海洋地殻の構造と形成プロセス

Abstract

Recent progress of studies on oceanic crust is reviewed focusing on the architecture and its formation process. Although crustal thickness does not vary with spreading rates with the exception of very-slow spread oceanic crust, architectures vary with spreading rates. The thickness of the upper oceanic crust (lava and sheeted dike complex) decreases with increasing spreading rates at intermediate to ultra-fast spread oceanic crusts. This implies that the gabbroic section thickens with spreading rates. The lava layer seems to become thicker, while sheeted dike complex becomes thinner with spreading rates. Thus, a systematic change of the crustal structures is expected with spreading rates for intermediate to ultra-fast spread oceanic crusts. However, such systematics are not followed at a slow to ultra-slow spread oceanic crust, where the sheeted dike complex might occur only locally. Therefore, the magmatic systems controlling the formation of the crustal architecture differ significantly between slow to ultra-slow and intermediate to ultra-fast spread oceanic crusts. The difference is also shown in lower oceanic crusts, i.e., small and isolated numerous gabbroic intrusions at slow to ultra-slow spread crusts, and thick successive gabbroic layer at intermediate to ultra-fast spread oceanic crusts. Recent dense sampling from extensive areas near the East Pacific Rise and drilling at Hole 1256D show that off-axis magmatism plays an important role to in making the lava layer thicker. It should be emphasized that the upper succession in the lava layer might correspond to magmatism from on-axis to off-axis toward the upside. Therefore, vertical variations in the upper portion of the lava layer show a lateral igneous variation from on-axis to off-axis magmatism. Another important result obtained at Hole 1256D is the appearance of granoblastic dikes that are recrystallized under high temperature conditions up to pyroxene hornfels facies. Petrographical and petrological observations suggest that dehydration partial melting occurred in the metamorphosed dikes due to invasion by gabbros. Gabbroic rocks recently obtained from Hole U1309D of the Atlantis massif near the Mid-Atlantic Ridge, give a new constraint for the formation process of slow spread oceanic crusts. Down hole variation at this site confirms that the lower crusts of slow spread crusts are composed of many gabbroic intrusions. However, it is noted that the most primitive gabbroic rocks such as troctolite occur in the upside of the drill hole as was the case for Hole 735B of the Atlantis bank. It is not understood yet what mechanism produces such primitive gabbros at the upside. The accretion fashion of the lower crust beneath intermediate to ultra-fast spreading ridges is still controversial with two end models proposed, gabbro glacier model and sheeted sill model. Hybrid models based on thermal analyses and petrological data have also been proposed. Segmentation structures might also affect also for the accretion style of the lower crust. Much deeper drillings and detailed studies of ophiolites are required to solve the accretion style of lower oceanic crusts beneath intermediate to ultra-fast spreading ridges.