The Lau Basin is a

back-arc basin A back-arc basin is a type of geologic Structural basin, basin, found at some convergent boundary, convergent plate boundaries. Presently all back-arc basins are submarine features associated with island arcs and subduction zones, with many found ...

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

subducting Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second pla ...

under the

Australian plate The Australian plate is or was 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 Indi ...

. The Tonga-Kermadec Ridge, 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. The basin has a raised transition area to the south where it joins the Havre Trough.

History

Lau Basin is a young basin (much is less than 5 million years old) that separates a previously continuous

island arc Island arcs are long archipelago, chains of active volcanoes with intense earthquake, seismic activity found along convergent boundary, convergent plate tectonics, tectonic plate boundaries. Most island arcs originate on oceanic crust and have re ...

by extensional rifting and spreading. During the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 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 million years ago, 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 million years ago, 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 ...

started in this region and eventually formed the Lau Basin between the separated ridges. In the north the basin reaches its maximum width of with a triangular shape to the south understood to be the result of the southward propagation of the main extensional centers and their asymmetric, predominantly westward opening.

Spreading centers

Southward propagating

The V-shaped Lau Basin was opened by two southward propagating spreading centers: 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 about /year. It erupts

mid-ocean ridge A mid-ocean ridge (MOR) is a undersea mountain range, 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 ...

basalt Basalt (; ) is an aphanite, 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 planetary surface, surface of a terrestrial ...

(MORB). 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 earthquakes. There is an Intermediate Lau spreading center (ILSC) between the two and to the east of the ELSC which has now four characterised segments. In the 3rd segment of the ELSC there is a transition in ridge morphology, associated with a substantial decrease of basin depth, from to which has been correlated with the appearance of an axial magma chamber reflector in the southern part of the ELSC. Recent measurements have shown that the opening rates are increasing at ELSC and CLSC. At present, the spreading rate of Lau Basin is about /year and as an example of a fast-spreading

much additional study has been undertaken which has identified additional spreading centers. As we come south down the Lau Basin spreading rates decrease being for the CLSC /year, just to the north of the ELSC at the ILSC /year, at the start of the Valu Fa Ridge (VFR) to the south /year, and at its southern end /year. Some authors have combined the VFR as part of the ELSC but the geology is slightly different. The southernmost spreading segment (it has two segments) of the VFR approaches to within of the arc to its east at about 24°S and is only deep. These spreading centers have now partially dismembered the Lau Ridge. South of the VFR, the back-arc region is mainly an area of stretched arc crust with abundant normal faulting but no obvious spreading and is called the Southern Lau Rift (SLR), an area of current active shallow earthquakes. Similarly to the south the Havre Trough has currently only rifting. To the north east is the southern section of the Fonualei Rift and Spreading Center (FRSC) which is southward propagating but to the north the interactions of the FRSC appear more complex and are mentioned below.

Otherwise propagating

From the north of the CLSC we have a northeast orientated Lau Extensional Transform Zone (LETZ) which joins up to the Peggy Ridge which is a fairly linear SW to NE orientated ridge greater than in the central Lau Basin(Labelled PR in diagram of basin on this page). The LETZ accommodates east to west extension but so does the FRSC to its east and such a double parallel arrangement has not been identified in any other back-arc basin. There is considerable complexity at the northern part of the Lau Basin where presently five independent oceanic tetectonic plates are interacting. The northwest aspect of the Lau basin has the Northwest Lau Spreading Center (NWLSC). This is spreading at /year. The Rochambeau Rifts to the NWLSC's northeast are moving apart at /year. To the east of the Rochambeau Rifts is an area of sea floor spreading between the Niuafo'ou plate and northern Tonga plate. From the north south we have the Northeast Lau Spreading Center (NELSC) separating at /year, the area south of the Mangatolu Triple Junction (MTJ, also known as the Kings Triple Junction) which is separating at /year and the FRSC whose first northern segment is propagating northwards with a spreading rate of /year in the north east Lau Basin but down to /year where the last segment of the FRCS intercepts the Tofua volcanic arc to the west of the Tonga Ridge. A prominent NW-trending formation of young volcanic structures that includes the Niuafo'ou shield volcano crosses the northern Lau Basin approximately west of the MTJ and is called the Western Rift Margin (WRM). To the east of the WRM the seafloor has multiple NNW trending elongated ridges of roughly the same orientation as the WRM, while to its west the seafloor is more chaotic with much volcanism.

