Chemical remanent magnetism related to the Dead Sea Rift: Evidence from Precambrian igneous rocks of Mount Timna, southern Israel

Abstract

A paleomagnetic and mineralogical study of shallow intrusive basement rocks on Mount Timna (Arabo Nubian Massif in Sinai) shows that although all the igneous rocks are of late Precambrian age, a remanent magnetic direction similar to the subrecent field (Miocene to present) is identified in samples of quartz‐monzodiorite, monzodiorite, dikes of various composition, and altered gabbro. The average direction of these rock units is (D/I) 359°/41°, α95=4°, and the pole is at 83.6°N,223.2°E. The “subrecent” direction appears both as an overprint direction and as the sole stable vector in dolerite, rhyolite, and andesite dikes. The magnetic mineral assemblage of these rocks includes secondary minerals, such as hematite and goethite, which formed by oxidation and hydration of the original magnetite and Ti‐magnetite. The subrecent direction is interpreted to have been acquired as a chemical remanent magnetization (CRM) by hydrothermal activity and circulation of thermal brines through fractures related to the adjacent Dead Sea transform. The hydrothermal activity occurred before uplift and erosion exposed the basement rocks, i.e., in the middle Miocene, during the early stages of activity of the Dead Sea rift. The north trending declinations indicate that Mount Timna has not rotated about a vertical axis after acquisition of the CRM, a conclusion that confirms a previous structural analysis. An absence of reversals implies that the duration of CRM acquisition was probably less than ∼1 m.y., the maximum length of normal polarities since the Oligocene. Other field directions were found in an alkali granite (one site, 343°/14°, α95=22°), in a dolerite dike (one site, 318°/0°, α95=2°) and in a gabbro (seven sites, 303°/56°, α95=6°). The field direction in the granite is similar to that for Early Cretaceous, a time of magmatic activity in Timna. Northwest trending declinations and shallow inclinations found in several samples of the dikes are carried by unaltered parts of the rocks. These directions are interpreted as late Precambrian‐Early Cambrian in age, indicating a near‐equatorial location of the region at that time.