Magnetic polarity structure of the lower oceanic crust

Allerton, Simon; Tivey, Maurice A.

doi:10.1029/2000gl008493

Cited by 36 publications

(55 citation statements)

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“…Magnetic anomaly "stripes" are detectable across the Atlantis II transverse ridge, including Atlantis Bank and its wave-cut platform (Dick et al, 1991b;Allerton and Tivey, 2001;Hosford et al, 2003;Baines et al, 2007Baines et al, , 2008. Whereas most of the platform is reversely magnetized, as are the gabbro intervals of Holes 735B and 1105A (Kikawa and Pariso, 1991), a narrow normal polarity zone ~2 km wide is present near its northern end.…”

Section: What Is the Nature Of Magnetic Anomalies?mentioning

confidence: 99%

“…Magnetic anomalies hosted within the gabbro massif indicate it was exposed on a 38.9 km continuously slipping fault over 2.7 My, beginning ~13 Ma and coinciding with accelerated spreading to ~17 mm/y, with sharply increased asymmetry of 14.1 mm/y to the south (Baines et al, 2008). Paleomagnetic remanence inclinations from Holes 735B and 1105A and JR31 seabed drill cores are consistently ~20° steeper than expected for the latitude of Atlantis Bank and are interpreted on the basis of the oriented BRIDGE drill cores as indicating a southward tilt of this magnitude subsequent to cooling of the section through the Curie temperature of magnetite (580°C) (Kikawa and Pariso, 1991;Pettigrew, Casey, Miller, et al, 1999;Allerton and Tivey, 2001).…”

Section: Geological Settingmentioning

confidence: 99%

“…Whereas most of the platform is reversely magnetized, as are the gabbro intervals of Holes 735B and 1105A (Kikawa and Pariso, 1991), a narrow normal polarity zone ~2 km wide is present near its northern end. This zone is equated to anomaly Chron C5r.2n (11.592-11.647 Ma; Allerton and Tivey, 2001;Baines et al, 2008) (Figure F7). The zone is detectable in sea-surface (Dick et al, 1991b;Hosford et al, 2003) and deep-towed magnetic profiles (Allerton and Tivey, 2001), as well as in the JR31 seabed drill cores; however, the precise location of the boundary in each case is slightly offset.…”

Section: What Is the Nature Of Magnetic Anomalies?mentioning

confidence: 99%

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2017

Proceedings of the International Ocean Discovery Program

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Section: What Is the Nature Of Magnetic Anomalies?mentioning

confidence: 99%

Section: Geological Settingmentioning

confidence: 99%

Section: What Is the Nature Of Magnetic Anomalies?mentioning

confidence: 99%

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2017

Proceedings of the International Ocean Discovery Program

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“…The only study of polarity boundary geometry in lower crust was done at Atlantis Bank on the South West Indian Ridge (SWIR) and showed the gabbro layer dipping away from the ridge axis at a very shallow angle of 5', after restoring a back tilt of 200 [Allerton and Tivey, 2001].…”

Section: Introductionmentioning

confidence: 99%

“…n) in lower crustal rocks is also exposed on the seafloor at Atlantis Bank (SWIR) [Dick et al, 1991]. This boundary is estimated to be dipping at -50 away from the ridge axis, after correction for crustal backtilt [Allerton and Tivey, 2001] An extensive bathymetric, magnetic and sea-floor sampling survey of the Kane Megamullion was carried out in 2004, onboard the RN Knorr (KN180-2). Seafloor samples were collected both in situ by the remotely operated vehicle (ROV) Jason II and dredged from the seafloor.…”

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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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Magnetic properties of variably serpentinized peridotites and their implication for the evolution of oceanic core complexes

Maffione

Morris

Plümper

et al. 2014

Geochem. Geophys. Geosyst.

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Serpentinization of ultramafic rocks during hydrothermal alteration at mid-ocean ridges profoundly changes the physical, chemical, rheological, and magnetic properties of the oceanic lithosphere. There is renewed interest in this process following the discovery of widespread exposures of serpentinized mantle on the seafloor in slow spreading oceans. Unroofing of mantle rocks in these settings is achieved by displacement along oceanic detachment faults, which eventually results in structures known as oceanic core complexes (OCCs). However, we have limited understanding of the mechanisms of serpentinization at the seafloor and in particular their relationship with the evolution of OCCs. Since magnetite is a direct product of serpentinization, the magnetic properties of variably serpentinized peridotites can provide unique insights into these mechanisms and their evolution in the oceanic lithosphere. Here we present new results from an integrated, rock magnetic, paleomagnetic, and petrological study of variably serpentinized peridotites from the first fossil OCC recognized in an ophiolite. Integration with existing data from mid-ocean ridge-related abyssal peridotites recovered from several scientific ocean drilling sites yields the first magnetic database from peridotites extending across the complete range (0-100%) of degrees of serpentinization. Variations in a range of magnetic parameters with serpentinization, and associated paleomagnetic data, provide: (i) key constraints on the mechanism(s) of serpentinization at mid-ocean ridges; (ii) insights on the potential for serpentinized peridotites to contribute to marine magnetic anomalies; and (iii) evidence that leads to a new conceptual model for the evolution of serpentinization and related remanence acquisition at OCCs.

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Magnetic polarity structure of the lower oceanic crust

Cited by 36 publications

References 32 publications

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

Magnetic properties of variably serpentinized peridotites and their implication for the evolution of oceanic core complexes

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