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 o ...

s situated along the Pacific coast of the Baja California peninsula in

Mexico Mexico (Spanish: México), officially the United Mexican States, is a country in the southern portion of North America. It is bordered to the north by the United States; to the south and west by the Pacific Ocean; to the southeast by Guatema ...

. The field formed by repeated eruptions beginning in the

Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...

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 The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million years ...

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-type

mafic A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...

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 a ...

c volcanism on the Baja California peninsula. The oldest cones mainly erupted primitive magmas with occasional small

xenolith A xenolith ("foreign rock") is a rock fragment (country rock) that becomes enveloped in a larger rock during the latter's development and solidification. In geology, the term ''xenolith'' is almost exclusively used to describe inclusions in igne ...

s. As the field evolved, differentiated magma became more common, although extremely pure primitive magmas – virtually devoid of xenoliths and unusually rich in

olivine The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...

phenocryst 300px, feldspathic phenocrysts. This granite, from the Switzerland">Swiss side of the Mont Blanc massif, has large white plagioclase phenocrysts, triclinic minerals that give trapezoid shapes when cut through). 1 euro coins, 1 euro coin (diameter ...

s – still dominated at the young cones. These primitive magmas originated from progressive

partial melting Partial melting occurs when only a portion of a solid is melted. For mixed substances, such as a rock containing several different minerals or a mineral that displays solid solution, this melt can be different from the bulk composition of the solid ...

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 alu ...

at unusually shallow levels in the mantle. The

ultramafic Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are composed ...

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 ...

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, with mi ...

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 websteri ...

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. When ...

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 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 An escarpment is a steep slope or long cliff that forms as a result of faulting or erosion and separates two relatively level areas having different elevations. The terms ''scarp'' and ''scarp face'' are often used interchangeably with ''escar ...

) 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 A raised beach, coastal terrace,Pinter, N (2010): 'Coastal Terraces, Sealevel, and Active Tectonics' (educational exercise), from 2/04/2011/ref> or perched coastline is a relatively flat, horizontal or gently inclined surface of marine origin,P ...

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, mainly ...

Evolution of areal faulting

From the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...

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 The Farallon Plate was an ancient oceanic plate. It formed one of the three main plates of Panthalassa, alongside the Phoenix Plate and Izanagi Plate, which were connected by a triple junction. The Farallon Plate began subducting under the west c ...

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 The North American Plate is a tectonic plate covering most of North America, Cuba, the Bahamas, extreme northeastern Asia, and parts of Iceland and the Azores. With an area of , it is the Earth's second largest tectonic plate, behind the Pacific ...

.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 subductio ...

. 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 * E ...

in the protogulf of the

Gulf of California The Gulf of California ( es, Golfo de California), also known as the Sea of Cortés (''Mar de Cortés'') or Sea of Cortez, or less commonly as the Vermilion Sea (''Mar Bermejo''), is a marginal sea of the Pacific Ocean that separates the Baja Ca ...

.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 A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust (geology), crust and the portion of the upper mantle (geology), mantle that behaves elastically on time sca ...

. 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 i ...

basalt Basalt (; ) is an aphanite, 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 planetary surface, surface of a terrestrial ...

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 A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a diverge ...

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 cap ...

'', 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 Diopside is a monoclinic pyroxene mineral with composition . It forms complete solid solution series with hedenbergite () and augite, and partial solid solutions with orthopyroxene and pigeonite. It forms variably colored, but typically dull ...

-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

andesite Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomi ...

s 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 e ...

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 A ...

, 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 b ...

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 Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only stat ...

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 plume A mantle plume is a proposed mechanism of convection within the Earth's mantle, hypothesized to explain anomalous volcanism. Because the plume head partially melts on reaching shallow depths, a plume is often invoked as the cause of volcanic hot ...

s.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 Partial melting occurs when only a portion of a solid is melted. For mixed substances, such as a rock containing several different minerals or a mineral that displays solid solution, this melt can be different from the bulk composition of the solid ...

in the mantle, or the progressive exhaustion of

incompatible element In petrology and geochemistry, an incompatible element is one that is unsuitable in size and/or charge to the cation sites of the minerals of which it is included. It is defined by the partition coefficient between rock-forming minerals and melt b ...

s in the source, led to a decrease in abundances of these volatiles as the field evolved.

Crustal contamination Crust may refer to: Common meanings Food * Crust (baking), the outer layer composed of pastry or salt * Crust, the bread foundation of pizza * Bread crust, the dense surface layer of bread Physical sciences * Crust (geology), the outer lay ...

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. * Defor ...

textures, which may have recorded plastic deformation that occurred with the

diapir A diapir (; , ) is a type of igneous intrusion in which a more mobile and ductily deformable material is forced into brittle overlying rocks. Depending on the tectonic environment, diapirs can range from idealized mushroom-shaped Rayleigh–T ...

ic 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 o ...

.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.

Description of the volcanic field

Regional geologic setting

Evolution of areal faulting

Volcanism

Geochemistry

Xenoliths

References

External links

Further reading