Farallon plate

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Simplified illustration of the fragmentation of the Farallon Plate in the Juan de Fuca Plate (including the Explorer and Gorda Plate) and Cocos Plate (including the Rivera Plate) as well as the formation of the San Andreas Fault and the California Peninsula and their attachment to the Pacific plate over the past 30 million years (north is left).

The Farallon Plate is a former large oceanic plate in the Eastern Pacific . It already existed in the earlier Mesozoic Era and from the Cretaceous to the Eocene it formed the eastern counterpart to the Pacific Plate to the west of the East Pacific Ridge . As a result of the progressive subduction of the Farallon Plate under the North American and South American Plate , it broke into several smaller plates in the further course of the Tertiary , of which the Cocos Plate and the Nazca Plate are the largest remaining today.

etymology

The name farallon plate was coined in 1969 by the US geophysicists Dan McKenzie and Jason Morgan in a work on the formation and development of tectonic triple points . It refers to the Farallon Islands off the coast of the US state California .

Geological development

Remnants of the northern part of the Farallon Plate (colored blue) in the deeper mantle below North America were modeled using geodynamic software. Note the large "slab window" that has opened as a result of the division into Juan-de-Fuca and coconut plates

The history of the Farallon plate can to a certain extent be reconstructed directly on the basis of its remains that are still on the surface of the earth's body today, but for the most part only on the basis of indirect evidence. The latter includes, above all, the pattern of magnetic anomalies in the crust of the Pacific plate ("seabed isochrones", see →  paleomagnetism ), from which the processes on the now largely subducted northern east Pacific ridge can be read. In addition, the age, chemistry, position and geometry of igneous provinces in the west of the North American Plate document both the “normal” subduction of oceanic lithosphere and the subduction of spreading zones (“slab windows”), and thus of plate boundaries.

The history of the Farallon Plate can be traced back well into the Mesozoic Era. At the turn of the Triassic to the Jurassic , about 200 million years ago, it probably formed a large part of the ocean floor of the eastern Panthalassa , the precursor ocean of the Pacific, and extended from high northern to high southern latitudes. At that time it was bounded by the Izanagi plate in the west, by the Phoenix plate * in the south and by the western Pangea or its offshore arches in the east. It is possible that at least parts of the Farallon Plate were already subducting to the east under Pangea at this time. The Pacific Plate emerged from the Izanagi-Phoenix-Farallon triple point during the early Jurassic .

In the late Cretaceous, about 80 million years ago, the Farallon Plate, which was now completely subducting to the east, bordered on the west with the rapidly growing Pacific Plate. The corresponding splay ridge thus represents a forerunner of today's East Pacific Ridge. To the north, the Farallon plate was limited at that time by a splay ridge against the so-called Kula plate. This probably emerged from the northernmost part of the Farallon Plate and thus represented the first step in its decay. Little is known about the development of the Kula plate or it is the subject of numerous speculations. It shall be presumed that it in the Eocene by a stop of seafloor spreading (Engl. "Ridge death") on the Pacific-Kula-back due to the conversion of the Kula-North American subduction zone in the still existing Queen Charlotte Fault (named after the Queen Charlotte Islands ) merged with the Pacific Plate. To the south, the Farallon Plate bordered the Antarctic Plate in the Upper Cretaceous after the remnants of the Phoenix Plate had merged with the surrounding plates .

In the Cenozoic fragmentation of the Farallon Plate, roughly three events can be distinguished. It began in the early Eocene about 55 million years ago with the decoupling of its northernmost part, the Juan de Fuca Plate (Vancouver Plate), which initially remained kinematically relatively closely associated with the rest of the Farallon Plate. The second major event took place 30 million years ago in the Oligocene . A protruding segment of the East Pacific Ridge reached the western edge of North America at what is now the south of the US state of California. Instead of the now subducting lithosphere of the Pacific plate, the subduction zone there turned into a transform fault, the precursor of the San Andreas Fault . The Juan de Fuca record was isolated from the rest of the Farallon record and began a cinematic life of its own. The current plate configuration finally developed at the beginning of the Miocene about 23 million years ago, when the "South Farallon Plate" was divided into the precursor of the Cocos Plate (Guadalupe Plate) and the Nazca Plate through the formation of a spreading zone. The cause of this is the differently directed "slab pull" due to the different orientation of the subduction zones on the continental margin of Central and South America and the weakening of the plate by the Galapagos hotspot.

Geological importance

Physical geography of the western United States. The more than 1500 km wide mountain system between the Pacific coast and the Great Plains is largely due to the subduction of the Farallon Plate.

The subduction of the Farallon Plate and the plates that emerged from it had a decisive influence on the geological development of the western edges of the two large American plates, in some cases as far as the interior of the continents. Among other things, the Andes and the Central American Cordillera go back to them. For North America it is assumed that in the late Cretaceous the subduction took place at a very shallow angle and the subducted part of the plate ("slab") extended almost horizontally far below the continent. In addition to the normal coastal cordillera ( Sierra Nevada , Cascade Mountains ), this led to the formation of a wide highland, which includes the central and southern Rocky Mountains , the Colorado Plateau and the Basin and Range Province . Furthermore, the shallow subduction of the Farallon plate for the high Cretaceous subsidence rates that occurred in a wide strip in the interior of North America (English "long-wavelength, high-amplitude subsidence"), and thus for the formation of the Western Interior Seaway and of the corresponding marine sediments. The subduction of an oceanic plateau is considered to be the cause of the plate's low diving angle . Such plateaus are made of rock that has a lower density than normal oceanic crust, as a result of which the slab in the sublithospheric mantle experiences more uplift.

