The stratigraphic framework of the Lima and Yaquina forearc basins offshore Peru, determined from multichannel seismic (MCS) data, reveals significantly different Neogene histories in the two basins. Miocene deposition in the Lima Basin was controlled mainly by variations in the relative subsidence rates. Depositional processes, particularly contour currents, may have had a major influence during the Pliocene-Pleistocene. In contrast, the Yaquina Basin shows no evidence of assumed contourites. Severe disruption of the reflectors indicates active tectonism throughout the Neogene in the Yaquina Basin, while reflectors representing late Miocene and younger strata in the Lima Basin are largely undeformed, which indicates relative quiescence. Most sequences in the Lima Basin demonstrate the presence of a hinge line that separates the relatively thin, wedgeshaped landward part from a much thicker, lens-shaped seaward part. Generally, this hinge appears to represent a paleoslope break. Such hinge lines are not evident in the Yaquina Basin. Both basins exhibit migration of the depocenters of the various sequences through time. The movement is to the south and landward in the Lima Basin, while migration is northward in the Yaquina Basin. These migrations appear to be the result of variations in the relative subsidence rates within the basins. In the Lima Basin these movements are closely related to structural features in the basement. In addition to a structural trend that is oriented parallel to the margin, we observed a secondary structural trend that is oriented east-west. Development of structural features along this trend led to the development of two distinct depocenters in most of the stratigraphic sequences in the Lima Basin.

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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 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 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 (T e ~ 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.

NOTICE PORTIONS OF THIS REPORT ARE ILLEGIBLE. It has been reproduced from the best availab!e copy to permit the broadest posslble availa bllity,

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