Variations In Upper Mantle Structure Research Articles

S-wave velocity structure beneath the High Lava Plains, Oregon, from Rayleigh-wave dispersion inversion

The High Lava Plains (HLP) “hotspot” track is a prominent volcanic lineament that extends from the southeast corner of Oregon in the northern Great Basin to Newberry volcano in the eastern Cascades. With the age of silicic volcanism decreasing along track to the northwest, the HLP and Newberry volcano are a rough mirror image to the Eastern Snake River Plain and Yellowstone but, in the case of the HLP, at an orientation strongly oblique to North American plate motion. Since this orientation is incompatible with plate motion over a fixed hotspot, other proposed origins for the HLP, such as asthenospheric inflow around a steepening slab, residual effects of a Columbia River/Steens plume, backarc spreading, and Basin and Range extension, relate it to various tectonic features of the Pacific Northwest. To begin distinguishing between these hypotheses, we image upper-mantle structure beneath the HLP and adjacent tectonic provinces with fundamental-mode Rayleigh waves recorded by stations of the USArray Transportable Array, the recently-initiated HLP seismic experiment, the United States National Seismograph Network, and the Berkeley seismic network. We estimate phase velocities along nearly 300 two-station propagation paths that lie within and adjacent to the HLP and cross the region along two azimuths, parallel to and perpendicular to the HLP track. The dispersion curves, which typically give robust results over the period range 16–171 seconds, are grouped by tectonic region, and the composite curves are inverted for S-wave velocity as a function of depth. We also fit a single plane wave to phase delays across the region to identify laterally homogeneous subregions. The resulting variations in upper-mantle structure correlate with variations in surface volcanism and tectonics. The lowest velocities (∼ 4.1 km/s) occur at ∼ 50 km depth in the SE corner of Oregon, where there has been extensive basaltic volcanism in the past 2–5 kyr, and suggest uppermost mantle temperatures sufficient to produce basaltic partial melting. While the seismic velocities of the uppermost mantle beneath the volcanic High Lava Plains are low relative to the standard Tectonic North America (TNA) model, they are only slightly lower than those found for the adjacent northern Great Basin and they appear to be significantly higher than upper-mantle velocities beneath the Eastern Snake River Plain. Our results provide no evidence for a residual plume signature beneath the HLP region, leaving open questions as to the origin of the HLP volcanic track itself.

Surface wave waveform inversion for variation in upper mantle structure beneath Iceland

We study the structure of the upper mantle beneath Iceland using surface wave waveforms recorded at pairs of stations lying approximately on the same great circles as the sources used. We invert for local, path-average Vs variations between the station pairs. The method used in this study is an extension of an algorithm proposed by Kushnir et al. (1989), which uses only the phase of the seismograms. In our waveform inversion not only the phases but also the amplitudes of the surface waves are used as structural constraints. We illustrate the resolution power of the new algorithm with synthetic examples. We apply the method to study upper mantle structure beneath Iceland using recordings of three events with northerly, south-southwesterly and easterly orientated paths and 19 station pairs. Depending on the separation distance of the stations, we invert waveforms in the frequency range 0.0166–0.08 Hz and 0.01–0.08 Hz. Resolution is limited by the penetration depth of the surface wave fundamental mode, and is good down to ∼150 km for the narrower frequency band and ∼200 km for the wider band. Although the inversions of the differential waveforms only provide information on lateral Vs variations between station pairs, the main structural features of the upper mantle beneath Iceland are retrieved. We confirm that the strongest negative Vs anomalies of up to ∼−5 per cent underlie central Iceland, and extend down to the limit of our resolution at ∼200 km. The rift zones away from central Iceland are underlain by low velocities in the depth range ∼50–100 km and high velocities below this, indicating that they are shallowly sourced. Such a structure also underlies northwest Vatnajokull, where a mantle plume is traditionally assumed to lie. Beneath intraplate areas, mantle structural variations are small. Using our method, smaller-scale mantle structures are detectable than is possible with teleseismic tomography, which tends to smear anomalies throughout larger volumes.

