Syntectonic remagnetization in the southern Methow block: resolving large displacements in the southern Canadian Cordillera

The Upper Cretaceous Ventura Member of the Goat Wall unit in the southern Methow block of southern British Columbia and northern Washington State holds a syntectonic magnetization. Eight new sites from Manning Park in British Columbia give a mean direction of D = 27.5[degrees], I = 60.1[degrees], k = 304.7, [[alpha].sub.95] = 3.2[degrees] after optimal partial tilt correction. Of five groups of bedded sites from farther south in the basin reported by Bazard et al. [1990], four have a syntectonic remanence with a direction similar to what we observe. The exception is one group which has optimal concentration of remanence directions on >100% untilting and an abherent direction which must be rejected. Combining the accepted sites, the optimal differential syntilting direction is D = 11.8[degrees], I = 61.5[degrees], k = 39.3, [[alpha].sub.95] = 3.4[degrees] (N = 47), giving a mean pole of 79.8[degrees]N, 359.2[degrees]E, K = 19.5, and [A.sub.95] = 4.8[degrees]. The age of the remagnetization is constrained to be between 88 and 80 Ma. Compared to cratonic North America, this result indicates that the southern Methow block was displaced from the south by 1800 [+ or -] 500 km, meaning it lay south of the Siena Nevada subduction zone but well north of other paleomagnetically constrained Cretaceous rock units from the Insular superterrane, including correlative strata of the Mount Tatlow area in the northern Methow block. Among several possibilities to reconcile this discrepancy, the most plausible has the whole Methow block translated coherently but with the southern Methow block strata remagnetized during transit. INDEX TERMS: 1525 Geomagnetism and Paleomagnetism: Paleomagnetism applied to tectonics (regional, global); 8102 Tectonophysics: Continental contractional orogenic belts; 9350 Information Related to Geographic Region: North America; 9609 Information Related to Geologic Time: Mesozoic; 1533 Geomagnetism and Paleomagnetism: Remagnetization; KEYWORDS: paleomagnetism, orogenic belts, North America, Mesozoic, remagnetization