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Depth Extent of the Lau Back-Arc Spreading Center and Its Relation to Subduction Processes

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Science  10 Oct 1997:
Vol. 278, Issue 5336, pp. 254-257
DOI: 10.1126/science.278.5336.254

Abstract

Seismic tomography and wave form inversion revealed that very slow velocity anomalies (5 to 7 percent) beneath the active Lau spreading center extend to 100-kilometer depth and are connected to moderately slow anomalies (2 to 4 percent) in the mantle wedge to 400-kilometer depth. These results indicate that geodynamic systems associated with back-arc spreading are related to deep processes, such as the convective circulation in the mantle wedge and deep dehydration reactions in the subducting slab. The slow regions associated with the Tonga arc and the Lau back arc are separated at shallow levels but merge at depths greater than 100 kilometers, suggesting that slab components of back-arc magmas occur through mixing at these depths.

Knowledge of the seismic structure beneath back-arc spreading centers is important because the width and depth of the slow-velocity regions below spreading centers provide constraints on the origin of back-arc spreading (1,2), the geochemical source of arc and back-arc magmas (3), the interaction between subduction and back-arc spreading (1), whether the mantle upwelling beneath spreading centers is passive or active, and to what depth the upwelling persists (2). A subduction zone with an associated back-arc spreading center and the existence of deep earthquakes immediately beneath the center provide an ideal geometry to image and understand back-arc spreading processes. The Tonga-Fiji region, which contains two-thirds of all deep earthquakes in the world, represents an optimal region for exploring these questions. Previous studies have discussed the seismic velocity anomalies due to the Tonga slab (4,5), but this work has been hampered by the poor distribution of seismic stations.

The installation (6) of 12 broadband stations in the Tonga and Fiji islands from November 1993 through December 1995 and a related 3-month deployment of 25 ocean bottom seismometers (OBS) (7) in the Lau back arc and the Tonga forearc provided a unique opportunity to determine high-resolution three-dimensional (3D) structure in this region (Fig. 1A). We used 41,471 arrival times from 926 earthquakes that occurred in the Tonga-Fiji region during the seismic experiment (Fig. 1B). Most of the events were associated with the subduction of the Tonga slab; they had a relatively uniform distribution in the entire upper mantle. This uniform distribution is an advantageous feature over other subduction zones, such as Japan and Alaska, where most of the seismicity is concentrated at depths shallower than 250 km (8, 9). We picked about 8200 arrival times at the 12 land stations from the 926 earthquakes and about 2900 arrivals at the 25 OBS stations from 250 earthquakes that occurred during the OBS deployment. The picking accuracy is estimated to be 0.05 to 0.3 s. The remaining arrival times were recorded by stations reporting to the Preliminary Determination of Epicenters (PDE) with epicentral distances up to 90°. The PDE arrival times have lower quality (picking accuracy of 0.2 to 0.5 s), so they were assigned less than half the weight of the local data. All of the 926 earthquakes were recorded by more than 20 stations, and their hypocentral locations have a statistical accuracy of ±3 to 9 km. We also picked 450 arrival times at the 12 land and 25 OBS stations from 45 large (magnitude of 6.0 to 8.0) teleseismic events with epicentral distances from 30° to 90°, which were assigned the same weight as the local data in the inversion.

Figure 1

(A) Map showing the seismometer deployments in the Fiji-Tonga region. Twelve broadband instrument island sites, 25 OBS sites, and two PDE sites (at Samoa and Fiji) recorded the data used in this study. A 2-year sample of deep earthquakes (depths of 300 to 680 km and m b > 4.8) (dots) delineates the deep Tonga slab. PASSCAL, Program for Array Seismic Studies of the Continental Lithosphere; IRIS, Incorporated Research Institutions in Seismology; GSN, Global Seismographic Network. (B) Hypocentral distribution of the 926 earthquakes used in this study.

We used a tomography method (9) to determine the 3DP wave velocity structure in the Tonga-Fiji region (9, 10) (Figs. 2and 3). To confirm that the major velocity features were adequately resolved by the inversion, we conducted checkerboard resolution tests (11) (Fig.4). The checkerboard test with a grid spacing of 50 km indicates good resolution for the area in and around the subducting Tonga slab and along the main line of OBSs (Fig. 4, A and B). For the test with a grid spacing of 70 km, the resolution is good for all the areas discussed (Fig. 4, C and D). We also conducted a number of inversions and resolution tests by changing the grid spacing, the grid configuration, and the initial model (10). The results show that the velocity structure in the study area (Fig. 3) can be resolved with a resolution of 50 to 70 km. This resolution scale is better than the 100- to 200-km resolution obtained in previous studies (5).

Figure 2

East-west vertical cross section of a P wave velocity image from 0- to 700-km depth along the line AB (1220-km length) in Fig. 3A. Red and blue colors denote slow and fast velocities, respectively. Solid triangles denote active volcanoes. CLSC denotes the location of the Central Lau Spreading Center and ELSC denotes the location of the Eastern Lau Spreading Center. Earthquakes within a 40-km width from the cross section are shown as white circles. The velocity perturbation scale is shown at the bottom.

Figure 3

P wave velocity images at (A) 25-, (B) 60-, (C) 100-, (D) 140-, (E) 230-, and (F) 430-km depths. Earthquakes within a 20-km depth range of the slice are shown as white circles. The long contour line to the right shows the Tonga trench. The lines in the middle show the back-arc spreading centers. The short contour lines to the left and in the upper right-hand corner show islands. Line AB in Fig. 3A shows the location of the cross section in Fig. 2. All other labeling is the same as in Fig. 2.

