Formation of uppermost oceanic crust

The uppermost portion of oceanic crust, extrusive basalts underlain and fed by diabasic dikes, represents the chilled roof zone or lid to mid‐oceanic spreading center magma chambers. This rigid lid forms and evolves through a steady state process of flexure, subsidence, and internal rotation, all occurring within a couple of kilometers of the spreading axis. The lid accommodates lateral spreading with dikes emplaced along a unique axis. Dikes tap underlying magma to feed overlying flows. Basalts accumulating along the axis of the lid load the lid, which responds by flexing down and away from the intrusive axis. This subsidence affects lid structure in the following ways. Emplaced dikes are tilted away from, and basalt flows are tilted toward, the intrusive axis. Lid subsidence and section rotation extend the base of the lid more rapidly at the axis than elsewhere. Consequently, dikes preferentially intrude at the existing axis to maintain existing axis shape and relative position through continued spreading. Loading and flexure of the lid, plus faster accretion at the base of the lid as a result of lid rotation, cause the lid to be uplifted distal to the axis. This creates flanking marginal highs and generates extension structures (fissuring and faulting) flanking and paralleling the loading (extrusive) axis. The crustal structure and geometry produced by this process is compatible with structure and topography observed at present‐day mid‐oceanic spreading centers and with structure observed in ophiolites, particularly the Bay of Islands Ophiolite.