Franciscan Complex

Last updated
Franciscan Complex
Stratigraphic range: Late Jurassic to Late Cretaceous
Chevron fold ribbon cherts.jpg
Chevron folds in ribbon chert of the Marin Headlands, California. Geologist Christie Rowe for scale.
Typevaried; primarily metamorphic (low grade), but also sedimentary, igneous and high-pressure metamorphic
Underliesvarious
Overliesbasement; Coast Range Ophiolite in some areas
Lithology
Primary schist (incl. serpentinite), sandstone, basalt, greywackes
Other shale, chert
Location
Region California Coast Ranges, northern Transverse Ranges
CountryUnited States
Type section
Named for San Francisco, California

The Franciscan Complex or Franciscan Assemblage is a geologic term for a late Mesozoic terrane of heterogeneous rocks found throughout the California Coast Ranges, and particularly on the San Francisco Peninsula. It was named by geologist Andrew Lawson, who also named the San Andreas fault that defines the western extent of the assemblage. [1]

Contents

The Franciscan Complex is dominated by greywacke sandstones, shales and conglomerates which have experienced low-grade metamorphism.  Other important lithologies include chert, basalt, limestone, serpentinite, and high-pressure, low-temperature metabasites (blueschists and eclogites) and meta-limestones. Fossils like radiolaria are found in chert beds of the Franciscan Complex. These fossils have been used to provide age constraints on the different terranes that constitute the Franciscan. The mining opportunities within the Franciscan are restricted to deposits of cinnabar and limestone.

The outcrops of the formation have a very large range, extending from Douglas County, Oregon to Santa Barbara County, California. [2] Franciscan-like formations may be as far south as Santa Catalina Island. The formation lends its name to the term describing high-pressure regional metamorphic facies, the Franciscan facies series. [3]

Geologic history

Map modified from Irwin (1990) showing distribution of Great Valley Sequence and Franciscan Complex (in blue). Great Valley Sequence map.gif
Map modified from Irwin (1990) showing distribution of Great Valley Sequence and Franciscan Complex (in blue).
Diagram (modified from Fig 3.11 in Irwin, 1990) showing the depositional setting of the Franciscan Assemblage and the contemporaneous Great Valley Sequence,. Franciscan subduction model.gif
Diagram (modified from Fig 3.11 in Irwin, 1990) showing the depositional setting of the Franciscan Assemblage and the contemporaneous Great Valley Sequence,.

The Franciscan Complex is an assemblage of metamorphosed and deformed rocks, associated with east-dipping subduction zone at the western coast of North America. [6] Although most of the Franciscan is Early/Late Jurassic through Cretaceous in age (150-66 Ma), [7] some Franciscan rocks are as old as early Jurassic (180-190 Ma) age and as young as Miocene (15 Ma). [8] The different age distribution represents the temporal and spatial variation of mechanisms that operated within the subduction zone. [9] Franciscan rocks are thought to have formed prior to the creation of the San Andreas Fault when an ancient deep-sea trench existed along the California continental margin. This trench, the remnants of which are still active in the Cascadia and Cocos subduction zone, resulted from subduction of oceanic crust of the Farallon tectonic plate beneath continental crust of the North American Plate. As oceanic crust descended beneath the continent, ocean floor basalt and sediments were subducted and then tectonically underplated to the upper plate. [10] This resulted in widespread deformation with the generation of thrust faults and folding, and caused high pressure-low temperature regional metamorphism. [10] In the Miocene, the Farallon-Pacific spreading center reached the Franciscan trench and the relative motion between Pacific-North America caused the initiation of the San Andreas Fault. Transform motion along the San Andreas Fault obscured and displaced the subduction related structures, resulting in overprinting of two generations of structures. [11]

Description

Shale matrix melange with clasts of sandstone and greenstone on Marshall's Beach, San Francisco Shale matrix melange with clasts of sandstone and greenstone on Marshall's Beach, San Francisco, USA.jpg
Shale matrix mélange with clasts of sandstone and greenstone on Marshall's Beach, San Francisco

