Understanding the Spreading and Formation of Oceanic Plates by Drilling in the Upper Crust off Hawaii

The style of crustal extension is governed by M, which is the ratio of magma consumed to relax the crustal strain caused by plate spreading. Bathymetric profiles across ridges are repro-duced well by changing M . However, what determines the value of M has not been explained. Fast-spread oceanic crust comprises dense sheet flows underlain by thin dense sheeted dikes compared to magmas. This density structure increases the magma extruded, allowing the crust to be extended solely by magmatic accretion; whereas, the intermediate-spread crust consists of less dense pillow lavas, yielding an apparent level of neutral buoyancy that traps magma to develop the sheeted dikes below. Consequently, the crust extends through dike intrusions in the lower levels and faults at shallow levels. Thus, the density structure of the oceanic crust determines the style of plate spreading, or the value of M . Because the spreading rate or the strain rate does not vary within the same ridge segment, intrasegment variations in crustal structure depend directly on the supply rate of magma, or the value of M , which decreases with the thin-ning of extrusive layers and the thickening of sheeted dikes along the Galapagos Spreading Center and the East Pacific Rise. This tendency is supported by the crustal architecture observed in holes 504B and 1256D, the Hess Deep and the Oman Ophiolite. The density structure of the upper crust can be discerned by the proportion of sheet flows among extrusive rocks on the ridge axis, which is drastically reduced with spreading rates from 10 cm/a to 7 cm/a. This spreading rate interval coincides with the change in axial magma chamber depth. Throughout this rate interval, the key observation is whether the style of crustal extension from magmatic accre-tion-dominant details ） . Sediments and lavas emplaced off ridge （ uppermost basement 100 m in Hole 1256D （ Umino et al ., 2008a ） and 50 m in Hole 504B （ Ayadi et al ., 1998 ） , respectively ） are excluded from density estimates. The 1256D crust on the East Pacific Rise spread at an ultrafast rate of 22 cm/a comprises high-density sheet flows, yielding almost the same lithostatic and magmastatic pressures. The 504B crust on the Costa Rica Rift spread at an intermedi-ate rate of 6.6 cm/a has an apparent level of neutral buoyancy （ LNB ） , where lithostatic pressure is lower than magmastatic pressure. Magma uprising from the axial magma chamber （ AMC ） is preferentially trapped in the LNB and forms the