Back‐arc rifting in the Izu‐Bonin Island Arc: Structural evolution of Hachijo and Aoga Shima Rifts

Abstract Multi‐ and single‐channel seismic profiles are used to investigate the structural evolution of back‐arc rifting in the intra‐oceanic Izu‐Bonin Arc. Hachijo and Aoga Shima Rifts, located west of the Izu‐Bonin frontal arc, are bounded along‐strike by structural and volcanic highs west of Kurose Hole, North Aoga Shima Caldera and Myojin Sho arc volcanoes. Zig‐zag and curvilinear faults subdivide the rifts longitudinally into an arc margin (AM), inner rift, outer rift and proto‐remnant arc margin (PRA). Hachijo Rift is 65 km long and 20–40 km wide. Aoga Shima Rift is 70 km long and up to 45 km wide. Large‐offset border fault zones, with convex and concave dip slopes and uplifted rift flanks, occur along the east (AM) side of the Hachijo Rift and along the west (PRA) side of the Aoga Shima Rift. No cross‐rift structures are observed at the transfer zone between these two regions; differential strain may be accommodated by interdigitating rift‐parallel faults rather than by strike‐ or oblique‐slip faults. In the Aoga Shima Rift, a 12 km long flank uplift, facing the flank uplift of the PRA, extends northeast from beneath the Myojin Knoll Caldera. Fore‐arc sedimentary sequences onlap this uplift creating an unconformity that constrains rift onset to ∼1‐2Ma. Estimates of extension (∼3km) and inferred age suggest that these rifts are in the early syn‐rift stage of back‐arc formation. A two‐stage evolution of early back‐arc structural evolution is proposed: initially, half‐graben form with synthetically faulted, structural rollovers (ramping side of the half‐graben) dipping towards zig‐zagging large‐offset border fault zones. The half‐graben asymmetry alternates sides along‐strike. The present ‘full‐graben’ stage is dominated by rift‐parallel hanging wall collapse and by antithetic faulting that concentrates subsidence in an inner rift. Structurally controlled back‐arc magmatism occurs within the rift and PRA during both stages. Significant complications to this simple model occur in the Aoga Shima Rift where the east‐dipping half‐graben dips away from the flank uplift along the PRA. 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. Pre‐existing structures have little influence on rift evolution; the rifts cut across large structural and volcanic highs west of the North Aoga Shima Caldera and Aoga Shima. Large, rift‐elongate volcanic ridges, usually extruded within the most extended inner rift between arc volcanoes, may be the precursors of sea floor spreading. As extension continues, the fissure ridges may become spreading cells and propagate toward the ends of the rifts (adjacent to the arc volcanoes), eventually coalescing with those in adjacent rift basins to form a continuous spreading centre. Analysis of the rift fault patterns suggests an extension direction of N80°E ± 10° that is orthogonal to the trend of the active volcanic arc (N10°W). The zig‐zag pattern of border faults may indicate orthorhombic fault formation in response to this extension. Elongation of arc volcanic constructs may also be developed along one set of the possible orthorhombic orientations. Border fault formation may modify the regional stress field locally within the rift basin resulting in the formation of rift‐parallel faults and emplacement of rift‐parallel volcanic ridges. The border faults dip 45–55° near the surface and the majority of the basin subsidence is accommodated by only a few of these faults. Distinct border fault reflections decreases dips to only 30° at 2.5 km below the sea floor (possibly flattening to near horizontal at 2.8 km although the overlying rollover geometry shows a deeper detachment) suggesting that these rifting structures may be detached at extremely shallow crustal levels.