Structure of the European upper mantle revealed by adjoint tomography

NASA Astrophysics Data System (ADS)

Zhu, Hejun; Bozdağ, Ebru; Peter, Daniel; Tromp, Jeroen

2012-07-01

Images of the European crust and upper mantle, created using seismic tomography, identify the Cenozoic Rift System and related volcanism in central and western Europe. They also reveal subduction and slab roll back in the Mediterranean-Carpathian region. However, existing tomographic models are either high in resolution, but cover only a limited area, or low in resolution, and thus miss the finer-scale details of mantle structure. Here we simultaneously fit frequency-dependent phase anomalies of body and surface waveforms in complete three-component seismograms with an iterative inversion strategy involving adjoint methods, to create a tomographic model of the European upper mantle. We find that many of the smaller-scale structures such as slabs, upwellings and delaminations that emerge naturally in our model are consistent with existing images. However, we also derive some hitherto unidentified structures. Specifically, we interpret fast seismic-wave speeds beneath the Dinarides Mountains, southern Europe, as a signature of northeastward subduction of the Adria plate; slow seismic-wave speeds beneath the northern part of the Rhine Graben as a reservoir connected to the Eifel hotspot; and fast wave-speed anomalies beneath Scandinavia as a lithospheric drip, where the lithosphere is delaminating and breaking away. Our model sheds new light on the enigmatic palaeotectonic history of Europe.

Mantle transition zone thinning beneath eastern Africa: Evidence for a whole-mantle superplume structure

NASA Astrophysics Data System (ADS)

Mulibo, Gabriel D.; Nyblade, Andrew A.

2013-07-01

to S conversions from the 410 and 660 km discontinuities observed in receiver function stacks reveal a mantle transition zone that is ~30-40 km thinner than the global average in a region ~200-400 km wide extending in a SW-NE direction from central Zambia, across Tanzania and into Kenya. The thinning of the transition zone indicates a ~190-300 K thermal anomaly in the same location where seismic tomography models suggest that the lower mantle African superplume structure connects to thermally perturbed upper mantle beneath eastern Africa. This finding provides compelling evidence for the existence of a continuous thermal structure extending from the core-mantle boundary to the surface associated with the African superplume.

Structure of Lithospheric and Upper Mantle Discontinuities beneath Central Mongolia from Receiver Functions

NASA Astrophysics Data System (ADS)

Cui, Z.; Meltzer, A.; Fischer, K. M.; Stachnik, J. C.; Munkhuu, U.; Tsagaan, B.; Russo, R. M.

2017-12-01

The origin and preservation of high-elevation low-relief surfaces in continental interiors remains an open questions. Central Mongolia constitutes a major portion of the Mongolian Plateau and is an excellent place to link deep earth and surface processes. The lithosphere of Mongolia was constructed through accretionary orogenesis associated with the Central Asian Orogenic Belt (CAOB) from the late Paleozoic to the early Triassic. Alkaline volcanic basalt derived from sublithospheric sources has erupted sporadically in Mongolia since 30 Ma. Constraining the depth variation of lithospheric and upper mantle discontinuities is crucial for understanding the interaction between upper mantle structure and surface topography. We conducted receiver functions (RF) analyses suitable data recorded at112 seismic broadband stations in central Mongolia to image the LAB and mantle transition zone beneath Central Mongolia. A modified H-κ stacking was performed to determine crustal average thickness (H) and Vp/Vs ratio (κ). Central Mongolia is characterized by thick crust (43-57 km) enabling use of both P wave RF and to S wave RF to image the LAB. The PRF traces in the depth domain are stacked based on piercing point locations for the 410 and 660 discontinuities using 0.6 ° × 0.6 ° bins in a grid. From south to north, the average lithospheric thickness is 85km in Gobi Altai gradually thinning northeastward to 78km in the southern Hangay Dome, 72 km in the northern Hangay Dome then increases to 75km in Hovsgol area. While there is overall thinning of the lithosphere from SW to NE, beneath the Hangay, there is a slight increase beneath the highest topography. The thickness of the mantle transition zone (MTZ) beneath central Mongolia is similar to global averages. This evidence argues against the hypothesis that a mantle plume exists beneath Central Mongolia causing low velocity anomalies in the upper mantle. To the east of the Hovsgol area in northern Mongolia, the MTZ thickens 10-15 km mainly due to depression in the 660-km discontinuity, perhaps representing a relict of subducted plate during CAOB.

Three-dimensional shear wave velocity structure in the Atlantic upper mantle

NASA Astrophysics Data System (ADS)

James, Esther Kezia Candace

Oceanic lithosphere constitutes the upper boundary layer of the Earth's convecting mantle. Its structure and evolution provide a vital window on the dynamics of the mantle and important clues to how the motions of Earth's surface plates are coupled to convection in the mantle below. The three-dimensional shear-velocity structure of the upper mantle beneath the Atlantic Ocean is investigated to gain insight into processes that drive formation of oceanic lithosphere. Travel times are measured for approximately 10,000 fundamental-mode Rayleigh waves, in the period range 30-130 seconds, traversing the Atlantic basin. Paths with >30% of their length through continental upper mantle are excluded to maximize sensitivity to the oceanic upper mantle. The lateral distribution of Rayleigh wave phase velocity in the Atlantic upper mantle is explored with two approaches. One, phase velocity is allowed to vary only as a function of seafloor age. Two, a general two-dimensional parameterization is utilized in order to capture perturbations to age-dependent structure. Phase velocity shows a strong dependence on seafloor age, and removing age-dependent velocity from the 2-D maps highlights areas of anomalously low velocity, almost all of which are proximal to locations of hotspot volcanism. Depth-dependent variations in vertically-polarized shear velocity (Vsv) are determined with two sets of 3-D models: a layered model that requires constant VSV in each depth layer, and a splined model that allows VSV to vary continuously with depth. At shallow depths (˜75 km) the seismic structure shows the expected dependence on seafloor age. At greater depths (˜200 km) high-velocity lithosphere is found only beneath the oldest seafloor; velocity variations beneath younger seafloor may result from temperature or compositional variations within the asthenosphere. The age-dependent phase velocities are used to constrain temperature in the mantle and show that, in contrast to previous results for the Pacific, phase velocities for the Atlantic are not consistent with a half-space cooling model but are best explained by a plate-cooling model with thickness of 75 km and mantle temperature of 1400°C. Comparison with data such as basalt chemistry and seafloor elevation helps to separate thermal and compositional effects on shear velocity.

East African upper mantle shear wave velocity structure derived from Rayleigh wave tomography

NASA Astrophysics Data System (ADS)

O'Donnell, J.; Nyblade, A.; Adams, A. N.; Mulibo, G.; Tugume, F.

2011-12-01

An expanded model of the three-dimensional shear wave velocity structure of the upper mantle beneath East Africa is being developed using data from the latest phases of the AfricaArray East African Seismic Experiment in conjunction with data from preceding studies. The combined dataset encompasses seismic stations which span Tanzania, Uganda and Zambia. From the new data, fundamental mode Rayleigh wave phase velocities are being measured at periods ranging from 20 to 180 seconds using the two-plane-wave method. These measurements will be combined with similarly processed measurements from previous studies and inverted for an upper mantle three-dimensional shear wave velocity model. In particular, the model will further constrain the morphology of the low velocity anomaly which underlies the East African Plateau extending to the southwest beneath Zambia.

The African Lithosphere

NASA Astrophysics Data System (ADS)

Priestley, K.; Debayle, E.; McKenzie, D.; Pilidou, S.

2007-12-01

There have been a number of prior, large scale surface wave studies of Africa, the majority of which rely on fundamental mode observations. In this study we use a large data set of multi-mode surface waves recorded over epicentral distances most of which are shorter than 6000 km, to investigate the Sv wave speed heterogeneity of the upper mantle beneath Africa. The inclusion of the higher mode data allow us to build an upper mantle model for the African plate with a horizontal resolution of a few hundred kilometers and a vertical resolution of a few tens of kilometers extending to about 400 km depth. Our tomographic images of the upper mantle beneath Africa displays significant shear velocity features, much of which correlate with surface geology. High velocity mantle persists beneath the West African and Congo cratons to 225-250 km depth, but the high velocity root beneath Kalahari Craton extends to only about 175 km depth. Low velocity upper mantle underlies the Pan- African terranes of Africa with the exception of the Damara mobile belt separating the Congo and Kalahari Cratons. The Damara mobile belt is underlain by a thick high velocity upper mantle lid which is indistinguishable from that beneath the Congo Craton to the north and the Kalahari Craton to the south. Low velocity upper mantle underlie the Hoggar, Tebesti and Darfur volcanic areas of northern Africa, and very low velocities underlie the Afar region to at least 400 km depth. We use the relationship between shear velocity and temperature of Priestley & McKenzie (2006) to derive a model for the African thermal lithosphere. Two types of lithosphere underlie Africa. Thick lithosphere underlies most of western Africa and all of southern Africa; in the latter the extent of the thick lithosphere is significantly different from the distribution of Archean crust mapped at the surface. Thick lithosphere forms one continuous structure beneath the Congo and Kalahari Cratons. Other than the Pan-African Damara mobile belt, the only Pan-African terrane of Africa free of recent (<30 Ma) volcanism, all of the Pan- African is underlain by lithosphere whose thickness is too thin to be resolved by our current surface wave analysis.

Mantle transition zone structure beneath India and Western China from migration of PP and SS precursors

NASA Astrophysics Data System (ADS)

Lessing, Stephan; Thomas, Christine; Rost, Sebastian; Cobden, Laura; Dobson, David P.

2014-04-01

We investigate the seismic structure of the upper-mantle and mantle transition zone beneath India and Western China using PP and SS underside reflections off seismic discontinuities, which arrive as precursors to the PP and SS arrival. We use high-resolution array seismic techniques to identify precursory energy and to map lateral variations of discontinuity depths. We find deep reflections off the 410 km discontinuity (P410P and S410S) beneath Tibet, Western China and India at depths of 410-440 km and elevated underside reflections of the 410 km discontinuity at 370-390 km depth beneath the Tien Shan region and Eastern Himalayas. These reflections likely correspond to the olivine to wadsleyite phase transition. The 410 km discontinuity appears to deepen in Central and Northern Tibet. We also find reflections off the 660 km discontinuity beneath Northern China at depths between 660 and 700 km (P660P and S660S) which could be attributed to the mineral transformation of ringwoodite to magnesiowuestite and perovskite. These observations could be consistent with the presence of cold material in the middle and lower part of the mantle transition zone in this region. We also find a deeper reflector between 700 and 740 km depth beneath Tibet which cannot be explained by a depressed 660 km discontinuity. This structure could, however, be explained by the segregation of oceanic crust and the formation of a neutrally buoyant garnet-rich layer beneath the mantle transition zone, due to subduction of oceanic crust of the Tethys Ocean. For several combinations of sources and receivers we do not detect arrivals of P660P and S660S although similar combinations of sources and receivers give well-developed P660P and S660S arrivals. Our thermodynamic modelling of seismic structure for a range of compositions and mantle geotherms shows that non-observations of P660P and S660S arrivals could be caused by the dependence of underside reflection coefficients on the incidence angle of the incoming seismic waves. Apart from reflections off the 410 and 660 km discontinuities, we observe intermittent reflectors at 300 and 520 km depth. The discontinuity structure of the study region likely reflects lateral thermal and chemical variations in the upper-mantle and mantle transition zone connected to past and present subduction and mantle convection processes.

Upper-mantle origin of the Yellowstone hotspot

USGS Publications Warehouse

Christiansen, R.L.; Foulger, G.R.; Evans, J.R.

2002-01-01

Fundamental features of the geology and tectonic setting of the northeast-propagating Yellowstone hotspot are not explained by a simple deep-mantle plume hypothesis and, within that framework, must be attributed to coincidence or be explained by auxiliary hypotheses. These features include the persistence of basaltic magmatism along the hotspot track, the origin of the hotspot during a regional middle Miocene tectonic reorganization, a similar and coeval zone of northwestward magmatic propagation, the occurrence of both zones of magmatic propagation along a first-order tectonic boundary, and control of the hotspot track by preexisting structures. Seismic imaging provides no evidence for, and several contraindications of, a vertically extensive plume-like structure beneath Yellowstone or a broad trailing plume head beneath the eastern Snake River Plain. The high helium isotope ratios observed at Yellowstone and other hotspots are commonly assumed to arise from the lower mantle, but upper-mantle processes can explain the observations. The available evidence thus renders an upper-mantle origin for the Yellowstone system the preferred model; there is no evidence that the system extends deeper than ???200 km, and some evidence that it does not. A model whereby the Yellowstone system reflects feedback between upper-mantle convection and regional lithospheric tectonics is able to explain the observations better than a deep-mantle plume hypothesis.

Peeling linear inversion of upper mantle velocity structure with receiver functions

NASA Astrophysics Data System (ADS)

Shen, Xuzhang; Zhou, Huilan

2012-02-01

A peeling linear inversion method is presented to study the upper mantle (from Moho to 800 km depth) velocity structures with receiver functions. The influences of the crustal and upper mantle velocity ratio error on the inversion results are analyzed, and three valid measures are taken for its reduction. This method is tested with the IASP91 and the PREM models, and the upper mantle structures beneath the stations GTA, LZH, and AXX in northwestern China are then inverted. The results indicate that this inversion method is feasible to quantify upper mantle discontinuities, besides the discontinuities between 3 h M ( h M denotes the depth of Moho) and 5 h M due to the interference of multiples from Moho. Smoothing is used to overcome possible false discontinuities from the multiples and ensure the stability of the inversion results, but the detailed information on the depth range between 3 h M and 5 h M is sacrificed.

New constraints on the upper mantle structure of the Slave craton from Rayleigh wave inversion

NASA Astrophysics Data System (ADS)

Chen, Chin-Wu; Rondenay, Stéphane; Weeraratne, Dayanthie S.; Snyder, David B.

2007-05-01

Rayleigh wave phase and amplitude data are analyzed to provide new insight into the velocity structure of the upper mantle beneath the Slave craton, in the northwestern Canadian Shield. We invert for phase velocities at periods between 20 s-142 s (with greatest sensitivity at depths of 28-200 km) using crossing ray paths from events recorded by the POLARIS broadband seismic network and the Yellowknife array. Phase velocities obtained for the Slave province are comparable to those from other cratons at shorter periods, but exceed the global average by ~2% at periods above 60 s, suggesting that the Slave craton may be an end member in terms of its high degree of mantle depletion. The one-dimensional inversion of phase velocities yields high upper-mantle S-wave velocities of 4.7 +/- 0.2 km/s that persist to 220 +/- 65 km depth and thus define the cratonic lithosphere. Azimuthal anisotropy is well resolved at all periods with a dominant fast direction of N59°E +/- 20°, suggesting that upper mantle anisotropy beneath the Slave craton is influenced by both lithospheric fabric and sub-lithospheric flow.

Upper mantle velocity structure beneath southern Africa from modeling regional seismic data

NASA Astrophysics Data System (ADS)

Zhao, Ming; Langston, Charles A.; Nyblade, Andrew A.; Owens, Thomas J.

1999-03-01

The upper mantle seismic velocity structure beneath southern Africa is investigated using travel time and waveform data which come from a large mine tremor in South Africa (mb 5.6) recorded by the Tanzania broadband seismic experiment and by several stations in southern Africa. The waveform data show upper mantle triplications for both the 410- and 670-km discontinuities between distances of 2100 and 3000 km. Auxiliary travel time data along similar profiles obtained from other moderate events are also used. P wave travel times are inverted for velocity structure down to ˜800-km depth using the Wiechert-Herglotz technique, and the resulting model is evaluated by perturbing it at three depth intervals and then testing the perturbed model against the travel time and waveform data. The results indicate a typical upper mantle P wave velocity structure for a shield. P wave velocities from the top of the mantle down to 300-km depth are as much as 3% higher than the global average and are slightly slower than the global average between 300- and 420-km depth. Little evidence is found for a pronounced low-velocity zone in the upper mantle. A high-velocity gradient zone is required above the 410-km discontinuity, but both sharp and smooth 410-km discontinuities are permitted by the data. The 670-km discontinuity is characterized by high-velocity gradients over a depth range of ˜80 km around 660-km depth. Limited S wave travel time data suggest fast S wave velocities above ˜150-km depth. These results suggest that the bouyant support for the African superswell does not reside at shallow depths in the upper mantle.

Evidence for long-lived subduction of an ancient tectonic plate beneath the southern Indian Ocean: Ancient Slab Beneath the Indian Ocean

DOE Office of Scientific and Technical Information (OSTI.GOV)

Simmons, N. A.; Myers, S. C.; Johannesson, G.

In this study, ancient subducted tectonic plates have been observed in past seismic images of the mantle beneath North America and Eurasia, and it is likely that other ancient slab structures have remained largely hidden, particularly in the seismic-data-limited regions beneath the vast oceans in the Southern Hemisphere. Here we present a new global tomographic image, which shows a slab-like structure beneath the southern Indian Ocean with coherency from the upper mantle to the core-mantle boundary region—a feature that has never been identified. We postulate that the structure is an ancient tectonic plate that sank into the mantle along anmore » extensive intraoceanic subduction zone that migrated southwestward across the ancient Tethys Ocean in the Mesozoic Era. Slab material still trapped in the transition zone is positioned near the edge of East Gondwana at 140 Ma suggesting that subduction terminated near the margin of the ancient continent prior to breakup and subsequent dispersal of its subcontinents.« less

Evidence for long-lived subduction of an ancient tectonic plate beneath the southern Indian Ocean: Ancient Slab Beneath the Indian Ocean

DOE PAGES

Simmons, N. A.; Myers, S. C.; Johannesson, G.; ...

2015-11-14

In this study, ancient subducted tectonic plates have been observed in past seismic images of the mantle beneath North America and Eurasia, and it is likely that other ancient slab structures have remained largely hidden, particularly in the seismic-data-limited regions beneath the vast oceans in the Southern Hemisphere. Here we present a new global tomographic image, which shows a slab-like structure beneath the southern Indian Ocean with coherency from the upper mantle to the core-mantle boundary region—a feature that has never been identified. We postulate that the structure is an ancient tectonic plate that sank into the mantle along anmore » extensive intraoceanic subduction zone that migrated southwestward across the ancient Tethys Ocean in the Mesozoic Era. Slab material still trapped in the transition zone is positioned near the edge of East Gondwana at 140 Ma suggesting that subduction terminated near the margin of the ancient continent prior to breakup and subsequent dispersal of its subcontinents.« less

Heterogeneous seismic anisotropy in the transition zone and uppermost lower mantle: evidence from South America, Izu-Bonin and Japan

NASA Astrophysics Data System (ADS)

Lynner, Colton; Long, Maureen D.

2015-06-01

Measurements of seismic anisotropy are commonly used to constrain deformation in the upper mantle. Observations of anisotropy at mid-mantle depths are, however, relatively sparse. In this study we probe the anisotropic structure of the mid-mantle (transition zone and uppermost lower mantle) beneath the Japan, Izu-Bonin, and South America subduction systems. We present source-side shear wave splitting measurements for direct teleseismic S phases from earthquakes deeper than 300 km that have been corrected for the effects of upper mantle anisotropy beneath the receiver. In each region, we observe consistent splitting with delay times as large as 1 s, indicating the presence of anisotropy at mid-mantle depths. Clear splitting of phases originating from depths as great as ˜600 km argues for a contribution from anisotropy in the uppermost lower mantle as well as the transition zone. Beneath Japan, fast splitting directions are perpendicular or oblique to the slab strike and do not appear to depend on the propagation direction of the waves. Beneath South America and Izu-Bonin, splitting directions vary from trench-parallel to trench-perpendicular and have an azimuthal dependence, indicating lateral heterogeneity. Our results provide evidence for the presence of laterally variable anisotropy and are indicative of variable deformation and dynamics at mid-mantle depths in the vicinity of subducting slabs.

Crustal and mantle velocity models of southern Tibet from finite frequency tomography

NASA Astrophysics Data System (ADS)

Liang, Xiaofeng; Shen, Yang; Chen, Yongshun John; Ren, Yong

2011-02-01

Using traveltimes of teleseismic body waves recorded by several temporary local seismic arrays, we carried out finite-frequency tomographic inversions to image the three-dimensional velocity structure beneath southern Tibet to examine the roles of the upper mantle in the formation of the Tibetan Plateau. The results reveal a region of relatively high P and S wave velocity anomalies extending from the uppermost mantle to at least 200 km depth beneath the Higher Himalaya. We interpret this high-velocity anomaly as the underthrusting Indian mantle lithosphere. There is a strong low P and S wave velocity anomaly that extends from the lower crust to at least 200 km depth beneath the Yadong-Gulu rift, suggesting that rifting in southern Tibet is probably a process that involves the entire lithosphere. Intermediate-depth earthquakes in southern Tibet are located at the top of an anomalous feature in the mantle with a low Vp, a high Vs, and a low Vp/Vs ratio. One possible explanation for this unusual velocity anomaly is the ongoing granulite-eclogite transformation. Together with the compressional stress from the collision, eclogitization and the associated negative buoyancy force offer a plausible mechanism that causes the subduction of the Indian mantle lithosphere beneath the Higher Himalaya. Our tomographic model and the observation of north-dipping lineations in the upper mantle suggest that the Indian mantle lithosphere has been broken laterally in the direction perpendicular to the convergence beneath the north-south trending rifts and subducted in a progressive, piecewise and subparallel fashion with the current one beneath the Higher Himalaya.

Upper mantle structure of central and West Antarctica from array analysis of Rayleigh wave phase velocities

NASA Astrophysics Data System (ADS)

Heeszel, David S.; Wiens, Douglas A.; Anandakrishnan, Sridhar; Aster, Richard C.; Dalziel, Ian W. D.; Huerta, Audrey D.; Nyblade, Andrew A.; Wilson, Terry J.; Winberry, J. Paul

2016-03-01

The seismic velocity structure of Antarctica is important, both as a constraint on the tectonic history of the continent and for understanding solid Earth interactions with the ice sheet. We use Rayleigh wave array analysis methods applied to teleseismic data from recent temporary broadband seismograph deployments to image the upper mantle structure of central and West Antarctica. Phase velocity maps are determined using a two-plane wave tomography method and are inverted for shear velocity using a Monte Carlo approach to estimate three-dimensional velocity structure. Results illuminate the structural dichotomy between the East Antarctic Craton and West Antarctica, with West Antarctica showing thinner crust and slower upper mantle velocity. West Antarctica is characterized by a 70-100 km thick lithosphere, underlain by a low-velocity zone to depths of at least 200 km. The slowest anomalies are beneath Ross Island and the Marie Byrd Land dome and are interpreted as upper mantle thermal anomalies possibly due to mantle plumes. The central Transantarctic Mountains are marked by an uppermost mantle slow-velocity anomaly, suggesting that the topography is thermally supported. The presence of thin, higher-velocity lithosphere to depths of about 70 km beneath the West Antarctic Rift System limits estimates of the regionally averaged heat flow to less than 90 mW/m2. The Ellsworth-Whitmore block is underlain by mantle with velocities that are intermediate between those of the West Antarctic Rift System and the East Antarctic Craton. We interpret this province as Precambrian continental lithosphere that has been altered by Phanerozoic tectonic and magmatic activity.

Upper mantle structure of the Tonga-Lau-Fiji region from Rayleigh wave tomography

NASA Astrophysics Data System (ADS)

Wei, S. Shawn; Zha, Yang; Shen, Weisen; Wiens, Douglas A.; Conder, James A.; Webb, Spahr C.

2016-11-01

We investigate the upper mantle seismic structure in the Tonga-Lau-Fiji region by jointly fitting the phase velocities of Rayleigh waves from ambient-noise and two-plane-wave tomography. The results suggest a wide low-velocity zone beneath the Lau Basin, with a minimum SV-velocity of about 3.7 ± 0.1 km/s, indicating upwelling hot asthenosphere with extensive partial melting. The variations of velocity anomalies along the Central and Eastern Lau Spreading Centers suggest varying mantle porosity filled with melt. In the north where the spreading centers are distant from the Tonga slab, the inferred melting commences at about 70 km depth, and forms an inclined zone in the mantle, dipping to the west away from the arc. This pattern suggests a passive decompression melting process supplied by the Australian plate mantle from the west. In the south, as the supply from the Australian mantle is impeded by the Lau Ridge lithosphere, flux melting controlled by water from the nearby slab dominates in the back-arc. This source change results in the rapid transition in geochemistry and axial morphology along the spreading centers. The remnant Lau Ridge and the Fiji Plateau are characterized by a 60-80 km thick lithosphere underlain by a low-velocity asthenosphere. Our results suggest the removal of the lithosphere of the northeastern Fiji Plateau-Lau Ridge beneath the active Taveuni Volcano. Azimuthal anisotropy shows that the mantle flow direction rotates from trench-perpendicular beneath Fiji to spreading-perpendicular beneath the Lau Basin, which provides evidence for the southward flow of the mantle wedge and the Samoan plume.

