palaeomagnetism of early miocene basaltic eruptions in the areas east and west of cairo

Pergamon Joumd of African Earth Sciences. Vol. 21, No. 3. pp. 407.419, 1995

Copyright 0 1995 Elsevier Science Lid

Fmnted in Great Britain. All rights reserved

0899.5362!95 $9.50 + 0.M)

Palaeomagnetism of Early Miocene basaltic eruptions in the areas east and west of Cairo

H. I. LOTFY), 2 R. VAN DER VOO,2 C. M. HALL,2 0. A. KAMEL’ and A. Y. ABDEL AALl

‘Department of Geology, Faculty of Science, El-Minia University, Egypt 2Department of Geological Sciences, University of Michigan, Ann Arbor, USA

(Received 11 October 1994; revised version received 16 June 1995)

Abstract - Two characteristic magnetic components (Al and A2) with distinctive non-overlapping bimodal distributions were palaeomagnetically isolated from what has long been considered as one single phase of post- Oligocene pre-Late Miocene basaltic volcanicity in the ‘east- and west-of-Cairo areas’. Al yields a mean Dec/Inc=198”/-24”, a95=2.6” and K=417 based on the site-means in tilt-corrected coordinates with a corresponding palaeomagnetic north pole at 66”N/167”E and A95=2.3” using site-mean pole averaging. A2 yields a distinct mean Dec/Inc=197”/-51” with a95=3.1” and K=270 corresponding to a palaeomagnetic north pole at 76”I$/lll”E and A95=3”. Aland A2 both pass the tilt test; the two components are never recorded at the same site and no in- termediate direction was observed.

The bi-modal grouping of the Al-A2 directions and their non-overlapping cones of 95% confidence indicate that the group means of Al and A2 are significantly different in tilt-corrected co-ordinates at the 95% level df confidence and represent two different ages of magnetization. Therefore, we argue that two distinct basaltic episodes occurred, which were not previously distinguished in the areas ‘east- and west-of-Cairo’. 39Ar/40Ar whole rock analyses appear to yield Early Miocene ages, but there are too many problems with the argon release spectra to rely on the integrated ages for more precise temporal definition.

A petrological study subsequently carried out on the two distinct basaltic episodes revealed that the Al episode is represented by very fine-grained porphyritic olivine-bearing basalts with dust-like groundmass rich in opaques made up of homogeneous magnetite grains that are rarely intergrown with homogeneous ilmenite lam&lae. In contrast, the A2 episode consists of medium- to coarse-grained holocrystalline non-porphyritic doleritic basalts with inhomogeneous magnetite and ilmenite grains that show a variety of intergrowths.

The two poles of this study are clearly virtual geomagnetic poles because of imperfect averaging of the secular variation of the geomagnetic field due to the limited number of exposures and the rapid cooling of the flows. Even so, a comparison with the APWP of Africa shows that the segment joining the two poles of the present study has a similar pattern as that of the APWP derived from DSDP sediments of the Atlantic part of Africa or those rotated from the North American craton and stable Europe and allows the authors to calibrate part of this APWP as Early Miocene. The available palaeomagnetic results imply a northward movement of Africa during the Tertiary followed by a clockwise rotation with respect to the axial geocentric dipole.

Resume - Les deux composantes magnetiques caracteristiques (Al et A2) etudiees possedent des distributions bimodales distinctes sans recouvrement. Elles ont ainsi et6 isolees paleomagnetiquement de ce qui a ete longtemps consider& comme une seule phase de volcanisme basaltique post-ohgoceneet p&-Miocene sup&eur dans lgregion uericairote la YE et a l’W du &ire). Al fournit une movenne DeclInc=198”/-24”, a95=2.6” and K=417. en se basant 1 I , I sur les moyennes de chaque site dans des coordonnees corrigees pour l’inchnaison. Ceci correspond a un nord paleomagnetique de 66”N/167”E avec A95=2.3” en utilisant une moyenne des poles moyens de chaque ‘site. A2 foumit une moyenne distincte (Dec/Inc=197”/-51” avec a95=3.1” and K=270), ce qui correspond a un nord pal6omagnetique de 76”N/lll”E avec A95=3”. Al et A2 ont tout deux subi le test de l’inclinaison; les deux composantes n’ont jamais Cte enregistrees sur le meme site et aucune direction intermediaire n’a et6 observee.

La disposition bimodale des directions Al et A2 et l’absence de recouvrement de leur cane a 95% de confiance indiquent que les moyennes de Al et A2 sont significativement differentes dans les coordonnees corrig6es pour l’inclinaison. Ils representent done deux ages differents de magn&isation. En consequence, nous soutenons l’existence de deux episodes basaltiques distincts, ce qui n’a jamais ete propose pour la region pericairote. Des analyses 39Ar/40Ar sur roche totale foumissent des ages Miocene inferieur mais les spectres de liberation de l’Ar sont affect& de trap de problemes que pour posseder une definition temporelle suffisante dans le probleme qui nous occupe.

