Paleomagnetic results from granitic basement rocks in the Cajon Pass Scientific Drillhole

McWilliams, Michael; Pinto, Michael J.

doi:10.1029/gl015i009p01069

Cited by 6 publications

(5 citation statements)

References 7 publications

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“…The process that produces the DIRMs appears to affect many rock types; both igneous and sedimentary, and so is unlikely to be a chemical effect. Some authors (Audunsson andLevi, 1989, Me Williams andPinto, 1988) have suggested an IRM origin for this component, resulting from magnetic contamination from the drill string, although they also recognize that piezoremanent magnetism (PRM) and shock effects may also contribute (e.g., Kodama, 1984). AF demagnetization of cores from Leg 148, again to Hole 504B (Alt, Kinoshita, Stokking, et al, 1993) has not identified any consistent drilling-induced component.…”

Section: Drilling-induced Remanent Magnetization (Dirm)mentioning

confidence: 99%

Origin of the Natural Remanent Magnetism of Sheeted Dikes in Hole 504B Cored during Legs 137 and 140

Allerton¹,

Pariso²,

Stokking³

et al. 1995

Proceedings of the Ocean Drilling Program, 137/140 Scientific Results

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The natural remanent magnetism of samples from the sheeted dike complex generated at the Costa Rica Rift, drilled in Deep Sea Drilling Project/Ocean Drilling Program Hole 504B, can be resolved into a vertical drilling induced remanent magnetization and a shallow stable remanence by alternating field and thermal demagnetization. We investigate the unblocking temperature and coercivity spectra of these components, and compare these to spectra of thermal, induced, and viscous magnetizations imparted in the laboratory. The drilling-induced remanence is not a simple thermal, induced, or viscous remanence, but probably a piezoremanence acquired in an elevated field. The stable remanence may be a thermal remanence. Comparison of the stable remanence in altered and fresh samples indicates that natural remanent intensity and bulk susceptibility are reduced in more altered samples, but the stability and direction are similar to those in fresh samples. Whereas the stable components of the sheeted dike complex as a whole have a mean inclination of-18° ± 17°, individual lithologic units exhibit much less scatter and significantly different inclinations. A plausible explanation is that the stable remanence is a thermal remanence or a rapidly acquired chemical remanence, which does not average secular variation. The distribution of the inclination data suggests that dikes may be intruded as multiples.

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Section: Drilling-induced Remanent Magnetization (Dirm)mentioning

confidence: 99%

Origin of the Natural Remanent Magnetism of Sheeted Dikes in Hole 504B Cored during Legs 137 and 140

Allerton¹,

Pariso²,

Stokking³

et al. 1995

Proceedings of the Ocean Drilling Program, 137/140 Scientific Results

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“…Kodama (1984) noted high intensity and steep DIRM in drill cores from granites and concluded that it was similar to isothermal remanent magnetization (IRM), but had an even closer resemblance to piezo remanent magnetization (PRM). McWilliams & Pinto (1988) identified DIRM in a granite drill core, which they associated with higher fields near the tip of the drill string, and Ozdemir et al (1988) suggested that hightemperature IRM/VRM might have produced the DIRM they observed in a predominantly granodiorite drill core.…”

Section: Introductionmentioning

confidence: 97%

Drilling-induced remanent magnetization in basalt drill cores

Audunsson¹,

Levi²

1989

Geophysical Journal International

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Drilling-induced remanent magnetization (DIRM) in drill cores can limit their use for magnetostratigraphic studies and preclude the use of secondary viscous remanence for their azimuthal orientation. DIRM was studied in a drill core of a thick Miocene basalt flow now buried at 0.45 km. Due to zonation of the magnetic properties within the flow, DIRM was observed in specimens whose remanence is controlled by grains ranging from multidomain (MD) to single domain (SD). DIRM in this drill core has the following properties: (1) it is characterized by high intensity and low stability; (2) the D I M intensity increases by at least a factor of 5 from the centre of the drill core to the drill string's cutting surface, where it appears to have been produced; (3) it is directed down and radially inward towards the centre of the drill core; and (4) it is relatively more dominant and more intense in magnetically less stable MD grains.The observed DIRM can be modelled as a pure IRM acquired in a field of the order of 10mT. Therefore, the DIRM in this drill core is most easily explained as having been produced during the initial drilling by a strong non-uniform field concentrated near the cutting rim of the drill string. Other processes which might contribute to DIRM production include tearing of grains and possible changes in strain, mechanical shocks and piezo remanent magnetization (PRM).In this drill core, DIRM in the magnetically less stable grains was more effectively cleaned by alternating fields (AF) than by thermal demagnetization, and judicious AF demagnetization was usually successful at defining the primary remanence, especially for specimens from the centre of the drill core, which are less affected by DIRM overprinting. The use of a non-magnetic drill string would further reduce, and might possibly eliminate, DIRM production.

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“…Ideally, the horizontal components of the stable remanent magnetization vectors of samples taken from borehole cores can be assumed to indicate north at the time of formation if the magnetization was acquired during a period of normal geomagnetic field polarity (south if during a reversed polarity period), and structures may therefore be oriented relative to this direction (e.g., McWilliams and Pinto, 1988;Dick, Erzinger, Stokking, et al, 1992). Several problems, however, are inherent in using Paleomagnetism in this way, not least that a strong magnetic field emanating from the drill string frequently has the effect of partially remagnetizing the samples, giving rise to a steeply inclined magnetic component in the core.…”

Section: Paleomagnetismmentioning

confidence: 99%

Reorientation of Cores Using the Formation Microscanner and Borehole Televiewer: Application to Structural and Paleomagnetic Studies with the Ocean Drilling Program

Macleod¹,

Parson²,

Sager³

1994

Proceedings of the Ocean Drilling Program

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Structural and paleomagnetic studies of Ocean Drilling Program (ODP) cores are severely hampered by the fact that the recovered cores can rarely be reoriented with any degree of confidence. Although several techniques for orienting cores have been used, none is without limitations. Core orientation devices used during drilling are expensive to operate and are usually unreliable. A common alternative, used when no core orientation tools are available, is to assume that the horizontal component of the magnetization direction of the core pointed north, and thus that features may be oriented relative to this direction. However, this neglects the effects of secular variations in the Earth's magnetic field and of later tectonic rotation on the sample. In this paper we outline a technique for core reorientation that matches distinctive inclined planar features measured on the core with their images on Formation MicroScanner and Borehole Televiewer wireline logs. It allows reorientation of structural and magnetic data, and hence the possibility of detecting vertical axis tectonic rotations using Paleomagnetism. Its methodology is described and an example, from ODP Leg 135, presented to illustrate its application.

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Paleomagnetic results from granitic basement rocks in the Cajon Pass Scientific Drillhole

Cited by 6 publications

References 7 publications

Origin of the Natural Remanent Magnetism of Sheeted Dikes in Hole 504B Cored during Legs 137 and 140

Origin of the Natural Remanent Magnetism of Sheeted Dikes in Hole 504B Cored during Legs 137 and 140

Drilling-induced remanent magnetization in basalt drill cores

Reorientation of Cores Using the Formation Microscanner and Borehole Televiewer: Application to Structural and Paleomagnetic Studies with the Ocean Drilling Program

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