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The San Quintín Volcanic Field is a collection of ten or eleven volcanic
cinder cone A cinder cone (or scoria cone) is a steep conical hill of loose pyroclastic fragments, such as volcanic clinkers, volcanic ash, or scoria that has been built around a volcanic vent. The pyroclastic fragments are formed by explosive eruptions or ...
s situated along the Pacific coast of the Baja California peninsula in Mexico. The field formed by repeated eruptions beginning in the Pleistocene and ending about 3000 years ago.Gorsline, D. S. and R. A. Stewart, Benthic marine exploration of Bahía de San Quintín, Baja California, 1960-61. Marine and Quaternary geology. ''Pac. Nat., 3'', 282-319, 1962.Woodford, A. O.
The San Quintín volcanic field, lower California
''
American Journal of Science The ''American Journal of Science'' (''AJS'') is the United States of America's longest-running scientific journal, having been published continuously since its conception in 1818 by Professor Benjamin Silliman, who edited and financed it himself ...
'' 15:337-345, 1928.
It is one of several known Quaternary
period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, entertainment, and media * Period (music), a concept in musical composition * Periodic sentence (or rhetorical period), a concept ...
volcanic fields in Baja. The lava shields appear to have first grown as subaqueous volcanoes that emerged as islands.Luhr, J. F., Aranda-Gómez, J. J., and Housh, T. B. San Quintín Volcanic Field, Baja California Norte, México: Geology, petrology, and geochemistry. ''
Journal of Geophysical Research The ''Journal of Geophysical Research'' is a peer-reviewed scientific journal. It is the flagship journal of the American Geophysical Union. It contains original research on the physical, chemical, and biological processes that contribute to the un ...
'' 100:10353–10380, 1995.
Currently located in a shallow bay, the cones are found in two groups and two isolated islands. The Northern group is composed of two cones, formed 126,000-90,000 years ago; the Southern group is larger, with two cones in the lagoon itself, and formed over a much larger and undetermined span of time. Most volcanic complexes in the field have a well-preserved scoria cone and lava apron, dotted with eruptive vents and lava flows. 42 eruptive units can be seen on the ground today. The San Quintín field is the only known location of
intraplate In geology, anorogenic magmatism is the formation, intrusion or eruption of magmas not directly connected with orogeny (mountain building). Anorogenic magmatism occurs, for example, at mid-ocean ridges, hotspots and continental rifts. This contrasts ...
-type mafic
alkali In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a basic, ionic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of ...
c volcanism on the Baja California peninsula. The oldest cones mainly erupted primitive magmas with occasional small xenoliths. As the field evolved, differentiated magma became more common, although extremely pure primitive magmas – virtually devoid of xenoliths and unusually rich in olivine phenocrysts – still dominated at the young cones. These primitive magmas originated from progressive partial melting of
spinel Spinel () is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula in the cubic crystal system. Its name comes from the Latin word , which means ''spine'' in reference to its pointed crystals. Properties S ...
lherzolite Lherzolite is a type of ultramafic igneous rock. It is a coarse-grained rock consisting of 40 to 90% olivine along with significant orthopyroxene and lesser amounts of calcic chromium-rich clinopyroxene. Minor minerals include chromium and a ...
at unusually shallow levels in the
mantle A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that. Mantle may refer to: *Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear **Mantle (vesture), an Eastern Orthodox ve ...
. The ultramafic xenoliths found are mostly spinel lherzolite, with the final 20%
harzburgite Harzburgite, an ultramafic, igneous rock, is a variety of peridotite consisting mostly of the two minerals olivine and low-calcium (Ca) pyroxene (enstatite); it is named for occurrences in the Harz Mountains of Germany. It commonly contains a f ...
s,
dunite Dunite (), also known as olivinite (not to be confused with the mineral olivenite), is an intrusive igneous rock of ultramafic composition and with phaneritic (coarse-grained) texture. The mineral assemblage is greater than 90% olivine, w ...
s, and
pyroxenite Pyroxenite is an ultramafic igneous rock consisting essentially of minerals of the pyroxene group, such as augite, diopside, hypersthene, bronzite or enstatite. Pyroxenites are classified into clinopyroxenites, orthopyroxenites, and the webst ...
s. The San Quintín field is also the only source of peridotitic and granulitic xenoliths on the peninsula. The largest and most abundant xenoliths are found in differentiated magmas.