L’etude petrologique qui a et6 menke sur les deux groupes distingues ci-dessus revele que l’episode Al est compose de basaltes a olivine porphyrique a grains tres fins, avec matrice d’aspect poussiereuse et sont riches en opaques (grains de magnetite homogene rarement en intercroissance avec des lamelles d’ilm&ite homogene). Par contre, l’episode A2 cons&e en basalte doleritique non porphyrique moyennement a grossierement grenu, sans verre, et comprenant des grains de magnetite et d’ilmenite heterogenes B structures d’intercroissances variees.

Les deux poles mesures dans cette etude sont clairement des poles geomagnetiques virtuels etant donne la moyenne imparfaite de la variation seculaire du champ geomagnetique resultant du nombre lirnite d’afflemements et du refroidissement rapide des coulees. N&mrnoins, une comparaison avec l’APWP de l’Afrique montre que le segment joignant les deux poles de cette etude est similaire h I’APWP construit a partir des sediments D!SbP de la partie atlantique de l’Afrique ou a partir de ceux, apres rotation, du craton d’Am&ique du Nord ou de I’Europe

408 H. I. LOTM et al.

stable. Ceci permet d’attribuer cette partie de I’APWP au MiocPne infhieur. Les rksultats p&omagn&iques disponibles indiquent un mouvement de YAfrique vers le nord au Tertiaire, suivi par une rotation dans les sens des aiguilles d’une montre en ce qui conceme le dipBle ghcentrique axial.

INTRODUCTION

A review of published Tertiary palaeomagnetic poles for Africa reveals that the problem is not the scarcity of the data but the scatter of these poles and the overlap of their circles of confidence to the extent that they cannot define a clear APWP for Africa during the Tertiary (Table 1, Fig. 1). Although many of these poles are probably based on primary magnetizations, their obvious dispersion can be explained in several ways:

i) Ignoring the use of a sun compass for the orientation of the highly magnetic basaltic rocks that con- stitute the main source of the African palaeomagnetic database leads to likely azimuthal errors in the observed palaeomagnetic declinations.

ii) Neglecting the application of structural correc- tions may lead to anomalous directions. This is of im- portance for basaltic flows in tectonically active areas that are characterized by rift-related extensional faulting and rotational sliding of flows along fault planes, such as those from the Ethiopian, Kenyan and Red Sea rifts and the Gulf of Suez region.

iii) Bracketing of the poles within a wide and of- ten debatable age range causes scatter in the pole distribution. In the case of the Ethiopian traps, for ex- ample, ages were quoted by McElhinny et al. (1968) as being Upper Cretaceous to Eocene on the basis of K-Ar determinations of 69 Ma. This age was later ar- gued by Brock et al. (1970) as being not older than Eo- cene-Oligocene.

iv) Applying stepwise demagnetization to only a few pilot samples followed by single-step demagnetization to all samples at one blanket-level may fail to remove unwanted overprints. This can result in scat- tered characteristic directions if the overprint’s hard- ness varies in different samples.

v) Preferring the use of Alternating Field (AF) demagnetization despite its inability to completely demagnetize the samples and/or resolve all magnetic components as well as its possible introduction of a spurious Anhysteretic Remanent Magnetization (ARM; Irving et al., 1961; Doe11 and Cox, 1967) will affect the obtained directions and may aggravate problem (iv) discussed above. The present study is, therefore, designed in order to isolate and resolve accurate and reliable palaeomagnetic directions from the Tertiary basaltic flows in the areas to the east and west of Cairo, through the application of sophisticated sampling, demagnetization and data analysis techniques, by avoiding the above- mentioned causes of scatter.

GEOLOGY AND SAMPLING

A detailed surface geological study was carried out (Lotfy, 1992) as a preliminary step before sampling the basalt for palaeomagnetic study, in order to delineate the stratigraphical, geomorphological and structural characteristics of the areas to the east and west of Cairo (Fig. 2). It was found that, within the stratigraphic sections exposed in the ‘east- and west- of-Cairo areas’, which range from Upper Cretaceous to Quaternary, the basaltic flows always overlie pre- Lower Miocene rocks but never penetrate Middle Miocene or younger strata.

The structural patterns in the investigated areas, careful analysis of low-altitude aerial photographs and detailed field studies revealed that the ‘east-of- Cairo area’ (Fig. 2a) reflects lateral tensional stresses resulting in differential vertical movements, whereas the ‘west-of-Cairo area’ is strongly affected by an Up- per Cretaceous compressional phase, which resulted in the formation of the Abu Roash anticlinal structure as a characteristic landmark (Fig. 2b).

