107V.Unit+4.Geology+Chap.All.Pacific+Islands

107V.Unit+4.Geology+Chap.All.Pacific+Islands - CHAPTER 3...

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Unformatted text preview: CHAPTER 3 Geology Gerard]. Fryer and Patricia Fryer », Introduction The islands of the Pacific and the landmasses around it have been crafted by plate tectonics and volcanism.The Pacific Plate, part of the brittle outer layer of the Earth, is in constant north- westward motion. Where it passes over stationary sources of molten rock (hot spots) volcanoes grow. Dozens of linear island chains are scattered across the Pacific, marking the passage of 120'E 140'E 150‘E 180' the plate over such hot spots.Along the western margins of the ocean are islands very different in character. These islands are generally in arcuate chains bordered by deep ocean trenches, the greatest ocean depths on Earth.They are home to explosive volcanoes, and are wracked by earthquakes. Such violent processes are characteristic of locales where plates converge. When fragments of continents get caught up in the midst of such processes, changes occur in the geometry of convergence zones.The fragments themselves are deformed and may be sun- dered or uplifted along great faultsAs we shall see, the Pacific and its western margins exhibit all of these geologic processes. 120'W 160'W 140'W Figure 3.1. Lithosplreric plates oftlie Pacific basin. Subduction zones are marked by triangles on the overriding plate. Arrows show relative plate motions across plate boundaries. Relative plate speeds are given in centimeters per year. SAP is the San Andreas Fault; AF is the Alpine Fault. Western Pacific plate margins are taken from Scheibner, et al. (I 991). The Pacific Islands 33 Plate Tectonics Many Pacific Island chains show a clear age progression: young, volcanically active islands at the southerly end of the chain and old, extinct, and eroded islands to the north.j.Tuzo Wilson (1963) first showed that the age progressions mean that the sea floor is moving over stationary, episodically erupting hot spots.The idea ofa horizontally moving sea floor, together with the much older observation that the contihents themselves move (Wegener 1929), led to the realization that continents and sea floor together are carried along passively as part of the brittle outer layer of the Earth. The brittle outer layer of the Earth is the lithosphere. The idea that the lithosphere moves is the basis of the theory of plate tectonics.According to this theory, the lithosphere is organized into eight major plates and several smaller ones that together cover the Earth. Motion of the plates relative to each other explains earthquakes, volcanoes, mountain building, and the origins of islands and continents.The lithospheric plates them— selves are the surface expression of giant convection cells in the Continental Crust Partial Melting Oceanic Crust Ocean-Continent Convergence, Accretionary Margin Oceanic Crust Ocean-Ocean Convergence, Nonaccretionary Margin Oceanic Crustai Remnants Continent-Continent Convergence, Mountain Building Figure 3.2. Sketch of three dimerth types of convergent plate boundaries: Ocean—continent, ocean—ocean and continenecontinent. mantle ofthe Earthflthe slow circulation of rock by which the Earth rids itself of internal heat. The sea floor is created by volcanism at midocean ridges. These ridges stretch for thousands of kilometers, defining the diverging edges of the lithospheric plates. Here hot, upwelling rock from deep in the mantle begins to melt.The melt, called magma, inflates the ridges, stretching the lithosphere until it breaks in a long, narrow fissure. Lava erupts at the sea floor and forms vertical dikes (thin sheets of solidified magma) beneath the surface. As the sea floor is rafted away to either side, the underlying magma chamber is cut off, to be replaced by a new magma chamber.The surface lavas, the dikes, and the old, frozen magma chambers together form the oceanic crust, which is abOut 5 km thick. Beneath the crust, the mantle is hot and deformable. As the sea floor spreads and cools, the underlying upper mantle also cools and becomes stiffer. Together with the overlying oceanic crust, this rigid part of the upper mantle comprises the lithos— phere. Because of‘the cooling, the lithosphere thickens with age; it grows from a thickness of only a few kilometers at the ridge to about 70 km beneath old sea floOr. The lithosphere of most of the Pacific was formed, and continues to form, at the East Pacific Rise (Figure 3.1). ' Lithospheric spreading at one location requires convergence elsewhere. Where plates converge, lithosphere either sinks beneath an overriding plate or is compressed and uplifted to form mountain ranges (Figure 3.2).The plate~to—plate interac— tions in convergence zones cause earthquakes and feed the vol— canoes that make up island arcs. Subduction, the sinking of cold lithosphere into the mantle, draws down the sea floor to form deep ocean trenches.The sinking lithosphere forms the down— welling limb of a mantle convection cell; the upwelling limbs provide the source material for sea—floor spreading. Where plates neither converge nor diverge, bot simply