Backarc Basins

Backarc (or marginal) basins are relatively small basins of either oceanic or continental affinity that form behind the volcanic arc in the overriding plate of a subduction zone (Fig. 9.3). Oceanic varieties are most common in the western Pacific, but are also found in the Atlantic behind the Caribbean and Scotia arcs. In all of these settings, the basins reside on the inner, concave side of the island arc and many are bounded on the side opposite the arc by a backarc ridge (remnant arc). Most of these basins are associated with extensional tectonics and high heat flow, and the majority of oceanic varieties contain sea floor spreading centers where new oceanic crust is generated. In continental settings, extensional backarc basins have been described in the context of Andean-type convergent margins (Section 10.2). Some of the best preserved examples of this type formed along the western margin of South America during

Figure 9.30 Paired metamorphic belts in the circum-Pacific region. Dotted lines, high pressure belts; solid lines, low pressure belts (redrawn from Miyashiro, 1973, with permission from Elsevier)

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Mesozoic times (Section 10.2.1). An example of an active continental backarc basin is the extensional Taupo volcanic zone on the North Island of New Zealand (Stern, 1987; Audoine et al., 2004).

Karig (1970) was one of the first to suggest that backarc basins form by the rifting of an existing island arc along its length, with the two halves corresponding to the volcanic and remnant arcs. This interpretation is based on observations in the Lau basin (Fig. 9.31), which lies west of the Tonga-Kermadec arc and is flanked on its western side by the Lau ridge. Karig (1970) concluded that extension was important during basin formation on the basis of the following observations: (i) the asymmetric cross-section of both the arc and ridge, which are mirrored across the center of the basin; (ii) the basin's topographic features, which are aligned parallel to both the arc and the ridge; (iii) the considerable sediment thickness present to the seaward side of the arc and landward side of the ridge and the absence of sediment within the basin; and (iv) the continuation of the arc-basin-ridge system to the south where it correlates with a zone of active backarc exten sion in the Taupo volcanic zone of New Zealand (Fig. 9.31). Further support for extension comes from the subsidence of remnant arcs as their dynamic support is removed after the development of backarc basins, earthquake focal mechanism solutions, and the segmented geometry of normal faults and spreading ridges, which also characterize continental rifts (Section 7.2) and mid-ocean ridges. The Woodlark Rift (Fig. 7.39b), which records the transition from rifting to sea floor spreading above a Neogene subduction zone (Section 7.8.2), illustrates this segmentation especially well.

In general, the composition of the crust in oceanic backarc basins is broadly similar to that of other ocean basins, although in some cases layer 1 is unusually thick. Net accretion rates are similar to those deduced for mid-ocean ridges, and range from approximately 160 mm a-1 in the northern Lau basin (Bevis et al., 1995) to 70 mm a-1 in the East Scotia Sea (Thomas et al., 2003) and 20-35 mm a-1 in the Mariana Trough (Martinez et al., 2000). The crust in these settings commonly shows substantial thinning by normal faulting, although the total crustal thickness also depends upon the rate of

Aerial Map Battle Hastings

Figure 9.31 Map showing the location of backarc basins in a part of the southwest Pacific, including the Lau basin, the South Fiji basin, the New Caledonian basin, and the Taupo volcanic zone (modified from Collins, 2002b, by permission of the American Geophysical Union. Copyright © 2002 American Geophysical Union). Box shows location of Fig. 9.32.

Figure 9.31 Map showing the location of backarc basins in a part of the southwest Pacific, including the Lau basin, the South Fiji basin, the New Caledonian basin, and the Taupo volcanic zone (modified from Collins, 2002b, by permission of the American Geophysical Union. Copyright © 2002 American Geophysical Union). Box shows location of Fig. 9.32.

magma addition and the age of the crust. In the Mariana Trough (Fig. 9.1), for example, crustal thicknesses range from 3.4 to 6.9 km, with the thinnest values corresponding to either slow spreading centers or magma-starved regions, and the thickest crust corresponding to regions of high magmatic activity (Kitada et al., 2006).