The ‘east-of-Cairo area’ (Fig. 2a) is dominated by high-angle, step-like normal faults that form ex- tended en echelon belts with two conspicuous trends: an east-west and a northwest-southeast trend. These trends usually swing and merge into one another, controlling the landscape and expose structural fea- tures including folds and igneous activity. Folds are subsidiary to faults and follow their trends (east-west and northwest-southeast). These folds are not proper folds due to lateral crustal compression, but rather they are tension-controlled, low-angle, asymmetric, wave-like noses, undulations, rolls, fault-drags, shallow superjacent warps and monoclines. They are seemingly just surface reflections of the draping of the section over subsurface normal fault blocks and are best explained by the stretching of strata to ac- commodate the strains resulting from subsurface vertical movements. To the west of Cairo (Fig. 2b), the most prominent structure is the Upper Cretaceous Abu Roash anticlinal structure, which forms an element of the east- northeast-west-southwest trending ‘Syrian arc sys- tem’ of compression-related folds that traverse northeast Africa and extend northeastward to the Levant. It is a set of complex en echelon southwesterly plung- ing, intensively faulted and overthrusted, nearly parallel anticlines and intervening synclines, that owe their origin to an Upper Cretaceous folding episode that affected the region during the ‘Laramide’ revolu- tion. Faults, secondary to folding, are either thrust or

Palaeomagnetism of Early Miocene basaltic eruptions in the areas east and west of Cairo 409

Table 1. Tertiary palaeomagnetic poles of Africa

SchuIt et al. (1981) Hussain and Azir (1978) &huh (1974) Hussain and A& (1983) Mussett and Ade-HaII (1975) Brock ef al. (1970) Mean of 4 studies in Lotfy (199; Mean of 5 studjes in Lotfy (199; Mean of 4 studies in Lotfy (199; Ressetar et al. (1981) Hussain et al. (1978) Raja and Vise (1973) Wassif (1991) Wassif (1989) Wassif (1988) ReiBy et al. (1976) Pate1 and Raja (1979) Watkins (1973) Pate1 and Gacii (1972) Bobier and Robin (1969) Reilly et al. (1976) Pouchan and Roche (197l) Schult ans Soffel(l973) Storetvedt et al. (1979) Hamzeh and WestphaI (1973) Schult (1974) Schult and Soffel(l973) Ade-HaB et al. (1975) ReiIIy et al. (1976) Pate1 and Raja i(1979)

Vo. ILocation and Formation Aee N Dec/Inc PaIaeouole A95 Reference 1A IBaharya Oasis sediments, Fe ore, 28.2N, 28.9E U. Eocene 7Si 188j-42 821144 7 18 Baharya Oasis basalt, 28.2N, 28.9E CU. Eocene 2Si 191/+5 2 Ethiopian South Plateau, basalt, 9.1N, 41E 25-40 Ma 22Si 192/+1 3 East El-Oweinat basalt, Egypt, 23.2N, “9.5E Tertiary 5Si 12/+24 4 Ethiopian Plateau lava, 24N, 39E 27.7f1.6 Ma 139Si 5 West Ethiopian flood basalt, 9.5N 38.5E 49f15Ma 20Si 187/+7 6 Abu ZaabaI basah, Egypt, 20.18N, 21.11E Oligocene 132sa 202/-58 7 G. Qatrani El-Fayoum basalt, Egypt, 29.42N, 30.4OE 27f3 Ma >llSi 203/-59 8 Abu Roash basalt, Egypt, 30.02N, 31.08E Oligocene 114sa 198/-57 9 Quesir, tholeiitic basalt, Egypt, 25.9N, 34.4E 25.5-34 Ma 3Si 211/-39

10 Abu Tereifiya baslat, Egypt, 25.9N, 32.1E Eo-OIigcen 12Si 188/-21 11 Tororo ring complex, Uganda 28-58 Ma 3Si S/-25 12A Westcentral Sinai basalt-NukhuI, Egypt 22-25 Ma 8Si 174/-61 128 Westcentral Sinai basalt-w. Taiba, Egypt 22.8fl Ma 5Si 188/-10 12C West-central Sinai basalt-c. Wadi MatuBa, Egypt 23.7*2 Ma 4Si 194/-56 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Turkana volcanics, Kenya, l.OS, 36E Narosura/Magadi voicanics, Kenya Canary Islands volcanics, 30N, 343E Kapiti phonolith, Kenya Algerian volcanics Rift valley basalt, Kenya, IS, 38E Afar and Issas volcanics, Ethiopia Jebel soda basalt, Libya, 28.7N, 15.6E Canary Islands volcanics Moroccan volcanics DanakiI Depr. volcanics, Ethiopia, 12.7N, 42.5E Jebel Nafousa basalt, Libya Garian basalt, Libya, 32N, 13.4E Rift valley basalt, Kenya, IS, 36E Narosura/Magadi volcanics, Kenya

14-32Ma 62Si 4/+2 12-15 Ma 14Si 182/-19 5-25 Ma 99Si 4/+31

12-13 Ma 12Si 189/+4 Miocene 13Si 1/+55 11-13 Ma 22Si 2/4

Eo-Miocene 24Si 194/-21 10.5-12 Ma 12Si 180/-32

Miocene 51Si 184/-31 6-9 Ma 5Si 175/-47

Pliocene 26Si 174/-S 4alOMa 20Si 183/-48 Pliocene 23Si 182/-50 1.8-7 Ma 102Si 3/-5 1.6-7 Ma 161Si 175/+5

IRift valley volcanics, Kenya, IS, 36E O-l.8 Ma 54Si l/-2 89/1&l 3 ReiIIy et al. (1976) No. corresponds to those of Fig. 1. N=number of samples (sa) or sites (Si). Dec/Inc (in degrees) are the declination and inclination.

