and structural geologic observations. INDEX TERMS: 1525 Geomagnetism and Paleomagnetism: Paleomagnetism applied to tectonics (regional, global); 1527 Geomagnetism and Paleomagnetism: Paleomagnetism applied to geologic processes; 8105 Tectonophysics: Continental margins and sedimentary basins; 8110 Tectonophysics: Continental tectonics-general (0905); KEYWORDS: Paleomagnetism, Cretaceous, British Columbia, Tectonics 1. Introduction [2] Some of the first discordant paleomagnetic directions in Cretaceous rocks of the North American Cordillera were discov¬ ered by Symons [1974] in plutons near Prince Rupert, British Columbia. Discordant paleomagnetic directions were observed in the Late Cretaceous Ecstall and Butedale plutons and the mid- Cretaceous plutons of Stephens Island and Pitt Island (Figures 1 and 2). These Cretaceous plutons all have paleomagnetic directions with shallow inclinations and clockwise-rotated declinations when compared to expected Cretaceous directions. These discordant directions were initially interpreted to indicate northeast-side-up tilting of these plutonic rocks [Symons, 1977]. Later, when dis¬ cordant paleomagnetic directions were found to be widespread within the North American Cordillera, the discordance was attrib¬ uted to tectonic motion of crustal blocks involving ^3000 km of northward transport and clockwise vertical axis rotation [Beck, 1976, 1980; Beck el al. , 1981; Irving el al. , 1985, 1996]. North- 'Now at Department of Earth Sciences, Syracuse University. Syracuse, New York, USA. Copyright 2002 by the American Geophysical Union. 0 1 48-0227/02/200 1 JB000270S09.00 ward transport is interpreted to have occurred in the interval from 90 Ma to 50 Ma. [3] Interpretation of discordant paleomagnetic directions from plutonic rocks is not straightforward because paleohorizontal at the time of magnetization can only be inferred indirectly. There are no direct lithologic recordings of paleohorizontal as with stratified rocks. As a result, a “tilt versus translation’’ controversy has ensued regarding the discordant paleomagnetic directions from plutonic rocks of the North Cascades and Coast Mountains bath- olilh [Butler et al., 1989; Brown and Burmester, 1991; Vandall, 1993; Ague and Brandon, 1996, 1997; Anderson, 1997]. [4] Discordant paleomagnetic directions have also been observed in Cretaceous sedimentary and volcanic rocks of the northern North American Cordillera [Wynne et a!., 1995; Ward et al.. 1997]. Some workers have concluded that the consistently shallow paleomagnetic inclinations confirm the transport interpre¬ tation of discordant paleomagnetic directions from the plutonic rocks. The tectonic transport interpretation has become known as the Baja British Columbia hypothesis [UmhoeJ'er, 1987; Cowan, 1994; Cowan el al., 1997]. This is because the implied mid- Cretaceous location of the Insular superterrane and at least the western portion of the Coast Mountains and North Cascades is in the position relative to North America now occupied by Baja California. However, concerns about compaction shallowing of paleomagnetic inclination in sedimentary rocks and about reli- 3 - I EPM
Title
Paleomagnetism and geochronology of the Ecstall pluton in the Coast Mountains of British Columbia: Evidence for local deformation rather than large-scale transport
Author
Robert F. Butler, George E. Gehrels, Suzanne L. Baldwin, Cameron Davidson
Publication Date
2002
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