Description of the volcanic field

The San Quintín Volcanic Field is found about south of the U.S. border and south of Ensenada, Baja California, at a latitude of ~30.5°N. Today, the cones rest on over a kilometer of
unconsolidated Soil consolidation refers to the mechanical process by which soil changes volume gradually in response to a change in pressure. This happens because soil is a two-phase material, comprising soil grains and pore fluid, usually groundwater. Whe ...
Plio-Quaternary sediment overlaid on the volcanic
basement A basement or cellar is one or more floors of a building that are completely or partly below the ground floor. It generally is used as a utility space for a building, where such items as the furnace, water heater, breaker panel or fuse box ...
of the lower Cretaceous Alistos Formation.Espindola-Cardeña, J. M., J. M. Romo-Jones, and M. Almeida-Vega, Gravimetria y estructura del Valle de San Quintín, B. C. ''GEOS Bol. Inf., 11'', pp 10-15, Union Geofis. Mex. Mexico City, 1991. The ten volcanic complexes recognized in the San Quintín field include Media Luna and Woodford in a northern group; a southern group consisting of Basu, Riveroll, Kenton, Picacho Vizcaino, Sudoeste, and Ceniza; and the isolated complexes Monte Mazo and Isla San Martín. A seacliff north of Basu exposes what may be an eleventh complex.


Regional geologic setting

The Santillán and Barrera Line, an important tectonic boundary, is found about NE of the volcanic field. At the boundary is an emerged portion of the Continental Borderland, a section of marine
deposition Deposition may refer to: * Deposition (law), taking testimony outside of court * Deposition (politics), the removal of a person of authority from political power * Deposition (university), a widespread initiation ritual for new students practiced f ...
marking the west coast of the Baja Peninsula and the eastern border of the depositional trough. On the other side lies the Stable Peninsula Province, an area mostly devoid of faulting (as opposed to the Gulf of California escarpment) and covered in pre-batholithic and
batholith A batholith () is a large mass of intrusive igneous rock (also called plutonic rock), larger than in area, that forms from cooled magma deep in Earth's crust. Batholiths are almost always made mostly of felsic or intermediate rock types, such ...
ic crystalline rocks.Gastil, R. G., R. P. Phillips, and C. C. Allison, Reconnaissance geology of the State of Baja California, ''Mem. Geol. Soc. Am., 140'', 170 pp. 1975. Another marine terrace escarpment is found to the east, rising above the coastal plain, with more late Cretaceous sediments overlain by early
Tertiary Tertiary ( ) is a widely used but obsolete term for the geologic period from 66 million to 2.6 million years ago. The period began with the demise of the non-avian dinosaurs in the Cretaceous–Paleogene extinction event, at the start ...
conglomerates and Pliocene
marine sediments Marine sediment, or ocean sediment, or seafloor sediment, are deposits of insoluble particles that have accumulated on the seafloor. These particles have their origins in soil and rocks and have been transported from the land to the sea, mai ...
.


