A model for the process of formation of the upper oceanic crust

It appears that certain features of ophiolites can only have been formed by conveyor belt type sea-floor spreading. Computer simulations of these features are used to derive a model for the formation of the upper oceanic crust. Detailed structural studies of complete ophiolites reveal a remarkably consistent structure of their upper portions: lavas with occasional dykes grade down through a rapid transition into a sheeted dyke layer, which in turn grades down into gabbro through a second equally sharp transition. Where sufficient sections can be measured sheeted dyke units show consistent one way chilling, there usually being about 10 per cent more margins chilled one way than the other. Two entirely independent computer simulations (of one way chilling, and of the increase in dyke percentage with depth in ophiolites) show that most of the dykes in their sheeted dyke units were intruded at spreading axes in zones less than 50 m wide. The dykes are intruded vertically, and subsequently both dykes and lavas may be rotated by tectonic activity and buried by more lavas as they migrate out of the roughly 4-km wide zones of lava formation. This model of formation of oceanic crust based on ophiolite data is consistent with observations on the present mid-ocean ridges. It appears that similar processes involving a narrow zone of dyke intrusion are developed at all spreading plate boundaries where the spreading rate is greater than about 10 mm/yr, irrespective of possible variations in spreading rate or tectonic setting of the spreading axis. From the model a number of features of the oceanic crust can be explained: (1) the shape of the boundaries between the blocks that produce magnetic anomalies, (2) upwelling of hydrothermal circulation will be most intense in the axial 1 km at spreading axes, (3) sulphide deposits are most likely to be formed in this axial 1 km and will then be buried by subsequent lava flows and (4) metamorphism of the upper oceanic crust will be greatest in this axial zone so that metamorphic layering is produced as an integral part of the formation of the oceanic crust and not as a result of subsequent burial.