Remarks

* The extensive Mesozoic Phoenix Plate, named after Jurassic seabed isochrones on the Pacific Plate near the Phoenix Islands in the Western Pacific, is considered to be the forerunner of the relatively small oceanic crust fragment , also known as the Drake or Aluk plate , which today belongs to the Antarctic Plate and west of the Scotia Plate on Drake Street .

Individual evidence

  1. Dan P. McKenzie, W. Jason Morgan: Evolution of Triple Junctions. Nature. Vol. 224, 1969, pp. 125-133, doi : 10.1038 / 224125a0 .
  2. a b c d e f M. Seton, RD Müller, S. Zahirovic, C. Gaina, T. Torsvik, G. Shephard, A. Talsma, M. Gurnis, M. Turner, S. Maus, M. Chandler: Global continental and ocean basin reconstructions since 200 Ma. Earth Science Reviews. Bd. 113, No. 3–4, 2012, pp. 212–270, doi : 10.1038 / 224125a0 (alternative full text access : EarthByte ).
  3. ^ A b c d e Tanya M. Atwater: Tectonics of the North East Pacific Region. Pp. 265-280 in: Albert W. Bally, Allison R. Palmer (Eds.): The Geology of North America, Vol A: The Geology of North America - an Overview. Geological Society of America, Boulder (CO) 1989, ISBN 0-8137-5207-8 , p. 269 ff.
  4. Peter J. Haeussler, Dwight C. Bradley, Ray E. Wells, Marti L. Miller: Life and death of the Resurrection plate: Evidence for its existence and subduction in the northeastern Pacific in Paleocene-Eocene time. Geological Society of America Bulletin. Vol. 115, No. 7, 2003, pp. 867-880, doi : 10.1130 / 0016-7606 (2003) 115 <0867: LADOTR> 2.0.CO; 2 (alternative full text access : USGS Alaska Science Center ).
  5. ^ Tanya Atwater, Joann Stock: Pacific-North America Plate Tectonics of the Neogene Southwestern United States: an Update. Pp. 393-421 in: Wallace G. Ernst, Clemens A. Nelson (Eds.): Integrated Earth and Environmental Evolution of the Southwestern United States. Bellwether Publishing, Columbia (MD) 1998, ISBN 0-9665869-0-5 , p. 409.
  6. ^ Tanya Atwater: Implications of Plate Tectonics for the Cenozoic Tectonic Evolution of Western North America. Geological Society of America Bulletin. Vol. 81, No. 12, 1970, pp. 3513-3536, doi : 10.1130 / 0016-7606 (1970) 81 [3513: IOPTFT] 2.0.CO; 2 (alternative full text access : UCSC ).
  7. ^ William P. Irwin: Geology and Plate Tectonic Development. Pp. 61-224 in Robert E. Wallace (Ed.): The San Andreas Fault System, California. US Geological Survey Professional Paper 1515. US Geological Survey, Department of the Interior, Washington, DC 1990 ( online ).
  8. ^ A b Peter Lonsdale: Creation of the Cocos and Nazca plates by fission of the Farallon plate. Tectonophysics. Vol. 404, No. 3-4, 2005, pp. 237-264, doi : 10.1016 / j.tecto.2005.05.011 (alternative full-text access : ResearchGate ).
  9. ^ A b Eugene Humphreys: Relation of flat subduction to magmatism and deformation in the western United States. Pp. 85-98 in: Suzanne Mahlburg Kay, Víctor A. Ramos, William R. Dickinson (Eds.): Backbone of the Americas: Shallow Subduction, Plateau Uplift, and Ridge and Terrane Collision. Geological Society of America Memoirs. Vol. 204, 2009, doi: 10.1130 / 2009.1204 (04) (alternative full text access : University of Oregon unedited manuscript without illustrations).
  10. ^ JX Mitrovica, C. Beaumont, GT Jarvis: Tilting of continental interiors by dynamical effects of subduction. Tectonics. Vol. 8, No. 5, 1989, pp. 1079-1094, doi : 10.1029 / TC008i005p01079 .
  11. ^ Lijun Liu, Sonja Spasojević, Michael Gurnis: Reconstructing Farallon Plate Subduction Beneath North America Back to the Late Cretaceous. Science. Vol. 322, No. 5903, 2008, pp. 934-938, doi : 10.1126 / science.1162921 (alternative full text access : ResearchGate ).
  12. ^ Georg Kleinschmidt: Geological Structure and Evolution of Antarctica. Pp. 430–437 in: Beau Riffenburgh (Ed.): Encyclopedia of the Antarctic, Volume 1 (A – K). Routledge (Taylor & Francis), Abingdon 2007, ISBN 978-0-415-97024-2 , p. 435.
  13. Roy Livermore, Juan Carlos Balanyá, Andrés Maldonado, José Miguel Martínez, José Rodríguez-Fernández, Carlos Sanz de Galdeano, Jesús Galindo Zaldívar, Antonio Jabaloy, Antonio Barnolas, Luis Somoza, Javier Hernández-Molina, Emma Suriñach, César Viseras: Autopsy on a dead spreading center: The Phoenix Ridge, Drake Passage, Antarctica. Geology. Vol. 28, No. 7, 2000, pp. 607-610, doi : 10.1130 / 0091-7613 (2000) 28 <607: AOADSC> 2.0.CO; 2 .