La Reforma Caldera

Coordinates: 27°30′29″N 112°23′31″W / 27.508°N 112.392°W / 27.508; -112.392[1]

Land next to the sea, with a darker semicircle just right of centre surrounding a whiter region
The caldera of La Reforma and neighbouring volcanoes

La Reforma is a Pliocene-Pleistocene caldera on the Baja California Peninsula in Mexico. It is part of about eleven volcanoes in Baja California, which formed when the Gulf of California formed starting from about ten million years ago. Previously, a volcanic arc had existed on Baja California peninsula. The basement the caldera is formed upon was formed between the Cretaceous and the Miocene.

The caldera has a diametre of 10 kilometres (6.2 mi); its formation was accompanied by the eruption of a 5–10 cubic kilometres (1.2–2.4 cu mi) ignimbrite. After the eruption, volcanic activity continued in and around the caldera. The centre of the caldera was uplifted by about 300 metres (980 ft). Other volcanoes in the area include El Aguajito and Tres Virgenes.

Geography and structure

La Reforma lies in central-eastern Baja California,[2] just south of the frontier between Baja California and Baja California Sur,[3] in Mexico,[1] 20 kilometres (12 mi) north of Santa Rosalia.[4] Other volcanic centres in the neighbourhood are Tres Virgenes and El Aguajito west of La Reforma;[5] the existence of the latter was first proposed in 1984 as "Santa Ana caldera".[6]

Volcanism in the area is linked to the formation of the Gulf of California about ten million years ago, as part of the tectonic activity of the Basin and Range province. The formation of the Gulf was accompanied by volcanic activity, at first calc-alkaline and later alkaline and tholeiitic.[7] Later, the interaction between the San Andreas Fault and the East Pacific Rise triggered the formation of a transform boundary in the gulf since 3.5 million years ago.[8] Presently, Baja California moves northwestward at a rate of 5.6 centimetres per year (2.2 in/year).[9] As part of this event, the Santa Rosalia basin formed and was filled by various Miocene-Pleistocene formations, some of which are exposed in La Reforma.[7] The formation of this basin was influenced by faults that trend NW-SE. Other fault systems are also active in the area.[10]

The La Reforma caldera has a diametre of 10 kilometres (6.2 mi),[2] the height of its rim ranges 100–500 metres (330–1,640 ft).[11] It is formed by a semicircular structure around Cabo Virgenes that reaches the coast of the Gulf of California at La Reforma in the northwest and at Punta Las Cuevas in the southeast.[12][1] A central block rises 700 metres (2,300 ft) above the rim of the caldera; it is formed by welded tuffs and forms a resurgent dome.[3] The highest point of this dome is 1,200 metres (3,900 ft) high.[11]

Alternative theories consider the La Reforma structure either a dome which was subsequently eroded in a circular pattern, or a set of tectonic blocks. The dome itself was considered to be formed by the Comondú volcanics, but appears to be instead made by products of the La Reforma activity.[3]

Geology

Subduction was active in the region until 12,500,000 - 11,000,000 years ago; volcanic activity occurring since then is because of tectonic changes associated with the development of the Gulf of California. The previous subduction related volcanism formed the so-called Comondú volcanics.[3]

When subduction was active, it triggered the formation of andesitic rocks named "Andesite of Sierra Santa Lucía" in the area of La Reforma.[13] These are the local manifestation of the Comondú volcanics.[14] This activity occurred 24-13 million years ago. The last subduction-related volcanism is named "Santa Rosalia dacite", it was erupted between 13 and 12 million years ago. The subsequent rifting-related volcanism laid down the Boleo basalts-basaltic andesite (11-9 million years ago), the El Morro tuff (9-8 million years ago) and the Cerro San Lucas unit (9.5-7.7 million years ago).[13]

La Reforma is one among 11 volcanic centres that are found in the Baja California Peninsula, which other than La Reforma-Tres Virgenes include Cerro Prieto, El Pinacate, San Quintin volcanic field, Puertecitos volcanic province, San Luis Island, Jaraguay volcanic field, San Borja volcanic field, Punta Púlpito, Mencenares volcanic complex and La Purísima.[15]

