Cecily J. Wolfe

Seismic anisotropy of oceanic upper mantle: Shear wave splitting methodologies and observations

We develop methodologies to obtain accurate measurements of shear wave splitting and apply these techniques to examine the pattern of oceanic upper mantle anisotropy. To obtain high-quality estimates of receiver splitting at island stations, we devise a stacking method that finds the optimum splitting parameters and 95% error region for teleseismic shear-wave phases from a suite of earthquake events. To obtain additional measurements in oceanic regions, we develop techniques to measure splitting parameters and errors for SS phases that sample upper mantle anisotropy at their bounce points. However, we find the data are often of low resolution, and anomalous characteristics are sometimes found that make splitting difficult to interpret. Ten second splitting of SS is observed across the BANJO seismic array, but we cannot unambiguously attribute this signal to mantle anisotropy at the bounce. The receiver splitting methods are used to assess the adequacy of a two anisotropic layer model for the Pacific region, with fast polarization azimuth (ϕ) in the lithosphere oriented in the fossil spreading direction and ϕ in the asthenosphere oriented in the absolute plate motion direction. This model has been proposed to explain surface wave data in the Pacific Ocean, but our splitting results demonstrate that oceanic anisotropy patterns are more heterogeneous than would be predicted. While the island splitting measurements could reflect the influence of individual hotspot upwellings, hotspot effects do not appear to be universally dominant. We propose that splitting observations alternatively indicate broad-scale differences in the underlying character of oceanic upper mantle anisotropy, associated with coherant patters of lithospheric structure and asthenospheric flow. In particular, splitting, surface wave models, and regional studies all support a model where lithospheric anisotropy throughout the South Pacific has been erased or reoriented toward the absolute plate motion direction, whereas more limited observations in the North Pacific indicate that the fossil lithospheric signature appears to be preserved.

Seismic evidence for a lower-mantle origin of the Iceland plume

Iceland, one of the most thoroughly investigated hotspots, is generally accepted to be the manifestation of an upwelling mantle plume. Yet whether the plume originates from the lower mantle or from a convective instability at a thermal boundary layer between the upper and lower mantle near 660 km depth, remains unconstrained. Tomographic inversions of body-wave delay times show that low seismic velocities extend to at least 400 km depth beneath central Iceland,, but cannot resolve structure at greater depth. Here we report lateral variations in the depths of compressional-to-shear wave conversions at the two seismic discontinuities marking the top and bottom of the mantle transition zone beneath Iceland. We find that the transition zone is 20 km thinner than in the average Earth beneath central and southern Iceland, but is of normal thickness beneath surrounding areas, a result indicative of a hot and narrow plume originating from the lower mantle.

Snail2 is an Essential Mediator of Twist1-Induced Epithelial Mesenchymal Transition and Metastasis

To metastasize, carcinoma cells must attenuate cell-cell adhesion to disseminate into distant organs. A group of transcription factors, including Twist1, Snail1, Snail2, ZEB1, and ZEB2, have been shown to induce epithelial mesenchymal transition (EMT), thus promoting tumor dissemination. However, it is unknown whether these transcription factors function independently or coordinately to activate the EMT program. Here we report that direct induction of Snail2 is essential for Twist1 to induce EMT. Snail2 knockdown completely blocks the ability of Twist1 to suppress E-cadherin transcription. Twist1 binds to an evolutionarily conserved E-box on the proximate Snail2 promoter to induce its transcription. Snail2 induction is essential for Twist1-induced cell invasion and distant metastasis in mice. In human breast tumors, the expression of Twist1 and Snail2 is highly correlated. Together, our results show that Twist1 needs to induce Snail2 to suppress the epithelial branch of the EMT program and that Twist1 and Snail2 act together to promote EMT and tumor metastasis.

Mantle flow, melting, and dehydration of the Iceland mantle plume

Abstract Recent studies have shown that the extraction of water from the mantle due to partial melting beneath mid-ocean ridges may increase the viscosity of the residuum by 2–3 orders of magnitude. We examine this rheological effect on mantle flow and melting of a ridge-centered mantle plume using three-dimensional numerical models. Results indicate that the viscosity increase associated with dehydration prevents buoyancy forces from contributing significantly to plume upwelling above the dry solidus. Consequently, upwelling in the primary melting zone is driven passively by plate spreading and melt production rates are substantially lower than predicted by models that do not include the rheological effect of dehydration. Predictions of along-axis crustal thickness, bathymetric, and gravity variations are shown to be consistent with observations at Iceland and along the Mid-Atlantic Ridge. Furthermore, these predictions result from a model of a plume with relatively high excess temperature (180°C) and narrow radius (100 km) — properties that are consistent with estimates previously inferred from geochemical and seismological observations. Calculations of incompatible trace-element concentrations suggest that observed along-axis geochemical anomalies primarily reflect incompatible element heterogeneity of the plume source.

