Nature and growth rate of the Northern Izu–Bonin (Ogasawara) arc crust and their implications for continental crust formation

The bulk composition of the continental crust throughout geological history is thought by most previous workers to be andesitic. This assumption of an andesitic bulk composition led to an early hypothesis by 72 ) that the continental crust was created by arc magmatism. This hypothesis for the origin of continental crust was challenged by several authors because: (i) the mean rate of arc crust addition obtained by 50 ) is too small to account for some certain phases of rapid crustal growth; and (ii) the bulk composition of ocean island arcs, the main contributor to the Archean and early Proterozoic crust, is basaltic rather than andesitic ( 4 ; 49 ). New data from the Northern Izu–Bonin arc are presented here which support the 72 ) hypothesis for the origin of the continental crust by andesitic arc magma. A geological interpretation of P wave crustal structure obtained from the Northern Izu–Bonin arc by 66 ) indicates that the arc crust has four distinctive lithologic layers: from top to bottom: (i) a 0.5–2-km-thick layer of basic to intermediate volcaniclastic, lava and hemipelagite (layer A); (ii) a 2–5-km-thick basic to intermediate volcaniclastics, lavas and intrusive layer (layer B); (iii) a 2–7-km-thick layer of felsic (tonalitic) rocks (layer C); and (iv) a 4–7-km-thick layer of mafic igneous rocks (layer D). The chemical composition of the upper and middle part of the northern Izu–Bonin arc is estimated to be similar to the average continental crust by 73 ). The rate of igneous addition of the Northern Izu–Bonin arc since its initial 45-Ma magmatism was calculated as 80 km³/km per million years. This rate of addition is considered to be a reasonable estimate for all arcs in the western Pacific. Using this rate, the global rate of crustal growth is estimated to be 2.96 km³/year which exceeds the average rate of crustal growth since the formation of the Earth (1.76 km³/year). Based on this estimate of continental growth and the previously documented sediment subduction and tectonic erosion rate (1.8 km³/year, 24 ), several examples of growth curves of the continental crust are presented here. These growth curves suggest that at least 50% of the present volume of the continental crust can be explained by arc magmatism. This conclusion indicates that arc magmatism is the most important contributor to the formation of continental crust, especially at the upper crustal level.     

Analytical study of turbidites, Otadai Formation, Boso Peninsula, Japan

On the Boso Peninsula, southeast of Tokyo, individual turbidite layers have been traced over about 38 km with the aid of tuff marker-beds. Sandstones are generally much more variable in thickness than the interbedded mudstones, showing lenticular shapes with great lateral continuities up to 30 km or more. But the latter have greater extents than the former, forming extensive sheets with a slight upslope thickening. Most sandstones show vertical and lateral grading, while mudstones gradually coarsen upslope. Sandstones have a variety of sedimentary structures typical of turbidites, whereas mudstones are virtually devoid of them throughout. Sandstones contain intensely abraded shallow-water fauna in contrast with mudstones with well-preserved deep-water fauna. Thus, mudstones are mostly of hemipelagic origin, and turbiditic mudstones, if present, are of negligible amounts. In general, individual sandstone beds are much more variable in thickness on the upstream side than on the downstream side. This is probably because of the existence of some minor channels and the adjacent levee-like embankments on the upslope side. At the base of thicker sandstone beds local penecontemporaneous erosion has usually taken place, giving rise to a few minor channels bordered by small levee-like embankments on both sides. The downslope side of the largest channel is commonly flanked by the greatest embankment that is coincident with the thickest part of each sandstone layer. Sandstones in the minor channels and levee-like embankments contain a lot of mud lumps torn out of the substratum, which decrease in size and amount and improve in roundness with increasing distance from the channels. In general, thicker sandstones show Bouma abcde sequences around the thickest part. Bouma d-division and convolution are developed preferentially on the downstream side of the thickest part. With decreasing bed thickness laterally, the complete depositional sequences arc successively replaced by Bouma bcde, then cde, and de sequences downslope, while by Bouma bc/c and then c/e sequences upslope. The facies transition of the horizon studied probably represents a section extending from the downslope end of the channelized portion of a suprafan to the outer portion with a smooth surface.