Paleomagnetic Data from Late Jurassic to mid-Cretaceous Plutons Southwest of Prince Rupert, British Columbia: Changes in Attitude or Changes in Latitude?

* Butler, R F (1), Gehrels, G E (1), Hart, W (1)

(1) Department of Geosciences, University of Arizona, Tucson, AZ 85721

Paleomagnetic samples were collected from a transect through Mesozoic plutonic rocks southwest of Prince Rupert, British Columbia. Characteristic directions were acquired from 32 normal-polarity sites on Pitt Island (U/Pb age = 107 Ma). The mean paleomagnetic direction is:Inclination [I] = $45.1\deg$; Declination [D] = $18.9\deg$. The observed mean paleomagnetic direction from 22 normal-polarity sites on McCauley Island (U/Pb age = 114 Ma) is: I=$51.8\deg$; D=$42.7\deg$. Sixteen sites of dominantly reversed polarity from Late Jurassic rocks of Banks Island (U/Pb age = 148 Ma) yield a mean paleomagnetic direction: I=$70.9\deg$; D=$47.5\deg$. Compared to the mid-Cretaceous expected direction (I=$78\deg$; D=$333\deg$), the meanpaleomagnetic directions from both the Pitt Island pluton and the McCauley Island pluton are shallow and clockwise rotated. However, compared to the Late Jurassic expected direction (I=$70\deg$; D=$317\deg$), the mean paleomagnetic direction from Banks Island has a concordant inclination but discordant declination. On the assumption that present horizontal approximatespaleohorizontal, the paleomagnetic data from Pitt and McCauley islands can be interpreted to indicate $\sim$4000 km of post-mid-Cretaceous northwards transport coupled with $50\deg$ to $60\deg$ of clockwise vertical-axis rotation (= Baja British Columbia hypothesis). However interpreting paleomagnetic data from all Mesozoic plutons in this region in terms of latitudinal transport requires: (1) intrusion and magnetization of the Late Jurassic Banks Island pluton essentially in its present latitudinal position; (2) $\sim$4000 km of southward motion prior to intrusion and magnetization of Pitt and McCauley islands plutons in mid-Cretaceous; and (3) $\sim$4000 km of northwardstransport prior to intrusion and magnetization of the $\sim$70 Ma Quottoon pluton. On the other hand, east- or northeast-side-up tilts can explain the discordant paleomagnetic directions from all Mesozoic plutons in the Prince Rupert region without complex latitudinal motion histories. Given that east-side-up tilting has affected portions of the Quottoon pluton in this region, east- ornortheast-side-up tilting during uplift of the Mesozoic plutons is the preferred tectonic explanation for the observed paleomagnetic directions.

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