Deep seismic structure and tectonics of northern Alaska: Crustal-scale duplexing with deformation extending into the upper mantle

USGS Publications Warehouse

Fuis, G.S.; Murphy, J.M.; Lutter, W.J.; Moore, Thomas E.; Bird, K.J.; Christensen, N.I.

1997-01-01

Seismic reflection and refraction and laboratory velocity data collected along a transect of northern Alaska (including the east edge of the Koyukuk basin, the Brooks Range, and the North Slope) yield a composite picture of the crustal and upper mantle structure of this Mesozoic and Cenozoic compressional orogen. The following observations are made: (1) Northern Alaska is underlain by nested tectonic wedges, most with northward vergence (i.e., with their tips pointed north). (2) High reflectivity throughout the crust above a basal decollement, which deepens southward from about 10 km depth beneath the northern front of the Brooks Range to about 30 km depth beneath the southern Brooks Range, is interpreted as structural complexity due to the presence of these tectonic wedges, or duplexes. (3) Low reflectivity throughout the crust below the decollement is interpreted as minimal deformation, which appears to involve chiefly bending of a relatively rigid plate consisting of the parautochthonous North Slope crust and a 10- to 15-km-thick section of mantle material. (4) This plate is interpreted as a southward verging tectonic wedge, with its tip in the lower crust or at the Moho beneath the southern Brooks Range. In this interpretation the middle and upper crust, or all of the crust, is detached in the southern Brooks Range by the tectonic wedge, or indentor: as a result, crust is uplifted and deformed above the wedge, and mantle is depressed and underthrust beneath this wedge. (5) Underthrusting has juxtaposed mantle of two different origins (and seismic velocities), giving rise to a prominent sub-Moho reflector. Copyright 1997 by the American Geophysical Union.

Thermal and mechanical structure of the upper mantle: A comparison between continental and oceanic models

NASA Technical Reports Server (NTRS)

Froidevaux, C.; Schubert, G.; Yuen, D. A.

1976-01-01

Temperature, velocity, and viscosity profiles for coupled thermal and mechanical models of the upper mantle beneath continental shields and old ocean basins show that under the continents, both tectonic plates and the asthenosphere, are thicker than they are beneath the oceans. The minimum value of viscosity in the continental asthenosphere is about an order of magnitude larger than in the shear zone beneath oceans. The shear stress or drag underneath continental plates is also approximately an order of magnitude larger than the drag on oceanic plates. Effects of shear heating may account for flattening of ocean floor topography and heat flux in old ocean basins.

Receiver function imaging of the mantle discontinuties beneath Fennoscandia and northern Europe

NASA Astrophysics Data System (ADS)

Frassetto, Andrew; Thybo, Hans

2010-05-01

Receiver functions from the Mantle Investigations of Norwegian Uplift Structure experiment (MAGNUS) are depth-converted using interval wavespeeds from AK-135 for the 410-km and 660-km discontinuities and combined using common-conversion-point stacking. This preliminary work shows a potentially complex mantle-transition-zone beneath southern Norway, with reduction in the amplitude of the 410-arrival and 20-30 km of shallowing of the 660-arrival beneath the axis of the Oslo Rift. To refine these measurements and place them in a regional context, we incorporate the MAGNUS dataset with permanent stations and previous temporary seismic deployments across Fennoscandia and northern Europe. New constraints on the depth to the lithosphere-asthenosphere boundary and character of the mantle-transition-zone will aid in understanding the causes for potentially recent uplift in the southern Scandes and the region of unusually slow upper mantle resolved beneath the region (Weidle and Maupin, 2008).

Imaging Lithospheric-scale Structure Beneath Northern Altiplano in Southern Peru and Northern Bolivia

NASA Astrophysics Data System (ADS)

Kumar, A.; Wagner, L. S.; Beck, S. L.; Zandt, G.; Long, M. D.

2014-12-01

The northern Altiplano plateau of southern Peru and northern Bolivia is one of the highest topographic features on the Earth, flanked by Western and Eastern Cordillera along its margin. It has strongly influenced the local and far field lithospheric deformation since the early Miocene (Masek et al., 1994). Previous studies have emphasized the importance of both the crust and upper mantle in the evolution of Altiplano plateau (McQuarrie et al., 2005). Early tomographic and receiver function studies, south of 16° S, show significant variations in the crust and upper mantle properties in both perpendicular and along strike direction of the Altiplano plateau (Dorbath et. al., 1993; Myers et al., 1998; Beck and Zandt, 2002). In order to investigate the nature of subsurface lithospheric structure below the northern Altiplano, between 15-18° S, we have determined three-dimensional seismic tomography models for Vp and Vs using P and S-wave travel time data from two recently deployed local seismic networks of CAUGHT and PULSE. We also used data from 8 stations from the PERUSE network (PERU Subduction Experiment). Our preliminary tomographic models show a complex variation in the upper mantle velocity structure with depth, northwest and southeast of lake Titicaca. We see the following trend, at ~85 km depth, northwest of lake Titicaca: low Vp and Vs beneath the Western Cordillera, high Vs beneath the Altiplano and low Vp and Vs beneath the Eastern Cordillera. This low velocity anomaly, beneath Eastern Cordillera, seems to coincide with Kimsachata, a Holocene volcano in southern Peru. At depth greater than ~85 km: we find high velocity anomaly beneath the Western Cordillera and low Vs beneath the Altiplano. This high velocity anomaly, beneath Western Cordillera, coincides with the well-located Wadati-Benioff zone seismicity and perhaps represents the subducting Nazca slab. On the southeast of lake Titicaca, in northern Bolivia, we see a consistently high velocity anomaly that continues deeper as a westward dipping high velocity structure in the upper mantle. Our further in depth modelling and tomographic constraints on Vp/Vs would allow us to better resolve the lithospheric features that we see in our preliminary tomographic models and their possible correlations with the evolution of northern Altiplano plateau.

Control of the Lithospheric Mantle on intracontinental Deformation: Revival of Eastern U.S. Tectonism

NASA Astrophysics Data System (ADS)

Biryol, C. B.; Wagner, L. S.; Fischer, K. M.; Hawman, R. B.

2016-12-01

The present tectonic configuration of the southeastern United States is a product of earlier episodes of arc accretion, continental collision and breakup. This region is located in the interior of the North American Plate, some 1500 km away from closest active plate margin. However, there is ongoing tectonism across the area with multiple zones of seismicity, rejuvenation of the Appalachians of North Carolina, Virginia, and Pennsylvania, and Cenozoic intraplate volcanism. The mechanisms controlling this activity and the modern-day state of stress remain enigmatic. Two factors often regarded as major contributors are plate strength and preexisting inherited structures. Recent improvements in broadband seismic data coverage in the region associated with the South Eastern Suture of the Appalachian Margin Experiment (SESAME) and EarthScope Transportable Array make it possible to obtain detailed information on the structure of the lithosphere in the region. Here we present new tomographic images of the upper mantle beneath the Southeastern United States, revealing large-scale structural variations in the upper mantle. Our results indicate fast seismic velocity patterns that can be interpreted as ongoing lithospheric foundering. We observe an agreement between the locations of these upper mantle anomalies and the location of major zones of tectonism, volcanism and seismicity, providing a viable explanation for modern-day activity in this plate interior setting long after it became a passive margin. Based on distinct variations in the geometry and thickness of the lithospheric mantle and foundered lithosphere, we propose that piecemeal delamination has occurred beneath the region throughout the Cenozoic, removing a significant amount of reworked/deformed mantle lithosphere. Ongoing lithospheric foundering beneath the eastern margin of stable North America explains significant variations in thickness of lithospheric mantle across the former Grenville deformation front.

A Review of Geophysical Constraints on the Deep Structure of the Tibetan Plateau, the Himalaya and the Karakoram, and their Tectonic Implications

NASA Astrophysics Data System (ADS)

Molnar, P.

1988-09-01

The Tibetan Plateau, the Himalaya and the Karakoram are the most spectacular consequences of the collision of the Indian subcontinent with the rest of Eurasia in Cainozoic time. Accordingly, the deep structures beneath them provide constraints on both the tectonic history of the region and on the dynamic processes that have created these structures. The dispersion of seismic surface waves requires that the crust beneath Tibet be thick: nowhere less than 50 km, at least 65 km, in most areas, but less than 80 km in all areas that have been studied. Wide-angle reflections of P-waves from explosive sources in southern Tibet corroborate the existence of a thick crust but also imply the existence of marked lateral variations in that thickness, or in the velocity structure of the crust. Thus isostatic compensation occurs largely by an Airy-type mechanism, unlike that, for instance, of the Basin and Range Province of western North America where a hot upper mantle buoys up a thin crust. The P-wave and S-wave velocities in the uppermost mantle of most of Tibet are relatively high and typical of those of Precambrian shields and stable platforms: Vp = 8.1 km s-1 or higher, and Vs≈ 4.7 km s-1. Travel times and waveforms of S-waves passing through the uppermost mantle of much of Tibet, however, require a much lower average velocity in the uppermost mantle than that of the Indian, or other, shields. They indicate a thick low-velocity zone in the upper mantle beneath Tibet, reminiscent of tectonically active regions. These data rule out a shield structure beneath northern Tibet and suggest that if such a structure does underlie part of the plateau, it does so only beneath the southern part. Lateral variations in the upper-mantle structure of Tibet are apparent from differences in travel times of S-waves from earthquakes in different parts of Tibet, in the attenuation of short-period phases, Pn and Sn, that propagate through the uppermost mantle of Tibet, and in surface-wave dispersion for different paths. The notably lower velocities and the greater attenuation in the mantle of north--central Tibet than elsewhere imply higher temperatures there and are consistent with the occurrence of active and young volcanism in roughly the same area. Surface-wave dispersion across north--central Tibet also requires a thinner crust in that area than in most of the plateau. Consequently the relatively uniform height of the plateau implies that isostatic compensation in the north--central part of Tibet occurs partly because the density of the relatively hot material in the upper mantle is lower than that elsewhere beneath Tibet, the mechanism envisioned by Pratt. Several seismological studies provide evidence consistent with a continuity of the Indian Shield, and its cold thick lithosphere, beneath the Himalaya. Fault-plane solutions and focal depths of the majority of moderate earthquakes in the Himalaya are consistent with their occurring on the top surface of the gently flexed, intact Indian plate that has underthrust the Lesser Himalaya roughly 80-100 km or more. P-waves from explosions in southern Tibet and recorded in Nepal can be interpreted as wide-angle reflections from this fault zone. P-wave delays across the Tarbela network in Pakistan from distant earthquakes indicate a gentle dip of the Moho beneath the array without pronounced later variations in upper-mantle structure. High Pn and Sn velocities beneath the Himalaya and normal to early S-wave arrival times from Himalayan earthquakes recorded at teleseismic distances are consistent with Himalaya being underlain by the same structure that underlies India. Results from explosion seismology indicate an increase in crustal thickness from the Indo--Gangetic Plain across the Himalaya to southern Tibet, but Hirn, Lepine, Sapin and their co-workers inferred that the depth of the Moho does not increase smoothly northward, as it would if the Indian Shield had been underthrust coherently beneath the Himalaya. They interpreted wide-angle reflections as evidence for steps in the Moho displaced from one another on southward-dipping faults. Although I cannot disprove this interpretation, I think that one can recognize a sequence of signals on their wide-angle reflection profiles that could be wide-angle reflections from a northward-dipping Moho. Gravity anomalies across the Himalaya show that both the Indo--Gangetic Plain and the Himalaya are not in local isostatic equilibrium. A mass deficit beneath the plain is apparently caused by the flexure of the Indian Shield and by the low density of the sedimentary rock in the basin formed by the flexure. The mass excess in the Himalaya seems to be partly supported by the strength of the Indian plate, for which the flexural rigidity is particularly large. An increase in the Bouguer gravity gradient from about 1 mGal km-1 (1 mGal = 10-3 cm s-2) over the Indo--Gangetic Plain to 2 mGal km-1 over the Himalaya implies a marked steepening of the Moho, and therefore a greater flexure of the Indian plate, beneath the Himalaya. This implies a northward decrease in the flexural rigidity of the part of the Indian plate underlying the range. Nevertheless, calculations of deflections of elastic plates with different flexural rigidities and flexed by the weight of the Himalaya show larger deflections and yield more negative gravity anomalies than are observed. Thus, some other force, besides the flexural strength of the plate, must contribute to the support of the range. A bending moment applied to the end of the Indian plate could flex the plate up beneath the range and provide the needed support. The source of this moment might be gravity acting on the mantle portion of the subducting Indian continental lithosphere with much or all of the crust detached from it. Seismological studies of the Karakoram are consistent with its being underlain by particularly cold material in the upper mantle. Intermediate-depth earthquakes occur between depths of 70 and 100 km but apparently do not define a zone of subducted oceanic lithosphere. Rayleigh-wave phase velocities are particularly high for paths across this area and imply high shear wave velocities in the upper mantle. Isostatic gravity anomalies indicate a marked low of 70 mGal over the Karakoram, which could result from a slightly thickened crust pulled down by the sinking of cold material beneath it. Geophysical constraints on the structure of Tibet, the Himalaya and the Karakoram are consistent with a dynamic uppermost mantle that includes first, the plunging of cold material into the asthenosphere beneath southern Tibet and the Karakoram, as the Indian plate slides beneath the Himalaya, and second, an upwelling of hot material beneath north--central Tibet. The structure is too poorly resolved to require such dynamic flow, but the existence for both a hot uppermost mantle beneath north--central Tibet and a relatively cold uppermost mantle beneath southern Tibet and the Karakoram seem to be required. Both group and phase velocities of Rayleigh waves and Love waves are delayed along paths crossing Tibet. The low velocities require a crustal thickness in excess of 50 km, and for most regions in excess of 60 km. Crustal thicknesses in excess of 80 km can be ruled out for all paths studied, and for most of Tibet, a crustal thickness of 65-70 km seems required. Clear evidence for lateral heterogeneity beneath Tibet is provided not only by body waves (discussed below) but also by surface waves (Brandon & Romanowicz 1986), which show an area of lower uppermost shear-wave velocity and thinner crust in north--central Tibet than elsewhere in the plateau. These variations might explain the differences in group velocities measured by different workers, and the different structures that they deduced, but if so, they also render the regionalization of surface-wave dispersion into arbitrary tectonic provinces risky. Although Rayleigh-wave phase velocities can resolve large differences in upper-mantle velocities for regions the size of Tibet, constraints on these velocities are best derived from body waves. Thus, with the exceptions of Brandon & Romanowicz's (1986) detailed investigation of north--central Tibet, the study of southernmost Tibet by Jobert et al. (1985) and that of Romanowicz (1982) for the northeasternmost part of the plateau, I do not think that surface waves have placed an important bound on the velocity in the upper mantle beneath Tibet. The seismic data are broadly consistent with partial melting of the uppermost mantle of north--central Tibet, where recent volcanism has been observed. Correspondingly, there is no suggestion of such low velocities, and such high temperatures, in the mantle elsewhere beneath Tibet, for which late-Cainozoic volcanism has not been reported. The results are also consistent with a slightly thinner crust in north--central Tibet than farther south, suggesting that both Airy and Pratt isostasy share compensation for north--central Tibet's great height. Finally, the average shear-wave velocity in the upper mantle of southern Tibet seems to be higher than that in northern Tibet, but neither is the degree of difference well determined, nor is the location of the transition from one to the other well mapped.

New insight into the Upper Mantle Structure Beneath the Pacific Ocean Using PP and SS Precursors

NASA Astrophysics Data System (ADS)

Gurrola, H.; Rogers, K. D.

2013-12-01

The passing of the EarthScope Transportable array has provided a dense data set that enabled beam forming of SS and PP data that resultes in improved frequency content to as much a 1 Hz in the imaging of upper mantle structure. This combined with the application of simultaneous iterative deconvolution has resulted in images to as much as 4 Hz. The processing however results in structure being averaged over regions of 60 to 100 km in radius. This is becomes a powerful new tool to image the upper mantle beneath Oceanic regions where locating stations is expensive and difficult. This presentation will summarize work from a number of regions as to new observations of the upper mantle beneath the Pacific and Arctic Oceans. Images from a region of the Pacific Ocean furthest from hot spots or subduction zones (we will refer to this as the 'reference region'). show considerable layering in the upper mantle. The 410 km discontinuity is always imaged using these tools and appears to be a very sharp boundary. It does usually appear as an isolated positive phase. There appears to be a LAB at ~100 km as expected but there is a strong negative phase at ~ 200 km with a positive phase 15 km deeper. This is best explained as a lens of partial melt as expected for this depth based on the geothermal gradient. If so this should be a low friction point and so we would expect it to accommodate plate motion. Imaging of the Aleutian subduction zone does show the 100 km deep LAB as it descends but this 200 km deep horizon appears as a week descending positive anomaly without the shallower negative pulse. In addition to the 410, 100 and 200 km discontinuities there are a number of paired anomalies, between the 200 and 400 km depths, with a negative pulse 15 to 20 km shallower then the positive pulse. We do not believe these are side lobes or we would see side lobes on the 100 km and 410 km discontinuities. We believe these to be the result of friction induced partial melt along zones of critical failure to accommodate differential mantle flow with depth. The paired layers disappear beneath the Hawaiian Island chain. We believe heat from the hot spot warms the mantle beneath the Hawaiian island chain so flow is more easily accommodated. As a result the lenses of melt disappear in the region near hot spots.

New Constraints on Upper Mantle Structure Underlying the Diamondiferous Central Slave Craton, Canada, from Teleseismic Body Wave Tomography

NASA Astrophysics Data System (ADS)

Esteve, C.; Schaeffer, A. J.; Audet, P.

2017-12-01

Over the past number of decades, the Slave Craton (Canada) has been extensively studied for its diamondiferous kimberlites. Not only are diamonds a valuable resource, but their kimberlitic host rocks provide an otherwise unique direct source of information on the deep upper mantle (and potentially transition zone). Many of the Canadian Diamond mines are located within the Slave Craton. As a result of the propensity for diamondiferous kimberlites, it is imperative to probe the deep mantle structure beneath the Slave Craton. This work is further motivated by the increase in high-quality broadband seismic data across the Northern Canadian Cordillera over the past decade. To this end we have generated a P and S body wave tomography model of the Slave Craton and its surroundings. Furthermore, tomographic inversion techniques are growing ever more capable of producing high resolution Earth models which capture detailed structure and dynamics across a range of scale lengths. Here, we present preliminary results on the structure of the upper mantle underlying the Slave Craton. These results are generated using data from eight different seismic networks such as the Canadian National Seismic Network (CNSN), Yukon Northwest Seismic Network (YNSN), older Portable Observatories for Lithospheric Analysis and Reseach Investigating Seismicity (POLARIS), Regional Alberta Observatory for Earthquake Studies Network (RV), USArray Transportable Array (TA), older Canadian Northwest Experiment (CANOE), Batholith Broadband (XY) and the Yukon Observatory (YO). This regional model brings new insights about the upper mantle structure beneath the Slave Craton, Canada.

Lithospheric radial anisotropy beneath the Gulf of Mexico

NASA Astrophysics Data System (ADS)

Chu, Risheng; Ko, Justin Yen-Ting; Wei, Shengji; Zhan, Zhongwen; Helmberger, Don

2017-05-01

The Lithosphere-Asthenosphere Boundary (LAB), where a layer of low viscosity asthenosphere decouples with the upper plate motion, plays an essential role in plate tectonics. Most dynamic modeling assumes that the shear velocity can be used as a surrogate for viscosity which provides key information about mantle flow. Here, we derive a shear velocity model for the LAB structure beneath the Gulf of Mexico allowing a detailed comparison with that beneath the Pacific (PAC) and Atlantic (ATL). Our study takes advantage of the USArray data from the March 25th, 2013 Guatemala earthquake at a depth of 200 km. Such data is unique in that we can observe a direct upward traveling lid arrival which remains the first arrival ahead of the triplications beyond 18°. This extra feature in conjunction with upper-mantle triplication sampling allows good depth control of the LAB and a new upper-mantle seismic model ATM, a modification of ATL, to be developed. ATM has a prominent low velocity zone similar to the structure beneath the western Atlantic. The model contains strong radial anisotropy in the lid where VSH is about 6% faster than VSV. This anisotropic feature ends at the bottom of the lithosphere at about the depth of 175 km in contrast to the Pacific where it extends to over 300 km. Another important feature of ATM is the weaker velocity gradient from the depth of 175 to 350 km compared to Pacific models, which may be related to differences in mantle flow.

The thermal regimes of the upper mantle beneath Precambrian and Phanerozoic structures up to the thermobarometry data of mantle xenoliths

NASA Astrophysics Data System (ADS)

Glebovitsky, V. A.; Nikitina, L. P.; Khiltova, V. Ya.; Ovchinnikov, N. O.

2004-05-01

The thermal state of the upper mantle beneath tectonic structures of various ages and types (Archaean cratons, Early Proterozoic accretionary and collisional orogens, and Phanerozoic structures) is characterized by geotherms and by thermal gradients (TG) derived from data on the P- T conditions of mineral equilibria in garnet and garnet-spinel peridotite xenoliths from kimberlites (East Siberia, Northeastern Europe, India, Central Africa, North America, and Canada) and alkali basalts (Southeastern Siberia, Mongolia, southeastern China, southeastern Australia, Central Africa, South America, and the Solomon and Hawaiian islands). The use of the same garnet-orthopyroxene thermobarometer (Theophrastus Contributions to Advanced Studies in Geology. 3: Capricious Earth: Models and Modelling of Geologic Processes and Objects 2000 44) for all xenoliths allowed us to avoid discrepancies in estimation of the P- T conditions, which may be a result of the mismatch between different thermometers and barometers, and to compare the thermal regimes in the mantle in various regions. Thus, it was established that (1) mantle geotherms and geothermal gradients, obtained from the estimation of P- T equilibrium conditions of deep xenoliths, correspond to the age of crust tectonic structures and respectively to the time of lithosphere stabilization; it can be suggested that the ancient structures of the upper mantle were preserved within continental roots; (2) thermal regimes under continental mantle between the Archaean cratons and Palaeoproterozoic belts are different today; (3) the continental mantle under Neoproterozoic and Phanerozoic belts is characterized by significantly higher values of geothermal gradient compared to the mantle under Early Precambrian structures; (4) lithosphere dynamics seems to change at the boundary between Early and Mezo-Neoproterozoic and Precambrian and Phanerozoic.

Seismic evidence for a tilted mantle plume and north-south mantle flow beneath Iceland

USGS Publications Warehouse

Shen, Y.; Solomon, S.C.; Bjarnason, I. Th; Nolet, G.; Morgan, W.J.; Allen, R.M.; Vogfjord, K.; Jakobsdottir, S.; Stefansson, R.; Julian, B.R.; Foulger, G.R.

2002-01-01

Shear waves converted from compressional waves at mantle discontinuities near 410- and 660-km depth recorded by two broadband seismic experiments in Iceland reveal that the center of an area of anomalously thin mantle transition zone lies at least 100 km south of the upper-mantle low-velocity anomaly imaged tomographically beneath the hotspot. This offset is evidence for a tilted plume conduit in the upper mantle, the result of either northward flow of the Icelandic asthenosphere or southward flow of the upper part of the lower mantle in a no-net-rotation reference frame. ?? 2002 Elsevier Science B.V. All rights reserved.