Mantle Shear-Wave Velocity Structure Beneath the Hawaiian Hot Spot

Earths Plume Plumbing Volcanic hot spots, such as the one that continues to build the Hawaiian Islands, are thought to form by one of two mechanisms: Either mantle plumes bring hot, buoyant material to the surface from deep within the Earths interior, or extensive processing of the upper mantle by plate tectonics causes localized volcanism in stressed or heterogeneous crust. Wolfe et al. (p. 1388; see the cover; see the news story by Kerr) used an extensive array of ocean-bottom and land-based seismometers to reveal the structure of the mantle beneath Hawaii. These high-resolution images reveal a high-temperature plume originating from the lower mantle. Extensive seismological data support a mantle plume origin for the Hawaiian volcanic hot spot. Defining the mantle structure that lies beneath hot spots is important for revealing their depth of origin. Three-dimensional images of shear-wave velocity beneath the Hawaiian Islands, obtained from a network of sea-floor and land seismometers, show an upper-mantle low-velocity anomaly that is elongated in the direction of the island chain and surrounded by a parabola-shaped high-velocity anomaly. Low velocities continue downward to the mantle transition zone between 410 and 660 kilometers depth, a result that is in agreement with prior observations of transition-zone thinning. The inclusion of SKS observations extends the resolution downward to a depth of 1500 kilometers and reveals a several-hundred-kilometer-wide region of low velocities beneath and southeast of Hawaii. These images suggest that the Hawaiian hot spot is the result of an upwelling high-temperature plume from the lower mantle.

Microearthquake characteristics and crustal velocity structure at 29°N on the Mid-Atlantic Ridge: The architecture of a slow spreading segment

We report the results of a microearthquake and seismic tomography experiment conducted along the southern half of the Mid-Atlantic Ridge segment at 29°N and aimed at investigating the relationship of earthquake and seismic structural characteristics to spreading processes. The seismic velocity structure is obtained from two-dimensional (2-D) and three-dimensional (3-D) tomographic inversions of travel times from shots along an axial refraction line. Inversion solutions indicate that the velocity structure in the lower crust is heterogeneous, with higher velocities and relatively thin crust near the segment end and lower velocities and a thicker layer 3 near the central bathymetrie high. The thickness of the lower crust at the segment end is asymmetric across axis, with thinner crust beneath the inside corner. The indicated variations in crustal thickness are consistent with those inferred from mantle Bouguer gravity anomalies. The microearthquakes located along axis during the 41-day recording period cluster in three separate along-axis regions: (1) the southern segment end near 28°55′N, (2) the central along-axis topographic high at 29°11′N, near and north of the Broken Spur hydrothermal vent field, and (3) a region midway between, beneath a volcano near 29°02′N. The greatest level of microearthquake activity was in a diffuse zone off axis beneath the inside corner of a nontransform offset. This pattern of off-axis microearthquake activity, and the cross-axis asymmetry in crustal thickness at the segment end, support tectonic models in which normal faulting and consequent crustal thinning occur preferentially at inside corner regions. Anomalous focal mechanisms for microearthquakes beneath the along-axis volcano and the significant seismicity beneath the axial volcanic ridge at the segment center, in contrast, may be the result of volcanic and hydrothermal processes, such as magma movement or thermal stresses generated near cooling plutons. A comparison of microearthquake characteristics with residual gravity data and velocity structure leads to the hypothesis that microearthquakes associated with areas of thin crust near the segment end and inside corner are dominantly tectonic in nature, whereas microearthquakes associated with volcanic and hydrothermal processes are more likely to occur toward the segment center in areas of greater rates of magma supply and thicker crust. Along axis, well-resolved focal depths determined with a 3-D velocity model range from 3 to 6 km beneath the seafloor and do not shoal toward the segment center. These observations indicate that the thermal structure of the crust along this slow spreading ridge segment is not in steady state.

The Marquesas archipelagic apron: Seismic stratigraphy and implications for volcano growth, mass wasting, and crustal underplating

