Upper mantle discontinuity structure in the region of the Tonga Subduction Zone (original) (raw)

Detailed structure and sharpness of upper mantle discontinuities in the Tonga subduction zone from regional broadband arrays

1] Recordings of deep Tonga earthquakes from two arrays of 12 broadband seismographs each in the Fiji and Tonga islands are stacked and searched for reflections and conversions from upper mantle discontinuities in the Tonga subduction zone. The arrays operated as part of the Seismic Arrays in Fiji and Tonga (SAFT) experiment from July 2001 to August 2002. In comparison with the commonly used teleseismic approaches, the short path lengths for the local data provide smaller Fresnel zones and high-frequency content for precise mapping of discontinuity topography and sharpness. To enhance the low-amplitude discontinuity phases s410p, P660p and S660p, deconvolved seismograms from each event/array pair are aligned on the maximum amplitude of the direct P wave and subsequently slant stacked. For the 410-km discontinuity, the results show no systematic variations in depth with distance to the cold slab. The 660-km discontinuity varies between 656 and 714 km in depth. For the southern and central parts of the subduction zone, the largest depths occur in the core of the Tonga slab. For the northern part, two separate depressions of the 660-km discontinuity are observed. These anomalies are interpreted as being induced by the active, steeply subducting Tonga deep zone and a subhorizontally lying remnant of subducted lithosphere from the fossil Vityaz trench, respectively. Interpreting the deflections of the 660-km discontinuity in terms of local temperatures implies a thermal anomaly of À800°K to À1200°K at 660 km depth. Except for the southern region where it may thicken, the width of the depressed 660-km discontinuity region implies that the Tonga slab seems to penetrate the 660-km discontinuity with little deformation. Waveform modeling suggests that both the 410and 660-km discontinuities are sharp. The 660-km discontinuity is at most 2 km thick in many parts of the region, and a first-order discontinuity cannot be precluded. The 410-km discontinuity thickness shows somewhat more variability and ranges from 2 to 10 km outside the slab and is at most 10 km thick within the slab. This suggests that the subduction process does not produce dramatic effects on the sharpness of the discontinuities.

Reconciling Pacific 410 and 660km discontinuity topography, transition zone shear velocity patterns, and mantle phase transitions

Earth and Planetary Science Letters, 2010

We observe that the depths of the 410 and 660 km seismic discontinuities are, on average, slightly positively correlated globally. This is due in large part to a modestly depressed 660 km discontinuity and a large depression of the 410 km discontinuity across the Pacific. The Clapeyron slope (dP/dT), the change in pressure (depth) of a phase transition with a change in temperature, can be used to predict the depth of a phase transformation assuming lateral temperature variations. The phase change of olivine to β-spinel is well understood experimentally, almost certainly produces the 410 km discontinuity, and has a positive Clapeyron slope. At the base of the transition zone, both the olivine component of the mantle (γ-spinel) and the pyroxene component (garnet) transform to perovskite and periclase. Observations of 660 km discontinuity depths are often consistent with the negative Clapeyron slope of the γ-spinel to perovskite and periclase transition, with an apparent anti-correlation with the depth of the 410 km discontinuity. However, under the Pacific, the depression of the 410 km discontinuity and slow seismic velocities indicate that the mantle is warmer than average. Using a negative Clapeyron slope for the perovskite-forming reaction, the 660 km discontinuity is predicted to be shallow in this region. However, we observe that it is either depressed or has little deflection from its average depth. We find that if the Clapeyron slope associated with the 660 km discontinuity changes sign from negative to positive between 1920 and 2020 K, we can explain the correlation of discontinuity structure with seismic velocities in the transition zone. This shift in sign is in accord with the dominant 660 km transition-forming reaction shifting from γ-spinel to garnet near this temperature.

