The high pore pressure gradients inherent to accretionary complexes affect the force balance of the wedge via seepage force, which acts in the direction of flow and is proportional to the pressure (head) gradient. If sufficiently large, this seepage force can offset gravity and friction and lead to failure. At the toe of the wedge sediments are weak, slopes are over-steepened by folding and faulting, and fluid pressure gradients can be high; these conditions are conducive to seepage-induced failure. For the 14 -16 ø slope at the toe of the southern Cascadia wedge, the pore pressure gradient necessary to initiate failure is )•=0.74-0.86. The gradient necessary to cause failure is sensitive to surface slope and sediment strength, but is insensitive to porosity. Reasonable estimates of sediment strength for most accretionary wedges require pore pressure gradients ranging from 10 to 60% of lithostatic to cause failure. These values are within the range of modeled and measured pore pressures in accretionary complexes, suggesting that seepage-induced slope failure should be an expected feature in this environment. If these failure features are observed, then their presence can be used to constrain the pore pressure gradient within the wedge, independent of any assumptions regarding fluid discharge or permeability. If seepage failure repeats and is localized in the same region, then it can lead to channel, gully, and canyon formation. Two convergent margins, southern Cascadia and northern I-Iispaniola, show many regularly spaced headless canyons that cannot be attributed to downslope erosive flow. We suggest that these canyons are forming from internally driven seepage-induced failure. Both the Oregon and Hispaniola accretionary wedges also contain evidence for non-uniform fluid flow based on the observed and inferred presence of vents. Using Darcy's Law, the pore pressure constraint from the slope failure analysis and an estimate of the total fluid discharge, we examine the relationship between wedge and vent permeabilities, the areal extent of focused fluid venting, and the percent of the total fluid discharge that flows out of vents. Given reasonable estimates of the total fluid discharge out of the southern Cascadia wedge, we find that the wedge must be less permeable than 2 x 1047 m 2 in order for focused fluid venting to occur at all.If the permeability of the vents is much higher than the wedge permeability, then the vents will occur over a very small percentage of the wedge; these vents, however, could accommodate much of the fluid flowing out of the wedge. Using Permeability measurements from samples collected at the toe of the Oregon margin [Horath, 1989], we estimate that vents at the toe of the southern Cascadia accretionary complex comprise less than 0.2% of the wedge area, but that these vents can accommodate up to 60% of the total fluid discharge. et al., 1985; Kulm et al., 1986; Le Pichon et al., 1987; J. C. Moore et al., 1988, 1990], heat flow anomalies [Reck, 1987; Fisher and Hounslow, 1990; Langseth e...
The 350-km-long, east-west trending Wetar back arc thrust belt in eastern Indonesia is a result of the collision of Australia with the Indonesian island arc. Along the northwest margin of Wetar Island a short (50 km) section of the thrust belt trends northeast, coincident withthe offset of the Indonesian arc by the newly discovered left-lateral Wetar-Atauro fault, which runs along the shelf region and trends northeast. The Wetar-Atauro fault may be viewed as a large-scale lateral ramp or wrench fault separating the eastern Wetar thrust belt from the western Wetar thrust belt. The interaction of strike-slip faulting and back arc thrust faulting creates several arc "blocks" whose geometry strongly affects the structure of the deformed wedge of sediment accreting in the Wetar back arc thrust belt. The varying orientations of the arc "backstop" make the Wetar back arc thrust belt a perfect laboratory for the study of oblique convergence. Along most of its length the Wetar thrust belt parallels the arc slope. However, along the northeast trending offset in the thrust belt, SeaMARC II side-scan sonar plus seismic reflection data show that the trend of the thrust belt is not parallel to the trend of the arc slope; rather, it is intermediate between the trend of the arc slope and the perpendicular to the expected convergence direction. The side-scan images allow us to map the geometry of four main thrust faults: the Wetar, Liran, Atauro, and Alor faults. The Wetar, Liran, and Atauro faults trend northeast, parallel to the thrust front, and do not have an en echelon relationship. The westernmost fault in the survey area, the Alor fault, trends almost east-west. The orientation of the Wetar-Atauro fault is consistent with a maximum principal stress within the arc oriented between 8 ø and 23 ø west of north, depending on the internal coefficient of friction. This agrees well with the P-axes of nearby earthquake focal mechanisms which trend consistently west of north, suggesting that the regional principal tectonic compression is oriented west of north. Because the thrust belt is oblique to the arc slope, we infer the structural directions are influenced by both the arc backstop and by the regional tectonic stress. The tectonic compression due to arc-continent collision in this region may be modified by the arc geometry in several ways: (1) the stress necessary to support the arc topography may be significant close to the arc, (2) the change in thickness of the elastic part of the lithosphere may cause a concentration of tectonic stress, and (3) the difference in material properties between the relatively rigid arc "bulldozer" and the weak basin sedimentary fill may be an important factor in producing a thrust belt that conforms roughly to the shape of the arc. The structure of the Wetar back arc thrust zone demonstrates that the development of small rigid blocks within a major collision zone may produce complex structural patterns and local directions of shortening that are highly oblique to the main direction of plate conv...
Using the SeaMARC II seafloor
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