Seismic Constraints on the Mantle Viscosity Structure beneath Antarctica

NASA Astrophysics Data System (ADS)

Wiens, Douglas; Heeszel, David; Aster, Richard; Nyblade, Andrew; Wilson, Terry

2015-04-01

Lateral variations in upper mantle viscosity structure can have first order effects on glacial isostatic adjustment. These variations are expected to be particularly large for the Antarctic continent because of the stark geological contrast between ancient cratonic and recent tectonically active terrains in East and West Antarctica, respectively. A large misfit between observed and predicted GPS rates for West Antarctica probably results in part from the use of a laterally uniform viscosity structure. Although not linked by a simple relationship, mantle seismic velocities can provide important constraints on mantle viscosity structure, as they are both largely controlled by temperature and water content. Recent higher resolution seismic models for the Antarctic mantle, derived from data acquired by new seismic stations deployed in the AGAP/GAMSEIS and ANET/POLENET projects, offer the opportunity to use the seismic velocity structure to place new constraints on the viscosity of the Antarctic upper mantle. We use an Antarctic shear wave velocity model derived from array analysis of Rayleigh wave phase velocities [Heeszel et al, in prep] and examine a variety of methodologies for relating seismic, thermal and rheological parameters to compute a suite of viscosity models for the Antarctic mantle. A wide variety of viscosity structures can be derived using various assumptions, but they share several robust common elements. There is a viscosity contrast of at least two orders of magnitude between East and West Antarctica at depths of 80-250 km, reflecting the boundary between cold cratonic lithosphere in East Antarctica and warm upper mantle in West Antarctica. The region beneath the Ellsworth-Whitmore Mtns and extending to the Pensacola Mtns. shows intermediate viscosity between the extremes of East and West Antarctica. There are also significant variations between different parts of West Antarctica, with the lowest viscosity occurring beneath the Marie Byrd Land (MBL). The MBL Dome and adjacent coastal areas show extremely low viscosity (~1018Pa-s) for most parameterizations, suggesting that low mantle viscosity may produce a very rapid response to ice mass loss in this region.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Adams, Aubreya N.; Wiens, Douglas A.; Nyblade, Andrew A.

The Cameroon Volcanic Line (CVL) is a 1800 km long volcanic chain, extending SW-NE from the Gulf of Guinea into Central Africa, that lacks the typical age progression exhibited by hot spot-related volcanic tracks. Our study investigates the upper mantle seismic structure beneath the CVL and surrounding regions to constrain the origin of volcanic lines that are poorly described by the classic plume model. Rayleigh wave phase velocities are measured at periods from 20 to 182 s following the two-plane wave methodology, using data from the Cameroon Seismic Experiment, which consists of 32 broadband stations deployed between 2005 and 2007.more » These phase velocities are then inverted to build a model of shear wave velocity structure in the upper mantle beneath the CVL. Our results show that phase velocities beneath the CVL are reduced at all periods, with average velocities beneath the CVL deviating more than –2% from the regional average and +4% beneath the Congo Craton. This distinction is observed for all periods but is less pronounced for the longest periods measured. Inversion for shear wave velocity structure indicates a tabular low velocity anomaly directly beneath the CVL at depths of 50 to at least 200 km and a sharp vertical boundary with faster velocities beneath the Congo Craton. Finally, these observations demonstrate widespread infiltration or erosion of the continental lithosphere beneath the CVL, most likely caused by mantle upwelling associated with edge-flow convection driven by the Congo Craton or by lithospheric instabilities that develop due to the nearby edge of the African continent.« less

Mantle transition zone structure beneath the Canadian Shield

NASA Astrophysics Data System (ADS)

Thompson, D. A.; Helffrich, G. R.; Bastow, I. D.; Kendall, J. M.; Wookey, J.; Eaton, D. W.; Snyder, D. B.

2010-12-01

The Canadian Shield is underlain by one of the deepest and most laterally extensive continental roots on the planet. Seismological constraints on the mantle structure beneath the region are presently lacking due to the paucity of stations in this remote area. Presented here is a receiver function study on transition zone structure using data from recently deployed seismic networks from the Hudson Bay region. High resolution images based on high signal-to-noise ratio data show clear arrivals from the 410 km and 660 km discontinuities, revealing remarkably little variation in transition zone structure. Transition zone thickness is close to the global average (averaging 245 km across the study area), and any deviations in Pds arrival time from reference Earth models can be readily explained by upper-mantle velocity structure. The 520 km discontinuity is not a ubiquitous feature, and is only weakly observed in localised areas. These results imply that the Laurentian root is likely confined to the upper-mantle and if any mantle downwelling exists, possibly explaining the existence of Hudson Bay, it is also confined to the upper 400 km. Any thermal perturbations at transition zone depths associated with the existence of the root, whether they be cold downwellings or elevated temperatures due to the insulating effect of the root, are thus either non-existent or below the resolution of the study.

Seismic Structure of Mantle Transition Zone beneath Northwest Pacific Subduction Zone and its Dynamic Implication

NASA Astrophysics Data System (ADS)

Li, J.; Guo, G.; WANG, X.; Chen, Q.

2017-12-01

The northwest Pacific subduction region is an ideal location to study the interaction between the subducting slab and upper mantle discontinuities. Various and complex geometry of the Pacific subducting slab can be well traced downward from the Kuril, Japan and Izu-Bonin trench using seismicity and tomography images (Fukao and Obayashi, 2013). Due to the sparse distribution of seismic stations in the sea, investigation of the deep mantle structure beneath the broad sea regions is very limited. In this study, we applied the well- developed multiple-ScS reverberations method (Wang et al., 2017) to analyze waveforms recorded by the Chinese Regional Seismic Network, the densely distributed temporary seismic array stations installed in east Asia. A map of the topography of the upper mantle discontinuities beneath the broad oceanic regions in northwest Pacific subduction zone is imaged. We also applied the receiver function analysis to waveforms recorded by stations in northeast China and obtain the detailed topography map beneath east Asia continental regions. We then combine the two kinds of topography of upper mantle discontinuities beneath oceanic and continental regions respectively, which are obtained from totally different methods. A careful image matching and spatial correlation is made in the overlapping study regions to calibrate results with different resolution. This is the first time to show systematically a complete view of the topography of the 410-km and 660-km discontinuities beneath the east Asia "Big mantle wedge" (Zhao and Ohtani, 2009) covering the broad oceanic and continental regions in the Northwestern Pacific Subduction zone. Topography pattern of the 660 and 410 is obtained and discussed. Especially we discovered a broad depression of the 410-km discontinuity covering more than 1000 km in lateral, which seems abnormal in the cold subducting tectonic environment. Based on plate tectonic reconstruction studies and HTHP mineral experiments, we argue that the east-retreat trench motion of the subducting Pacific slab might play an important role in the observed broad depression of the 410-km discontinuity.

Rheological properties of the lower crust and upper mantle beneath Baja California: a microstructural study of xenoliths from San Quintin

NASA Astrophysics Data System (ADS)

Van der Werf, Thomas F.; Chatzaras, Vasileios; Tikoff, Basil; Drury, Martyn R.

2016-04-01

Baja California is an active transtensional rift zone, which links the San Andreas Fault with the East Pacific Rise. The erupted basalts of the Holocene San Quintin volcanic field contain xenoliths, which sample the lower crust and upper mantle beneath Baja California. The aim of this research is to gain insight in the rheology of the lower crust and the upper mantle by investigating the xenolith microstructure. Microstructural observations have been used to determine the dominant deformation mechanisms. Differential stresses were estimated from recrystallized grain size piezometry of plagioclase and clinopyroxene for the lower crust and olivine for the upper mantle. The degree of deformation can be inferred from macroscopic foliations and the deformation microstructures. Preliminary results show that both the lower crust and the upper mantle have been affected by multiple stages of deformation and recrystallization. In addition the dominant deformation mechanism in both the lower crust and the upper mantle is dislocation creep based on the existence of strong crystallographic preferred orientations. The differential stress estimates for the lower crust are 10-29 MPa using plagioclase piezometry and 12-35 MPa using clinopyroxene piezometry. For the upper mantle, differential stress estimates are 10-20 MPa. These results indicate that the strength of the lower crust and the upper mantle are very similar. Our data do not fit with the general models of lithospheric strength and may have important implications for the rheological structure of the lithosphere in transtensional plate margins and for geodynamic models of the region.

African hot spot volcanism: small-scale convection in the upper mantle beneath cratons.

PubMed

King, S D; Ritsema, J

2000-11-10

Numerical models demonstrate that small-scale convection develops in the upper mantle beneath the transition of thick cratonic lithosphere and thin oceanic lithosphere. These models explain the location and geochemical characteristics of intraplate volcanos on the African and South American plates. They also explain the presence of relatively high seismic shear wave velocities (cold downwellings) in the mantle transition zone beneath the western margin of African cratons and the eastern margin of South American cratons. Small-scale, edge-driven convection is an alternative to plumes for explaining intraplate African and South American hot spot volcanism, and small-scale convection is consistent with mantle downwellings beneath the African and South American lithosphere.

Radar-Sounding of Icy Mantles and Comets Using Natural Radio Noise

NASA Astrophysics Data System (ADS)

Winebrenner, D. P.; Sahr, J. D.

2011-10-01

Radar-sounding of ice sheets on Earth yields crucial information on ice history and dynamics, including discoveries of subglacial lakes beneath 3-4 km of ice [1]. Mars Express and the Mars Reconnaissance Orbiter (MRO) have now demonstrated the corresponding power of orbital radar sounding for planetary exploration, in particular by imaging structures within and beneath kilometers of Martian water ice [2-4]. Based on this experience, a sophisticated orbital radar sounder is planned for a flagship mission to Europa, with the aim of imaging stratigraphy, faults, diapirs and other geological structure in the upper few kilometers of the water-ice mantle there, and possibly even detecting the upper surface of the (likely) underlying ocean [5]. Recent modeling of the formation and evolution of volatilerich bodies suggests that oceans or lakes of liquid water occur beneath water-ice mantles in a surprising variety of places, including Ceres in the outer asteroid belt [6], 3 of the 4 Galilean moons of Jupiter as well as Enceladus and Titan in the Saturnian system [7], and possibly even Pluto [8]. Thus there is now a wide scope for low-cost missions to bodies of exceptional interest, and for radar sounding of icy mantles to image near-surface structural geology related to underlying water (whether past or present).

The upper mantle beneath the Cascade Range: A comparison with the Gulf of California

NASA Technical Reports Server (NTRS)

Walck, M. C.

1984-01-01

Seismograms from 22 earthquakes along the northeast Pacific rim recorded in southern California form the data set for investigation of the upper mantle beneath the Cascade Range-Juan de Fuca region, a transitional area encompassing both very young ocean floor and a continental margin. These data consist of 853 seismograms (6 deg delta 42 deg) which produce 1068 travel times and 40 ray parameter estimates. These data are compared directly to another large suite of records representative of structure beneath the Gulf of California, an active spreading center. The spreading center model, GCA, was used as a starting point in WKBJ synthetic seismogram modeling and perturb GCA until the northeast Pacific data are matched. Application of wave field continuation to these two groups of data provides checks on model's consistency with the data as well as an estimate of the resolvability of differences between the two areas. Differences between the models derived from these two data sets are interpretable in terms of lateral structural variation beneath the two regimes.

The North American upper mantle: density, composition, and evolution

USGS Publications Warehouse

Mooney, Walter D.; Kaban, Mikhail K.

2010-01-01

The upper mantle of North America has been well studied using various seismic methods. Here we investigate the density structure of the North American (NA) upper mantle based on the integrative use of the gravity field and seismic data. The basis of our study is the removal of the gravitational effect of the crust to determine the mantle gravity anomalies. The effect of the crust is removed in three steps by subtracting the gravitational contributions of (1) topography and bathymetry, (2) low-density sedimentary accumulations, and (3) the three-dimensional density structure of the crystalline crust as determined by seismic observations. Information regarding sedimentary accumulations, including thickness and density, are taken from published maps and summaries of borehole measurements of densities; the seismic structure of the crust is based on a recent compilation, with layer densities estimated from P-wave velocities. The resultant mantle gravity anomaly map shows a pronounced negative anomaly (−50 to −400 mGal) beneath western North America and the adjacent oceanic region and positive anomalies (+50 to +350 mGal) east of the NA Cordillera. This pattern reflects the well-known division of North America into the stable eastern region and the tectonically active western region. The close correlation of large-scale features of the mantle anomaly map with those of the topographic map indicates that a significant amount of the topographic uplift in western NA is due to buoyancy in the hot upper mantle, a conclusion supported by previous investigations. To separate the contributions of mantle temperature anomalies from mantle compositional anomalies, we apply an additional correction to the mantle anomaly map for the thermal structure of the uppermost mantle. The thermal model is based on the conversion of seismic shear-wave velocities to temperature and is consistent with mantle temperatures that are independently estimated from heat flow and heat production data. The thermally corrected mantle density map reveals density anomalies that are chiefly due to compositional variations. These compositional density anomalies cause gravitational anomalies that reach ~250 mGal. A pronounced negative anomaly (−50 to −200 mGal) is found over the Canadian shield, which is consistent with chemical depletion and a corresponding low density of the lithospheric mantle, also referred to as the mantle tectosphere. The strongest positive anomaly is coincident with the Gulf of Mexico and indicates a positive density anomaly in the upper mantle, possibly an eclogite layer that has caused subsidence in the Gulf. Two linear positive anomalies are also seen south of 40°N: one with a NE-SW trend in the eastern United States, roughly coincident with the Grenville-Appalachians, and a second with a NW-SE trend beneath the states of Texas, New Mexico, and Colorado. These anomalies are interpreted as being due to (1) the presence of remnants of an oceanic slab in the upper mantle beneath the Grenville-Appalachian suture and (2) mantle thickening caused by a period of shallow, flat subduction during the Laramie orogeny, respectively. Based on these geophysical results, the evolution of the NA upper mantle is depicted in a series of maps and cartoons that display the primary processes that have formed and modified the NA crust and lithospheric upper mantle.

High resolution image of uppermost mantle beneath NE Iran continental collision zone

NASA Astrophysics Data System (ADS)

Motaghi, K.; Tatar, M.; Shomali, Z. H.; Kaviani, A.; Priestley, K.

2012-10-01

We invert 3775 relative P wave arrival times using the ACH damped least square method of Aki et al. (1977) to study upper mantle structure beneath the NE Iran continental collision zone. The data for this study were recorded by 17 three component broad-band stations operated from August 2006 to February 2008 along a profile from the center of Iranian Plateau, near Yazd, to the northeastern part of Iran on the Turan Platform just north of the Kopeh Dagh Mountains. The results confirm the previously known low velocity upper mantle beneath Central Iran. Our tomographic model reveals a deep high velocity anomaly. The surficial expressions of this anomaly are between the Ashkabad and Doruneh Faults, where the resolution and ray coverage are good. A transition zone in uppermost mantle is recognized under the Binalud foreland that we interpreted as suture zone between Iran and Turan platform. Our results indicate that Atrak Valley which is the boundary between the Binalud and Kopeh Dagh Mountains can be considered as the northeastern suture of the Iranian Plateau where Eurasia and Turan Platform under-thrust beneath the Binalud range and Central Iran.

Magmatic plumbing system from lower mantle of Hainan plume

NASA Astrophysics Data System (ADS)

Xia, Shaohong; Sun, Jinlong; Xu, Huilong; Huang, Haibo; Cao, Jinghe

2017-04-01

Intraplate volcanism during Late Cenozoic in the Leiqiong area of southernmost South China, with basaltic lava flows covering a total of more than 7000 km2, has been attributed to an underlying Hainan plume. However, detailed features of Hainan plume, such as morphology of magmatic conduits, depth of magmatic pool in the upper mantle and pattern of mantle upwelling, are still enigmatic. Here we present seismic tomographic images of the upper 1100 km of the mantle beneath the southern South China. Our results show a mushroom-like continuous low-velocity anomaly characterized by a columnar tail with diameter of about 200-300 km that tilts downward to lower mantle beneath north of Hainan hotspot and a head that spreads laterally near the mantle transition zone, indicating a magmatic pool in the upper mantle. Further upward, this head is decomposed into small patches, but when encountering the base of the lithosphere, a pancake-like anomaly is shaped again to feed the Hainan volcanism. Our results challenge the classical model of a fixed thermal plume that rises vertically to the surface, and propose the new layering-style pattern of magmatic upwelling of Hainan plume. This work indicates the spatial complexities and differences of global mantle plumes probably due to heterogeneous compositions and changefully thermochemical structures of deep mantle.

Lithospheric structure beneath Eastern Africa from joint inversion of receiver functions and Rayleigh wave velocities

NASA Astrophysics Data System (ADS)

Dugda, Mulugeta Tuji

Crust and upper mantle structure beneath eastern Africa has been investigated using receiver functions and surface wave dispersion measurements to understand the impact of the hotspot tectonism found there on the lithospheric structure of the region. In the first part of this thesis, I applied H-kappa stacking of receiver functions, and a joint inversion of receiver functions and Rayleigh wave group velocities to determine the crustal parameters under Djibouti. The two methods give consistent results. The crust beneath the GEOSCOPE station ATD has a thickness of 23+/-1.5 km and a Poisson's ratio of 0.31+/-0.02. Previous studies give crustal thickness beneath Djibouti to be between 8 and 10 km. I found it necessary to reinterprete refraction profiles for Djibouti from a previous study. The crustal structure obtained for ATD is similar to adjacent crustal structure in many other parts of central and eastern Afar. The high Poisson's ratio and Vp throughout most of the crust indicate a mafic composition, suggesting that the crust in Afar consists predominantly of new igneous rock emplaced during the late synrift stage where extension is accommodated within magmatic segments by diking. In the second part of this thesis, the seismic velocity structure of the crust and upper mantle beneath Ethiopia and Djibouti has been investigated by jointly inverting receiver functions and Rayleigh wave group velocities to obtain new constraints on the thermal structure of the lithosphere. Crustal structure from the joint inversion for Ethiopia and Djibouti is similar to previously published models. Beneath the Main Ethiopian Rift (MER) and Afar, the lithospheric mantle has a maximum shear wave velocity of 4.1-4.2 km/s and extends to a depth of at most 50 km. In comparison to the lithosphere away from the East African Rift System in Tanzania, where the lid extends to depths of ˜100-125 km and has a maximum shear velocity of 4.6 km/s, the mantle lithosphere under the Ethiopian Plateau appears to have been thinned by ˜30-50 km and the maximum shear wave velocity reduced by ˜0.3 km/s. Results from a 1D conductive thermal model suggest that the shear velocity structure of the lithosphere beneath the Ethiopian Plateau can be explained by a plume model, if a plume rapidly thinned the lithosphere by ˜30--50 km at the time of the flood basalt volcanism (c. 30 Ma), and if warm plume material has remained beneath the lithosphere since then. About 45-65% of the 1-1.5 km of plateau uplift in Ethiopia can be attributed to the thermally perturbed lithospheric structure. In the final part of this thesis, the shear-wave velocity structure of the crust and upper mantle beneath Kenya has been obtained from a joint inversion of receiver functions, and Rayleigh wave group and phase velocities. The crustal structure from the joint inversion is consistent with crustal structure published previously by different authors. The lithospheric mantle beneath the East African Plateau in Kenya is similar to the lithosphere under the East African Plateau in Tanzania. Beneath the Kenya Rift, the lithosphere extends to a depth of at most ˜75 km. The lithosphere under the Kenya Plateau is not perturbed when compared to the highly perturbed lithosphere beneath the Ethiopian Plateau. On the other hand, the lithosphere under the Kenya Rift is perturbed as compared to the Kenya Plateau or the rest of the East African Plateau, but is not as perturbed as the lithosphere beneath the Main Ethiopian Rift or the Afar. Although Kenya and Ethiopia have similar uplift and rifting histories, they have different volcanic histories. Much of Ethiopia has been affected by the Afar Flood Basalt volcanism, which may be the cause of this difference in lithospheric structure between these two regions.

Slab-controlled Tectonomagmatism of the Pacific Northwest: A Holistic view of Columbia River, High Lava Plains, and Snake River Plain/Yellowstone Volcanism

NASA Astrophysics Data System (ADS)

James, D. E.; Fouch, M. J.; Long, M. D.; Druken, K. A.; Wagner, L. S.; Chen, C.; Carlson, R. W.

2012-12-01

We interpret post-20 Ma tectonomagmatism across the U.S. Pacific Northwest in the context of subduction related processes. While mantle plume models have long enjoyed favor as an explanation for the post 20-Ma magmatism in the region, conceptually their support has hinged almost entirely on two major features: (1) Steens/Columbia River flood basalt volcanism (plume head); and (2) The Snake River Plain/Yellowstone hotspot track (plume tail). Recent work, synthesized in this presentation, suggests that these features are more plausibly the result of mantle dynamical processes driven by southerly truncation of the Farallon/Juan de Fuca subduction zone and slab detachment along the evolving margin of western North America (Long et al., 2012; James et al., 2011). Plate reconstructions indicate that shortening of the subduction zone by the northward migration of the Mendocino triple junction resulted in a significant increase in the rate of trench retreat and slab rollback ca 20 Ma. Both numerical modeling and physical tank experiments in turn predict large-scale mantle upwelling and flow around the southern edge of the rapidly retreating slab, consistent both with the observed Steens/Columbia River flood volcanism and with the strong E-W mantle fabric observed beneath the region of the High Lava Plains of central and eastern Oregon. The High Lava Plains and Snake River Plain time-progressive volcanism began concurrently about 12 Ma, but along highly divergent tracks and characterized by strikingly different upper mantle structure. Crustal and upper mantle structure beneath the High Lava Plains exhibits evidence typical of regional extension; i.e. thin crust, flat and sharp Moho, and an uppermost mantle with low velocities but otherwise largely devoid of significant vertical structure. In contrast, the Snake River Plain exhibits ultra-low mantle velocities to depths of about 180 km along the length of the hotspot track. Seismic images of the upper mantle in the depth range 300-600 km show that a northern segment of the orphaned Farallon plate lies sub-horizontally in the mantle transition zone parallel to and along the length of the SRP. The images also provide evidence for present-day upwelling from the deep upper mantle around the northern edge of the remnant slab beneath SRP as well as around its leading tip beneath Yellowstone. These results, coupled with petrologic and geochemical constraints, provide compelling support for a subduction model that accounts for virtually all post-20 Ma Cenozoic volcanism and structural deformation in the Cascadian back arc. James, D.E., Fouch, M.J., Carlson, R.W., Roth, J.B., 2011. Slab fragmentation, edge flow, and the origin of the Yellowstone hotspot track. Earth and Planetary Science Letters 311, 124-135. Long, M.D., Till, C.B., Druken, K.A., Carlson, R.W., Wagner, L.S., Fouch, M.J., James, D.E., Grove, T.L., Schmerr, N., Kincaid, C., 2012. Mantle dynamics beneath the Pacific Northwest and generation of voluminous back-arc volcanism. G-cubed in press.

Constraints on upper mantle Vp/Vs ratio variations beneath eastern North China from receiver function tomography

NASA Astrophysics Data System (ADS)

Si, Shaokun; Tian, Xiaobo; Gao, Rui

2017-05-01

To detect the thinning, modification, and replacement of the basement of the lithosphere is a key step in understanding the destruction mechanism of the North China lithosphere. The difference of the basement of the lithosphere is mainly displayed by the variation of the peridotite composition and its physical state. Vp/Vs ratio (hereafter referred to as velocity ratio) is more sensitive to this change than Vp or Vs alone. By means of the strong dependence of the travel-time of the wave converted at the 410-km discontinuity (P410s) observed in the receiver function (RF) on the velocity ratio in the upper mantle, we developed a new mapping method to constrain the velocity ratio between the Moho and 410-km discontinuity. Using the RFs extracted from 246 broadband stations beneath the North China Craton (NCC), we obtained a high-resolution velocity ratio image of the upper mantle. The abnormal velocity ratio indicates a strong lateral variation of the mineral composition in the upper mantle beneath North China. Two low-velocity-ratio patches are imaged at the top of the upper mantle and the 410 km depth, respectively. The former may be related to the orthopyroxene enrichment in the lithospheric mantle, and the latter may reflect the stagnant Pacific slab in the mantle transition zone (MTZ). A prominent high-velocity-ratio anomaly is also imaged in the upper mantle beneath the Shaanxi-Shanxi rift system in the central NCC, with the highest anomaly reaching 10%. We speculate that the high velocity ratio of upper mantle is related to convective flow due to slab dehydration in the MTZ. The dehydration of the retained slab in the MTZ results in partial melting and upwelling of upper mantle materials. Such convective flow and their melting are closely related to the Cenozoic basalt eruption in the northern section of the Shaanxi-Shanxi rift system.

Lithospheric instability and the source of the Cameroon Volcanic Line: Evidence from Rayleigh wave phase velocity tomography

DOE PAGES

Adams, Aubreya N.; Wiens, Douglas A.; Nyblade, Andrew A.; ...

