S14C-01
Seismic and petrological properties of the upper mantle between 300 and 400 km depth
We compare the traveltime data from the long range seismic profiles and from the earthquakes recorded to the offset of 3000 km with theoretical traveltimes predicted by standard seismological models: PREM, IASP- 91, AK-135 and especially from seismo-petrological model PREF (Cammarano and Romanowicz - 2007). We try to compare our models to earlier studies by Thybo and Perchuc (1997a). Our data suggests that for several events in the distance range 2000-2800 km, the first-arrivals are characterized by a relatively high velocity of 8.7–8.9 km/s. It is about 2.5% higher than P-wave velocity of the Lehmann phases, observed in the nearest offset and about 3% smaller than velocity below 410 km discontinuity. We suggest that this is a new first-order seismological boundary which can be interpreted as a top of the mantle transition zone. Seismological arguments for the existence of such a boundary are as follows: refracted waves with velocity 8.7-8.9 km/s and reflected waves find by Warren at al. (1967) and by Thybo and Perchuc (1997b). Also the interpretation of the SS precursor phases (Deuss and Woodhouse 2002) suggested a reflection boundary around 300 km (our interpretation). Depth of this boundary strongly depends on the thermal state of the mantle in particular regions. In conclusion we can say that the mantle transition zone starts much earlier and the lower part of the upper mantle is much faster than predicted by purely pyrolitic mantle model. Several petrological studies suggest influences of fluids (especialy H2O) on the character of the 410 km discontinuity and of the transition zone. All the differences in experimental data can be explained by the effect of temperature on the phase transformations within the olivine-wadsleyite system.
S14C-02 INVITED
Inversion of Long Period Waveforms and SS Precursor Data for Thermal and Compositional Models of the Upper Mantle.
The knowledge of the thermal (T) and compositional (C) state of the mantle transition zone is a key factor for understanding the evolution of our planet and the surface manifestations of the processes occurring in its interior. Long period seismic waveforms, including overtones, which are particularly sensitive to the transition zone, are included in the datasets from which the Berkeley elastic and anelastic S tomographic models have been derived in the last 15 years. Recently, we developed an inversion approach which includes constraints from mineral physics and allows us to obtain upper mantle 3D models expressed directly in terms of lateral variations in temperature and composition. However, with our standard long period dataset, we have limited resolution in the vicinity of mantle discontinuities. On the other hand, somewhat shorter period SS precursors waveform data are sensitive to the topography and impedance jump of mantle discontinuities, and are particularly useful to provide additional constraints on the temperature and composition of the transition zone. We here show some preliminary results of incorporating these data into our tomographic inversion. We proceed as follows: in the first step, we refine our average 1D model, through a parameter search for temperature and composition that best satisfies the average global stack of SS precursor waveforms together with long-period waveforms, free oscillation average frequencies and body wave travel times. In a second step, we include constraints from SS precursor travel time data to simultaneously invert for 3D lateral variations in temperature and topography of the upper mantle discontinuities. Finally, we plan to add further constraints from SS precursor waveforms to iteratively constrain lateral variations in temperature and composition in the upper mantle.
S14C-03 INVITED
Seismic Imaging of Meta-stable Olivine Wedge in the Subducting Slab Beneath Japan via Vectorial Receiver Function
To image the subducting Pacific plate better, we have extended the treatment of Kawakatsu and Watada (2007, Science) in which they had corrected for the effect of a dipping interface on seismic receiver functions (RF) to image a low-velocity layer atop of the subducting slab. The dip angle of the Pacific plate estimated from seismicity is employed to correct the effect of the dipping interface. For each potential conversion point, only a P-S conversion which satisfies the Snell's law on the dipping interface is used for RF. Two horizontal component RFs are then rotated to the direction of expected polarization of P-S converted waves from the interface, and stacked at the conversion point in such a way that the amplitude corresponds to the possible S-wave velocity jump at the interface. This method is applied to Hi-net recording of teleseismic events from 2001 to the end of 2006. The total number of events analyzed is 681, and the number of RFs is more than 300,000. The results show a clear image of a bottom boundary of the subducting Pacific plate; the thickness of the plate is estimated to be ~80km (Tonegawa et al., 2006, EPSL). Below 350km right beneath central/southwestern Japan, there also exist signatures inside of the slab which we attribute to those originated from the postulated meta-stable olivine wedge (MOW; Iidaka and Suetsugu, 1992, Nature). We observe both velocity decrease (from shallow to deep) and increase corresponding respectively to the upper and lower edge of the MOW which is expected to have several percent slower seismic velocity relative to the surrounding normal slab (Kaneshima et al., 2007, EPSL). The catalogue seismicity by JMA indicates that deep earthquakes are located along the lower edge of the MOW. The detailed investigation of the relative locations of these features should give a tight constraint on the origin of deep earthquakes. The existence of the MOW indicates insignificant amount of water present in the subducting slab in the region (Hosoya et al., 2005, GRL); together with the observed deep depression (~40km) of the 660-km discontinuity in the same area, the effective Clapeylon slope of dry slab for the 660km discontinuity should be significantly steeper than those predicted by recent high-pressure experiments (e.g., Katsura et al., 2003, PEPI).