slide past each other, the boundary between the two plates is a trans— form fault, which may stretch for hundreds of kilometers. California’s San Andreas Fault is the best—known example, though the Alpine Fault of New Zealand is ofcomparable size. Hot Spots and Linear island Chains Mantle rock rising beneath a midocean ridge does so as a vertical sheet, but the source rock for hot spots rises in confined plumes. The plumes are thought to form over topographic irregularities at the core—mantle boundary. These irregularities concentrate heat, making the overlying mantle rock hotter than adjacent rock. The hot rock rises buoyantly through the entire mantle, forming a path, the plume, along which more material can rise. As rock in the plume rises to depths shallower than 70 km it begins to melt, because the reduction of pressure near the surface lowers the melting point. Less than 5% of the upwelling rock is melted, but this is enough to provide the source magma from which to build island chains (Figure 3.3) such as the Hawaiian Islands. The composition of the magmas formed depends on the composition and degree of melting of the source rock. When mantle rock is partially melted, the resulting magma is basaltic. 34 The Pacific Islands increasin g erosron, Increasing SUb . Sldence Eefnloflnt. Atoll Island with / ‘ ’ a ’3:\ BarrierReef 5‘: 3 ,. r- m a H island wilh " r A (l g—JFringing Fleef l-¢_ ‘, r ‘3 Active * Volcano ocean floor solid lines = subaerial suriaces dashed lines = submarine surlaces Rising melt taut Spa! source Figure 3.3. A schematic cross section of an oceanic lithospheric plate showing how plumes of rising melt iii-ed hot spots. Volcanoes formed above the hot spot are rafted away in the direction of plate movement and thus increase in age with distance from the hot spot. Basalt is the most common rock type in the Pacific basin. It is a dark gray to black rock (Color Plate 3A) with a low silica (SiOz) content and a low content of volatile (low melting point) compounds such as water and carbon dioxide. Basalt erupts at high temperatures, more than 1100°C (Macdonald et al 1983). The first lavas from a hot spot will be erupted on the sea floor. The surface of lavas erupted underwater is rapidly chilled to form a glassy skin.The skin swells and splits to release more lava, which in turn chills over. This repetitive process forms round extrusions of lava and discrete blobs that break off and tumble downslope.When exposed in cross section, these under— water lava forms look like a stack of pillows, so they are called pillow lavas. The pile of pillow lavas is weak, so gravity begins to pull the growing volcano apart. Tension produces radial cracks, called rifts. Once rifts form, much magma is diverted from summit eruptions and instead injected through rifts to feed flank erup— tions.The magma left in a rift solidifies to form a dike. Rift zones expand and are filled repeatedly as new pulses of magma rise into the volcano. Eventually, the rift becomes a zone several kilometers wide underlain by solid dike rock.The dike complexes are more competent than the intervening rubbly flanks of pillow lava and thus act as buttresses, imparting localized internal strength to the'seamount. Many seamounts on the Pacific Ocean floor are star-shaped, with arms extend- ing along the rift zones. Collapsed flanks, compoSEd of piles of talus (mainly pillow lava fragments), lie between the arms. As the seamount approaches sea level, pressure is reduced, and steam boiling off the hot lava expands explosively. Sea water becomes involved in the eruption, and explosions shoot volu Canic ash (fine, solidified magma particles) into the atmosphere. Falling ash is swept away by waves and currents, but eventually the volcano breaches the surface. Once a solid surface has formed above sea level, fluid lava flows predominate, common— ly from rift zone eruptions.The freely flowing lava forms broad sheets, characterized by a smooth, ropy surface (prihoehoe), or a jagged, clinkery one (‘a‘sU (Macdonald et al. 1983). The low silica, low gas content and high temperature of basaltic lavas gives them low viscosity, which results in relative— ly gentle eruptions with very little ash. The low viscosity also produces thin flows, often less than a meter thick. Repeated gentle eruptions and thin lava flows form broad, convex shield volcanoes (Color Plate 3B). From initial eruption at the sea floor to formation of an island shield volcano, the process takes at least a million years. Between the rift zones, the unbuttressed flanks of an island (built on weak, rubbly pillow lava) may collapse if shaken loose by an earthquake or if oversteepened as the volcano inflates with magma. Fault scarps (cliffs) oriented approximately parallel to the shoreline indicate that the flank has failed; the cliffs are headwalls of large slump blocks. Slump blocks may fail cata— strophically as giant landslides (Figure 3.4). Sonar mapping of the sea floor around the Hawaiian Islands shows slide deposits extending two hundred kilometers from the islands (Moore et a1 1989). Similar evidence for large—scale slope failure is found around most volcanic islands. Figure 3.4. Landslide paths