Misplaced Pages

#212787

55-620: Lau Basin is a young basin (much is less than 5 million years old) that separates a previously continuous island arc by extensional rifting and spreading. During the Pliocene , the Pacific Plate was subducting beneath the Australian Plate. 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

110-477: A crust which is either oceanic or intermediate between the normal oceanic crust and that typical of continents; heat flow in the basins is higher than in normal continental or oceanic areas. Some arcs, such as the Aleutians, pass laterally into the continental shelf on the concave side of the arc, while most of the arcs are separated from the continental crust. Movement between two lithospheric plates explains

165-554: A previously subducted Louisville Seamount Chain volcano may have been a factor in the explosive nature of the 2022 eruption . ʻAta is about 50 km (31 mi) 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 95 km (59 mi) to

220-605: Is a fairly linear SW to NE orientated ridge greater than 200 km (120 mi) 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

275-404: Is a plane that dips under the overriding plate where intense volcanic activity occurs, which is defined by the location of seismic events below the arc. Earthquakes occur from near surface to ~660 km depth. The dip of Benioff zones ranges from 30° to near vertical. An ocean basin may be formed between the continental margin and the island arcs on the concave side of the arc. These basins have

330-460: 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 2.7 km (1.7 mi) to 2.1 km (1.3 mi) which has been correlated with the appearance of an axial magma chamber reflector in

385-412: Is now believed that water acts as the primary agent that drives partial melting beneath arcs. It has been shown that the amount of water present in the down-going slab is related to the melting temperature of the mantle. The greater the amount of water present, the more the melting temperature of the mantle is reduced. This water is released during the transformation of minerals as pressure increases, with

440-404: Is related to the age of the subduction zone and the depth. The tholeiitic magma series is well represented above young subduction zones formed by magma from relative shallow depth. The calc-alkaline and alkaline series are seen in mature subduction zones, and are related to magma of greater depths. Andesite and basaltic andesite are the most abundant volcanic rock in island arc which is indicative of

495-428: The subduction zone. They are the principal way by which continental growth is achieved. Island arcs can either be active or inactive based on their seismicity and presence of volcanoes. Active arcs are ridges of recent volcanoes with an associated deep seismic zone. They also possess a distinct curved form, a chain of active or recently extinct volcanoes, a deep-sea trench , and a large negative Bouguer anomaly on

550-399: 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 started in this region and eventually formed the Lau Basin between the separated ridges. In the north the basin reaches its maximum width of 500 km (310 mi) with a triangular shape to the south understood to be the result of

605-559: The CLSC 120 mm (4.7 in)/year, just to the north of the ELSC at the ILSC 102 mm (4.0 in)/year, at the start of the Valu Fa Ridge (VFR) to the south 69 mm (2.7 in)/year, and at its southern end 48 mm (1.9 in)/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

SECTION 10

#1732765934213

660-467: The ELSC, with basalt and andesite present. Further south the eruptives of the rift valley east of the SLR are mainly andesitic and/or dacitic while the western margin of the SLR has andesites and basalts. 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 -type basalt filled

715-437: 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

770-480: 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 Tonga are located in the latitudinal range of the ELSC, notably in view of its recent eruptive history Hunga Tonga–Hunga Haʻapai , 80 km (50 mi) away. It has been suggested that carbonate sediments deposited on

825-847: The Lau basin has the Northwest Lau Spreading Center (NWLSC). This is spreading at 75 mm (3.0 in)/year. The Rochambeau Rifts to the NWLSC's northeast are moving apart at 110 mm (4.3 in)/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 42 mm (1.7 in)/year,

880-417: 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

935-568: The VFR approaches to within 20 km (12 mi) of the arc to its east at about 24°S and is only 1,700 m (5,600 ft) 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

990-466: 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 M w  5 it has been possible to group

1045-406: The arcs shows that they have always existed at their present position with respect to the continents, although evidence from some continental margins suggests that some arcs may have migrated toward the continents during the late Mesozoic or early Cenozoic . They are also found at oceanic-oceanic convergence zones, in which case the older plate will subduct under the younger one. The movement of

