Statistical averaging of marine magnetic anomalies and the aging of oceanic crust

Blakely, Richard J.

doi:10.1029/jb088ib03p02289

Cited by 33 publications

(14 citation statements)

References 32 publications

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“…Although the thickness of the magnetized layer responsible for the magnetic lineation varied (see Blakely, 1983, for the tabulation), the vertical boundary had been assumed until the drilling of the oceanic basement of DSDP and ODP revealed a more complicated structure of the upper part of the oceanic crust. In DSDP and ODP holes in the Atlantic Ocean, the magnetization of the basalts is not homogeneous and normally and reversely magnetized sections were observed in a single hole.…”

Section: Discussionmentioning

confidence: 99%

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

Hamano¹,

Kinoshita²

1990

Proceedings of the Ocean Drilling Program

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Three components of the magnetic field in Hole 395A have been measured by a newly developed magnetometer. Observed variations of the vertical and the horizontal components of the magnetic field were interpreted by use of simple model calculations. The results indicate that the size of the homogeneously magnetized body around the hole is about 100 m, and that the normal-reverse boundary observed at 260 m sub-bottom depth in Hole 395A is tilted in an east-west direction by about 30°.

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Section: Discussionmentioning

confidence: 99%

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

Hamano¹,

Kinoshita²

1990

Proceedings of the Ocean Drilling Program

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“…However, side-scan sonar imagery south of the Atlantis transform, find evidence that eruptive features near the transform were fed by a magma source closer to the segment center [Smith et al, 1998] Rock magnetization [Johnson and Atwater, 1977;Bleil and Peterson, 1983] and magnetic anomaly studies [Blakely, 1983;Wittpenn et al, 1989] Hussenoeder et al, 1996b;Weiland et al, 1995]. The cross-axis character of magnetic anomalies are often interpreted in terms of layer 2A thickness…”

Section: Construction Of the Upper Crustmentioning

confidence: 99%

Seismic and magnetic constraints on the structure of upper oceanic crust at fast and slow spreading ridges

Hussenoeder¹

1998

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The upper ocean crust contains a comprehensive record of the shallow geological processes active along the world's mid-ocean ridge system. This thesis examines the magnetic and seismic structure of the upper crust at two contrasting ridges-the fast spreading East Pacific Rise (EPR) and the slow spreading Mid-Atlantic Ridge (MAR)-to build a more complete understanding about the roles of volcanic emplacement, tectonic disruption and hydrothermal alteration in the near-ridge environment.A technique that inverts potential field measurements. directly from an uneven observation track is developed and applied to near-bottom magnetic data from the spreading segments north of the Kane transform on the MAR. It is concluded that the central anomaly magnetization high marks the locus of focused volcanic emplacement. A cyclic faulting model is proposed to explain the oscillatory magnetization pattern associated with discrete blocks of crust being transported out of the rift valley between intensely altered fault zones. Seismic waveform and amplitude analyses of the magma sill along the EPR reveal it to be a thin «100 m) body of partial melt. These characteristics have important implications for melt availability and transport within the cycle of eruption and replenishment. A genetic algorithm-based seismic waveform inversion technique is developed and applied to on-and near-axis multichannel data from 17°20 's on the EPR and the spreading segment south of the Oceanographer transform (MAR) to map and compare for the first time the detailed velocity structure of the upper crust at two different spreading rates. Combined with conventionally processed seismic profiles, our results show that, while final extrusive thickness is comparable at all spreading ridges (300-500 m), the style of thickening may vary. While a thin (:S100 m) extrusive carapace quadruples in thickness within 1-4 km of the EPR crest, the extrusive section at the MAR achieves its final thickness within the inner valley. Both show evidence for a narrow zone of volcanic emplacement. Vigorous hydrothermalism at the EPR may produce a more rapid increase in basement velocities relative to the MAR. Rapid modification of the extrusive/dike transition at both ridges indicates that hydrothermalism is enhanced in this interval. Along-axis transport of lavas may thicken the extrusive pile at slow spreading segment ends, strengthening the magnetic highs generated by lava chemistry. Biographical SketchIn order to wax only partially rhapsodic, let me start at my formative age as a marine geophysicist ... the age of six. The first hint that I wasn't going to grow up to be a successful businessman was dropped in Germany, where my best friend and I rejected the idea of opening a lemonade stand in favor of selling fossils and geodes on the street corner. We soon went bankrupt. Our village in southern Germany was not far from Solnhofen, home of the famed Archaeopteryx. The countryside was full of geological treasures exposed in streambeds or along roadsides. Our final m...

