PALEOMAGNETISM OF EARLY MIOCENE BASALTIC ERUPTIONS IN THE AREAS EAST AND WEST OF CAIRO

Two characteristic magnetic components (A1 and A2) with distinctive no n-overlapping bimodal distributions were palaeomagnetically isolated f rom what has long been considered as one single phase of post-Oligocen e pre-late Miocene basaltic volcanicity in the 'east- and west-of-Cair o areas'. A1 yields a mean Dec/Inc=198 degrees/-24 degrees, alpha 95=2 .6 degrees and K=417 based on the site-means in tilt-corrected co-ordi nates with a corresponding palaeomagnetic north pole at 66 degrees N/1 67 degrees E and A95=2.3 degrees using site-mean pole averaging. A2 yi elds a distinct mean Dec/Inc=197 degrees/-51 degrees with alpha 95=3.1 degrees and K=270 corresponding to a palaeomagnetic north pole at 76 degrees N/111 degrees E and A95=3 degrees. A1 and A2 both pass the til t test; the two components are never recorded at the same site and no intermediate direction was observed. The bi-modal grouping of the A1-A 2 directions and their non-overlapping cones of 95% confidence indicat e that the group means of Al and A2 are significantly different in til t-corrected coordinates at the 95% level of confidence and represent t wo different ages of magnetization. Therefore, we argue that two disti nct basaltic episodes occurred, which were not previously distinguishe d in the areas 'east- and west-of-Cairo'. Ar-39/Ar-40 whole rock analy ses appear to yield Early Miocene ages, but there are too many problem s with the argon release spectra to rely on the integrated ages for mo re precise temporal definition.A petrological study subsequently carri ed out on the two distinct basaltic episodes revealed that the A1 epis ode is represented by very fine-grained porphyritic olivine-bearing ba salts with dust-like groundmass rich in opaques made up of homogeneous magnetite grains that are rarely intergrown with homogeneous ilmenite lamellae. In contrast, the A2 episode consists of medium- to coarse-g rained holocrystalline non-porphyritic doleritic basalts with inhomoge neous magnetite and ilmenite grains that show a variety of intergrowth s. The two poles of this study are dearly virtual geomagnetic poles be cause of imperfect averaging of the secular variation of the geomagnet ic field due to the limited number of exposures and the rapid cooling of the Bows. Even so, a comparison with the APWP of Africa shows that the segment joining the two poles of the present study has a similar p attern as that of the APWP derived from DSDP sediments of the Atlantic part of Africa or those rotated from the North American craton and st able Europe and allows the authors to calibrate part of this APWP as E arly. Miocene. The available palaeomagnetic results imply a northward movement of Africa during the Tertiary followed by a clockwise rotatio n with respect to the axial geocentric dipole.