Evolution of areal faulting

From the Cretaceous period until about 29 million years ago (29 Ma), the
oceanic Oceanic may refer to: *Of or relating to the ocean *Of or relating to Oceania **Oceanic climate **Oceanic languages **Oceanic person or people, also called "Pacific Islander(s)" Places * Oceanic, British Columbia, a settlement on Smith Island, ...
Farallon Plate
subducted Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...
eastward beneath the west coast of the North American Plate.Atwater, T. A., Implications of plate tectonics for the Cenozoic evolution of western North America, ''Geol. Soc. Am. Bll., 81'', 3513-3536, 1970. The arrival of the Pacific-Farallon spreading center at the trench formed two
triple junction A triple junction is the point where the boundaries of three tectonic plates meet. At the triple junction each of the three boundaries will be one of three types – a ridge (R), trench (T) or transform fault (F) – and triple junctions can b ...
s that migrated in opposite directions, northward and southward, along the coast. This stopped subduction and formed an interplate
transform boundary A transform fault or transform boundary, is a fault along a plate boundary where the motion is predominantly horizontal. It ends abruptly where it connects to another plate boundary, either another transform, a spreading ridge, or a subduction ...
. The southward-migrating triple junction passed San Quintín about 17 Ma and tip of Baja around 12 Ma, ending subduction in the Baja peninsula.Mammerickx, J. and K. D. Klitgord, Northern East Pacific Rise: Evolution from 25 m.y.B.P. to the Present, ''J. Geophys. Res., 87'', 295-302, 1993. The cessation of subduction forced the Pacific and North American plates along the fault zones created alongside the San Benito and the Tosco-Abreojos faults, which run approximately parallel to the west coast of Baja.Spencer, J. E. and W. R. Normark, Tosco-Abreojos fault zone: A Neogene transform plate boundary within the Pacific margin of southern Baja California, México, ''Geology, 7'', 554-557, 1979. Over time, the direction of relative motion between these plates swung west, accommodated by
extension Extension, extend or extended may refer to: Mathematics Logic or set theory * Axiom of extensionality * Extensible cardinal * Extension (model theory) * Extension (predicate logic), the set of tuples of values that satisfy the predicate * Ext ...
in the protogulf of the Gulf of California.Hausback, B. P., Cenozoic volcanic and tectonic evolution of Baja California Sur, México, in ''Geology of the Baja California Peninsula Publ. 39'', edited by V. A. Frizzel Jr., pp 219-236, Society of Economic Paleontologists and Mineralogists, Pacific Section, Bakersfield, Calif., 1984. Simultaneously, normal faulting throughout the protogulf further weakened the areal lithosphere. The northern protogulf was flooded as early as 13 Ma,Smith, J. T., Cenozoic marine mollusks and the paleogeography of the Gulf of California, in The Gulf and Peninsular Province of the Californias, edited by J. P. Dauphin and B. A. Simoneit, ''AAPG Mem., 47'', 637-666, 1991. and by 10 Ma, the region was experiencing eruptions of
tholeiitic The tholeiitic magma series is one of two main magma series in subalkaline igneous rocks, the other being the calc-alkaline series. A magma series is a chemically distinct range of magma compositions that describes the evolution of a mafic magma ...
basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of a ...
s. By 3.5 Ma, the gulf region was the center of most of the Pacific-North American plate motion, generating new seafloor from mid-ocean ridge basalts (MORB) at spreading centers following areal transform faults.Curray, J. R., et al., Leg 64 seeks evidence of development of basins, ''Geotimes, 24'', 18-20, 1979. The fault planes of Baja, going NNW-SSE, now accommodate plate boundary displacementAngelier, J. B., et al., Fault tectonics of the Baja California Peninsula and the opening of the Sea of Cortez, México, ''J. Struct. Geol., 3'', 347-357, 1981. and are currently active. In 1975, a collection of seismic events occurred after a dearth of events for a third of a century about northwest of the field.Rebollar, C. J., A. Reyes, and M. Reichle, Estudio del enjambre de San Quintín, Baja California, México, ocurrido durante 1975, ''Geofís, Int., 21'' (4), 331-358, 1982. The ten volcanic complexes of San Quintín are all aligned N-S to NW-SE, parallel to an offshore fault, the Santillán and Barrera line, and the main escarpment of the Gulf of California. The cones are therefore related to faulting, but no direct evidence for extensional faulting has been found. Indirect evidence is found in the shared characteristics of the field with other Quaternary volcanic fields linked to extensional faulting found throughout the province. This includes intraplate geochemical signatures in the magmas, and the presence of mantle and crustal xenoliths.Lynch, D. J., T. E. Musselman, J. T. Gutmann, and P. J. Patchett, Isotopic evidence for the origin of Cenozoic volcanic rocks in the Pinacate volcanic field, northwestern México, ''
Lithos Lithos is a glyphic sans-serif typeface designed by Carol Twombly in 1989 for Adobe Systems. Lithos is inspired by the unadorned, geometric letterforms of the engravings found on Ancient Greek public buildings. The typeface consists of only capi ...
'', 29, 295-302, 1993.


