Petrology and geochemistry of the Izu-Bonin Forearc tephras

Numerous marine tephra layers cored at Sites 792 and 793 in the Izu-Bonin forearc region offer additional information about the timing and spatial characteristics of arc volcanism and the evolution of island arcs. Explosive volcanism along the Izu-Bonin Arc, with maxima just before rifting of the arc at ~40 and 5-0 Ma, produced black and white tephras of variable grain sizes and chemical compositions. Most of the tephras belong chemically to low-K and low-alkali tholeiitic rock series with a few tephra of the high-K and alkalic rock series. Most of the tephras (low-K series) were derived from the Izu-Bonin Arc, although a few were produced far to the west of the Izu-Bonin Arc (e.g., from the Ryukyu Arc). Black tephras may have come from nearby sources, such as Aogashima, Sumisu, and Torishima islands. The high-K series of tephras, within the sediments younger than 3 Ma, may reflect thickening of the island-arc crust.

Petrology and geochemistry of Quaternary tephras of the Izu-Bonin Arc

Quaternary marine tephras in the Izu-Bonin Arc offer significant information about explosive volcanic activities of the arc. Visual core descriptions, petrographic examinations, and chemical and grain-size analyses were conducted on tephras of backarc, arc, and forearc origin. Tephras are black and white and occur in simple and multiple modes with mixed and nonmixed ashes of black and white glass shards. The grain size distributions of the tephras are classified into three categories: coarse, white pumiceous, and fine white and black well-sorted types. The frequency of occurrence of the white and black tephras differs within the tectonic settings of the arc. Chemically, the Quaternary tephras in this region belong to low-alkali tholeiitic series with lower K2O and TiO2 than normal ordinary arc volcanic materials. Several tephras from different sites along the forearc correlate with each other and with tephras in the Shikoku Basin site and with Aogashima volcanics. These volcanic ashes resemble those in other backarc rifting areas, such as in the Fiji, Okinawa (Ryukyu), and Mariana regions.

(Table 1) Basin area, average late-summer speedup, number of sinks and runoff of West Greenland Ice Sheet catchments

We use interferometric synthetic aperture radar observations recorded in a land-terminating sector of western Greenland to characterise the ice sheet surface hydrology and to quantify spatial variations in the seasonality of ice sheet flow. Our data reveal a non-uniform pattern of late-summer ice speedup that, in places, extends over 100 km inland. We show that the degree of late-summer speedup is positively correlated with modelled runoff within the 10 glacier catchments of our survey, and that the pattern of late-summer speedup follows that of water routed at the ice sheet surface. In late-summer, ice within the largest catchment flows on average 48% faster than during winter, whereas changes in smaller catchments are less pronounced. Our observations show that the routing of seasonal runoff at the ice sheet surface plays an important role in shaping the magnitude and extent of seasonal ice sheet speedup.