Figure 4

Results of checkerboard resolution tests for P wave velocity structure at 25-km (A and C) and 480-km depths (B andD). The grid spacing is 50 km in (A) and (B) and 70 km in (C) and (D). Open and solid circles denote low and high velocities, respectively. The perturbation scales are shown at the bottom.

The subducting Tonga slab was imaged as a 100-km-thick zone with a P wave velocity that is 4 to 6% higher than the surrounding mantle (Fig. 2). Beneath the Tonga arc and the Lau back arc, low-velocity anomalies of up to 7% are visible (Figs. 2 and 3). The slow-velocity anomaly beneath the Tonga arc represents a dipping region about 30 to 50 km above the slab, extending from the surface to about 140-km depth (Fig. 2). This feature is similar to the low-velocity features found beneath the Japan and Alaska volcanic fronts (8, 9). This slow anomaly probably represents the source zone for island arc magmas. Volatiles released from the subducting slab may reduce the melting point of the rock above and allow partial melting to produce arc magmas (3,12). Slow anomalies beneath areas of the active Central Lau Spreading Center (CLSC) and the Eastern Lau Spreading Center (ELSC) extend to depths of about 100 km. These depths correspond to regions where the primary magma genesis is expected to take place beneath an oceanic spreading center (13, 14). The maximum heterogeneity of P wave velocity between the Lau back-arc basin and the Pacific Plate is about 13% at these depths. Slow anomalies are located to the west of the CLSC and ELSC (Figs. 2 and 3). Beneath 100-km depth, the amplitude of the back-arc anomalies is reduced, but a moderately slow anomaly (−2 to −4%) exists down to a depth of at least 400 km. To investigate the depth extent of slow anomalies in the Lau back arc with a different methodology, we inverted 16 wave forms from seven regional earthquakes recorded at the land stations to determine the 1D S wave structure beneath the Lau Basin (15). The inversion results (Fig.5) show a similar level of velocity heterogeneity and depth distribution of the back-arc anomalies to that found in the P wave tomography. The level of Swave velocity heterogeneity reaches a maximum of about 18% between the Lau Basin and the old Pacific lithosphere at depths of 40 to 90 km (Fig. 5). The velocity difference decreases to about 2% at 180-km depth, but a small, poorly resolved difference persists to greater depths (15). There has been disagreement concerning the depth extent of slow-velocity anomalies at mid-ocean ridges (MORs) (13, 16). Our results show that, at least for back-arc spreading centers, moderately slow velocity anomalies extend to depths of at least 400 km. These anomalies may reflect either the depth extent of oceanic spreading centers due to the depth of the associated upwelling patterns or processes endemic to back-arc spreading centers, perhaps due to interactions between the slab and the back arc.

Figure 5

S wave velocity models determined by inversions of entire regional vertical and radial wave forms recorded at island broadband seismic stations for various tectonic regions of the southwest Pacific.

The slow-velocity anomalies at depths of 300 to 400 km (Fig. 2) could be caused by upwelling flow patterns in the back-arc region or by volatiles resulting from the deep dehydration reactions occurring in the subducting Tonga slab. Volatiles would have the effect of lowering the melting temperature and the seismic velocity and may produce small amounts of partial melt (17). Temperatures in fast subducting slabs like Tonga are low enough for water to reach the stability depths of dense hydrous magnesian silicate phases (18), which may allow water penetration down to depths of 660 km (18, 19). The phase diagrams of important hydrous phases, the associated reaction kinetics, and the relevant mantle conditions (slab temperature and composition) are not known sufficiently well enough to predict the depth at which dehydration would occur. Partial melting of the back-arc region by volatiles from the deep slab may be important in localizing low seismic velocities in the back arc; the slow anomalies we observed at depths of 300 to 400 km may represent this process.

The slowest anomaly in the back-arc region is not found beneath the spreading center, but rather to the west. This is similar to observations from several recent experiments along MORs, which showed smaller delay times for arrivals at stations near the MORs than for stations on the flanks (20, 21). The faster arrivals near the ridge axis have been attributed to the alignment of anisotropic minerals in the mantle, with fast propagation for vertically traveling P waves caused by focused vertical flow beneath the spreading center. This effect may cause the arrivals at OBS stations near the spreading center to be faster than those off the ridge, causing the slowest anomalies to be displaced off the spreading center. The actual magma chamber beneath the spreading center would be expected to be less than 10 km in width (22), too small to image in this study.

Anisotropy may explain why the slowest velocities are not found immediately beneath the ridge, but it cannot explain why the western flank of the spreading center is slower than the eastern flank. This observation may be related to the ongoing tectonic processes in the Lau Basin. The CLSC, toward the west, is lengthening southward by rift propagation at the expense of the older ELSC, transferring the spreading activity westward (23). Our results suggest that this transfer may be favored by the proximity of the western ridge to upper mantle with the slowest velocities, which is also presumably the hottest mantle and the best source region for magma. Thus, the ridge propagation may be an attempt by the tectonic system to maintain the spreading center at or near the upper mantle magma source region.

The slow-velocity regions beneath the Tonga arc and the Lau back arc seem to be separated at shallow levels but merge at deeper levels (compare Fig. 3, A and C). This behavior suggests that although the arc and back-arc magma systems are separated at shallow levels, where most of the magma is generated, there may be some interchange between the magma systems at depths greater than 100 km. Interchange with slab-derived volatiles at depths greater than 100 km may help to explain some of the unique features in the petrology of back-arc magmas relative to typical MOR basalts, including excess volatiles and large ion lithophile enrichment (24).

  • * To whom correspondence should be addressed. E-mail: dzhao{at}usc.edu

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