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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.

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This study documents lowering of the surf zone (i.e. the upper shoreface) leading to intra-shoreface erosion, following two rapid relative sea-level falls along a tectonically uplifted coast during the Holocene, and the characteristics of the resultant prograding shoreface deposits. These findings are based on high-resolution analysis and radiocarbon dating of three new drill cores obtained from the Kujukuri strand plain, Pacific coast of eastern Japan, combined with previously published borehole data and information on modern shoreline profile adjustments. A shallowing-upward sandy succession composed of lower and upper shoreface facies, foreshore and backshore facies was recognized in the drill cores. Two rapid falls in relative sea-level at 2AE3 to 2AE6 and 1AE8 to 2AE0 ka are recorded by downstepping of the base of the foreshore facies, and farther seawards by the lowering of an erosional boundary between the upper and lower shoreface facies. Superimposed bed profiles of an adjacent modern beach define an envelope, the base of which reflects shore-normal migration of longshore bars and troughs. The base of the envelope represents an erosional surface that divides the surface mobile layer above from preserved deposits beneath. The surface is concave upwards and steeper than the mean beach profile, and exhibits a flat platform approximately at the lower limit of the upper shoreface equating to the storm surf zone. The seaward transition of this surface, rather than the mean equilibrium profile, controls the metre-scale to decimetre-scale internal structure of the Kujukuri shoreface deposits. Depositional models for sea-level fall based on an exponential equilibrium profile do not adequately account for the presence and migration of longshore bars and troughs.

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This paper presents a case study that assessed spatial variations in the tectonic uplift rates of beach deposits in the relict Kujukuri strand plain, situated on the northeastern coast of the Boso Peninsula, eastern Japan. The southern Boso Peninsula is tilted downward to the northeast due to plate subduction along the Sagami Trough. However, the cause of the northeastern coast uplift creating the relict strand plain is unclear, due to the absence of a Holocene raised marine terrace sequence. Elevations and ages of beach deposits were collected from drilled cores and ground-penetrating radar profiles along three shore-normal sections in the southern Kujukuri strand plain. From this, alongshore variations in the relative sea level since the mid-Holocene could be seen. These corresponded to north-to-northeast downward tilting at a rate of 0.4 m/ka for an interval 10 km and are concordant with the longer term tilting of the last interglacial marine terrace surrounding the plain. Although it is difficult to assess shore-normal variations of uplift based on the present dataset, the recognized tilting apparently continues to the tilting of the southern Boso Peninsula, implying the Sagami Trough probably affects the uplift of the Kujukuri coast.