An example of very consistent brittle deformation over a wide intracontinental area: The late Pan-African fracture system of the Tuareg and Nigerian shield

Brittle deformation during the latter part of the Pan-African orogeny in the Hoggar and Nigeria was characterized by a conjugate strike slip fault system, consisting of NE—SW dextral and NW—SE sinistral trending sets of faults. This system is considered to be the result of a horizontal maximum stress axis $\text{σ}{}_1$ orientated E—W.In the crystalline basement, the faults have offset the N—S trending Pan-African mylonitic shear zones, and have cut the late orogenic granites (Taourirt) and ring complexes of Adrar des Iforas.In the western Hoggar, folding of the post-orogenic molassic “série pourprée” considered as Cambrian in age (Caby, 1970), is probably associated with the same stress regime. A similar fault system also affects the Phanerozoic sedimentary cover of the North-Saharian platform, the Ténéré desert, and the Djado basin. Later reactivation of this fault system by phanerozoic epeirogenic events, and recent uplift is demonstrated.The geometry of this late Pan-African brittle deformation is very consistent over a large area and may reflect the stress field operative during indentation of a “plastic” Pan-African shield, by a more rigid West-African craton.     

A stochastic approach to sieving kinetics

A model of the batch sieving process based on a stochastic approach is presented. The model indicates that there is an inherent uncertainty associated with sieving results which is a function of the mean sieving rate, degree of choking and sieving time. The model suggests that choking of sieve apertures may lead to a disproportionately high degree of uncertainty. Experimental data for a sonic sifter device gave good semi-quantitative agreement with the model, while that for a Rotap device gave qualitative agreement.     

The geochemistry of basalts from a back-arc spreading centre in the East Scotia Sea

Rapid sea floor spreading has taken place over the last 8 Myr behind the South Sandwich island arc, producing a regular set of magnetic lineations. Suites of fresh basalts have been dredged from four widely separated localities along the spreading axis. Dredges 20 and 23 yielded sub-alkaline olivine tholeiites, dredge 22 recovered vesicular tholeiites with minor normative olivine, while dredge 24 contained a fractionated suite of highly vesicular quartz-normative basalts with higher $\text{Fe}\text{Mg}$. The concentrations of the incompatible elements Ti, P, Zr, Hf, Nb, Ta, Y and the REE increase systematically from dredge 24 through dredges 22 and 20 to dredge 23 and there is a comparable increase in $\text{Ce}{}_{\mathrm{N}}\text{Yb}{}_{\mathrm{N}}$. Quantitative modelling suggests that all the basalts can be derived from an essentially similar mantle source (with respect to these elements) through varying degrees of partial melting, but involving some residual clinopyroxene. Basalts from dredge 24 have unusually low concentrations of Ti, P, Zr, Nb, Y, REE and Ni, similar to the values in arc tholeiites, and the more primitive dredge 24 liquids seem to have been generated through high degrees of partial melting (~ 30%) leaving a dunitic residue. Transitional arc tholeiite characteristics are also apparent in the relatively high K, Rb, Ba contents and ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios of dredge 24 and 22 basalts, though Nd isotope ratios are uniform. It is considered that fluids derived from the dehydrating subducted slab may have locally penetrated the source regions of the back-arc basalts, carrying K, Rb, Ba and seawater-enriched ⁸⁷Sr, and producing conditions of magma generation similar to that of arc tholeiites. However, it is unlikely that the sources for these and other marginal basin basalts differ fundamentally from the range of mantle sources feeding normal mid-ocean ridges.     

Trace elements in the Allende meteorite—IV. Amoeboid olivine aggregates

INAA data for Ca, Sc, Hf, La, Ce, Sm, Eu, Tb, Yb, Lu, Os, Ir, Ru, Na, Cl, Br, Fe, Mn, Cr, Co, Au, As, and Sb are presented for ten amoeboid aggregates from the Allende meteorite. Only one lacks olivine. Seven of the remainder, as a group, have cosmic proportions of refractory lithophile and siderophile elements and appear to have formed when coarse-grained Allende inclusion material underwent partial reaction with a low-temperature nebular gas and mixture with FeO-rich olivine. The other two have highly fractionated abundances of refractory elements relative to one another compared to Cl chondrites, including Group II REE patterns, and probably formed by the mixing of fine-grained Allende inclusion material with FeO-rich olivine. Non-refractory siderophile components are also different in composition in each type of amoeboid olivine aggregate.     

Absolute ages of Quaternary radiolarian datum levels in the equatorial Pacific

Radiolarian assemblages were examined in two Quaternary cores (V24-58; RC11-209) from the tropical Padific Ocean. Eight radiolarian datum levels were identified in each core, and “absolute” ages were estimated for these levels by interpolation between paleomagnetic reversal boundaries previously established for the cores. The tropical radiolarian zonation for the Quaternary proposed by Nigrini (1971) appears to be most useful in terms of the reliability and ease of identification of the proposed zonal boundaries. Our estimated ages for the base of each of these zones are: Buccinosphaera invaginata Zone (Zone 1): 210,000 yr BP; Collosphaera tuberosa Zone (Zone 2): 370,000 yr BP; Amphirhopalum ypsilon Zone (Zone 3): 940,000 yr BP; Anthocyrtidium angulare Zone (Zone 4): 1,700,000 yr BP.A comparison of our age estimates with those of Quaternary radiolarian datum levels in cores from other regions suggests that significant diachroneity on a scale of up to several hundred thousand years may exist for some (and perhaps all) of these “events.” Diachroneity is most readily studied and documented in late Neogene cores where the absolute ages of the magnetic polarity reversals are known most precisely, but may also exist (though difficult to recolve) in earlier Cenozoic sediments. The existence of such diachroneity, if demonstrated through further studies, would have significant implications for our understanding of evolutionary patterns of planktonic communities in different biogeographic regions.     

Comment on “the troodos ophiolitic complex was probably formed in an island arc”, by A. Miyashiro

The use of bulk chemical analyses from the Troodos complex to determine the chemical affinity of the ophiolites is unjustified since these rocks exhibit conspicuous alteration in thin section. Very similar rocks on mainland Greece can be shown, on mineralogical grounds, to have been mainly tholeiitic originally, with a subordinate alkaline trend. This is compatible with their origin at an oceanic ridge. Regional geological evidence provides strong support for this origin. However, bulk chemical analyses from them could be used, like those from Troodos, to argue against a mid-ocean ridge origin. This suggests that the use of bulk chemical analyses from these rocks is very unreliable. In the presence of other, more reliable data, inferences from bulk chemistry can be rejected. The Troodos ophiolites are very similar, in both petrology and geological position, to those in Greece and probably originated in the same way.     

Radiolaria as paleoclimatic indicators: Pleistocene climatic fluctuations in the equatorial Pacific Ocean

Late Pleistocene radiolarian assemblages were examined in two cores (V24-58 and RC11-209) from the eastern equatorial Pacific Ocean, using Nigrini's 1970 method of recurrent group analysis. The radiolarian “climatic index” curves obtained by this analysis were in close agreement with fluctuations in carbonate levels established previously for the same cores. This correspondence suggests that equatorial radiolarian assemblages changed significantly in response to Pleistocene climatic oscillations. The radiolarian “climatic indices” of the two cores showed close agreement with each other. Our data indicate that equatorial radiolarian assemblages may prove useful for regional correlations between cores, and for paleoclimatic interpretations.