Abstract The Izu-Bonin-Mariana (IBM) arc system extends 2800 km from the Izu Peninsula to Guam Island (Fig. 2f.1) and provides an excellent example of an intra-oceanic convergent margin where the effects of crustal anatexis and assimilation are considered to be minimal (Stern et al. 2003; Tatsumi Stern 2006). The current IBM activity is caused by subduction of the Pacific Plate beneath the Philippine Sea Plate, which dips at 35° at the northern tip of this system and ia nearly vertical at Mariana. Evolution of the IBM Arc since 50 Ma has strongly in?uenced the present architecture of the Japanese archipelago. One important event during this arc system’s evolution is back-arc rifting from 15-25 Ma that separated the IBM Arc system from the remnant Kyushu-Palau Ridge by c. 500 km and created the young oceanic lithosphere of the Shikoku Basin that is now being subducted beneath the SW Japan Arc. It should be further stressed that recent geophysical and geological survey results suggest this intra-oceanic arc is an active site of both creation and growth of the continental crust. This chapter will highlight the tectonic and structural evolution of the IBM arc system.

Abstract Pleistocene basalts from Daisen and Mengameyama in the SW Japan volcanic arc of western Honshu are characterized by an abundance of olivine crystals with Fe-rich rims. At Daisen, these have previously been interpreted to have formed from their host melt by equilibrium crystal fractionation and by disequilibrium fractionation during supercooling. Here we use combined electron probe microanalysis, isotopography, transmission electron microscopy and selected area electron diffraction to show that crystal rims are significantly enriched in aluminium (up to c. 1 wt%) and hydrogen (up to c. 10 000 ppm) hosted in oriented low-density amorphous domains. These domains are interpreted to have formed by melting of deuteric and/or post-deuteric metasomatic alteration minerals upon uptake of older olivine crystals into fresh, initially aphyric host melts up to a few hours prior to eruption. It is argued that uptake of variably altered crystals into initially aphyric or sparsely phyric melts may be a common process at subduction zones, and can account for typical disequilibrium textures displayed by arc magmas erupted in SW Japan and elsewhere. Analyses of the altered crystal cargo in arc volcanic rocks therefore provides an important tool for understanding subvolcanic hydrothermal systems and the interaction of ascending melts with such systems. Supplementary material: Olivine mineral chemistry data from two typical Daisen basalts and one typical Mengameyama basalt, and a figure showing the locations of all focussed ion beam (FIB) sections studied here, are available at http://www.geolsoc.org.uk/SUP18760 .

Abstract Minerals of mafic rocks from the SW Japan arc have been studied to deduce P – T –X H2O conditions and their variations in mafic arc magmas. Two-pyroxene thermobarometry of magmas from several volcanoes yields constant temperatures and variable pressures. MELTS fractional crystallization modelling is employed to show that such ‘pseudo-decompression paths’ (PDPs) are artefacts that derive from uptake of pyroxene antecrysts formed at a range of crustal levels by isobaric cooling of previously intruded mafic melts. It is shown that PDPs can be used to constrain oxygen fugacities and initial water contents of the intruded magmas. These constraints, and plagioclase hygrometry, indicate that initial melt H 2 O contents change systematically along the SW Japan arc. Direct determination of hydrogen in olivine by secondary ion mass spectrometry yields consistently low olivine H 2 O contents of 11±4 ppm (1σ), with little, if any, along-arc variations. MELTS modelling indicates that crystallization of calcic plagioclase and olivine dominantly occurs during upper crustal differentiation of mafic melts. The combined data indicate that aphyric melts are released from the mantle wedge, taking up most if not all crystals from previously intruded plutonic rocks during rapid magma ascent to the surface.

Abstract Regional stress has a significant impact on fault development during the formation of salt dome structures. To examine such effects of wrenching, we conducted a series of analog experiments of updoming using dry, granular materials and observed the deformation on the top free surface. The experiments included three deformation styles: (1) updoming followed by wrenching, (2) simultaneous updoming and wrenching, and (3) wrenching followed by updoming. In the first series of the experiments, the faults produced by simple updoming were overprinted by two strikeslip fault systems that were generated by the subsequent wrenching. The second series of experiments with the configuration of simultaneous updoming and wrenching generated normal faults in a direction perpendicular to relative extension by the wrench. In the third series of experiments, the Riedel and anti-Riedel shear faults formed by wrenching were deformed by the subsequent updoming and were overprinted by the faults related to the updoming. These experimental results are applied to the fault systems observed above dome structures in the United Arab Emirates region, where extensive faults in the northwest— southeast direction have developed. By analogy, these faultswere probably formed during an updoming and simultaneous wrenching. The direction of simple shear inferred from a comparison of real faults and experimental results suggests that dextral wrenching caused by the Oman stress regime during the Late Cretaceous affected the region at the time of the updoming.