Volcanism

In Baja California, high-volume volcanism ended around 15 Ma, shortly after the end of subduction.Gastil, R. Gordon, Daniel Krummenacher, and John Minch. The record of Cenozoic volcanism around the Gulf of California, ''Geol. Soc, Am. Bull., 90'', 839-857, 1979. The third oldest volcano has reliably (using 40Ar/39Ar step-heating) been dated to 126,000 years ago. Following eruptions in the northern gulf area covered the San Quintin Volcanic Field in alkalic, diopside-rich basalts.Sawlan, M. G., Magmatic evolution of the Gulf of California rift, ''AAPG Mem., 47'', 301-369, 1991. General characteristics of these basalts, known as ''bajaites'',Rogers, G., et al., Geochemistry of Holocene volcanic rocks associated with ridge subduction in Baja California, México, ''Nature, 315'', 389-392, 1985.Saunders, A. D., et al., Geochemistry of Cenozoic volcanic rocks, Baja California, México: Implications for the petrogenesis of post-subduction magmas, ''J. Volcanol. Geotherm. Res., 32'', 223-245, 1987. are also similar to the ''alkali basalts'' found in Baja California Sur, as well as high-magnesium andesites of the
Aleutians The Aleutian Islands (; ; ale, Unangam Tanangin,”Land of the Aleuts", possibly from Chukchi ''aliat'', "island"), also called the Aleut Islands or Aleutic Islands and known before 1867 as the Catherine Archipelago, are a chain of 14 large vo ...
.Kay, R. W., Aleutian magnesian andesites: Melts from subducted Pacific Ocean crust, ''J. Volcanol. Geotherm. Res., 4'', 117-132, 1978 These ''bajaites'', mostly found a bit south of the San Quintín volcanic field, have high Mg, Ni, Cr, and Sr, low Rb, and high K/Rb and La/Yb ratios. The characteristics may suggest that one source is
slab Slab or SLAB may refer to: Physical materials * Concrete slab, a flat concrete plate used in construction * Stone slab, a flat stone used in construction * Slab (casting), a length of metal * Slab (geology), that portion of a tectonic plate that i ...
-derived melts, i.e. a subducting plate adds volatiles to the ultramafic rock of the
upper mantle The upper mantle of Earth is a very thick layer of rock inside the planet, which begins just beneath the crust (at about under the oceans and about under the continents) and ends at the top of the lower mantle at . Temperatures range from appro ...
such that it melts. It has been proposed that the slab fully subducted in the relevant latitudes of the San Quintín field, so that a " no-slab window" allowed the plate to directly access asthenospheric mantle. This is the most obvious explanation of the observered intraplate characteristics of the alkali basalts, with lherzolite. Alternatively, the ''bajaites'' could be formed from partial melting in the mantle of metasomatized mantle
apatite Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl− ions, respectively, in the crystal. The formula of the admixture of the three most common ...
and
amphibole Amphibole () is a group of inosilicate minerals, forming prism or needlelike crystals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Its IMA symbol is Am ...
, followed by postsubduction rifting. This explanation leaves the field untouched by a second subduction episode and leads more naturally to the
ocean island basalt Ocean island basalt (OIB) is a volcanic rock, usually basaltic in composition, erupted in oceans away from tectonic plate boundaries. Although ocean island basaltic magma is mainly erupted as basalt lava, the basaltic magma is sometimes modified by ...
s that are also present.