The units of the Franciscan complex are aligned parallel to the active margin between the North American and Pacific plates. [12] The Franciscan Complex is in contact with the Great Valley Sequence, which was deposited on the Coast Range Ophiolite, along its eastern side. [13] [14] The type area of Franciscan rocks in San Francisco consists of metagraywackes, gray claystone and shale, thin bedded ribbon chert with abundant radiolarians, altered submarine pillow basalts (greenstone) and blueschists. [15] Broadly, the Franciscan can be divided into two groups of rocks. Coherent terranes are internally consistent in metamorphic grade and include folded and faulted clastic sediments, cherts and basalts, ranging from sub-metamorphic to prehnite-pumpellyite or low-temperature blueschist (jadeite-bearing) grades of metamorphism. Mélange terranes are much smaller, found between or within the larger coherent terranes and sometimes contain large blocks of metabasic rocks of higher metamorphic grade (amphibolite, eclogite, and garnet-blueschist). [10] The mélange zones in the Franciscan usually have a block in matrix appearance with higher grade metamorphic blocks (blueschist, amphibolite, greenschist, eclogite) embedded within the mélange matrix. [16] The matrix material of the mélanges are mudstone or serpentinite. Geologists have argued for either a tectonic or olistostormal origin. [17] In the northern Coast Ranges, the Franciscan has been divided into the Eastern, Central and Coastal Belts based on metamorphic age and grade, with the rocks younging and the metamorphic grade decreasing to the west. [18] [19] [10] The Franciscan varies along strike, because individual accreted elements (packets of trench sediment, seamounts, etc.) did not extend the full length of the trench. Different depths of underplating, distribution of post-metamorphic faulting, and level of erosion produced the present-day surface distribution of high P/T metamorphism. [9] [10]

Fossils

Franciscan sediments contain a sparse, but diverse assemblage of fossils. The most abundant fossils by far are microfossils, particularly in the cherts, which contain single-celled organisms called radiolarians that have exoskeletons of silica. There are also in some of the shales microfossils of planktonic foraminifera that have exoskeletons of carbonate. These microfossils, by and large, indicate deposition in an open-water setting where deep-water conditions exist. [20] Vertebrate fossils in the Franciscan are extremely rare, but include three Mesozoic marine reptiles that are shown in the table below. [21] Again, these indicate an open-water, and therefore deep-marine setting. Although rare, a few shallow-marine fossils have been found as well, and include extinct oysters (Inoceramus) and clams (Buchia). [20] Microfossils in the Calera Limestone member of the Franciscan exposed at the Permanente and Pacifica cement quarries also indicate a shallow-marine setting, with deposition on top of a seamount in the tropical Pacific Ocean and subsequent transport and accretion by the Pacific Plate onto the California continental margin. [22] Thus, even though most of the Franciscan appears to have been deposited in a deep-water setting, it is a complex and diverse assemblage of rocks, and shallow-water settings, though not the norm, existed as well.

Mesozoic Vertebrate Fossils of the Franciscan Complex
GenusSpeciesNotes
Ichthyosaurus californicus [23] Name means "fish-lizard of California." Found in 1935 in Stanislaus County in a piece of Franciscan chert from the Coast Ranges washed into the Great Valley.
franciscanus [23] Name means "fish-lizard of the Franciscan." Found in 1940 in San Joaquin County in a piece of Franciscan chert from the Coast Ranges washed into the Great Valley.
Plesiosaurus hesternus [23] Name means "one who is near to being a lizard of the West coast." Found in 1949 in San Luis Obispo County in a limestone concretion in Franciscan-Knoxville shales.

Economic importance

Although no significant accumulations of oil or gas have been found in the Franciscan, other opportunities have been exploited over the years. During the 19th century when gold mining was one of the main industries in California, cinnabar associated with serpentine in the Franciscan and Great Valley Group was mined for quicksilver (mercury) needed to process gold ore and gold-bearing gravels. Some of the more important mines were those at New Idria and New Almaden, the Sulphur Bank Mine at Clearlake Oaks, and the Knoxville Mine (cf. McLaughlin Mine) and others at Knoxville. The Franciscan also contains large bodies of limestone pure enough for making cement, and the Permanente Quarry near Cupertino, California is a giant open-pit mine in a body of Franciscan limestone that supplied most of the cement for building the Shasta Dam across the Sacramento River. [24] The Rockaway Quarry in Pacifica is another example of a major limestone quarry in the Franciscan.

