Mid-Ocean Ridges: Discontinuities, Segments and Giant Cracks

Macdonald, Ken C.; Scheirer, Daniel S.; Carbotte, S. M.

doi:10.1126/science.253.5023.986

Cited by 273 publications

(203 citation statements)

References 68 publications

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“…As the CAMH represents a time-integrated signal, a magnetic discontinuity would indicate ridge processes that were stable on timescales of a few thousand to tens of thousands of years [Macdonald et al, 1991;; Lee et al, 1996;White et al, 2002]. A correlation between the location of a present day bathymetric discontinuity and a magnetic discontinuity would provide evidence for the relatively stable position of the ridge discontinuity over these timescales.…”

Section: Magnetic Surveys Betweenmentioning

confidence: 99%

“…Indeed, documented eruptions in both 1991 and 2006 between 90 51 '-46' N [Haymon et al, 1993;Gregg et al, 1996;Tolstoy et al, 2006; and extensive hydrothermal activity in the area However, any correlation between the CAMH and the ridge morphology, and consequently crustal accretion processes, has remained ambiguous because of the inherent low resolution of the sea surface magnetic data used in these studies and the difficulty in mapping the NVZ. As the CAMH represents a time-integrated signal, a magnetic discontinuity would indicate ridge processes that were stable on timescales of a few thousand to tens of thousands of years [Macdonald et al, 1991;Lee et al, 1996; White et al, 2002]. A correlation between the location of a present day bathymetric discontinuity and a magnetic discontinuity would provide evidence for the relatively stable position of the ridge discontinuity over these timescales.…”

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confidence: 99%

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Oceanic lithosphere magnetization : marine magnetic investigations of crustal accretion and tectonic processes in mid-ocean ridge environments

Williams¹

2007

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The origin of symmetric alternating magnetic polarity stripes on the seafloor is investigated in two marine environments; along the ridge axis of the fast spreading East Pacific Rise (EPR) (90 25'-90 55'N) and at Kane Megamullion (KMM) (230 40'N), near the intersection of the slow-spreading Mid Atlantic Ridge with Kane Transform Fault. Marine magnetic anomalies and magnetic properties of seafloor samples are combined to characterize the magnetic source layer in both locations. The EPR study suggests that along-axis variations in the observed axial magnetic anomaly result from changing source layer thickness alone, consistent with observed changes in seismic Layer 2a. The extrusive basalts of the upper crust therefore constitute the magnetic source layer along the ridge axis and long term crustal accretion patterns are reflected in the appearance of the axial anomaly. At KMM the C2r.2r/C2An. In (-2.581 Ma) polarity reversal boundary cuts through lower crust (gabbro) and upper mantle (serpentinized peridotites) rocks exposed by a detachment fault on the seafloor, indicating that these lithologies can systematically record a magnetic signal. Both lithologies have stable remanent magnetization, capable of contributing to the magnetic source layer. The geometry of the polarity boundary changes from the northern to the central regions of KMM and is believed to be related to changing lithology. In the northern region, interpreted to be a gabbro pluton, the boundary dips away from the ridge axis and is consistent with a rotated conductively cooled isotherm. In the central region the gabbros have been removed and the polarity boundary, which resides in serpentinized peridotite, dips towards the ridge axis and is thought to represent an alteration front. The linear appearance of the polarity boundary across both regions indicates that the two lithologies acquired their magnetic remanence during approximately the same time interval. Seismic events caused by detachment faulting at Kane and Atlantis Transform Faults are investigated using hydroacoustic waves (T-phases) recorded by a hydrophone array. Observations and ray trace models of event propagation show bathymetric blockage along propagation paths, but suggest current models of T-phase excitation and propagation need to be improved to explain observed characteristics of T-phase data. AcknowledgementsA thesis may have your name on the title page, but it is truly the joint effort of many people that gets you through to the end of a Ph.D.. I am very grateful to my main thesis advisor Maurice Tivey, who taught me so much about magnetics and always had time to talk about my research when I knocked on his door. Learning to write a scientific paper has been a long learning curve and I really appreciate Maurice's editing skills and stamina for reading drafts of my papers. I have been lucky enough to be in the right place at the right time to work on some excellent datasets, particularly the Kane Megamullion data that makes up the majority of my thesis, and I thank Mau...

