ABSTRACT: Structural Evolution of Sumisu Rift, Izu-Bonin Arc
Adam Klaus, Brian Taylor, Gregory Moore, Mary McKay
Seismic, side-scan, and SeaBeam data document the structural evolution of Sumisu Rift. The ^sim100 km long rift basin is bounded by large-offset (total throw >2 km) normal fault zones dipping 30-40° creating asymmetric rift profiles that alternate along the strike of the rift.
Two accommodation zones (AZs) segment the rift into three half-graben. The northern two have master fault zones and maximum flank uplifts on the east; this switches to the west in the southern half-graben. A volcanic cross-chain has intruded along, and differential basin subsidence occurs across, the northern AZ. A switch in rift asymmetry and fault orientations, as well as differential basin subsidence, occurs across the southern AZ.
The primary fault trends (N30°W and N10°E) indicate orthorhombic fault formation in response to N80°E extension (orthogonal to the volcanic arc, N10°W). Maximum subsidence is presently concentrated in the eastern portion of the basin. Horizontal offsets across normal faults account for ^sim2-3 km of extension (3-5% strain).
Rift basin development is characterized by an increasing concentration of basin subsidence in the east, concentration of extension along master fault zones, and basin symmetry owing to hanging wall collapse and antithetic faulting. Rift elongate volcanics intruded into the rift basins may be the earliest precursors of seafloor spreading. Thick (>2 km) pre-rift volcaniclastic sections imaged beneath the arc margin are obscured by syn-rift faulting and volcanism.
AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990