DIKES EMPLACED INTO FRACTURED BASEMENT, TIMNA IGNEOUS COMPLEX, ISRAEL

Dikes are usually envisioned as arrays of parallel segments dilated pe rpendicular to the direction of the least compressive stress. We descr ibe here four dikes of highly irregular shape intruded in the fracture d basement in the Timna Igneous Complex, southern Israel. The dikes in clude a doleritic dike, 2.3 lan long and 1.6 m to 32 m thick, and thre e andesitic dikes, up to 1.5 km long and 8 m thick. The dikes each dis play significant variations of dip (up to 60 degrees), strike (up to 1 60 degrees) and thickness. The thickness variations correlate better w ith the segment attitude than with the position along the dikes. We sh ow that the irregular shapes of the Timna dikes are the result of empl acement into fractured host rock under different paleostress states an d driving pressures. Three dilation styles that differ by the geometry of the initial cracks are analyzed: an array of randomly oriented cra cks (style A), a single linear crack (style B), and an array of interc onnected, nonparallel cracks (style C). The analysis of style A provid es the stress state during dike emplacement, including the orientation s of the three principal stresses (sigma(1) greater than or equal to s igma(2) greater than or equal to sigma(3)), the stress ratio phi = (si gma(2) - sigma 3)/(sigma(1) - sigma(3)), and the normalized driving pr essure R = (P-m - sigma(3))/(sigma(1) - sigma(3)). The stress ratio ph i indicates the shape of the stress ellipsoid and it ranges from phi = 0 for sigma 2 = sigma(3) (prolate ellipsoid) to phi = 1 for sigma(1) = sigma(2), (oblate ellipsoid). The normalized driving pressure R indi cates the relative magnitude of the internal magma pressure P-m with r espect to the tectonic stresses, and it ranges from R = 0 for P-m = si gma(3) to R = 1 for P-m = sigma(1). We found that for three dikes in T imna, phi similar to 0.25, indicating small differences between the tw o horizontal principal stresses, and for one dike phi similar to 0.9, indicating a large difference between the two horizontal principal str esses. The normalized driving pressure R is about 0.08 in two horizont ally propagating dikes and about 0.25 in two vertically propagating di kes. Style B predicts an elliptical thickness profile along the dike d ue to dilation of a linear crack; this prediction agrees with the prof ile of one of the dikes, The predicted thicknesses due to dilation of the interconnected array of cracks (style C) are in good agreement wit h the thickness variations of the doleritic dike, and in fair agreemen t with two of the andesitic dikes. Deviations from the ideal geometry suggest separate stages of propagation and dilation in some of the dik e segments.