2015-03-24

The Cameroon Volcanic Line (CVL) is a 1800 km long volcanic chain, extending SW-NE from the Gulf of Guinea into Central Africa, that lacks the typical age progression exhibited by hot spot-related volcanic tracks. Our study investigates the upper mantle seismic structure beneath the CVL and surrounding regions to constrain the origin of volcanic lines that are poorly described by the classic plume model. Rayleigh wave phase velocities are measured at periods from 20 to 182 s following the two-plane wave methodology, using data from the Cameroon Seismic Experiment, which consists of 32 broadband stations deployed between 2005 and 2007.more » These phase velocities are then inverted to build a model of shear wave velocity structure in the upper mantle beneath the CVL. Our results show that phase velocities beneath the CVL are reduced at all periods, with average velocities beneath the CVL deviating more than –2% from the regional average and +4% beneath the Congo Craton. This distinction is observed for all periods but is less pronounced for the longest periods measured. Inversion for shear wave velocity structure indicates a tabular low velocity anomaly directly beneath the CVL at depths of 50 to at least 200 km and a sharp vertical boundary with faster velocities beneath the Congo Craton. Finally, these observations demonstrate widespread infiltration or erosion of the continental lithosphere beneath the CVL, most likely caused by mantle upwelling associated with edge-flow convection driven by the Congo Craton or by lithospheric instabilities that develop due to the nearby edge of the African continent.« less

Mantle Structure Beneath East Africa and Zambia from Body Wave Tomography

NASA Astrophysics Data System (ADS)

Mulibo, G.; Nyblade, A.; Tugume, F.

2011-12-01

In this study, P and S travel time residuals from teleseismic earthquakes recorded on over 60 temporary AfricaArray seismic stations deployed in Uganda, Kenya, Tanzania and Zambia between 2007 and 2011 are being inverted, together with travel time residuals from previous deployments, for a 3D image of mantle wave speeds variations extending to a depth of 1200 km. Preliminary results show that at depths of 200 km of less, low wave speed anomalies are well developed beneath the Eastern and Western Branches of the East African Rift System. At deep depths, the low wave speed anomalies focus under the center and southern part of the East African Plateau and extend into the transition zone. At transition zone depths and within the top part of the lower mantle, the low wave speed anomaly shifts to the southwest beneath Zambia, indicating that the low wave speed anomaly is continuous across the transition zone and that it extends into the lower mantle. This result suggests that the upper mantle low wave speed anomaly beneath East Africa is connected to the African superplume anomaly in the lower mantle beneath southern Africa.

Determining the Upper Mantle Seismic Structure beneath the Northern Transantarctic Mountains from Regional P- and S-wave Tomography

NASA Astrophysics Data System (ADS)

Brenn, G.; Hansen, S. E.; Park, Y.

2016-12-01

Stretching 3500 km across Antarctica, the Transantarctic Mountains (TAMs) are the largest non-compressional mountain range on Earth. It has been suggested that the TAMs may have served as a nucleation point for the large-scale glaciation of Antarctica, and understanding their tectonic history has important implications for ice sheet modeling. However, the origin and uplift mechanism associated with the TAMs is controversial, and multiple models have been proposed. Seismic investigations of the TAM's subsurface structure can provide key constraints to help evaluate these models, but previous studies have been primarily focused on the central TAMs near Ross Island. Using data from the new 15-station Transantarctic Mountain Northern Network as well as data from several smaller networks, this study investigates the upper mantle velocity structure beneath a previously unexplored portion of the northern TAMs through regional body wave tomography. Relative travel-times were calculated for 11,182 P-wave and 8,285 S-wave arrivals from 790 and 581 Mw ≥ 5.5 events, respectively, using multi-channel cross correlation, and these data were then inverted for models of the upper mantle seismic structure. Resulting P- and S-wave tomography images reveal two focused low velocity anomalies beneath Ross Island (RI; δVP= -2.0%; δVS=-1.5% to -4.0%) and Terra Nova Bay (TNB; δVP=-1.5% to -2.0%; δVS= -1.0% to -4.0%) that extend to depths of 200 and 150 km, respectively. The RI and TNB slow anomalies also extend 50-100 km laterally beneath the TAMs front and sharply abut fast velocities beneath the EA craton (δVP=0.5% to 2%; δVS=1.5% to 4.0%). A low velocity region (δVP= -1.5%), centered at 150 km depth beneath the Terror Rift (TR) and primarily constrained within the Victoria Land Basin, connects the RI and TNB anomalies. The focused low velocities are interpreted as regions of partial melt and buoyancy-driven upwelling, connected by a broad region of slow (presumably warm) upper mantle associated with Cenozoic extension along the TR. Dynamic topography estimates based on the imaged S-wave velocity perturbations are consistent with observed surface topography in the central and northern TAMs, thereby providing support for uplift models that advocate for thermal loading and a flexural origin for the mountain range.

Sub-crustal seismic activity beneath Klyuchevskoy Volcano

NASA Astrophysics Data System (ADS)

Carr, M. J.; Droznina, S.; Levin, V. L.; Senyukov, S.

2013-12-01

Seismic activity is extremely vigorous beneath the Klyuchevskoy Volcanic Group (KVG). The unique aspect is the distribution in depth. In addition to upper-crustal seismicity, earthquakes take place at depths in excess of 20 km. Similar observations are known in other volcanic regions, however the KVG is unique in both the number of earthquakes and that they occur continuously. Most other instances of deep seismicity beneath volcanoes appear to be episodic or transient. Digital recording of seismic signals started at the KVG in early 2000s.The dense local network reliably locates earthquakes as small as ML~1. We selected records of 20 earthquakes located at depths over 20 km. Selection was based on the quality of the routine locations and the visual clarity of the records. Arrivals of P and S waves were re-picked, and hypocentral parameters re-established. Newl locations fell within the ranges outlined by historical seismicity, confirming the existence of two distinct seismically active regions. A shallower zone is at ~20 km depth, and all hypocenters are to the northeast of KVG, in a region between KVG and Shiveluch volcano. A deeper zone is at ~30 km, and all hypocenters cluster directly beneath the edifice of the Kyuchevskoy volcano. Examination of individual records shows that earthquakes in both zones are tectonic, with well-defined P and S waves - another distinction of the deep seismicity beneath KVG. While the upper seismic zone is unquestionably within the crust, the provenance of the deeper earthquakes is enigmatic. The crustal structure beneath KVG is highly complex, with no agreed-upon definition of the crust-mantle boundary. Rather, a range of values, from under 30 to over 40 km, exists in the literature. Similarly, a range of velocity structures has been reported. Teleseismic receiver functions (RFs) provide a way to position the earthquakes with respect to the crust-mantle boundary. We compare the differential travel times of S and P waves from deep events observed at a site closest to the epicenter to delay times of Ps phases in RFs that we associate with the crust-mantle transition. Both observations are essentially differences between travel times of S and P waves originating at the same place, and traversing the same velocity structure. Consequently, the uncertainty of the velocity structure beneath the KVG does not influence the comparison. For all events nominally located at 28-30 km beneath KVG the S-P time at the nearest site (CIR) significantly exceeds 4 seconds. Given that crust-mantle boundary Ps times at nearby sites are ~3 s, these earthquakes take place in the upper mantle. Both recent RFs and wide-angle reflection (Deep Seismic Sounding) studies in the late 1970s identified additional boundaries beneath KVG at depths in excess of 40 km. The nature of these boundaries is unclear, however their sharpness suggests chemical changes or the presence of fluids or melts. Chemistry of Klyuchevskoy lavas suggests sub-crustal origin with no clear magma chamber within the crust. Sub-crustal earthquakes we describe show that processes in the magma conduit at the base of the crust beneath KVG are vigorous enough to promote brittle failure in the surrounding mantle rock. The complex seismic structure of the uppermost mantle beneath KVG may reflect a history of magma injection that is accompanied by seismic energy release.

P wave velocity of Proterozoic upper mantle beneath central and southern Asia

NASA Astrophysics Data System (ADS)

Nyblade, Andrew A.; Vogfjord, Kristin S.; Langston, Charles A.

1996-05-01

P wave velocity structure of Proterozoic upper mantle beneath central and southern Africa was investigated by forward modeling of Pnl waveforms from four moderate size earthquakes. The source-receiver path of one event crosses central Africa and lies outside the African superswell while the source-receiver paths for the other events cross Proterozoic lithosphere within southern Africa, inside the African superswell. Three observables (Pn waveshape, PL-Pn time, and Pn/PL amplitude ratio) from the Pnl waveform were used to constrain upper mantle velocity models in a grid search procedure. For central Africa, synthetic seismograms were computed for 5880 upper mantle models using the generalized ray method and wavenumber integration; synthetic seismograms for 216 models were computed for southern Africa. Successful models were taken as those whose synthetic seismograms had similar waveshapes to the observed waveforms, as well as PL-Pn times within 3 s of the observed times and Pn/PL amplitude ratios within 30% of the observed ratio. Successful models for central Africa yield a range of uppermost mantle velocity between 7.9 and 8.3 km s-1, velocities between 8.3 and 8.5 km s-1 at a depth of 200 km, and velocity gradients that are constant or slightly positive. For southern Africa, successful models yield uppermost mantle velocities between 8.1 and 8.3 km s-1, velocities between 7.9 and 8.4 km s-1 at a depth of 130 km, and velocity gradients between -0.001 and 0.001 s-1. Because velocity gradients are controlled strongly by structure at the bottoming depths for Pn waves, it is not easy to compare the velocity gradients obtained for central and southern Africa. For central Africa, Pn waves turn at depths of about 150-200 km, whereas for southern Africa they bottom at ˜100-150 km depth. With regard to the origin of the African superswell, our results do not have sufficient resolution to test hypotheses that invoke simple lithospheric reheating. However, our models are not consistent with explanations for the African superswell invoking extensive amounts of lithospheric thinning. If extensive lithospheric thinning had occurred beneath southern Africa, as suggested previously, then upper mantle P wave velocities beneath southern Africa would likely be lower than those in our models.

Shield volcanism and lithospheric structure beneath the Tharsis plateau, Mars

NASA Technical Reports Server (NTRS)

Blasius, K. R.; Cutts, J. A.

1976-01-01

The heights of four great shield volcanoes, when interpreted as reflecting the local hydrostatic head on a common source of upwelling magma, provide significant constraints on models of lithospheric structure beneath the Tharsis plateau. If Bouguer gravity anomalies are modeled in terms of a variable thickness crust, and a two-component (crust/mantle) earth-like structure is assumed for the Martian lithosphere, the derived model lithosphere beneath the Tharsis plateau has the following properties: (1) the upper low-density 'crustal' component is thickened beneath the Tharsis plateau; (2) the lower high-density 'mantle' component is thinned beneath the Tharsis plateau; and (3) there is a net gradient on the base of the Martian lithosphere directed downward away from beneath the summit of the Tharsis plateau. A long history of magmatic intrusion is hypothesized to have been the cause of the updoming of the Tharsis plateau and the maintenance of the plateau in a state of only partial compensation.

Deep structure of Medicine Lake volcano, California

USGS Publications Warehouse

Ritter, J.R.R.; Evans, J.R.

1997-01-01

Medicine Lake volcano (MLV) in northeastern California is the largest-volume volcano in the Cascade Range. The upper-crustal structure of this Quaternary shield volcano is well known from previous geological and geophysical investigations. In 1981, the U.S. Geological Survey conducted a teleseismic tomography experiment on MLV to explore its deeper structure. The images we present, calculated using a modern form of the ACH-inversion method, reveal that there is presently no hint of a large (> 100 km3), hot magma reservoir in the crust. The compressional-wave velocity perturbations show that directly beneath MLV's caldera there is a zone of increased seismic velocity. The perturbation amplitude is +10% in the upper crust, +5% in the lower crust, and +3% in the lithospheric mantle. This positive seismic velocity anomaly presumably is caused by mostly subsolidus gabbroic intrusive rocks in the crust. Heat and melt removal are suggested as the cause in the upper mantle beneath MLV, inferred from petro-physical modeling. The increased seismic velocity appears to be nearly continuous to 120 km depth and is a hint that the original melts come at least partly from the lower lithospheric mantle. Our second major finding is that the upper mantle southeast of MLV is characterized by relatively slow seismic velocities (-1%) compared to the northwest side. This anomaly is interpreted to result from the elevated temperatures under the northwest Basin and Range Province.

Upper mantle structure beneath the northern part of the East African Plateau using data from the NE Uganda temporary seismic network

NASA Astrophysics Data System (ADS)

Bressers, C. A.; Nyblade, A.; Tugume, F.

2017-12-01

Data from a newly installed temporary seismic array in northeastern Uganda are incorporated into an existing body wave tomography model of eastern Africa to improve imaging of the upper mantle beneath the northern part of the East African Plateau. Nine temporary broadband stations were installed in January 2017 and will be operated through 2018 to obtain data for resolving structure under the northern part of the plateau as well as the East African rift in northern Kenya. Preliminary tomography models incorporate several months of data from stations in NE Uganda, plus many years of data from over 200 seismic stations throughout eastern Africa used in previously published body wave tomography models. The data come from teleseismic earthquakes with mb ≥ 5.5 at a distance from each station of 30° to 90°. P and S wave travel time residuals have been obtained using a multichannel cross correlation method and inverted using VanDecar's method to produce 3D tomographic images of the upper mantle. The preliminary results exhibit better resolved structure under the northern part of the East African Plateau than pervious models and suggest that the fast-wave speed anomaly in the upper mantle associated with the Tanzanian Craton—which is bounded by the Western and Eastern branches of the rift system—extends across most of northern Uganda.

Crustal and Mantle Structure beneath the Okavango and Malawi Rifts and Its Geodynamic Implications

NASA Astrophysics Data System (ADS)

Gao, S. S.; Liu, K. H.; Yu, Y.; Reed, C. A.; Mickus, K. L.; Moidaki, M.

2017-12-01

To investigate crustal and mantle structure beneath the young and incipient sections of the East African Rift System and provide constraints on rifting models, a total of 50 broadband seismic stations were placed along three profiles across the Okavango and Malawi rifts, with a total length of about 2500 km. Results to date suggest minor crustal thinning and nearly normal seismic velocities in the upper mantle beneath both rifts. The thickness of the mantle transition zone is comparable to the global average, suggesting the lack of thermal upwelling from the lower mantle beneath the rifts. In addition, shear-wave splitting analysis found no anomalies in either the fast polarization orientation or the splitting time associated with the rifts, and thus has ruled out the existence of small-scale mantle convection or plume-related mantle flow beneath the rifts. While the Okavango rift has long been recognized to be located in a Precambrian orogenic zone between the Kalahari and Congo cratons, our results suggest that the Malawi Rift is also developing along the western edge of a lithospheric block with relatively greater thickness relative to the surrounding area. Those seismological and gravity modeling results are consistent with a passive rifting model, in which rifts develop along pre-existing zones of lithospheric weakness, where rapid variations of lithospheric thickness is observed. Lateral variations of dragging stress applied to the bottom of the lithosphere are the most likely cause for the initiation and development of both rifts.

Tomographic imaging of subducted lithosphere below northwest Pacific island arcs

USGS Publications Warehouse

Van Der Hilst, R.; Engdahl, R.; Spakman, W.; Nolet, G.

1991-01-01

The seismic tomography problem does not have a unique solution, and published tomographic images have been equivocal with regard to the deep structure of subducting slabs. An improved tomographic method, using a more realistic background Earth model and surf ace-reflected as well as direct seismic phases, shows that slabs beneath the Japan and Izu Bonin island arcs are deflected at the boundary between upper and lower mantle, whereas those beneath the northern Kuril and Mariana arcs sink into the lower mantle.

Detachments of the subducted Indian continental lithosphere based on 3D finite-frequency tomographic images

NASA Astrophysics Data System (ADS)

Liang, X.; Tian, X.; Wang, M.

2017-12-01

Indian plate collided with Eurasian plate at 60 Ma and there are about 3000 km crustal shortening since the continental-continental collision. At least one third of the total amount of crustal shortening between Indian and Eurasian plates could not be accounted by thickened Tibetan crust and surface erosion. It will need a combination of possible transfer of lower crust to the mantle by eclogitization and lateral extrusion. Based on the lithosphere-asthenosphere boundary images beneath the Tibetan plateau, there is also at least the same amount deficit for lithospheric mantle subducted into upper/lower mantle or lateral extrusion with the crust. We have to recover a detailed Indian continental lithosphere image beneath the plateau in order to explain this deficit of mass budget. Combining the new teleseismic body waves recorded by SANDWICH passive seismic array with waveforms from several previous temporary seismic arrays, we carried out finite-frequency tomographic inversions to image three-dimensional velocity structures beneath southern and central Tibetan plateau to examine the possible image of subducted Indian lithosphere in the Tibetan upper mantle. We have recovered a continuous high velocity body in upper mantle and piece-wised high velocity anomalies in the mantle transition zone. Based on their geometry and relative locations, we interpreted these high velocity anomalies as the subducted and detached Indian lithosphere at different episodes of the plateau evolution. Detachments of the subducted Indian lithosphere should have a crucial impact on the volcanism activities and uplift history of the plateau.

Regional variations in upper mantle compressional velocities beneath southern California 1. Post-shock temperatures: Their experimental determination, calculation, and implications, 2.. Ph.D. Thesis

NASA Technical Reports Server (NTRS)

Raikes, S. A.

1978-01-01

The compressional velocity within the upper mantle beneath Southern California is investigated through observations of the dependence of teleseismic P-delays at all stations of the array on the distance and azimuth to the event. The variation of residuals with azimuth was found to be as large as 1.3 sec at a single station; the delays were stable as a function of time, and no evidence was found for temporal velocity variations related to seismic activity in the area. These delays were used in the construction of models for the upper mantle P-velocity structure to depths of 150 km, both by ray tracing and inversion techniques. The models exhibit considerable lateral heterogeneity including a region of low velocity beneath the Imperial Valley, and regions of increased velocity beneath the Sierra Nevada and much of the Transverse Ranges. The development is described of a technique for the experimental determination of post-shock temperatures, and its application to several metals and silicates shocked to pressures in the range 5 to 30 GPa. The technique utilizes an infra-red radiation detector to determine the brightness temperature of the free surface of the sample after the shock wave has passed through it.

Seismological evidence of the Hales discontinuity in northeast India

NASA Astrophysics Data System (ADS)

Anand, Aakash; Bora, Dipok K.; Borah, Kajaljyoti; Madhab Borgohain, Jayanta

2018-04-01

The crust and upper mantle shear wave velocity structure beneath the northeast India is estimated by joint inversion of Rayleigh wave group velocity and receiver function, calculated from teleseismic earthquakes data recorded at nine broadband seismic stations. The Assam valley and the Shillong-Mikir plateau are the two important tectonic blocks in the northeast India, which are surrounded by the Himalayan collision zone in the north, Indo-Burma subduction zone in the east and by the Bengal basin in the south. The joint inversion followed by forward modeling reveal crustal thicknesses of 30-34 km beneath the Shillong plateau, 36 km beneath the Mikir hills and 38-40 km beneath the Assam valley with an average shear wave velocity (Vs) of 3.4-3.5 km/s. The estimated low upper mantle shear wave velocity (Vsn) 4.2-4.3 km/s may be due to the rock composition or grain size or increased temperature and partial melt (<1%) in the upper mantle, or an effect of all. Also, we report for the first time, the existence of the Hales discontinuity at depths 56-74 km with Vs ∼4.4-4.6 km/s. Variable depth of the Hales discontinuity may be explained by the geotherm and/or addition of Cr3+ and Fe2+ in the spinel-garnet system.

Geophysical study of the structure and processes of the continental convergence zones: Alpine-Himalayan Belt

NASA Technical Reports Server (NTRS)

Toksoez, M. N.

1981-01-01

The seismic wave velocity structure in the crust and upper mantle region beneath the Tibetan plateau was studied in detail. Also, a preliminary study of the uppermost mantle P wave velocity beneath Iran and Turkey was carried out, and the results are compared with those for the Tibetan plateau. These two studies compose the bulk of the efforts on the observational aspects of continental collision zones in addition to satellite derived data. On the theoretical aspects the thermal evolution of converging plate boundaries was explored using a finite difference scheme.

Upper Mantle Discontinuity Structure Beneath the Western Atlantic Ocean and Eastern North America from SS Precursors

NASA Astrophysics Data System (ADS)

Schmerr, N. C.; Beghein, C.; Kostic, D.; Baldridge, A. M.; West, J. D.; Nittler, L. R.; Bull, A. L.; Montesi, L.; Byrne, P. K.; Hummer, D. R.; Plescia, J. B.; Elkins-Tanton, L. T.; Lekic, V.; Schmidt, B. E.; Elkins, L. J.; Cooper, C. M.; ten Kate, I. L.; Van Hinsbergen, D. J. J.; Parai, R.; Glass, J. B.; Ni, J.; Fuji, N.; McCubbin, F. M.; Michalski, J. R.; Zhao, C.; Arevalo, R. D., Jr.; Koelemeijer, P.; Courtier, A. M.; Dalton, H.; Waszek, L.; Bahamonde, J.; Schmerr, B.; Gilpin, N.; Rosenshein, E.; Mach, K.; Ostrach, L. R.; Caracas, R.; Craddock, R. A.; Moore-Driskell, M. M.; Du Frane, W. L.; Kellogg, L. H.

2015-12-01

Seismic discontinuities within the mantle arise from a wide range of mechanisms, including changes in mineralogy, major element composition, melt content, volatile abundance, anisotropy, or a combination of the above. In particular, the depth and sharpness of upper mantle discontinuities at 410 and 660 km depth are attributed to solid-state phase changes sensitive to both mantle temperature and composition, where regions of thermal heterogeneity produce topography and chemical heterogeneity changes the impedance contrast across the discontinuity. Seismic mapping of this topography and sharpness thus provides constraint on the thermal and compositional state of the mantle. The EarthScope USArray is providing unprecedented access to a wide variety of new regions previously undersampled by the SS precursors. This includes the boundary between the oceanic plate in the western Atlantic Ocean and continental margin of eastern North America. Here we use a seismic array approach to image the depth, sharpness, and topography of the upper mantle discontinuities, as well as other possible upper mantle reflectors beneath this region. This array approach utilizes seismic waves that reflect off the underside of a mantle discontinuity and arrive several hundred seconds prior to the SS seismic phase as precursory energy. In this study, we collected high-quality broadband data SS precursors data from shallow focus (< 30 km deep), mid-Atlantic ridge earthquakes recorded by USArray seismometers in Alaska. We generated 4th root vespagrams to enhance the SS precursors and determine how they sample the mantle. Our data show detection of localized structure on the discontinuity boundaries as well as additional horizons, such as the X-discontinuity and a potential reflection from a discontinuity near the depth of the lithosphere-asthenosphere boundary. These structures are related to the transition from predominantly old ocean lithosphere to underlying continental lithosphere, as while deeper reflectors are associated with the subduction of the ancient Farallon slab. A comparison of the depth of upper mantle discontinuities to changes in seismic velocity and anisotropy will further quantify the relationship to mantle flow, compositional layering, and phases changes.

Crustal and upper mantle velocity structure of the Salton Trough, southeast California

USGS Publications Warehouse

Parsons, T.; McCarthy, J.

1996-01-01

This paper presents data and modelling results from a crustal and upper mantle wide-angle seismic transect across the Salton Trough region in southeast California. The Salton Trough is a unique part of the Basin and Range province where mid-ocean ridge/transform spreading in the Gulf of California has evolved northward into the continent. In 1992, the U.S. Geological Survey (USGS) conducted the final leg of the Pacific to Arizona Crustal Experiment (PACE). Two perpendicular models of the crust and upper mantle were fit to wide-angle reflection and refraction travel times, seismic amplitudes, and Bouguer gravity anomalies. The first profile crossed the Salton Trough from the southwest to the northeast, and the second was a strike line that paralleled the Salton Sea along its western edge. We found thin crust (???21-22 km thick) beneath the axis of the Salton Trough (Imperial Valley) and locally thicker crust (???27 km) beneath the Chocolate Mountains to the northeast. We modelled a slight thinning of the crust further to the northeast beneath the Colorado River (???24 km) and subsequent thickening beneath the metamorphic core complex belt northeast of the Colorado River. There is a deep, apparently young basin (???5-6 km unmetamorphosed sediments) beneath the Imperial Valley and a shallower (???2-3 km) basin beneath the Colorado River. A regional 6.9-km/s layer (between ???15-km depth and the Moho) underlies the Salton Trough as well as the Chocolate Mountains where it pinches out at the Moho. This lower crustal layer is spatially associated with a low-velocity (7.6-7.7 km/s) upper mantle. We found that our crustal model is locally compatible with the previously suggested notion that the crust of the Salton Trough has formed almost entirely from magmatism in the lower crust and sedimentation in the upper crust. However, we observe an apparently magmatically emplaced lower crust to the northeast, outside of the Salton Trough, and propose that this layer in part predates Salton Trough rifting. It may also in part result from migration of magmatic spreading centers associated with the southern San Andreas fault system. These spreading centers may have existed east of their current locations in the past and may have influenced the lower crust and upper mantle to the east of the current Salton Trough.