in the Hawaiian Islands. The hurnnrocky terrain of the sea bottom north of Moloka ‘i is characteristic of a landslide deposit. The entire north flank ofMoloka‘i has collapsed into the sea, as has the northwest flank of the Ko‘olau Volcano of O‘ahu. Tuscaloosa Sea-mount is a piece of the Ko'oiau Volcano that slid over 70 ion flow its original location. These landslides are the largest known landslides on Earth. Such landslides, together with the slow, continuous erosion by surf, rain, and wind, Whittle away at the island.As long as the island is fed by the hot spot, volcanism will counter the erosion. Inevitably, however, sea—floor spreading carries the island away from its source. Volcanism declines, then stops, and erosion reduces the island to sea level.Thus, in a midocean chain of islands, the degree of erosion correlates with distance from the active hot spot (Figure 3.3). As islands age, there is also an increase in the size of coral reefs (Darwin 1860). Because reef communities require sun— light, they must initially form in the shallow waters near shore to form fringing reefs. More severely eroded volcanoes are sur— rounded by a ring of shallow coralline shoals, a barrier reef, separated from the remnant of the volcano by a lagoon. In still older islands (atolls) the central volcanic remnants are missing altogether, and only a ring ofcoral and coralline islands remains around a shallow lagoon. W The Pacific Islands 35 The progressive aging continues.With the advent ofacoustic profiling in the 19405 many seamounts with flat tops (called guyots) were discovered. Some, though not all, have coralline caps that thicken and increase in depth below sea level with dis— tance from the hot spot. Erosion can explain the flattening of the tops of the volcanoes as the land surface is worn down to sea level, but it cannot explain the increasing depths. What is happening is that the sea floor itself is sinking. Ocean lithosphere migrates away from midocean ridges at rates of a few centimeters per year. As the plate ages it cools, increas— es in density, and subsides relative to the ridge crest. As it sub— sides, overlying islands and guyots subside with it. Ocean floor subsidence, if slow enough, will allow continued growth of coral, but subsidence faster than coral can grow (or transport of an atoll beyond the tropical belt of warm water) will result in a guyot. As the volcanoes grow, they become an increasing burden on the lithosphere beneath. which sags under the load (Wessel 1993).This sagging creates a moat (“deep”) around the volcano on the sea floor, forming a broad arch in the surrounding lithos— phere (Figure 3.5). Because the currently active volcanoes are least eroded, they form the most concentrated load. As each island in turn is rafted away from the hot spot, it rises over the arch formed by the new volcanic center before settling back into a slow subsidence. Rising over the arch puts the island in tension, which may trigger a posterosional rejuvenation of vol— canism. Posterosional features are scattered small cones of cin— der and ash or small shield volcanoes. Diamond Head on the Hawaiian island of O‘ahu is a typical example. 22'N 20'N 18'N 160°W 158"W 156'W Gradual subsidence of guyots is the normal aging process, but there are variations. A guyot may be transported over the arch around another hot spot and so be exposed again. Flexure (bending) of the lithosphere prior to its subduction into a trench forms similar arches or upwarps. A guyot carried up onto such an arch may reemerge as an island. The Loyalty Islands (see Figure 3.6) have been uplifted by upwarping of the North Loyalty Basin prior to its subduction into the New Hebrides Trench (Nunn 1994). Niue Island, an uplifted atoll east of the Tonga Trench, was created in similar fashion. The lagoon was elevated well above sea level and is now dry. An apparent vertical motion is also imparted by variations in sea level. During the last maximum in glaciation. about 21 thousand years ago, sea level was at least 110 m below its pre— sent level (Peltier 1996). Sea level variations from glaciation over the last two million years are thought to have oscillated between —130 m and +10 m. Glacial variations in sea level cou— pled with continuous uplift has produced elevated erosional benches on many. Pacific islands (as on Tongatapu Island, Tonga). Similarly, sea level variations coupled with continuous subsidence will produce a succession of submarine terraces.All Hawaiian islands display such terraces; those on the northwest slope of the island of Hawai‘i descend to 4 km depth. Some intraplate island groups (notably the Cook Islands) appear not to Show a clear age progression, and thus have cast doubt on the hot—spot hypothesis.Wessel and Kroenke (1997), however, have demonstrated that confusing age progressions result from changes over geological time in Pacific plate motion (so that the islands trail off from the hot spot in varying Ocean Depth (m) 154'W 152'W Figure 3.5. Bathymetric map of the Hawaiian Islands showing the Hawaiian Ridge, Deep and Arch. 36 The Pacific Islands W directions). Moreover, episodic rejuvenation of volcanism can result as islands rise over