Lithospheric structure and along-strike segmentation of the Central Andean Plateau: seismic Q, magmatism, flexure, topography and tectonics

The Central Andean plateau is a 300-km-wide, nearly 4-km-high plateau which is situated above a 30°E-dipping segment of the subducted Nazca plate. Systematic changes in the topography, the upper mantle seismic attenuation ( Q) structure, the lithospheric flexural rigidity and the distribution and chemistry of back-arc lavas indicate that the upper mantle structure changes along strike beneath the plateau. South of 23°S, the upper mantle beneath the plateau becomes hotter, and the lithosphere becomes thinner and weaker. This lateral variation in upper mantle structure coincides with variations in the tectonic style and timing of deformation in two physiographically distinct segments of the plateau and its adjacent foreland thrust belt to the east: the Bolivian Altiplano and Subandean ranges in the north and the Argentine Puna and Santa Barbara system in the south. The segments' different mantle lid thicknesses result in an Altiplano supported mainly by crustal thickening and a Puna supported by a combination of a crustal root and a thermal mantle root. A thicker, stronger foreland lithosphere in the north favors thin-skinned underthrusting of the foreland beneath the plateau, while to the south, the thinner, weaker lithosphere allows more basement-involved shortening within the foreland and less underthrusting beneath the plateau. A weaker lithosphere results in a longer period of compressional deformation and greater local and structural relief in the Puna than in the relatively stable Altiplano to the north. Ultimately, this observed NS-directed segmentation on the plateau and in the foreland may reflect the different responses of strong and weak continental lithosphere to an “Andean” type subduction margin.

Variations In Upper Mantle Structure Under Northern Australia

SUMMARY Temporary array deployments of short-period seismometers in northern Australia have been used to build up composite record sections for waves interacting with the upper mantle. Stable measures of the seismic wavefield are provided by stacking the complex envelopes of all the seismic waveforms falling in a 10km distance interval away from the source. Two groups of sources (a) along the Flores Arc, Indonesia with propagation under northwestern Australia, and (b) in New Guinea with paths to the NNE of the array, have been used to construct composite record sections for both P and SV waves over the distance range 1300–2800 km. the timing and amplitude distributions for P waves from the two regions show noticeable differences. Detailed modelling of the record sections yields velocity models with significant variation in velocity for the two sets of propagation paths for which the midpoints are separated by about 1000km. The short-period SV-wave sections indicate efficient propagation of highfrequency S waves in a lithosphere extending down to 210km. Arrivals from the deeper mantle cannot be correlated with confidence because of a loss in high-frequency content revealed by broad-band observations. This requires a significant attenuation zone for S beneath 210 km.

Attenuation of high‐frequency seismic waves beneath the central Andean Plateau

Observed patterns of high‐frequency seismic wave attenuation suggest that near 22°S, the upper mantle structure beneath the central Andean plateau changes along strike to the south. Contrasting regions of high and low upper mantle seismic wave attenuation beneath the plateau are identified based on striking azimuthal variations in the character and frequency content of P and S waves propagating beneath the plateau to a portable seismic network deployed in Jujuy Province, Argentina (24°S, 65°W). Ray paths from intermediate depth earthquakes located north and northwest of the network transmit seismic waves with a higher frequency content than ray paths from earthquakes at similar depths and distances but located west and south of the network. The estimated apparent Q values fall into two categories: Qp >500 and Qs >350 for the high‐Q paths, and Qp <350 and Qs < 200 for the low‐Q paths. In addition, Sn phases from regional crustal earthquakes in the Subandean foreland fold‐thrust belt to the north propagate efficiently to the Jujuy network, while Sn is not observed from foreland earthquakes located at similar distances to the south of the network. These observations combined with data from previously reported wave propagation studies suggest that south of about 22° S, the upper mantle beneath the plateau and its adjacent foreland thrust belt is more highly attenuating than the upper mantle farther north. Forward modeling of the Q measurements made at Jujuy indicates that the observations can be explained either by a variable thickness high‐Q upper plate beneath the plateau, or by a thin, variable width, very low‐Q zone in the asthenospheric wedge above the subducted slab. This lateral variation in upper mantle structure coincides with two physiographically distinct segments of the central Andean plateau and its adjacent foreland thrust belt to the east: the Bolivian Altiplano and Subandean ranges in the north and the Argentine Puna and Santa Barbara system in the south. We interpret the north‐south change in upper mantle attenuation and the corresponding changes in physiography, topography, and tectonic style at the surface to be due to a mantle lid that is thicker beneath the Altiplano and the Subandean belt than beneath the Puna and the Santa Barbara ranges.