See also

Notes

  1. ↑ Bailey, Irwin and Jones (1964), Franciscan and related rocks and their significance in the geology of western California. California Division of Mines and Geology, v. 183 p. 15-17.
  2. ↑ Oregon Coast Range simplified geologic map
  3. ↑ Tulane University - Regional Metamorphism
  4. ↑ Irwin, William P. (1990). Wallace, Robert E. (ed.). "Geology and plate-tectonic Development". The San Andreas Fault System, California-U.S. Geological Survey Professional Paper. 1515: 61–82.
  5. ↑ Irwin, William P. (1990). Wallace, Robert E. (ed.). "Geology and plate-tectonic development". The San Andreas Fault System, California. U.S. Geological Survey Professional Paper. 1515: 74.
  6. ↑ HAMILTON, WARREN (1969). "Mesozoic California and the Underflow of Pacific Mantle". Geological Society of America Bulletin. 80 (12): 2409. Bibcode:1969GSAB...80.2409H. doi:10.1130/0016-7606(1969)80[2409:mcatuo]2.0.co;2. ISSN   0016-7606.
  7. ↑ Bailey, Irwin and Jones (1964), p. 142-146; Blome and Irwin (1983), p. 77-89.
  8. ↑ McLaughlin (1982), p. 595-605.
  9. 1 2 Mulcahy, Sean R.; Starnes, Jesslyn K.; Day, Howard W.; Coble, Matthew A.; Vervoort, Jeffrey D. (May 2018). "Early Onset of Franciscan Subduction". Tectonics. 37 (5): 1194–1209. Bibcode:2018Tecto..37.1194M. doi: 10.1029/2017tc004753 . ISSN   0278-7407.
  10. 1 2 3 4 5 Wakabayashi, John (1992-01-01). "Nappes, Tectonics of Oblique Plate Convergence, and Metamorphic Evolution Related to 140 Million Years of Continuous Subduction, Franciscan Complex, California". The Journal of Geology. 100 (1): 19–40. Bibcode:1992JG....100...19W. doi:10.1086/629569. ISSN   0022-1376. S2CID   140552742.
  11. ↑ Wentworth et al. (1984), p. 163-173; Irwin (1990), p. 61-82.
  12. ↑ Wassmann, Sara; Stöckhert, Bernhard (2012-09-28). "Matrix deformation mechanisms in HP-LT tectonic mélanges — Microstructural record of jadeite blueschist from the Franciscan Complex, California". Tectonophysics. Chaos and Geodynamics: Melanges, Melange Forming Processes and Their Significance in the Geological Record. 568–569: 135–153. Bibcode:2012Tectp.568..135W. doi:10.1016/j.tecto.2012.01.009. ISSN   0040-1951.
  13. ↑ Ernst, W. G. (1970). "Tectonic contact between the Franciscan Mélange and the Great Valley Sequence—Crustal expression of a Late Mesozoic Benioff Zone". Journal of Geophysical Research. 75 (5): 886–901. Bibcode:1970JGR....75..886E. doi:10.1029/JB075i005p00886. ISSN   2156-2202.
  14. ↑ Turner, Francis J. (1981). Metamorphic petrology: mineralogical, field, and tectonic aspects (2d ed.). Washington: Hemisphere Pub. Corp. ISBN   0-07-065501-4. OCLC   5894059.
  15. ↑ Wahrhaftig, Clyde (1984). A Streetcar to Subduction and Other Plate Tectonic Trips by Public Transport in San Francisco. Washington, D. C.: American Geophysical Union. doi:10.1029/sp022. ISBN   0-87590-234-0.
  16. ↑ Hsü, K. Jinghwa (1968-08-01). "Principles of Mélanges and Their Bearing on the Franciscan-Knoxville Paradox". GSA Bulletin. 79 (8): 1063–1074. doi:10.1130/0016-7606(1968)79[1063:POMATB]2.0.CO;2. ISSN   0016-7606.
  17. ↑ Wakabayashi, John (August 2011), "Mélanges of the Franciscan Complex, California: Diverse structural settings, evidence for sedimentary mixing, and their connection to subduction processes", Mélanges: Processes of Formation and Societal Significance, Geological Society of America Special Papers, vol. 480, Geological Society of America, pp. 117–141, doi:10.1130/2011.2480(05), ISBN   978-0-8137-2480-5
  18. ↑ James O. Berkland (2), Loren A. Ray (1972). "What is Franciscan?". AAPG Bulletin. 56. doi:10.1306/819a421a-16c5-11d7-8645000102c1865d. ISSN   0149-1423.
  19. ↑ Blake, M.C.; Howell, D.G.; Jones, David Lawrence (1982). "Preliminary tectonostratigraphic terrane map of California". Open-File Report. doi: 10.3133/ofr82593 . ISSN   2331-1258.
  20. 1 2 Bailey, Irwin and Jones (1965), p. 115-123; Blome and Irwin (1983), p. 77-89.
  21. ↑ Hilton (2003), p. 223-225.
  22. ↑ Tarduno et al. (1985), p. 345-347.
  23. 1 2 3 Hilton (2003), "Appendix: Summary of the Mesozoic Reptilian Fossils of California," p. 272-273.
  24. ↑ Austin, Donna (26 June 2009). "Kaiser dug for cement and hit aluminum foil". Cupertino News (newspaper - online edition). Retrieved 14 June 2013. Also see the following online anonymous article "Henry Kaiser's Legacy Woven into Rich California Tapestry". Kasier Permanente. 26 November 2009. Retrieved 14 June 2013.

