The Lau Basin is a

back-arc basin A back-arc basin is a type of geologic basin, found at some convergent plate boundaries. Presently all back-arc basins are submarine features associated with island arcs and subduction zones, with many found in the western Pacific Ocean. Most o ...

(also addressed as "interarc basin") at the Australian-Pacific plate boundary. It is formed by the

Pacific plate The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At , it is the largest tectonic plate. The plate first came into existence 190 million years ago, at the triple junction between the Farallon, Phoenix, and I ...

subducting Subduction is a geological process in which the oceanic 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 ...

under the

Australian plate The Australian Plate is a major tectonic plate in the eastern and, largely, southern hemispheres. Originally a part of the ancient continent of Gondwana, Australia remained connected to India and Antarctica until approximately when India brok ...

. The

Tonga-Kermadec Ridge The Tonga-Kermadec Ridge is an oceanic ridge in the south-west Pacific Ocean underlying the Tonga- Kermadec island arc. It is the most linear, fastest converging, and most seismically active subduction boundary on Earth, and consequently has the ...

, a frontal arc, and the Lau-Colville Ridge, a remnant arc, sit to the eastern and western sides of the basin, respectively.Gill, J. B. 1976. "Composition and Age of Lau Basin and Ridge Volcanic Rocks: Implications for Evolution of an Interarc Basin and Remnant Arc." Bulletin of the Geological Society of America 87 (10): 1384–1395.

History

Lau Basin is a young basin (<= 5 m.y. old) that separates a previously continuous

island arc Island arcs are long chains of active volcanoes with intense seismic activity found along convergent tectonic plate boundaries. Most island arcs originate on oceanic crust and have resulted from the descent of the lithosphere into the mantle alon ...

by extensional rifting. During the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58 The slab of the Pacific plate melted as it was thrust down, and then rose to form the original Tonga-Kermadec Ridge. Around 25 Ma B.P, the Pacific plate started to drift away from the Australian plate, thus splitting the volcanic ridge. The rifting was initially caused by extension until 6 Ma B.P, by which time

seafloor spreading Seafloor spreading or Seafloor spread is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. History of study Earlier theories by Alfred Wegener a ...

started in this region and eventually formed the Lau Basin between the separated ridges.

Spreading centers

The V-shaped Lau Basin was opened by two southward propagating

spreading center A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a diver ...

s: the Central Lau Spreading Center (CLSC) and the East Lau Spreading Center (ELSC). The initial ELSC was oriented north–south and has a spreading rate of ~100mm/yr. The northeastern tip of ELSC propagated southward faster than the other part and produced a pseudofault oriented 170 degree. The ELSC rotated 15–25 degree clockwise and continued to propagate towards the south. Then the CLSC, as well as an extensional transform zone (ETZ) linking the two spreading centers were formed. The CLSC propagated southwards and replaced the northern segment ELSC. The region of overlap of CLSC and ELSC is characterized by

strike-slip earthquake In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

s. Recent measurements have shown that the opening rates are increasing at ELSC and CLSC. At present, the spreading rate of Lau Basin is about 150mm/year. It is an example of a fast-spreading

Petrology

Lau Basin volcanics are mainly andesites and dacites that were erupted 6.4 to 9.0 Ma. Most mafic rocks found are 55% SiO2 basaltic

andesite Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predo ...

s. The whole basin floor is mostly composed of MORB-like rocks, but the westmost 80~120 km of the basin floor contains a mixture of MORB, transitional and arc-like basalts. This western region has a different composition because it was formed by extension and rifting between the Lau and Tonga ridges before seafloor spreading started. The

graben In geology, a graben () is a depressed block of the crust of a planet or moon, bordered by parallel normal faults. Etymology ''Graben'' is a loan word from German, meaning 'ditch' or 'trench'. The word was first used in the geologic conte ...

s in this region was then filled by fresh magma from a

mantle A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that. Mantle may refer to: *Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear **Mantle (vesture), an Eastern Orthodox ve ...

source that is different from the mantle source for CLSC/ELSC.

Mantle source

The source of mantle melt to the Lau Basin is centered west of the spreading centers at shallow depth. This source may have directly supplied the western part of Lau Basin. The

MORB A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a diver ...

-type

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...

filled the grabens that were originally formed by extension in western Lau Basin. Asymmetric melt supply gave rise to the asymmetric thickness of crust at different sections of the basin. This melt supply may still be continuing today as indicated by a low-velocity anomaly in the

upper mantle The upper mantle of Earth is a very thick layer of rock inside the planet, which begins just beneath the crust (at about under the oceans and about under the continents) and ends at the top of the lower mantle at . Temperatures range from appr ...

beneath the western Lau Basin.

Mantle convection

At the subduction boundary between Pacific plate and Tonga-Kermadec plate, the roll-back of the Tonga trench and Pacific slab caused compensating flow of the mantle beneath the Lau Basin. This fertile mantle then encounters the water released from the dehydrated subducting Pacific slab and undergoes

partial melting Partial melting occurs when only a portion of a solid is melted. For mixed substances, such as a rock containing several different minerals or a mineral that displays solid solution, this melt can be different from the bulk composition of the soli ...

. This results in the creation of a batch of depleted mantle between the fertile mantle and subducting slab. An upward flow of the depleted layer is then induced by

back-arc spreading A back-arc basin is a type of geologic basin, found at some convergent plate boundaries. Presently all back-arc basins are submarine features associated with island arcs and subduction zones, with many found in the western Pacific Ocean. Most of ...

and slab subduction towards corner region where the mantle is hydrated. The enhanced melting in this region prevents the depleted mantle from getting re-enriched and thus allows it to flow until it overturns. It is then carried back down beneath the back-arc as subduction continues. The ELSC located right on top of the highly depleted mantle thus experiences a diminished magma supply which results in a thinner layer of crust and a faster spreading rate. The CLSC, on the other hand, has thicker crust because it overlies the fertile mantle that is largely removed from effect of the volcanic front. Unlike ELSC, CLSC has characteristics that are much more similar to a mid-ocean ridge.

Crustal structure

Crustal thickness increases from 6 km in the east to 9 km in the west. All of the Lau basin crust has a thicker midcrustal section than is seen in the pacific plate. The Lau Basin crust can be divided into eastern, central and western sections according to their thickness (5.5–6.5, 7.5–8.5 and 9 km, respectively). crust in the eastern section is similar to the one in the Pacific Plate with a thicker midcrustal layer and a thinner lower crustal layer. This suggests that it is composed of oceanic crust that was created more than 1.5Ma at the ELSC. The boundary between the eastern and central sections coincides with the boundary between the ELSC crust and CLSC crust, implying the internal structures in these two spreading ridges are, or were different. The central section has relatively thicker crust that formed within the past 1.5Ma at the CLSC. The boundary between the central and western crustal sections lies in the middle of ELSC crust, suggesting that the western section contains crust created both by oceanic spreading at ELSC and by island arc extension from the original Lau Basin.

Volcanoes and earthquakes

At present, the Lau Basin is still an active back-arc that is rapidly evolving in time. Six of the seven volcanoes in the Lau Basin are still active. Earthquakes in this region are mostly crustal earthquakes. Small earthquakes from the basin are barely recorded on land because of high mantle attenuation. Most of the earthquakes, as well as volcanic activities locate at the east boundary of Lau Basin, along the Tonga Ridge which is very volcanically active.

References

{{Coord, 19, S, 176, W, display=title Oceanic basins of the Pacific Ocean Landforms of Oceania Back-arc basins