Upper mantle anisotropy beneath Peru from SKS splitting: Constraints on flat slab dynamics and interaction with the Nazca Ridge

NASA Astrophysics Data System (ADS)

Eakin, Caroline M.; Long, Maureen D.; Wagner, Lara S.; Beck, Susan L.; Tavera, Hernando

2015-02-01

The Peruvian flat slab is by far the largest region of flat subduction in the world today, but aspects of its structure and dynamics remain poorly understood. In particular, questions remain over whether the relatively narrow Nazca Ridge subducting beneath southern Peru provides dynamic support for the flat slab or it is just a passive feature. We investigate the dynamics and interaction of the Nazca Ridge and the flat slab system by studying upper mantle seismic anisotropy across southern Peru. We analyze shear wave splitting of SKS, sSKS, and PKS phases at 49 stations distributed across the area, primarily from the PerU Lithosphere and Slab Experiment (PULSE). We observe distinct spatial variations in anisotropic structure along strike, most notably a sharp transition from coherent splitting in the north to pervasive null (non-split) arrivals in the south, with the transition coinciding with the northern limit of the Nazca Ridge. For both anisotropic domains there is evidence for complex and multi-layered anisotropy. To the north of the ridge our *KS splitting measurements likely reflect trench-normal mantle flow beneath the flat slab. This signal is then modified by shallower anisotropic layers, most likely in the supra-slab mantle, but also potentially from within the slab. To the south the sub-slab mantle is similarly anisotropic, with a trench-oblique fast direction, but widespread nulls appear to reflect dramatic heterogeneity in anisotropic structure above the flat slab. Overall the regional anisotropic structure, and thus the pattern of deformation, appears to be closely tied to the location of the Nazca Ridge, which further suggests that the ridge plays a key role in the mantle dynamics of the Peruvian flat slab system.

Regional Vp, Vs, Vp/Vs, and Poisson's ratios across earthquake source zones from Memphis, Tennessee, to St. Louis, Missouri

USGS Publications Warehouse

Catchings, R.D.

1999-01-01

Models of P- and S-wave velocity, Vp/Vs ratios, Poisson's ratios, and density for the crust and upper mantle are presented along a 400-km-long profile trending from Memphis, Tennessee, to St. Louis, Missouri. The profile crosses the New Madrid seismic zone and reveals distinct regional variations in the crustal velocity structure north and south of the latitude of New Madrid. In the south near Memphis, the upper few kilometers of the crust are dominated by upper crustal sedimentary basins or graben with P-wave velocities less than 5 km/sec and S-wave velocities of about 2 km/sec. P-wave velocities of the upper and middle crust range from 6.0 to 6.5 km/sec at depths above 25 km, and corresponding S-wave velocities range from 3.5 to 3.7 km/sec. The lower crust consists of a high-velocity layer (Vp = 7.4 km/sec; Vs ~4.2 km/sec) that is up to 20-km thick at the latitude of New Madrid but thins to about 15 km near Memphis. To the north, beneath the western-most Illinois basin, low-velocity (Vp < 5 km/sec; Vs < 2.3 km/sec) sedimentary basins are less than 1-km deep. The average velocities (Vp = 6.0 km/sec; Vs = 3.5 km/sec) of the underlying, near-surface rocks argue against large thickness of unconsolidated noncarbonate sediments within 50 km of the western edge of the Illinois basin. Most of the crust beneath the Illinois basin is modeled as one layer, with velocities up to 6.8 km/sec (Vs = 3.7 km/sec) at 37-km depth. The thick, high-velocity (Vp = 7.4 km/sec; Vs ~4.2 km/sec) lower crustal layer thins from about 20 km near New Madrid to about 6 km beneath the western Illinois basin. Refractions from the Moho and upper mantle occur as first arrivals over distances as a great as 160 km and reveal upper mantle layering to 60 km depth. Upper mantle layers with P-wave velocities of 8.2 km/sec (Vs = 4.5 km/sec) and 8.4 km/sec (Vs = 4.7 km/sec) are modeled at 43 and 60 km depth, respectively. Crustal Vp/Vs ratios range between 1.74 and 1.83, and upper mantle Vp/V s ratios range from 1.78 to 1.84. Poisson's ratios range from about 0.26 to 0.33 in the crust and from about 0.27 to 0.29 in the upper mantle. Modeled average densities range from about 2.55 in the sedimentary basins to 3.43 in the upper mantle. Geophysical characteristics of the crust and upper mantle within the New Madrid seismic zone are consistent with other continental rifts, but the crustal structure of the Illinois basin is not characteristics of most continental rift settings. Seismic and gravity data suggest a buried horst near the middle of Reelfoot rift, beneath which is a vertical zone of seismicity and velocity anomalies. The relative depth of the Reelfoot rift north and south of the Reelfoot graben suggests that the rift and its bounding faults may extend eastward beneath the city of Memphis.

Of Mantle Plumes, Their Existence, and Their Nature: Insights from Whole Mantle SEM-Based Seismic Waveform Tomography

NASA Astrophysics Data System (ADS)

Romanowicz, B. A.; French, S. W.

2014-12-01

Many questions remain on the detailed morphology of mantle convection patterns. While high resolution P wave studies show a variety of subducted slab behaviors, some stagnating in the transition zone, others penetrating into the lower mantle (e.g. Fukao & Obayashi, 2013), low velocity structures - the upwelling part of flow - are more difficult to resolve at the same scale. Indeed, depth extent and morphology of the low velocity roots of hotspot volcanoes is still debated, along with the existence of "mantle plumes". Using spectral element waveform tomography, we previously constructed a global, radially anisotropic, upper mantle Vs model (SEMum2, French et al., 2013) and have now extended it to the whole mantle by adding shorter period waveform data (SEMUCB-WM1, French & Romanowicz, GJI, in revision). This model shows long wavelength structure in good agreement with other recent global Vs models derived under stronger approximations (Ritsema et al. 2011; Kustowski, et al. 2008), but exhibits better focused, finer scale structure throughout the mantle. SEMUCB-WM1 confirms the presence in all major ocean basins of the quasi-periodic, upper mantle low velocity anomalies, previously seen in SEMum2. At the same time, lower mantle low velocity structure is dominated by a small number (~15 globally) of quasi-vertical anomalies forming discrete "column"" rooted at the base of the mantle. Most columns are positioned near major hotspots, as defined by buoyancy flux, and are wider (~800-1000 km diameter) than expected from the thermal plume model - suggestive of thermo-chemical plumes, which may be stable for long times compared to purely thermal ones. Some columns reach the upper mantle, while others deflect horizontally near 1000 km - the same depth where many slabs appear to stagnate. As they reach the transition zone, the wide columnar structure can be lost, as these "plumes" appear to meander through the upper mantle, perhaps entrained by more vigorous, lower viscosity, convection. Most "plumes" in the Pacific LLSVP region appear as isolated columns rising from the CMB, such as beneath Hawaii (rooted near a known ultra low velocity zone, Cottaar & Romanowicz, 2012). Conversely, the African LLSVP region appears more massive up to mid-mantle depths, with isolated "plumes" at its borders, including that beneath Iceland.

Seismic structure of the lithosphere and upper mantle beneath the ocean islands near mid-oceanic ridges

NASA Astrophysics Data System (ADS)

Haldar, C.; Kumar, P.; Kumar, M. Ravi

2014-05-01

Deciphering the seismic character of the young lithosphere near mid-oceanic ridges (MORs) is a challenging endeavor. In this study, we determine the seismic structure of the oceanic plate near the MORs using the P-to-S conversions isolated from quality data recorded at five broadband seismological stations situated on ocean islands in their vicinity. Estimates of the crustal and lithospheric thickness values from waveform inversion of the P-receiver function stacks at individual stations reveal that the Moho depth varies between ~ 10 ± 1 km and ~ 20 ± 1 km with the depths of the lithosphere-asthenosphere boundary (LAB) varying between ~ 40 ± 4 and ~ 65 ± 7 km. We found evidence for an additional low-velocity layer below the expected LAB depths at stations on Ascension, São Jorge and Easter islands. The layer probably relates to the presence of a hot spot corresponding to a magma chamber. Further, thinning of the upper mantle transition zone suggests a hotter mantle transition zone due to the possible presence of plumes in the mantle beneath the stations.

Nonlinear 1D and 2D waveform inversions of SS precursors and their applications in mantle seismic imaging

NASA Astrophysics Data System (ADS)

Dokht, R.; Gu, Y. J.; Sacchi, M. D.

2016-12-01

Seismic velocities and the topography of mantle discontinuities are crucial for the understanding of mantle structure, dynamics and mineralogy. While these two observables are closely linked, the vast majority of high-resolution seismic images are retrieved under the assumption of horizontally stratified mantle interfaces. This conventional correction-based process could lead to considerable errors due to the inherent trade-off between velocity and discontinuity depth. In this study, we introduce a nonlinear joint waveform inversion method that simultaneously recovers discontinuity depths and seismic velocities using the waveforms of SS precursors. Our target region is the upper mantle and transition zone beneath Northeast Asia. In this region, the inversion outcomes clearly delineate a westward dipping high-velocity structure in association with the subducting Pacific plate. Above the flat part of the slab west of the Japan sea, our results show a shear wave velocity reduction of 1.5% in the upper mantle and 10-15 km depression of the 410 km discontinuity beneath the Changbaishan volcanic field. We also identify the maximum correlation between shear velocity and transition zone thickness at an approximate slab dip of 30 degrees, which is consistent with previously reported values in this region.To validate the results of the 1D waveform inversion of SS precursors, we discretize the mantle beneath the study region and conduct a 2D waveform tomographic survey using the same nonlinear approach. The problem is simplified by adopting the discontinuity depths from the 1D inversion and solving only for perturbations in shear velocities. The resulting models obtained from the 1D and 2D approaches are self-consistent. Low-velocities beneath the Changbai intraplate volcano likely persist to a depth of 500 km. Collectively, our seismic observations suggest that the active volcanoes in eastern China may be fueled by a hot thermal anomaly originating from the mantle transition zone.

Crustal Structure of the Flood Basalt Province of Ethiopia from Constrained 3-D Gravity Inversion

NASA Astrophysics Data System (ADS)

Mammo, Tilahun

2013-12-01

The Oligocene Afar mantle plume resulted in the eruption of a large volume of basaltic magma, including major sequences of rhyolitic ignimbrites, in a short span of time across Ethiopia. In order to assess the impact of these magmatic processes on the crust and to investigate the general crustal configuration beneath the Ethiopian plateau, northern part of the Main Ethiopian Rift and the Afar depression, analysis and modeling of the gravity field have been conducted. The Bouguer gravity map is dominated by long-wavelength anomalies that primarily arise from the isostatic compensation of the topography. Consequently, anomalies within the crust/upper mantle are masked and quantitative interpretation becomes difficult. The long-wavelength anomalies are approximated using admittance technique and subsequently removed from the Bouguer anomalies to obtain the residual isostatic anomalies. The residual map contains both short- and intermediate-wavelength anomalies related to geologic and tectonic features. The long-wavelength regional isostatic field is used to map the crust-mantle interface and the results are in good agreement with those determined by other geophysical methods. Seismic constrained gravity inversion was performed on the isostatic residual field and series of three-dimensional models have been constructed for the structures of the crust and upper mantle beneath the uplifted and rifted flood basalt province of northern Ethiopia. The inversion results have shown that the NW plateau has thick crust that rests on normal lithospheric mantle. Afar, On the other hand, is marked by thin stretched crust resting on a low-density upper mantle indicating a hotter thermal regime and partial melt. No lithospheric mantle is observed beneath Afar. The models further indicate the presence of an extensive sub-crustal thick (~12 km on average) and high-density (~3.06 gm/cc) mafic accreted igneous layer of fractionated cumulate (magmatic underplating) beneath the NW plateau. The study suggests that the underplate was fundamental to the accretion process and may have played a role in compensating most of the plateau uplift and in localizing stresses.

Structure of crust and upper mantle beneath NW Himalayas, Pamir and Hindukush by multi-scale double-difference seismic tomography

NASA Astrophysics Data System (ADS)

Bhatti, Zahid Imran; Zhao, Junmeng; Khan, Nangyal Ghani; Shah, Syed Tallataf Hussain

2018-08-01

The India-Asia collision and subsequent subduction initiated the evolution of major tectonic features in the Western Syntaxis. The complex tectonic structure and shallow to deep seismicity have attracted geoscientists over the past two decades. The present research is based on a 3D tomographic inversion of P-wave arrival time data to constrain the crustal and upper mantle structure beneath the NW Himalayas and Pamir-Hindukush region using the Double-difference tomography. We utilized a very large multi-scale dataset comprising 19,080 earthquakes recorded at 397 local and regional seismic stations from 1950 to 2017. The northward dipping seismic zone coinciding with the low velocity anomaly suggests the subduction of the Indian lower crust beneath the Hindukush. The extent of the northward advancing Indian slab increases from east to west in this region. We observed no signs of northward subduction of the Indian plate under the Hindukush beyond 71°E longitude. The Indian plate overturns due south after interacting with the Asian plate beneath the southern Pamir, which correlates with the counter-clockwise rotation of the Indian plate. The Asian plate is also imaged as a southward subducting seismic zone beneath the southern Pamir. In the NW Himalayas, the northward subducting Indian plate appears as a gently dipping low velocity anomaly beneath the Karakoram Block. The stresses caused by the collision and subduction along the Shyok Suture and Indus Suture are translated to the south. The crustal scale seismicity and high velocity anomalies indicate an intense deformation in the crust, which is manifested by syntaxial bends and thrust faults to the south of the Main Mantle Thrust.

Seismological Constraints on Lithospheric Evolution in the Appalachian Orogen

NASA Astrophysics Data System (ADS)

Fischer, K. M.; Hopper, E.; Hawman, R. B.; Wagner, L. S.

2017-12-01

Crust and mantle structures beneath the Appalachian orogen, recently resolved by seismic data from the EarthScope SESAME Flexible Array and Transportable Array, provide new constraints on the scale and style of the Appalachian collision and subsequent lithospheric evolution. In the southern Appalachians, imaging with Sp and Ps phases reveals the final (Alleghanian) suture between the crusts of Laurentia and the Gondwanan Suwannee terrane as a low angle (<15°) southward-dipping interface that soles into a flat-lying mid-crustal detachment. The suture location near the top of the crust coincides closely with the northern limit of the Suwannee terrane reconstructed from its lower Paleozoic shelf strata (Boote and Knapp, 2016). The observed suture geometry implies over 300 km of head-on shortening across a plate boundary structure similar in scale to the Himalayan mid-crustal detachment. While the suture and other structures from the Alleghanian collision are preserved in the upper and mid-crust, the lower crust and mantle lithosphere beneath this region have been significantly modified by later processes. Ps receiver functions, wavefield migration and SsPmp modeling reveal that crustal thickness reaches a maximum of 58 km (beneath high elevations in the Blue Ridge terrane) and decreases to 29-35 km (beneath lower elevations in the Carolina and Suwannee terranes). Given metamorphic estimates of unroofing (Duff and Kellogg, 2017) isostatic arguments indicate crustal thicknesses were 15-25 km larger at the end of the orogeny, indicating a thick crustal root across the region. The present-day residual crustal root beneath the Blue Ridge mountains is estimated to have a density contrast with the mantle of only 104±20 kg/m3. This value is comparable to other old orogens but lower than values typical of young or active orogens, indicating a loss of lower crustal buoyancy over time. At mantle depths, the negative shear velocity gradient that marks the transition from lithosphere to asthenosphere, as illuminated by Sp phases, varies across the Appalachian orogen. This boundary is shallow beneath the northeastern U.S. and in the zone of Eocene volcanism in Virginia, where low velocity anomalies occur in the upper mantle. These correlations suggest recent active lithosphere-asthenosphere interaction.

Crust and Upper Mantle Structure of Antarctica from Rayleigh Wave Tomography

NASA Astrophysics Data System (ADS)

Wiens, D. A.; Heeszel, D. S.; Sun, X.; Chaput, J. A.; Aster, R. C.; Nyblade, A.; Anandakrishnan, S.; Wilson, T. J.; Huerta, A. D.

2012-12-01

We combine data from three temporary arrays of seismometers (AGAP/GAMSEIS 2007-2010, ANET/POLENET 2007-2012, TAMSEIS 2001-2003) deployed across Antarctica, along with permanent stations in the region, to produce a large scale shear velocity model of the continent extending from the Gamburtsev Subglacial Mountains (GSM) in East Antarctica, across the Transantarctic Mountains (TAM) and West Antarctic Rift System (WARS) to Marie Byrd Land (MBL) in West Antarctica. Our combined dataset consists of Rayleigh wave phase and amplitude measurements from 112 stations across the study region. We first invert for 2-D Rayleigh wave phase velocities using the two-plane wave method. These results are then inverted for shear velocity structure using crustal thicknesses derived from ambient noise tomography and teleseismic receiver functions. We refine our shear velocity model by performing a Monte Carlo simulation that explores the tradeoff between crustal thickness and upper mantle seismic velocities. The resulting model is higher resolution than previous studies (~150 km resolution length) and highlights significant differences in crustal and uppermost mantle structure between East and West Antarctica in greater detail than previously possible. East Antarctica is underlain by thick crust (reaching ~55 km beneath the GSM) and fast, cratonic lithosphere. West Antarctica is defined by thinner crust and slow upper mantle velocities indicative of its more recent tectonic activity. The observed boundary in crustal thickness closely follows the TAM front. MBL is underlain by a thicker lithosphere than that observed beneath the WARS, but slow mantle velocities persist to depths greater than 200 km, indicating a 'deep seated' (i.e. deeper than the deepest resolvable features of our model) thermal source for volcanism in the region. The slowest seismic velocities at shallow depths are observed in the Terror Rift region of the Ross Sea along an arc following the TAM front, where the most recent extension has occurred, and in another region of active volcanism. The Ellsworth-Whitmore Mountains are underlain by relatively thick crust and an intermediate thickness lithosphere, consistent with its hypothesized origin as a remnant Precambrian crustal block. We also produce upper mantle viscosity models for the study region using a temperature-dependent rheology, assuming that mantle seismic anomalies are dominated by temperature variations. Initial results closely correlate with the velocity model, with viscosities beneath West Antarctica inferred to be orders of magnitude lower than beneath East Antarctica. These viscosity results have important implications for our understanding of glacial isostatic adjustment, which is of particular interest in producing models of past and future changes in the Antarctic Ice Sheets.

Resolving mantle structure beneath the Pacific Northwest

NASA Astrophysics Data System (ADS)

Darold, A. P.; Humphreys, E.; Schmandt, B.; Gao, H.

2011-12-01

Cenozoic tectonics of the Pacific Northwest (PNW) and the associated mantle structures are remarkable, the latter revealed only recently by EarthScope seismic data. Over the last ~66 Ma this region experienced a wide range of tectonic and magmatic conditions: Laramide compression, ~75-53 Ma, involving Farallon flat-slab subduction, regional uplift, and magmatic quiescence. With the ~53 Ma accretion of Siletzia ocean lithosphere within the Columbia Embayment, westward migration of subduction beginning Cascadia, along with initiation of the Cascade volcanic arc. Within the continental interior the Laramide orogeny was quickly followed by a period of extension involving metamorphic core complexes and the associated initial ignimbrite flare-up (both in northern Washington, Idaho, and western Montana); interior magmo-tectonic activity is attributed to flat-slab removal and (to the south) slab rollback. Rotation of Siletzia created new crust on SE Oregon and, at ~16 Ma, the Columbia River Flood Basalt (CRB) eruptions renewed vigorous magmatism. We have united several EarthScope studies in the Pacific Northwest and have focused on better resolving the major mantle structures that have been discovered. We have tomographically modeled the body waves with teleseismic, finite-frequency code under the constraints of ambient noise tomography and teleseismic receiver function models of Gao et al. (2011), and teleseismic anisotropy models of Long et al. (2009) in order to resolve structures continuously from the surface to the base of the upper mantle. We now have clear imaging of two episodes of subduction: Juan De Fuca slab deeper than ~250 km is absent across much of the PNW, and it has an E-W tear located beneath northern Oregon; Farallon slab (the "Siletzia curtain") is still present, hanging vertically just inboard of the core complexes, and with a basal tear causing the structure to extend deeper (~600 km) beneath north-central Idaho than beneath south-central Idaho and northern Washington (~300 km). Lying just west of the Siletzia curtain, beneath NE Oregon, is a prominent high-velocity body centered on 250 km depth. Its nearly circular plan view corresponds with the area of intense Columbia River Basalt eruptions and with the circular topographic bull's eye centered on the recently uplifted (post CRB) Wallowa Mountains. Finally, we are investigating a very low-velocity volume of mantle present between the E-W Juan de Fuca tear and the high-velocity body beneath the Wallowa Mountains. At 250 km depth this is the strongest low-velocity anomaly beneath the western U.S. Presently we are completing resolution testing on the structures revealed through our imaging in order to resolve their structural details. These synthetic resolution tests along with the high resolution imaging of the crust and upper mantle will clarify several previously cited structures as well as strengthen our conclusions on the tectonic history and geodynamical evolution of the mantle while aiding in putting together a comprehensive story for the area.

Upper mantle structure under western Saudi Arabia from Rayleigh wave tomography and the origin of Cenozoic uplift and volcanism on the Arabian Shield

DOE Office of Scientific and Technical Information (OSTI.GOV)

Park, Y; Nyblade, A; Rodgers, A

2007-11-09

The shear velocity structure of the shallow upper mantle beneath the Arabian Shield has been modeled by inverting new Rayleigh wave phase velocity measurements between 45 and 140 s together with previously published Rayleigh wave group velocity measurement between 10 and 45 s. For measuring phase velocities, we applied a modified array method that minimizes the distortion of raypaths by lateral heterogeneity. The new shear velocity model shows a broad low velocity region in the lithospheric mantle across the Shield and a low velocity region at depths {ge} 150 km localized along the Red Sea coast and Makkah-Madinah-Nafud (MMN) volcanicmore » line. The velocity reduction in the upper mantle corresponds to a temperature anomaly of {approx}250-330 K. These finding, in particular the region of continuous low velocities along the Red Sea and MMN volcanic line, do not support interpretations for the origin of the Cenozoic plateau uplift and volcanism on the Shield invoking two separate plumes. When combined with images of the 410 and 660 km discontinuities beneath the southern part of the Arabian Shield, body wave tomographic models, a S-wave polarization analysis, and SKS splitting results, our new model supports an interpretation invoking a thermal upwelling of warm mantle rock originating in the lower mantle under Africa that crosses through the transition zone beneath Ethiopia and moves to the north and northwest under the eastern margin of the Red Sea and the Arabian Shield. In this interpretation, the difference in mean elevation between the Platform and Shield can be attributed to isostatic uplift caused by heating of the lithospheric mantle under the Shield, with significantly higher region along the Red Sea possibly resulting from a combination of lithosphere thinning and dynamic uplift.« less

Constraints on the shear speed, crust thickness and residual topography of western Tibet from surface wave tomography and virtual deep seismic sounding

NASA Astrophysics Data System (ADS)

Matchette-Downes, H.; van der Hilst, R. D.; Priestley, K. F.

2017-12-01

We have estimated the thickness of the crust in western Tibet by measuring the time delays between the direct S and the SsPmp seismic phases. We find that the thickness of the crust increases from around 50 km beneath the Tethyan Himalayas to around 80 km beneath the Lhasa block, and then decreases to around 70 km beneath the Qiangtang terrane.This method, virtual deep seismic sounding (VDSS), also yields robust estimates of the contribution of crust buoyancy to elevation. By subtracting the predicted elevation from the real topography, we find there is no observable deviation from hydrostatic topography beneath the Tethyan Himalaya, but there is negative residual topography of 1.5 to 2.0 km beneath the Lhasa and Qiangtang terranes. It is also known that the interior of the Plateau is isostatically compensated, as it has small free air gravity anomalies.Additionally, we have estimated the 3D shear speed structure of the crust and upper mantle. This model is derived from maps of the fundamental mode Rayleigh wave phase speed dispersion in the period range from 20 to 140 s, obtained from a standard two-plane-wave inversion constrained with receiver functions and group speeds from ambient noise. The observations agree with previous observations of a low-wavespeed zone in the mid-crust and a gradual Moho. Furthermore, the long-period Rayleigh waves detect a high-wavespeed upper mantle.Together, the observations of high upper mantle wavespeeds, negative residual topography, and small free air gravity anomalies support the hypothesis that cold, dense Indian lithosphere has underthrust the Plateau in this region. However, in the presentation we also consider contributions to residual topography from plate flexure, lower crustal flow, or deeper mantle flow (dynamic topography).

