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Seamounts along California's continental margin
Davidson Seamount is one of several seamounts along the California continental margin. It is located about 120 km southwest of Monterey, CA, and rises to within 1300 m of the sea surface. It is oriented northeast-southwest and is a complex series of cones on parallel ridges without a summit caldera, not a typical conical shape. Lavas are basaltic, and range from tholeiite to trachyte. Hyaloclastite deposits indicate that mildly explosive eruptions occurred.
Other California continental margin seamounts are similar to Davidson in shape, lithologies, and range of ages. They are different from typical ocean-island volcanoes or near-ridge seamounts: they do not line up with each other and are not progressively older like a hot spot chain; they erupted onto much older crust, meaning that they were not associated with mid-ocean ridges. They instead are thought to have erupted along existing zones of weakness because of melting in response to strain on the tectonic plate.
A study of deep sea coral distributions at Davidson Seamount is reported on the eclectic topics page. More information about Davidson Seamount is under the MBARI mapping program. This seamount is proposed to become a national marine sanctuary.
Xenolith translates to "foreign rock"; it is a clast of another composition entrained in the erupted lavas and can offer clues to the magmatic processes in the volcano. Xenoliths are also discussed in Hot spot: Magmatic processes.
Our research on California's continental margin seamounts
The discussions below are paraphrased from abstracts of papers published by the Submarine Volcanism group.
Diverse origin of xenoliths
CALIFORNIA CONTINENTAL MARGIN - A diverse assemblage of small mafic and ultramafic xenoliths occurs in alkalic lava from Davidson and Pioneer seamounts located at the continental margin of central California. Based on mineral compositions and textures, they form three groups: (1) mantle xenoliths of lherzolite, pyroxenite, and dunite with olivine of >FO90; (2) ocean crust xenoliths of dunite with olivine <FO90, troctolite, pyroxenegabbro, and anorthosite with low-K2O plagioclase; (3) cumulates of seamount magmas of alkalic gabbro with primary amphibole and biotite and anorthosites with high-K2O plagioclase. The alkalic cumulates are genetically related to, but more evolved than, their host lavas and probably crystallized at the margins of magma reservoirs. Modeling and comparison with experimentally derived phases suggest an origin at moderate pressures (~0.5-0.9 GPa). The high volatile contents of the alkalic host lavas may have pressurized the magma chambers and helped to propel the xenolith-bearing lavas directly from deep storage at the base of the lithosphere to the eruption site on the ocean floor, entraining fragments of the upper mantle and ocean crust cumulates from the underlying abandoned spreading center.
Reference: Davis, A.S., D.A. Clague, J.B. Paduan (2007) Diverse origins of xenoliths from seamounts at the continental margin, offshore central California, Journal of Petrology; doi: 10.1093/petrology/egm003. [Abstract] [Article]
Hyaloclastite deposits
CALIFORNIA CONTINENTAL MARGIN - Hyaloclastite deposits are abundant on mid-Miocene volcanic seamounts offshore central California. The glass compositions are predominantly evolved hawaiite and megearite, although minor amounts of tholeiitic to alkalic basalt are also present. Textural features give evidence for different eruption styles. For the evolved, alkalic compositions, fragmentation occurred primarily in response to exsolution of magmatic gases as the magma approached and erupted on the seafloor. Textural features of pyroclasts suggest formation of lava fountains of limited size and height, depending on water depth. Monomict, clast-supported hyaloclastite of highly vesicular pyroclasts suggest limited dispersal and deposition near vent sites. Matrix-supported, polymict breccias are reworked and displaced by currents into deeper water. The narrow range of glass compositions extending over multiple layers of volcanic sandstone suggests deposition from a slurry of tephra and water directly related to an eruption. Basaltic hyaloclastite of vesicle-free, angular glass fragments from the deepest site apparently formed from quench granulation. Sulfur contents suggest eruption depths ranging from near sea level to over 2000 m for samples collected from 1300 to over 3400 m depth, implying large amounts of subsidence for these seamounts.
Reference: A.S. Davis and D.A. Clague (2003) Hyaloclastite from miocene seamounts offshore central California: compositions, eruption styles, and depositional processes, in: Explosive Subaqueous Volcanism, J.D.L. White, J.L. Smellie, and D.A. Clague (eds), Geophysical Monograph 140, American Geophysical Union, 129-142.
Decompression melting along existing faults
CALIFORNIA CONTINENTAL MARGIN - Davidson, Guide, Pioneer, Gumdrop, and Rodriguez seamounts are located at the continental margin offshore California between latitudes 37.5oN and 34oN. They may represent a previously unrecognized type of intraplate oceanic volcanism. They are morphologically unlike typical ocean island volcanoes or near-ridge seamounts: they consist of a series of cones aligned along parallel ridges separated by sediment-filled troughs, and lack pit craters, summit calderas, or isolated cones. They are oriented northeast-southwest, perpendicular to the direction of plate motion but parallel to the structure of the underlying oceanic crust.
40Ar/39Ar laser fusion ages of minerals separated from the lavas indicate at least two episodes of volcanism at ~16 and ~12 million years ago, younger by 7 to 10 million years than the underlying ocean crust. The rocks are predominantly differentiated alkalic basalt, hawaiite, and mugearite. They lack coherent liquid lines-of-descent, consistent with small batches of magma forming, fractionally crystallizing, and erupting in isolation from the previous and subsequent batches. The presence of mantle xenoliths suggests that magmas originated in the upper mantle, and xenoliths of alkalic cumulates and xenocrysts of feldspar, amphibole, and titanomagnetite are also consistent with fractionation in the upper mantle. Sr, Nd, and Pb isotopic ratios of some samples are from MORB-like depleted mantle sources; others indicate more variably enriched mantle sources. Abundances of highly incompatible elements, such as Nb, Ta, and Th, suggest that magmas formed by small percentage partial melting of variably enriched MORB-mantle.
Basalts erupted onshore in coastal California during the mid-Miocene, coeval with those of the seamounts offshore, have a similar isotopic range but trace elements show a subduction-related signature of crustal contamination. The onshore basalts are proposed to have ascended from shallow crustal reservoirs in a window behind a descending subducted slab. The chemistry of the volcanic rocks offshore, however, was not influenced by a slab component. Although a slab-window origin is plausible for the mid-Miocene volcanism, later episodes of small, sporadic eruptions on- and offshore probably resulted from decompression melting of mantle rising along existing zones of weakness undergoing extension related to continued movement along transform fault systems.
Reference: A.S. Davis, D.A. Clague, W.A. Bohrson, G.B. Dalrymple, and H.G. Greene (2002) Seamounts at the continental margin of California: a different kind of oceanic intra-plate volcanism, Geological Society of America Bulletin: Vol. 114, No. 3, pp. 316–333. [Abstract]
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