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 predomina ...

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 depression (geology), depressed block of the Crust (geology), crust of a planet or moon, bordered by parallel normal faults. Etymology ''Graben'' is a loan word from German language, German, meaning 'ditch' or 't ...

s in this region was then filled by fresh magma from a mantle source that is different from the mantle source for CLSC/ELSC. In the north eastern portion of the basin there is over of dacite lava north of the

Niuatahi Niuatahi or also called Volcano O is a submarine volcano located in the far northern territory of Tonga. Since the cone in the middle is named Motutahi, the volcano is sometimes referred to as Niuatahi-Motutahi. Despite not having any record of a ...

seafloor caldera which appears to have come from seafloor activity not associated with the caldera which on its flanks also has some dacite eruptives. The southern basin volcanics and that of the

ʻAta Ata is a depopulated island in the far southern end of the Tonga archipelago, situated approximately south-southwest of Tongatapu. It is distinct from Atā, an uninhabited, low coral island in the string of small atolls along the Piha passag ...

volcano can be associated with recycling from the subducted portions of the

Louisville Seamount Chain The Louisville Ridge, often now referred to as the Louisville Seamount Chain, is an underwater chain of over 70 seamounts located in the Southwest portion of the Pacific Ocean. One of the longest seamount chains on Earth, it stretches some Vander ...

. Dredged lavas from the FRSC are almost identical to lavas from the nearby arc volcanoes. To the south the lavas in this part of the Lau Basin are more arc-like than 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 div ...

at the ELSC, with basalt and

present. Further south the eruptives of the rift valley east of the SLR are mainly

andesitic 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 predomina ...

and/or dacitic while the western margin of the SLR has andesites and basalts.

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

-type

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 (geology), crust (at about under the oceans and about under the continents) and ends at the top of the lower mantle (Earth), lower man ...

beneath the western Lau Basin.

Mantle convection

At the subduction boundary between Pacific plate and the

Tonga Tonga, officially the Kingdom of Tonga, is an island country in Polynesia, part of Oceania. The country has 171 islands, of which 45 are inhabited. Its total surface area is about , scattered over in the southern Pacific Ocean. accordin ...

and Kermadec plates, the roll-back of the

Tonga Trench The Tonga Trench is an oceanic trench located in the southwestern Pacific Ocean. It is the deepest trench in the Southern hemisphere and the second deepest on Earth after the Mariana Trench. The fastest plate-tectonic velocity on Earth is occurri ...

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 is the phenomenon that occurs when a rock is subjected to temperatures high enough to cause certain minerals to melt, but not all of them. Partial melting is an important part of the formation of all igneous rocks and some metamorp ...

. 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 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 in the east to 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 1.5 million years ago 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 1.5 million years 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. In the ELSC further studies have shown that the back-arc crust created at less than from the volcanic arc front is unusually thick at 8 to 9 km) and has a thick upper crustal layer and a lower crustal layer ("Domain II crust", “hydrous” crust) due to slab-derived water input into the subaxial melting regime of the back-arc spreading center. Seismic studies show that back-arc crust created at distances greater than from the volcanic arc front is thinner and more similar to typical oceanic crust ("Domain III crust"). The crust in the southern FRSC was created by extension of arc crust with variable input of

magmatism Magmatism is the emplacement of magma within and at the surface of the outer layers of a terrestrial planet, which solidifies as igneous rocks. It does so through magmatic activity or igneous activity, the production, intrusion and extrusion of ...

and magmatic underplating is found in some parts of the southern Niuafo'ou microplate.

