Dikes emplaced into fractured basement, Timna Igneous Complex, Israel

Abstract

Dikes are usually envisioned as arrays of parallel segments dilated perpendicular to the direction of the least compressive stress. We describe here four dikes of highly irregular shape intruded in the fractured basement in the Timna Igneous Complex, southern Israel. The dikes include a doleritic dike, 2.3 km long and 1.6 m to 32 m thick, and three andesitic dikes, up to 1.5 km long and 8 m thick. The dikes each display significant variations of dip (up to 60°), strike (up to 160°) and thickness. The thickness variations correlate better with the segment attitude than with the position along the dikes. We show that the irregular shapes of the Timna dikes are the result of emplacement into fractured host rock under different paleostress states and driving pressures. Three dilation styles that differ by the geometry of the initial cracks are analyzed: an array of randomly oriented cracks (style A), a single linear crack (style B), and an array of interconnected, nonparallel cracks (style C). The analysis of style A provides the stress state during dike emplacement, including the orientations of the three principal stresses (σ1 ≥ σ2 ≥ σ3), the stress ratio ϕ = (σ2 ‐ σ3)/(σ1 ‐ σ3), and the normalized driving pressure R = (Pm ‐ σ3)/(σ1 ‐ σ3). The stress ratio ϕ indicates the shape of the stress ellipsoid and it ranges from ϕ = 0 for σ2 = σ3 (prolate ellipsoid) to ϕ = 1 for σ1 = σ2 (oblate ellipsoid). The normalized driving pressure R indicates the relative magnitude of the internal magma pressure Pm with respect to the tectonic stresses, and it ranges from R = 0 for Pm = σ3 to R = 1 for Pm = σ1. We found that for three dikes in Timna, ϕ ∼ 0.25, indicating small differences between the two horizontal principal stresses, and for one dike ∼ 0.9, indicating a large difference between the two horizontal principal stresses. The normalized driving pressure R is about 0.08 in two horizontally propagating dikes and about 0.25 in two vertically propagating dikes. Style B predicts an elliptical thickness profile along the dike due to dilation of a linear crack; this prediction agrees with the profile of one of the dikes. The predicted thicknesses due to dilation of the interconnected array of cracks (style C) are in good agreement with the thickness variations of the doleritic dike, and in fair agreement with two of the andesitic dikes. Deviations from the ideal geometry suggest separate stages of propagation and dilation in some of the dike segments.