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

Abstract

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.

Gabbro Fabrics from Site 894, Hess Deep: Implications for Magma Chamber Processes at the East Pacific Rise

“…Preliminary cuts made parallel to the foliation provided favorably oriented sections for identification of a lineation; thin sections were then made parallel to this lineation, both parallel and peΦendicular to the foliation. Following the procedure described by MacLeod et al (1992MacLeod et al ( ,1994summarized in Gillis, Mével, Allan, et al, 1993), the axis of the borehole was considered to be vertical, and azimuths measured relative to artificial "core-liner coordinates." "North" was defined as perpendicular to the cut surface and lying in the working half of the core.…”

Structures in Peridotites from Site 895, Hess Deep: Implications for the Geometry of Mantle Flow beneath the East Pacific Rise

“…6B). Given the greater uncertainties inherent in recalculating strikes from apparent dip data (MacLeod et al, 1994), we do not think that the dispersion of strikes is significant; instead, the strikes will all be regarded as more or less north-south, within the resolution of our measurements. In contrast, the restored lineation measurements, at least when considered together, show a range of orientations, with no immediately apparent preference for either steep or shallow plunges (Fig.…”

“…Nevertheless, the dispersion of inclinations suggests that some rotation has certainly occurred after magnetization; and, thus, it is more relevant to consider the orientations of the peridotite fabrics (which necessarily predate acquisition of the "primary" remanence) restored relative to a common magnetic declination and inclination direction. Although this restoration does not tell us the present-day orientation of the cores as they were immediately before drilling, in the absence of logging data (and consequent azimuthal reorientation of cores to present-day geographical coordinates; MacLeod et al, 1994;MacLeod, Boudier, et al, this volume;MacLeod, Célérier, et al, this volume), it does provide us with the best estimate of the original orientations of fabrics relative to the ridge reference frame, because the angular relationships between the fabrics and remanence direction is unlikely to have changed.…”

“…Traces of a foliation, principally defined by elongation of spinel and, to a lesser extent, orthopyroxene, were identified using a binocular microscope on the surfaces of the core. Planes were reconstructed from two apparent dip measurements, in some cases also with a strike measurement from horizontal surfaces, in the manner described by MacLeod et al (1994) and Gillis, Mével, Allan, et al (1993). Identification of a lineation was rarely possible in hand specimen, other than to note that the fabric was stronger in certain directions than in others.…”

“…Preliminary cuts made parallel to the foliation provided favorably oriented sections for identification of a lineation; thin sections were then made parallel to this lineation, both parallel and perpendicular to the foliation. Fabric orientations were measured using the procedure described by MacLeod et al (1994) and summarized in Gillis, Mével, Allan, et al (1993). The axis of the borehole was considered to be vertical (no borehole deviation measurements were made at Site 895), and azimuths were measured relative to artificial "coreliner coordinates": "north" was defined as perpendicular to the cut surface and lying in the working half of the core.…”