Comment on “Tectonic rotations in extensional regimes and their paleomagnetic consequences for ocean basalts” by Kenneth L. Verosub and Eldridge M. Moores

Cande, S. C.; Kent, Dennis V.

doi:10.1029/jb090ib06p04647

Cited by 27 publications

(26 citation statements)

References 21 publications

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“…Implications Perpendicular to the anomaly trend, the shape of SFS magnetic anomalies may yield seafloor tectonic information because this shape is related to the skewness angles of both the present and ancient geomagnetic inclinations [e.g., Cande, 1976;Cande and Kent, 1985]. Cande [1978] attributes skewness of SFS anomalies to a combination of the following: (1) anomalous behavior of the ancient geomagnetic field, (2) tectonic tilting, and (3) broad transition zones between the normal and re- Our observations of the magnetic terrain effect associated with volcanic cracks (Figures 8 and 9) suggest that injected basalts in dikes or in SFS-type seafloorspreading modes may have their NRM significantly deflected away from the ambient geomagnetic field [Baag and Xu, 1991;Baag et al 1990].…”

Section: Inclination and Skewness Of Seafloor Spreading (Sfs) Magnetimentioning

confidence: 94%

Deflection of paleomagnetic directions due to magnetization of the underlying terrain

Baag¹,

Helsley²,

Xu³

et al. 1995

J. Geophys. Res.

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In the summers of 1989 and 1991 we made 344 near‐ground level measurements of the ambient geomagnetic field above recent basalts on the island of Hawaii using a three‐component fluxgate magnetometer. We studied 12 surface features, including a lava pond, lava channels, long tilted blocks, smooth sloping surfaces, two fissures, and a deep U‐shaped road cut. We observed substantial differences (up to 20°) between the observed and expected (International Geomagnetic Reference Field, IGRF) magnetic field directions over these features except those composed of shelly pahoehoe and a flat (horizontally) thin lava pond. We also observed inclinations that were systematically shallower than the IGRF field by up to 5°. We show that these shallower inclinations can be explained by the magnetization of the regionally sloping surface of the southern side of the island. We found that all of the observed inclination deflections can be explained by simple two‐dimensional models which assume uniform induced and remanent magnetization parameters in the local terrain. Our observations imply that the inclination deflections cannot be corrected without a complete knowledge of the preexisting terrain and the remanence in the underlying flows upon which the lavas cooled. Since this information is rarely available, it is difficult or impossible to discriminate between dispersion of paleomagnetic directions caused by the magnetic terrain effect and dispersion due to other factors such as paleosecular variation (PSV). We therefore conclude that PSV dispersion parameters cannot be accurately determined from paleomagnetic measurements on highly magnetic volcanic flows. We also suggest that some of the geomagnetic excursions inferred from measurements on volcanic rocks may be at least in part due to the magnetic terrain effect. It is unnecessary to invoke ad hoc mechanisms such as clastic, block, or crustal rotations, distortion of the top crust, or flow deformation to explain the large between‐site dispersions or inclination anomalies observed in many of the paleomagnetic data from volcanic rocks. Our observations also bring into question the general reliability of paleomagnetic pole positions inferred from volcanic rocks, as a systematic inclination deflection due to local and regional slopes and irregular terrain, such as those we observed, would lead to a corresponding error in. the inferred paleolatitude. The magnetic terrain effects also offer alternative explanations for anomalous paleomagnetically inferred plate motions.

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Section: Inclination and Skewness Of Seafloor Spreading (Sfs) Magnetimentioning

confidence: 94%

Deflection of paleomagnetic directions due to magnetization of the underlying terrain

Baag¹,

Helsley²,

Xu³

et al. 1995

J. Geophys. Res.

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“…Instead, dipole field intensity would continue to change during otherwise constant polarity periods, producing sloped tops and bottoms on the supposedly square wave magnetization distribution. Other solutions invoked have called for sloping vertical block boundaries in the lower part of the magnetized layer due to a time lag in thermoremanent magnetization acquisition (TRM) at that level (Blakely, 1976;Cande and Kent, 1976) or to chemical remanent magnetization build-up (Raymond and LaBrecque, 1987) for several million years after TRM acquisition. However, the latter two solutions do not explain the long-term time dependency.…”

Section: Discussionmentioning

confidence: 99%

“…The potential space problem created by such strongly tilted blocks also argues against this possibility. Some portions of the marine magnetic record do not exhibit anomalous skewness, such as the M0 to Ml On sequences analyzed here (Larson and Chase, 1972;Cande and Kent, 1985), so the concept of time-varying anomalous skewness was proposed by Cande (1978). He suggested that this would result from the Earth's magnetic dipole field intensity behaving in a non-"square wave" manner during times of Notes: Strike of lineations is positive cross-strike direction plus 90°.…”

