Overlapping rift zones at the 5.5°S offset of the East Pacific Rise

A Seabeam and magnetometer survey of the Pacific-Nazca plate boundary around 5.5°S mapped a “nontransform offset” whose geology and kinematics seem typical of a whole class of structures formed where fast-spreading rises are laterally offset for distances less than the width of a subaxial magma chamber. Though the spreading center is abruptly displaced by 15 km right laterally, there is no trace of strike-slip faulting. Instead the ends of two 100 km-long axial volcanic ridges veer 15° toward each other and overlap for more than 20 km. In this overlapping region, between 5°22′S and 5°32′S, accretion of the upper oceanic crust by dike injection and eruption is partitioned between two adjacent and parallel rift zones with oblique and one-sided spreading. A continuous but dog-legged magma chamber is inferred to underlie both rift zones and an intervening 9-km wide volcano-studded basin. A strong magnetic signature from the crust, which accreted around this oblique chamber identifies the trace of the offset on the adjacent rise flank; in the past 0.5×106 years the southern axial ridge has propagated north at 40 mm/yr, 55% of the spreading half-rate.

[1]  Roger N. Anderson,et al.  A model for the horst and graben structure of Midocean ridge crests based upon spreading velocity and basalt delivery to the Oceanic crust , 1973, Geofísica Internacional.

[2]  W. Duffield.  Structure and origin of the Koae fault system, Kilauea Volcano, Hawaii , 1975 .

[3]  D. Gray,et al.  Tectonic evolution of the FAMOUS area of the Mid-Atlantic Ridge, lat 35°50′ to 37°20′N , 1977 .

[4]  R. Searle,et al.  Sonar studies of the Mid-Atlantic Ridge and Kurchatov Fracture Zone , 1977 .

[5]  J. Karson,et al.  Geometry of transform zones , 1980, Nature.

[6]  G. L. Johnson,et al.  Recent plate motions in the Galapagos area , 1977 .

[7]  K. Nelson.  A simple thermal-mechanical model for mid-ocean ridge topographic variation , 1981 .

[8]  B. Nordlie.  Morphology and Structure of the Western Galápagos Volcanoes and a Model for Their Origin , 1973 .

[9]  D. Rea.  Analysis of a fast-spreading rise crest: The East Pacific Rise, 9° to 12° south , 1976 .

[10]  J. G. Moore,et al.  The 1962 flank eruption of Kilauea volcano and structure of the east rift zone , 1964 .

[11]  B. Rosendahl.  Evolution of oceanic crust: 2. Constraints, implications, and inferences , 1976 .

[12]  P. J. Fox,et al.  Overlapping spreading centres: new accretion geometry on the East Pacific Rise , 1983, Nature.

[13]  P. Lonsdale.  Regional shape and tectonics of the equatorial East Pacific Rise , 1977 .

[14]  Kazuaki Nakamura,et al.  EN ECHELON FEATURES OF ICELANDIC GROUND FISSURES , 1970 .

[15]  D. Rea.  Model for the Formation of Topographic Features of the East Pacific Rise Crest , 1975 .

[16]  A. Furumoto.  Nature of the magma conduit under the east rift zone of Kilauea Volcano, Hawaii , 1978 .

[17]  K. Macdonald.  Mid-Ocean Ridges: Fine Scale Tectonic, Volcanic and Hydrothermal Processes Within the Plate Boundary Zone , 1982 .

[18]  Ken C. Macdonald,et al.  Deep-tow studies of the structure of the Mid-Atlantic Ridge crest near lat 37°N , 1977 .

[19]  N. Kusznir.  Thermal evolution of the oceanic crust; its dependence on spreading rate and effect on crustal structure , 1980 .

[20]  G. A. Macdonald,et al.  Hawaiian Volcanoes During 1955 , 1964 .

[21]  N. Sleep,et al.  Topography and tectonics of Mid‐Oceanic Ridge axes , 1979 .

[22]  P. Lonsdale.  Structural geomorphology of a fast-spreading rise crest: The East Pacific Rise near 3°25′S , 1977 .

[23]  P. Lonsdale.  Near-Bottom Reconnaissance of a Fast-Slipping Transform Fault Zone at the Pacific-Nazca Plate Boundary , 1978, The Journal of Geology.

[24]  R. Searle,et al.  Fine-Scale Sonar Study of Tectonics and Volcanism on the Reykjanes Ridge , 1981 .

[25]  Richard N. Hey,et al.  Propagating rifts on midocean ridges , 1980 .

[26]  Douglas S. Wilson,et al.  The Galapagos axial magnetic anomaly: Evidence for the Emperor Event within the Brunhes and for a two-layer magnetic source , 1981 .

[27]  David K. Rea,et al.  Asymmetric sea-floor spreading and a nontransform axis offset: The East Pacific Rise 20°S survey area , 1978 .

[28]  D. Rea.  Changes in the axial configuration of the East Pacific Rise near 6°S during the past 2 m.y. , 1976 .

[29]  D. W. Peterson,et al.  Storage, migration, and eruption of magma at Kilauea volcano, Hawaii, 1971–1972 , 1982 .

[30]  G. Walker.  Excess spreading axes and spreading rate in Iceland , 1975, Nature.

[31]  D. Rea.  Tectonics of the Nazca-Pacific divergent plate boundary , 1981 .

[32]  R. E. Wilcox,et al.  Basic Wrench Tectonics , 1973 .

[33]  J. Delaney,et al.  A detailed study of the Cobb Offset of the Juan de Fuca Ridge: Evolution of a propagating rift , 1983 .

[34]  T. Juteau,et al.  Basaltic pillars in collapsed lava-pools on the deep ocean-floor , 1979, Nature.

[35]  R. Blakely,et al.  Short-wavelength magnetic anomalies in a region of rapid seafloor spreading , 1975, Nature.

[36]  I. Gibson.  The structure and volcanic geology of an axial portion of the Main Ethiopian Rift , 1969 .

[37]  S. Jakobsson,et al.  Petrology of the Western Reykjanes Peninsula, Iceland , 1978 .

[38]  P. Lipman.  Rates of volcanic activity along the southwest rift zone of Mauna Loa volcano, Hawaii , 1980 .

[39]  J. Delaney,et al.  Petrologic consequences of rift propagation on oceanic spreading ridges , 1983 .

[40]  Donald A. Swanson,et al.  The February 1969 east rift eruption of Kilauea Volcano, Hawaii , 1976 .

[41]  E. D. Jackson,et al.  Orientation and growth of Hawaiian volcanic rifts: the effect of regional structure and gravitational stresses , 1972, Proceedings of the Royal Society of London. A. Mathematical and Physical Sciences.

[42]  M. McNutt,et al.  A near-bottom geophysical traverse of the Reykjanes Ridge , 1978 .

[43]  F. Spiess,et al.  Deep-Tow Observations at the East Pacific Rise, 8°45'N, and Some Interpretations , 1980 .

[44]  Roger N. Anderson,et al.  Topography and evolution of the East Pacific Rise between 5°S and 20°S , 1972 .

[45]  H. Schouten,et al.  The memory of the accreting plate boundary and the continuity of fracture zones , 1982 .

[46]  Donald A. Swanson,et al.  Displacement of the south flank of Kilauea Volcano; the result of forceful intrusion of magma into the rift zones , 1976 .