Along-axis variations in seafloor spreading in the MARK area

Karson, Jeffrey A.; Thompson, Geoffrey; Humphris, Susan E.; Edmond, John M.; Bryan, W. B.; Brown, Jennifer R.; Winters, Alec T.; Pockalny, R. A.; Casey, John F.; Campbell, Andrew C.; Klinkhammer, Gary P.; Palmer, Martin R.; Kinzler, Rosamond J.; Sulanowska, Margaret

doi:10.1038/328681a0

Cited by 310 publications

(257 citation statements)

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“…Certainly many normal faults and grabens exist on the Mid-Atlantic Ridge [Karson et al, 1987;Kong et al, 1988;Mutter et al, 1988;Head et al, 1996], but it is difficult to associate specific structures with individual intrusion events, because (1) no historical eruptions have been documented, (2) the neovolcanic zone is wider and less focused across axis, (3) it is harder to differentiate purely tectonic from dike-induced structures, and (4) axial volcanic ridges are the product of multiple events which coalesce, overlap, and often produce small seamounts which bury associated structures [Smith and Cann, 1993]. So, although we speculate that grabens which form on submarine slow spreading ridges tend to be wider than those on intermediate and fast ridges (as observed on land), there are not enough data from the seafloor to provide specific examples.…”

Section: Discussionmentioning

confidence: 99%

Graben formation associated with recent dike intrusions and volcanic eruptions on the mid‐ocean ridge

Chadwick¹,

Embley²

1998

J. Geophys. Res.

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Abstract. Grabens have been mapped immediately adjacent to recently erupted submarine lava flows on the Cleft and CoAxial segments of the Juan de Fuca Ridge (JdFR). The grabens are 10-100 m wide and 5-15 m deep and are located uprift and/or downrift of the eruptive vents that fed the flows. We interpret that these structures formed (or were reactivated) directly over the dike that fed the eruptions as it was intruding toward the surface. These graben structures are primary conduits for diffuse hydrothermal venting on the seafloor during the cooling of newly intruded dikes. The axial summit "graben" or "caldera" on the East Pacific Rise (EPR) in some locations has similar dimensions to the grabens observed on the JdFR, and we interpret that it too may be a dike-induced graben structure (although often buried by subsequent eruptions where it is narcowest). These grabens on the JdFR and the EPR are distinctly narrower and deeper than grabens that formed during well-documented dike intrusions on slow spreading rifts on land (Iceland and Afar). Mechanical modeling suggests that narrow grabens would form when a dike is at shallow depth where it imposes a high stress perturbation on the ambient stress state to cause faulting. Therefore the narrow grabens on the JdFR and EPR imply that a relatively high level of horizontal compressive stress typically exists perpendicular to the ridges, and this must be overcome by high dikeinduced perturbations to cause faulting. This is probably because gradual plate spreading rarely gets enough time to lower the compressive stress significantly due to the high frequency of dike intrusion events relative to the plate spreading rate. The dikes that do intrude in this environment must have relatively high internal magma pressure. Therefore the size and character of dike-induced grabens that form at the surface on intermediate to fast spreading ridges reflect the fact that volcanism dominates over tectonism in regulating the local stress state where a robust magma supply is available.

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Section: Discussionmentioning

confidence: 99%

Graben formation associated with recent dike intrusions and volcanic eruptions on the mid‐ocean ridge

Chadwick¹,

Embley²

1998

J. Geophys. Res.

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“…Crust with a significant thickness of such cumulates may thus appear anomalously thin seismically. There is also evidence in the Atlantic for substantial disruption of crust, and the presence of abundant serpentinized peridotite at high crustal levels [e.g., Karson et al, 1987;Bougault, personal communication]. In this case, seismic "crustal thickness" may reflect tectonic disturbance or depth of hydrothermal alteration rather than the volume of magma derived from the mantle.…”

Section: 1mentioning

confidence: 99%

Petrological Systematics of Mid-Ocean Ridge Basalts: Constraints on Melt Generation Beneath Ocean Ridges

