Magnetization of the Oceanic Crust Inferred from Magnetic Logging in Hole 395A

Hamano, Yozo; Kinoshita, Hajimu

doi:10.2973/odp.proc.sr.106109.148.1990

Cited by 15 publications

(17 citation statements)

References 7 publications

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“…To extract crustal magnetic signals, we first subtracted x‐, y‐, and z‐ values of the Earth's magnetic field at the drilled site from the raw GPIT data. The GPIT logs clearly show steel drill pipe joints as well as the induced magnetic field from the bottom of the pipe (Figure S2 in the) similar to results reported in previous studies [e.g., Hamano and Kinoshita , 1990]. Weakly magnetized intervals are observed within sediment interbeds (Figure 2a) and the GPIT sediment‐basalt interface data show diagnostic curves at the top and bottom of sediment intervals, suggesting that these contacts are influenced by the induced magnetic field from the more strongly magnetized basaltic rock layers [cf Tivey et al , 2005…”

Section: Methodssupporting

confidence: 88%

“Equator Crossing” of Shatsky Rise?: New insights on Shatsky Rise tectonic motion from the downhole magnetic architecture of the uppermost lava sequences at Tamu Massif

Tominaga¹,

Evans²,

Iturrino³

2012

Geophysical Research Letters

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Shatsky Rise is a Large Igneous Province (LIP) currently located in the northwestern Pacific. New downhole magnetic logging data from Integrated Ocean Drilling Program (IODP) Hole U1347A at Tamu Massif of Shatsky Rise captured the magnetic architecture in the uppermost lava sequence, providing a rare opportunity to investigate a time series of the intra‐plate volcanism in conjunction with the Pacific plate construction history centered at the triple junction. Logging data results indicate that Tamu Massif was formed during normal polarity periods south of the paleoequator and crossed the equator at some point in the M19–M17 period. Combining these new observations with previous interpretations of the massif's tectonic history, a time series of the latitudinal tectonic motion of a LIP and the underlying Pacific plate during the plateau formation is postulated.

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Section: Methodssupporting

confidence: 88%

“Equator Crossing” of Shatsky Rise?: New insights on Shatsky Rise tectonic motion from the downhole magnetic architecture of the uppermost lava sequences at Tamu Massif

Tominaga¹,

Evans²,

Iturrino³

2012

Geophysical Research Letters

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“…Following the approach of Hamano and Kinoshita [1990], we can calculate the apparent inclination from the downhole log component data. A perfectly cylindrical hole radius R 1 , surrounded by a larger radius R 2 , horizontal cylindrical disc of homogenously magnetized crust bounded on the top and bottom of the disc at depths Z 1 and Z 2 , respectively, will have the following expression for the vertical F Z and horizontal F H magnetic fields in the hole at Z = Z 0 : and where z is depth, m is magnetization and I is magnetization inclination.…”

Section: Logging Data Interpretation and Modelingmentioning

confidence: 99%

“…The apparent inclination is calculated assuming horizontal layers, when in fact, according to the FMS data, the layers are tilted. Hamano and Kinoshita [1990] give modified field equations for a tilted layer, which predict a change in shape of the magnetic field components at the boundaries. For example, a horizontal layer magnetized only in a horizontal direction will have zero vertical component.…”

Section: Logging Data Interpretation and Modelingmentioning

confidence: 99%

“…[9] Following the approach of Hamano and Kinoshita [1990], we can calculate the apparent inclination from the downhole log component data. A perfectly cylindrical hole radius R 1 , surrounded by a larger radius R 2 , horizontal cylindrical disc of , and the polarity unit summary from previous work and this study, (d) calculated apparent inclination (black dots), where red dots indicate normal polarity and blue dots are reverse polarity estimates, (e) shipboard rock core NRM measurements of continuous core from Leg 185 (http://www.oceandrilling.org) and discrete sample measurements for Leg 129 [Wallick and Steiner, 1992], (f) diagrammatic version of computed inclination with red arrows (left-pointing) representing normal polarity and blue arrows (right-pointing) representing reverse polarity, and (g) lithologic sequence identification for reference.…”

Section: Geochemistry Geophysicsmentioning

confidence: 99%

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Downhole magnetic measurements of ODP Hole 801C: Implications for Pacific oceanic crust and magnetic field behavior in the Middle Jurassic