adjacent hot—spot arches.Thus, super— ficial lava flows on the more distant, older islands may yield younger geological dates than islands closer to the hot spot. Islands at Spreading Centers Young sea floor is hot, buoyant, and shallower than the sur— rounding sea floor.Thar is why divergent plate boundaries (also called spreading centers) form ridges. Sometimes a spreading center, or a volcano that first grew at a spreading center, breach— es the sea surface to form an island.This can happen either because a nearby hot spot has fed additional magma to the spreading center, or because the spreading center itself is in shallower than normal water. In the Pacific, only Easter Island is associated with a true midocean ridge spreading center, the East Pacific Rise (Figure 3.1). Not all divergent plate boundaries occur in the middle of large ocean basins, however.There is also spreading going on in several of the shallower seas around the western margin of the Pacific (we shall consider the formation of these marginal seas in a later section). Islands formed in such an environment include the Niua Group (Tonga),Tikopia (Solomon Islands), and the Witu Islands (Papua New Guinea). Volcanism at spreading centers is responsible for rich ore deposits. Circulation of heated sea water into fissures in hot rock leaches out metals (e.g., copper, zinc, and lead), which are deposited at hot springs on the sea floor. Most such deposits lie at water depths greater than 2 km and cannot be mined eco- nomically. Sometimes, however, a spreading center is carried into a subduction zone and caught up in a collision between a subducting landmass and the island arc. Fragments of the spreading center may then be scraped off and upthrust onto land, taking the ore deposits with them.The metal deposits of New Britain (Papua New Guinea) and Malaita (Solomon Islands) were formed in this way. Island Arcs The Pacific Plate is being subducted all along its western boundary. Subduction zones tend to establish themselves in broad, sweeping curves, so island chains formed along subduc— tion zones-are called island arcs. Island arc volcanism is driven by magma generation related to the subduction, as we shall discuss. But in a subduction environment, islands can also form for other reasons. Flexing of the downgoing plate may upwarp the sea floor, lifting guyots to become islands, as we have already seen. If the subducting lithospheric plate has abundant sediment, that sediment my be scraped off onto the overriding plate. The scraped—off sediment builds up into a wedge called a melange (see Figure 3.2).The wedge may build up so much that it pro— duces a ridge that dominates the forearc, the region between the island arc and the trench axis. If the forearc is uplifted by tec~ tonic forces, the ridge may be lifted above the surface to form islands. Examples of such islands include Guam,Yap, and ‘Eua . (Tonga). To explain how the island arc itself forms, it is necesssary to consider the origin of island arc magmas. Earthquake studies and seismic imaging show that the subducting plate penetrates to depths of 700 km or more. As the lithosphere subducts into the mantle, increases in pressure and temperature drive off water and carbon dioxide (this is essentially distillation of the plate). The presence of these distillates lowers the melting point of overlying mantle rock so much that magma is producedThe magma then rises, because it is less dense than the surrounding solid mantle. Resultant volcanism at the sea floor builds a vol— canic arc above the subduction zone.The islands that make up the Mariana are, for example, are the summits of active volcaa noes that formed on the sea floor about 100 km above the top of the subducting Pacific Plate. Along a volcanic arc, volcanoes are spaced roughly every 50 km. This characteristic spacing seems to be controlled by how much melt it takes to sustain a conduit from the melt zone. Melting begins where the subducting slab reaches a depth of 100 km, so, as already pointed out for the Marianas, volcanoes grow above that point. Since volcanoes grow directly above the melt zone, the distance of the volcanic are from the trench axis depends on the dip of the subducting plate (Gill 1981). If the plate dips steeply, as it does beneath the Bismarck Archipelago, the arc is close to the trench—in the Bismarcks the arc—trench distance is only 50 km. More typical are the Marianas, where the volcanic arc is 200 km from the trench axis (corresponding to a clip on the subducting plate of 30°).The shallowest dips of all occur below Western South America; in Chile, the arc— trench distance is 500 km. Magma formed above subducting plates is most commonly of basaltic composition, at least initially. The magmas of island arcs can also be andesitic. Andesite is a medium gray to green rock often containing visible white to light gray crystals (e.g., Macdonald et a1. 