Benue Trough the Central African Shear Zone and for the volcanic island of St. Helena in the Atlantic ocean. Three periods of magmatic activity (volcanic action) have been identified, 147–106 Ma, 97–81 Ma and 68–49 Ma. The first is prominent in the north of the trough, and contemporary with magmatism in Brazil, probably occurring during a period of crustal extension before the Atlantic started to open. The second is found only in the south of the trough, and may belong to a period when the extension of the Atlantic had slowed down, ending with a period of compression. The third and last
Mariana Trough Iwo Jima), and Iwo Jima . Rifting and spreading are inferred to be propagating northward at a rate of 10 to 40 cm/year , so the variations in tectonic and magmatic style seen along-strike north of 18°N provide an example of the sequence of events that occur at any one section across the back-arc basin as the rift evolves from updoming through rifting to true spreading. Mariana Trough The Mariana Trough is an active back-arc basin in the western Pacific Ocean . It is an integral part of the Izu-Bonin-Mariana Arc system. The Mariana Trough stretches 1300 km from north