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Figure 2. Clastic dike 18 m high, filledwith green clayey sediment crosscuttingLisan Formation and branchingtoward surface. Dike architectureresembles that of bifurcated dynamicfracture during upward propagation(e.g., Bahat, 1991) and shows geometrysimilar to that of injection dikepresented by McCalpin (1996, p. 366).Lisan laminae are not displacedacross dike, indicating that clasticdike is extensional fracture.tory Fig. DR1 1 ). The dikes exposed in thecanyon walls of Wadi Perazim within the LisanFormation are extensional fractures (Figs.2 and 3), indicating brittle Holocene fracturing.The dikes are as long as 1 km, 30 m high,and 0.4 m wide, and are arranged mainly inradial and tangential geometry. The radialtraces, which span a sector of 70, convergetoward the Black Hill dome. This led Marco1GSA Data Repository item 2006019, FigureDR1, detailed location map of clastic dikes, FiguresDR2–DR4, rock magnetic data, and FigureDR5, streaked AMS fabric of an injection dike, isavailable online at www.geosociety.org/pubs/ft2006.htm, or on request from editing@geosociety.org or Documents Secretary, GSA, P.O.Box 9140, Boulder, CO 80301-9140, USA.Figure 3. Wide (~0.4 m) clastic dikeopen at surface filled with brownishsilt. Source of fill is veneer of eolianand fluvial sediments, which coverAmi’az Plain.et al. (2002) to suggest that the dike patternis related to the local stress exerted bydoming.We distinguish between two types of dikes.Most abundant are dikes composed of greenclay, silty quartz, and aragonite, with a compositionsimilar to that of the lower layers ofthe Lisan Formation. In many dikes, a continuousconnection between the dike fill and agreen clayey layer of the Lisan Formation isobserved, clearly indicating that these structuresare injection dikes. Several of thesedikes branch toward the surface (Fig. 2). Occasionally,these dikes thin upward; some failto reach the surface. Less common are depositionaldikes composed of brownish silt(which occur sporadically with horizontal beddingplanes), which resembles the veneer ofsurface sediments (Fig. 3). These dikes alwaysintersect the present topographic surface andcommonly have a large opening in their upperpart.AMS APPLICATION FOR CLASTICDIKESHypothesisFoliation and lineation of a magnetic fabricmay form as a result of transport, deposition,and deformation of rocks (Borradaile andHenry, 1997). These features are commonlyassociated with AMS, which has been used toresolve current directions in sediments (Tarlingand Hrouda, 1993; Liu et al., 2001) andflow directions in magmas (Baer, 1995; Abelsonet al., 2001; Aïfa and Lefort, 2001). AMShas also been correlated with strain in rocksand tectonic deformation of sediments (Paréset al., 1999), and has been used to characterizesoft-sediment deformation (Schwehr andTauxe, 2003).We use AMS to distinguish between depositionaland injection clastic dikes. We adoptTauxe’s (1998) terminology, where the eigenvalues 1 , 2 , and 3 correspond to maximum,intermediate, and minimum values of themagnetic susceptibility, and the principal eigenvectorsare V 1 ,V 2 , and V 3 , respectively.In sedimentary rocks, we expect a wellgroupedvertical V 3 direction and dispersed V 1and V 2 directions within a horizontal plane(hereafter termed sedimentary fabric). Thevalues of the associated 1 and 2 are indistinguishableand characterized by an oblate AMSellipsoid. In moderate currents, grain imbricationresults in slightly off-vertical V 3 directions,and V 1 directions (in lower-hemisphereprojection) are antiparallel to the flow direction(Tauxe, 1998; Liu et al., 2001). In highenergycurrents with particles entrained, V 1directions are perpendicular to the flow direction,and V 3 directions are commonlystreaked, resulting in prolate or triaxial AMSellipsoids (Tauxe, 1998, and referencestherein).On the basis of the above-mentioned previousworks, we hypothesize that depositionaldikes will display a sedimentary AMS fabric,and that injection dikes will display prolate ortriaxial AMS ellipsoids. In the latter case, twotypes of AMS ellipsoids may occur, dependingon the flow velocity of the injected clastics.Under moderate flow velocities, V 1 is expectedto be parallel to the flow vector, whereasunder high flow velocity, V 1 is expected to beperpendicular to the flow direction andstreaked V 3 distribution may evolve. In thelatter case, the flow direction will be indicatedby either the V 2 or V 3 direction (Tauxe, 1998;Moreira et al., 1999). In both cases, the eigenvectorsshould be well grouped, characterizinga flow fabric.Sampling Strategy and MethodsWe recovered 312 samples from 14 clasticdikes and country rocks. We carved 2.5 cmcylinder pedestals with a sharp knife andplaced on them on plastic araldite glue–coatedcylinders with no AMS signal. The dikes weresampled across their width and along theirheight, 8–35 specimens in each. On the basisof field observations, two of the dikes are depositionaland four are injection dikes; theother eight seem to be injection dikes, but70 GEOLOGY, February 2006