1983). Andesite has higher silica and more volatile content than basalt. Andesite can form either by incor— poration of crustal materials into basaltic magma or by the cooling and partial crystallization of basaltic magmas in a magma chamber. In partial crystallization the early—formed crystals settle out from the magma, leaving a melt with a differ— ent composition (more andesitic) than the parent basaltic magma. Andesitic lavas are chiefly found landward of the trenches that mark the circum—Paciflc subduction zones. For this reason, the ring of volcanoes that encircles the Pacific is sometimes called the “Andesite Line.” Volumetrically, andesites make up only a small proportion of island arcs, but that proportion is commonly exposed at the summits of the volcanoes. Subduction zones sometimes generate magmas with silica and volatile contents even higher than andesite. The resulting lava is rhyolite. Rhyolites are pink or light gray and often have visible quartz crystals. Magma formation in island arcs is another process that concentrates metal—bearing minerals to potential ore status. Metals are concentrated in fluids associated with the late stages of crystallization of magmas, especially andesitic and rhyolitic magmas. If these late-stage magmas crystallize underground and subsequently undergo partial melting, the new magmas M The Pacific Islands 37 may have an even greater concentration of ore minerals and metals. Repeated episodes ofmagrna generation in arc environ- ments can produce exceptionally rich ore—bearing deposits, such as the copper ores ofBougainville Island. Island arc volcanoes are composed of lava flows interlayered between deposits of ash or coarser volcaniclastic material (tephra).Alternating lavas and volcanic debris layers permit the volcano to build up steep slopes (around 35°), forming a strato— volcano or composite cone (Color Plate 3C). Eruptions are often explosive, a characteristic of andesitic and rhyolitic volca- noes.These eruptive explosions occur because the magma is highly charged with gases (derived from the subducting plate) and because the higher silica content of andesitic and rhyolitic magmas results in higher viscosity. Gases dissolved in a magma come out of solution as the magma rises and pressure decreas- es. Higher viscosity makes the rising magma more resistant to the expansion of gas bubbles. If the amount of expanding gas is great enough, it can blow the magma apart very near the sur— face, hurling gas and fragmented rock debris high into the atmosphere.The result is a voluminous ash deposit (Color Plate 3D). 15's 20°S 165°E 170'E Figure 3.6. Regional structure map oftbe Vanuatu island arc. If subduction occurs at a constant rate and in a constant direction, volcanoes tend to form a single arc above the magma source. Such simple geometry is easily disrupted.When a large, buoyant object, such as a continental fragment or an ancient island arc (both less dense than oceanic lithosphere), is rafted into the subduction zone, subduction may be blocked. If convergence between the plates continues, a new subduction zone must form away from the obstruction. Further, any change in motion at one margin of plate will affect all other margins. Such changes influence the location and nature of volcanism above the subduction zone. The Vanuatu (New Hebrides) island arc, for example, has experienced a complex sequence of volcanic successions and structural disturbances in response to changes in the direction of subduction.Vanuatu consists of three separate volcanic arcs that lie parallel to one another (Figure 3.6).The western vol— canic belt is the oldest (Kroenke 1984, Nunn 1994).Volcanic centers on these islands were active from about 15 to 11 Ma (millions of years ago), during a period in which the oceanic plate to the east of the chain was being subducted toward the west (Figure 3.7).At the end of this period subduction stopped, cutting off the magma supply to the volcanoes. A new sub— duction zone formed to accommodate continued convergence between the plates, but the new subduction was in the opposite direction. The plate to the west of the islands began subducting east— ward beneath the Western Belt. This subduction uplifted and deformed the Western Belt and induced extension of sea floor east ofit. By about 8 Ma, the eastward subduction had gener— ated enough magma to form a new volcanic chain: the Eastern Belt. About 5 Ma the eastward—dipping, downgoing plate adopted a steeper angle of subduction.The locus of active vol» canism, following its magma source, migrated westward to its present position and formed yet another chain of volcanoes, the currently active Central Chain. Such complex evolution ofsubduction zones may concenu trate minerals to produce ore bodies. For example, in New Caledonia, ultramafic rock (dark rock rich in iron and magne— sium) is exposed at the surface. This is now recognized as deep oceanic crustal material upthrust on land when it was squeezed between an island arc and a continent. Subsequent erosion and leaching of silica from the ultramafic rock enriched the residual surface layers in nickel and other metals.The nickel now supw plies a thriving mining industry. Similar ultramafics are exposed in New Guinea and New Zealand, although there metal con— centration is not to ore grade. New Caledonia, New Guinea, and New Zealand are all continental fragments; we discuss such landmasses in a later section. Marginal Seas and Backarc Basins Around