These observations and the evidence for extension and sea floor spreading imply that backarc crust in oceanic settings is generated in a manner similar to that occurring at mid-ocean ridges (Section 6.10). However, there are many important differences in the processes that form basaltic crust in these two environments.

Structures corresponding to a mid-ocean ridge are not always present in backarc basins, and magnetic lineations may be poorly developed (Weissel, 1981). Those lineations that are present can be correlated with the magnetic polarity timescale (Section 4.1.6), although they tend to be shorter, of lower amplitude and less clearly defined than oceanic anomalies. In the southern Lau Basin, for example, individual magnetic anomaly lineations cannot be traced for more than 30 km along the strike of the basin (Fujiwara et al., 2001). This short length probably reflects the small size of basement faults, which typically display segmented, en echelon patterns. Geochemical studies also indicate that backarc lavas commonly display greater compositional variations, including higher water contents, than mid-ocean ridge basalts (Taylor & Martínez, 2003). Many backarc lavas are chemically related to the lavas that form the adjacent island arc. These observations suggest that crustal accretion in backarc basins is strongly influenced by processes related to subduction (e.g. Kitada et al., 2006).

Tomographic images of the mantle beneath active backarc spreading centers have confirmed the important linkages that exist between backarc crustal accretion and subduction. In one of the best-studied arc-backarc systems, Zhao et al. (1997) showed that very slow seismic velocities beneath the active Lau spreading center and moderately slow anomalies under the Tonga arc are separated at shallow (<100 km) depths in the mantle wedge but merge below 100 km to depths of 400 km (Plate 9.1 between pp. 244 and 245). The magnitude of the velocity anomalies is consistent with the presence of approximately 1% melt at depths of 30-90 km (Wiens & Smith, 2003). At greater depths the anomalies may result from the release of volatiles originating from the dehydration of hydrous minerals. These results indicate that backarc spreading is related to convective circulation in the mantle wedge and dehydration reactions in the subducting slab. They also suggest that backarc magma production is separated from the island arc source region within the depth range of primary magma production. By contrast, below 100 km, backarc magmas originate through mixing with components derived from slab dehydration and may help to explain some of the unique features in the petrology of backarc magmas relative to typical mid-ocean ridge basalts.

A wide variety of mechanisms has been postulated to explain the formation of backarc basins. One common view is that the extension and crustal accretion that characterize these environments occur in response to regional tensional stresses in the overriding lithosphere of the subduction zone (Packham & Falvey, 1971). These stresses may result from the trench suction force as the subducting slab steepens or "rolls-back" beneath the trench (Chase, 1978; Fein & Jurdy, 1986) (Section 12.6). Such a roll-back mechanism has been postulated to occur in subduction systems where the "absolute" direction of movement of the overriding plate is away from the trench (e.g. Figs. 10.9b,c, 10.37). Other sources of the tension could include convection in the upper mantle wedge induced by the descent of the underthrusting slab (Hsui & Toksoz, 1981; Jurdy & Stefanick, 1983) or an increase in the angle of subduction with depth (Section 12.6). Although these and other mechanisms controlling the evolution of backarc basins are often debated, most authors agree that basin evolution is strongly influenced by the pattern of flow, partial melting and melt transport in the upper mantle wedge above a subduction zone. Geodynamic models increasingly have appealed to three-dimensional circulation patterns associated with trench migration and slab roll-back to explain the thermal evolution of the wedge and the production of melt within it (Kincaid & Griffiths, 2003; Wiens & Smith, 2003).

