East Pacific Rise from Siqueiros to Orozco Fracture Zones: Along‐strike continuity of axial neovolcanic zone and structure and evolution of overlapping spreading centers

Abstract

We have used the Sea Beam multibeam echo-sounding system to survey the East Pacific Rise (EPR) from 8ø20'N to 18ø30'N, obtaining complete coverage of the EPR axial neovolcanic zone and all intervening transform faults. Here we focus on the EPR neovolcanic zone and the definition of overlapping spreading centers (OCS's) between the Orozco and Siqueiros transform faults. The neovolcanic zone is narrow [0.5-2.0 kin) and continuous along strike and occurs within and near a strike-continuous axial summit graben. The neovolcanic zone and summit graben reside along the crest of a volcanic axial high which is 2-10 km wide and which continues uninterrupted along strike for 40-140 kin. Within 20-50 km of an intersection with a transform fault the axial neovolcanic zone narrows and deepens, and within only 3-6 km of the intersection the neovolcanic zone turns sharply into the transform valley. At seven locations between the Siqueiros and Orozco transform faults the axial neovolcanic zone is discontinuous and is laterally offset a short distance (1-15 km). In contrast with a classic ridge/transform/ridge plate boundary, however, offset rise terminations overlap each other by a distance approximately 3 times greater than their offset. They curve toward each other, and often one merges into the other along strike. Separating the OSC's is an elliptical overlap basin up to 600 m deep whose long axis is subparallel to the adjacent spreading centers. The overlap basin is characterized by volcanic constructional edifices. A key difference between these offsets and small transform faults in the Atlantic is that there are no transform fault structures in the overlap basin which link the offset rise axes. A continuous axial depth profile reveals a long-wavelength (9.0-60 km) undulation of the rise axis which may be associated with variations in the magmatic budget along strike. The lowest points in the axial depth profile occur near transform faults and at OSC's, suggesting that these features are associated with along-strike minima in the magmatic budget. We suggest that major phases of volcanic activity propagate episodically along the axis away from centers of magmatie inflation and that OSC's develop where magmatic pulses fail to meet due to misalignment of fracture systems, sinuous bends in the rise axis, or other heterogeneities. Transform faults fail to develop on fast spreading centers where the lateral offsets are small (<15 kin) because the lithosphere is too thin and weak to maintain a classic rigid plate ridge/transform fault pattern. The OSC geometry. is unstable and evolves rapidly, and the overlapping rises may propagate at rates faster than the local spreading rate. One of the two OSC's prevails, while the other is abandoned. While the locations of OSC's may not be fixed relative to the rise axis, they may tend to recur near the same place, creating scars in older lithosphere which resemble fracture zones, but whose fine-scale structure is quite different. There is excellent agreement between the observed structure of ...

Manganese and methane in hydrothermal plumes along the East Pacific Rise, 8°40′ to 11°50′N

“…The southern third of the northern segment, the western limb of the 11°45'N OSC, and the eastern limb of the 9"03'N OSC are also the only sections that have a triangular, uninflated cross-section and lack a summit caldera and a magma reflector (Macdonald and Fox, 1988) All of these features suggest that these sections are magma starved. By contrast, the other limbs of the OSC's, which are continuous with the next ridge segments to the north and south rather than with those in the survey area, appear to be magmatically robust (Macdonald et al, 1984).…”

“…While the eastern limb is broad and has a domed cross-section, an axial summit caldera, and a shallow magma reflector, it is deep ( >2750 m) . By contrast, the western limb is narrow and triangular and lacks a summit caldera and magma reflector, but it is relatively shallow (<2600 m) (Macdonald et al, 1984;Macdonald and Fox, 1988). A hydrocast (S-35) in the nodal basin, which lies between the two limbs of the OSC and exceeds 3160 m in depth, showed a hydrothermal plume maximum (4.4 nM Mn, 0.6 nM CH,) at the same water depth as those on the limbs, indicating that the plume almost certainly originated elsewhere and was carried over the basin by currents.…”

Seismic structure and indicators of magma budget along the Southern East Pacific Rise

“…There is evidence both in support of, and against, a simple magma supply model as is illustrated in Figure 1 [Macdonald et al, 1984;Lonsdale, 1989b]. These variations in axial depth are associated with systematic changes in the shape of the axial high [Macdonald and Fox, 1988;Scheirer and Macdonald, 1993].…”

Repulsion and Attraction between a Pair of Cracks in a Plastic Sheet

“…The stress field around the crack tip is then modified by the presence of the other cracks leading to kinked trajectories. In hierarchical fracturing processes [13], it is usually observed that cracks coalesce around a 90°angle, as for example in dessication [14][15][16] or in fault dynamics [17][18][19][20]. It has also been shown that two collinear approaching cracks (submitted to a uniaxial stress in mode I) can repel each other instead of merging tip to tip [21], resulting in a curved, "hookshaped" path.…”