MECHANICS OF EMPLACEMENT AND TECTONIC IMPLICATIONS OF THE RAMON DIKE SYSTEMS, ISRAEL

A radial system comprising more than 200 basaltic and trachytic dikes and two minor systems of parallel dikes intruded the Ramon area, southern Israel, during the Early Cretaceous. Field relations between dikes and fractures in the radial system indicate that the dikes intruded self-generated fractures, and thus indicate the directions of the tectonic stresses. Other field observations indicate that the dikes propagated in subhorizontal directions up to distances of 15 km from their source. Our analysis of the emplacement mechanics of these dikes shows that the horizontal propagation is best explained by the density differences between the intruding magma and the host rocks. The measured mean density for basement rocks at depths greater than 2.4 km is 2.55 +/- 0.07 g/cm3, and it is 2.36 +/- 0.21 g/cm3 for the sedimentary cover above. For the magma to be propagating horizontally at its neutral buoyancy level requires a mean magma density of about 2.5 g/cm3. The large distance of horizontal propagation requires a low viscous pressure drop, below 0.1 MPa/km within the dike, and an overpressure of about 1 MPa in the magma chamber. We computed stress trajectories using a two-dimensional elastic model for a pressurized hole in a regional stress field and compared it to the Ramon radial system. The model reveals that the dikes originated at a central intrusion of about 3 km diameter and intruded under a predominantly radial state of stress with negligible regional stress field. This period of weak tectonic stresses and intensive magmatism falls between the early Mesozoic extensional regime and the late Mesozoic-Cenozoic compressional regime in Israel. The calculated center of the radial system is offset from a large magnetic anomaly south of the Ramon area, suggesting a 3 km right-lateral displacement along the Ramon fault after the intrusion of the radial system.