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The northern Channel Islands form a regional gravity high which forms a break in the dominant northwest-southeast gravity pattern south of the islands and the east-west gravity and structural pattern found on the north in the Transverse Ranges province of California. Both northwest-southeast and east-west components are apparent in the gravity and structure on the islands. Thus, the northern Channel Islands lie on the border between Transverse Ranges structure and the northwest-trending structures characteristic of the rest of California.
The complete Bouguer anomaly values across the northern Channel Islands range from 20 to 80 mgal. North of the Santa Cruz Island fault there is little variation in the complete Bouguer anomaly except toward the east across the Anacapa Passage, where the complete Bouguer anomaly decreases sharply. This decrease reflects the deepening of the basement rock under the Santa Barbara Channel.
South of the Santa Cruz Island fault, a continuous gravity pattern extends from the schist exposures on Santa Cruz Island to Point Bennett on San Miguel Island. This continuity in gravity suggests that the basement rock and the Santa Rosa basin also are continuous from the southwestern part of Santa Cruz Island to the western tip of San Miguel Island. The lowest gravity values are found in the Santa Cruz Passage, a fact that indicates that the center of the Santa Rosa basin is between Santa Cruz and Santa Rosa Islands. This gravity low is an extension of the gravity low in the Santa Cruz basin on the south and suggests that the Santa Rosa basin is a northern continuation of the Santa Cruz basin. A gravity high south of San Miguel Island and a high north of Santa Rosa Island give evide ce for two possible preexisting source areas, which supplied sediments to the area of San Miguel and Santa Rosa Islands.
Sparker profiles in the Santa Cruz Passage show the presence of a northwest-trending fault along Santa Cruz Canyon, but the continuity of the gravity data across this region shows that this fault is unimportant. A second fault, the Santa Rosa Island fault, cannot be seen on the sparker profiles, nor is it reflected in the complete Bouguer anomaly values over Santa Rosa Island. Therefore, this fault also is insignificant in regional structure. The Santa Cruz Island fault, in contrast, is reflected markedly in the gravity pattern over the island, and may have produced a large amount of lateral offset.
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