Palaeopole coordinates are in north latitude and east longitude. A95 is the statistical parameter associated with the mean palaeopole.

58/187 75/170 74/160 82/200 81/168 76/93 68/87 74/93

62/126 69/189 76/196 79113 65/58 76/89

85/163 80/34

82/114 81/118 88/154 87/187 76/135 78/196 78/144 83/212 80/258 85/152 88/125 871148 84/297

6 6 4 4 8 7 6

22 5

10 10

8 2 9 4 17 2 6 5 7 3

14 3 4 5 2 4

reverse faults associated with and simultaneous to the development of the Abu Roash anticlinal structure, or are younger normal faults along the same trends dominating the area to the east of Cairo.

A total of 348 separately oriented blocks were collected at 29 basaltic exposures in the areas to the east and west of Cairo (Fig. 2). The structural atti- tudes (strike/dip) were easily measured from sedimentary strata to allow, in a later step, the structural rotation of magnetic vectors back to their hypotheti- cal pre-tilt horizontal position and the application of the tilt test to constrain their age of magnetization. A sun compass was used exclusively for orientation to avoid the azimuth errors inherent in the magnetic compass directions under the effect of the dominating remanent magnetic field of the strongly magnetized basal& The direction of true geographic north was then calculated using the sun compass readings and the nautical almanac. To average-out random orientation errors, 12 samples were collected from each site and this density was maintained at alI sampling sites. Due to the remarkable homogeneity of the flows, only one core needed to be extracted from each block sample, which then was used to slice only one stan-

dard 2.2 height x 2.5 diameter specimen. After trim- ming, the specimens were stored in a magnetically shielded room with a rest magnetic field of less than 200 nT at the University of Michigan alaeomagnetic

iZo laboratory before and during all la, ratory measurements in order to avoid induction of secondary viscous magnetizations.

LABORATORY METHODS AND PALAEOMAGNETIC RESULTS

The measurement of the NRMs of all the samples used a Schonstedt SSM.lA spinner magnetometer housed in the field-free room. Intensities ranged from 14 to 26,000 mA m-i.

Four prototype samples were then selected from the twelve samples collected at each site and subjected to a detailed stepwise progressive demagnetization. Two of each four prototypes selected were subjected to progressive stepwise thermal demagnetization using a Schonstedt TSD-1 furnace until the complete demagnetization of the samples. The orientation of the specimens in the furnace was alternated at each demagnetization step to average-out the effect

410

b)

H. I. LOTFY et al.

Figure 1. (a) Palaeopoies for Tertiary rocks from Africa, numbered according to the list of Table 1. jb) same, with cones of 95% confidence.

of any small magnetic field existing inside the furnace. The other two of the four representative specimens per site were progressively treated with a Schonstedt GSD-IAC single axis AF demagnetizer up to 100 mT. Specimen orientations, with respect to the three demagnetized axes, were reversed at each step to minimize the effect of spurious ARM components possibly introduced in the samples at higher demagnetization fields.

Although both thermal and AF techniques revealed all magnetic components, thermal demagnetization was superior in that it was possible to completely demagnetize the samples yielding well- resolved linear trajectories decaying to the origin. In an AF demagnetization up to a 100 mT peak field, on the other hand, the samples could not be completely demagnetized and sometimes displayed the high coercivity characteristic components as noisy, zigzag- like trajectories decaying only partly towards the origin. Subsequently, thermal demagnetization was exclusively applied to all samples in at least 9 steps until complete demagnetization, but with larger increments at low temperatures. Small increments (10°C) were maintained when approaching the Curie

temperatures of the magnetic carriers. Zijderveld (1967) diagrams (Fig. 3), stereographic projections and intensity-decay plots were used to visually m- spect the magnetic records in a search for characteristic linear trends and linear trajectories that represented the number and directions of the resolved magnetic components.

