A Palaeomagnetic Study of 143 Ma Kimberlite Dikes In Central New York State

Abstract

SUMMARY A palaeomagnetic study of 143 Ma serpentinized kimberlite dikes near Ithaca, New York, yields a dual-polarity, high unblocking temperature, and high coercivity magnetization which passes the reversal test and two baked contact tests. The mean pole position (58N, 203E; A95 = 3.8, N = 7) differs from published late Jurassicearly Cretaceous North American poles currently used to define the apparent polar wander path. The angular dispersion in mean directions (&=3.5) is low but the presence of reversals argues that the Ithaca kimberlites magnetization should represent sufficient time for averaging of palaeosecular variation. Similar findings apparently typify palaeomagnetic studies of other serpentinized kimberlite, supporting the suggestion that thermo-chemical remanent magnetization in this lithology prolongs the duration of magnetization acquisition sufficiently to average secular variation per dike. A consistent but weak foliation in anisotropy of ARM parallels the N-S and vertical orientation of the Ithaca dikes, but is apparently unrelated to the northwest-down or southeast-up remanence. The Ithaca kimberlites pole may therefore record a previously undocumented sharp bend or 'cusp' at -143Ma, and the initiation of a Cretaceous and Cenozoic interval of apparent polar wander that generally follows a great circle along the 200E meridian to geographic north. A coeval (-145 Ma) kimberlite pole from southern Africa transferred to North America agrees with the Ithaca kimberlites pole position whereas reported poles from the Berriasian stratotype (southern France) and 144Ma Svalbard dolerites provide less diagnostic tests of the Ithaca kimberlites pole due in part to uncertainties in Europe-North America reconstructions.