Morphology of a ‘superfast’ Mid-Ocean Ridge crest and flanks: The East Pacific Rise, 7°–9° S

Detailed bathymetric data from a Hydrosweep multibeam sonar survey of a 250 km-long portion of the superfast-spreading southern East Pacific Rise crest and flanks show that the along-axis variation in morphology and axial depth differs significantly from that observed at the fast-spreading northern East Pacific Rise. While the deep mantle upwelling pattern is similar under the northern and southern East Pacific Rise, our observations require that the connectivity of the shallow, subcrestal plumbing system be more efficient beneath the super-fast spreading southern East Pacific Rise than beneath the slower spreading northern East Pacific Rise.     

Segmentation and disruption of the East Pacific Rise in the mouth of the Gulf of California

Analysis of new multibeam bathymetry and all available magnetic data shows that the 340 km-long crest of the East Pacific Rise between Rivera and Tamayo transforms contains segments of both the Pacific-Rivera and the Pacific-North America plate boundaries. Another Pacific-North America spreading segment (Alarcon Rise) extends 60 km further north to the Mexican continental margin. The Pacific-North America-Rivera triple junction is now of the RRR type, located on the risecrest 60 km south of Tamayo transform. Slow North America-Rivera rifting has ruptured the young lithosphere accreted to the east flank of the rise, and extends across the adjacent turbidite plain to the vicinity of the North America-Rivera Euler pole, which is located on the plate boundary. The present absolute motion of the Rivera microplate is an anticlockwise spin at 4° m.y.^–1 around a pole located near its southeast corner; its motion has recently changed as the driving forces applied to its margins have changed, especially with the evolution of the southern margin from a broad shear zone between Rivera and Mathematician microplates to a long Pacific-Rivera transform. Pleistocene rotations in spreading direction, by as much as 15° on the Pacific-Rivera boundary, have segmented the East Pacific Rise into a staircase of en echelon spreading axes, which overlap at lengthening and migrating nontransform offsets. The spreading segments vary greatly in risecrest geomorphology, including the full range of structural types found on other rises with intermediate spreading rates: axial rift valleys, split shield volcanoes, and axial ridges. Most offsets between the segments have migrated southward, but within the past 1 m.y. the largest of them (with 14–27 km of lateral displacement) have shown dueling behavior, with short-lived reversals in migration direction. Migration involves propagation of a spreading axis into abyssal hill terrain, which is deformed and uplifted while it occupies the broad shear zones between overlapping spreading axes. Tectonic rotation of the deformed crust occurs by bookshelf faulting, which generates teleseismically recorded strike-slip earthquakes. When reversals of migration direction occur, plateaus of rotated crust are shed onto the rise flanks.     

Structure and topography of the Siqueiros transform fault system: Evidence for the development of intra-transform spreading centers

The Siqueiros transform fault system, which offsets the East Pacific Rise between 8°20N–8°30N, has been mapped with the Sea MARC II sonar system and is found to consist of four intra-transform spreading centers and five strike-slip faults. The bathymetric and side-looking sonar data define the total width of the transform domain to be 20km. The transform domain includes prominent topographic features that are related to either seafloor spreading processes at the short spreading centers or shearing along the bounding faults. The spreading axes and the seafloor on the flanks of each small spreading center comprise morphological and structural features which suggest that the two western spreading centers are older than the eastern spreading centers. Structural data for the Clipperton, Orozco and Siqueiros transforms, indicate that the relative plate motion geometry of the Pacific-Cocos plate boundary has been stable for the past 1.5 Ma. Because the seafloor spreading fabric on the flanks of the western spreading centers is 500 000 years old and parallels the present EPR abyssal hill trend (350°) we conclude that a small change in plate motion was not the cause for intra-transform spreading center development in Siqueiros. We suggest that the impetus for the development of intra-transform spreading centers along the Siqueiros transform system was provided by the interaction of small melt anomalies in the mantle (SMAM) with deepseated, throughgoing lithospheric fractures within the shear zone. Initially, eruption sites may have been preferentially located along strike-slip faults and/or along cross-faults that eventually developed into pull-apart basins. Spreading centers C and D in the eastern portion of Siqueiros are in this initial pull-apart stage. Continued intrusion and volcanism along a short ridge within a pull-apart basin may lead to the formation of a stable, small intra-transform spreading center that creates a narrow swath of ridge-parallel structures within the transform domain. The morphology and structure of the axes and flanks of spreading centers A and B in the western and central portion of Siqueiros reflect this type of evolution and suggest that magmatism associated with these intra-transform spreading centers has been active for the past 0.5–1.0 Ma.     

