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Remagnetizations in Late Permian and Early Triassic Rocks from Southern Africa and Their Implications for Pangeareconstructions

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Remagnetizations in Late Permian and Early Triassic Rocks from Southern Africa and Their Implications for Pangeareconstructions 412 Earth and Planetary Science Letters, 79 (1986) 412-418 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 151 Remagnetizations in Late Permian and Early Triassic rocks from southern Africa and their implications for Pangea reconstructions Martha M. Ballard ‘, Rob Van der Voo * and I.W. HZlbich 2 ’ Department of Geological Sciences, University of Michigan, Am Arbor, MI 48109 (U.S.A.) ’ Department of Geology, University of Stellenbosch, Stellenbosclr (Republic of South Africa) Received March 31,1985; revised version received June 24, 1986 -A paleomagnetic study of late Paleozoic and early Mesozoic sedimentary rocks from southern Africa suggests wide-spread remagnetization of these rocks. Samples of the Mofdiahogolo Formation in Botswana and of the Lower Beaufort Group in South Africa were treated using thermal, alternating field and chemical demagnetization. The Mofdiahogolo redbeds show a univectoral decay of the remanence revealing a characteristic direction of D = 340°, I = - 58O, k = 64, a9s = 12O. The Lower Beaufort sandstones, using thermal and alternating field demagnetization, show a very similar direction of D = 337O, I = -63”, k = 91, ags = 6O. A fold test on the Beaufort rocks is negative indicating that this magnetization is secondary and acquired after the Permo-Triassic Cape Belt folding event. Previous studies have reported similar directions in the Upper Beaufort redbeds as well as in the Kenyan Maji ya Chumvi Formation of Early Triassic age. The poles of these studies have been used in testing the validity of the various Pangea reconstructions for the Late Permian and the Early Triassic. Our results suggest that these poles may also be based on remagnetized data and that their use to document the position of Gondwana in Pangea reconstructions should be treated with caution. 1. Introduction base for the Late Permian and the Early Triassic is small and, more importantly, the ages of the mag- Paleomagnetic data indicate that the Wegener netic directions for these rocks are not well-con- fit, or Pangea A as it has come to be known, while strained. The present paleomagnetic study of the probably valid for the earliest Jurassic, is not folded Late Permian Lower Beaufort Group from supported by the paleopoles for late Paleozoic and the Karoo Basin in southern Africa serves to test Early Triassic time [l]. Several alternative pre- the validity of the previously published poles for Jurassic reconstructions have been proposed. Pan- this same time period. gae A2 [2] fits northwestern South America tightly into the present day Gulf of Mexico and juxtapo- 2. Geology ses northwest Africa against the eastern U.S. sea- board. This is achieved by rotating Gondwana The Cape Fold Belt is part of a Gondwana-wide 20° clockwise about a pole in the Sahara relative orogen that is preserved in South America, south- to the Wegener fit [3]. Pangea B, proposed by ern Africa, Antarctica and Australia. In southern Irving [4], requires a 35” clockwise rotation of Africa, the deformation stylk is predominantly one Gondwana relative to Laurasia. This places north- of east-west trending major folds that verge to the western South America against the eastern U.S. north (Fig. lb). Although metamorphic giade in- seaboard and Africa somewhere south of the Bal- creases gradually southward up to the lowermost kans. Pangea A2 agrees well with Carboniferous greenschist facies, the fold intensity may change and Early Permian paleomagnetic data while the abruptly across the trend to produce a zoned Late Permian and Early Triassic data are in better orogen. This is also reflected in two types of Pan agreement with Pangea B [l]. African basement underlying the Cape Fold Belt Thus, paleomagnetic data cannot at present as inferred from geophysical studies of the region. eliminate either reconstruction. However, the data An east-west trending highly conductive crustal 0012-821X/86/$03.50 0 1986 Elsevier Science Publishers B.V. Karoo Basin b) I LLITE CRYSTALLINITY Group Formation C&rite Beaufort ‘.,:,‘. 7. .: $ . .: . ---------- Karoo Ecca ----_---__ -_--_----_ bYb AAA # Witteberg Cape Bokkeveld Table Mountain Granitoids Kaaimans hkm) nkm Kango (nk) l-k DIAGI1AMMA’iIC CRUSTAL SECI’ION TIIIIOUGII TIIE CAPE FOLD BELT Fig. 1. (a) A generalized geologic map of the Karoo and Tuli Basins in southern Africa. Inset maps show sampling locations along with sampling sites, represented as closed 2 circles, in greater detail. (b) Diagrammatic cross section through the Cape Fold Belt after Walbich [7]. The Bcaufort samples were taken from an area just north of the Beattie w Magnetic Anomaly and south of the dolerite intrusion. 