its western margin, the Pacific proper is separated from adjacent continents by marginal basins (seas). Marginal basins separate islands made ofcontinental rocks from their par- ent continents; backarc basins form on the overriding-plate side of island arcs. Marginal basins, such as the Tasman Sea, which separates 38 The Pacific Islands Western Belt Uplifted torearc 15-11 Ma Western Bell Western Belt Eastern Belt volcanism Younger basin sediments Eastern Belt volcanics Central Chain volcanics BEE Upliited forearc Basin sediments Figure 3.7. Cross sections showing the development of the Vanuatu are as a consequence of reversals in direction of subduction (redrawn after Nunn 1994). New Zealand from Australia, are the result of rifting and breakup of a continent. Roughly 150 Ma the southern super— continent Gondwana started to break up to form present—day Australia, South America, Africa, India, and Antarctica. The rifts between these continent—sized pieces became centers for sea—floor spreading, creating the Indian, South Atlantic, and Southern oceans. But smaller fragments were rifted off too, to produce marginal seas. Continental rifting and the formation ofoceans may be trig— gered by the continent’s drifting over a hot spot (Morgan 1981).The formation of marginal basins seems to have a simiw lat origin: the Tasman Sea and the Coral Sea opened after Gondwana drifted over the Lord Howe hot spot (Yan and Kroenke 1993). Continental rifting, and the formation of ocean basins and marginal seas, are part of the same process that forms and sustains midocean ridges.That process is thought to be driven by deep convection of the mantle. Backarc basins also owe their origins to sea—floor spreading, but they involve processes operating at shallower depths. Backarc basins form from rifting and spreading behind island arcs, phenomena that are driven by subduction and which do not appear to have any direct link to deep convection. The origin of the driving forces that initiate and sustain backarc basins is a matter ofsome debate. Karig (1971) suggest— ed that rising diapirs (plumes) of mantle material behind sub— ducting plates provide the extensional forces necessary for the formation of backarc basins. Moberly (1972) suggested that subducting plates “roll back" toward the ocean basins from which they come and in so doing drag the subduction systems oceanward. Such trench rollback and the associated migration of the outer edge of the overriding plate produce tension forces sufficient to cause backarc extension. Backarc basins follow a two—stage evolution of rifting and spreading (Fryer 1995).Along the island arc, volcanic eruptions and intrusions (magma solidified at depth) build up a thick, brittle crust. The brittle crust is weak, so any tension in the overriding plate will cause it to rift (i.e., to split) along the line of active volcanoes. Magma rises up to seal the rifts and build new-lithosphere, which again rifts, because it is the weakest region of the overriding plate.With repeated rifting and sealing, the new lithosphere grows, separating the still—active line of arc volcanoes from the remnant arc, the ridge that was cleaved off from the volcanic are by the initial rifting. During the initial stage of extension, lavas erupted in the rifts may be of virtually any composition from basaltic through rhyolitic (Fryer et al 1990). In the initial stages, volcanism occurs over a broad region of the backarc basin. Eventually the extension in the backarc region widens the basin sufficiently, so that sea—floor spreading, like that at rnidocean ridges, becomes the mechanism by which the backarc basin grows. These processes may occur repeatedly, producing a series of island arcs, backarc basins, and remnant arcs. The Mariana convergent margin (Figure 3.8) provides an example (Fryer 1996).The Mariana system was an established convergent margin prior to 50 Ma.A rifting event split the vol— canic arc between about 31—15 Ma, isolating the Palau—Kyushu Ridge as a remnant arc and producing the central Parece Vela Basin until spreading ceased at 17—15 Ma. A second phase of rifting began about 10 Ma, separating an active volcanic arc from what was to become another remnant arc, the West Mariana Ridge. Extension of the Mariana Trough over the last 6—8 m.y. contributed to the increasingly bow—like shape of the arc as the rifting and sea—floor spreading continued to separate the active Mariana Ridge from the extinct West Mariana Ridge. Ore formation often occurs in backarc basins. In part, this is because the periodicity of volcanism in arc environments pror vides opportunities for repeated enrichment episodes. As in midocean ridge complexes, metals are concentrated by hydrothermal circulation (temperature—driven circulation of sea water through the crust). Moreover, large volumes of ash from the nearby volcanoes creates a thick blanket of volcaniclastic sediment, significantly slowing heat loss and making ore pro— duction more efficient. W The Pacific Islands 39 SEA OF JAPAN Depth (m) G 2000 4000 30'N ocAsawnnA - PLATEan 20'N 130'E 14D'E 150 Figure 3.8. Features ol‘the Philippine Sea region. Plate margins are denoted as in Figure I. Dashed double