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

We have used the Sea Beam multibeam echo-sounding system to survey the East Pacific Rise (EPR) from 8°20′N to 18°30′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 km) 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 km. 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 (20–60 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 magmatic 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 km) 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 OSC's and laboratory studies of OSC's using wax models, photoelastic studies of overlapping cracks, and boundary element modeling of overlapping cracks. Seismic refraction and reflection near two OSC's suggests that large offset OSC's have separate magma chambers beneath the offset rise axes, while small offset OSC's may share a single axial magma chamber. Three-dimensional inversion of magnetic anomalies suggests that significant petrologic anomalies may occur near some OSC tips. Waxing and waning of overlapping magma chambers may give rise to the eruption of highly fractionated FeTi basalts and associated magnetic anomalies.