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“…Lowrie, 1973) such that the original magnetizations are completely masked in present-day normal polarity overprint. Blakely (1983) has called on a progressive demagnetization of the source layer with time. A common characteristic of these theories is the-notion that Jurassic basalts have crossed an age threshold, beyond which the magnetic signal is diminished.…”

Section: The Jurassic Quiet Zonementioning

confidence: 99%

Paleomagnetic and Rock Magnetic Properties of Jurassic Quiet Zone Basalts, Hole 801C . . .

Wallick¹,

Steiner²

1992

Proceedings of the Ocean Drilling Program

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The paleomagnetic and rock magnetic properties of 51 Jurassic basalts from Ocean Drilling Program (ODP) Hole 801C have been examined. Magnetic properties vary with lithologic composition; alkalic rocks and hydrothermally-altered tholeiites are much weaker in intensity and generally contain higher coercivity magnetic components than the older and less-altered tholeiites at the base of the hole. For the entire column, the Jurassic basalts have an average initial natural remanent magnetization (NRM) intensity of approximately 1.24 A/m and average median destructive fields (MDF) of 8.31 mT. These values and the mean Koenigsberger ratio of 1.7 are very similar to results obtained for Jurassic basalts from the Atlantic (DSDP Leg 76). The similarities suggest that the basalts of both sites and their remanence characteristics are representative of Jurassic oceanic crust.The most profound discovery in these samples was the presence of 5 inclination zones, each showing consistent positive (or negative) polarity opposite the overlying and underlying polarity bands. We interpret these to represent a record of change in polarity of the EarüYs magnetic field and, because of the large number over such a short interval (60 m) of crust, we assert that the rapid change in polarity during the Jurassic is the probable reason behind the origin of the Jurassic Quiet Zone. INTRODUCTIONSeveral attempts have been made in the past to reach Jurassic oceanic crust within the region referred to as the Jurassic Quiet Zone in order to examine the magnetic character of samples of that age. Sites 198 (25.8°N, 154.6°E) and 199 (13.5°N, 156.2°E) were terminated due to drilling complications (Heezen, MacGregor, et al., 1973) while nearby Site 585 (13.5°'N, 156.8°E) was stopped in a thickerthan-expected sequence of Lower Cretaceous turbidites (Shipboard Scientific Party, 1985a). Most disappointing was the recovery of a thick sequence of Cretaceous intrusive and extrusive rocks from Hole 462A (17°14.5'N, 165°1.9'E) in the Nauru basin, a site that lies beyond a magnetic anomaly correlated with chron M26, thus indicating a Late Jurassic age (Shipboard Scientific Party, 1981). Further drilling of Hole 462A on Leg 89 yielded similar results (Shipboard Scientific Party, 1985b). Even on the present leg, both Site 800 (21°55.4'N, 152°19.4'E) and Site 802 (12°5.8'N, 153°12.6'E) were terminated in Cretaceous volcanic rocks . The third site, Site 801 (18°38.6'N, 156°21.6'E), however, reached its intended target and finally laid to rest any doubts that the magnetic smooth zone in this region did hold, at least in part, remnants of the Middle and Late Jurassic Pacific plate.While remote sensing studies of variations of the geomagnetic field will continue to play a major role in our understanding of crustal movements and development, they can play only a partial role in our understanding of the origins of the Jurassic Quiet Zone. Only analyses of samples collected directly from the Jurassic Quiet Zone can provide answers to the questions concerning the ...

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Statistical averaging of marine magnetic anomalies and the aging of oceanic crust

Cited by 33 publications

References 32 publications

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

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

Seismic and magnetic constraints on the structure of upper oceanic crust at fast and slow spreading ridges

Paleomagnetic and Rock Magnetic Properties of Jurassic Quiet Zone Basalts, Hole 801C . . .

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