Bouvet Triple Junction in the South Atlantic: Geology and evolution

The South American, African, and Antarctic lithospheric plates meet in the Bouvet Triple Junction (BTJ) located in the South Atlantic near the island of Bouvet. Multibeam, magnetic, gravimetric, and seismic reflection data have been used to understand the evolution of the three accretionary/transform boundaries that converge in the BTJ. The easternmost segments of the American-Antarctic Ridge (AAR) have a spreading full rate of 19.5 mm/yr for the last 8 m.y. They are oriented N-S, except for some NE-SW segments, probably created by magma-poor extension. The southernmost portion of Mid-Atlantic Ridge (MAR) (spreading full rate of 30.5 mm/yr for the last 9 m.y.) has elevated topographic anomalies; it is segmented by transform and overlapping discontinuities and shows evidence of axial propagation. The MAR axial valley bifurcates at its southern tip in two branches oriented 115° and 180° that are, or have been up to recently, loci of crustal accretion. The bifurcation represents a former ridge-ridge-ridge (RRR) triple junction. The westernmost segments of the Southwest Indian Ridge (SWIR) are anomalously high. The segment adjacent to the island of Bouvet (spreading rate 14.5 mm/yr) is shallower than normal by almost 1 km due to the influence of the Bouvet hot spot. The westernmost SWIR segment (Spiess Ridge) consists of a "swollen" volcanic ridge that reaches 320 m below sea level and has a deep caldera on its summit. Spiess Ridge narrows and deepens to the NW; V-shaped topographic and magnetic lineations suggest that it propagates NW at a rate of 40 to 50 mm/yr. The Spiess magmatic event started at roughly 1 Ma, when it caused deactivation of the 115° spreading branch. Therefore the Antarctic, South American, and African plates meet presently not in a triple point but in a broad zone of diffuse deformation. An area of extensional deformation observed east of Spiess Ridge may be caused by excess crustal formation at Spiess Ridge that cannot be accommodated by motion of rigid plates. The evolution of the BTJ since 10 Ma involves stages of RRR, RFF and RRF configurations with highly variable geometry of the accretionary/transform boundaries. Topographic anomalies, anomalously thick crust and excess volcanism suggest that the upper mantle below this region is affected by widespread, strong thermal anomalies that have influenced the configuration of the BTJ, and determined indirectly intraplate deformation in wide areas of the BTJ region. The thermal anomaly that gave rise to the SWIR-Spiess excess magmatism is the prime cause of the recent disruption of a former RRR configuration, and of the imminent establishment of a new RRR Triple Junction to the north.