414 segment today underlies the north-central zone of north traverse from gently folded rocks nearer the the fold belt and its basement (Fig. lb). The Cape orogen toward the undeformed, horizontal northern boundary of the segment at about 32’45’s strata further north (Fig. la). along 22”30’E coincides with the Beattie magnetic Karoo deposits are also found in the partially anomaly [5]. De Beer et al. [6] interpret this seg- fault-bounded Tuli Basin in eastern-central ment as weak, heavy crust that located the axial Botswana (see Fig. la). These deposits include the zones of the Cape and Karoo Basins and, during undeformed mudstones of the Mofdiahogolo For- the Cape orogeny, helped to control tectonic mation, which are interbedded with more aren- movements in time and space. aceous layers and a few ironstones [13] and lie The available evidence for the Cape folding uncomformably on Precambrian gneissic base- points to a short-lived intra-cratonic erogenic epi- ment. These are unconformably overlain by coarse sode [7]. The conspicuous absence of synorogenic elastics which have been correlated with the Seswe magmatic activity has also been explained by a Formation. The Seswe Formation, in turn, is inter- flat-plate subduction model which positions a sub- preted as the successional equivalent of the Ful- duction zone - 1000 km to the south of the Cape ton’s Drift Mudstones found in the continuation Fold Belt [8], Halbich et al. [9] interpret the Cape of the Tuli Basin into Zimbabwe where Glos- folding as a semi-continuous deformation with sopteris, Phyllotheca and Calamites have been four compressive pulses that began in the Early found [14]. On this basis, a Late Permian age has Permian (278 Ma), with concomitant Ecca Group been assigned to the Mofdiahogolo Formation. deposition in the Karoo Basin that developed Capping the entire sedimentary section is a series north of the rising mountain chain. A climax was of subaerial basalts, the Bobonong Lava Forma- reached in the middle Permian (258 Ma) with the tion, with a K/Ar age of 181 f 4 Ma [15]. Lime- beginning of Beaufort sedimentation. The third stones intercalated in the lavas are assigned an (247 Ma-latest Permian) and fourth (230 Ma- Early Jurassic age based on palynomorphologic Late Triassic) erogenic pulses were synchronous study [15]. Fig. 1 shows the locations of four sites with the deposition of the Upper Beaufort beds from which fourteen hand-samples were taken and higher Karoo strata respectively. from the basal mudstones. Underlain by Permo-Carboniferous glaciogenic deposits (Dwyka Formation) and a thick sequence of Permian elastic deposits ranging from starved 3. Laboratory techniques basin muds and turbidites (Ecca Group) that grade upward to fluvial channel facies, the Beaufort Group is made of alternating layers of gray/green Remanent magnetizations of all the samples sands and reddish mudstones becoming coarser- collected were measured using either a Supercon- grained higher in the section. Smith [lo] interprets ducting Technology (ScT) cryogenic magnetome- these deposits as channel and flood basin facies ter or a Schonstedt SSM-1A spinner magnetome- which probably developed in front of the rising ter. To isolate the characteristic magnetic direc- Cape orogen to the south. tions, several stepwise demagnetization techniques The occurrence of Endothiodon and Ciste- were used. Thermal demagnetizations were done cephalus [ll] ties the age of the Lower Beaufort to in a SchSnstedt TSD-1 model oven. A Schonstedt the latest Permian/earliest Triassic. The results of GSD-1 AC Specimen Demagnetizer was used for 39Ar/40Ar age dating of the Lower Beaufort rocks alternating field (AF) demagnetizations. Chemical [9] suggest that the Lower Beaufort was subse- demagnetizations were performed by leaching quently deformed during the fourth pulse of the vertically notched samples in 8N HCl while in a Cape folding event, 230 Ma ago. In the central reduced magnetic field space. part of the Karoo Basin, the Beaufort Group is The magnetic direction remaining after each intruded by Karoo dolerite sheets and dykes, dated treatment was plotted using the demagnetization from 159 to 190 Ma [12]. diagrams described .by Zijderveld [16]. Character- Thirty-seven hand samples from seven sites were istic magnetic directions were, in all cases, de- collected in the Lower Beaufort along a south- termined by principal component analysis [17]. 415 4. Results atures less than 350°C. In the remainder of the samples, the characteristic magnetization was al- All fourteen mudstone samples collected in most completely eliminated at temperatures below Botswana possessed a north-northwesterly and 350°C. Chemical demagnetization revealed a simi- steeply up natural remanent magnetization lar, univectoral decay of the remanence. (NRM). Due to the high coercivity of the mag- The mean characteristic magnetization for all netic carriers, AF demagnetization was ineffective fourteen samples is D = 340” and I = -58”. in cleaning these rocks. Samples from this collec- Sample directions are averaged in Table 1 and tion were treated with either thermal or chemical plotted in Fig. 3b. step-wise demagnetization techniques. With both NRM directions for the Lower Beaufort are all techniques, the magnetic remanence decayed lin- north-northwesterly and steeply up. In AF demag- early to the origin (Fig.
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