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Sheeted dykes present in exposed ophiolite

A sheeted dyke complex or sheeted dike complex is a series of oceanic crust intrusions by igneous volcanic rock created below the seafloor in a sub-parallel formation . At mid-ocean ridges, dykes are formed when magma beneath areas of tectonic plate divergence does not reach the surface of oceanic crust and cools below the seafloor forming upright columns of igneous rock. Magma continues to cool, pushing existing seafloor away from the area of divergence, and as additional magma rises and cools, existing igneous rock is pushed in outward, often intruding upon continental crust, forming ophiolite.

The creation of sheeted dykes is a driving factor of seafloor spreading. Areas of oceanic crust with high volcanic activity are subject to the formation of igneous rock, encouraging the creation of new seafloor. Sheeted dykes make up a large portion of oceanic crust due the existing extrusive rock and oceanic crust being moved out of the way for new seafloor to made.

Dyke Formation

Magma rises through oceanic crust at mid-ocean ridges

Sheeted dyke complexes are most commonly found at divergent plate boundaries marked by the presence of mid-ocean ridges. These subaqeous mountain ranges are made up of newly-created oceanic crust due to tectonic plates moving away from each other. In response to the separation of plates, magma from the asthenosphere is subject to upwelling, pushing hot magma up towards the seafloor. The magma that reaches the surface is subject to fast cooling and creates basaltic formations such as pillow lava, a common extrusive rock created near areas of volcanic activity on the seafloor. Although some magma is able to reach the surface of oceanic crust, a considerable amount of magma solidifies within the crust. Dykes are formed when the rising magma that does not reach the surface cools into upright columns of igneous rock beneath areas of divergence.

Ophiolites

Dykes are perpetually formed as long as magma continues to flow through the plate boundary, creating a distinct, stratigraphic-like sequences of rocky columns within the seafloor. Ophiolites are formed when these sections of oceanic crust are revealed above sea level and embedded within coastal crust. Older dykes formed near divergence zones are pushed away as new seafloor is created, a phenomenon known as seafloor spreading, and over time, the oldest dykes are pushed far enough from convergence zones to be exposed above sea level.

Seafloor Spreading and Continental Drift

(Top) Creation of rift valley due to low spreading rate. (Middle and bottom) Creation of mid-ocean ridges due to higher spreading rate.

The creation of sheeted dykes is a perpetual and continuous process that promotes the phenomenon known as seafloor spreading. Seafloor spreading is the creation of new oceanic crust by volcanic activity at mid-ocean ridges, and as magma continues to rise and solidify at mid-ocean ridges, the existing older dykes are pushed out of the way to make room for newer seabed. The rate at which new oceanic crust is created is referred to as spreading rate, and variations in spreading rate determine the geometry of the mid-ocean ridge being created at plate boundaries.

Fast-Spreading Ridge

Mid-ocean ridges with a spreading rate greater than or equal to 90 mm/year are considered to be fast-spreading ridges. Due to the large amounts magma being expelled from the asthenosphere in a relatively short period of time, these formations typically protrude much higher from the seafloor.

Slow-Spreading Ridge

Mid-ocean ridges with a spreading rate less than or equal to 40 mm/year are considered to be slow-spreading ridges. These formations are typically characterized by a large depression in the seafloor, known as rift valleys, and are formed due to the lack of magma present to solidify.

List of Researched Sheeted Dyke Complexes

• Isua Supracrustal Belt (IBS), Greenland - This 3.8 billion-year-old formation located in Greenland contains sheeted dykes that provide the oldest known evidence for seafloor spreading.
• Maydan Syncline, Oman - A sheeted dyke complex on the coast of Oman has been discovered to have been formed during a single sea-floor spreading episode.
• Hole 504b, Coasta Rica - Hole 504b is a scientific ocean drilling program that burrowed 1562.3 m below the seafloor directly through layers of sediment exposing sheeted dykes and pillow lava.
  

Bibliography

González-Jiménez, José María; Griffin, William L.; Gervilla, Fernando; Proenza, Joaquín A.; O'Reilly, Suzanne Y.; Pearson, Norman J. (2014). "Chromitites in ophiolites: How, where, when, why? Part I. A review and new ideas on the origin and significance of platinum-group minerals." Lithos. 189 (15): 127-139.

Karson, Jeffrey A. (2018). "From Ophiolites to Oceanic Crust: Sheeted Dike Complexes and Seafloor Spreading." Dyke Swarms of the World: A Modern Perspective. Springer: Singapore. ISBN 978-981-13-1665-4. https://doi.org/10.1007/978-981-13-1666-1_13

Karson, Jeffrey A; Hurst, Stephen D.; Lonsdale, Peter (1992). “Tectonic Rotations of Dikes in Fast-Spread Oceanic Crust Exposed near Hess Deep.” Geology. 20 (8): 685. https://pubs.geoscienceworld.org/gsa/geology/article/20/8/685/189870/tectonic-rotations-of-dikes-in-fast-spread-oceanic

Marinoni, Laura B. (2001). "Crustal Extension from Exposed Sheet Intrusions: Review and Method Proposal." Journal of Volcanology and Geothermal Research. 107 (1-3): 27-46. https://doi.org/10.1016/S0377-0273(00)00318-8

Phillips-Lander, Charity M; Dilek, Yildirim (2009). “Structural Architecture of the Sheeted Dike Complex and Extensional Tectonics of the Jurassic Mirdita Ophiolite, Albania.” Lithos. 108 (1-4): 192–206.https://www.sciencedirect.com/science/article/pii/S0024493708002181

Robinson, Paul T.; Malpas, John; Dilek, Yildirim; Zhou, Mei-fu (2008). “The Significance of Sheeted Dike Complexes in Ophiolites.” GSA Today : a Publication of the Geological Society of America. 18 (11): 4. https://www.geosociety.org/gsatoday/archive/18/11/abstract/i1052-5173-18-11-4.htm

Schmincke, Hans-Ulrich (2004). Volcanism. New York: Springer-Verlag Berlin Heidelberg New York. pp. 61–62. ISBN 3-540-43650-2.