Jean-Pierre Bouillin,2 Gérard Poupeau,2 Christophe Basile,2 Erika Labrin,2 and Jean Mascle3


Fission-track data obtained on apatite grains from Early Cretaceous sequences from Ocean Drilling Program Leg 159 drill sites and from deep dives sampling along the Côte d’Ivoire-Ghana Marginal Ridge have been used to assess the thermal history of the Côte d’Ivoire-Ghana Transform Margin.
Measurements demonstrate that all the apatite grains were heated above 120°C and cooled quickly during the Cretaceous. Apatite fission-track dating are distributed into three groups:
1. A group, characterized by ages ranging ~110 Ma, has been only observed on Leg 159 samples. These apatites were found either in the deepest drilled strata, which were heated above 120°C as also indicated by hydrothermalism evidences, or in Upper Cretaceous dated strata. In the second case, the apatites are obviously reworked. We believe that the thermal event, postdated by the 110-Ma cooling age, would have been generated by mechanical frictions along an intracontinental transform, active between the African and Brazilian parting basement. The Lower Cretaceous heated formations would subsequently have been locally eroded, to the south and/or the west of the Leg 159 sites. The eroded and reworked material, including 110-Ma apatite grains, would have been then redeposited within the Upper Cretaceous sediments.
2. A group of samples, whose ages are centered ~90 Ma, characterize both drilled sediments and slope outcrops. We tentatively explain this new thermal event by a second discontinuous hydrothermal episode, which may have occurred up to Turonian times, along the southward-shifted active transform.
3. Cooling ages between 80 and 70 Ma, apparently restricted within the central and western part of the Côte d’Ivoire-Ghana Marginal Ridge, may postdate a new localized heating, which we tentatively interpret as a consequence of a contact between the transform margin and a southern passing oceanic accretionary center.

1Mascle, J., Lohmann, G.P., and Moullade, M. (Eds.), 1998. Proc. ODP, Sci. Results, 159: College Station, TX (Ocean Drilling Program).
2UPRES-A5025 CNRS Laboratoire de Géodynamique des Chaînes Alpines, Université de Grenoble-I, 15 rue Gignoux, F38031 Grenoble Cedex, France. bouillin@ujf-grenoble.fr
3Geosciences-Azur, Laboratoire de Geodynamique sous-marine, BP 48, F06230 Villefranche/Mer, France.