Tectonics

The processes of

formation were first proposed by Daniel (Dan) Karig in 1970 from studies of the Lau Basin. The possibility of there being in this region several tectonic plates and triple junctions was suggested by Clement Chase the next year. The Lau Basin presently has oceanic crust from the

to its east, the Niuafo'ou plate to its north east and the clockwise rotating Tonga plate to its west. The Futuna microplate is in close relationship to the north in this most active tectonic area. In the northern Lau Basin, the extensional motion between the reference points of

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

and

is accommodated by multiple zones of active rifting and spreading that are located along the boundaries of the Niuafo'ou microplate. These are so complex, especially towards the north that other smaller microplates may currently exist and certainly some of the plate boundaries are zones of deformation or for other reasons are ill defined. There is a overlapping spreading center from the northernmost segment of the FRSC in the east to the southernmost segment of the Mangatolu Triple Junction in the west. The relationships between seafloor and crustal properties, that were established based on observations made at mid-ocean ridges such as distance to spreading center, water depth and crustal age may not be strictly applicable in the back-arc basin setting. In particular the complexity of the northern section is best explained if the spreading in back-arc basins is not as linear a process as it is along mid-ocean ridges and rather back arc spreading has the potential for newly emerging or jumping spreading centers. The west dipping Pacific slab whose bed rock is about 110 millions years old is presently being subducted under the independent Tonga microplate whose spreading center from the Australian Plate are those of the southern Lau Basin. The seismogenic zone below the Lau Basin is very displaced from the

, so that the slab is at about depth under the Lau Basin spreading axis. The southern limit of the basin is related to the subduction of the Louisville Ridge below the Kermadec-Tonga subduction zone.

Volcanoes

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. The island volcano of Niuafoʻou has erupted multiple times since historic records began. To the east some islands of

are located in the latitudinal range of the ELSC, notably in view of its recent eruptive history Hunga Tonga–Hunga Haʻapai, away. It has been suggested that carbonate sediments deposited on a previously subducted

volcano may have been a factor in the explosive nature of the 2022 eruption.

is about east of the Valu Fa Ridge and compositional analysis of its volcanics have identified that these are associated with the subducted portions of the Louisville seamounts. Kao which has the highest point of Tonga and Tofua are about to the east of the most northern segment of the ELSC. The large

caldera is in the northeast of the basin. The eastern side of the basin has the Tofua volcanic arc along the western side of the Tonga Ridge.

Earthquakes

Earthquakes in this region are mostly crustal earthquakes. Small earthquakes from the basin are barely recorded on land because of high mantle attenuation. However low‐magnitude seismicity (i.e. mainly less than 5) has been recorded along the active spreading centers in the Lau Basin by ocean bottom seismometers. 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. In the Southern Lau Rift shallow earthquake swarms have occurred. In terms of shallow and thus crustal earthquakes greater than it has been possible to group the earthquakes into stress domains: *Northern Lau Basin region (dominated by right‐lateral strike‐slip faulting in diffuse deformation zones and left‐lateral strike‐slip faulting along prominent crustal‐scale plate boundary faults) **North east Fiji stress domain (southern end FSC and Fiji Transform Zone) **Central Futuna stress domain (Futuna Central Zone) **Northern Lau stress domain (centered on the Rochambeau Rifts) **North east Lau and Tonga forearc stress domain (contains the MTJ and NELSC) *Eastern Lau Basin region **Northern FRSC stress domain (dominated by strike‐slip faulting) **Southern FRSC stress domain (dominated by extension at the propagating tip) *Central Lau Basin **Peggy Ridge stress domain (to its west where plate boundary as east is inactive now, with the most western portion having transform to extension stresses) **Central Lau stress domain (with the plate boundary of the LETZ having transform to extension stresses and the more southern CLSC to ELSC regions having extension to transform stresses) *Southern Lau Basin region **Tofua stress domain (with zone of compression to the west of

Tongatapu Tongatapu is the main island of Tonga and the site of its capital, Nukuʻalofa, Nukualofa. It is located in Tonga's southern island group, to which it gives its name, and is the country's most populous island, with 74,611 residents (2016), 70.5% o ...

Island, the main island of

, and transtension to the southeast, adjacent to the active Tofua arc) **Valu Fa stress domain (extension at the Valu Fa propagating tip)

References

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