Seismic Migration Imaging of the Crust and Upper Mantle Discontinuity Structure beneath Southern Taiwan

NASA Astrophysics Data System (ADS)

Liu, Y.-S.; Kuo, B.-Y.

2009-04-01

Taiwan is located in the convergent plate boundary zone where the Philippine Sea plate has obliquely collided on the Asian continental margin, initiating the arc-continent collision and subsequent mountain-building in Taiwan. Receiver function has been a powerful tool to image seismic velocity discontinuity structure in the crust and upper mantle which can help illuminate the deep dynamic process of active Taiwan orogeny. In this study, we adopt backprojection migration processing of teleseismic receiver functions to investigate the crust and upper mantle discontinuities beneath southern Taiwan, using the data from Southern Taiwan Transect Seismic Array (STTA), broadband stations of Central Weather Bureau (CWB), Broadband Array in Taiwan for Seismology (BATS), and Taiwan Integrated Geodynamics Research (TAIGER). This composite east-west trending linear array has the aperture of about 150 km with the station spacing of ~5-10 km. Superior to the common midpoint (CMP) stack approach, the migration can properly image the dipping, curved, or laterally-varying topography of discontinuous interfaces which very likely exist under the complicated tectonic setting of Taiwan. We first conduct synthetic experiments to test the depth and lateral resolution of migration images based on the WKBJ synthetic waveforms calculated from available source and receiver distributions. We will next construct the 2-D migration image under the array to reveal the topographic variation of the Moho and lithosphere discontinuities beneath southern Taiwan.

Upper Mantle Shear Wave Structure Beneath North America From Multi-mode Surface Wave Tomography

NASA Astrophysics Data System (ADS)

Yoshizawa, K.; Ekström, G.

2008-12-01

The upper mantle structure beneath the North American continent has been investigated from measurements of multi-mode phase speeds of Love and Rayleigh waves. To estimate fundamental-mode and higher-mode phase speeds of surface waves from a single seismogram at regional distances, we have employed a method of nonlinear waveform fitting based on a direct model-parameter search using the neighbourhood algorithm (Yoshizawa & Kennett, 2002). The method of the waveform analysis has been fully automated by employing empirical quantitative measures for evaluating the accuracy/reliability of estimated multi-mode phase dispersion curves, and thus it is helpful in processing the dramatically increasing numbers of seismic data from the latest regional networks such as USArray. As a first step toward modeling the regional anisotropic shear-wave velocity structure of the North American upper mantle with extended vertical resolution, we have applied the method to long-period three-component records of seismic stations in North America, which mostly comprise the GSN and US regional networks as well as the permanent and transportable USArray stations distributed by the IRIS DMC. Preliminary multi-mode phase-speed models show large-scale patterns of isotropic heterogeneity, such as a strong velocity contrast between the western and central/eastern United States, which are consistent with the recent global and regional models (e.g., Marone, et al. 2007; Nettles & Dziewonski, 2008). We will also discuss radial anisotropy of shear wave speed beneath North America from multi-mode dispersion measurements of Love and Rayleigh waves.

Determining the upper mantle seismic structure beneath the northern Transantarctic Mountains, Antarctica, from regional P- and S-wave tomography

NASA Astrophysics Data System (ADS)

Brenn, Gregory Randall

Stretching 3,500 km across Antarctica, with peak elevations up to 4,500 m, the Transantarctic Mountains (TAMs) are the largest non-compressional continental mountain range on Earth and represent a tectonic boundary between the East Antarctica (EA) craton and the West Antarctic Rift System. The origin and uplift mechanism associated with the TAMs is controversial, and multiple models have been proposed. Seismic investigations of the TAM's subsurface structure can provide key constraints to help evaluate these models, but previous studies have been primarily focused only on the central TAMs near Ross Island. Using data from the new 15-station Transantarctic Mountain Northern Network as well as data from several smaller networks, this study investigates the upper mantle velocity structure beneath a previously unexplored portion of the northern TAMs through regional body wave tomography. Relative travel-times were calculated for 11,182 P-wave and 8,285 S-wave arrivals from 790 and 581 Mw ≥ 5.5 events, respectively, using multi-channel cross correlation, and these data were then inverted for models of the upper mantle seismic structure. Resulting P- and S-wave tomography images reveal two focused low velocity anomalies beneath Ross Island (RI; deltaVP ≈ -2.0%; deltaV S ≈ -1.5% to -4.0%) and Terra Nova Bay (TNB; deltaVP ≈ -1.5% to -2.0%; deltaVS ≈ -1.0% to -4.0%) that extend to depths of 200 and 150 km, respectively. The RI and TNB slow anomalies also extend 50-100 km laterally beneath the TAMs front and sharply abut fast velocities beneath the EA craton (deltaVP ≈ 0.5% to 2%; deltaV S ≈ 1.5% to 4.0%). A low velocity region (deltaVP ≈ -1.5%), centered at 150 km depth beneath the Terror Rift (TR) and primarily constrained within the Victoria Land Basin, connects the RI and TNB anomalies. The focused low velocities are interpreted as regions of partial melt and buoyancy-driven upwelling, connected by a broad region of slow (presumably warm) upper mantle associated with Cenozoic extension along the TR. Dynamic topography estimates based on the imaged S-wave velocity perturbations are consistent with observed surface topography in the central and northern TAMs, thereby providing support for uplift models that advocate for thermal loading and a flexural origin for the mountain range.

CCP Receiver-Function Imaging of the Moho beneath Volcanic Fields in Western Saudi Arabia

NASA Astrophysics Data System (ADS)

Blanchette, A. R.; Mooney, W. D.; Klemperer, S. L.; Zahran, H. M.; El-Hadidy, S. Y.

2015-12-01

We are searching for structural complexity in the crust and upper mantle beneath the Neogene volcanic fields ('harrats') of western Saudi Arabia. We determined P-wave seismic receiver functions for 50 broadband seismographic stations located within or adjacent to three volcanic fields: Harrats Lunayyir, Rahat, and Khaybar. There are 18 seismographic stations within Lunayyir, 11 in Khaybar, and 15 in Rahat with average interstation spacing of 10 km, 30km, and 50 km. For each station we calculated 300 to 600 receiver functions with an iterative time-domain deconvolution; noisy receiver functions (outliers) were rejected by cross correlating each receiver function with a station stack; we only accepted those with a cross correlation coefficient ≥ 0.6. We used these receiver functions to create a common-conversion point (CCP) image of the crust and upper mantle. The Moho and lithosphere-asthenosphere boundary (LAB) are clearly imaged, particularly beneath Lunayyir, and have average depths of about 38 km and 60 km. We do not find any evidence for structural disruption of the Moho within our ~70 km x 70 km image of the Moho beneath Lunayyir. We image a clear crust-mantle boundary beneath Rahat and Khaybar also at ~38 km, 2-3 km deeper than anticipated from prior receiver function results outside of the harrats. Mid-crustal low velocity zones seen locally beneath all three harrats, most commonly at 10-15 km or 15-20 km in depth, may more likely represent silicic Precambrian basement than accumulations of magma. Estimates of up to ~0.5 km3 of magma erupted during each eruptive episode are consistent with the lack of a disrupted Moho. However, the total erupted volume of magma, e.g. > 1000 km3 at Rahat, together with associated intrusions from the mantle, is consistent with crustal thickening of ~2 km beneath the harrats.

Improved Seismic Images of the Pacific Northwest Interior, With a Focus on the Region of the Columbia River Flood Basalts and Central Idaho

NASA Astrophysics Data System (ADS)

Stanciu, A. C.; Humphreys, E.; Clayton, R. W.

2017-12-01

We construct a P-wave model of the upper mantle based on new and previously acquired data from the USArray-TA stations and regional deployments, including the HLP, ID-OR, and the currently recording Wallowa stations. Our teleseismic arrival times are corrected for crustal structure (based on surface wave, receiver function, and controlled-source models from the region). Our modeling incorporates 3-D ray tracing and several simple considerations of radial anisotropy on travel time. As imaged previously, we find high P-wave velocity anomalies located beneath the Wallowa Mountains and beneath the Idaho Batholith in central west Idaho. Our improved imaging finds that these two anomalies are located down to 350 km depth, and are clearly separated from one another and from a shallower fast anomaly in the uppermost mantle beneath the westernmost Snake River Plain. Our preferred interpretation includes a combination of delamination and slab fragments in this region. As fast (and presumably cool) structures, these upper-mantle anomalies are thought to have a lithospheric origin. The anomaly beneath central Idaho is interpreted as the leading edge of the Farallon slab associated with the accretion of Siletzia terrane to North America. This anomaly may include some North American lithosphere that delaminated from the Laramide-thickened lithospheric mantle, perhaps related to Challis magmatism. The Wallowa anomaly is likely to represent Farallon lithosphere that delaminated during the Columbia River flood basalt event. The small anomaly between the two deeper fast anomalies, occurring at depths above 150km, could represent an isolated lithospheric fragment or a structure created by the Columbia River flood basalt event.

Coupled anisotropic and isotropic tomography of the upper mantle beneath northern Fennoscandia - Application of a novel code AniTomo

NASA Astrophysics Data System (ADS)

Munzarova, Helena; Plomerova, Jaroslava; Kissling, Edi; Vecsey, Ludek; Babuska, Vladislav

2017-04-01

Seismological investigations of the continental mantle lithosphere, particularly its anisotropic structure, advance our understanding of plate tectonics and formation of continents. Orientation of the anisotropic fabrics reflects stress fields during the lithosphere origin and its later deformations. To contribute to studies of the large-scale upper-mantle anisotropy, we have developed code AniTomo for regional anisotropic tomography. AniTomo allows a simultaneous inversion of relative travel time residuals of teleseismic P waves for 3D distribution of isotropic-velocity perturbations and anisotropy in the upper mantle. Weak hexagonal anisotropy with symmetry axis oriented generally in 3D is assumed. The code was successfully tested on a large series of synthetic datasets and synthetic structures. In this contribution we present results of the first application of novel code AniTomo to real data, i.e., relative travel-time residuals of teleseismic P waves recorded during passive seismic experiment LAPNET in the northern Fennoscandia between 2007 and 2009. The region of Fennoscandia is a suitable choice for the first application of the new code. This Precambrian region is tectonically stable and has a thick anisotropic mantle lithosphere (Plomerova and Babuska, Lithos 2010) without significant thermal heterogeneities. In the resulting anisotropic model of the upper mantle beneath the northern Fennoscandia, the strongest anisotropy and the largest velocity perturbations concentrate in the mantle lithosphere. We delimit regions of laterally and vertically consistent anisotropy in the mantle-lithospheric part of the model. In general, the identified anisotropic regions correspond to domains detected by joint interpretation of lateral variations of the P- and SKS-wave anisotropic parameters (Plomerova et al., Solid Earth 2011). Particularly, the mantle lithosphere in the western part of the volume studied exhibits a distinct and uniform fabric that is sharply separated from the surrounding regions. The eastern boundary of this region gradually shifts westward with increasing depth in the tomographic model. We connect the retrieved domain-like anisotropic structure of the mantle lithosphere in the northern Fennoscandia with preserved fossil fabrics of the Archean micro-plates, accreted during the Precambrian orogenic processes.

Interaction of the Cyprus/Tethys Slab With the Mantle Transition Zone Beneath Anatolia

NASA Astrophysics Data System (ADS)

Thompson, D. A.; Rost, S.; Taylor, G.; Cornwell, D. G.

2017-12-01

The geodynamics of the eastern Mediterranean are dominated by northward motion of the Arabian/African continents and subduction of the oldest oceanic crust on the planet along the Aegean and Cyprean trenches. These slabs have previously been imaged using seismic tomography on a continental scale, but detailed information regarding their descent from upper to lower mantle and how they interact with the mantle transition zone have been severely lacking. The Dense Array for North Anatolia (DANA) was a 73 station passive seismic deployment active between 2012-2013 with the primary aim of imaging shallow structure beneath the North Anatolian Fault. However, we exploit the exceptional dataset recorded by DANA to characterise a region where the Cyprus Slab impinges upon the mantle transition zone beneath northern Turkey, providing arguably the most detailed view of a slab as it transits from the upper to lower mantle. We map varying depths and amplitudes of the transition zone seismic discontinuities (`410', `520' and `660') in 3D using over 1500 high quality receiver functions over an area of approximately 200km x 300km. The `410' is observed close to its predicted depth, but the `660' is depressed to >670 km across the entirety of the study region. This is consistent with an accumulation of cold subducted material at the base of the upper mantle, and the presence of a `520' discontinuity in the vicinity of the slab surface also suggests that the slab is present deep within the transition zone. Anomalous low velocity layers above and within the transition zone are constrained and may indicate hydration and ongoing mass/fluid flux between upper and lower mantle in the presence of subduction. The results of the study have implications not only for the regional geodynamics of Anatolia, but also for slab dynamics globally.

Tottori earthquakes and Daisen volcano: Effects of fluids, slab melting and hot mantle upwelling

NASA Astrophysics Data System (ADS)

Zhao, Dapeng; Liu, Xin; Hua, Yuanyuan

2018-03-01

We investigate the 3-D seismic structure of source areas of the 6 October 2000 Western Tottori earthquake (M 7.3) and the 21 October 2016 Central Tottori earthquake (M 6.6) which occurred near the Daisen volcano in SW Japan. The two large events took place in a high-velocity zone in the upper crust, whereas low-velocity (low-V) and high Poisson's ratio (high-σ) anomalies are revealed in the lower crust and upper mantle. Low-frequency micro-earthquakes (M 0.0-2.1) occur in or around the low-V and high-σ zones, which reflect upward migration of magmatic fluids from the upper mantle to the crust under the Daisen volcano. The nucleation of the Tottori earthquakes may be affected by the ascending fluids. The flat subducting Philippine Sea (PHS) slab has a younger lithosphere age and so a higher temperature beneath the Daisen and Tottori area, facilitating the PHS slab melting. It is also possible that a PHS slab window has formed along the extinct Shikoku Basin spreading ridge beneath SW Japan, and mantle materials below the PHS slab may ascend to the shallow area through the slab window. These results suggest that the Daisen adakite magma was affected by the PHS slab melting and upwelling flow in the upper mantle above the subducting Pacific slab.

S-P wave travel time residuals and lateral inhomogeneity in the mantle beneath Tibet and the Himalaya

NASA Technical Reports Server (NTRS)

Molnar, P.; Chen, W.-P.

1984-01-01

S-P wave travel time residuals were measured in earthquakes in Tibet and the Himalaya in order to study lateral inhomogeneities in the earth's mantle. Average S-P residuals, measured with respect to Jeffrey-Bullen (J-B) tables for 11 earthquakes in the Himalaya are less than +1 second. Average J-B S-P from 10 of 11 earthquakes in Tibet, however, are greater than +1 second even when corrected for local crustal thickness. The largest values, ranging between 2.5 and 4.9 seconds are for five events in central and northern Tibet, and they imply that the average velocities in the crust and upper mantle in this part of Tibet are 4 to 10 percent lower than those beneath the Himalaya. On the basis of the data, it is concluded that it is unlikely that a shield structure lies beneath north central Tibet unless the S-P residuals are due to structural variations occurring deeper than 250 km.

Full-waveform Inversion for Localized 3-D S-velocity Structure in D" Beneath the Caribbean using US-Array Data

NASA Astrophysics Data System (ADS)

Borgeaud, A. F. E.; Konishi, K.; Kawai, K.; Geller, R. J.

2015-12-01

The region beneath Central America is known to have significant lateral velocity heterogeneities from the upper mantle down to the lowermost mantle. It is also known for its long history of subducting oceanic plates and fragmented plate remnants that sunk to the lowermost mantle (e.g., Ren et al., 2007). In this study, we use localized full-waveform inversion to invert for the 3-D S-velocity beneath the Caribbean. We use the DSM (Kawai et al., 2006) to compute 1-D synthetic seismograms and the first-order Born approximation to compute the partial derivatives for 3-D structure. We use a larger dataset with better coverage than Kawai et al. (2014), consisting of S and ScS phases from US-Array data for events in South America. The resulting 3-D model can contribute to understanding whether the cause of the velocity anomalies is thermal, chemical, or due to phase transitions.

Velocity and Attenuation Structure of the Tibetan Lithosphere using Seismic Attributes of P-waves from Regional Earthquakes Recorded by the Hi-CLIMB Array

NASA Astrophysics Data System (ADS)

Nowack, R. L.; Bakir, A. C.; Griffin, J.; Chen, W.; Tseng, T.

2010-12-01

Using data from regional earthquakes recorded by the Hi-CLIMB array in Tibet, we utilize seismic attributes from crustal and Pn arrivals to constrain the velocity and attenuation structure in the crust and the upper mantle in central and western Tibet. The seismic attributes considered include arrival times, Hilbert envelope amplitudes, and instantaneous as well as spectral frequencies. We have constructed more than 30 high-quality regional seismic profiles, and of these, 10 events have been selected with excellent crustal and Pn arrivals for further analysis. Travel-times recorded by the Hi-CLIMB array are used to estimate the large-scale velocity structure in the region, with four near regional events to the array used to constrain the crustal structure. The travel times from the far regional events indicate that the Moho beneath the southern Lhasa terrane is up to 75 km thick, with Pn velocities greater than 8 km/s. In contrast, the data sampling the Qiangtang terrane north of the Bangong-Nujiang (BNS) suture shows thinner crust with Pn velocities less than 8 km/s. Seismic amplitude and frequency attributes have been extracted from the crustal and Pn wave trains, and these data are compared with numerical results for models with upper-mantle velocity gradients and attenuation, which can strongly affect Pn amplitudes and pulse frequencies. The numerical modeling is performed using the complete spectral element method (SEM), where the results from the SEM method are in good agreement with analytical and reflectivity results for different models with upper-mantle velocity gradients. The results for the attenuation modeling in Tibet imply lower upper mantle Q values in the Qiangtang terrane to the north of the BNS compared to the less attenuative upper mantle beneath the Lhasa terrane to the south of the BNS.

Crustal structure beneath western and eastern Iceland from surface waves and receiver functions

USGS Publications Warehouse

Du, Z.; Foulger, G.R.; Julian, B.R.; Allen, R.M.; Nolet, G.; Morgan, W.J.; Bergsson, B.H.; Erlendsson, P.; Jakobsdottir, S.; Ragnarsson, S.; Stefansson, R.; Vogfjord, K.

2002-01-01

We determine the crustal structures beneath 14 broad-band seismic stations, deployed in western, eastern, central and southern Iceland, using surface wave dispersion curves and receiver functions. We implement a method to invert receiver functions using constraints obtained from genetic algorithm inversion of surface waves. Our final models satisfy both data sets. The thickness of the upper crust, as defined by the velocity horizon Vs = 3.7 km s-1, is fairly uniform at ???6.5-9 km beneath the Tertiary intraplate areas of western and eastern Iceland, and unusually thick at 11 km beneath station HOT22 in the far south of Iceland. The depth to the base of the lower crust, as defined by the velocity horizon Vs = 4.1 km s-1 is ???20-26 km in western Iceland and ???27-33 km in eastern Iceland. These results agree with those of explosion profiles that detect a thinner crust beneath western Iceland than beneath eastern Iceland. An earlier report of a substantial low-velocity zone beneath the Middle Volcanic Zone in the lower crust is confirmed by a similar observation beneath an additional station there. As was found in previous receiver function studies, the most reliable feature of the results is the clear division into an upper sequence that is a few kilometres thick where velocity gradients are high, and a lower, thicker sequence where velocity gradients are low. The transition to typical mantle velocities is variable, and may range from being very gradational to being relatively sharp and clear. A clear Moho, by any definition, is rarely seen, and there is thus uncertainty in estimates of the thickness of the crust in many areas. Although a great deal of seismic data are now available constraining the structures of the crust and upper mantle beneath Iceland, their geological nature is not well understood.

3-D Upper-Mantle Shear Velocity Model Beneath the Contiguous United States Based on Broadband Surface Wave from Ambient Seismic Noise

NASA Astrophysics Data System (ADS)

Xie, Jun; Chu, Risheng; Yang, Yingjie

2018-05-01

Ambient noise seismic tomography has been widely used to study crustal and upper-mantle shear velocity structures. Most studies, however, concentrate on short period (< 50 s) surface wave from ambient noise, while studies using long period surface wave from ambient noise are limited. In this paper, we demonstrate the feasibility of using long-period surface wave from ambient noise to study the lithospheric structure on a continental scale. We use broadband Rayleigh wave phase velocities to obtain a 3-D V S structures beneath the contiguous United States at period band of 10-150 s. During the inversion, 1-D shear wave velocity profile is parameterized using B-spline at each grid point and is inverted with nonlinear Markov Chain Monte Carlo method. Then, a 3-D shear velocity model is constructed by assembling all the 1-D shear velocity profiles. Our model is overall consistent with existing models which are based on multiple datasets or data from earthquakes. Our model along with the other post-USArray models reveal lithosphere structures in the upper mantle, which are consistent with the geological tectonic background (e.g., the craton root and regional upwelling provinces). The model has comparable resolution on lithosphere structures compared with many published results and can be used for future detailed regional or continental studies and analysis.

Shear wave reflectivity imaging of the Nazca-South America subduction zone: Stagnant slab in the mantle transition zone?

NASA Astrophysics Data System (ADS)

Contenti, Sean; Gu, Yu Jeffrey; Ökeler, Ahmet; Sacchi, Mauricio D.

2012-01-01

In this study we utilize over 5000 SS waveforms to investigate the high-resolution mantle reflectivity structure down to 1200 km beneath the South American convergent margin. Our results indicate that the dynamics of the Nazca subduction are more complex than previously suggested. The 410- and 660-km seismic discontinuities beneath the Pacific Ocean and Amazonian Shield exhibit limited lateral depth variations, but their depths vary substantially in the vicinity of the subducting Nazca plate. The reflection amplitude of the 410-km discontinuity is greatly diminished in a ˜1300-km wide region in the back-arc of the subducting plate, which is likely associated with a compositional heterogeneity on top of the upper mantle transition zone. The underlying 660-km discontinuity is strongly depressed, showing localized depth and amplitude variations both within and to the east of the Wadati-Benioff zone. The width of this anomalous zone (˜1000 km) far exceeds that of the high-velocity slab structure and suggesting significant slab deformation within the transition zone. The shape of the 660-km discontinuity and the presence of lower mantle reflectivity imply both stagnation and penetration are possible as the descending Nazca slab impinges upon the base of the upper mantle.

Lithospheric Delamination or Relict Slab Beneath the Former North American Cratonic Margin in Idaho and Oregon? New Constraints From Seismic Tomography.

NASA Astrophysics Data System (ADS)

Stanciu, A. C.; Russo, R. M.; Mocanu, V. I.; VanDecar, J. C.; Hongsresawat, S.; Bremner, P. M.; Torpey, M. E.; Panning, M. P.

2016-12-01

We present a new high-resolution P-wave velocity model of the upper mantle beneath the former passive margin of the North American craton in Oregon and Idaho. We identify high velocity anomalies in the central part of the model and low velocity anomalies to the northwest and southeast. Our results derive from an integrated data set of teleseismic P waves recorded at 145 broadband stations, 85 deployed between 2011 and 2013 as part of the IDOR Passive experiment, and 60 USArray-TA stations. We determined 15,000 travel-times using multi-channel cross-correlation (VanDecar and Crosson, 1990). Phanerozoic tectonic events that affected upper mantle seismic structure here include subduction of Farallon and Juan de Fuca lithosphere, accretion of Blue Mountains terranes, Sevier and Laramide orogenies, Idaho batholith formation, Yellowstone and Columbia River volcanism, and Basin and Range extension. Our results indicate a high P-wave velocity anomaly located beneath the Idaho Batholith in west-central Idaho traceable down to 150-200 km depth. A similar anomaly identified by Schmandt and Humphrey (2011) beneath Washington and Montana was interpreted as a slab remnant from the accretion of Siletzia to North America. Alternatively, the fast Vp anomalies are delaminated North American craton lithosphere. Thickened lithosphere may have formed during Farallon subduction, terrane collision and accretion. Crust as much as 55 km thick present during Late Cretaceous (Foster et al., 2001; Gaschnig et al., 2011) is potentially indicative of lithospheric thickening leading to delamination. To the southeast, upper mantle low velocity anomalies occur beneath the Western Snake River Plain. We associate these low velocities with high temperatures generated by the Yellowstone mantle plume system. We observe a low velocity anomaly beneath the Wallowa Mountains starting at 150-200 km extending to depths below the resolution of our model.