Related Research Articles

<span class="mw-page-title-main">Subduction</span> A geological process at convergent tectonic plate boundaries where one plate moves under the other

Subduction is a geological process in which the oceanic lithosphere and some continental 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 heavier plate dives beneath the second plate and sinks into the mantle. A region where this process occurs is known as a subduction zone, and its surface expression is known as an arc-trench complex. The process of subduction has created most of the Earth's continental crust. Rates of subduction are typically measured in centimeters per year, with rates of convergence as high as 11 cm/year.

<span class="mw-page-title-main">Blueschist</span> Type of metavolcanic rock

Blueschist, also called glaucophane schist, is a metavolcanic rock that forms by the metamorphism of basalt and rocks with similar composition at high pressures and low temperatures, approximately corresponding to a depth of 15–30 km (9.3–18.6 mi). The blue color of the rock comes from the presence of the predominant minerals glaucophane and lawsonite.

<span class="mw-page-title-main">Antler orogeny</span> Tectonic event from the Late Devonian into the Mississippian and early Pennsylvanian

The Antler orogeny was a tectonic event that began in the early Late Devonian with widespread effects continuing into the Mississippian and early Pennsylvanian. Most of the evidence for this event is in Nevada but the limits of its reach are unknown. A great volume of conglomeratic deposits of mainly Mississippian age in Nevada and adjacent areas testifies to the existence of an important tectonic event, and implies nearby areas of uplift and erosion, but the nature and cause of that event are uncertain and in dispute. Although it is known as an orogeny, some of the classic features of orogeny as commonly defined such as metamorphism, and granitic intrusives have not been linked to it. In spite of this, the event is universally designated as an orogeny and that practice is continued here. This article outlines what is known and unknown about the Antler orogeny and describes three current theories regarding its nature and origin.

The Nevadan orogeny occurred along the western margin of North America during the Middle Jurassic to Early Cretaceous time which is approximately from 155 Ma to 145 Ma. Throughout the duration of this orogeny there were at least two different kinds of orogenic processes occurring. During the early stages of orogenesis an "Andean type" continental magmatic arc developed due to subduction of the Farallon oceanic plate beneath the North American Plate. The latter stages of orogenesis, in contrast, saw multiple oceanic arc terranes accreted onto the western margin of North America in a "Cordilleran type" accretionary orogen. Deformation related to the accretion of these volcanic arc terranes is mostly limited to the western regions of the resulting mountain ranges and is absent from the eastern regions. In addition, the deformation experienced in these mountain ranges is mostly due to the Nevadan orogeny and not other external events such as the more recent Sevier and Laramide Orogenies. It is noted that the Klamath Mountains and the Sierra Nevada share similar stratigraphy indicating that they were both formed by the Nevadan orogeny. In comparison with other orogenic events, it appears that the Nevadan Orogeny occurred rather quickly taking only about 10 million years as compared to hundreds of millions of years for other orogenies around the world.

<span class="mw-page-title-main">Mélange</span> Large-scale breccia typically consisting of a jumble of large blocks of varied lithologies

In geology, a mélange is a large-scale breccia, a mappable body of rock characterized by a lack of continuous bedding and the inclusion of fragments of rock of all sizes, contained in a fine-grained deformed matrix. The mélange typically consists of a jumble of large blocks of varied lithologies. Both tectonic and sedimentary processes can form mélange.