lines are relicr spreading Ct‘lift‘t‘s. Continental Islands The large islands of the southwest Pacific (New Guinea, New Caledonia, Fiji, and New Zealand) are Fragments of the ancient southern continent Gondwana, together with island arc accretions. The geology of these islands is complicated by extensive faulting, metamorphism, and magmatic intrusion. Much of the old geology has been further obscured by more recent volcanism. Nevertheless, guided by plate tectonics, a coherent story of how these islands evolved is beginning to emerge (Coleman 1997). The discussion here will focus on New Zealand, and will necessarily be brief. Since 300 Ma, the New Zealand area was the site ofa suc— cession of subduction systems, with associated trenches and island arcs, lying some distance off the east coast of Gondwana (Grindley and Davey 1982). Sediments from the continent accumulated in the intervening sea to form thick sequences of greywacke (sandstone) and argillite (siltstone), which were interlayered with volcanics.At 125 Ma subduction was blocked by the accumulated buoyant material, resulting in a major change in plate motions. The growing mass of sediment and volcanic accretions oil" Gondwana was compressed, uplifted, and intruded by granite (granite is mineralogically identical to rhyolite, but has larger crystals due to slower cooling). Slivers of underlying mantle—— ultramafic rockiwere also thrust up into these structures. Much rock was metamorphosed by being buried and subjected to higher pressure and temperature (which changes the miners als). This resulted in the metamorphic rocks greenschist and blueschist. Through such complex processes the margin of Gondwana was extended, forming the ancestral New Caledonia and New Zealand.The present Lord Howe Rise and Campbell Plateau were also part of this landmass (Figure 3.9). The final breakup of Gondwana occurred about 90 Ma when spreading began between Australia and Antarctica. At about 8” Ma. additional spreading rifted New Zealand away from Australia and Antarctica to create the Tasman Basin and the Southwestern Pacific Basin. The Tasman spreading ended around ()0 Ma, but the Southwestern Pacific Basin spreading continued, carrying New Zealand northward (Figure 3.10). During this time, the mountains of New Zealand were eroded away to form a large area of subdued relief, which was repeat— edly inundated by the sea. Organic matter deposited in coastal swamps Formed coal deposits, while the shells ot‘shallow—water marine organisms accumulated as limestone (Gage 1980). Since the separation from Gondwana, New Zealand has been carried north, rotated counterclockwise, and stretched out along the Alpine Fault. Today New Zealand straddles the boundary between the Pacific and Australian Plates (Figure 3.9).To the north the Pacific Plate is being subducted along the Kermadec and Hikurangi trenches.To the south, the Australian Plate is being subducted beneath the Pacific Plate along the ' CHALLENGER 40.5 PLATEAU 50‘s Auckland I5 9 \. CAMPBEL . Campbell I. 170'W IBO'E 170'E 180' Figure 3.9 The islands ofNeu' Zealand and surrounding submarine (Sutures. Volcanoes active \t'ttlnn the last l,l.l“nyt'.'11'h‘.‘ii"t‘ marked by white (mingle; 4E] The Pacific Islands present figure 3.10. Evolution ofthe Southwest Pacific over the last 70 million years. Continents are black; oceanic plateaus and submerged continental firagments are gray. Continental margins and subduction zones are represented as dark lines. At 70 Ma, spreading between Australia and Antarctica has been going on for 20 million years, but the spreading was slow so the continents are still close to each other. Spreading is going on in the Tasman Basin, separating New Zealand fi'om Australia. Spreading has just split South Island and the Campbell Plateau fiom Antarctica. At 35 Ma, Tasman spreading has ended, but more rapid spreading is going on between Australia and Antarctica and between Antarctica and New Zealand. New Zealand is being carried north. The Kermadec Trench is about to become active. In the present, New Zealand has been carried farther north, While South Island has been stretched along the Alpine Fault. (Simplified from Yan and Kroenlce 1993). Puysegur Trench. New Zealand is thus caught between two subduction systems with opposite polarity. Between these subduction systems, motion between the Australian and Pacific Plates is taken up along the Alpine Fault, which extends the length of South Island. The Alpine Fault has predominantly transform motion (horizontal slip parallel to the trace of the fault), with a total displacement of 450 km in the last 27 million years. But it also has a small component of convergence, which causes uplift. Uplift on the fault has raised the Southern Alps.These mounm rains, among the highest in the Pacific Islands region, have reached 3,700 m and continue to rise today. Volcanism has occurred episodically on both islands throughout New Zealand’s geological history. The modern volcanics of North Island are driven by subduction of the Pacific Plate into the Hikurangi Trench. Other volcanoes have also been active in the recent past: Mt. Egmont in the west and the Auckland and Bay of Islands volcanic