Structural development of Sumisu Rift, Izu‐Bonin Arc

Abstract

Geophysical swath mapping, multichannel seismic profiling, and ocean drilling data are used to document the structural evolution of Sumisu Rift and to analyze the pattern of strain resulting from extension of an intraoceanic island arc. The ∼120‐km‐long, 30–50‐km‐wide Sumisu Rift 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 dip slopes. The zig‐zag pattern of normal faults (average strikes 337° and 355°) indicates extension oriented 076°±10°, 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 sometimes by cross‐rift volcanism, rather than by strike‐ or oblique‐slip faults. From estimates of extension (2–5 km), the age of the rift (∼2 Ma), and the accelerating subsidence, we infer that Sumisu Rift is in the early synrift stage of back arc basin formation. Following an early sag phase, half graben formed with synthetically faulted, structural rollovers facing large‐offset (2–2.5 km throw) 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. Back arc volcanism, usually erupted along faults, occurs in the rift and along the protoremnant arc during both stages. Where drilled, the arc margin has been uplifted 1.1±0.5 km concurrently with ∼1.1 km of rift basin subsidence. Extremely high sedimentation rates, up to 6 m/kyr in the inner rift, have kept pace with synrift faulting, created a smooth basin floor, and resulted in sediment thicknesses that mimic the differential basin subsidence. 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.

Anisotropy and Flow in Pacific Subduction Zone Back-arcs

“…2). The orientation of normal faults in the Sumisu Rift (TAYLOR et al, 1991) and source mechanisms from the Harvard Centroid Moment Tensor Catalog for Sumisu Rift earthquakes are consistent with extension at a roughly E-W azimuth that is normal to the trench and lies within 30°of most observed fast directions. In the southern Kuril and Japan back-arcs, paleomagnetic data suggest that the overriding plates deformed along a roughly N-S dextral shear system during the Miocene.…”

“…1). In Izu-Bonin, back-arc spreading occurred in the Shikoku Basin from 29 to 15 Ma and has continued in the Sumisu Rift from 2 Ma to the present (TAYLOR et al, 1991;CLIFT, 1995;KOBAYASHI et al, 1995). The initial orientation of back-arc spreading in the Shikoku Basin was roughly ENE -WSW, but later spreading had an azimuth closer to E (KOBAYASHI et al, 1995) (Fig.…”

Cenozoic transverse zones and igneous belts in the Great Basin, Western United States: Their tectonic and economic implications

“…6), change strike (north-northwest to the north, north-northeast to the south) where they cross the transverse zone (Rowley, 1968), a characteristic feature of transverse zones (cf. Taylor et al, 1991).…”

Transient crustal movement in the northern Izu–Bonin arc starting in 2004: A large slow slip event or a slow back-arc rifting event?

“…Tamaki, 1985). In fact, a chain of topography suggesting active E-W rifting with width of~30 km have been identified to the west of the Izu volcanic arc (Taylor et al, 1991). In the southern Izu-Bonin Arc (beneath the Bonin Islands), such back-arc spreading does not occur.…”