The morphology and tectonics of the Mark area from Sea Beam and Sea MARC I observations (Mid-Atlantic Ridge 23° N)

Kong, Laura S. L.; Detrick, R. S.; Fox, Paul J.; Mayer, Larry A.; Ryan, William B. F.

doi:10.1007/bf02424661

Cited by 153 publications

(119 citation statements)

References 37 publications

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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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“…Gravity data obtained along the MAR axis suggest that such segments, which are typically a few tens of kilometers long, may result from a focusing of the ridge's magma supply beneath segment centers, with attenuation of magmatism near the segment ends (Kuo and Forsyth, 1988;Lin et al, 1990;Patriat et al, 1991;Deplus et al, 1992). The MARK area comprises a series of such segments, which have been distinguished based on ridge-axis morphology and tectonics Kong et al, 1988), seismic studies (Purdy and Detrick, 1986), and gravimetry (Morris and Detrick, 1991).…”

Section: Leg 153 Preliminary Reportmentioning

confidence: 99%

“…The northern ridge segment also has a continuous neovolcanic Site 648). The boundary between the two segments is a relatively wide transition zone (extending from 23°18'N to 23°05'N) which corresponds with both a probable offset of magnetic anomalies ) and a marked change of rift-valley morphology Kong et al, 1988). It has been interpreted as a zero-offset transform fault (Purdy and Detrick, 1986) or as an accommodation zone linking two regions of different spreading styles (Brown and Karson, 1988).…”

Section: Leg 153 Preliminary Reportmentioning

confidence: 99%

“…Deformation features bear witness to a complex strain history over progressively decreasing temperatures. These early, high-temperature features are overprinted in most samples by an anastomosing foliation, highlighted by thin, white Detrick and Purdy, 1980Purdy and Detrick, 1986Cormier et al, 1984Detrick et al, 1990Mutter and Karson, 1992Toomey et al, 1985Kong et al, 1988Searle et al, 1992Karson et al, 1992aKarson and Dick, 1983Karson et al, 1987Brown and Karson, 1988Karson et al, 1992bCannat et al, unpub. data Karson and Dick, 1983Karson et al, 1987Karson, 1988 Mével et al, 1991;Auzende et al, 1993 Leg 153 Preliminary Report Page 32 1-Serpentinized harzburgite; 2-Pyroxene-rich serpentinized harzburgite; 3-Serpentinized harzburgite; 4-Diabase; 5-Serpentinized harzburgite; 6-Oxide-rich metagabbro; 7-Amphibolite and gneissic gabbro.…”

Section: Leg 153 Preliminary Reportmentioning

confidence: 99%

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Mid Atlantic Ridge

Cannat¹,

Karson²,

Miller³

1994

ODP Preliminary Report

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The principal success of Leg 153 was the recovery of extensive sections of ultramafic and mafic rocks from a slow-spreading mid-ocean ridge. Drilling was undertaken at multiple sites along the western flank of the Mid-Atlantic Ridge (MAR), south of the Kane Transform (MARK). The principal objectives of this second in a series of offset drilling legs and subsequent shore-based studies were to investigate the chemical composition and evolution of the lower crust and mantle created at a slow-spreading ridge, the geometry of magmatic and deformation structures preserved in residual peridotite and in lower crustal rocks, and the intensity and diversity of metamorphism attending crustal evolution in this slow-spreading environment.Site 920, located at latitude 23°20.32'N, about 40 km south of the Kane Transform along the western wall of the Mid-Atlantic Ridge median valley, sampled predominantly serpentinized harzburgite, interpreted to represent uplifted and altered suboceanic mantle. Drilling at this site achieved the best penetration and recovery of any drilling efforts to date in massive serpentinized peridotite. Other rock types sampled from this site include serpentinized lherzolite, dunite, clinopyroxenite, and websterite, as well as variably metamorphosed intervals of gabbro, olivine gabbro, oxide mineral-rich gabbro and gabbronorite, and plagioclase-olivine phyric diabase.Despite intense alteration of the primary minerals to serpentine, primary mineralogy and textures are widely preserved in the recovered ultramafic rocks. The dominant texture is coarse grained porphyroclastic with lobate grain boundaries and frequent 120° triple junctions, suggesting efficient grain growth at close to solidus temperature. Most samples also display an anastomosing serpentine foliation that results from the preferred orientation of fine, discontinuous veins of serpentine and iron oxide minerals cutting the background mesh-textured serpentine.Paleomagnetic studies indicate that this anastomosing foliation probably dips moderately to the east, toward the MAR neovolcanic zone. Pyroxenite and gabbroic veins, and gabbroic intervals up to at least a few decimeters thick are ubiquitous in the cored ultramafic rocks, and cover a wide range of compositions from spinel-bearing interstitial melt segregations to altered, oxide mineral-, apatite-, and zircon-bearing gabbro.Sites 921 through 924 are located in outcrops of gabbroic rocks, also along the western wall of the Mid-Atlantic Ridge median valley near its intersection with the Kane Transform. These outcrops occur on the flank of a broad, dome-like uplift, referred to as an inside-corner high or Leg 153 Preliminary Report Page 8 ridge/transform intersection massif. Sites 921, 922, and 923 lie along a north-south-trending ridge axis parallel transect, such that variations in the igneous, metamorphic, and deformational features in the rocks recovered from these sites may reflect along-axis variability in the processes that create and modify lower crustal rocks. Site 924 is offs...