Depth variation of the mid-mantle seismic discontinuity

Geophysical Research Letters, 1997

Short-period array seismograms of deep events that occurred in the Indonesia, Japan and Izu-Bonin arcs are stacked and beam-formed to identify the near-source S-P converted waves that result from the mantle transition discontinuities. Most of the resulting images reveal the existence of a mid-mantle seismic discontinuity ("920 km discontinuity") in these regions. Of the 15 events analyzed, three that occurred at the western end of the Indonesia arc show clear S-P arrivals observable even in individual seismograms. The mid-mantle discontinuity is characterized by large depth variation (900 ~ 1080 km) and velocity contrast variation in different subduction zones. Especially, the depth variation of the mid-mantle discontinuity beneath the Indonesia arc, where the discontinuity deepens from 940 km at the eastern end to 1080 km at the western end, appears to be well correlated with the location of the high-velocity anomalies in recent tomographic models. However, the mid-mantle discontinuity cannot be simply coincided with the bottom of the high-velocity anomalies, because a velocity increase at the discontinuity is observed from the waveform analysis. Introduction Recent tomographic studies in the western Pacific region [Zhou and Clayton, 1990; van der Hilst et al., 1991; Fukao et al., 1992; $akurai el al., 1995] have found that high-velocity anomalies [hereafter referred to as HVAs] are deflected at the '660-km' discontinuity in some subduction zones and extend to at least several hundred kilometers below the '660-km' discontinuity, but no deeper than approximately 1100 km, in others. This feature is clearly shown in recent studies of the correlation between seismic tomography and subduction history [Wen and Anderson, 1995; K•valov& el al., 1995]. The result of tomographic studies showing that subducted slabs are trapped in the depth range of 800-1100 km suggests that there may exist a geodynamical boundary at this depth which resists slabs penetration. Meanwhile, we have reported that a mid-mantle seismic discontinuity exists at a depth of about 920 km beneath Tonga and two other subduction zones [Kawakatsu and Niu, 1994]. Therefore the nature of this "920 km discontinuity" may have significant implication for mantle dynamics; further detailed study of the "920 km discontinuity" in different subduction zones to delineate its relation with the HVAs appears to be essential.

Seismological evidence for a slab detachment in the Tonga subduction zone

Tectonophysics, 2009

The Tonga Wadati-Benioff zone is characterized by a large seismicity gap beneath the Lau Basin that raises the question of the slab continuity between the shallow to intermediate part (60-300 km) and the deep part (400-700 km). To address this problem, we investigated the Wadati-Benioff Zone geometry and stress regime through a detailed analysis of the spatial distribution of moment tensors and variation of the stress tensor, using the global seismicity [Engdahl, E., Van der Hilst, R., Buland, R., 1998. Global teleseismic earthquake relocation with improved travel times and procedures for depth determination. Bull. Seism. Soc. Am. 88, 722-743.] and the Centroid Moment Tensor solutions (CMT) catalogs [Dziewonski, A.M., Chou, T., Woodhouse, J.H., 1981. Determination of earthquake source parameters from waveform data for studies of global and regional seismicity. J. Geophys. Res. 86, 2825Res. 86, -2852. The stress tensors were computed using the Gephart's program . An improved method for determining the regional stress tensor using earthquake focal mechanism data: application to the San Fernando earthquake sequence. J. Geophys. Res. 89,[9305][9306][9307][9308][9309][9310][9311][9312][9313][9314][9315][9316][9317][9318][9319][9320]. The stress inversion results indicate that between 21°S and 27°S, and depths down to 700 km the slab is under homogeneous down dip compressional stress regime, while north of 21°S we found strong variations of the stress orientations between the intermediate and deep portions of the slab. We also show that between 14°and 19°S, the stresses at intermediate depth (60-300 km) can be resolved into two slab parallel domains, a thin upper part of the slab that is under downdip compression and the lower part that is under downdip extension. This pattern of two zones with opposite mechanical behavior is characteristic of a subducted plate with a free lower limit that does not interact with the 670-km depth boundary. These results together with the large seismicity gap within the slab argue for a slab detachment.

Seismological evidences for a slab detachment in the Tonga subduction zone

Direct evidence for the undulation of the 660-km discontinuity beneath Tonga: Comparison of Japan and California array data

Geophysical Research Letters, 1995

Short-period seismograms of Tonga deep earthquakes recorded by Japanese and Californian seismic networks are stacked to identify the S-P converted wave associated with the 660-km discontinuity. The travel-time difference between this S-P converted wave and the direct P wave is used to constrain the depth of the 660-km discontinuity. Analysis of a total of 29 events produced a detailed topographical map of the discontinuity beneath the Tonga subduction zone.

Multiple discontinuities near 660 km beneath Tonga area

Geophysical Research Letters, 2006

We study the discontinuities beneath Tonga subduction zone using the deep earthquakes recorded by the Pacific Northwest Seismic Network (PNSN). The multiple discontinuities around 660 km beneath Tonga subduction zone are found using converted phase SdP. The amplitude ratios of the converted phase with the direct P wave (Ac/Ap) are also determined. The largest number of robust converted phases with large Ac/Ap comes from the depth range of 660-690 km, its peak is at 680 km; the second one appears in the depth range of 700-745 km, its peak is at 740 km. The two discontinuities are obviously depressed by the subduction slab. According to experimental and numerical studies, a possible explanation of the observation is that the first discontinuity is formed by the transition of g-spinel to perovskite and magnesiowüstite and the second is formed by the transition of ilmenite to perovskite.