Lateral Variations in Upper Mantle Structure of the Philippine Sea Basin

The Chinkuashih-Shuinantung area is mainly composed of outcrops from theMiocene Nankang Formation.Crustal movements after the late Pliocene have ledto several high-angle faults and lifted the Shitih Formation to the surface.Duringthe Pleistocene,dacite intruded into Miocene sediments and thereafter hydrothermalgold-copper ore deposits were formed.These deposits are composed mostly of metalsulfides,including pyrite,enargite,galena,tetrahedrite,and sphalerite,togetherwith limited native gold.All these sulfides may produce acid mine drainages.Theriver bed course of the Chiufen Chi is rather close to the fault traces,indicating aclose relationship between water flow systems and tectonic movements.Water and soil samples were collected from several selected sites,includingthe upper,middle,and lower stream areas of the Chiufen Chi;along with thewest,south,and east banks of the Shuinantung Bay;in addition,mine watersdrained directly from the Penshan 5th adit and 7th adit were also collected neartheir portals.Geochemical data such as pH value,water temperature,turbidity,conductivity,dissolved oxygen,and selected ionic concentrations,were measuredlater.All samples were collected and analyzed during each season of a year.Theresults indicate that water drained from the two mine adits and the lower streamarea of the Chinfen Chi exhibit low pH values,high metal(esp.Cu and Fe)andsulfate contents,and great conductivities.The acid mine drainage effect is veryobvious,especially during the rainy season.The Transient Electro-Magnetic(TEM)method,using square loops 20 m on aside,was also applied in this area for prospecting purposes.Measured transientvoltages were inverted in terms of 1-D models.Four-layered models consistingof low-high-low-low resistivity patterns best fit the 49 coincident loop data.Theeffective exploration depth was between 5 m to 50m.Furthermore,resistivitiesmeasured along the Chiufen Chi varied with the season,indicating high porosity andpermeability in rocks underlying the study area,probably due to ore mineralization,faults or joints;on the other hand,resistivities measured along the ShuinantungBay are not affected by seasonal change,indicating a saturated aquifer underneaththe area near the sea.

A review of geophysical constraints on the deep structure of the Tibetan Plateau, the Himalaya and the Karakoram, and their tectonic implications

A review of geophysical constraints on the deep structure of the Tibetan Plateau, the Himalaya and the Karakoram, and their tectonic implications

Regional variations in upper mantle structure beneath Southern California

The large seismographic network in Southern California provides a unique opportunity for studying seismic velocity variations within this tectonically active region, which includes a major plate boundary whose surface expression is the San Andreas Fault. In this study, the compressional velocity within the upper mantle beneath Southern California is investigated using observations of the dependence of teleseismic P-wave delays at all stations on the distance and azimuth to the event. The variation of residuals with azimuth, which may be as large as 1.3 s at a single station, requires considerable heterogeneity within the upper mantle. Models for the velocity structure are derived from the delays by manual ray tracing and by automatic inversion techniques. These models are generally consistent, although the inversion technique leads to considerable smoothing, and include regions of low velocity beneath the Imperial Valley (∼ 30-100 km depth; Δv/v∼ - 4 per cent) and the central Mojave Desert and northern Salton Trough (∼ 100-180 km; Δv/v ∼ - 2.5 per cent) and high velocities beneath the south-western Sierra Nevada (∼ 50-180 km; Δv/v ∼ 5 per cent) and much of the Transverse Ranges (∼ 50-180 km; Δv/v ∼ 4 per cent). The last observation is in good agreement with the findings of Hadley & Kanamori. No evidence is found for velocity differences across the San Andreas Fault at any depth, and there is no marked lithosphere—asthenosphere boundary; however, the velocity variations do correlate with the general tectonic provinces within Southern California.