1100-667: The area south of the Mangatolu Triple Junction (MTJ, also known as the Kings Triple Junction) which is separating at 30 mm (1.2 in)/year and the FRSC whose first northern segment is propagating northwards with a spreading rate of 28 mm (1.1 in)/year in the north east Lau Basin but down to 9 mm (0.35 in)/year where the last segment of the FRCS intercepts the Tofua volcanic arc to

1155-404: 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

SECTION 20

#1732765934213

1210-499: The calc-alkaline magmas. Some Island arcs have distributed volcanic series as can be seen in the Japanese island arc system where the volcanic rocks change from tholeiite—calc-alkaline—alkaline with increasing distance from the trench. Several processes are involved in arc magmatism which gives rise to the great spectrum of rock composition encountered. These processes are, but not limited to, magma mixing, fractionation, variations in

1265-527: The caldera which on its flanks also has some dacite eruptives. The southern basin volcanics and that of the ʻAta volcano can be associated with recycling from the subducted portions of the Louisville Seamount Chain . 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 at

1320-479: 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 and Tonga 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

1375-405: The convex side of the volcanic arc. The small positive gravity anomaly associated with volcanic arcs has been interpreted by many authors as due to the presence of dense volcanic rocks beneath the arc. Inactive arcs are a chain of islands which contains older volcanic and volcaniclastic rocks . The curved shape of many volcanic chains and the angle of the descending lithosphere are related. If

1430-401: The crust is neither being consumed nor generated. Thus the present location of these inactive island chains is due to the present pattern of lithospheric plates. However, their volcanic history, which indicates that they are fragments of older island arcs, is not necessarily related to the present plate pattern and may be due to differences in position of plate margins in the past. Understanding

1485-564: The deepest features of ocean basins; the deepest being the Mariana trench (approximately 11,000 m or 36,000 ft). They are formed by flexing of the oceanic lithosphere, developing on the ocean side of island arcs. Back-arc basin : They are also referred to as marginal seas and are formed in the inner, concave side of island arcs bounded by back-arc ridges. They develop in response to tensional tectonics due to rifting of an existing island arc. Benioff zone or Wadati-Benioff zone : This

1540-418: The depth and degree of partial melting and assimilation. Therefore, the three volcanic series results in a wide range of rock composition and do not correspond to absolute magma types or source regions. Remains of former island arcs have been identified at some locations. The table below mention a selection of these. Niuafo%27ou Plate The Niuafoʻou Plate is a small tectonic plate located west of

1595-409: The earthquakes into stress domains: 19°S 176°W  /  19°S 176°W  / -19; -176 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 along

1650-466: The east of the most northern segment of the ELSC. The large Niuatahi 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 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 M w less than 5) has been recorded along

1705-462: 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 beneath the western Lau Basin. At the subduction boundary between Pacific Plate and the Tonga and Kermadec plates ,

Lau Basin - Misplaced Pages Continue

1760-637: 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 Tonga Trench , so that the slab is at about 250 km (160 mi) 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 . At present,

1815-551: The island arcs towards the continent could be possible if, at some point, the ancient Benioff zones dipped toward the present ocean rather than toward the continent, as in most arcs today. This will have resulted in the loss of ocean floor between the arc and the continent, and consequently, in the migration of the arc during spreading episodes. The fracture zones in which some active island arcs terminate may be interpreted in terms of plate tectonics as resulting from movement along transform faults , which are plate margins where

1870-471: The major features of active island arcs. The island arc and small ocean basin are situated on the overlying plate which meets the descending plate containing normal oceanic crust along the Benioff zone. The sharp bending of the oceanic plate downward produces a trench. There are generally three volcanic series from which the types of volcanic rock that occur in island arcs are formed: This volcanic series

1925-422: 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

1980-471: The mantle wedge. If hot material rises quickly enough so that little heat is lost, the reduction in pressure may cause pressure release or decompression partial melting . On the subducting side of the island arc is a deep and narrow oceanic trench, which is the trace at the Earth's surface of the boundary between the down-going and overriding plates. This trench is created by the downward gravitational pull of

