PALEOMAGNETISM OF THE EARLY TRIASSIC SEMEITAU IGNEOUS SERIES, EASTERN KAZAKSTAN

Abstract

Paleomagnetic results obtained from the early Triassic Semeitau igneous series (SIS) yield a high-quality pole for eastern Kazakstan that is concordant with reference poles for Siberia and Europe, strongly supporting the hypothesis that Kazakstan was already joined with Eurasia at this time. Of the 25 sites that were measured, data were straightforward for 20 sites, 2 of which recorded normal polarity and 18 of which recorded reverse; 5 sites were discarded because of severe overprinting, unstable demagnetizations, or extremely scattered within-site directions. Combining sites judged to be the same cooling unit and taking the average of 15 cooling units of equal weight, we calculated a pole located at 56°N, 139°E (N = 15, K = 24.6, α95 = 7.9) for the SIS. We consider this pole to be reliable. The sampling of temporally spaced extrusives and more slowly cooled intrusives and the presence of both polarities strongly suggest that this pole averages secular variation. The 40Ar/39Ar determinations on sanidine crystals from two extrusive units yielded highly consistent ages of 248.2 ± 0.5 and 248.8 ± 0.5 Ma. Comparison of our pole for the SIS with the Siberian reference pole shows relative translation (−8° ± 10°) and rotation (1°±16°) that are indistinguishable from zero at 95% confidence. This result fails to confirm hypothesized post-Permian rotation of the Siberian craton relative to its margins and provides no evidence for left-lateral motion on the east-west shear zones in which the Semeitau is located. The similarity of ages between the SIS and the Siberian Traps supports speculation of a genetic relationship between the two.