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“…Thus, intrusion of dikes plays a fundamental role in crustal accretion at spreading centers. Recent progress in geomorphological studies on volcanic landforms and tectonic features suggests that volcanic processes similar to those at mid-ocean ridges also operate at Iceland; volcanic landforms comprise major volcanic edifices probably fed through long, continuous fissures, and smaller volcanic cones covering the former edifices (Kong et al, 1989;Smith and Cann, 1992). Historical andprehistorical records of earthquakes and land deformations, as well as volcanic eruptions in Iceland, suggest that such extensive lava shields are formed by large eruptions taking place at intervals of several hundred year, while central volcanoes are foci of magma intrusions at rifting episodes every 100-150 yr (Sigurdsson and Sparks, 1978).…”

Section: Introductionmentioning

confidence: 99%

Downhole Variations in Grain Size at Hole 504B: Implications for Rifting Episodes at Mid-Ocean Ridges

Umino¹

1995

Proceedings of the Ocean Drilling Program, 137/140 Scientific Results

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The maximum grain sizes of plagioclase and magnetite in the groundmass of the sheeted dike complex drilled at Hole 504B have been measured. Downhole variations through a 440-m-long section show a crude zig-zag pattern consisting of a gradual decrease or increase followed by an abrupt jump. The gradual decrease or increase in grain size extends over many lithologic units, and hence, does not reflect variations in grain size within a single dike. Such a zig-zag pattern is well explained by grain-size variations through multiple dikes. By using the observed inclination of sheeted dikes of 81° ± 2.5°, thickness of the multiple dikes varies from 0.7 to 8.5 m and averages to 4 ± 1 m. The average thickness of individual dikes forming multiple dikes is 0.8 m. We expect such multiple dikes to be formed during rifting events beneath mid-oceanic spreading ridges. If the average expansion at rifting episodes is twice as wide as the average width of the multiple dike units, the full spreading rate of 7.2 cm/yr of Cocos Ridge gives 112 ± 33 yr for a time interval of the rifting.A simple one-dimensional conductive cooling model is applied to solidification of multiple dikes. Numerical simulations show that the grain-size variations observed through the drill hole are more consistent with a model where a new injection of a dike occurs periodically with a constant time interval rather than one where the next dike intrudes just after the solidification of the previous one. Grain-size variations within simple dikes from Iritono, Japan, and those for Makaopuhi lava lake, Hawaii, show that square root of crystallization time is linearly correlated with the logarithm of plagioclase size. By using an empirically derived relationship between these two variables, the variations of plagioclase size through Hole 504B are directly compared with the calculated times for crystallization. Each rifting episode at the Costa Rica Rift lasts for several years, and periodic injection of a new dike occurs into the center of a previously solidified multiple dike at time intervals varying from 1 to 12 months.

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The morphology and tectonics of the Mark area from Sea Beam and Sea MARC I observations (Mid-Atlantic Ridge 23° N)

Cited by 153 publications

References 37 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

Mid Atlantic Ridge

Downhole Variations in Grain Size at Hole 504B: Implications for Rifting Episodes at Mid-Ocean Ridges

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