2035-533: The margins of continents. Below are some of the generalized features present in most island arcs. Fore-arc : This region comprises the trench, the accretionary prism, and the fore-arc basin. A bump from the trench in the oceanward side of the system is present (Barbados in the Lesser Antilles is an example). The fore-arc basin forms between the fore-arc ridge and the island arc; it is a region of undisturbed flat-bedded sedimentation. Trenches : These are

2090-446: The mineral carrying the most water being serpentinite . These metamorphic mineral reactions cause the dehydration of the upper part of the slab as the hydrated slab sinks. Heat is also transferred to it from the surrounding asthenosphere. As heat is transferred to the slab, temperature gradients are established such that the asthenosphere in the vicinity of the slab becomes cooler and more viscous than surrounding areas, particularly near

2145-565: The north that other smaller microplates may currently exist and certainly some of the plate boundaries are zones of deformation or for other reassons 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

2200-555: The oceanic part of the plate is represented by the ocean floor on the convex side of the arc, and if the zone of flexing occurs beneath the submarine trench , then the deflected part of the plate coincides approximately with the Benioff zone beneath most arcs. Most modern island arcs are near the continental margins (particularly in the northern and western margins of the Pacific Ocean). However, no direct evidence from within

2255-405: 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 thickness increases from 6 km (3.7 mi) in the east to 9 km (5.6 mi) in the west. All of the Lau basin crust has a thicker midcrustal section than is seen in

Lau Basin - Misplaced Pages Continue

2310-425: 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

2365-458: The relatively dense subducting plate on the leading edge of the plate. Multiple earthquakes occur along this subduction boundary with the seismic hypocenters located at increasing depth under the island arc: these quakes define the Benioff zone . Island arcs can be formed in intra-oceanic settings, or from the fragments of continental crust that have migrated away from an adjacent continental land mass or at subduction-related volcanoes active at

2420-478: 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 . 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

2475-429: The seafloor is more chaotic with much volcanism. 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 andesites . 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

2530-429: The source of heat that causes the melting of the mantle was a contentious problem. Researchers believed that the heat was produced through friction at the top of the slab. However, this is unlikely because the viscosity of the asthenosphere decreases with increasing temperature, and at the temperatures required for partial fusion, the asthenosphere would have such a low viscosity that shear melting could not occur. It

2585-589: 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. From the north of the CLSC we have a northeast orientated Lau Extensional Transform Zone (LETZ) which joins up to the Peggy Ridge which

2640-494: The southern Niuafo'ou microplate . The processes of back-arc basin 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 Australian Plate to its east, the Niuafo'ou Plate to its north east and

2695-421: 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 150 mm (5.9 in)/year and as an example of a fast-spreading back-arc basin 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

2750-656: The southward propagation of the main extensional centers and their asymmetric, predominantly westward opening. 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 100 mm (3.9 in)/year. It erupts mid-ocean ridge basalt (MORB). The northeastern tip of ELSC propagated southward faster than

2805-470: The subaxial melting regime of the back-arc spreading center. Seismic studies show that back-arc crust created at distances greater than 70 km (43 mi) 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 and magmatic underplating is found in some parts of

SECTION 50

#1732765934213

2860-420: The upper part of the slab. This more viscous asthenosphere is then dragged down with the slab causing less viscous mantle to flow in behind it. It is the interaction of this down-welling mantle with aqueous fluids rising from the sinking slab that is thought to produce partial melting of the mantle as it crosses its wet solidus . In addition, some melts may result from the up-welling of hot mantle material within

2915-577: 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 75 km (47 mi) 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

2970-491: 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 50 km (31 mi) 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

3025-513: Was formed by extension and rifting between the Lau and Tonga ridges before seafloor spreading started. The grabens 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 402 km (155 sq mi) of dacite lava north of the Niuatahi seafloor caldera which appears to have come from seafloor activity not associated with

#212787