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Geology; February 2006; v. 34; no. 2; p. 69-72; DOI: 10.1130/G22001.1
© 2006 Geological Society of America
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Earthquake-induced clastic dikes detected by anisotropy of magnetic susceptibility

Tsafrir Levi1, Ram Weinberger2, Tahar Aïfa3, Yehuda Eyal4 and Shmuel Marco5

1 Department of Geological and Environmental Sciences, Ben Gurion University of the Negev, Beer Sheva, Israel, and Geological Survey of Israel, 30 Malkhe Yisrael Street, Jerusalem 95501, Israel, and Ramon Science Center, Ben-Gurion University of the Negev, P.O. Box 194, Mizpe Ramon 80600, Israel
2 Geological Survey of Israel, 30 Malkhe Yisrael Street, Jerusalem 95501, Israel
3 Géosciences-Rennes, CNRS UMR6118, Université de Rennes l, Campus de Beaulieu, 35042 Rennes cedex, France
4 Department of Geological and Environmental Sciences, Ben Gurion University of the Negev, Beer Sheva, Israel
5 Department of Geophysics and Planetary Sciences, Tel Aviv University, Tel Aviv 69978, Israel

Clastic dikes form either by passive deposition of clastic material into preexisting fissures or by fracturing and injection of clastic material during seismic shaking or passive overpressure. Because of their similar final geometry, the origin of clastic dikes is commonly ambiguous. We studied the mechanisms of clastic dike formation within the seismically active Dead Sea basin, where hundreds of clastic dikes crosscut soft rock of the late Pleistocene lacustrine Lisan Formation. We analyzed the anisotropy of magnetic susceptibility (AMS) of clastic dikes of known origin and defined characteristic AMS signatures of depositional or injection filling. We discovered that passively filled dikes, which contain brownish silt resembling local surface sediments, are characterized by an oblate AMS ellipsoid and vertical minimum susceptibility axis V3. Dikes that contain green clayey sediment connected to a mineralogically identical detrital layer of the Lisan Formation are characterized by a triaxial AMS ellipsoid, well grouped subhorizontal and parallel to the dike walls' maximum susceptibility axis V1, and subvertical intermediate susceptibility axis V2. Field evidence and AMS analysis indicate that most of these dikes were emplaced by injection inferred to be due to seismically triggered fluidization. This novel application of the AMS provides a petrofabric tool for distinguishing passively filled dikes from injection dikes and, where appropriate, for identifying the latter as seismites.

Key Words: clastic dikes • anisotropy of magnetic susceptibility • Dead Sea transform • seismites






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