Abstract New, high-resolution multi-beam bathymetric data from RV Sonne cruise SO251 show a widely variable surface morphology along the southern Kumano Basin and Nankai accretionary prism off SW Japan. Combined with a three-dimensional seismic volume, these data provide insight into the ubiquitous and varied nature of faulting typical of accretionary prism settings, a high number of submarine landslides across the entire study area that vary both spatially and temporally, a pronounced absence of slide deposit bathymetric manifestations, widely varied slope angles and a potential subducted seamount scar. We have mapped scars of 442 primary and 184 secondary landslides and have measured the areas evacuated by these slides. Most of the slides are completely disintegrative, so surficial landslide deposits are almost absent. The incidence with which temporally sequential slope failures and fault structures cross-cut themselves and one another provides evidence of potential failure pre-conditioning such as gas hydrates, pore fluid overpressures and bottom current activity. Seismic loading and slope over-steepening are then the most likely final trigger mechanisms to slope failure. The majority of observed landslides (64%) occur seawards of the outer ridge, providing insight into the relationship between surficial landsliding and subsurface tectonic processes along this accretionary prism.

Abstract 3D seismic data from the Nankai margin provide detailed imagery documenting the onset of deformation at an active sediment-dominated accretionary prism, including a previously unmapped network of normal faults. The Nankai margin off southwest Japan is characterized by active subduction, seismogenesis, and a large accretionary prism with fold-and-thrust belt structure. Imbricate thrusting is the dominant structural style of the outer 20 km of the prism. This structural domain develops at the prism toe, where an incipient imbricate thrust displays significant along-strike variability in dip, offset, and development of hangingwall anticlines. Compressional deformation is preceded by normal faulting that initiates seaward of the trench axis. Seismic data in this area reveal a complex, intersecting pattern of normal faults within the incoming hemipelagic sediments. Underlying the faulted section is a high-amplitude reflector interpreted as representing oceanic basement. This reflector contains elongate horsts and grabens oriented perpendicular to the margin interpreted as relict spreading centre fabric. Analysis of the orientation of normal faults within the Shikoku basin sequence shows a correlation between fault geometry and basement structure. This faulting is notably similar to layer-bound compaction faults, documented in the North Sea and elsewhere, attributed to both hydrofracturing and volumetric contraction of fine-grained sediments. Mapped normal faults may thus be the result of a combination of differential compaction of sediments above irregular, dipping oceanic basement and compactional dewatering seaward of the toe of the accretionary prism.

Seismic reflection profiles and SeaMARC II imagery from the southwest Panama margin demonstrate that oblique convergence is presently occurring along what had previously been thought of as a transform margin. Our seismic profiles image landward-dipping thrust faults and seaward-verging folds at the toe of the slope. The frontal deformation zone as imaged on the SeaMARC II mosaic is 12 to 15 km wide with individual east-west-trending folds and thrusts that are laterally continuous for 5 to 10 km. Much of the terrigenous trench sediment is offscraped and accreted, forming an accretionary prism (South Panama deformed belt). Three linear ridges (part of the Panama Fracture Zone complex) are being obliquely subducted along the southwest Panama margin. The oblique convergence causes the ridges to sweep eastward along the trench. The SeaMARC II mosaic shows that the regional structure of the South Panama deformed belt is dominated by east-west-trending trench segments that are separated by the north-south fracture zone ridges. The trench shallows where the ridges intersect the trench, and the deformation front is warped around the ridges. On the east side of each ridge the accretionary complex bends to a northwest-southeast trend, suggesting that the ridges are deforming the accretionary complex. As the accretionary prism rides up over each ridge, it thickens markedly. By the time the prism reaches the top of the ridge, its surface slope has been greatly oversteepened and large portions of accreted material slump into the trench. After passage of the ridge, the system returns to its “normal” state, and accretion resumes, adding the slumped material back into the accretionary prism. The accretionary prism is thus only temporarily disrupted by the subduction of the Panama Fracture Zone system ridges.

Abstract The Cenozoic geologic history of west-central Sumatra is governed by the northward movement of the Indian plate with respect to Southeast Asia. The morphology and structure of the western Sumatran margin reflects the cumulative effects of the resulting subduction and right-lateral slip, especially of that since the late Oligocene. The characteristics of this margin are similar to those of other arc systems. Nias, one of the islands of the trench slope break, consists of mid-Tertiary melange and younger, less deformed slope-basin strata. The trench slope break has been migrating westward since at least the mid-Miocene as shown by the Quaternary pattern of uplift and subsidence and by combined geological and geophysical data along the western flank of the forearc, or upper slope, basin. This forearc basin contains at least 4 km of Neogene sediments. On the western flank of the forearc basin, strata lie on a melange basement and are sharply flexed onto the trench slope break, but on the eastern flank the basin strata appear to lap onto an older continental slope and shelf. Seismic reflection profiles and drilling on the continental shelf delineate a marked unconformity that increases in depth from near zero at the coast to more than 2 km at the old shelf break. Subsidence and transgression of the old shelf by younger shelf sediments began in the early Miocene and may be continuing at present. Beneath the unconformity on the outer shelf there are Paleogene strata that may define an older forearc basin. The landward flank of this suspected basin, beneath the inner shelf, is probably underlain by Mesozoic and Paleozoic metamorphic and igneous rocks that are covered by littoral Paleogene strata. Scattered across the inner shelf and extending into the coastal mountains are numerous, probably Oligocene, andesitic intrusives and their associated extrusive debris. These igneous centers are anomalous in that they are closer to the trench than are the volcanic chains of younger and older ages. Both the uplift which cut the shelf unconformity and the andesites may have been related to the northward migration of a ridge-trench-trench triple junction along the Sunda arc. The Paleogene littoral strata along the west coast show increasing intensity of folding toward the Barisan Range and at the mountain front are sharply flexed and sheared. The rate of deposition in the offshore basins and the lack of deformation of the shelf unconformity indicate that the major uplift of the Barisan block occurred during the late Miocene and Pliocene, although it may still be proceeding. There is no evidence of a mid-Miocene orogeny in central Sumatra, but instead, of continuous subduction since at least the late Oligocene which has been accompanied by westward migration of morphotectonic units.