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 properties of a fast spreading ridge ophiolite. The paleomagnetism of the ultramafics (seven sites) and layered gabbros (two sites) of the Samail, Wadi Tayin‐Ibra, and Muscat massifs of Oman has been investigated, along with that of the Permian metabasites from the subophiolitic Saih Hatat continental window (five sites). Of the 202 studied samples, 183 exhibit stable components with a dominant NNW trend and a shallow, positive or, less frequently, negative inclination. Superposition of normal and reverse polarities was observed in one gabbro, three peridotite, and two metabasite sites. The in situ characteristic directions are similar in both the nappe and metamorphic window, except for a lone, low‐grade metabasite with a component (D = 168°, I = 35°) referred to a preserved Permian magnetization. The moderate directional scatter can be explained by postnappe tilting along the NNE trending axis. The high‐grade metabasites in the window were completely remagnetized during the metamorphism associated with nappe emplacement and subsequent exhumation tectonics between 80 and 70 Ma. Since the overlying ophiolites have similar, normal, or reverse directions, they were remagnetized contemporaneously. The magnetic mineralogy of the ultramafic‐layered gabbro sequence was likely modified extensively at that time by serpentinization 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 lithosphere.