A new conglomerate test in palaeomagnetism

The conglomerate test is widely used in palaeomagnetism to date components of natural remanent magnetization with respect to deposition of conglomerates. It has been demonstrated, however, that this test may be positive even if the data are strongly contaminated by a secondary remanence, especially for the commonly used small number of clasts Starkey & Palmer 1970). Here we show with the aid of numerical simulations that different statistical procedures employed in this test have similar low sensitivities to remagnetization. We suggest a new conglomerate test which incorporates additional information on the direction of a secondary palaeomagnetic component which is isolated from either clasts themselves or their host rocks. Numerical simulations show that this new test is about twice as sensitive to remagnetization as the previous procedures and is robust with respect to small errors in the direction of a secondary component.     

Mesozoic palaeomagnetism in Vestfjella, Dronning Maud Land, East Antarctica

Summary. Thermal and alternating field demagnetization of oriented samples of Mesozoic lava flows (200–230 Myr) and dykes (154–172Myr) collected from seven nunataks within the Vestfjella region, Dronning Maud Land, revealed stable directions of magnetization of normal and reverse polarity. Directional distributions of both polarities define tight groups along the same palaeomagnetic axis. Depending on whether the regional westward dip of the lava flows originated prior to, or after the intrusive igneous phase, two or one, respectively, palaeomagnetic pole(s) can be estimated. Both poles, however, are in general accord with previous Mesozoic poles from East Antarctica.     

The palaeomagnetism of the Tertiary igneous complex of Ardnamurchan

Summary. Samples have been taken systematically through the intrusive sequence for the three centres which form the Tertiary central intrusive ring complex. Palaeomagnetic measurements show that all the units are reversely magnetized as are the earlier lavas – an extension of those of Mull and Morvern – which they intrude. This is in contrast to Mull, Skye and Arran, where the igneous sequences have been shown to span at least three polarity intervals, but is similar to Rhum where the only normal units are a few late dykes. Since Rhum and Ardnamuchan both lie on the dyke swarm which passes through Muck, the two complexes are considered to be closely related. If the Ardnamurchan central complex was formed in a single reversed interval the total duration of igneous activity was less than 3 Myr.     

Thermochronometry and palaeomagnetism of the Archaean Nelshoogte Pluton, South Africa

²⁰⁷Pb/²⁰⁶Pb single-grain zircon, ⁴⁰Ar/³⁹Ar single-grain hornblende and biotite, and ⁴⁰Ar/³⁹Ar bulk-sample muscovite and biotite ages from the Nelshoogte trondhjemite pluton located in eastern Transvaal, South Africa, show that this granitoid had a protracted thermal history spanning 3213±4  Ma to about 3000  Ma. Whole-rock ⁴⁰Ar/³⁹Ar ages from cross-cutting dolerite dykes indicate that these were intruded at about 1900  Ma. There is no evidence of this or other, later events significantly affecting the argon systematics of the minerals from the pluton dated by the ⁴⁰Ar/³⁹Ar method.
  The pluton has a well-defined palaeomagnetic pole which is dated at 3179±18 (2 σ ) Ma by ⁴⁰Ar/³⁹Ar dating of hornblende. This pole (18°N, 310°E, A ₉₅=9°) yields a palaeolatitude of 0°, significantly different from other Archaean poles from the Kaapvaal Craton. The palaeolatitude difference implies that there was significant apparent polar wander during the Archaean. A second, overprinting magnetization seen in the pluton is also seen in the lower-Proterozoic dolerite dykes, and is consistent with other lower-Proterozoic (2150–1950  Ma) poles for southern Africa.     

The British Tertiary igneous province: palaeomagnetism of the Arran dykes

Summary. Five hundred and sixteen sites in Arran were sampled. After cleaning by heating and/or alternating fields 87 per cent yielded directions with α₉₅ < 10°, comprising 435 dykes, 12 sills and one granite; polarities could be determined for some of the remainder. Directions are thought to be primary and on average are shallower than corresponds to the mean centred axial dipole, with many shallow reversely-magnetized dykes present.
Seventeen per cent of the dykes are normally magnetized, 5 per cent have intermediate directions. Polarity does not correlate with petrography, thickness or trend, but does depend upon location. In particular, on the NE coast up to 70 per cent are normal, and possibly belong to a different swarm.
Normal polarities were found for both sites in the Northern Granite, for Holy Island and for some of the sills. In conjunction with the known stratigraphy this shows that the rocks were formed during an R-N-R sequence of polarities, or more probably R-N-R-N. Using the radiometric age of 58 Ma for the Northern Granite and the best estimate of the polarity time-scale the duration of activity was probably in the range 0.5 to 3.4 Ma if three polarity periods were involved, and 3.4 to 4.4 Ma if four were involved.     

