Obduction-related remagnetization at the base of an ophiolite: Paleomagnetism of the Samail nappe lower sequence and of its continental substratum, southeast Oman Mountains

This paper considers the effect of obduction tectonics on the magnetic prop erties of a fast spreading ridge ophiolite. The paleomagnetism of the ultra mafics (seven sites) and layered gabbros (two sites) of the Samail, Wadi Ta yin-Ibra, and Muscat massifs of Oman has been investigated, along with that of the Permian metabasites from the subophiolitic Saih Hatat continental w indow (five sites). Of the 202 studied samples, 183 exhibit stable componen ts with a dominant NNW trend and a shallow, positive or, less frequently, n egative inclination. Superposition of normal and reverse polarities was obs erved in one gabbro, three peridotite, and two metabasite sites. The in sit u characteristic directions are similar in both the nappe and metamorphic w indow, except for a lone, low-grade metabasite with a component (D = 168 de grees, I = 35 degrees) referred to a preserved Permian magnetization. The m oderate directional scatter can be explained by postnappe tilting along the NNE trending axis. The high-grade metabasites in the window were completel y remagnetized during the metamorphism associated with nappe emplacement an d subsequent exhumation tectonics between 80 and 70 Ma. Since the overlying ophiolites have similar, normal, or reverse directions, they were remagnet ized contemporaneously. The magnetic mineralogy of the ultramafic-layered g abbro sequence was likely modified extensively at that time by serpentiniza tion from the upward metamorphic fluid advection in the faulted ophiolite. Therefore the magnetic properties of the lower part of the southern Samail ophiolite cannot be used for modeling magnetic anomalies of oceanic lithosp here.