Role of Punctuated Subsidence and Structural Inversion in Creating East African Spice Islands

The coastal margin of Tanzania is classically thought to be an exemplar of passive margin that underwent passive (thermal) post-rift subsidence since the rifting of Madagascar from East Africa in the Middle Jurassic. However, the presence of three major anticlinal folds that create the large offshore islands of Pemba, Zanzibar and Mafia is curious and suggests that the evolution of the East African continental margin is more complex. With the recent surge in exploration activities, new seismic data allows the genesis of the islands to be understood and placed in their regional context for the first time. Results of our studies demonstrate that the folds were created by contractional deformation associated with fault reactivation and basin inversion from the Neogene (c.6-7Ma). Compression in the area has continued to the present day, evidenced by deformation at the seabed. Given that the timing of the contractional deformation that formed the island anticlines are coeval with the onset of extension in the East African Rift System that appears to be the likely driver for this long-lived yet punctuated tectonism. However, equally crucial is the occurrence of the Davie Ridge, which creates the eastern buttress for strain to accrue between.