Geochemistry

The basalt lavas of the San Quintín field have the same composition as ocean island basalts found in Hawaii and the
Azores ) , motto =( en, "Rather die free than subjected in peace") , anthem= ( en, "Anthem of the Azores") , image_map=Locator_map_of_Azores_in_EU.svg , map_alt=Location of the Azores within the European Union , map_caption=Location of the Azores wi ...
, where they are associated with mantle plumes.Storey, M., Rogers, G., Saunders, A.D. and Terrell, D.J. 1989. San Quintín volcanic field, Baja California, Mexico: ‘within-plate’ magmatism following ridge subduction. ''Terra Nova'' 1: 195–202. Isotopic composition ranges overlap for primitive and differentiated rocks, indicating that the latter was derived from fractional crystallization of the primitive magmas. Xenolith abundance in the differentiated magma hint that fractional crystallization occurred within the mantle, below entrained peridotite. Due to this depth and the estimated speed of ascent, fractionation-elevated volatiles may have been important in driving the differentiated magmas to the surface. Partial melting in the mantle, or the progressive exhaustion of incompatible elements in the source, led to a decrease in abundances of these volatiles as the field evolved. Crustal contamination can be inferred from samples taken from two cones, and small isotopic variations for the other cones indicate at least three mantle components. The primitive magmas differ from other reported intraplate-type mafic alkalic suites by having relatively high Al2O3 and Yb, as well as low ratios of La/Yb and CaO/Al2O3. Rising Al2O3 and falling CaO, along with decreasing incompatible element abundances, are consistent with progressive partial melting.


Xenoliths

The ultramafic xenoliths of the field are composed roughly of 80% spinel lherzolites, with lesser harzburgites, dunites, and pyroxenites. There is a high abundance of clinopyroxenes, about 35% by volume. Granulite xenoliths, interpreted as being derived from the lower crust, and peridotite xenoliths, interpreted as derived from the upper mantle, are particularly abundant in the Woodford, Media Luna, and Basu cones. The site exhibits strong
deformation Deformation can refer to: * Deformation (engineering), changes in an object's shape or form due to the application of a force or forces. ** Deformation (physics), such changes considered and analyzed as displacements of continuum bodies. * Defo ...
textures, which may have recorded plastic deformation that occurred with the diapiric rise of mantle through the
low-velocity zone The low-velocity zone (LVZ) occurs close to the boundary between the lithosphere and the asthenosphere in the upper mantle. It is characterized by unusually low seismic shear wave velocity compared to the surrounding depth intervals. This range ...
.Basu, A. R.
Hot-spots, mantle plumes and a model for the origin of ultramafic xenoliths in alkali basalts
''Earth Planet. Sci.Lett., 28'', 261-274, 1975.
Alternatively, more complicated models suggest that a shallow, active shear zone deformed the xenoliths and it was only later that the deeper, source magmas modified the chemical composition of the field.Cabanes, N., and J.-C. C. Mercier
Insight into the upper mantle beneath an active extensional zone: The spinel-peridotite xenoliths from San Quintín (Baja California, México)
''Contrib. Mineral. Petrol., 100'', 374-382, 1988.


References


External links

*


Further reading

* Luhr, James F., Paul Kimberly, Lee Siebert, J. Jorge Aranda-Gomez, Todd B. Housh, Giuseppina Kysar Mattietti. 2006. México's Quaternary volcanic rocks: Insights from the MEXPET petrological and geochemical database. In:
Neogene-Quaternary Continental Margin Volcanism: A Perspective from México
', Geological Society of America Special Paper 402, pp. 1–44. {{DEFAULTSORT:San Quintin Volcanic Field Volcanic fields Volcanoes of Baja California Volcanoes of Baja California Sur Volcanism of Mexico Landforms of Baja California Landforms of Baja California Sur