S14C-04
The 520 km Discontinuity: No Longer Just the Middle Child of the Transition Zone
The 520 km discontinuity is more difficult to image and changes character laterally more relative to its siblings the 410 km and 660 km discontinuities. The 520 km discontinuity is thought to result from the phase change of β-spinel to γ-spinel, a transition which has a smaller impedance contrast than the olivine to β-spinel or the γ-spinel to perovskite and periclase transitions that are associated with the 410 km and 660 km discontinuities, respectively. In addition, there are gradual phase changes occurring in the pyroxene component of the mantle between the 410 km and 660 km discontinuity which complicate imaging and interpretation of the 520 km discontinuity. The best global coverage of discontinuity depth comes from the stacking of SS precursors, seismic phases that bounce off the discontinuities and arrive as precursors to the main SS phase. A recent study by Houser et al. (2008) contains the largest compilation of SS precursor measurements. However, since the 520 km discontinuity is not ubiquitously observed, its characteristics were not addressed. Here, we examine the geographic distribution (which is global, but not even) of high quality 520 km discontinuity measurements derived from precursors. The principal result is that the depth of the 520 km discontinuity has a higher correlation with tomographically-imaged high and low seismic velocities than either the 410 km or 660 km discontinuities. This stronger temperature sensitivity is in accord with determinations of the Clapeyron slopes of the differing transitions. Furthermore, this correlation is observed in a wide range of tectonic environments, and the prospect thus exists that the 520 km discontinuity will, where it is observed, provide a tighter bound on temperature within the transition zone than either of the other major discontinuities. Furthermore, improved constraints on the temperature distribution at depth may produce improved insight into the relative roles of thermal and compositional effects on the 660 km disctoninuity.
S14C-05 INVITED
A Thermal Boundary Layer at the top of the Lower Mantle: the Tomographic Evidence
The idea that the 660 km discontinuity is a thermal boundary layer, maintained by a negative Clapeyron slope, a compositional increase in density, a jump in viscosity or (most likely) a combination of those factors, has been around for more than thirty years. The seismological evidence for slabs penetrating in the lower mantle made the hypothesis of a fundamental separation between upper- and lower mantle less likely. However, a closer look at the seismic evidence shows that there are, in fact, various observations that point to a thermal boundary layer at this depth: [1] High-resolution tomography of subducting slabs in the upper mantle shows an abundance of tears that significantly reduce the strength of the slab itself; penetration in the lower mantle should then be dominated by buoyancy, not by transmitted stresses. Many slab fragments are pictured to reside in the transition zone, indicating an absence of sufficient (negative) buoyancy. [2] High-resolution tomography of lower mantle plumes shows that at least some of them widen significantly below 660 km, indicative of a resisting force near the top of the lower mantle that leads to ponding, [3] Some slabs that penetrate into the lower mantle show low velocity anomalies below the upper mantle slab, consistent with an interpretation of a return flow of lower mantle material with a temperature that is above the adiabat.
