Paleomagnetic evidence for displacement from the south of the Coast Plutonic Complex, British Columbia

Abstract

The mid-Cretaceous Spuzzum and Porteau plutons of the Coast Plutonic Complex of British Columbia have two magnetizations, A and B. The A magnetization (eight sites, 83 specimens, D = 30.3", I = 56.7", ay, = 4.9", paleolatitude = 37 t SON, paleopole 65.0°N, 14.9OW, Ay, = 6.2") is considered to have been acquired in the age range 105-90 Ma. This result differs from the field established for cratonic North America in this time range. The difference could be caused either by previously undetected tilting about a horizontal axis of the plutons, or by their rotation about a vertical axis and lateral displacement relative to the craton. Previously observed mid-Cretaceous magnetizations from other rock units from the western Canadian Cordillera and the Cascades of Washington, United States, are similarly discordant with respect to the craton. This similarity over such a large area indicates that, although local undetected tilting could be partly responsible, it is unlikely to be the prime cause, and we argue therefore that lateral displacement and rotation have occurred. It would seem that much of the western part of the Canadian Cordillera has moved north by about 2400 km and rotated clockwise since the mid-Cretaceous. The paleolatitude of the southern Coast Plutonic Complex of British Columbia is statistically identical to that recently observed (39 * 3ON) for three plutons from the Central Sierra Nevada of California, which raises the possibility that the two complexes were much closer together at the time of their emplacement than at present. The second magnetization called B (four sites, 27 specimens, D = 5. lo, 1 = 67.6", a , , = 4.7", paleopole 86.S0N, 5 1.2"W) is parallel to the mid-Tertiary field, as previously determined from nearby intrusions, and is considered to be an overprint acquired during regional heating and low-grade metasomatism. Some earlier paleomagnetic studies of mid-Cretaceous rocks from the Coast Plutonic Complex indicated either an absence of displacement or uncertain evidence for it, and we attribute this to the nonrecognition, in this earlier work, of similar magnetically stable overprints of Tertiary age. Overprints in several Triassic rock units in the western Cordillera are parallel to the A magnetization, indicating that the mid-Cretaceous and the mid-Tertiary probably were periods of severe magnetic overprinting in British Columbia. Mid-Cretaceous and Late Triassic results from the western Cordillera of British Columbia are systematically different, indicating that movements relative to the craton occurred between these times.Les plutons de Spuzzum et de Porteau d'bge crktact moyen du Complexe plutonique cbtier de la Colombie-Britannique exhibent deux aimantations, A et B. L'aimantationA (huit sites, 83 spkcimens, D = 30,3", 1 = 56,7", ag5 = 4,9O, palkolatitude = 37 2 5"N, palkopole 65,0°N, 14,9"0, A,, = 6,2") semble avoir kt6 acquise entre 105 et 90 Ma. Ce rksultat differe du champ ktabli pour le craton de I'Amkrique du Nord relatif h cet Cpisode de temps. Cette diffkrence peut s'expli...

East-derived strata in the Methow basin record rapid mid-Cretaceous uplift of the southern Coast Mountains batholith

“…The Baja British Columbia hypothesis ("Baja BC" after Irving 1985) suggests that the Intermontane and Insular superterranes accreted to North America much farther south and were translated to their present positions along major dextral strike-slip faults (Champion et al 1984;Irving 1985;Irving et al 1985;Irving and Wynne 1990;Cowan et al 1997;Haggart et al 2006). In this model, the Insular superterrane would have accreted to North America as far south as present-day Baja California in western Mexico at ca.…”

Late Miocene remagnetization within the internal sector of the Northern Apennines, Italy

“…Paleomagnetic investigations, combined with other geophysical analytical methods, have provided useful information on the kinematics of orogenic regions such as the distribution and amount of vertical-axis rotation of thrust sheets (e.g., Channell, 1992;Channell et al, 1992), oroclinal bending (e.g., Lucente and Speranza, 2001;Schill et al, 2002), horizontal-axis tilting of intrusive rocks (e.g., Varga et al, 1999) and latitudinal displacements and rotations of allochthonous terranes (e.g., Beck, 1980;Irving et al, 1985;Hagstrum and Murchey, 1993). Partial to complete remagnetization of rocks, however, is common in orogenic settings, and can often obliterate primary components (i.e., remanent magnetizations acquired during or soon after deposition for sedimentary rocks).…”

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

“…These discordant directions were initially interpreted to indicate northeast-side-up tilting of these plutonic rocks [Symons, 1977]. Later, when discordant paleomagnetic directions were found to be widespread within the North American Cordillera, the discordance was attributed to tectonic motion of crustal blocks involving $3000 km of northward transport and clockwise vertical axis rotation [Beck, 1976[Beck, , 1980Beck et al, 1981;Irving et al, 1985Irving et al, , 1996. Northward transport is interpreted to have occurred in the interval from 90 Ma to 50 Ma.…”