THE PALEOMAGNETISM OF THE EARLY JURASSIC KAROO LARGE IGNEOUS PROVINCE, SOUTHERN AFRICA AND IMPLICATIONS FOR THE RECENTLY POSTULATED JURASSIC TRUE POLAR WANDER EVENT

A growing body of paleomagnetic data from North America as well as Africa/Adria support the hypothesis that the Early to Middle Jurassic experienced a large magnitude (~40o polar shift) true polar wander (TPW) event, between about 183 and 151 Ma. For Africa/Adria, as reported by Muttoni et al. (in press), this event is bracketed by data from the Early Jurassic (ca. 184 to 178 Ma) Karoo Large Igneous Province and other African igneous rocks and from Early Tithonian strata from Adria. Concurrent with studies of the fabrics, internal zonation and magma flow in mafic sills of the Karoo province, we have obtained paleomagnetic data from the laterally extensive gabbroic sills lying at or near the floor of the province, which consists of a complex array of intrusions and a thick sequence of flood basalts. Samples were collected at 27 regionally distributed sites in lower gabbroic sills in the floor of the complex (typical lateral spacing of some 70 km, total area covered ~ 50,000 km2), with 10-20 samples obtained per site. Most of the collection responds favorably to progressive demagnetization; a well-grouped characteristic magnetization (ChRM) of NNW declination and moderate negative inclination (normal polarity) is defined at most sites (e.g., D: 321.2°, I: -56.6°,α95: 3.5°,k: 122.2,N: 14 samples, Site K101; D: 327.8°, I: -56.3°,α95: 3.4°,k: 141.4,N: 13, Site K107; D: 347.3°, I: -53.4°,α95: 6.2°,k: 49.4,N: 11, Site K121). Median destructive fields for the ChRM at well-grouped sites range from ~ 20 to ~40 mT; the ChRM at some sites is essentially fully unblocked by 350C and others show a narrow range of unblocking below ~580C, implying that pyrrhotite and low-Ti magnetite are common ChRM carriers. A grand mean of D: 336.9°, I: -55.7°, α95: 3.8°,k: 66.2, is provided from well-defined mean directions from 22 sites; the paleomagnetic pole of 70.1°N, 273.5°E, VGP ASD of 4.6°, is consistent with other studies of both intrusions and lavas of the Karoo province. The dominance of normal polarity in the lower sills sampled in this study may imply their emplacement over a short time interval at about 184 Ma. An evaluation of all available and well-dated paleomagnetic data from the Karoo province and comparable age intrusions in Africa does not reveal a consistent pattern regarding defining the inception of the TPW event.