Tectonic controls on block rotation and sheeted sill emplacement in the Xigaze Ophiolite (Tibet): the construction mode of slow-spreading and ultraslow-spreading oceanic crusts

Abstract

The internal structure of oceanic crusts is not well understood due to the limitation of deep drilling. However, that of ophiolites, i.e., on-land ancient analogs of oceanic lithosphere, could be precisely mapped and measured. The Xigaze ophiolite in Tibet has been regarded as “peculiar”, due to the sheeted sill complex in its upper crust, and non-sheeted diabase sills/dikes crosscutting its mantle and lower crust, which are geometrically different from the primarily vertical sheeted dike complex. Based on extensive field observations, here we present petrological and geochemical data for the Xigaze ophiolite to decipher the origin of sheeted sill complex and its implications for the construction of oceanic crusts. Diabases in the Xigaze ophiolite could be subdivided into sheeted sills, Group 1 non-sheeted dikes, and Group 2 non-sheeted sills, based on their orientations. These diabases cut other lithologies, and hence belong to the latest-stage products. Based on petrological, geochemical, and structural data, we highlight the important role of detachment fault in the generation of sheeted and non-sheeted sills. During the formation of oceanic crust, large block exhumation, multi-stage rotations, and foundering are argued here as key mechanisms for the generation of Xigaze sheeted and non-sheeted dikes/sills, all of which are in the evolution of detachment fault systems. These processes are also not uncommon for asymmetrical segments at modern slow-spreading and ultraslow-spreading ridges, but are rare at symmetrical segments. Due to the evolution of detachment fault, the internal structures of (ultra)slow-spreading ridges are more complex than those at fast-spreading ridges.