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A series of rhombohedral basins along the 300-km (185-mi) long Blanco transform, surveyed recently by Sea Beam, provide good oceanic analogues to pull-apart basins described along major strike-slip/oblique-slip boundaries on the continent such as the San Andreas and Dead Sea areas. In the Blanco region, continual reorientation of the transform in response to changes in plate motion during the past few million years provides a mechanism for the formation of these basins. In plan view, the Blanco transform is similar to the Gulf of California; that is, there is a series of long strike-slip faults separated by short tensional basins (up to 20 km, 12 mi, long) with structures oriented at a high angle to the master faults. There are some preliminary indications that magmatic a tivity is occurring in some of the Blanco basins. In the region of the Cascadia depression, previous workers have documented at least 1 cm/yr of crustal sinking within the Holocene. The presence of turbidites and ash layers in this region should provide fruitful ground for detailed studies of the interplay between sedimentation and tectonics and lead toward a better understanding of the evolution of pull-apart structures.
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