Local

Based on outcrops, the basement is of Cretaceous age and formed by granitic rocks.[3] Other parts of the basement are Miocene marine sediments and the Comondú volcanics.[11] Andesite of Sierra Santa Lucía is found west-southwest of the Santa Rosalia basin.[13] The basaltic La Esperanza formation is also found in the area.[16]

Composition

Lava flows are made of andesite, basalt and dacite. Lava domes are made out of rhyolite.[1] Pliocene rocks contain clinopyroxene and feldspar. An additional rock is plagioclase. Apatite, magnetite and zirconia are also present.[17] The ignimbrites also contain feldspar and clinopyroxene.[18] Some of the last erupted rocks also contain amphibole.[19] The area is known for deposits of Cu and Mn.[4] Some rocks have been converted into palagonite.[17]

Elemental composition of La Reforma volcanic rocks resemble these of calc-alkaline volcanism.[3] This is consistent with the geochemistry of nearby Tres Virgenes, although one pyroclastic flow at La Reforma was considered to be peralkaline.[14]

Climate

Precipitation in the area amounts to 60 millimetres per year (0.075 in/Ms), dominated by winter precipitation which amounts to 40.6 millimetres (1.60 in). The climate is arid; most precipitation falls on Tres Virgenes.[12]

Eruptive history

Volcanism at La Reforma started in the Pliocene, with ash flows and subaqueous pumice flows. Later, activity became subaerial and also involved pillow lava.[2] Of the local volcanoes, La Reforma was the first to emerge from the sea.[20] 4-5 million years ago basaltic dykes were placed.[12] Overall, volcanic activity at La Reforma occurred between 1.6 and 1.4 million years ago.[21] At least 4 ignimbrites are found at La Reforma.[11]

A major ignimbrite-forming eruption occurred in the early Pleistocene. This eruption involved the formation of tuffs formed by pantellerite.[2] This eruption covered a surface area of about 200 square kilometres (77 sq mi) with 5–10 cubic kilometres (1.2–2.4 cu mi) of ignimbrite.[22] This ignimbrite is rich in fiammes.[11] Andesitic effusive activity occurred on the flanks of the caldera both before and after its collapse.[1]

Post-caldera activity formed lava domes and lava flows of rhyolitic composition along the ring faults of the caldera.[2] One of these lava flows was dated to be 1,090,000 ± 110,000 years old,[1] the domes range 1.4 - 1.2 million years ago.[12] Some La Reforma eruption products were later buried by activity from El Aguajito,[23] to which volcanic activity had later migrated.[20] Basaltic cones,[2] erupted 600,000 years ago,[12] on the flanks of the caldera are tectonically unrelated; like Isla Tortuga and Tres Virgenes they are controlled by the extension of the Sea of Cortez.[2] Fluvial deposits were laid down in the caldera.[24] Present-day seismic activity occurs in the area.[3] Future large scale explosive activity is possible.[17]

The centre of the caldera has been affected by resurgent doming, triggering the exposure of Miocene age rocks including diorites. Along with the uplift of the surrounding land, this has resulted in Pleistocene sea deposits being uplifted to over 300 metres (980 ft) of altitude. The average pace of this uplift is 240 millimetres per kiloare (8.8×10−6 in/sq ft).[2] The existence of a magma chamber beneath La Reforma is suggested by the water springs in the area, which contain minor amounts of magmatic water.[25] Temperatures in hot springs of the area range 21–98 °C (70–208 °F),[13] and the area has been investigated for the potential of geothermal energy.[15]