Adjoint tomography of crust and upper-mantle structure beneath Continental China

NASA Astrophysics Data System (ADS)

Chen, M.; Niu, F.; Liu, Q.; Tromp, J.

2013-12-01

Four years of regional earthquake recordings from 1,869 seismic stations are used for high-resolution and high-fidelity seismic imaging of the crust and upper-mantle structure beneath Continental China. This unprecedented high-density dataset is comprised of seismograms recorded by the China Earthquake Administration Array (CEArray), NorthEast China Extended SeiSmic Array (NECESSArray), INDEPTH-IV Array, F-net and other global and regional seismic networks, and involves 1,326,384 frequency-dependent phase measurements. Adjoint tomography is applied to this unprecedented dataset, aiming to resolve detailed 3D maps of compressional and shear wavespeeds, and radial anisotropy. Contrary to traditional ray-theory based tomography, adjoint tomography takes into account full 3D wave propagation effects and off-ray-path sensitivity. In our implementation, it utilizes a spectral-element method for precise wave propagation simulations. The tomographic method starts with a 3D initial model that combines smooth radially anisotropic mantle model S362ANI and 3D crustal model Crust2.0. Traveltime and amplitude misfits are minimized iteratively based on a conjugate gradient method, harnessing 3D finite-frequency kernels computed for each updated 3D model. After 17 iterations, our inversion reveals strong correlations of 3D wavespeed heterogeneities in the crust and upper mantle with surface tectonic units, such as the Himalaya Block, the Tibetan Plateau, the Tarim Basin, the Ordos Block, and the South China Block. Narrow slab features emerge from the smooth initial model above the transition zone beneath the Japan, Ryukyu, Philippine, Izu-Bonin, Mariana and Andaman arcs. 3D wavespeed variations appear comparable to or much sharper than in high-frequency P-and S-wave models from previous studies. Moreover our results include new information, such as 3D variations of radial anisotropy and the Vp/Vs ratio, which are expected to shed new light to the composition, thermal state, flow or fabric structure in the crust and upper mantle, as well as the related dynamical processes. We intend to use these seismic images to answer important tectonic questions, namely, 1) what controls the strength of the lithosphere; 2) how does lithosphere deform during the formation of orogens, basins and plateaus; 3) how pervasive is lithospheric delamination or partial removal beneath orogens and plateaus; 3) whether or not (and how) are slab segmentation and penetration into the lower mantle linked to upwellings associated with widespread magmatism in East Asia.

Lithospheric Instability beneath the Southeast Carpathians

NASA Astrophysics Data System (ADS)

Houseman, G. A.; Lorinczi, P.; Ren, Y.; Stuart, G. W.

2012-12-01

The South Carpathian Project, a major seismological experiment carried out during 2009-2011 by the University of Leeds, the National Institute of Earth Physics in Bucharest, the Eötvös Loránd Geophysical Institute in Budapest, and the Seismological Survey of Serbia in Belgrade, has resulted in the most detailed tomographic images yet obtained of the upper mantle structure beneath the Pannonian - Carpathian region (Ren et al., EPSL, 2012). These images illuminate the upper mantle over a wide region, but they specifically shed new light on the unique geological structure which is responsible for the damaging earthquakes that occur in the upper mantle beneath the Vrancea Zone of the South-east Carpathians. The earthquakes occur at the NE end of an asymmetric high velocity structure that extends upward to the SW, oblique to the southern edge of the South Carpathians. This sub-vertical high-velocity body is bounded by slow anomalies to the NW and SE, which extend down to the top of the Mantle Transition Zone. With increasing depth, the fast region becomes more circular in cross-section until about 400 km where the fast anomaly fades out. The main mass of fast (presumably dense) material is located directly beneath the seismic activity. The earthquakes are all characterised by near-vertical T-axes, which means they are caused by vertical stretching. The seismic moment release rate can be used to estimate vertical strain rates; these strain-rates imply that the mantle at 200 km is moving downward at about 20 mm/yr relative to the surface. The depth distribution of seismic-moment release rate follows a characteristic pattern that is most easily explained if this high velocity structure is produced by a Rayleigh-Taylor instability acting on an unstable stratification of mantle lithosphere above asthenosphere. Three-dimensional numerical experiments assuming viscous flow confirm that the drip-like structure that we image may be a natural consequence of a Rayleigh-Taylor instability triggered by recent convergence of Adria and Europe. One property of Rayleigh-Taylor instability is that rates of deformation increase with time during the peak development phase of the instability. The present high rate of tectonic activity in this region therefore is probably short-lived on a geological scale (< 1 Myr) but is likely to increase in energy before it is abates. These unique tomographic and seismological datasets provide clear evidence of lithospheric gravitational instability occurring on a short length-scale and fast time-scale via a mode that is best described as a form of Rayleigh-Taylor instability.

The p-wave upper mantle structure beneath an active spreading centre - The Gulf of California

NASA Technical Reports Server (NTRS)

Walck, M. C.

1984-01-01

Over 1400 seismograms of earthquakes in Mexico are analyzed and data sets for the travel time, apparent phase velocity, and relative amplitude information are utilized to produce a tightly constrained, detailed model for depths to 900 km beneath an active oceanic ridge region, the Gulf of California. The data are combined by first inverting the travel times, perturbing that model to fit the p-delta data, and then performing trial and error synthetic seismogram modelling to fit the short-period waveforms. The final model satisfies all three data sets. The ridge model is similar to existing upper mantle models for shield, tectonic-continental, and arc-trench regimes below 400 km, but differs significantly in the upper 350 km. Ridge model velocities are very low in this depth range; the model 'catches up' with the others with a very large velocity gradient from 225 to 390 km.

Crustal and uppermost mantle structure and deformation in east-central China

NASA Astrophysics Data System (ADS)

Li, H.; Yang, X.; Ouyang, L.; Li, J.

2017-12-01

We conduct a non-linear joint inversion of receiver functions and Rayleigh wave dispersions to obtain the crustal and upper mantle velocity structure in east-central China. In the meanwhile, the lithosphere and upper mantle deformation beneath east-central China is also evaluated with teleseismic shear wave splitting measurements. The resulting velocity model reveals that to the east of the North-South Gravity Lineament, the crust and the lithosphere are significantly thinned. Furthermore, three extensive crustal/lithospheric thinning sub-regions are clearly identified within the study area. This indicates that the modification of the crust and lithosphere in central-eastern China is non-uniform due to the heterogeneity of the lithospheric strength. Extensive crustal and lithospheric thinning could occur in some weak zones such as the basin-range junction belts and large faults. The structure beneath the Dabie orogenic belt is complex due to the collision between the North and South China Blocks during the Late Paleozoic-Triassic. The Dabie orogenic belt is generally delineated by a thick crust with a mid-crust low-velocity zone and a two-directional convergence in the lithospheric scale. Obvious velocity contrast exhibits in the crust and upper mantle at both sides of the Tanlu fault, which suggests the deep penetration of this lithospheric-scale fault. Most of our splitting measurements show nearly E-W trending fast polarization direction which is slightly deviating from the direction of plate motion. The similar present-day lithosphere structure and upper mantle deformation may imply that the eastern NCC and the eastern SCB were dominated by a common dynamic process after late Mesozoic, i.e., the westward subduction of Pacific plate and the retreat of the subduction plate. The westward subduction of the Philippine plate and the long-range effects of the collision between the Indian plate and Eurasia plate during Cenozoic may have also contributed to the present velocity structure and stress environment of eastern China.

Linking Upper Mantle Processes and Long-wavelength Topographic Swells in Cenozoic Africa

NASA Astrophysics Data System (ADS)

Nixon, S.; Maclennan, J.; White, N.; Fishwick, S.

2008-12-01

The topography of present day Africa is influenced by two different wavelengths of dynamic support. The South African Superplume sits beneath Sub-equatorial Africa and is thought to be supported by a lower mantle thermo-chemical anomaly. On a smaller scale a series of topographic domal swells, 1000km in diameter, occur across the continent. The swells are characterised by elevated dynamic topography, a positive long-wavelength gravity anomaly and a negative velocity perturbation from a higher mode surface wave tomography model. In addition, where the lithosphere is thinner than 100km, the swells are capped with volcanic products, erupted periodically since ~30 Ma. These areas include the Cameroon Volcanic line, Hoggar, Tibesti and Darfur in North Africa, and the Ethiopian Plateau and the Kenyan dome found along the East African Rift system. The given relationships suggest the domal swells result from and are supported by upper mantle convection. In order to investigate these relationships a database of 3000 geochemical analyses has been assembled for Cenozoic African volcanism, from both literature search and by new analyses of samples collected from the Al Haruj volcanic field in Libya. Incompatible trace element ratios and REE trends from primitive basalts (>7wt% MgO) erupted less then 10Ma, representing the products of mantle melting, are compared with the upper mantle velocity structure. At depths of 75-100km the greatest velocity perturbation is associated with the Afar/Ethiopia region, where as smaller perturbations are found beneath the North African swells of Hoggar, Tibesti and Darfur. The comparison of absolute velocities, taken from the higher mode tomography model, with trace element ratios has found the low seismic velocity Afar/Ethiopia region to have shallow melting at high melt fractions (La/Yb~9) whereas North African swells with faster seismic velocities at 100 km depth, show deeper melting with smaller melt fractions (La/Yb~30). This positive correlation continues to depths of 150km and is believed to represent variations in mantle potential temperature beneath the African continent. With further modelling of major, trace and REE data we hope to provide insights into variations in mantle potential temperature, melt fraction and velocity structure beneath the topographic swells across the African continent.

P wave anisotropic tomography of the Alps

NASA Astrophysics Data System (ADS)

Hua, Yuanyuan; Zhao, Dapeng; Xu, Yixian

2017-06-01

The first tomographic images of P wave azimuthal and radial anisotropies in the crust and upper mantle beneath the Alps are determined by joint inversions of arrival time data of local earthquakes and teleseismic events. Our results show the south dipping European plate with a high-velocity (high-V) anomaly beneath the western central Alps and the north dipping Adriatic plate with a high-V anomaly beneath the Eastern Alps, indicating that the subduction polarity changes along the strike of the Alps. The P wave azimuthal anisotropy is characterized by mountain chain-parallel fast-velocity directions (FVDs) in the western central Alps and NE-SW FVDs in the Eastern Alps, which may be caused by mantle flow induced by the slab subductions. Our results reveal a negative radial anisotropy (i.e., Vph < Vpv) within the subducting slabs and a positive radial anisotropy (i.e., Vph > Vpv) in the low-velocity mantle wedge, which may reflect the subvertical plate subduction and its induced mantle flow. The results of anisotropic tomography provide important new information on the complex mantle structure and dynamics of the Alps and adjacent regions.

Lowermost mantle anisotropy and deformation along the boundary of the African LLSVP

NASA Astrophysics Data System (ADS)

Lynner, Colton; Long, Maureen D.

2014-05-01

Shear wave splitting of SK(K)S phases is often used to examine upper mantle anisotropy. In specific cases, however, splitting of these phases may reflect anisotropy in the lowermost mantle. Here we present SKS and SKKS splitting measurements for 233 event-station pairs at 34 seismic stations that sample D″ beneath Africa. Of these, 36 pairs show significantly different splitting between the two phases, which likely reflects a contribution from lowermost mantle anisotropy. The vast majority of discrepant pairs sample the boundary of the African large low shear velocity province (LLSVP), which dominates the lower mantle structure beneath this region. In general, we observe little or no splitting of phases that have passed through the LLSVP itself and significant splitting for phases that have sampled the boundary of the LLSVP. We infer that the D″ region just outside the LLSVP boundary is strongly deformed, while its interior remains undeformed (or weakly deformed).

Adakitic-like volcanism in Southern Mexico and subduction of the Tehuantepec Ridge

NASA Astrophysics Data System (ADS)

Manea, M.; Manea, V. C.

2007-05-01

The origin of El Chichón volcano is poorly understood, and our attempt in this study is to demonstrate that Tehuantepec Ridge, a major tectonic discontinuity on the Cocos plate, plays a key role in the slab dehydration budget and therefore in partial melting of the mantle beneath El Chichón. Using marine magnetic anomalies we show that the upper mantle beneath TR undergo partial serpentinization, a 5-7 km thick serpentinized root extending along TR and below the oceanic crust. Another key aspect of the magnetic anomaly over southern México is a long-wavelength (~150 km) high amplitude (~500 nT) magnetic anomaly located between the trench and the coast. Using a 2D joint magnetic-gravity forward model, constrained by the subduction P-T structure, slab geometry and seismicity, we find a highly magnetic and low-density source located at 40-130 km depth. We interpret this result as a serpentinized mantle wedge by fluids expelled from the subducting Cocos plate beneath southern Mexico. Such a deep hydrated mantle requires a low temperature wedge (T<600° C) because serpentine is stable below this temperature and also the magnetic properties are preserved for temperature less than the Currie point for magnetite (~580° C). This result explains the lack of volcanism in southern México where the slab depth is ~ 100 km. Using phase diagrams for sediments, basalt and peridotite, and the subduction P-T structure beneath El Chichón we find that sediments and basalt dehydrate ~ 50% at depths corresponding with the location of serpentinized mantle wedge, whereas the serpentinized root beneath TR strongly dehydrates (60-80%) at higher depths (170-180 km) comparable with the slab depth beneath El Chichón. We conclude that this strong deserpentinization pulse of mantle lithosphere beneath TR at great depths triggers arc melting, explaining the unusual location and probably the adakitic signature of El Chichón.

Thin Lithosphere Beneath the Ethiopian Plateau Revealed by a Joint Inversion of Rayleigh Wave Group Velocities and Receiver Functions

NASA Astrophysics Data System (ADS)

Dugda, Mulugeta T.; Nyblade, Andrew A.; Julia, Jordi

2007-08-01

The seismic velocity structure of the crust and upper mantle beneath Ethiopia and Djibouti has been investigated by jointly inverting receiver functions and Rayleigh wave group velocities to obtain new constraints on the thermal structure of the lithosphere. Most of the data for this study come from the Ethiopia broadband seismic experiment, conducted between 2000 and 2002. Shear velocity models obtained from the joint inversion show crustal structure that is similar to previously published models, with crustal thicknesses of 35 to 44 km beneath the Ethiopian Plateau, and 25 to 35 km beneath the Main Ethiopian Rift (MER) and the Afar. The lithospheric mantle beneath the Ethiopian Plateau has a maximum shear wave velocity of about 4.3 km/s and extends to a depth of ˜70-80 km. Beneath the MER and Afar, the lithospheric mantle has a maximum shear wave velocity of 4.1-4.2 km/s and extends to a depth of at most 50 km. In comparison to the lithosphere away from the East African Rift System in Tanzania, where the lid extends to depths of ˜100-125 km and has a maximum shear velocity of 4.6 km/s, the mantle lithosphere under the Ethiopian Plateau appears to have been thinned by ˜30-50 km and the maximum shear wave velocity reduced by ˜0.3 km/s. Results from a 1D conductive thermal model suggest that the shear velocity structure of the Ethiopian Plateau lithosphere can be explained by a plume model, if a plume rapidly thinned the lithosphere by ˜30-50 km at the time of the flood basalt volcanism (c. 30 Ma), and if warm plume material has remained beneath the lithosphere since then. About 45-65% of the 1-1.5 km of plateau uplift in Ethiopia can be attributed to the thermally perturbed lithospheric structure.

Along-arc variation in water distribution in the upper mantle beneath Kyushu, Japan, as derived from receiver function analyses

NASA Astrophysics Data System (ADS)

Abe, Y.; Ohkura, T.; Hirahara, K.; Shibutani, T.

2013-12-01

The Kyushu district, Japan, under which the Philippine Sea (PHS) plate is subducting in a WNW direction, has several active volcanoes. On the volcanic front in Kyushu, a 110 km long gap in volcanism exists in the central part of Kyushu and volcanic rocks with various degrees of contamination by slab-derived fluid are distributed. To reveal the causes of the gap in volcanism and the chemical properties of volcanic rocks and to understand the process of magma genesis and water transportation, we should reveal along-arc variation in water distribution beneath Kyushu. We investigated the seismic velocity discontinuities in the upper mantle beneath Kyushu, with seismic waveform data from 65 stations of Hi-net, which are established by National Research Institute for Earth Science and Disaster Prevention, and 55 stations of the J-array, which are established by Japan Meteorological Agency, Kyushu University, Kagoshima University and Kyoto University. We used receiver function analyses developed especially for discontinuities with high dipping angles (Abe et al., 2011, GJI). We obtained the geometry and velocity contrasts of the continental Moho, the oceanic Moho, and the upper boundary of the PHS slab. From the geometry of these discontinuities and contrast in S wave velocities, we interpreted that the oceanic crust of the PHS slab has a low S wave velocity and is hydrated to a depth of 70 km beneath south Kyushu, to a depth of 80-90 km beneath central Kyushu, and to a depth of no more than 50 km beneath north Kyushu. We also interpreted that the fore-arc mantle beneath central Kyushu has a low velocity region (Vs < 3.2 km/s) that can contain hydrated materials and free aqueous fluid. Such a low velocity fore-arc mantle does not exist beneath north and south Kyushu. Beneath north Kyushu, the oceanic crust does not appear to convey much water in the mantle wedge. Beneath south Kyushu, water dehydrated from the slab could move to the back-arc side and cause arc volcanism, while it could move to the fore-arc side and cause a gap in volcanism and hydration of the fore-arc mantle materials.

Mantle wedge exhumation beneath the Dora-Maira (U)HP dome unravelled by local earthquake tomography (Western Alps)

NASA Astrophysics Data System (ADS)

Solarino, Stefano; Malusà, Marco G.; Eva, Elena; Guillot, Stéphane; Paul, Anne; Schwartz, Stéphane; Zhao, Liang; Aubert, Coralie; Dumont, Thierry; Pondrelli, Silvia; Salimbeni, Simone; Wang, Qingchen; Xu, Xiaobing; Zheng, Tianyu; Zhu, Rixiang

2018-01-01

In continental subduction zones, the behaviour of the mantle wedge during exhumation of (ultra)high-pressure [(U)HP] rocks provides a key to distinguish among competing exhumation mechanisms. However, in spite of the relevant implications for understanding orogenic evolution, a high-resolution image of the mantle wedge beneath the Western Alps is still lacking. In order to fill this gap, we perform a detailed analysis of the velocity structure of the Alpine belt beneath the Dora-Maira (U)HP dome, based on local earthquake tomography independently validated by receiver function analysis. Our results point to a composite structure of the mantle wedge above the subducted European lithosphere. We found that the Dora-Maira (U)HP dome lays directly above partly serpentinized peridotites (Vp 7.5 km/s; Vp/Vs = 1.70-1.72), documented from 10 km depth down to the top of the eclogitized lower crust of the European plate. These serpentinized peridotites, possibly formed by fluid release from the subducting European slab to the Alpine mantle wedge, are juxtaposed against dry mantle peridotites of the Adriatic upper plate along an active fault rooted in the lithospheric mantle. We propose that serpentinized mantle-wedge peridotites were exhumed at shallow crustal levels during late Eocene transtensional tectonics, also triggering the rapid exhumation of (U)HP rocks, and were subsequently indented under the Alpine metamorphic wedge in the early Oligocene. Our findings suggest that mantle-wedge exhumation may represent a major feature of the deep structure of exhumed continental subduction zones. The deep orogenic levels here imaged by seismic tomography may be exposed today in older (U)HP belts, where mantle-wedge serpentinites are commonly associated with coesite-bearing continental metamorphic rocks.

Lithosphere-asthenosphere interaction beneath Ireland from joint inversion of teleseismic P-wave delay times and GRACE gravity

NASA Astrophysics Data System (ADS)

O'Donnell, J. P.; Daly, E.; Tiberi, C.; Bastow, I. D.; O'Reilly, B. M.; Readman, P. W.; Hauser, F.

2011-03-01

The nature and extent of the regional lithosphere-asthenosphere interaction beneath Ireland and Britain remains unclear. Although it has been established that ancient Caledonian signatures pervade the lithosphere, tertiary structure related to the Iceland plume has been inferred to dominate the asthenosphere. To address this apparent contradiction in the literature, we image the 3-D lithospheric and deeper upper-mantle structure beneath Ireland via non-linear, iterative joint teleseismic-gravity inversion using data from the ISLE (Irish Seismic Lithospheric Experiment), ISUME (Irish Seismic Upper Mantle Experiment) and GRACE (Gravity Recovery and Climate Experiment) experiments. The inversion combines teleseismic relative arrival time residuals with the GRACE long wavelength satellite derived gravity anomaly by assuming a depth-dependent quasilinear velocity-density relationship. We argue that anomalies imaged at lithospheric depths probably reflect compositional contrasts, either due to terrane accretion associated with Iapetus Ocean closure, frozen decompressional melt that was generated by plate stretching during the opening of the north Atlantic Ocean, frozen Iceland plume related magmatic intrusions, or a combination thereof. The continuation of the anomalous structure across the lithosphere-asthenosphere boundary is interpreted as possibly reflecting sub-lithospheric small-scale convection initiated by the lithospheric compositional contrasts. Our hypothesis thus reconciles the disparity which exists between lithospheric and asthenospheric structure beneath this region of the north Atlantic rifted margin.

Crust and uppermost-mantle structure of Greenland and the Northwest Atlantic from Rayleigh wave group velocity tomography

NASA Astrophysics Data System (ADS)

Darbyshire, Fiona A.; Dahl-Jensen, Trine; Larsen, Tine B.; Voss, Peter H.; Joyal, Guillaume

2018-03-01

The Greenland landmass preserves ˜4 billion years of tectonic history, but much of the continent is inaccessible to geological study due to the extensive inland ice cap. We map out, for the first time, the 3-D crustal structure of Greenland and the NW Atlantic ocean, using Rayleigh wave anisotropic group velocity tomography, in the period range 10-80 s, from regional earthquakes and the ongoing GLATIS/GLISN seismograph networks. 1-D inversion gives a pseudo-3-D model of shear wave velocity structure to depths of ˜100 km with a horizontal resolution of ˜200 km. Crustal thickness across mainland Greenland ranges from ˜25 km to over 50 km, and the velocity structure shows considerable heterogeneity. The large sedimentary basins on the continental shelf are clearly visible as low velocities in the upper ˜5-15 km. Within the upper continental basement, velocities are systematically lower in northern Greenland than in the south, and exhibit a broadly NW-SE trend. The thinning of the crust at the continental margins is also clearly imaged. Upper-mantle velocities show a clear distinction between typical fast cratonic lithosphere (Vs ≥4.6 km s-1) beneath Greenland and its NE margin and anomalously slow oceanic mantle (Vs ˜4.3-4.4 km s-1) beneath the NW Atlantic. We do not observe any sign of pervasive lithospheric modification across Greenland in the regions associated with the presumed Iceland hotspot track, though the average crustal velocity in this region is higher than that of areas to the north and south. Crustal anisotropy beneath Greenland is strong and complex, likely reflecting numerous episodes of tectonic deformation. Beneath the North Atlantic and Baffin Bay, the dominant anisotropy directions are perpendicular to the active and extinct spreading centres. Anisotropy in the subcontinental lithosphere is weaker than that of the crust, but still significant, consistent with cratonic lithosphere worldwide.

Crustal shear wave velocity and radial anisotropy beneath the Rio Grande rift from ambient noise tomography

NASA Astrophysics Data System (ADS)

Fu, Yuanyuan V.; Li, Aibing

2015-02-01

Shear wave velocity and radial anisotropy beneath New Mexico are obtained from ambient seismic noise tomography using data from the Transportable Array. Besides the distinct seismic structure imaged across the Rio Grande rift from the Colorado Plateau to the Great Plains, both velocity and anisotropy models also reveal significant variations along the rift. The rift at Albuquerque is characterized by remarkably low velocity in the shallow crust, high velocity and strong positive anisotropy in the middle and lower crust, and low velocity in the upper mantle. These observations can be interpreted as magma accumulation in the shallow crust and significant mafic underplating in the lower crust with abundant melt supply from the hot mantle. We propose that the Albuquerque region has recently been experiencing the most vigorous extensional deformation in the rift. Positive anisotropy with Vsh > Vsv appears in the central and southern rifts with a stronger anisotropy beneath younger volcanoes, reflecting layering of magma intrusion due to past and recent rifting activities. The low velocities in the uppermost mantle are observed under high-elevation places, the Jemez Lineament, northern rift, and east rift boundary, implying that the buoyancy of hot mantle largely compensates the local high topography. Low mantle velocities appear at the boundary of the southern rift, corresponding to the large lithosphere thickness change, instead of the rift center, consistent with the prediction from the small-scale, edge-driven mantle convection model. We conclude that the edge-driven upper mantle convection is probably the dominant mechanism for the recent and current rifting and uplift in the Rio Grande rift.

