The Palaeomagnetism of the Gulf of Guinea Volcanic Province, West Africa

Abstract

Summary Palaeomagnetic results are presented for 205 sites and over 600 cores from Upper Tertiary to Recent lavas and intrusions in the Gulf of Guinea volcanic area of equatorial West Africa. The island of Annobon is built up of a suite of reversely magnetized lavas about 18.4 My in age which give a mean direction of D = 189-2, I = 11.2 (N = 7, a95 = 153). A younger phase of volcanism dated as 2.6My has produced lavas with a mean direction of magnetization of D = 337-1, I = -44.3 (N = 7, ag5 = 9.3). The accessible parts of the island of SHo Tom6 are covered by young lavas of Pliocene and younger age with both polarities. Forty nine significant sites from this island combine to give a mean direction of D = 359-2, I = -7.0 (ug5 = 4.7). The Island of Principe is an extensively eroded complex of lavas intruded by phonolites and tephrites believed to be Lower Miocene in age. Twenty five acceptable sites combine to give a mean direction of D = 187.2, I = - 3.2 (a95 = 8.9). Both polarities are present in the lavas and intrusions on this island. All but two of 52 sites on Fernando Po0 are normally magnetized and the bulk of this island is believed to have been built up in the Brunhes epoch. The sites combine to give a mean direction of D = 3599, I = -3.0 (a95 = 4.4) and the derived palaeomagnetic pole is significantly different from the present pole implying that the field here has not approximated to an axial geocentric dipole in Brunhes epoch times. Thirty-two sites distributed through the volcanic area of Cameroun combine to give a mean direction of D = 4@, I = 4.3 (ag5 = 6.1). The sites around li4t Cameroun are all normally magnetized and this volcano is believed to have grown in Brunhes epoch times. Analysis of the Upper Tertiary palaeomagnetic results from the African plate demonstrates no detectable movement of the plate relative to the geographic pole for about the past 25 My.