References

  1. 1 2 3 4 5 6 "La Reforma". Global Volcanism Program. Smithsonian Institution.
  2. 1 2 3 4 5 6 7 8 Demant, Alain; Ortlieb, Luc (January 1981). "Plio-pleistocene volcano-tectonic evolution of la Reforma Caldera, Baja California, Mexico". Tectonophysics. 71 (1-4): 194. doi:10.1016/0040-1951(81)90065-2.
  3. 1 2 3 4 5 6 7 Hook, Simon J.; Dmochowski, Jane E.; Howard, Keith A.; Rowan, Lawrence C.; Karlstrom, Karl E.; Stock, Joann M. (April 2005). "Mapping variations in weight percent silica measured from multispectral thermal infrared imagery—Examples from the Hiller Mountains, Nevada, USA and Tres Virgenes-La Reforma, Baja California Sur, Mexico". Remote Sensing of Environment. 95 (3): 279. doi:10.1016/j.rse.2004.11.020.
  4. 1 2 Demant 1984, p. 75.
  5. ↑ Garduño-Monroy, Vargas-Ledezma & Campos-Enriquez 1993, p. 48.
  6. ↑ Garduño-Monroy, Vargas-Ledezma & Campos-Enriquez 1993, p. 47.
  7. 1 2 Garduño-Monroy, Vargas-Ledezma & Campos-Enriquez 1993, p. 49.
  8. ↑ Zanchi, Andrea (December 1994). "The opening of the Gulf of California near Loreto, Baja California, Mexico: from basin and range extension to transtensional tectonics". Journal of Structural Geology. 16 (12): 1621. doi:10.1016/0191-8141(94)90131-7.
  9. ↑ Fabriol, Hubert; Delgado-Argote, Luis A; Dañobeitia, Juan José; Córdoba, Diego; González, Antonio; Garcı́a-Abdeslem, Juan; Bartolomé, Rafael; Martı́n-Atienza, Beatriz; Frias-Camacho, Vı́ctor (November 1999). "Backscattering and geophysical features of volcanic ridges offshore Santa Rosalia, Baja California Sur, Gulf of California, Mexico". Journal of Volcanology and Geothermal Research. 93 (1-2): 76. doi:10.1016/S0377-0273(99)00084-0.
  10. ↑ Portugal et al. 2000, p. 225.
  11. 1 2 3 4 5 García Sánchez,, Laura (2015-12-15). "STRATIGRAPHY OF REFORMA CALDERA, BAJA CALIFORNIA SUR, MEXICO". AGU.
  12. 1 2 3 4 5 Portugal et al. 2000, p. 227.
  13. 1 2 3 4 Del Rio-Salas, Rafael; Ochoa-Landín, Lucas; Eastoe, Christopher J.; Ruiz, Joaquín; Meza-Figueroa, Diana; Valencia-Moreno, Martín; Zúñiga-Hernández, Hugo; Zúñiga-Hernández, Luis; Moreno-Rodríguez, Verónica (2013-12-01). "Génesis de la mineralización de óxidos de manganeso en la región de Boleo y la Península de Concepción, Baja California Sur: restricciones a partir de isótopos de Pb-Sr y geoquímica de elementos de las tierras raras". Revista mexicana de ciencias geológicas. 30 (3): 482–499. ISSN 1026-8774.
  14. 1 2 Wong, Victor; Munguía, Luis (2006-03-01). "Seismicity, focal mechanisms, and stress distribution in the Tres Vírgenes volcanic and geothermal region, Baja California Sur, Mexico". Geofísica internacional. 45 (1): 23–37. ISSN 0016-7169.
  15. 1 2 Arango-Galván, Claudia; Prol-Ledesma, Rosa María; Torres-Vera, Marco Antonio (May 2015). "Geothermal prospects in the Baja California Peninsula". Geothermics. 55: 42. doi:10.1016/j.geothermics.2015.01.005.
  16. ↑ Demant 1984, p. 78.
  17. 1 2 3 Demant 1984, p. 83.
  18. ↑ Demant 1984, p. 85.
  19. ↑ Demant 1984, p. 87.
  20. 1 2 Garduño-Monroy, Vargas-Ledezma & Campos-Enriquez 1993, p. 53.
  21. ↑ Garduño-Monroy, Vargas-Ledezma & Campos-Enriquez 1993, p. 54.
  22. ↑ Demant 1984, p. 79.
  23. ↑ Garduño-Monroy, Vargas-Ledezma & Campos-Enriquez 1993, p. 51.
  24. ↑ Garduño-Monroy, Vargas-Ledezma & Campos-Enriquez 1993, p. 57.
  25. ↑ Portugal et al. 2000, p. 241.

Sources

This article is issued from Wikipedia - version of the 11/13/2016. The text is available under the Creative Commons Attribution/Share Alike but additional terms may apply for the media files.