Rifting of oceanic lithosphere at transforms and trench sloap seamounts.

摘要

Two geographically and geologically diverse environments provide examples of the effect of extension on the oceanic lithosphere, although neither one is in a traditional extensional regime. The Eltanin transform fault system is composed of right-stepping transforms Heezen and Tharp, which form the transform boundary separating the East Pacific Rise from The Pacific Antarctic Ridge. The Pacific plate underwent a change in direction of plate motion approximately 5 Ma, putting the Eltanin transforms into extension. The transforms responded to the opening change by each rupturing a rift zone in their inside corner, which has accommodated the change in plate motion, allowing the transforms to maintain their original strike. Heezen and Tharp are twins in terms of similar age, length, slip rate, and general morphology, as well as in the development of their inside corners as zones of accommodation of extensional stress. However, the magma chamber of the EPR at the Heezen ridge-transform intersection is much more robust than the corresponding magma chamber at Tharp, and this difference has created contrasts in morphology such as the construction of volcanic ridges in the inside corner (from rupture into the magma chamber) at Heezen, instead of the deep rifted valleys of Tharp. Inside corner rifting also is found at the Pacific-Antarctic transform Udintsev, supporting the theory that inside corner rifting needs long and relatively fast slipping transforms to develop.; Capricorn Seamount is on the eastern edge of the convergent boundary of the Tonga trench. The star-shaped seamount (actually a drowned atoll) is currently on the inner slope of the trench, and is undergoing intense extensional faulting, caused primarily by bending-induced extension along the slope. The summit of Capricorn is broken by the faulting, and also shows erosional features from its time above sea level. The southern flank of Capricorn has two bending-induced faults curving away from their previous trench-parallel strike, and curving into the radial arms of the seamount. The drowning and destruction of Capricorn may represent the future for the island of Niue, located 230 km to the east.