Martinez & Taylor (2002) developed a model of crustal accretion for the Lau basin that explains the mechanism of backarc magmatism and its relationship to magmatism in the Tonga arc. These authors observed that the various spreading centers in the basin (Fig. 9.32) display structural and compositional patterns that correlate with distance from the arc. As in most other intra-oceanic arc systems, the crust displays a general depletion of certain elements relative to mid-ocean ridge basalt that increases from the backarc toward the arc. In addition, the spreading center closest to the arc (the Valu Fa spreading ridge) displays a structure, depth and morphology indicating that it is characterized by an enhanced magma supply relative to other centers. Farther away from the arc, the East Lau and Central Lau spreading centers display diminished and normal magma supplies, respectively. Martinez & Taylor (2002) proposed that these variations result from the migration of magma source regions supplying the backarc spreading centers through the upper mantle wedge.

The model of Martinez & Taylor (2002) begins with the roll-back of the Pacific slab beneath the Tonga trench (large white arrow in Fig. 9.33a). This motion induces a compensating flow ofmantle material beneath the Lau basin (small black arrows). As the mantle encounters water that is released from the subducting slab (Section 9.8), it partially melts, leaving a residual mantle depleted of a certain melt fraction. The stipple in Fig. 9.33a indicates the region of hydrated mantle. The region of partial melt is shown as the white background beneath the stippling. Flow induced by subduction drives the depleted layer toward the upper corner of the wedge where increased water concentrations from the slab promote additional melting. This region of enhanced melting (outlined area in Fig. 9.33a) supplies the Valu Fa spreading ridge close to the volcanic

Lau Basin Focal Mechanisms

Figure 9.32 Location map of the Lau basin showing the backarc spreading centers (heavy lines), trench axis (dotted line), arc volcanoes (white triangles), and contours of the subducted slab (dashed lines) labeled in km (after Martínez & Taylor, 2002, with permission from Nature 416,417-20. Copyright © 2002 Macmillan Publishers Ltd). N, Niuafo'ou plate; T, Tonga plate; VFR, Valu Fa ridge; ELSC, east Lau spreading center; CLSC, central Lau spreading center; ETZ, extensional transform zone.

Figure 9.32 Location map of the Lau basin showing the backarc spreading centers (heavy lines), trench axis (dotted line), arc volcanoes (white triangles), and contours of the subducted slab (dashed lines) labeled in km (after Martínez & Taylor, 2002, with permission from Nature 416,417-20. Copyright © 2002 Macmillan Publishers Ltd). N, Niuafo'ou plate; T, Tonga plate; VFR, Valu Fa ridge; ELSC, east Lau spreading center; CLSC, central Lau spreading center; ETZ, extensional transform zone.

front, which receives melt that would otherwise supply the volcanic arc. The enhanced melting in this region also allows the depleted mantle (light gray in Fig. 9.33a) to remain weak enough to flow until it overturns and is carried back beneath the backarc basin as subduction proceeds. This return flow of depleted mantle results in diminished melt delivery to the East Lau spreading center farther from the volcanic arc because the melting regime is too far away to directly draw arc melts. Normal melting conditions occur at the Central Lau spreading ridge because this region overlies mantle that is farthest away from the volcanic front and from the effects of the slab. Consequently this latter spreading center displays a crustal thickness, morphology, and geochemistry like those of a typical mid-ocean ridge.

Taylor & Martínez (2003) generalized this model to include other oceanic backarc basins, including the Mariana, Manus and East Scotia Sea basins. Variations in basalt geochemistry in these basins also can be explained by the migration of melt source regions in the mantle wedge and by differences in the extents and depths of partial melting. The geochemical data also suggest that the mantle source regions for the Lau and Manus backarc basins are hotter than those of the Mariana and Scotia due to faster rates of subduction. These increased rates appear to induce greater

Outlined region of partial melting

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Outlined region of partial melting

Backarc Spreading

Figure 9.33 Model for the formation of the Lau backarc basin (after Martinez & Taylor, 2002, with permission from Nature 416,417-20. Copyright © 2002 Macmillan Publishers Ltd). Large white arrow indicates the roll-back of the Tonga trench and Pacific slab. Large black arrows show the subduction component of the Pacific slab, and small black arrows show flow in the mantle wedge induced by the slab subduction and backarc spreading. Stippled gradient indicates region of hydrated mantle, with water concentration increasing eastward toward slab. Region of partial melting is indicated.

