Evidence for large-scale rotations at spreading centers from the Josephine ophiolite

Abstract

Repetition of the Josephine ophiolite by large-scale open folding and imbricate faulting provides an excellent opportunity to study the geometric relationships between sheeted dikes, igneous layering, and bedding. Measurements of over 600 dike margins from the sheeted dike complex and high-level gabbro indicate that dikes are oriented at 20°–40° to bedding in metasedimentary rocks overlying the ophiolite. In addition, igneous layering is discordant to gross lithologic layering and is oriented at a large angle to both sheeted dikes and bedding. Similar relationships may occur in several other ophiolites. The “inclined” sheeted dikes and discordant igneous layering are best explained by a >45° rotation of new oceanic crust (ophiolite) immediately following formation at a spreading center. Rotation could have been accomplished by either subsidence or faulting. Further evidence for large-scale rotations of oceanic crust include frequent anomalous magnetic inclinations in drill cores of Layer 2 from the Atlantic.