Paleomagnetism of the Lesotho basalt, southern Africa

Kosterov, Andrei; Perrin, Mireille

doi:10.1016/0012-821x(96)00005-2

Cited by 28 publications

(50 citation statements)

References 33 publications

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“…Such trends are well fitted by a statistical model of the geomagnetic field in which the fluctuations of the non-axial dipole components are isotropic and independent from those of the axial dipole (Camps & Prévot 1996). Taking into account the weakness of the local steady palaeofied in the Early Jurassic (24 ± 11 µT according to Kosterov & Perrin 1996), the predicted transitional field intensity would have been particularly low. 9) suggests that the maximum decrease in the palaeofield intensity might have reached 80-90 per cent during the Lesotho reversal, which is comparable with the result obtained from direct palaeointensity measurements for the well-documented Cenozoic Steens Mountain reversal (Prévot et al 1985b).…”

Section: Discussion a N D C O N C L U S I O N Smentioning

confidence: 74%

“…The palaeogeographic reference frame is pinned to the global palaeopole position for the period 175-200 Ma (Prévot et al 2000). Star show the rotated position of the local palaeomagnetic pole obtained by Kosterov & Perrin (1996) just during the most transitional stages, the Lesotho reversal is incompletely recorded. No VGP very close to the palaeoequator is observed.…”

Section: Discussion a N D C O N C L U S I O N Smentioning

confidence: 99%

“…Each of these directions defines a 'palaeomagnetic unit'. Following the results of the palaeosecular variation study of the normal and reversed magnetozones of the Lesotho Basalt carried out by Kosterov & Perrin (1996) The transition path is obviously discontinuous. The limits of the transition zone were defined from the reversal angle (Prévot et al 1985b) of each palaeomagnetic unit.…”

Section: E S C R I P T I O N O F T H E C O M P O S I T E G E O M a mentioning

confidence: 97%

“…For each specimen, the direction of the characteristic remanent magnetization (ChRM) was calculated using principal-component analysis (Kirschvink 1980). Maps redrawn from Kosterov & Perrin (1996). no really reliable palaeointensity data could be obtained. Unfortunately, due to the poor magnetic stability of most of the Lesotho lava flows during heating (Kosterov & Prévot 1998 Figure 2.…”

Section: Laboratory Proceduresmentioning

confidence: 99%

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Revisiting the Jurassic geomagnetic reversal recorded in the Lesotho Basalt (Southern Africa)

Prévot¹,

Roberts²,

Thompson³

et al. 2003

Geophysical Journal International

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SUMMARY We carried out a detailed and continuous palaeomagnetic sampling of the reversed to normal geomagnetic transition recorded by some 60 consecutive flow units near the base of the Lesotho Basalt (183 ± 1 Ma). After alternating field or thermal cleaning the directions of remanence are generally well clustered within flow units. In contrast, the thermal instability of the samples did not allow us to obtain reliable palaeointensity determinations. The geomagnetic transition is incompletely recorded due to a gap in volcanic activity attested to both by eolian deposits and a large angular distance between the field directions of the flows underlying or overlying these deposits. The transition path is noticeably different from that reported in a pioneering work carried out in 1962. The most transitional virtual geomagnetic poles (VGPs) are observed after the volcanic hiatus. Once continents are replaced in their relative position 180 Myr, the post‐hiatus VGPs cluster over Russia. However, two successive rebounds from that cluster are found, with VGPs reaching repeatedly the Eastern Asia coast. Thus, the VGP path is not narrowly constrained in palaeolongitude. The decrease in intensity of magnetization as the field deviates from the normal or reversed direction suggests that the decrease in field magnitude during the reversal reached 80–90 per cent. We conclude that although the reversal is of a dipole of much weaker moment than that which existed on average during Cenozoic time, the characteristics of the reversing geodynamo seem to be basically similar.

