Magnetic properties of some young basalts from the East Pacific Rise

Sempéré, Jean‐Christophe; Meshkov, Alexandra; Thommeret, Michel; Macdonald, Ken C.

doi:10.1007/bf00369245

Cited by 20 publications

(7 citation statements)

References 38 publications

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“…Highly magnetized rocks have been dredged at propagating ridge tips, and at ridge-transform intersections where they are assumed to reflect fractionation within the distal ends of a shallow magma chamber (Sinton et al, 1983;Sempere et al, 1988). We have carried out three-dimensional analysis of the magnetic data within a region of the 31 ~ 34~ ' S survey in order to precisely locate these high amplitude anomalies and assess their possible association with a geochemical source.…”

Section: High Amplitude Magnetic Anomaliesmentioning

confidence: 99%

Spreading rates, rift propagation, and fracture zone offset histories during the past 5 my on the Mid-Atlantic Ridge; 25°–27°30′ S and 31°–34°30′ S

1991

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Abstract. The southern Mid-Atlantic Ridge (MAR) is spreading at rates (34-38mmyr -l) that fall within a transitional range between those which characterize slow and intermediate spreading center morphology. To further our understanding of crustal accretion at these transitional spreading rates, we have carried out analysis of magnetic anomaly data from two detailed SeaBeam surveys of the MAR between 25~176 and 31~176 S. Within these areas, the MAR is subdivided into 9 ridge segments bounded by large-and short-offset discontinuities of the ridge axis. From two-dimensional Fourier inversions of the magnetic anomaly data we establish the history of spreading within each ridge segment for the past 5 my and the evolution of the bounding ridge-axis discontinuities. We see evidence for the initiation and diminishment of small-offset discontinuities, and for the transition of rigid large-offset transform faults to less stable short-offset features. Individual ridge segments display independent spreading histories in terms of both the sense and amount of asymmetric spreading within each which have given rise to changes through time in the lengths of bounding ridge-axis discontinuities. Over the past 3-5 my, the short-offset discontinuities within the area have lengthened/shortened by approximately the same amount (~ 10 km). During this same time period, larger-offset transform faults have remained comparatively constant in length. A shift in plate motion at anomaly 3 time may have given rise to change in the length of short-offset second-order discontinuities. However, the pattern of lengthening/shortening short-offset discontinuities we see is not simply related to the geometry of the plate boundary in these regions which precludes a simply relationship between plate motion changes and response at the plate boundary. We document a case of rapid (minimum 60 mm yr-t) small-scale rift propagation, occurring between 2.5 and 1.8 my, associated with transition of the Moore transform fault to an oblique-trending ridge-axis discontinuity. Propagation across the Moore discontinuity and similar propagation within the 31~176 " S area may be associated with the reduced age contrast in lithosphere across second-order discontinuities.Total opening rates within our northern survey area decreased from anomaly 4' to 2 time and rates within both areas have increased since the Jaramillo. Total opening rates measured for anomaly intervals differ along the plate boundary significantly, more than expected with changing distance to the pole of rotation. These differences imply a degree of short-term non-rigid plate behaviour which may be associated with ridge segments acting as independent spreading cells. Magnetic polarity transition widths from our inversion studies may be used to infer a zone of crustal accretion which is 3-6 km wide, within the inner floor of the rift valley. A systematic increase of transition width with age would be expected if deeper crustal sources dominate the magnetic signal in older crust but this is not observed. W...

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Section: High Amplitude Magnetic Anomaliesmentioning

confidence: 99%

Spreading rates, rift propagation, and fracture zone offset histories during the past 5 my on the Mid-Atlantic Ridge; 25°–27°30′ S and 31°–34°30′ S

1991

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“…The rocks with the reversed polarity of RM have been considered as the spurious ones by the respected authors. The data of the submarine igneous rocks under consideration have been published by Bina (1990);Furuta (1993); Kelso et al (1996);Kirpatrik (1978); Levi (1983); Petersen (1979);Prévot et al (1979);Sager et al (2008); Sempere et al (1988).…”

Section: Experimental Results and Their Evaluationmentioning

confidence: 99%

“…Basalts from the ridge axis (age < 0.01 m.y), from 0.7 m.y. old crust on the flanks of the spreading Center, and basalts from the OSC -overlapping spreading centers near 9 • N on the East Pacific Rise -high magnetized zone (Sempere et al, 1988); 648BU -recent basalts (Bina, 1990); 396B -13.0 m.y. basalts (Petersen, 1979;Kirpatrick, 1979).…”

Section: Experimental Results and Their Evaluationmentioning

confidence: 99%

The self-reversal origin of the reversed remanent magnetization in the igneous rocks containing the oxidized Fe-Ti oxide solid solutions