The 660-km discontinuity within the subducting NW-Pacific lithospheric slab

Earth and Planetary Science Letters, 2002

The 660-km seismic discontinuity (660) in Earth's mantle is generally attributed to the breakdown of the ringwoodite phase of olivine, but other mineral reactions are also thought to occur near 660-km depth. Recently, complex arrivals of P660s waves (converted from P to s at the 660) in active and recently active subduction zones have been interpreted as evidence for additional seismic discontinuities caused by the garnet^perovskite and garnetî lmenite^perovskite phase transformations (gtCpv, gtCilCpv) at relatively low temperatures. Here we show that the P660s phases converting at the 660 within the subducting NW-Pacific slab beneath the station MDJ in Northeast China are clear and coherent, with no additional arrivals in the vicinity. P660s waves that convert near the boundaries of the area where the 660 occurs within the slab produce distinctly more complex, multiple arrivals, but they are more likely to be caused by small-scale topography rather than 'multiplicity' of the 660. Our observations suggest that the gtCpv transformation and the gtCilCpv, if it occurs in the mantle, are spread over tens of kilometers and do not have sharp onsets visible to short-period seismic waves. ß

Tomography and Dynamics of Western-Pacific Subduction Zones

Monographs on Environment, Earth and Planets, 2012

We review the significant recent results of multiscale seismic tomography of the Western-Pacific subduction zones and discuss their implications for seismotectonics, magmatism, and subduction dynamics, with an emphasis on the Japan Islands. Many important new findings are obtained due to technical advances in tomography, such as the handling of complex-shaped velocity discontinuities, the use of various later phases, the joint inversion of local and teleseismic data, tomographic imaging outside a seismic network, and P-wave anisotropy tomography. Prominent low-velocity (low-V) and high-attenuation (low-Q) zones are revealed in the crust and uppermost mantle beneath active arc and back-arc volcanoes and they extend to the deeper portion of the mantle wedge, indicating that the low-V /low-Q zones form the sources of arc magmatism and volcanism, and the arc magmatic system is related to deep processes such as convective circulation in the mantle wedge and dehydration reactions in the subducting slab. Seismic anisotropy seems to exist in all portions of the Northeast Japan subduction zone, including the upper and lower crust, the mantle wedge and the subducting Pacific slab. Multilayer anisotropies with different orientations may have caused the apparently weak shear-wave splitting observed so far, whereas recent results show a greater effect of crustal anisotropy than previously thought. Deep subduction of the Philippine Sea slab and deep dehydration of the Pacific slab are revealed beneath Southwest Japan. Significant structural heterogeneities are imaged in the source areas of large earthquakes in the crust, subducting slab and interplate megathrust zone, which may reflect fluids and/or magma originating from slab dehydration that affected the rupture nucleation of large earthquakes. These results suggest that large earthquakes do not strike anywhere, but in only anomalous areas that may be detected with geophysical methods. The occurrence of deep earthquakes under the Japan Sea and the East Asia margin may be related to a metastable olivine wedge in the subducting Pacific slab. The Pacific slab becomes stagnant in the mantle transition zone under East Asia, and a big mantle wedge (BMW) has formed above the stagnant slab. Convective circulations and fluid and magmatic processes in the BMW may have caused intraplate volcanism (e.g., Changbai and Wudalianchi), reactivation of the North China craton, large earthquakes, and other active tectonics in East Asia. Deep subduction and dehydration of continental plates (such as the Eurasian plate, Indian plate and Burma microplate) are also found, which have caused intraplate magmatism (e.g., Tengchong) and geothermal anomalies above the subducted continental plates. Under Kamchatka, the subducting Pacific slab shortens toward the north and terminates near the Aleutian-Kamchatka junction. The slab loss was induced by friction with the surrounding asthenosphere, as the Pacific plate rotated clockwise 30 Ma ago, and then it was enlarged by the slab-edge pinch-off by the asthenospheric flow. The stagnant slab finally collapses down to the bottom of the mantle, which may trigger upwelling of hot mantle materials from the lower mantle to the shallow mantle. Suggestions are also made for future directions of the seismological research of subduction zones.

Tonga slab deformation: The influence of a lower mantle upwelling on a slab in a young subduction zone

Geophysical Research Letters, 2000

There are fundamental geographic variations in the deformation of slabs in the transition zone. The seismic energy release and morphology of the Tonga slab show that it is deforming faster and has accumulated more deformation than any other slab. We show that Tonga overlies the edge of the large-scale Pacific superplume. There is no substantial aseismic penetration into the lower mantle beneath Tonga, consistent with initiation of subduction during the Eocene. Other major subduction systems overlay seismically fast structures. For long-lived subduction systems, the lower mantle tends to pull down on slabs while in Tonga the lower mantle pushes upward, partially accounting for the intense deformation. The perturbation to the state of slab stress due to large-scale mantle flow is 10 to 40 MPanearly as large as that expected from slab pull.

Upper mantle discontinuity structure in the region of the Tonga Subduction Zone (original) (raw)

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