S14C-06
Seismic imaging of the stagnant Pacific slab in the mantle transition zone under East Asia
We used regional and global seismic tomography to determine high-resolution 3-D P-wave velocity structure of the crust and mantle down to 1200 km depth under Western Pacific to East Asia (Zhao, 2004, 2007; Huang and Zhao, 2006). A large number of arrival times of P, pP, PP and PcP waves recorded by many seismic stations in East Asia are used in the tomographic inversions. The subducting Pacific slab is imaged clearly as a high-velocity zone from the oceanic trenches down to 670-km depth, and intermediate-depth and deep earthquakes are located within the slab. The Pacific slab becomes stagnant in the mantle transition zone under eastern China. The western edge of the stagnant slab is generally parallel with the Japan trench and the Ryukyu trench and roughly coincides with a prominent surface topographic boundary in East China. Although there are some discrepancies between the topographic boundary and the western edge of the stagnant slab, both of them are located approximately 1800 km west of the trenches. The entire Pacific slab is stagnant in the mantle transition zone under Northeast China (53-37 degree north latitude). Under 37-28 degree north latitude, however, some of the slab materials are visible below the 670-km discontinuity, though most of the slab materials are still in the transition zone, suggesting that part of the slab materials have started to drop down to the lower mantle. Under the Mariana arc, the Pacific slab penetrates directly down to the lower mantle. It is also visible that the Philippine Sea slab has subducted down to the mantle transition zone depth under western Japan and the Ryukyu back-arc region (Abdelwahed and Zhao, 2007). There are three active intraplate volcanoes in China. The Changbai and Wudalianchi volcanoes in Northeast China are underlain by significant slow anomalies in the upper mantle, above the stagnant Pacific slab, suggesting that the two active volcanoes are not hot spots but a kind of back-arc volcanoes associated with the deep subduction of the Pacific slab and its stagnancy in the transition zone as well as corner flow in the big mantle wedge (BMW) above the stagnant slab. The active Tengchong volcano in Southwest China is related to the eastward subduction of the Burma microplate. References: D. Zhao (2004) Phys. Earth Planet. Inter. 146, 3-34. D. Zhao (2007) Gondwana Research 12, 335-355. J. Huang, D. Zhao (2006) J. Geophys. Res. 111, B09305.
S14C-07
Slab Tearing To Be Stagnant
Slabs subducting from Kurile, Japan, Izu-Bonin and Mariana arcs show very complicated configuration. We report a phenomenon of slab tearing now occurring beneath southwest Japan, based on our three dimensional P-wave speed model. The North-Kurile and the Mariana slabs penetrate the 660-km seismic discontinuity into the lower mantle and are stagnant between ~800 - ~ 900 km depths. In particular, the Japan slab and the Izu-Bonin slabs shallow their dips at depths below the 410-km discontinuity to extend near-horizontally along the 660-km discontinuity although the bottoming depths are somewhat different. The near-horizontal slab is expected to move in the dip direction of the subducting slab, which is WNW for the Japan slab and WSW for the Izu-Bonin slab. The tomographic image shows a distinct gap in slab-associated fast anomaly below 300 km depth across the junction of the Japan slab and the Izu-Bonin slab. This gap is best explained by slab tearing caused by the difference in moving direction of the near-horizontal portions of the Japan and Izu-Bonin slabs. The deeper parts of these two slabs are now being torn apart from each other at their junction, where deep shocks 350 - 400 km deep in the vicinity of the junction show anomalous focal mechanisms with lateral tension along the slab strike. This lateral tension mechanism is in marked contrast to the down-dip compression mechanism of deep shocks occurring further away from the junction. The slab tearing creates two opposing walls in the slab along the junction. We detected S-P converted waves generated at the north-faced wall defining the northern end of the Izu-Bonin slab. This wall dips to the north at a steep angle of 75 deg.
S14C-08 INVITED
Can the transition zone test the Plate and Plume hypotheses?
In recent years it has been recognized that the assumption that melting anomalies such as Iceland and
Hawaii are fuelled by mantle plumes has by no means been proven. It has been proposed instead that they
result from shallow processes associated with plate tectonics–the "Plate" model. The mantle plume model
predicts that beneath melting anomalies, material with a temperature anomaly of 300 ± 100 K rises
through conduits that extend from the core-mantle boundary to the surface. They must thus pass through
both the 660-km and 410-km discontinuities. As a consequence, topographic anomalies are expected on the
discontinuities, along with seismic wave-speed anomalies above, within and below the transition zone. The
Plate model, in contrast, predicts that the structures and processes underlying melting anomalies are
confined to the upper mantle, usually the shallowest parts. Structures may occasionally extend through the
410-km discontinuity and on into the transition zone, but rarely, if ever, deeper and through the 660-km
discontinuity.
The 410-km discontinuity is depressed under some melting anomalies, but this observation is permitted by
both the Plate and Plume hypotheses. In principle, the presence or absence of topography on the 660-km
discontinuity is the most diagnostic indicator of whether or not downward-continuous, hot conduits exist. In
practice, however, there are difficulties. It is unclear what the net Clapeyron slope is for the several
mineralogical phase changes that occur in this region. Furthermore, the expected topographic anomaly may
be too small to detect in practice, except for unusually well-instrumented cases. It has also not been clearly
laid out what the effects of partial melt and compositional anomalies are on transition-zone discontinuity
topography. It is thus at present unclear to what extent study of the 660-km discontinuity can test the mantle
plume hypothesis.
http://www.mantleplumes.org/