Almost all samples are characterized by a bivec- torial decay with well-resolved linear trajectories (Fig. 3). Their directions have been calculated using prin- cipal component analysis techniques (Kirschvink, 1980). Upon removal of a soft, viscous, low-coercivity and low Tb component by up to 40 mT peak AF and 400°C (component B), the trajectories reverse their directions marking the beginning of a steady univecto- rial decay to the origin of the hard component. This stable component was characterized, in some samples, by a shallow, up inclination (called component Al) whereas in others it has a steeper, negative inclination (component A2). In contrast, component B has a northerly declination with a moderate to steep positive inclination, apparently parallel to the present-day field at the study areas. Each component was plotted in the co-ordinates in which it was most


Mnm

r - km 1 I

31'25' 31"30 31"35

2b" 3io 33"

16 Anticlinal W Stucture

Figure 2. Maps of the sampling sites in the areas east (a) and west (b) of Cairo. Black areas represent basalt exposures; K-Cretaceous, ErEocene, Ol-Oligocene, Mnm-non-marine Miocene, Mm-marine Miocene, Pl-Pliocene, Q-Quatemary, R-Recent, Up-Upper.

probably acquired (e.g. Fig. 4). The soft component B, which conforms to the present-day field, was likely acquired while the rocks were in their present-day position and is plotted in in situ co-ordinates. On the other hand, the characteristic components Al and A2 were acquired while the flows were in their pre-tilt position (because they pass the tilt-fold test as will be shown below) and are represented in tilt-corrected pm-deformation co-ordinates.

Components Al and A2 are distinguished by their inclinations (shallow vs steeper) andI occur in sites 1 to 11 and 12 to 29, respectively. Either Al or A2 (but never both) are isolated at a given site and mterme- diate directions between Al and A2 are not observed.

Considering each site as a spot reading in geological time, the group-mean directions of each of the separated components Al, A2 and B ~ were calculated together with the pertinent Fisherian statistical pa-

412 H. I. LOTFY et al.

W. ‘JP

u. UP

w. UP

Figure 3. Orthogonal demagnetization diagrams (Zijderveld, 1967) of representative samples from six sites, treated with alternating fields (samples from sites 3 and 4) and thermal methods. Open (closed) symbols represent projections onto the vertical (horizontal) plane.

Site l-l 1 Site l-4 Site I-1 2

Figure 4. Fqual angle projections of the directions isolated for three representative sites, plotted in the co-ordinates in which the magnetizations are thought to have been acquired. Closed (open) symbols represent projections onto the lower (upper) hemisphere.

rameters (k and ~95; Fisher, 1953) giving unit-weight to each observed direction. The site-means of all the 29 sites, together with their a95’s, are plotted on an equal area projection in Fig. 5. The site means of the hard components Al and A2 (Fig. 5), form a trimodal grouping in situ with a wide range of inclination but about the same declination in the southwest quad- rant. Upon tilt correction, the clusters are significantly

improved and a clear bimodal grouping representing Al and A2 with completely non-overlapping ~~95’s is obtained; this suggests that two populations of magnetic components are present that characterize two different volcanic episodes in the ‘east- and west-of- Cairo areas’.

Figure 6 presents an orientation-density plot (Schmidt, 1925), constructed on a standard equal area


Site means of the isolated components

Figure 5. Stereonets (as in Fig. 4) with site-mean directions of the B, Al and A2 components isolated during stepwise demagnetization, in in situ and tilt-corrected coordinates.

Tilt-Corrected I

Equal-area projection

N = 29 sites Tilt-Corrected

60

-10 -20 -30 -40 -50 -60 Inclination

n

lh p, ~

180190200210220 Declination

Figure 6. Density diagram of the Al and AZ characteristic directions in equal-area projection and histograms of the inclination and declination values after correction for the tilt of the basal&

net for the site-mean directions of the characteristic components Al and A2 in tilt-corrected co-ordinates and shows that their two populations are statistically distinct. The frequency per cent diagram of the site- mean inclinations and declinations for the 29 sites in tilt- corrected co-ordinates (Fig. 6) further shows that the site-mean inclinations reflect clear bimodal distributions for Al and A2 despite unimodal declinations.

450 - Com!yE:t Al

400 - I-1 to l-11 Y t 350- zi

fi 3oo g 250-

g 200- .- $ 150-1

; loo-

50-

00 Kl 10 20 30 40 50 60 70 60 90 I

Unfolding %

Component A2 18 Sites

I-1 2 to l-29

- 99%

1

- 95%

>

00 Kl 10 20 30 40 50 60 70 80 90 K2

Unfolding %

Figure 7. Incremental fold tests for the Al and A2 components &&rating that the magnetizations are pre-folding, i.e. the precision parameter k2 at 100% unfolding has the highest values.


@ Component A2 i

kof pang

180

270

0

Figure 8. Mean directions for the Al and A2 components with their 95% cones of confidence before and after tilt correction and the corresponding virtual geomagnetic poles.

The cones of 95% confidence (~~95’s) of the tilt- corrected site-means do not overlap, also proving that Al and A2 are significantly different at the 95% probability level.

ln order to test the stability and constrain the age of Al and A2, a tilt test (Graham, 1949) was applied separately to each of Al and A2; applying McEI- hinny’s statistical levels of confidence (McElhinny, 1964), the site-means of each of Al and A2 pass the fold test. The site-means of A2 pass the test at the 99% confidence level yielding K2/K1=3.11 (18 sites). The means of Al yielded K2/K1=2.62 (11 sites), passing the test at a percentage between 95% and 99%; the lower confidence level is due to the small amount of dip recorded and the small number of entries.