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Section: Discussion a N D C O N C L U S I O N Smentioning

confidence: 74%

Section: Discussion a N D C O N C L U S I O N Smentioning

confidence: 99%

Section: E S C R I P T I O N O F T H E C O M P O S I T E G E O M a mentioning

confidence: 97%

Section: Laboratory Proceduresmentioning

confidence: 99%

See 2 more Smart Citations

Revisiting the Jurassic geomagnetic reversal recorded in the Lesotho Basalt (Southern Africa)

Prévot¹,

Roberts²,

Thompson³

et al. 2003

Geophysical Journal International

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“…The palaeomagnetic investigations of van Zijl et al (1962), confirmed by Kosterov and Perrin (1996) and Hargraves et al (1997), have firmly established the occurrence of a magnetic reversal in the succession of lavas of the Drakensberg Group. The lowermost section of 300 m is reversely magnetized followed by a 200 m thick transitional section with intermediate polarities and an upper section of 900 m, which is normally magnetized.…”

Section: Zonementioning

confidence: 80%

Physical characteristics of the Karoo sediments and mode of emplacement of the dolerites

Zijl

2006

South African Journal of Geology

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The mode of emplacement of the dolerites and their relation with the overlying basalts are revisited in the light of new information provided by a recent resistivity study of the structure of the Karoo basin and augmented by an analysis of age and palaeomagnetic data. The different styles of intrusion from the bottom to the top are explained in terms of anisotropy, lithology and increasing upward bending stresses accompanied by a decrease in overburden thickness. The lowermost zone 3 consisting of flat lying dolerite sills of large extent occurs in well-laminated, homogeneous shales with a high degree of anisotropy. The thick doleriterich middle zone 2 extending from the Upper Ecca Subgroup through the Beaufort Group and partially into higher Formations is characterised by the occurrence of large basin structures of dolerite. A model, which assumes a central dyke as source and based on the principle that the propagating magma front always follows the path of least resistance is developed to explain the formation of a basin structure including the steepening and flattening of the rim. The model also explains the variations in shape and size such as are encountered in the field by taking into consideration the structure of the nexus of lenticular sandstones intercalated with shales and mudstones occurring throughout this zone. The dominant style of intrusion of, at least, the upper part of zone 1 that extends from the surface to a depth of about 700 m is that of dykes and steeply dipping sheets. Another striking feature of this zone is its dolerite-poor nature. An analysis of the magnetic polarities of the dolerites and basalts leads to the conclusion that the reversely polarized dolerites, which only occur in zone 1 were the feeders to the outpouring of the basalts. The younger normally polarized dykes that cut through the entire basalt succession are associated with the period of cooling and contraction following the main phase of dolerite intrusion. The implications, aided by well resolved age determinations are that the duration of intrusion of the Karoo magmatic event was short. Finally, the significant increase in both the amount of dolerite and the thickness of the dolerite bearing layer towards the east supports the view of a magma source in that direction.

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Eruptive history of the Karoo lava flows and their impact on early Jurassic environmental change

et al. 2017

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This paper reports new paleomagnetic and geochronologic data from a ~1500 m thick composite section belonging to the Drakensberg group, the thickest remnant of the Karoo lavas in Northern Lesotho. Flow‐by‐flow analysis of paleomagnetic directions reveals 21 magnetic directional groups, corresponding to single eruptive events, and 16 individual lava flows. The new age determinations of lava flows range from 180.1 ± 1.4 to 182.8 ± 2.6 Ma. These data, combined with previous results, allow us to propose that the main part of the Drakensberg group and the Karoo intrusive complex dated around 181–183 Ma may have been erupted over a period as short as 250 kyr and may have coincided with the two main phases of extinction in the Early Toarcian. This scenario agrees well with the discontinuous rhythm of environmental and biotic perturbations in the Late Pliensbachian‐Toarcian interval.

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Paleomagnetism of the Lesotho basalt, southern Africa

Cited by 28 publications

References 33 publications

Revisiting the Jurassic geomagnetic reversal recorded in the Lesotho Basalt (Southern Africa)

Revisiting the Jurassic geomagnetic reversal recorded in the Lesotho Basalt (Southern Africa)

Physical characteristics of the Karoo sediments and mode of emplacement of the dolerites

Eruptive history of the Karoo lava flows and their impact on early Jurassic environmental change

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