Section: °Smentioning

confidence: 99%

Skewness of Magnetic Anomalies M0 to M29 in the Northwestern Pacific

Larson¹,

Sager²

1992

Proceedings of the Ocean Drilling Program

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M-sequence magnetic anomalies from the Phoenix, Japanese, and Hawaiian lineation patterns in the western Pacific have been analyzed for cross-sectional skewness to determine a paleomagnetic apparent polar wander path for the Pacific plate for the Early Cretaceous and Late Jurassic. Results from magnetic anomalies MO to M29 have been averaged to yield pole locations ranging in age from 122 to 155 Ma both with and without the possibility of anomalous skewness. In either case these results show that the Pacific plate was moving south from 155 to 129 Ma (Kimmeridgian to Hauterivian). The plate then reversed this motion and began to move slowly to the north. The M-sequence skewness data have been combined with younger poles from seamounts to show that this slow northward drift continued until about 82 Ma, when the Pacific plate began moving more rapidly to the north. This northward drift has continued at varying rates since that time.A discrepancy in the skewness of the Phoenix lineations relative to the Hawaiian and Japanese lineations becomes apparent just prior to Ml On time and grows to a maximum at M29 time. This could be due to a yet-undiscovered plate (called here the Stealth plate) that lay between the Pacific plate and the Phoenix lineations from Ml On to M29 time. Alternatively, it could be due to time-varying anomalous skewness resulting, perhaps, from changes in magnetic field strength during normal or reversed polarity intervals, causing the magnetization pattern to differ from the assumed "square-wave" shape. Both interpretations imply similar types of north-south motion for the Pacific plate; however, the anomalous skewness interpretation also implies a large component of clockwise rotation from 145 to 82 Ma.

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“…Verosub & Moores 1981, 1985Cande & Kent 1985). The basis of recognition of tectonic tilting is that the initial orientation of pelagic sediments and basalt flows would presumably be horizontal, whereas the sheeted dyke complex would be intruded sub-vertically.…”

Section: Crustal Tiltingmentioning

confidence: 99%

The Josephine ophiolite: an ancient analogue for slow- to intermediate-spreading oceanic ridges

Alexander

Harper

1992

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The 162 Ma Josephine ophiolite, NW California and SW Oregon, consists of harzburgite tectonite (>800 km2), cumulates, high-level gabbro, a sheeted dyke complex having a consistent dyke orientation over hundreds of square kilometres, and pillow lavas. The ophiolite is conformably overlain by hemipelagics which grade upward into synorogenic turbidites. Open folding of the ophiolite and sediments provides ideal conditions to reconstruct the structural geometry of oceanic features in the ophiolite. Episodic axial magma chambers, structural extension, and episodic hydrothermal circulation is indicated by: (1) the local presence of both highly fractionated and very primitive lavas and dykes, (2) thick talus(?) breccias, and (3) multiple stacked massive sulphide deposits, each overlain by up to 5 m of mudstone within the pillow lavas. The effects of structural extension are evident from: (1) a 50 ° tilting of the entire crustal sequence except for the uppermost lava flows, (2) oceanic normal and transfer faults, and (3) shear zones within the harzburgite consisting of peridotite (+_ talc) mylonite and high-temperature serpentine mylonite. A regionally extensive, subhorizontal serpentine mylonite zone within the upper c. 1 km of the harzburgite formed at c. 500°C and may represent an oceanic detachment fault. The 50 ° tilting of the entire crustal sequence occurred prior to eruption of the uppermost lava flows and implies extreme attenuation (c. 100%) at the spreading ridge. This extension would account for the present thin crustal sequence of the Josephine ophiolite (c. 3 km thick). It is important to note that where deep faulting occurs the actual oceanic crustal thickness, as defined by seismic velocities, is likely to coincide with the lower limit of hydrothermal serpentine alteration in the harzburgite tectonite.Subseafloor hydrothermal alteration resulting from flow of discharging fluids is especially localized along brittle oceanic faults in the Josephine ophiolite. Seafloor massive sulphide deposits in the pillow lavas and mineralized stockworks in the upper dyke complex apparently formed by discharging black smoker fluids localized along oceanic fault zones. Relatively late oceanic faults have been observed in the sheeted dyke complex and are extensively altered to quartz + sulphides + sericite. They are tentatively interpreted as off-axis faults and may represent pathways for lower temperature fluids which vented to the seafloor and formed metalliferous sediments 8-23 m above the uppermost pillow basalts of the ophiolite. Although the regional setting and geochemistry of the Josephine ophiolite indicate that it formed in a suprasubduction zone setting, the following features suggest that it may provide a useful structural analogue for slow-to intermediate-spreading mid-ocean ridges: (1) episodic magmatism, (2) extensive faulting and talus deposits, (3) normal faulting extending into the upper mantle, (4) fault-controlled hydrothermal discharge, and (5) large-scale tilting of the entire crustal sequence. The hig...

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Comment on “Tectonic rotations in extensional regimes and their paleomagnetic consequences for ocean basalts” by Kenneth L. Verosub and Eldridge M. Moores

Cited by 27 publications

References 21 publications

Deflection of paleomagnetic directions due to magnetization of the underlying terrain

Deflection of paleomagnetic directions due to magnetization of the underlying terrain

Skewness of Magnetic Anomalies M0 to M29 in the Northwestern Pacific

The Josephine ophiolite: an ancient analogue for slow- to intermediate-spreading oceanic ridges

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