No abstract

Nontransform ridge axis discontinuities, such as propagating rifts, overlapping spreading centers (OSCs) and saddle points, occur frequently along the mid-ocean ridge system. We used Sea Beam and deep-tow to study an example of a relatively large nontransform O • offset near 20 40 S on the East Pacific Rise (EPR). The two spreading center tips are 15 km apart and separated by a diffuse shear zone. A broad V-shaped wake of disturbed topography and magnetics occurs on the flanks of the EPR near the offset. On a fine scale, the offset is tectonically much more complex than either the typical OSC or propagating rift structures for two reasons. First, it is a "dueling propagator" in which structures produced by successive short-term episodes of northward and southward propagation have overprinted each other, with migration toward the south prevailing over the long term. Second, its offset distance is such that its tectonic behavior may be transitional from that of a propagating rift to that of an OSC system. The offset's rate of migration to the south has been '20 mm/yr over the last 2 m.y., far less than total opening rate of 162 mm/yr. Distinct propagation events may have been much more rapid, however, surging north and south at rates of 160-230 mm/yr. In the process, several fossil spreading ridges 30-50 km long have been abandoned and rafted to the side. We propose a fine scale tectonic evolution in which a propagating spreading center either curves inside or runs outside of a soon to be abandoned, propagating rift tip. We call this process 'self-decapitation.' Dueling propagation at 20ø40'S m•y be caused b• competing magmatic sources at shallow points along the EPR: one to the north near 20øS and one to the south near 21"30'S. The 20"40'S dueling propagator occurs at a long-wavelength low in the axial depth profile between these competing sources. In general, the division of spreading centers into segments separated by ridge axis discontinuities occurs at low points along the axis and may be caused by the pattern of upper mantle convection and the factors governing flow of magma along the strike of the ridge. overlap basin striking parallel to the overlapping spreading centers, whose depth is typically 5-10% of the Copyright 1988 by the American Geophysical Union. width of the offset. The temporal offset at OSCs is Paper number 6B6256 generally less than 0.2 m.y., so that the offset distance is 0148-0227/88/006B-6256505.00 generally less than 10-15 km, depending on the spreading 2875 2876 MACDONALD ET AL..' DUELING PROPAGATING RIDGES, EAST PACIFIC RISE 120 ø W 110 ø 1 O0 ø 90 ø RIVERA 84 m__•_m • yr OROZCO 104 mm • CLIPPERTON 120 m__•m • yr SIQUEIROS COCOS