Regionalized shear velocity models of the Pacific upper mantle from observed Love and Rayleigh wave dispersion

Summary. Group and phase velocities for periods between 16 and 110 s have been determined using the single-station method along 33 Rayleigh-and 30 Love-wave paths across the Pacific. These dispersion data were used to detect variations in upper mantle structure beneath the Pacific. Taking both regional changes and azimuthal anisotropy into consideration, regionalized group and phase velocities were derived for four regions of differing age. The boundaries of the regions were chosen to correspond to isochrons of 20, 50 and 100 Myr, as inferred from geomagnetic lineations. A small, but significant degree of azimuthal anisotropy (less than 0.8 per cent) was found to occur throughout the Pacific if the degree and direction of maximum velocity were assumed to be uniform everywhere. The average direction of maximum velocity was found to be 85°± 6° as measured clockwise from north. Regionalized group and phase velocities for Rayleigh waves at the larger periods increase systematically with increasing age of the ocean floor. For Love waves, departures from a systematic increase occur in the youngest (0–20 Myr) and oldest (> 100 Myr) portions of the Pacific. The dispersion data were corrected for the effects of anelasticity and were inverted to obtain shear velocity models for each of the four regions using the stochastic inversion method. A comparison of the shear wave models derived from Rayleigh waves in the four regions indicates that average shear velocities at depths between 30 and 110 km increase rapidly with age. This increase occurs mainly because the depth to the base of the lithosphere increases; however, the shear velocities of the lithosphere also increase for the regions out to 100 Myr in age. Slightly lower velocities in the upper part of the lithosphere for greater ages may be due to a large influx of mid-plate volcanism during the Cretaceous. Separate inversions of Love-and Rayleigh-wave velocities indicate that polarization anisotropy can be resolved for the lithosphere. Polarization anisotropy cannot, be resolved for the low-velocity zone beneath the Pacific using these data, except possibly in the youngest regions, where lateral complexities of the East Pacific Rise are apt to affect the data adversely. Assuming that we can adequately invert our Love-and Rayleigh-wave data separately to obtain information on anisotropy, we find that average SV velocities in the lithosphere increase from about 4.3 to 4.6 km/s, and average SH velocities increase from about 4.5 to 4.7 km/s, from the youngest to oldest regions. Average velocities in the upper portion of the low-velocity zone increase from about 4.1 to about 4.25 km/s for the same regions. The existence of anisotropy in the lithosphere is constant with the results of seismic refraction experiments in the Pacific and supports peroxide as the predominant upper mantle material. The occurrence of anisotropy through-out the entire lithosphere, suggests a situation in which periodite crystallizes at the base of the lithosphere from a partially molten low-velocity zone and olivine crystals become aligned in response to the stress field within the Pacific plate.

Lateral variations in upper-mantle structure around India as obtained from Gauribidanur seismic array data

Summary. In order to understand the tectonic processes responsible for the motion of lithospheric plates, it would be desirable to resolve as much information as possible on the detailed vertical and lateral structure that may be present in the upper mantle. From the point of view of continental drift, the Indian subcontinent is of particular interest because of the manner that it has rapidly moved northwards and collided with the Asian continent. The Himalayan mountains to the north, the mid-oceanic ridges to the south and the earthquake belts surrounding the Indian plate all show that the uppermantle structure in that area must have some very significant lateral variations. The Gauribidanur medium aperture seismic array (GBA) in southern India is a very well located array for obtaining data for upper-mantle research. In this paper we present the results from a study of over 300 earthquakes (distance 14°-36° and azimuth 0°-360°) recorded at this array. Adaptive processing techniques were employed on the first 36 s of the short-period P-wavetrains in an attempt to not only determine lateral variations in velocity but also to locate and identify the positions of the triplications on the upper-mantle travel-the branches which are associated with the major phase discontinuities or high-velocity gradient zones of the upper mantle. Four seismic zones defined by the following azimuthal ranges were considered: (a) Himalayan (0°-90°), (b) Java trench (90°-180°), (c) Midoceanic ridge (180° -280°) and (d) Hindukush (280°-360°). Inversion of the data has revealed that there is a considerable evidence for a larger lowvelocity or low-Q layer along the oceanic paths as compared to the continental paths. Further evidence for the existence of ‘400-km’ and ‘650-km’ discontinuities was found with the exception of the Himalayan region. In this region the replacement of the latter discontinuity by a broad high-velocity gradient zone was much more consistent with the observed data. Consistent slowness anomaly patterns, presented in the form of an array diagram, were observed for the Java trench and the Mid-Indian oceanic ridge. The most significant feature of the azhuthal anomaly pattern was the distinct absence of any positive anomalies from events occurring in the Ridge. Although upper-mantle velocity measurements were obtained over all directions about the array, it was not possible to carry out multi-directional velocity-measurements in the same region. However, it is felt that this study will compliment any other future work on anisotropy around India. There is very little evidence from our analysis for the existence of anistropy at depths below 500 km.