Analysis of the modes of directional data with particular reference to palaeomagnetism

Summary. Advantages of using the mode in analysis of palaeomagnetic vectors are discussed, and a computer technique is described for contouring and precisely locating the modes of vector distributions that may be highly skewed. In contrast to conventional determinations of the mode, unit vectors from a given data set are treated not as discrete points, but as identical Fisherian probability density functions defined (at an angle θ from the unit vector) by: p = exp [ sk (cos θ– 1)], where kis the estimate of the Fisherian concentration parameter, and s is an arbitrarily assigned 'smoothing parameter'. A grid, representing the cumulative probability distribution of the total sample of vectors, is contoured to provide a graphical display of the distribution around the most probable value, the mode. By repeatedly contouring the same sample of vectors with successively larger values of s, and by treating the mode as a vector with length given by the total probability value at the mode, 'progressive modal diagrams' can be constructed, to aid in determining the stable position of the mode of skewed distributions. In addition, a new statistic β₉₅ is suggested as an error estimator for the mode. The statistic β₉₅ is derived from the largest subset of the total sample that has a mean identical with the mode of the total sample; this statistic is defined as the Fisherian half-angle of the cone of 95 per cent confidence for the mean of this subset.     

The palaeomagnetism and magnetic fabric of sediments from Peak Cavern, Derbyshire

Summary. This paper describes the magnetic remanence and susceptibility anisotropy of 42 samples of laminated clay from Peak Cavern. The sediments are shown to have acquired a stable remanent magnetization due to the depositional alignment of magnetite grains. The magnetic fabric data confirm the existence of alteration within a reddened surface layer. A record of geomagnetic secular variation in the underlying clay is compared to a dated lake sediment magnetostratigraphy to suggest that deposition occurred around 7000 yr bp . A method for estimating water current directions based on the remanence and magnetic fabric data is critically assessed before deducing the sense of palaeoflow in the cave.     

The palaeomagnetism of the central zone of the Lewisian foreland, north-west Scotland

Summary. Three principal directions of magnetization are recognized in the central part of the Lewisian metamorphic terrain of north-west Scotland. The first ('A') magnetization is a high blocking temperature component residing in magnetite and imposed during post-Laxfordian uplift and cooling. Fifty sites yield an overall mean D = 285.9°, I = 54.9° and palaeomagnetic pole at 273.2° E, 37.6° N ( dp = 3.7°, dm = 5.2°); this magnetization was probably acquired at crustal depths of 6–10 km and is linked to K—Ar uplift ages averaging 1650–1625 Ma. The second ('B') magnetizations are defined by E—W directions and also reside in high blocking temperature components; they are, however, dipolar, have some properties distinct from the 'A' magnetizations, and are correlated with late stages in the history of the complex at 1400–1200 Ma. The third ('C') NE directed magnetizations reside predominantly in low blocking temperature components in pyrrhotite and possibly maghemite, and were probably acquired at a late stage of the regional uplift; they do not correlate with post-1450 Ma magnetizations from the Laurentian Shield and probably relate to the as yet undefined interval 1600–1450 Ma. The collective palaeomagnetic data and certain geologic data suggest that the Lewisian foreland should be rotated by 30° clockwise about a local axis of rotation on the conventional reconstruction of the North Atlantic continents; this rotation is associated with Lower Palaeozoic trans-current movements and may be related to a fourth ('D') magnetization of viscous origin.
A collective assessment of 1850–1600 Ma palaeomagnetic data for the Laurentian Shield defines a large apw loop; there is widespread agreement between data from the constituent structural provinces of the Shield although different metamorphic regions define complementary segments of the loop related to uplift over different intervals of time.