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Section: Magnetic Surveys Betweenmentioning

confidence: 99%

mentioning

confidence: 99%

Oceanic lithosphere magnetization : marine magnetic investigations of crustal accretion and tectonic processes in mid-ocean ridge environments

Williams¹

2007

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“…As seafloor maps are made at higher levels of resolution and with more complete spatial coverage, progressively finer scales of segmentation become apparent. Classification of segments based on segment length, RAD offset size, geochemical variations, and longevity of segments and their bounding RADs permit the recognition of similar orders of segmentation on different ridges [Langmuir et al, 1986;Macdonald et al, 1988Macdonald et al, , 1991]. Yet the origins of these scales of ridge segmentation remain elusive.…”

Section: Introductionmentioning

confidence: 99%

Basaltic lava domes, lava lakes, and volcanic segmentation on the southern East Pacific Rise

White¹,

Macdonald²,

Haymon³

2000

J. Geophys. Res.

145

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Abstract. Meter-scale DSL-120 sonar mapping and coregistered Argo II photographic observations reveal changes in eruptive style that closely follow the third-order structural segmentation of the ridge axis on the southern East Pacific Rise, 17ø11'-18ø37'S. Near segment ends we observe abundant basaltic lava domes which average 20 rn in height and 200 rn in basal diameter and have pillow lava as the dominant lava morphology. The ubiquity of pillow lava suggests low effusion rate eruptions. The abundance of lava domes suggests that the fissure eruptions were of sufficient duration to focus and produce a line of volcanic edifices. Near segment centers we observe fewer but larger lava domes, voluminous drained and collapsed lava lakes, and smooth lobate and sheet lava flows with very little pillow lava. The abundance of sheet flows suggests that high effusion rate eruptions are common. Fewer lava domes and large lava lakes suggest that fissure eruptions do not focus to point sources. This pattern was observed on eight third-order ridge segments suggesting that a fundamental volcanic segmentation of the ridge occurs on this scale. The third-order segment boundaries also correlate with local maxima in the seismic axial magma chamber reflector depth throughout the study area and decreased across-axis width of the region of seismic layer 2A thickening along the one segment where sufficient cross-axis seismic lines exist. The geochemically defined magmatic segment boundaries in the study area match the locations of our volcanic segment boundaries, although rock sampling density is not adequate to constrain the variation across all the third-order volcanic segments that we identify. These observations suggest that variation in the processes of crustal accretion along axis occurs at a length scale of tens of kilometers on superfast spreading (> 140 km/Myr full rate) mid-ocean ridges.

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“…In order to understand how a favourable tectonic and volcanic setting may develop at NTDs for hydrothermal circulation, the geometrical influence on NTD stresses and volcanotectonic structure need to be understood. A geometrical classification of NTDs based on observation and structural criteria has already been attempted [4,18]. Three distinct types of NTDs were identified along the MidAtlantic Ridge.…”

Section: Introductionmentioning

confidence: 99%

Numerical modelling of non-transform discontinuity geometry: Implications for ridge structure, volcano-tectonic fabric development and hydrothermal activity at segment ends

Tyler

Bull

Parson

et al. 2007

Earth and Planetary Science Letters

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Ocean ridge discontinuities partition and offset spreading centres at a range of scales. Large scale discontinuities (10's-100's km) are synonymous with first-order transform faults, which have well defined linear fault zone valleys. In contrast, NonTransform Discontinuities (NTDs) are diffuse, smaller scale offsets (0 to b 20 km), characterised by central basins or topographic highs. The geometry of NTD offsets can be categorised by the sense of offset, either right-stepping or left-stepping, and by the relative positions of the segment tips. The segment tip configurations include under-lapping, over-lapping or simple across-axis jumps or stepping in the ridge axis. In this study finite difference software is used to model segment geometry at a slow-spreading ridge under a normal tensile-stress within a homogeneous and isotropic medium. Along-and across-axis segment separations were varied incrementally for left-and right-stepping senses. The results show that the ratio of along-axis to across-axis segment tip separation is a dominant control of stress field rotation within an NTD. Features which most clearly show rotation within an NTD include basins and tectonically controlled constructional ridges. The obliquity of these features along with measurements of the surrounding fault fabrics are used as a way of observing and determining stress rotations within NTDs along the Central Indian Ridge (CIR). These rotations were used to obtain segment geometries from models where the central tensor showed an equivalent rotation. The results show that geometry has a profound effect on stress field rotation under which large-and small-scale volcanotectonic fabrics form. In addition, a shortfall of the predicted model tip relative to interpreted positions, along with morphology and observation of the ridge fabrics at the terminations to some segments, suggests the existence of a zone, broadly analogous to the process zone observed in fracture mechanics, which we call a damage zone. Given the criteria for the promotion of hydrothermal circulation, this damage zone would have a greater potential for hosting hydrothermal activity.

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Mid-Ocean Ridges: Discontinuities, Segments and Giant Cracks

Cited by 273 publications

References 68 publications

Oceanic lithosphere magnetization : marine magnetic investigations of crustal accretion and tectonic processes in mid-ocean ridge environments

Oceanic lithosphere magnetization : marine magnetic investigations of crustal accretion and tectonic processes in mid-ocean ridge environments

Basaltic lava domes, lava lakes, and volcanic segmentation on the southern East Pacific Rise

Numerical modelling of non-transform discontinuity geometry: Implications for ridge structure, volcano-tectonic fabric development and hydrothermal activity at segment ends

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