The Farallon Trench was a subduction related tectonic formation located off the coast of the western California continental margin during the late to mid Cenozoic era, around 50 miles southeast of modern-day Monterey Bay. The time duration of subduction began from around 165 Ma when the Farallon Plate replaced the Mezcalera promontory, until the San Andreas Fault straightening around 35 Ma. As data accumulated over time, a common view developed that one large oceanic plate, the Farallon Plate, acted as a conveyor belt, conveying accreted terranes onto the North American west coast. As the continent overran the subducting Farallon Plate, the denser plate became subducted into the mantle below the continent. When the plates converged, the dense oceanic plate sank into the mantle to form a slab below the lighter continent. Rapid subduction under the southwestern North America continent began 40 to 60 million years ago (Ma), during the mid Paleocene to mid Eocene epochs. This convergent subduction margin created a distinctive geomorphologic feature called an oceanic trench, which occurs at a convergent plate boundaries as a heavy metal rich, lithospheric plate moves below a light silica rich continental plate. The trench marks the position at which the flexed subducting slab begins to descend beneath and deform the continental plate margin. By 43 Ma, during the Eocene, worldwide plate motions changed and the Pacific Plate began to move away from North America and subduction of the Farallon Plate slowed dramatically. By around 36 Ma, the easternmost part of the East Pacific Rise, located between the Pioneer and Murray fracture zones at that time, approached the trench and the young, hot, buoyant lithosphere appears to have clogged part of the subduction zone, resulting in widespread dramatic uplift on land. The eventual complete subduction of this plate, consequential contact of the Pacific Plate with the California continental margin, and creation of the Mendocino Triple Junction (MTJ), took place around 30 to 20 Ma. The partial complete subduction and division of the Farallon Plate by the Pacific Plate, created the Juan de Fuca Plate to the north and the Cocos Plate to the south. The final stages of the evolution of California's continental margin was the growth of the San Andreas transform fault system, which formed as the Pacific Plate came into contact with the continental margin and the MTJ was formed. As subduction of the Pacific Plate continued along this margin, and the contact zone grew, the San Andreas proportionally grew as well.

<span class="mw-page-title-main">Accretionary wedge</span> The sediments accreted onto the non-subducting tectonic plate at a convergent plate boundary

An accretionary wedge or accretionary prism forms from sediments accreted onto the non-subducting tectonic plate at a convergent plate boundary. Most of the material in the accretionary wedge consists of marine sediments scraped off from the downgoing slab of oceanic crust, but in some cases the wedge includes the erosional products of volcanic island arcs formed on the overriding plate.

The Lachlan Fold Belt (LFB) or Lachlan Orogen is a geological subdivision of the east part of Australia. It is a zone of folded and faulted rocks of similar age. It dominates New South Wales and Victoria, also extending into Tasmania, the Australian Capital Territory and Queensland. It was formed in the Middle Paleozoic from 450 to 340 Mya. It was earlier known as Lachlan Geosyncline. It covers an area of 200,000 km2.

<span class="mw-page-title-main">Ring Mountain (California)</span> Mountain in United States of America

Ring Mountain is an elevated landform on the Tiburon Peninsula in Marin County, California. This mountain was named for George E. Ring, who served as a Marin County Supervisor from 1895 to 1903.

The Sonoma orogeny was a period of mountain building in western North America. The exact age and structure of the Sonoma orogeny is controversial. The orogeny is generally thought to have occurred during the Permian / Triassic transition, around 250 million years ago, following the Late Devonian Antler orogeny. The Sonoma orogeny was one of a sequence of accretionary events along the Cordilleran margin, possibly caused by the closure of the basin between the island arc of Sonomia and the North American continent. Evidence of this event has been reported throughout western North America, but most distinctly in northwest Nevada.

<span class="mw-page-title-main">Bangong suture</span>

The Bangong suture zone is a key location in the central Tibet conjugate fault zone. Approximately 1,200 km long, the suture trends in an east–west orientation. Located in central Tibet between the Lhasa and Qiangtang terranes, it is a discontinuous belt of ophiolites and mélange that is 10–20 km wide, up to 50 km wide in places. The northern part of the fault zone consists of northeast striking sinistral strike-slip faults while the southern part consists of northwest striking right lateral strike-slip faults. These conjugate faults to the north and south of the Bangong intersect with each other along the Bangong-Nujiang suture zone.