Figure 9.33 Model for the formation of the Lau backarc basin (after Martinez & Taylor, 2002, with permission from Nature 416,417-20. Copyright © 2002 Macmillan Publishers Ltd). Large white arrow indicates the roll-back of the Tonga trench and Pacific slab. Large black arrows show the subduction component of the Pacific slab, and small black arrows show flow in the mantle wedge induced by the slab subduction and backarc spreading. Stippled gradient indicates region of hydrated mantle, with water concentration increasing eastward toward slab. Region of partial melting is indicated.

- Pacific

Backarc Spreading

150-120 Ma

Figure 9.34 Schematic plate reconstruction showing the location of early Mesozoic extensional backarc and marginal basins in South America (modified from Mpodozis & Allmendinger, 1993, with permission from the Geological Society of America). Marginal basins in Colombia and southern Chile (Rocas Verdes) are floored by oceanic crust. Casma-Huarmey in Peru and the central Chile basin are aborted marginal basins developed on thinned continental crust. Tarapaca and Neuquen are backarc basins with dominantly sedimentary infill.

- Pacific

150-120 Ma

Figure 9.34 Schematic plate reconstruction showing the location of early Mesozoic extensional backarc and marginal basins in South America (modified from Mpodozis & Allmendinger, 1993, with permission from the Geological Society of America). Marginal basins in Colombia and southern Chile (Rocas Verdes) are floored by oceanic crust. Casma-Huarmey in Peru and the central Chile basin are aborted marginal basins developed on thinned continental crust. Tarapaca and Neuquen are backarc basins with dominantly sedimentary infill.

transport of heat within the mantle wedge (England & Wilkins, 2004). Thus, many of the fundamental differences between crustal accretion in backarc and mid-ocean ridge settings can be explained by the structure and dynamics of flow in the upper mantle wedge. However, it is important to realize that, in addition to processes related to subduction, it is probable that some backarc basins are influenced by the specific configurations of plate boundaries in their vicinity. An example of this may be the North Fiji basin where the backarc spreading direction is oriented at unusually low angles (10-30°) to the trend of the arc. Schellart et al. (2002) suggested that this unusual spreading direction could be related to an asymmetric opening of the basin and to collisional processes occurring along the plate boundary. The evolution of the Woodlark Rift (Section 7.8.2) also is strongly influenced by local boundary conditions, including rheological weaknesses in the lithosphere.

Variability in the structure and magmatic characteristics of backarc basins also is common in continental settings. Along the western margin of South America, for example, a series of extensional basins formed during a period of Mesozoic extension above a long-lived subduction zone (Fig. 9.34) (Section 10.2.1). In most of these basins, extension and backarc rifting occurred without the formation of a basaltic basin floor (Mpodozis & Allmendinger, 1993). Only in Colombia and southernmost Chile did extension proceed to a stage where complete rupture of continental crust (Section 7.5) occurred and oceanic-type spreading centers developed. A possible modern analogue of these oceanic basins is the Bransfield basin, located behind the South Shetland trench near the Antarctic Peninsula (Fig. 9.1). This latter basin is asymmetric and displays evidence for having opened by rift propagation through pre-existing arc crust beginning 4-5 million years ago (Barker et al., 2003). Mora-Klepeis & McDowell (2004) discuss the geochemical signatures of rocks that record a similar transition from arc volcanism to rifting in the Baja California region of northwestern México.

Although common, not all continental backarcs are associated with extension or rifting. Many zones of ocean-continent convergence, including the modern Andean margin (Section 10.2), record shortening and orogenesis in the backarc environment. Regardless of the style of deformation they record, most continental backarcs are characterized by relatively thin, hot lithosphere (Hyndman et al., 2005) (e.g. Fig. 10.7) whose properties greatly affect the mechanical evolution of the convergent margin (e.g. Sections 10.2.5,10.4.6).

Orogenic belts

Backarc Region

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