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 ha
ve been used to understand the evolution of the three accretionary/transfor
m boundaries that converge in the BTJ. The easternmost segments of the Amer
ican-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, probabl
y 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 el
evated topographic anomalies; it is segmented by transform and overlapping
discontinuities and shows evidence of axial propagation. The MAR axial vall
ey bifurcates at its southern tip in two branches oriented 115 degrees and
180 degrees that are, or have been up to recently, loci of crustal accretio
n. The bifurcation represents a former ridge-ridge-ridge (RRR) triple junct
ion. The westernmost segments of the Southwest Indian Ridge (SWIR) are anom
alously 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-s
haped 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 degrees spreading branch. Therefor
e 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 extens
ional deformation observed east of Spiess Ridge may be caused by excess cru
stal formation at Spiess Ridge that cannot be accommodated by motion of rig
id plates. The evolution of the BTJ since 10 Ma involves stages of RRR, RFF
and RRF configurations with highly variable geometry of the accretionary/t
ransform boundaries. Topographic anomalies, anomalously thick crust and exc
ess volcanism suggest that the upper mantle below this region is affected b
y widespread, strong thermal anomalies that have influenced the configurati
on of the BTJ, and determined indirectly intraplate deformation in wide are
as 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 R
RR configuration, and of the imminent establishment of a new RRR Triple Jun
ction to the north.