Orlický

2011

Contributions to Geophysics and Geodesy

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I study the magnetic and paleomagnetic characteristics of the Pleistocene to Quaternary basalts from the West Carpathian Mountains. I have revealed the existence of the so-called self-reversal remanent magnetization bearing disordered-antiferromagnetic Fe-Ti phases, with the Néel temperatures of either TN = 430−450• C (the Ti-Mgh phases) or with TN = 460 − 470• C (Hem-Ilm phases). I have suggested for the first time to name of these phases in this article. It is very important that these phases are the only sources of the reversed RM, of the self-reversal origin, probably of the chemical remanent magnetization (CRM) origin. The basalts of Southern Slovakia were differentiated to these of the magnetite and Ti-rich Ti-Mt bearing rocks with normal polarity of RM, and those with the presence of the self-reversal RM bearing disordered-antiferromagnetic Ti-Mgh phases of the TN = 430 to 450• C, and those of the disordered-antiferromagnetic Hem-Ilm phases of the TN = 460 − 470• C, both of only reversed RM of the self-reversal origin. I present the original model based on the principe that the magnetite, hematite and Ti-rich titanomagnetite bearing rocks, without any disordered-antiferromagnetic phases, carry only normal -positive RM. The self-reversal remanent magnetization bearing disorderedantiferromagnetic magnetic phases of Fe-Ti oxide solid solutions carry only reversed remanent magnetization in the rocks. This model has been involved to explain the reversed or normal RM of submarine igneous rocks published in the literature. We anticipate that in all continental or submarine reversally magnetized rocks the disordered-antiferromagnetic Fe-Ti phases would be revealed, either in their initial, complete developed stages, or as the remnants of these phases.We suggest that the reverse remanent magnetization of rocks is due to the existence of the self-reversal remanent magnetization bearing disordered-antiferromagnetic magnetic phases of Fe-Ti solid solutions in the rocks, but not due to the existence of the reversals of the geomagnetic field.

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“…A similar explanation for a large scale axial low in the measured anomaly was proposed by Tamsett and Girdler (1982) for magnetic data from the Gulf of Aden. For very fresh pillows and flows, the Curie temperature is approximately 180~ (Prevot et al, 1979;Johnson and Atwater, 1977;Sempere et al, 1988) while it is approximately 575~ for gabbros (Kent et al, 1978;Dunlop and Prevot, 1982). Shallow Curie isotherms may be present in the area resulting from the thermal structure of the crust at such an extreme spreading rate.…”

Section: Axial Highsmentioning

confidence: 96%

Deep-Tow magnetics near 20°S on the East Pacific Rise: A study of short wavelength anomalies at a very fast spreading center

1990

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Abstract. Six Deep-Tow magnetic profiles across the axis of the East Pacific Rise [EPR] in two small areas between 19~ ' and 20~ were collected during the 1983 Protea 1 cruise of the R/V Melville. These near-bottom profiles are of extremely high resolution allowing the interpretation of very short wavelength features. We have inverted the magnetic field data to determine the rock magnetization distribution near the axis of this ultrafast speading center (162 mm yr t). The solutions reveal large amplitude (up to 35 A m -l) short wavelength (I-3 km) variations in magnetization. Specifically all crossings show a narrow (0.5 to 1.5 km) low in magnetization superimposed on a broader (2.5 to 4 km) high directly over the ridge axis. Four profiles in the northern area (19~ ' to 19'~33'S) also show symmetrical near-axis (within 4 km) lows which are remarkably continuous along strike. Explanations for the short-wavelength variations are discussed which fall into the following categories: (1) variations in the thickness of the magnetized layer, (2) variations in rock chemistry (e.g. alteration due to hydrothermal activity), and (3) paleofield intensity variations. None of the mechanisms discussed alone adequately explain the observed phenomena in the study area or on a world-wide scale. Further sampling and high resolution surveying will be required in order to accurately determine the relative importance of the mechanisms discussed.

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Magnetic properties of some young basalts from the East Pacific Rise

Cited by 20 publications

References 38 publications

Spreading rates, rift propagation, and fracture zone offset histories during the past 5 my on the Mid-Atlantic Ridge; 25°–27°30′ S and 31°–34°30′ S

Spreading rates, rift propagation, and fracture zone offset histories during the past 5 my on the Mid-Atlantic Ridge; 25°–27°30′ S and 31°–34°30′ S

The self-reversal origin of the reversed remanent magnetization in the igneous rocks containing the oxidized Fe-Ti oxide solid solutions

Deep-Tow magnetics near 20°S on the East Pacific Rise: A study of short wavelength anomalies at a very fast spreading center

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