<span class="mw-page-title-main">High pressure metamorphic terranes along the Bangong-Nujiang Suture Zone</span>

High pressure terranes along the ~1200 km long east-west trending Bangong-Nujiang suture zone (BNS) on the Tibetan Plateau have been extensively mapped and studied. Understanding the geodynamic processes in which these terranes are created is key to understanding the development and subsequent deformation of the BNS and Eurasian deformation as a whole.

<span class="mw-page-title-main">Lhasa terrane</span> Fragment of crustal material that forms present-day southern Tibet

The Lhasa terrane is a terrane, or fragment of crustal material, sutured to the Eurasian Plate during the Cretaceous that forms present-day southern Tibet. It takes its name from the city of Lhasa in the Tibet Autonomous Region, China. The northern part may have originated in the East African Orogeny, while the southern part appears to have once been part of Australia. The two parts joined, were later attached to Asia, and then were impacted by the collision of the Indian Plate that formed the Himalayas.

The geology of Alaska includes Precambrian igneous and metamorphic rocks formed in offshore terranes and added to the western margin of North America from the Paleozoic through modern times. The region was submerged for much of the Paleozoic and Mesozoic and formed extensive oil and gas reserves due to tectonic activity in the Arctic Ocean. Alaska was largely ice free during the Pleistocene, allowing humans to migrate into the Americas.

The geology of Ecuador includes ancient Precambrian basement rock and a complex tectonic assembly of new sections of crust from formerly separate landmasses, often uplifted as the Andes or transformed into basins.

The geology of Guatemala encompasses rocks divided into two tectonic blocks. The Maya Block in the north has igneous and metamorphic North American Craton basement rocks, overlain by late Paleozoic metasedimentary rocks, which experienced deformation during the Devonian. Red beds, evaporites and marine limestone from the Mesozoic overlie these rocks. A karst landscape formed in the thick limestone units across the north of the country. During a collisional orogeny, these Paleozoic and Mesozoic rocks were uplifted, thrust and folded as the Central Guatemalan Cordillera. Paleogene rocks from the early Cenozoic include volcanic and marine clastic rocks, associated with high rates of erosion.

The Superior Craton is a stable crustal block covering Quebec, Ontario, and southeast Manitoba in Canada, and northern Minnesota in the United States. It is the biggest craton among those formed during the Archean period. A craton is a large part of the Earth's crust that has been stable and subjected to very little geological changes over a long time. The size of Superior Craton is about 1,572,000 km2. The craton underwent a series of events from 4.3 to 2.57 Ga. These events included the growth, drifting and deformation of both oceanic and continental crusts.

The geology of California is highly complex, with numerous mountain ranges, substantial faulting and tectonic activity, rich natural resources and a history of both ancient and comparatively recent intense geological activity. The area formed as a series of small island arcs, deep-ocean sediments and mafic oceanic crust accreted to the western edge of North America, producing a series of deep basins and high mountain ranges.

<span class="mw-page-title-main">Ashe Metamorphic Suite</span>

The Ashe Metamorphic Suite, also referred to as the Ashe Formation, was named after its type locality, Ashe County, North Carolina. The Ashe Metamorphic Suite is located in the Eastern Blue Ridge providence that extends from North Carolina up to South-Western Virginia. It is a collection of metamorphic rocks of both sedimentary and volcanic origin. Zircon dating indicates an age of 470 to 335 Ma for the unit. The protolith of the Ashe Metamorphic Suite was deposited during the Late Proterozoic and reaching its cooling age during the end of the Devonian. The Ashe Metamorphic Suite is overwhelmingly composed of amphibolites and mica schists.

<span class="mw-page-title-main">Geology of New Caledonia</span>

The geology of New Caledonia includes all major rock types, which here range in age from ~290 million years old (Ma) to recent. Their formation is driven by alternate plate collisions and rifting. The mantle-derived Eocene Peridotite Nappe is the most significant and widespread unit. The igneous unit consists of ore-rich ultramafic rocks thrust onto the main island. Mining of valuable metals from this unit has been an economical pillar of New Caledonia for more than a century.

References

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.