Multichannel seismic lines, sonobuoy and gravity data across the Marquesas Islands are used to study volcano growth, island mass wasting, and crustal underplating at island chains with overfilled moats. The Marquesas bathymetry reflects the changing thickness of the sedimentary infill rather than the basement topography. The moat contains two major regions of differing seismic stratigraphy: (1) the moat edges, where a unit of continuous layered reflectors is present containing minor lenses of chaotic diffractors and, (2) the central moat, where the deep moat basins are overfilled by an acoustically opaque unit of discontinuous reflectors of up to 2 km thickness, in places capped by a ponded unit. Plate flexure models require broad underplating of the crust by low-density (crustal?) material at the Marquesas Islands to explain the depth to volcanic basement and gravity observations. The seismic velocities and seismic stratigraphy, as well as the general structure of the islands and surrounding seafloor, indicate the apron is mostly debris from island mass wasting. Reflectors of the outermost moat generally onlap the flexural arch in the lower section and offlap and overfill it in the upper section. In the central moat, reflectors change shape from concave up in the lower section to convex in the upper section. Three-dimensional diffusion models of sedimentation, which incorporate a time-dependent seafloor deflection from progressive island loading and vary sediment influx as islands are formed and mass waste, suggest that three main factors make the moat stratigraphy at the Marquesas different from Hawaii: (1) the Marquesas moat is overfilled, while the Hawaiian moat is underfilled, (2) sediment diffusivities are lower at the Marquesas, and (3) the Marquesas islands are separated by deep sedimentary basins, in contrast to Hawaii, where islands are separated by a shallow ridge. The lower sediment diffusivity at the Marquesas may reflect a larger proportion of “blocky”, massive material in the central Marquesas moat or alternatively a change in the dominant process of sediment transport. While there is similar sediment supply for a given along-moat distance at both the Marquesas and Hawaii, the underfilled moat at Hawaii is apparently a consequence of greater moat volumes due to the larger size of the Hawaiian volcanoes, and possibly variations in underplating, that load the plate. The difference in sediment/edifice ratios is likely related to the larger eruption rates at Hawaii and different styles of volcano construction between Hawaii and the Marquesas.

Initial results from the ICEMELT Experiment: Body‐wave delay times and shear‐wave splitting across Iceland

We present results from the first stage of the ICEMELT broadband seismometer experiment designed to determine upper mantle structure beneath Iceland, a hotspot located on the Mid-Atlantic Ridge. Relative delays of teleseismic body waves across Iceland are in excess of l s for P waves and as large as 3 s for S waves. The patterns of P and S wave delays suggest a low-velocity anomaly in the upper few hundred kilometers beneath central Iceland, consistent with the signature of mantle upwelling beneath a hotspot. Shear-wave splitting measurements of the fast polarization direction ϕ and the delay time δt between the fast and slow shear waves have been obtained at several network stations. Splitting times range from 0.7 to 1.7 s, and fast directions are generally between N20°W and N45°W. While splitting times of this magnitude must be primarily signatures of the anisotropy of the Icelandic upper mantle, the directions of fast polarization are inconsistent with simple models of horizontally diverging flow either in the plate spreading direction or radially from the center of the hotspot. A hypothesis consistent with splitting data obtained to date is that the dominant contribution to upper mantle anisotropy is from the large-scale mantle flow field of the North Atlantic.

On the Mathematics of Using Difference Operators to Relocate Earthquakes

This article examines the properties of difference operators that are used to relocate earthquakes and remove path anomaly biases. There are presently three established algorithms based on such techniques: (1) the method of Jordan and Sverdrup (1981), (2) the double-difference method of Got et al. (1994), and (3) the modified double-difference method of Waldhauser and Ellsworth (2001). We show that the underlying mathematics of these three methods are similar, although there are distinct contrasts in how each is adapted. Our results provide insight into the performance of individual methods. Both the Jordan and Sverdrup (1981) and double difference methods (Got et al. , 1994; Waldhauser and Ellsworth, 2001) remove the average path anomaly bias in a set of events, but the equation weighting is more ideal in the first method. Distance dependent weighting in the Waldhauser and Ellsworth (2001) method does not reduce earthquake location-dependent path anomaly bias unless damping is applied, but damping causes the locations between earthquakes spaced far apart to be less well resolved. Alternatively, the results using Jordan and Sverdrup (1981) and Got et al. (1994) only remove a constant bias across a model subregion and cannot resolve the relative locations between subregions. The results of this study indicate that differencing operators contain the fundamental limitation that when the path anomalies from velocity heterogeneity change stongly with earthquake position, the bias effects can be reduced in the relative locations between closely spaced earthquakes, but the effects cannot be reduced in the relative locations between earthquakes spaced far apart. Manuscript received 2 July 2001.

Shear-wave splitting at central Tien Shan: Evidence for rapid variation of anisotropic patterns

At active collisional belts, the fast polarization axis of shear-wave splitting is generally aligned parallel to the strike of the belt, which has been proposed to indicate mantle strain that is coherent with crustal deformation. A notable exception is central Tien Shan, where anomalous patterns of splitting have previously been observed. We here analyze shear-wave splitting of SKS phases across north central Tien Shan using digital data from the Kyrgyzstan Broadband Seismic Network (KNET). We find a pattern of short-wavelength anisotropic heterogeneity that supports complex mantle flow due to small-scale convection. The along-strike variations in mantle structure contrast with the coherent pattern of crustal shortening, and indicate that mantle flow is not directly coupled to crustal deformation in this region.