zones on the north» ern peninsula of North Island.Thc Taranaki Plain (around Mt. Egmont) and the city and suburbs of Auckland remain regions of significant volcanic hazard. Subduction is also causing vol— canism in the Kermadec Islands. Backatc spreading is active in the Havre Trough and extends onshore into the Taupo Volcano Zone. Concluding Remarks The Pacific region displays each of the major processes of plate tectonics: hot~spot volcanism, divergent and convergent plate margin processes, and transform fault motion. All of the plate boundaries throughout the Pacific shift through time, adjusting to changes in plate motion. Such changes will con— tinue. The modern southwest Pacific is a clutter of rifted continental fragments, abandoned island arcs, and shallow oceanic plateaus.This material is so resistant to subduction that a future adjustment in plate motions is inevitable. That adjust- ment may already be happening.A diffuse band of earthquakes has developed on the Pacific plate between Yap and Samoa, far north of the current subduction zones that stretch from New Guinea to Tonga (Kroenke and Walker 1986). Nearby, a bathymettic deep has developed on the sea floor. This may mark the birthplace of the Pacific’s newest subduction zone, the Micronesian Trench. The Pacific Islands 41 EE— Bibliography Coleman, P._l. 1997. Australia and the Melanesian arcs: a review of tectonic settingsJournal ofAustralian Geology and Geophysics 17:113-125. Darwin, C. R. 1860. A naturalist's voyage (journal ofresearch into the natural history and geology of the countries Visited during the voyage oFHMS ‘Beagle' round the world). London: Henry Colburn, final revised edition. Reprinted in 1906 as The voyage ofthe Beagle, London:j.M. Dent. E Fryer, P., B.Taylor, C. Langmuir, and A. Hochstaedter. 1990. Petrology and geochemistry oflavas from the Sumisu and Torishima backarc rifts. Earth and Planetary Science Letters 1002161478. Fryer, P 1995. Geology of the Mariana Trough. in Backarc basins; tectonics and niagmatism, ed. B.Taylor. New York: Plenum Press, 237—279. Fryer, P. 1996.Tectonic evolution of the Mariana convergent margin. Reviews of'Ceophysics 34:89—125. Gage, M. 1980. Legends in the rocks: an outline ofNew Zealand geology. New Zealand:Whitcoulls Publishers. Gill,]. B. 1981. Orogenic andesites. BerlinrHeidelbergz Springer—Verlag. Grindley, G.W, and F .j. Davey. 1982.The reconstruction of New Zealand, Australia, and Antarctica. In Antarctic Geosciencc, ed. C. Craddock. Madison,Wisconsin: University ofWisconsin Press. Karig, D. E. 1971. Origin and development of marginal basins in the western PacificJournal ofGeophysical Research 76:2452u2561, Kroenke, LW'. 1984. Cenozoic tectonic development of the Southwest Pacific, UN. ESCAP, CCOP/SOPAC Technical Bulletin 6. Kroenke LW 1996. Plate tectonic development of the western and south- western Pacific: rnesozoic to the present. In The origin and evolution of Pacific Island biotas, New Guinea to eastern Polynesia: patterns and processes, ech.A. Keasr and S. E. Miller. Amsterdam: SPB Academic Publishing, 19—34. Kroenke, LW, and D.A.Walker. 1986. Evidence for the formation of'a new trench in the western Pacific. Eos, Transactions ofthe American Geophysical Union 67:145—146. W 42 Moberly, R. 1972. Origin of lithosphere behind island arcs, With reference to the western Pacific. in Studies in Earth and space sciences, eds. R. Shagam, et a1. Geological Society ofAmerica Memoir 132. Boulder, Colorado: Geological Society ofAmerica, 35755. MooreJ. G., D. A. Clague, 1LT. Holcomb, 12W Lipman,W’. R. Normark, and M. E.Torresan. 1989. Prodigious submarine landslides on the Hawaiian Ridge._jourtial of Geophysical Research 941146541484. Morgan,WJ. 1981. Horspot tracks and the opening of the Atlantic and Indian Oceans. In The sea,Vol. 7: The oceanic lithosphere, ed. C. Eniiliani. New York:j.Wiley, 443—487. Macdonald, G.A., A.T. Abbott, and E L. Peterson 1983. Volcanoes in the sea: the geology ofHawaii, Second Edition. Honolulu: University of Hawai‘i Press. Nunn, P .D. 1994. Oceanic islands. Oxford: Blackwell Publishers. Peltier, RAW. 1996. Mantle viscosity and ice-age ice sheet topography. Science 273213594364. Scheibner, E.,T. Sato, and C. Craddock. 1991 .Tectonic map of the Circum— Pacific region, Southwest Quadrant, Circurn Pacific Map Series, Map CP- 37. Denver, Colorado: US Geological Survey. Wessel, P. 1993. Observational constraints on models of the Hawaiian hot spot swelljournal of'Geophysical Research 98:16,095—16,104. Wessel, P. and LW Kroenke. 1997. A geometric technique for relocating hotspots and refining absolute plate motions. Nature 3873654369. Wegener,A. 1929. Die Eantehung der Kontinente und Ozeane, 4th edition (reviSed), translated by J. Biram 1967, The Origin of continents and oceans. London: Methuen. Wilson,j. T. 1963. Evidence from islands on the spreading of ocean floors. Nature 197536-538. Yan, C.Y., and L.W. Kroenke. 1991A plate tectonic reconstruction of the Southwest Pacific, 0—100 Ma. Proceedings of the Ocean Drilling Program, Scientific Results, 130:697—709. HIGP Publication 1011. SOEST Publication 4667. The Pacific Islands ...
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