Deep‐tow studies of the overlapping spreading centers at 9°03′N on the East Pacific Rise

Abstract

The deeply‐towed instrument package of the Scripps Institution of Oceanography was used to study for the first time the fine scale structure of an overlapping spreading center (OSC) system: the 9°03’N OSC on the East Pacific Rise (EPR). The eastern and western limbs of this OSC system which are 8 km apart and overlap by 27 km show marked differences. Away from the tips, the spreading centers are highly tectonized and differ greatly from the ridge tips. The western ridge tip exhibits a few fissures but no sign of recent volcanic activity whereas the eastern ridge tip is highly fissured and has experienced more recent volcanic episodes which are confined to a narrow zone within this highly fissured area. In both cases, the ridge tip fissures are 10°–15° oblique to the strike of the faults that occur on the flanks of the adjacent portion of the opposite spreading axis, indicating a recent change in the direction of deviatoric tension and suggesting that the eastern and western spreading centers have been propagating southward and northward respectively. The eastern ridge tip fissures are transecting the flank of the western spreading axis suggesting that the eastern spreading center is about to become the through‐going trace of the East Pacific Rise. In contrast, the western spreading center disappears into abyssal hill terrain that was presumably created at the eastern ridge axis. Some of the very few tectonic features observed in the overlap basin may reflect the existence of a short‐lived shear couple due to the interaction of the two overlapping spreading centers. Sediment cover over the whole area and especially in the overlap basin is relatively thick, supporting the idea of a time‐averaged deficit in the magmatic budget at this OSC. We suggest that the 9°03’N OSC developed where two magmatic pulses, independent in space and time and propagating along the strike of the East Pacific Rise away from the loci of melt emplacement, failed to meet. Misalignment of the magmatic conduits resulted in the propagation of the two spreading centers past each other and the development of the ensuing OSC geometry. Independent lines of evidence derived from the three‐dimensional inversion of the magnetic field at the 9°03’N OSC (Sempere et al., 1984), from numerical modelling of the growth of two en echelon elastic cracks in a tensile stress field (Sempere and Macdonald, 1986) and from geochemical data (Langmuir et al., 1986) support our interpretation of the Deep‐Tow data. We suggest that deviations from axial linearity of the spreading centers (DEVALs; Langmuir et al., 1986) and small nonoverlapping offsets (SNOOs; Batiza and Margolis, 1986) are simply local lows or saddle points along the axial depth profile that arise when two magmatic pulses propagating toward one another meet head on. OSCs and some saddle points (i.e., DEVALs, SNOOs) are all small, rapidly evolving ridge axis discontinuities which may represent the surficial expression of the distal ends of small scale longitudinal convection cells beneath the East Pacific Rise.