Tivey

Larson

Schouten

et al. 2005

Geochem Geophys Geosyst

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[1] Downhole horizontal and vertical magnetic field measurements within the 474 m thick Jurassic crustal section drilled at ODP Hole 801C in the western Pacific show anomalies that are ''in phase,'' indicating that this site formed in the southern hemisphere and moved across the paleoequator to its present location at 18.6°N. The inclination computed from the horizontal and vertical anomaly logging data varies significantly downhole and can be explained by progressive rotation of the lava sequence as it is buried during accretion. This is compatible with the observed dips in the Formation Micro Scanner (FMS) data, which show up to 42°of rotation toward the fossil spreading axis. We restore the magnetic inclination to prerotated values and obtain an estimated pretilt inclination of 39.9°± 6.6°, corresponding to a paleolatitude of 22.7°± 5°S. The magnetic logging data document six polarity units within the drilled volcanic extrusive crust. The upper 132 m thick logged section of lavas contains four polarity units, which apparently formed 7 Myr after the formation of the underlying basement. The lower 212 m logged section of the hole is reversely magnetized with an intervening normal polarity zone. More than one reversal downhole would indicate a rapid reversal rate given the time it takes to form the extrusive section of oceanic crust: $45,000 years at a fast spreading rate of 66 km/Myr. Previous results from a contemporaneous section in Spain suggest rapid reversals throughout the late Bajocian-early Bathonian Stage (170-165 Ma). Nevertheless, the complicated formation history prevents quantification of the geomagnetic reversal rate for the 801C crustal section.

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“…The outer surfaces of the lineated segments of oceanic crust of same polarity would more appropriately be modeled by prisms instead of cylinders; however, the difference between models is small for a large lateral extent (Hamano and Kinoshita, 1990).…”

Section: -3880mentioning

confidence: 99%

Implications for the Sources of Marine Magnetic Anomalies Derived from Magnetic Logging in Holes 504B and 896A

Ulrich¹,

Voehm²,

Bosum³

1996

Proceedings of the Ocean Drilling Program, 148 Scientific Results

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The magnetic fields in the deepest oceanic drill hole, DSDP/ODP Hole 504B, and in the neighboring Hole 896A, have been logged continuously with a calibrated three-axis magnetometer so that horizontal and vertical field anomalies could be determined. The extrusive basalts produce the largest anomalies (up to 5000 nT) but the amplitudes vary strongly with short wavelengths, whereas the sheeted dikes are associated with 500 to 1500 nT anomalies. The horizontal anomalies are predominantly negative and confirm the inferred inverse polarity of the drilled oceanic crust. At the base of the extrusives (oceanic Layer 2A) there are positive anomalies, presumably from basalts with normal magnetization. However, steeply dipping layers with inverse polarity could also cause positive anomalies. The vertical extent of negative anomalies in Layer 2A is only 200 m for Hole 896A and 475 m for Hole 504B, indicating a strong lateral variability in the thickness of this source layer. The fine structures of the anomalies in both holes bear little resemblance to each other but the average amplitudes of the horizontal anomalies are both around 2000 nT in the upper parts. Simple forward models as horizontal cylinders allow us to determine average magnetization values of 3 A/m for the extrusive basalts and 1.7 A/m for the sheeted dikes (Layer 2B). The former is only half the average measured rock magnetization, whereas sheeted dike values agree. At depths that correspond to the transitions in drilling from Legs 111/137 and 140/148 there are clear changes in the horizontal anomalies. These are inteΦreted as a result of drilling-induced magnetizations caused by strongly magnetic drill pipes used during Legs 137 and 140. The deduced average magnetization intensities have been used as parameters to model surface anomalies. Layers 2A and 2B contribute approximately equally to the surface anomalies with maximum amplitudes of 160 nT that agree well with the measured anomalies; thus gabbros are unlikely to contribute to marine lineated magnetic anomalies.

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Magnetization of the Oceanic Crust Inferred from Magnetic Logging in Hole 395A

Cited by 15 publications

References 7 publications

“Equator Crossing” of Shatsky Rise?: New insights on Shatsky Rise tectonic motion from the downhole magnetic architecture of the uppermost lava sequences at Tamu Massif

“Equator Crossing” of Shatsky Rise?: New insights on Shatsky Rise tectonic motion from the downhole magnetic architecture of the uppermost lava sequences at Tamu Massif

Downhole magnetic measurements of ODP Hole 801C: Implications for Pacific oceanic crust and magnetic field behavior in the Middle Jurassic

Implications for the Sources of Marine Magnetic Anomalies Derived from Magnetic Logging in Holes 504B and 896A

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