Morphology and tectonics of the Galapagos Triple Junction

We describe the results of GLORIA and SEABEAM surveys, supplemented by other marine geophysical data, of the Galapagos Triple Junction where the Pacific, Cocos and Nazca plates meet. The data allowed detailed topographic and tectonic maps of the area to be produced. We located each spreading axis with a precision of about 1 km. All three plate boundaries change character as the triple junction is approached to take on morphologies typical of slower spreading axes: the fast-spreading East Pacific Rise develops the morphology of a medium-spreading rise, and the medium-spreading Cocos-Nazca Rise takes on the appearance of a slow-spreading ridge. The axis of the East Pacific Rise was found to be completely continuous throughout the survey area, where it runs along the 102°05 W meridian. The Cocos-Nazca axis, however, fails to meet it, leaving a 20-km-wide band of apparently normal East Pacific Rise crust between its tip and the East Pacific Rise axis. As a consequence there must be considerable intra-plate deformation within the Cocos and Nazca plates. A further 40 km of the Cocos-Nazca axis is characterised by oblique faulting that we interpret to be a sign of rifting of pre-existing East Pacific Rise crust. We infer that true sea-floor spreading on the Cocos-Nazca axis does not begin until 60 km east of the East Pacific Rise axis. Other areas of similar oblique faulting occur on the Pacific plate west of the triple junction and along the rough-smooth boundaries of the Galapagos Gore. We present a model involving intermittent rifting, rift propagation, and sea-floor spreading, to explain these observations.     

Regional shape and tectonics of the equatorial East Pacific Rise

The geography of the East Pacific Rise (EPR) between 10°N and 6°S, redetermined by new surface ship surveys, is characterized by long spreading axes orthogonal to infrequent transform faults. Near 2°10N the EPR is intersected by the Cocos-Nazca spreading center at the Galapagos triple junction. The present pattern was established 27-5.5 m.y.b.p. by a complex sequence of rise-crest jumps and reorientations from a section of the Pacific-Farallon plate boundary. Transverse profiles of the rise flanks can be matched by thermal contraction curves for aging lithosphere, except between the triple junction and 4°S, where the east flank is anomalously shallow and almost horizontal. Most sections of spreading axis have the 10–30 km wide, 100–400 m high, axial ridge that is characteristic of fast spreading centers. However, within 60 km of the triple junction the rise crest structure is atypical, with an axial rift valley and elevated rift mountains, despite a spreading rate of 140 mm/yr. With the exception of this atypical section, the bathymetric profile along the spreading axis is remarkably even, with continuous, gentle slopes for hundreds of kilometers between major transform faults, where step-like offsets in axial depths occur. Most of the observations can be accommodated by a model in which the long spreading axes are underlain by continuous crustal magma chambers that allow easy longitudinal flow of magma, and whose size controls the style and dimensions of EPR crestal topography.Contribution of the Scripps Institution of Oceanography, new series.     

Structural geomorphology of a fast-spreading rise crest: The East Pacific Rise near 3°25′S

A deeply-towed instrument package was used in a detailed survey of the crest of the East Pacific Rise (EPR) near 3°25S, where the Pacific and Nazca plates are separating at 152 mm/yr. A single 90 km-long traverse of the rise crest extends near-bottom observations onto the rise flanks. A ridge at the spreading axis is defined by its steep regional slopes, probably caused by rapid crustal contraction as the spreading magma chamber freezes, rather than by outward-facing fault scarps. It can be divided into a marginal horst-and-graben zone with low (<50 m), symmetric fault blocks, and a 2 km-wide elongate axial shield volcano that is unfaulted except for a narrow crestal rift zone. This has a summit graben (10–35 m deep) probably formed by caldera collapse, and narrow pillow basalt walls built over vent fissures. Small, low (<50 m) volcanic peaks occur on the shield volcano and the horst-and-graben zone, and some may have been built away from the spreading axis. Major plate-building lava flows issue from the crestal rift zone and flow an average of 500 m down the sides of the volcano. The marginal horst-and-graben zone results from tensional faulting of a thin crust of lava, and evolves by progressive shearing on inclined fault planes. Crustal extension continues at least as far as 20 km from the axis, and the large, long fault blocks formed in thicker crust beyond the subaxial magma chamber develop into abyssal hills. Pelagic sedimentation, at a maximum rate of 22 m/10⁶ years, gradually infills open fissures and smooths the small-scale roughness of the fault blocks. Off-axis volcanism has also resulted in smoother crust, and built seamounts.Comparison of the EPR at 3°25S with the Famous Rift and Galapagos Rift reveals a similarity in the processes and small-scale landforms at fast, medium and slow-spreading ridges. There are significant differences in the medium-scale landforms, probably because plate-boundary volcanic and tectonic processes act on crust of very different strength, thickness, and age.Contribution of the Scripps Institution of Oceanography, new series.