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Fractures and Stress |
1 Institute of Oceanographic Sciences, Deacon Laboratory, Wormley, Surrey, GU8 5UB, UK
2 Department of Oceanography, Texas A&M University, College Station, Texas 77843-6331, USA
Palaeomagnetic studies on land have shown that block rotations on a variety of scales are commonplace in many tectonic regimes. In the oceans, however, analogous rotations have rarely been documented. This is due principally to the difficulty of their measurement rather than to their likely absence. Palaeomagnetic measurements can be, and are, readily made on borehole cores. However, the difficulty in reliably reorienting these cores, and hence their magnetization vectors, back to geographical coordinates severely hampers their use for the purposes of addressing tectonic rotations. Several techniques for orienting cores are commonly in use, none of which is without its limitations for palaeomagnetic purposes. A technique is outlined here that potentially allows the reliable orientation of sections of core by matching distinctive inclined planar features measured on the core with their images on Formation MicroScanner and Borehole Televiewer wireline logs. Its methodology is described and an example, from Ocean Drilling Program Leg 135, presented to illustrate its application.
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