Structural and Stratigraphic Evolution of the Sumisu Rift, Izu-Bonin Arc

The Sumisu Rift, which is ~ 120 km long and 30-50 km wide, is bounded to the north and south by structural and volcanic highs west of the Sumisu and Torishima calderas and longitudinally by curvilinear border fault zones with both convex and concave dips. The zigzag pattern of normal faults (average strikes N23°W and N5°W) indicates fault formation in orthorhombic symmetry in response to N76° ± 10°E extension, orthogonal to the volcanic arc. Three oblique transfer zones divide the rift along strike into four segments with different fault trends and uplift/subsidence patterns. Differential strain across the transfer zones is accommodated by interdigitating, rift-parallel faults and some cross-rift volcanism, rather than by strikeor oblique-slip faults. From estimates of extension (2-5 km), the age of the rift (~2 Ma), and the accelerating subsidence, we infer that the Sumisu Rift is in the early syn-rift stage of backarc basin formation. Following an early sag phase, a half graben formed with a synthetically faulted, structural rollover facing large-offset border fault zones. In the three northern rift segments, the largest faults are on the arc side and dip 60°-75°W, whereas in the southern segment they are on the west side and dip 25°-50°E. The present, "full graben" stage is dominated by hanging-wall antithetic faulting, basin widening by footwall collapse, and a concentration of subsidence in an inner rift. The hanging wall collapses but not necessarily as a result of border fault propagation from adjacent rift segments. Whereas the border faults may penetrate the Theologically weak lithosphere (Te ~ 3 km), many of the hanging-wall and footwall collapse structures are detached only a few kilometers below the seafloor. Where drilled, the arc margin has been uplifted 1.1 ±0.5 km concurrently with ~l.l km of rift basin subsidence. Backarc volcanism, usually erupted along faults, occurs in the rift and along the proto-remnant arc during both stages. A linear zone of weakness caused by the greater temperatures and crustal thickness along the arc volcanic line controls the initial locus of rifting. Rifts are better developed between the arc edifices; intrusions may be accommodating extensional strain adjacent to the arc volcanoes. No obvious correlations are observed between the rift structures and preexisting cross-arc trends. Episodic eruptions of the arc volcanoes are the dominant sediment source; sediment is primarily fed into the northern and southern ends of the rift basin. Sediments are distributed throughout the region by eruptive events and redeposited in the rift basin by unconfined mass flows. Extremely high sedimentation rates, up to 6 m/1000 yr in the inner rift, have kept pace with syn-rift faulting, create a smooth basin floor, and result in sediment thicknesses that mimic the differential basin subsidence. INTRODUCTION AND PREVIOUS WORK Although the active rifting of intraoceanic island arcs was first proposed about 20 yr ago (Karig, 1971; Packham and Falvey, 1971), the extensional mechanisms associated with stretching arc lithosphere remain poorly known. Investigations using swath-mapping, seismic, sedimentologic, petrologic, and heat-flow techniques have begun to reveal the surficial pattern of extensional strain, the nature and structural control of rift volcanism, sedimentation, and hydrothermal circulation, as well as the influence of the line of active arc volcanoes on rifting processes (Sibuet et al., 1987; Taylor et al., 1990a; Wright et al., 1990). One of the better studied active arc rift systems occurs south of Honshu, Japan (Fig. 1). Bathymetric depressions west of the Izu-Bonin island arc were first observed by Mogi (1968) and Hotta (1970). Investigations using reconnaissance seismic and bathymetric data (Karig and Moore, 1975; Honza and Tamaki, 1985) were sufficient to identify the presence and gross morphology of these fault-bounded basins and allowed the hypothesis that they were extensional in origin. Sumisu Rift (Fig. 1), in the Izu-Bonin island arc, has been the focus of recent investigations by the Hawaii Institute of Geophysics and the Taylor, B. , Fujioka, K. , et al., 1992. Proc. ODP, Sci. Results, 126: College Station, TX (Ocean Drilling Program). 2 Department of Geology and Geophysics, University of Hawaii, 2525 Correa Rd., Honolulu, HI 96822, U.S.A. 3 Present address: Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano, Tokyo 164, Japan. Department of Geology, University of Toronto, 170 College Street, Toronto, Ontario M5S 1 Al, Canada. 5 Geological Survey of Japan, 1-1-3 Higashi, Tsukuba, Ibaraki 305, Japan. Geological Survey of Japan. Densely spaced single-channel seismic data have been collected to define shallow rift structures (Murakami, 1988; Brown and Taylor, 1988). SeaMARC II sidescan sonar imagery and bathymetry (Brown and Taylor, 1988; Taylor et al., 1988a, 1988b) and SeaBeam bathymetry (Taylor et al., 1990a) investigations mapped the surficial distribution of faults, volcanoes, and sediments. Additional sedimentologic (Nishimura and Murakami, 1988), volcanic (Fryer et al., 1990; Hochstaedter et al., 1990), and heat-flow (Yamazaki, 1988; Taylor et al., 1990a) studies have begun to document the sediment and volcanic rock types, processes, and distribution, as well as the nature of hydrothermal circulation. Extension of the Izu-Bonin Arc has resulted in the formation of asymmetric rift basins located adjacent to the active volcanic arc that are 20-60 km wide by 20-120 km long. The rifts are bounded by high-angle, normal fault zones and are separated along strike by structural highs and chains of submarine volcanoes (Honza and Tamaki, 1985; Taylor et al., 1984; Brown and Taylor, 1988; Murakami, 1988). Individual rift basins are segmented along strike by cross-trending transfer zones that link opposing border fault zones (Taylor et al., 1990a; Taylor and Shipboard Scientific Party, 1990). Structural control of volcanism is commonly observed in these backarc rift basins where extrusive volcanic rocks (1) are concentrated along cross-trending accommodation zones, (2) are elongate parallel to rift faulting, (3) form clusters of aligned vents along rift faults, and (4) extend outward onto the rift margins (Brown and Taylor, 1988; Taylor et al., 1990a). Within the rift basins, the presence of geochemically "backarc basin basalts" indicates that a magma source with little influence by arc components is present even at the earliest stages of arc rifting (Fryer et al., 1990; Hochstaedter et al., 1990; Gill et al., this volume). Active backarc rift basin hydrothermal