Langmuir

Klein

Plank

2013

Geophysical Monograph Series

671

390

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Mid-ocean ridge basalts (MORB) are a consequence of pressure-release melting beneath ocean ridges, and contain much information concerning melt formation, melt migration and heterogeneity within the upper mantle. MORB major element chemical systematics can be divided into global and local aspects, once they have been corrected for low pressure fractionation and interlaboratory biases. Regional average compositions for ridges unaffected by hot spots ("normal" ridges) can be used to define the global correlations among normalized Na20, FeO, TiO2 and Si02 contents, CaO/Al203 ratios, axial depth and crustal thickness. Back-arc basins show similar correlations, but are offset to lower FeO and TiO2 contents. Some hot spots, such as the Azores and Galapagos, disrupt the systematics of nearby ridges and have the opposite relationships between FeO, Na 20 and depth over distances of 1000 km.Local variations in basalt chemistry from slow-and fast-spreading ridges are distinct from one another. On slow-spreading ridges, correlations among the elements cross the global vector of variability at a high angle. On the fast-spreading East Pacific Rise (EPR), correlations among the elements are distinct from both global and slow-spreading compositional vectors, and involve two components of variation. Spreading rate does not control the global correlations, but influences the standard deviations of axial depth, crustal thickness, and MgO contents of basalts.Global correlations are not found in very incompatible trace elements, even for samples far from hot spots. Moderately compatible trace elements for normal ridges, however, correlate with the major elements. Trace element systematics are significantly different for the EPR and the mid-Atlantic Ridge (MAR). Normal portions of the MAR are very depleted in REE, with little variability; hot spots cause large long wavelength variations in REE abundances. Normal EPR basalts are significantly more enriched than MAR basalts from normal ridges, and still more enriched basalts can erupt sporadically along the entire length of the EPR. This leads to very different histograms of distribution for the data sets as a whole, and a very different distribution of chemistry along strike for the two ridges. Despite these differences, the mean Ce/Sm ratios from the two ridges are identical.Existing methods for calculating the major element compositions of mantle melts [Klein and Langmuir, 1987;McKenzie and Bickle, 1988;Niu and Batiza, 1991] are critically examined. New quantitative methods for mantle melting and high pressure fractionation are developed to evaluate the chemical consequences of melting and fractionation processes and mantle heterogeneity. The new methods rely on new equations for partition coefficients for the major elements between mantle minerals and melts. The melting calculations can be used to investigate the chemical compositions produced by small extents of melting or high pressures of melting that cannot yet be determined experimentally. Application of the new models to the ...

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“…Along the Mid-Atlantic Ridge (MAR), south of the Kane Transform (MARK area), serpentinized peridotites crop out in a belt approximately 2 km wide and 20 km long along the western median valley wall [Karson et al, 1987]. These serpentinites likely were exposed during an episode of amagmatic plate separation in which little gabbroic crust was formed and detachment faulting occurred [Karson and Lawrence, 1997].…”

Section: Modeling Thermal Cracking In the Oceanic Lithospherementioning

confidence: 99%

“…These serpentinites likely were exposed during an episode of amagmatic plate separation in which little gabbroic crust was formed and detachment faulting occurred [Karson and Lawrence, 1997]. Serpentinites collected along the seafloor in this region using Alvin and Nautile include harzburgites, peridotites containing primarily olivine and orthopyroxene, that are altered to form massive or schistose serpentinites [Karson et al, 1987]. During ODP Leg 153, serpentinized harzburgites were sampled to depths of 200 m in this area [Cannat et al, 1995].…”

Section: Modeling Thermal Cracking In the Oceanic Lithospherementioning

confidence: 99%

Experimental and seismological constraints on the rheology, evolution, and alteration of the lithosphere at oceanic spreading centers

deMartin¹

2007

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Oceanic spreading centers are sites of magmatic, tectonic, and hydrothermal processes. In this thesis I present experimental and seismological constraints on the evolution of these complex regions of focused crustal accretion and extension. Experimental results from drained, triaxial deformation experiments on partially molten olivine reveal that melt extraction rates are linearly dependent on effective mean stress when the effective mean stress is low and non-linearly dependent on effective mean stress when it is high. Microearthquakes recorded above an inferred magma reservoir along the TAG segment of the Mid-Atlantic Ridge delineate for the first time the arcuate, subsurface structure of a long-lived, active detachment fault. This fault penetrates the entire oceanic crust and forms the high-permeability pathway necessary to sustain long-lived, high-temperature hydrothermal venting in this region. Long-lived detachment faulting exhumes lower crustal and mantle rocks. Residual stresses generated by thermal expansion anisotropy and mismatch in the uplifting, cooling rock trigger grain boundary microfractures if stress intensities at the tips of naturally occurring flaws exceed a critical stress intensity factor. Experimental results coupled with geomechanical models indicate that pervasive grain boundary cracking occurs in mantle peridotite when it is uplifted to within 4 km of the seafloor. Whereas faults provide the high-permeability pathways necessary to sustain high-temperature fluid circulation, grain boundary cracks form the interconnected network required for pervasive alteration of the oceanic lithosphere. This thesis provides fundamental constraints on the rheology, evolution, and alteration of the lithosphere at oceanic spreading centers.

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Along-axis variations in seafloor spreading in the MARK area

Cited by 310 publications

References 0 publications

Graben formation associated with recent dike intrusions and volcanic eruptions on the mid‐ocean ridge

Graben formation associated with recent dike intrusions and volcanic eruptions on the mid‐ocean ridge

Petrological Systematics of Mid-Ocean Ridge Basalts: Constraints on Melt Generation Beneath Ocean Ridges

Experimental and seismological constraints on the rheology, evolution, and alteration of the lithosphere at oceanic spreading centers

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