Volcanic Petrology and Geologic History of Northeast Bank, Southern California Borderland

Evidence that three seamounts off southern California were ancient islands

“…Palmer (1964) observed the morphology and sediments on the summit of Rodriguez, and suggested it grew to sea level, was truncated by wave erosion, and then subsided. Hawkins et al (1971) suggested the same for Northeast Bank, and dredged shallow-water bivalves fossilized in freshly erupted volcanic material from more than 500 m depth. The summit of San Juan is not smooth and sediment covered, although it shares other characteristics, described below, suggesting it too was above sea level.…”

“…San Juan Seamount was surveyed using 12 kHz multibeam sonar. Northeast Bank has incomplete swath bathymetric coverage and low-resolution coverage with Coast and Geodetic Survey soundings data (Hawkins et al, 1971). The seamounts were observed and sampled on dives using the MBARI ROV Tiburon in 2003 and 2004 with 6 dives on Rodriguez, 3 on San Juan, and 2 on Northeast Bank (Fig.…”

“…The initial suggestion that Rodriguez Seamount (Palmer, 1964) and Northeast Bank (Hawkins et al, 1971) had been subaerial was based on the flattened shape of the summit platforms and rounded breaksin-slope detected in single beam bathymetric data. The change in slope of Hawaiian islands was interpreted by Mark and Moore (1987) as a transition from gentler subaerial to steeper submarine caused by effects on volcanic processes.…”

“…A group of seamounts lie offshore California, and Rodriguez Seamount (Palmer, 1964) and Northeast Bank (Hawkins et al, 1971) were proposed to have been islands at one time based mainly on the flattened shape of their summits. These two, plus an additional 7 seamounts, have now been explored with a remotely-operated vehicle (ROV).…”

Evidence for interaction of a spreading ridge with the outer California borderland

“…At Northeast Bank, a high just south of the study area, a younger volcanic episode may have followed the 10 Ma episode, based on a fission-track age of ∼ 4 Ma and late Pliocene fauna included in the volcaniclastic sediment (Hawkins, 1970;Hawkins et al, 1971). Coeval (< 16 Ma) volcanic suites show a subduction-related signature (e.g., basalts from the Santa Maria province (Cole and Basu, 1995) and calkalkaline volcanic rocks northeast of the Inner Borderland (Weigand and Savage, 1993;Weigand et al, 2002)) although subduction had clearly ceased.…”