Magmatic underplating

Description

Magmatic underplating occurs when basaltic magmas are trapped at the Mohorovičić discontinuity, within the crust, or even in crust-mantle transition zones. Large amounts of magma reach the surface via volcanoes, but there are times when the magma gets trapped and solidifies in one of these zones. This phenomenom is due to relative densities of the rising magma and the surrounding rock. Magmatic underplating is important for reasons such as the magnitude of an igneous or volcanic event and also that underplating can be responsible for the thickening of the crust when the magma solidifies. Evidence for magmatic underplating comes from studies in igneous petrology, geochemistry and the geophysical seismic studies which utilize the differences in densities between the underplated material and surrounding lithosphere.^[1]

Evidence

There are three arguments that can be made to support the phenomena of magmatic underplating. One is that "evidence of gabbro fractionation in erupted basaltic sequences"^[1] allows us to know the smallest possible mass of concealed material. The second is that, in the Karoo Province of southern Africa, "large volumes of rhyolite along the S.E. continental margin were generated from basaltic precursors.^[1] The third being that studies of geomorphology in this area suggest that "at least one kilometer of permanent uplift associated with the vulcanism" was experienced in this area.^[1] Also, studies have been done on the Laccadive Islands in the Northwest Indian Ocean and results have shown that between the Cretaceous and Oligocene the Indian Plate drifted over a mantle plume causing thermal erosion of the lithosphere which, in turn, created a mass of melt between the lithosphere and asthenosphere. Magma was quickly transported to the surface. The Moho is a boundary that hinders the partial melt from ascending and this material is proven to be between 16-24 km below the Laccadive islands due to it being exposed as the high velocity layer.^[2] There are many other ways for geologists to prove this phenomena in different areas. In studies in the Kutch District of Northwest India, seismic tomographic studies prove the existence of a large mafic body in the lower part of the crust, very close to the mantle.^[3]

Effects

Studies have been done on the phenomenom of magmatic underplating in various areas around the world. In northern Italy, the effects of magmatic underplating were studied along a traverse through the Strona-Ceneri Zone and the Ivrea Verbano Zone. The studies included a thermal modeling method which split the cross section up into three different sections: the upper crust, the lower crust, and the upper mantle. The model displayed multiple magmatic intrusions spreading over time, which resulted in the heating up of the lower crust causing metamorphism and anatexis, and even managed to moderately heat up the top of the lower crust. The results also showed that final heating began at the same time as extension in shallower crustal levels, while in deeper parts, extension occurred later than the thermal peak of metamorphism. It was also shown that magmatic underplating during a time period of about thirty million years was strong enough to erase all tectono–metamorphic history in the Ivera Verbano Zone. This information was preserved in the Strona-Ceneri Zone due to the fact that areas in the upper crust were not affected nearly as much.^[4] Other research has been conducted in the Kutch District of Northwest India. It was concluded that the uplift that occurred in the area was due to intrusions of magma in the lower part of the crust. This uplift occured because of two separate processes. One of these processes is due to magmatic underplating, while the other involves only isostasy. Research has shown that during the Oxfordian Stage a peak transgressive event occurred which was followed by the deposition of shale and sandstone. It is possible that the lower units may represent a rise in sea level. The sea began to withdraw because of the uplift related to the magmatic underplating.^[3]

Denudation

Studies have been done in the British Isles linking (Paleogene) denudation with magmatic underplating. It has been shown that the wavelength and amplitude of denudation can be determined by the density and distribution of the underplating in a given area. Modeling of data brought on by studies of the British Isles shows that a large amount of high velocity material occurs around the Mohorovičić discontinuity under the Irish Sea. Epeirogenic uplift is a long-wavelength form of uplift and be split up into two separate categories, which is transient and permanent. Permanent epeirogenic uplift is possibly mainly produced by magmatic underplating, while transient uplift is more involved with mantle convection. Magmatic underplating is important for causing quick epeirogenic uplift in certain areas. It has been argued that the greatest denudation happened in the Paleogene due to records of clastic deposition in sedimentary basins. Some of these sedimentary basins include the North Sea Basin and the Porcupine Basin off the southwest coast of Ireland. It has also been argued that Paleogene denudation was mainly caused by magmatic underplating.^[5]

Magma storage

Unsolidified areas of magmatic underplating have the possibility of being a feeder to volcanoes on the surface of the crust. An example of where this is possibly occurring is in the Rajmahal Traps. Studies that have been done on the area show that there is a 10-15 km thick igneous layer at the base of the crust beneath this area. The thickness of the layer is different in various parts of the area. It is in the center, where the thickness is the largest, where it is possible that the magma is being fed to the Rajmahal Traps up above.^[6]

Assimilation

Assimilation is basically when rock is melted into a host magma incorporating all of the chemicals that were in it into the host magma. This is relevant to magmatic underplating because assimilation of fragments of crust is important to crust-mantle mixing. If a xenolith were to undergo assimilation, it would start to incorporate all of itself into the host magma, disbanding the entire xenolith. Assimilation is improved in hydrous magma because of hydration crystallization. There are two types of assimilation. One of these is called bulk assimilation, which is when the rock remains solid, as opposed to melt assimilation, which is when it is in magma form. Arguments for assimilation mainly are derived from chemical evidence like radiogenic isotopes but it is possible for there to be physical evidence conserved in xenoliths. Only recently has assimilation become more accepted due to isotopic studies of certain minerals that show proof of magma mixing and hybridization.^[7]

Examples

• Hawaiian Islands
• Réunion
• Ninety East Ridge
• Kutch District
• Deccan Traps ^[3]
• British Isles^[5]
• Laccadive Islands^[2]
• Rahjamal Traps^[6]

References

1. ^ ^{a b c d} Cox, K.G. (1993). "Continental Magmatic Undeplating". Philosophical Transactions of the Royal Society: 155-166. doi:10.1098. 
2. ^ ^{a b} Gupta, Sandeep; Rai Mishra (August 2010). "Magmatic Underplating of Crust Beneath the Laccadive Island, NW Indian Ocean". Geophysical Journal International (183): 536-542. 
3. ^ ^{a b c} Karmalkar, N.R.; Kale, Duraiswami, Jonalgadda (25). "Magma underplating and storage in the crust-builiding process beneath the Kutch region, NW India". Current Science 94 (12): 1582-1588. 
4. ^ Henk, Andreas; Franz, Teufel, Oncken (1997). "Magmatic Underplating, Extension, and Crustal Reequilibration: Insights from a Cross-Section through the Ivrea Zone and Strona-Ceneri Zone, Northern Italy". The Journal of Geology 105: 367-377. 
5. ^ ^{a b} Tiley, Richard; White, Al-Kindi (January 2004). "Linking Paleogene denudation and magmatic underplating beneath the British Isles". Geology Magazine 3: 345-351. doi:10.1017/S0016756804009197. 
6. ^ ^{a b} Singh, A P; Kumar, Singh (December 2004). "Magmatic underplating beneath the Rajmahal Traps: Gravity signature and derived 3-D configuration". Proc, indian Acad. Sci. 113 (4): 759-769. 
7. ^ Beard, James; Ragland, Crawford (August 2005). "Reactive bulk assimilation: A model for crust-mantle mixing in silicic magmas". Geology 33 (8): 681-684.