When tracing the change of the precision parameter K, each of the characteristic components Al and A2 yielded its highest K-value at 100% unfolding when a stepwise incremental unfolding (Fig. 7) is applied. Clearly, the characteristic components Al and A2 are constrained to be pre-deformation or, more precisely, pre-faulting in age and, given a Middle Miocene age for the tilt of the basal& it can be concluded that the magnetizations Al and A2 are stable, pre-Middle Miocene magnetic signatures.

Based on their stability, positive fold tests, signifi-

cant difference from each other and from the present- day field and their internal consistency in their re- spective sites, the characteristic components Al and A2 are considered as primary magnetic directions re- flecting the palaeomagnetic field at the time when their magmatic phases occurred. The Al and A2 site- means were then combined into two group means to estimate the characteristics of the geomagnetic field in the study areas at the time when each of the re- spective magmatic episodes occurred (Fig. 8 and Ta- ble 2). The magmatic episode characterized by component Al was recorded at 11 sites/l32 samples (sites I-l to I-11) to yield a mean palaeomagnetic direction based on site-means as Dec=198”, Inc=-24” with a95=2.6” and K=417. The palaeomagnetic pole is at 66”N, 167”E with A95=2.3” (Fig. 8 and Table 2). The magmatic episode characterized by component A2, on the other hand, was observed at 18 sites/216 samples (sites I-12 to I-29) and yields a mean palaeomagnetic direction based on site-means of Dec=197”, Inc=-51” with a95=3.1” and K=270. The palaeomagnetic pole is at 76”N, 111”E with A95=3” using VGP averaging (Fig. 8, Table 2).

CHRONOLOGY AND PETROLOGY

In order to determine the ages of the rocks as well as the two palaeomagnetic poles, 39Ar/J’JAr whole rock age dating was attempted. Samples were pack- aged in pure Al foil and irradiated in position L67 of the Phoenix Ford Memorial nuclear reactor at the University of Michigan. The samples were subsequently degassed in a double-walled Ta furnace con- nected on-line to a MAP215 mass spectrometer. Both Faraday and electron multiplier collectors were used and all analyses are extrapolated to inlet time and corrected for mass discrimination. For each heating step, the temperature of the samples was ramped up at approximately 150°C per minute and held at the target temperature for 20 minutes.

The argon release spectra and commensurate Ca/K ratios are shown for the two samples (representing the Al and A2 episodes) in Fig. 9. Inte- grated ages are about 23 and 18 Ma, but the age spectra are clearly rather disturbed and could represent similar ‘plateau’ ages of 22.5 Ma with the A2 sample showing possible later alteration at about 5 Ma. It is unlikely that the odd structures in the age spectra are due to total redistribution of 39Ar from re- coil, because the low ages are not only associated with the high Ca part of the spectrum but also with the high K part of the spectrum. However, the pos- sibility of total internal 39Ar redistribution due to re- coil cannot be excluded, especially given the distinct difference in palaeomagnetic directions seen in the two samples. It is therefore tempting to use the total gas ages (equivalent to conventional K-Ar results) as


Table 2. Site-mean results and site-mean palaeopoles

B component A component Site Strike/ Dip Dec/Inc Dec/Inc a95 k Dec/Inc Dec/Inc a95 k Pole

in situ TC in situ TC ites with Al components I-l O/O 355163 355163 8 37 2001-27 200/-27 3 195 66/158 I-2 9515 352/ 70 348/75 7 49 1981-17 1981-22 3 222 6§/166 I-3 12515 357159 351163 7 35 200/-17 200/-22 3 289 641183 I-4 98/2 360152 360/54 9 27 2021-21 2021-23 4 139 631159 I-5 9516 353/53 351/59 10 20 1981-14 198/-20 3 173 641168 I-6 O/O 353153 353153 10 20 198/-28 1981-28 2 460 68/168 I-7 5513 349/64 352167 9 25 1981-23 1991-25 2 521 66/162 I-8 85/8 357159 358167 9 29 195/-11 196/-19 3 208 65/172 I-9 O/O 355157 355157 9 25 1941-29 1941-29 2 513 711167

