Palaeomagnetic Evidence Bearing on the Evolution of the Canadian Cordillera [and Discussion]

E. Irving, P. J. Wynne, P. F. Hoffman, A. Trench, A. H. F. Robertson
1990 Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences  
Palaeomagnetic data from Permian, Triassic and Jurassic bedded rocks, to which attitudinal corrections can be applied, yield palaeolatitudes concordant with those of ancestral North America, but very large predominantly anticlockwise rotations about vertical axes. Data from Cretaceous rocks yield apparent palaeolatitudinal displacements that increase westward. Small or negligible displacements are obtained from the Omineca Belt. Intermediate displacements (1000-2000 km) from the Intermontane
more » ... the Intermontane Belt, are based on data from Cretaceous bedded sequences. Further to the west in the Coast Belt, larger apparent displacements (greater than 2000 km) have been obtained from plutons for which no attitudinal control is yet available. Data from Eocene rocks are concordant. Possibilities to consider are as follows: (a) little or no displacement and tilting to the southwest at about 30°; (b) large (greater than 2000 km in the Coast Belt) northward displacement since mid-Cretaceous time preceded by southward displacement of comparable magnitude in Juro-Cretaceous time; (r) lesser (1000-2000 km) overall displacement coupled with variable and lesser tilts to the south and southeast of plutons of the Coast Belt. Under hypothesis ( ) the western Cordillera was formed and has remained in approximately its present position relative to ancestral North America; data from bedded volcanics of the Intermontane Belt are not consistent with this hypothesis. From the evidence currently available we favour hypotheses (b) or (r), although more data from bedded sequences are required. It is noteworthy that hypotheses (a) and (c) predict tilt directions that differ by about 90° and hence ought to be distinguishable by geological studies. [ 31 ] 35 Vol. 331. A 488 E. IRVING AND P. J. WYNNE (compare figures 1 and 2) . T able 1. Craton reference palaeopoles T (Ma) craton T (Ma) cordillera lat.° N, long.® E (495) rel. A Eocene 54-48 55-48 83, 176 (3) H KE, KA, SP, FL B Palaeocene 67-62 Palaeocene (66-58) 81, 185 (6) L CA C latest Cretaceous and Palaeocene 73-63 70 78, 186 (8) L CK D mid-Cretaceous 136-85 120-95 71, 196 (5) H SC, SS, SA, AX, SB, CK, CS, SP, PP, MS E early Jurassic 195-191 Sinemurian-T oarcian (204-187) 65, 082 (4) M HZ, BZ F late Triassic Norian-Carnian (230-208) late Ladinian-early Norian (332-323) 53, 098 (7) M ST, KX, NI G early Permian 285-255 late Sakmarian-early Artinskian (270-265) 46, 119 (4) H AS Notes for table 1. First column gives ages for which data are available from both craton and cordillera. Age spans of respective data are given in the second and third columns, in numbers if determined radiometrically, and by geological stages if determined stratigraphically followed by numerical ages estimated from the timescale of Palmer (1983). Palaeopoles and errors (P = 0.05) are followed by an estimate of reliability (H, high; M, moderate; L, low). The last column gives the cordilleran rock-units for which displacements and rotations have been calculated in table 2. Terranes and belts The Western Cordillera in British Columbia and adjacent areas of northern Washington and Alaska comprise several morpho-geologic belts which trend general NNW to SSE (figure 1). Within each belt are several fault-bounded terranes (figure 2). Some terranes are confined within a single belt, others are not. For example, the Wrangellia terrane straddles the boundary between the Intermontane and Coast belts, and Quesnellia occurs on both sides of the boundary between the Intermontane and Omineca belts
doi:10.1098/rsta.1990.0085 fatcat:e2gyrslnizdlrlq7fun5jhgydm