I-10 O/O 359/69 359169 5 84 W/-27 W/-27 3 179 661160 I-11 95/3 360170 359173 5 65 197/-20 1971-23 3 235 661167 /lean of Al component (in situ k=159, N=ll) 1981-21 198/-24 3 417 661167 lites wiith A2 components I-12 8517 17160 23166 10 20 2001-43 2031-49 4 155 701118 I-13 90/4 8/56 9/59 6 47 1951-47 1961-51 3 171 78/111 I-14 8714 l/57 l/60 9 24 1851-47 1861-51 4 153 851103 I-15 90/5 5/62 5/67 7 40 W/-45 200/-50 2 512 731114 I-16 260/9 10165 5158 9 25 2121-61 2031-54 3 216 701103 I-17 260112 11/61 8150 15 9 2151-60 2041-51 4 129 69/111 I-18 260110 355/64 354154 13 13 1931-61 1881-52 5 89 83199 I-19 9015 l/47 l/52 15 9 196/-43 1971-48 4 105 751121 I-20 8316 353/54 353/60 11 17 181/-45 182/-51 4 103 $8177 I-21 27317 6/62 5/55 14 11 203/-57 200/-50 4 120 731114 I-22 270/9 357165 358158 11 28 2181-56 2111-49 3 193 631114 I-23 270111 344172 250161 10 32 1981-61 1941-50 4 101 78/114 I-24 285/7 198/-60 197/-53 4 133 75/103 I-25 28516 204/-57 203/-51 3 212 70/111 I-26 270/8 6/57 5151 14 12 1961-56 194/-50 4 134 781114 I-27 260/2 358159 358157 13 16 1991-54 1981-52 4 104 74/107 I-28 280/3 l/79 2178 4 137 192/-54 1921-51 3 169 80/109 I-29 28014 3/55 3151 13 13 1891-57 189/-53 4 128 82/94 /lean of A2 component (in situ k=87, N=18) 1981-54 197/-51 3 270 761111

Dec/Inc is the decIination/incIination in degrees, TC=tiIt corrected; k and a95 are the statistical parameters associated with the mean; Pole=virtual geomagnetic pole calculated for each site.

415

being the ages of formation for these basal& Unfor- tunately, at this stage the argon isotope results cannot rule out either interpretation.

A detailed comparative petrographical study of 29 thin-polished sections (one section/site) (Lotfy, 1992), revealed that the two Miocene basaltic episodes dis- play two different groups of igneous textures. The Al basaltic episode (sites I-l to I-11) is characterized by very fine-g-rained basal& whereas the A2 volcanic episode (sites I-12 to I-29) consists of medium- and coarse-grained holocrystaIIine non-porphyritic flows.

The Al fine-grained basaltic episode is dominantly porphyritic with a dark cryptocry@Iine or glassy groundmass rich in dust-like opaques, giving the thin sections their characteristic dark appearance.

The A2 coarse-grained flows, on ‘the other hand, are characterized by holo-microcry$taIIine groundmass, ophitic/subophitic and doleritic textures. Phe- nocrysts include plagioclase, pyroxene, olivine and opaque minerals, in decreasing order of abundance. The plagioclase is mainly labradorite and is usually present as fresh, euhedral, zoned mbular laths. The

H. I. LOTFY et al.

--- 22.4 Ma ___

0.2 0.4 0.6 0.8 0.2 0.4 0.6 0.8

Fraction of 39Ar Released Fraction of 39At Released

30

20

10

0

r - 22.6m-‘1 0.2 0.4 0.6 0.8

Fraction of 39& Released

301 . . . . 1 ,

20 -

?I 10 -

-I 1

o+ . .

0.2 0.4 0.6 0.8

Fraction of 39Ar Released

Figure 9. (Left) Whole-rock age spectra and (right) Ca/K versus the fraction of 3gAr released, for a sample with an Al palaeomagnetic direction (top) and one with an A2 direction (bottom).

pyroxene occurs as subhedral to rounded zoned ir- regularly corroded plates of augite and pigeonite. The olivine grains are usually granules altered along cracks and rims into iddingsite and chlorite as pseudomorphs after olivine.

The opaque minerals are mainly magnetite and ihnenite showing different textural relationships with subsidiary hematite (martite) and maghemite developed as secondary alteration products. Magnetite is usually present in two phases: as medium to coarse, platy, star-shaped, skeletal and fish-bone cracked mi- crophenocrysts of magmatic (primary) magnetite and as fine-grained, dust-like aggregates of secondary magnetite developed along altered olivine crystals.

The magnetite is commonly altered to maghemite and sometimes to hematite (martite). Martitization usually starts along the rims and proceeds inward, parallel to the octahedral planes, developing pseudomorphs. Ilmenite is present as granules and elongated acicular lamellae as well as large homogeneous prismatic crystals.

The fine-grained basalts of the Al episode (sites I- 1 to I-11) are generally characterized by homogeneous (titano-) magnetite grains that are occasionally intergrown with homogeneous ilmenite lamellae and .myrmekitic or. graphic-textured silicates with magnetite phenocrysts.

In the A2 coarse-grained basaltic episode (sites I-12

Table 3. Tertiary mean poles of North America and Europe rotated to African coordinates

Age interval Mean pole Mean pole Euler rotation Euler rotation Rot-pole Rot pole NAM EUR NAM-Am EUR-NAM from NAM from EUR

O-20 Ma 86/28 85/165 79.1/77.95/+2.4 65/137/-2.3 86/26 85/180 20-30 Ma 79/129 79/114 79.6/37.8/+5.3 66.5/135/-5.6 80/134 80/123 30-50 Ma 80/ 148 75/159 76.4/7.1/+9.8 68.7/136/-7.7 81/170 741177 50-65 Ma 79/163 n/177 80.6/70.5/+18.1 62.4/141/-12.5 79/ 196 72/ 209 NAM=North America, EUR=Europe, MR=Africa; Euler rotation parameters are given as latitude/longitude/rotation angle (positive

when counterclockwise); Rot-poles are the rotated poles in African coordinates.


Figure 10. Tertiary mean palaeopoles rotated from North American and European into African co-ordinates (see Table 3) compared with the AF%T of Tauxe et al. (1983) and the two virtual geomagnetic poles of this study.

to I-29), on the other hand, the magnetite and ilmenite grains are inhomogeneous and show different kinds of intergrowths, such as intergranular, coarse-trellis, network and patchy, developing two-phase intergrowths of elongated acicular ferri-ilmenite and octahedral titanomagnetite. The latter is commonly oxi- dized to titanomaghemite as pseudomorphs and rarely to martite (hematite) particularly along periph- eries. Graphic texture with silicates and the exsolu- tion of ihnenite and rutile as worm-like aggregates of blitz texture are also recorded in advanced stages of alteration. Pseudobrookite is frequently developed as pseudomorphs after ferri-ilmenite intergrown with titanohematite.

IMPLICATIONS FOR AFRICA’S APWP

A comparison of the two poles (VGP’s) and the APWP for the African plate derived from DSDP sediments in the Atlantic part of the African plate (Tauxe et al., 1983) shows a similar trend (Fig. 10). A small offset between the segment of the present study and that of the DSDP sediments could be due to:

i) shallowing of the palaeomagnetic inclinations in sediments due to compaction (Qpdyke, 1961);

ii) discrepancies in the observed declination of the azimuthly unoriented DSDP cores (in which the declination of the characteristic component is calculated by aligning the secondary component to the PDF direction in the study area); or

iii) incomplete averaging-out of the secular variation in our VGPs. APWPs for Africa can also be constructed by rotating those of the North American craton or stable Europe into African co-ordinates. because there is a relatively large dataset of Tertiary poles from the stable North American and European cratons, their Tertiary results were grouped into shorter time windows (Table

3). Only poles that withstood the scrutiny of modern reliability tests (McElhinny and Embleton, 1976; van der Voo, 1988) and have a Q value ~2 (van der Voo, 1990), as well as A95 <lo”, are included in the means. As the intervening plate margin between Europe and Africa is convergent, there is no direct information on the Euler pole to rotate the European poles into the African co-ordinates. Thus, the stable !European mean poles are first rotated to North American co- ordinates, using the rotation parameters listed in Rowley and Lottes (1988).

The mean poles for the appropriate time windows of both stable North America and Europe (rotated to the North American co-ordinates), are then rotated to the African co-ordinates using the Euler poles and rotation angles listed in Klitgord and Schouten (1986). The results (Table 3) are plotted in Rig. 10 on equal- area polar projections to illustrate the Tertiary APWPs of Africa as inferred from the rotated North American and European poles, where they can be compared to the Tertiary APWP based on palaeomagnetic poles derived from the African plate itself (Tauxe et al., 1983) and the two poles of the present study in the central part of Fig. 10.

The Early Miocene segment of the APWP between our two poles reveals a similar pattern to that ob- tamed from the well-resolved Al%&’ of the DSDP sediments of the African plate (Tauxe et al., 1983) and the poles rotated from the North American and European cratons.

CONCLUSIONS

This palaeomagnetic investigation means that it is possible to distinguish, for the first time, two different magmatic episodes during the Early Miocene in the areas to the east and west of Cairo. The authors infer that the beginning of the Early’ Miocene volcan-


ism is characterized by component Al [Dec/Inc= 198”/-24”, in tilt-corrected co-ordinates, yielding a north pole at 66”N/167”E and a colatitude=77” based on 11 sites/l32 samples (sites I-l to Irll)]. This episode was found to be characterized by olivine- bearing, fine-grained, porphyritic basal&

The later Early Miocene volcanism in the Cairo’ area is characterized by component A2 [Dec/Inc= 197”/-51”, in tilt-corrected co-ordinates, resulting in a pole at 76”N/lll”E and colatitude=59” based on 18 sites/216 samples (sites I-12 to I-29)]. This episode was found to be represented by coarse-grained, holo- cyrstalhne, non-porphyritic, olivine-free, doleritic basalt.

Taking into consideration that the results are VGPs, which may not have averaged the secular variation of the geomagnetic field, the palaeolatitude or colatitude differences between the two basaltic episodes suggests that Africa underwent a northward translation during the Miocene. This northward movement of Africa during the Tertiary is also rec- ognized in the palaeomagnetic results from the stratigraphically well-constrained DSDP sediments from the African plate, although of lesser magnitude. Subsequently, the African plate rotated in a clockwise sense with respect to the present-day dipole field during the past -20 Ma.

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palaeomagnetism of early miocene basaltic eruptions in the areas east and west of cairo

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