Receiver function imaging of mantle transition zone discontinuities beneath the Tanzania Craton and the Eastern and Western Branches of the East African Rift System

NASA Astrophysics Data System (ADS)

Sun, M.; Liu, K. H.; Fu, X.; Gao, S. S.

2017-12-01

To investigate the mechanism of initiation and development of the Eastern African Rifting System (EARS) circumfluent the Tanzania Craton (TC), over 7,100 P-to-S radial receiver functions (RFs) recorded by 87 broadband seismic stations are stacked to map the topography of mantle transition zone (MTZ) discontinuities beneath the TC and the Eastern and Western Branches of the EARS. After time-depth conversion using the 1-D IASP91 Earth model, the resulting 410 km (d410) and 660 km (d660) discontinuity apparent depths are found to be greater than the global averages beneath the whole study area, implying slower than normal upper mantle velocities. The mean thickness of the MTZ beneath the Western Branch and TC is about 252 km, which is comparable to the global average and is inconsistent with the existence of present-day thermal upwelling originating from the lower mantle. In contrast, beneath the Eastern Branch, an 30 km thinning of the MTZ is observed from an up to 50 km and 20 km apparent depression of the d410 and d660, respectively. On the basis of previous seismic tomographic results and empirical relationships between velocity and thermal anomalies, we propose that the most plausible explanation for the observations beneath the volcanic Eastern Branch is the existence of a low-velocity layer extending from the surface to the upper MTZ, probably caused by decompression partial melting associated with continental rifting. The observations are in general agreement with an upper mantle origin for the initiation and development of both the Western and Eastern Branches of the EARS beneath the study area.

P-wave velocity structure beneath the northern Antarctic Peninsula: evidence of a steeply subducting slab and a deep-rooted low-velocity anomaly beneath the central Bransfield Basin

NASA Astrophysics Data System (ADS)

Park, Yongcheol; Kim, Kwang-Hee; Lee, Joohan; Yoo, Hyun Jae; Plasencia L., Milton P.

2012-12-01

Upper-mantle structure between 100 and 300 km depth below the northern Antarctic Peninsula is imaged by modelling P-wave traveltime residuals from teleseismic events recorded on the King Sejong Station (KSJ), the Argentinean/Italian stations (JUBA and ESPZ), an IRIS/GSN Station (PMSA) and the Seismic Experiment in Patagonia and Antarctica (SEPA) broad-band stations. For measuring traveltime residuals, we applied a multichannel cross-correlation method and inverted for upper-mantle structure using VanDecar's method. The new 3-D velocity model reveals a subducted slab with a ˜70° dip angle at 100-300 km depth and a strong low-velocity anomaly confined below the SE flank of the central Bransfield Basin. The low velocity is attributed to a thermal anomaly in the mantle that could be as large as 350-560 K and which is associated with high heat flow and volcanism in the central Bransfield Basin. The low-velocity zone imaged below the SE flank of the central Bransfield Basin does not extend under the northern Bransfield Basin, suggesting that the rifting process in that area likely involves different geodynamic processes.

Layered Crustal and Mantle Structure and Anisotropy beneath the Afar Depression and Malawi Rift Zone

NASA Astrophysics Data System (ADS)

Reed, Cory Alexander

Although a wealth of geophysical data sets have been acquired within the vicinity of continental rift zones, the mechanisms responsible for the breakup of stable continental lithosphere are ambiguous. Eastern Africa is host to the largest contemporary rift zone on Earth, and is thus the most prominent site with which to investigate the processes which govern the rupture of continental lithosphere. The studies herein represent teleseismic analyses of the velocity and thermomechanical structure of the crust and mantle beneath the Afar Depression and Malawi Rift Zone (MRZ) of the East African Rift System. Within the Afar Depression, the first densely-spaced receiver function investigation of crustal thickness and inferred velocity attenuation across the Tendaho Graben is conducted, and the largest to-date study of the topography of the mantle transition zone (MTZ) beneath NE Africa is provided, which reveals low upper-mantle velocities beneath the Afar concordant with a probable mantle plume traversing the MTZ beneath the western Ethiopian Plateau. In the vicinity of the MRZ, a data set comprised of 35 seismic stations is employed that was deployed over a two year period from mid-2012 to mid-2014, belonging to the SAFARI (Seismic Arrays For African Rift Initiation) experiment. Accordingly, the first MTZ topography and shear wave splitting analyses were conducted in the region. The latter reveals largely plate motion-parallel anisotropy that is locally modulated by lithospheric thickness abnormalities adjacent to the MRZ, while the former reveals normal MTZ thicknesses and shallow discontinuities that support the presence of a thick lithospheric keel within the MRZ region. These evidences strongly argue for the evolution of the MRZ via passive rifting mechanisms absent lower-mantle influences.

Anisotropy beneath the Southern Pacific - real or apparent?

NASA Astrophysics Data System (ADS)

Prasse, Philipp; Thomas, Christine

2016-04-01

Anisotropy of the lowermost mantle beneath the South- to Central Pacific is investigated using US-Array receivers and events located near the Tonga-Fiji subduction zones. Differential splitting in three different distance ranges (65° -85° , 90° -110° and >110°) of S-ScS, SKS-S, SKS-Sdiff phases is used. By utilizing differential splitting technique, it was possible to correct for upper mantle, as well as source- and receiver side anisotropy and effectively quantify shear wave splitting originating in the lowermost mantle. Delay times of horizontal (SH) and vertical polarized (SV) shear waves show that predominantly the SH wave is delayed relative to the SV wave. Motivated by the discrepancy in previous Pacific studies investigating the lowermost mantle beneath the Pacific the possibility of isotropic structure producing the observed splitting is tested. Synthetic seismograms are computed, based on various isotropic models and the resulting synthetics are analysed in the same way as the real data. While simple layered models do not produce splitting and therefore apparent anisotropy, models in which the lowermost mantle is represented as a negative gradient in P- and S-wave velocity, produce clear apparent anisotropy. Thus, this study presents a possible alternative way of explaining the structure of the D" region.

Icelandic-type crust

USGS Publications Warehouse

Foulger, G.R.; Du, Z.; Julian, B.R.

2003-01-01

Numerous seismic studies, in particular using receiver functions and explosion seismology, have provided a detailed picture of the structure and thickness of the crust beneath the Iceland transverse ridge. We review the results and propose a structural model that is consistent with all the observations. The upper crust is typically 7 ?? 1 km thick, heterogeneous and has high velocity gradients. The lower crust is typically 15-30 ?? 5 km thick and begins where the velocity gradient decreases radically. This generally occurs at the V p ??? 6.5 km s-1 level. A low-velocity zone ??? 10 000 km2 in area and up to ??? 15 km thick occupies the lower crust beneath central Iceland, and may represent a submerged, trapped oceanic microplate. The crust-mantle boundary is a transition zone ???5 ?? 3 km thick throughout which V p increases progressively from ???7.2 to ???8.0 km s-1. It may be gradational or a zone of alternating high- and low-velocity layers. There is no seismic evidence for melt or exceptionally high temperatures in or near this zone. Isostasy indicates that the density contrast between the lower crust and the mantle is only ???90 kg m-3 compared with ???300 kg m-3 for normal oceanic crust, indicating compositional anomalies that are as yet not understood. The seismological crust is ???30 km thick beneath the Greenland-Iceland and Iceland-Faeroe ridges, and eastern Iceland, ???20 km beneath western Iceland, and ???40 km thick beneath central Iceland. This pattern is not what is predicted for an eastward-migrating plume. Low attenuation and normal V p/V s ratios in the lower crust beneath central and southwestern Iceland, and normal uppermost mantle velocities in general, suggest that the crust and uppermost mantle are subsolidus and cooler than at equivalent depths beneath the East Pacific Rise. Seismic data from Iceland have historically been interpreted both in terms of thin-hot and thick-cold crust models, both of which have been cited as supporting the plume hypothesis. This suggests that the plume model for Iceland is an a priori assumption rather than a hypothesis subject to testing. The long-extinct Ontong-Java Plateau, northwest India and Parana??, Brazil large igneous provinces, beneath which mantle plumes are not expected are all underlain by mantle low-velocity bodies similar to that beneath Iceland. A plume interpretation for the mantle anomaly beneath Iceland is thus not required.

Differentiating flow, melt, or fossil seismic anisotropy beneath Ethiopia

NASA Astrophysics Data System (ADS)

Hammond, J. O. S.; Kendall, J.-M.; Wookey, J.; Stuart, G. W.; Keir, D.; Ayele, A.

2014-05-01

Ethiopia is a region where continental rifting gives way to oceanic spreading. Yet the role that pre-existing lithospheric structure, melt, mantle flow, or active upwellings may play in this process is debated. Measurements of seismic anisotropy are often used to attempt to understand the contribution that these mechanisms may play. In this study, we use new data in Afar, Ethiopia along with legacy data across Ethiopia, Djibouti, and Yemen to obtain estimates of mantle anisotropy using SKS-wave splitting. We show that two layers of anisotropy exist, and we directly invert for these. We show that fossil anisotropy with fast directions oriented northeast-southwest may be preserved in the lithosphere away from the rift. Beneath the Main Ethiopian Rift and parts of Afar, anisotropy due to shear segregated melt along sharp changes in lithospheric thickness dominates the shear-wave splitting signal in the mantle. Beneath Afar, away from regions with significant lithospheric topography, melt pockets associated with the crustal and uppermost mantle magma storage dominate the signal in localized regions. In general, little anisotropy is seen in the uppermost mantle beneath Afar suggesting melt retains no preferential alignment. These results show the important role melt plays in weakening the lithosphere and imply that as rifting evolves passive upwelling sustains extension. A dominant northeast-southwest anisotropic fast direction is observed in a deeper layer across all of Ethiopia. This suggests that a conduit like plume is lacking beneath Afar today, rather a broad flow from the southwest dominates flow in the upper mantle.

The mantle transition zone beneath the Afar Depression and adjacent regions: implications for mantle plumes and hydration

NASA Astrophysics Data System (ADS)

Reed, C. A.; Gao, S. S.; Liu, K. H.; Yu, Y.

2016-06-01

The Afar Depression and its adjacent areas are underlain by an upper mantle marked by some of the world's largest negative velocity anomalies, which are frequently attributed to the thermal influences of a lower-mantle plume. In spite of numerous studies, however, the existence of a plume beneath the area remains enigmatic, partially due to inadequate quantities of broad-band seismic data and the limited vertical resolution at the mantle transition zone (MTZ) depth of the techniques employed by previous investigations. In this study, we use an unprecedented quantity (over 14 500) of P-to-S receiver functions (RFs) recorded by 139 stations from 12 networks to image the 410 and 660 km discontinuities and map the spatial variation of the thickness of the MTZ. Non-linear stacking of the RFs under a 1-D velocity model shows robust P-to-S conversions from both discontinuities, and their apparent depths indicate the presence of an upper-mantle low-velocity zone beneath the entire study area. The Afar Depression and the northern Main Ethiopian Rift are characterized by an apparent 40-60 km depression of both MTZ discontinuities and a normal MTZ thickness. The simplest and most probable interpretation of these observations is that the apparent depressions are solely caused by velocity perturbations in the upper mantle and not by deeper processes causing temperature or hydration anomalies within the MTZ. Thickening of the MTZ on the order of 15 km beneath the southern Arabian Plate, southern Red Sea and western Gulf of Aden, which comprise the southward extension of the Afro-Arabian Dome, could reflect long-term hydration of the MTZ. A 20 km thinning of the MTZ beneath the western Ethiopian Plateau is observed and interpreted as evidence for a possible mantle plume stem originating from the lower mantle.

Upper mantle shear wave velocity structure beneath northern Victoria Land, Antarctica: Volcanism and uplift in the northern Transantarctic Mountains

NASA Astrophysics Data System (ADS)

Graw, Jordan H.; Adams, Aubreya N.; Hansen, Samantha E.; Wiens, Douglas A.; Hackworth, Lauren; Park, Yongcheol

2016-09-01

The Transantarctic Mountains (TAMs) are the largest non-compressional mountain range on Earth, and while a variety of uplift mechanisms have been proposed, the origin of the TAMs is still a matter of great debate. Most previous seismic investigations of the TAMs have focused on a central portion of the mountain range, near Ross Island, providing little along-strike constraint on the upper mantle structure, which is needed to better assess competing uplift models. Using data recorded by the recently deployed Transantarctic Mountains Northern Network, as well as data from the Transantarctic Mountains Seismic Experiment and from five stations operated by the Korea Polar Research Institute, we investigate the upper mantle structure beneath a previously unexplored portion of the mountain range. Rayleigh wave phase velocities are calculated using a two-plane wave approximation and are inverted for shear wave velocity structure. Our model shows a low velocity zone (LVZ; ∼4.24 km s-1) at ∼160 km depth offshore and adjacent to Mt. Melbourne. This LVZ extends inland and vertically upwards, with more lateral coverage above ∼100 km depth beneath the northern TAMs and Victoria Land. A prominent LVZ (∼4.16-4.24 km s-1) also exists at ∼150 km depth beneath Ross Island, which agrees with previous results in the TAMs near the McMurdo Dry Valleys, and relatively slow velocities (∼4.24-4.32 km s-1) along the Terror Rift connect the low velocity anomalies. We propose that the LVZs reflect rift-related decompression melting and provide thermally buoyant support for the TAMs uplift, consistent with proposed flexural models. We also suggest that heating, and hence uplift, along the mountain front is not uniform and that the shallower LVZ beneath northern Victoria Land provides greater thermal support, leading to higher bedrock topography in the northern TAMs. Young (0-15 Ma) volcanic rocks associated with the Hallett and the Erebus Volcanic Provinces are situated directly above the imaged LVZs, suggesting that these anomalies are also the source of Cenozoic volcanic rocks throughout the study area.

Evidence for strong lateral seismic velocity variation in the lower crust and upper mantle beneath the California margin

USGS Publications Warehouse

Lai, Voon; Graves, Robert; Wei, Shengji; Helmberger, Don

2017-01-01

Regional seismograms from earthquakes in Northern California show a systematic difference in arrival times across Southern California where long period (30–50 seconds) SH waves arrive up to 15 seconds earlier at stations near the coast compared with sites towards the east at similar epicentral distances. We attribute this time difference to heterogeneity of the velocity structure at the crust-mantle interface beneath the California margin. To model these observations, we propose a fast seismic layer, with thickness growing westward from the San Andreas along with a thicker and slower continental crust to the east. Synthetics generated from such a model are able to match the observed timing of SH waveforms better than existing 3D models. The presence of a strong upper mantle buttressed against a weaker crust has a major influence in how the boundary between the Pacific plate and North American plate deforms and may explain the observed asymmetric strain rate across the boundary.

Understanding the nature of mantle upwelling beneath East-Africa

NASA Astrophysics Data System (ADS)

Civiero, Chiara; Hammond, James; Goes, Saskia; Ahmed, Abdulhakim; Ayele, Atalay; Doubre, Cecile; Goitom, Berhe; Keir, Derek; Kendall, Mike; Leroy, Sylvie; Ogubazghi, Ghebrebrhan; Rumpker, Georg; Stuart, Graham

2014-05-01

The concept of hot upwelling material - otherwise known as mantle plumes - has long been accepted as a possible mechanism to explain hotspots occurring at Earth's surface and it is recognized as a way of removing heat from the deep Earth. Nevertheless, this theory remains controversial since no one has definitively imaged a plume and over the last decades several other potential mechanisms that do not require a deep mantle source have been invoked to explain this phenomenon, for example small-scale convection at rifted margins, meteorite impacts or lithospheric delamination. One of the best locations to study the potential connection between hotspot volcanism at the surface and deep mantle plumes on land is the East African Rift (EAR). We image seismic velocity structure of the mantle below EAR with higher resolution than has been available to date by including seismic data recorded by stations from many regional networks ranging from Saudi Arabia to Tanzania. We use relative travel-time tomography to produce P- velocity models from the surface down into the lower mantle incorporating 9250 ray-paths in our model from 495 events and 402 stations. We add smaller earthquakes (4.5 < mb < 5.5) from poorly sampled regions in order to have a more uniform data coverage. The tomographic results allow us to image structures of ~ 100-km length scales to ~ 1000 km depth beneath the northern East-Africa rift (Ethiopia, Eritrea, Djibouti, Yemen) with good resolution also in the transition zone and uppermost lower mantle. Our observations provide evidence that the shallow mantle slow seismic velocities continue trough the transition zone and into the lower mantle. In particular, the relatively slow velocity anomaly beneath the Afar Depression extends up to depths of at least 1000 km depth while another low-velocity anomaly beneath the Main Ethiopian Rift seems to be present in the upper mantle only. These features in the lower mantle are isolated with a diameter of about 400 km indicating deep multiple sources of upwelling that converge in broader low-velocity bodies along the rift axis at shallow depths. Moreover, our preliminary models show that the low-velocity feature in the transition zone and uppermost lower mantle beneath Afar trends to the northeast beneath the Red Sea and Saudi Arabia as opposed to being linked to the African Superplume towards the southwest.

Small-scale convection beneath the transverse ranges, California: Implications for interpretation of gravity anomalies

NASA Technical Reports Server (NTRS)

Humphreys, E. D.; Hager, B. H.

1985-01-01

Tomographic inversion of upper mantle P wave velocity heterogeneities beneath southern California shows two prominent features: an east-west trending curtain of high velocity material (up to 3% fast) in the upper 250 km beneath the Transverse Ranges and a region of low velocity material (up to 4% slow) in the 100 km beneath the Salton Trough. These seismic velocity anomalies were interpreted as due to small scale convection in the mantle. Using this hypothesis and assuming that temperature and density anomalies are linearly related to seismic velocity anomalies through standard coefficients of proportionality, leads to inferred variations of approx. + or - 300 C and approx. + or - 0.03 g/cc.

Fine crustal and uppermost mantle S-wave velocity structure beneath the Tengchong volcanic area inferred from receiver function and surface-wave dispersion: constraints on magma chamber distribution

NASA Astrophysics Data System (ADS)

Li, Mengkui; Zhang, Shuangxi; Wu, Tengfei; Hua, Yujin; Zhang, Bo

2018-03-01

The Tengchong volcanic area is located in the southeastern margin of the collision zone between the Indian and Eurasian Plates. It is one of the youngest intraplate volcano groups in mainland China. Imaging the S-wave velocity structure of the crustal and uppermost mantle beneath the Tengchong volcanic area is an important means of improving our understanding of its volcanic activity and seismicity. In this study, we analyze teleseismic data from nine broadband seismic stations in the Tengchong Earthquake Monitoring Network. We then image the crustal and uppermost mantle S-wave velocity structure by joint analysis of receiver functions and surface-wave dispersion. The results reveal widely distributed low-velocity zones. We find four possible magma chambers in the upper-to-middle crust and one in the uppermost mantle. The chamber in the uppermost mantle locates in the depth range from 55 to 70 km. The four magma chambers in the crust occur at different depths, ranging from the depth of 7 to 25 km in general. They may be the heat sources for the high geothermal activity at the surface. Based on the fine crustal and uppermost mantle S-wave velocity structure, we propose a model for the distribution of the magma chambers.

Upper mantle seismic structure beneath central East Antarctica from body wave tomography: Implications for the origin of the Gamburtsev Subglacial Mountains

NASA Astrophysics Data System (ADS)

Lloyd, Andrew J.; Nyblade, Andrew A.; Wiens, Douglas A.; Hansen, Samantha E.; Kanao, Masaki; Shore, Patrick J.; Zhao, Dapeng

2013-04-01

The Gamburtsev Subglacial Mountains (GSM), located near the center of East Antarctica, are the highest feature within the East Antarctic highlands and have been investigated seismically for the first time during the 2007/2008 International Polar Year by the Gamburtsev Mountains Seismic Experiment. Using data from a network of 26 broadband seismic stations and body wave tomography, the P and S wave velocity structure of the upper mantle beneath the GSM and adjacent regions has been examined. Tomographic images produced from teleseismic P and S phases reveal several large-scale, small amplitude anomalies (δVp = 1.0%, δVs = 2.0%) in the upper 250 km of the mantle. The lateral distributions of these large-scale anomalies are similar in both the P and S wave velocity models and resolution tests show that they are well resolved. Velocity anomalies indicate slower, thinner lithosphere beneath the likely Meso- or Neoproterozoic Polar Subglacial Basin and faster, thicker lithosphere beneath the likely Archean/Paleoproterozoic East Antarctic highlands. Within the region of faster, thicker lithosphere, a lower amplitude (δVp = 0.5%, δVs = 1.0%) slow to fast velocity pattern is observed beneath the western flank of the GSM, suggesting a suture between two lithospheric blocks possibly of similar age. These findings point to a Precambrian origin for the high topography of the GSM, corroborating other studies invoking a long-lived highland landscape in central East Antarctica, as opposed to uplift caused by Permian/Cretaceous rifting or Cenozoic magmatism. The longevity of the GSM makes them geologically unusual; however, plausible analogs exist, such as the 550 Ma Petermann Ranges in central Australia. Additional uplift may have occurred by the reactivation of pre-existing faults, for example, during the Carboniferous-Permian collision of Gondwana and Laurussia.

Crustal and upper-mantle structure of South China from Rayleigh wave tomography

NASA Astrophysics Data System (ADS)

Shan, B.; Xiong, X.; Zhao, K. F.; Xie, Z. J.; Zheng, Y.; Zhou, L.

2017-03-01

In this study, we image the crust and upper-mantle seismic velocity structures in South China using teleseismic Rayleigh waves recorded at 354 stations from the Chinese provincial networks (CEArray). We process Rayleigh wave data from 1087 teleseismic events and construct phase velocity maps at periods of 40-150 s. By combining dispersion curves at 6-70 s from Zhou et al. and at 40-150 s from the teleseismic surface wave tomography of this study, we construct a 3-D shear velocity model of the crust and upper mantle of South China. Distinct seismic structures are revealed from the eastern part of South China (including the South China Fold System and the eastern Yangtze Craton) to the western Yangtze Craton. The South China Fold System and eastern Yangtze Craton are characterized by lower velocities and shallow lithosphere-asthenosphere boundary (∼90 km), which are similar to the lithospheric thermal and seismic velocity structures of the North China basin. These observations may imply that the lithospheric destruction and thinning occurred not only beneath the North China Craton, but also beneath the eastern part of South China. The western Yangtze Craton, including the Sichuan Basin and Jiangnan Orogen, is underlain by a thicker and colder lithosphere with high velocities. The contrast of the lithosphere structure between the western Yangtze Craton and other parts of South China indicates that the lithospheric destruction and thinning of the east and southeast parts of South China may terminate at the boundary of the Jiangnan Orogen.

A geophysical perspective on mantle water content and melting: Inverting electromagnetic sounding data using laboratory-based electrical conductivity profiles

NASA Astrophysics Data System (ADS)

Khan, A.; Shankland, T. J.

2012-02-01

This paper applies electromagnetic sounding methods for Earth's mantle to constrain its thermal state, chemical composition, and "water" content. We consider long-period inductive response functions in the form of C-responses from four stations distributed across the Earth (Europe, North America, Asia and Australia) covering a period range from 3.9 to 95.2 days and sensitivity to ~ 1200 km depth. We invert C-responses directly for thermo-chemical state using a self-consistent thermodynamic method that computes phase equilibria as functions of pressure, temperature, and composition (in the Na2O-CaO-FeO-MgO-Al2O3-SiO2 model system). Computed mineral modes are combined with recent laboratory-based electrical conductivity models from independent experimental research groups (Yoshino (2010) and Karato (2011)) to compute bulk conductivity structure beneath each of the four stations from which C-responses are estimated. To reliably allocate water between the various mineral phases we include laboratory-measured water partition coefficients for major upper mantle and transition zone minerals. This scheme is interfaced with a sampling-based algorithm to solve the resulting non-linear inverse problem. This approach has two advantages: (1) It anchors temperatures, composition, electrical conductivities, and discontinuities that are in laboratory-based forward models, and (2) At the same time it permits the use of geophysical inverse methods to optimize conductivity profiles to match geophysical data. The results show lateral variations in upper mantle temperatures beneath the four stations that appear to persist throughout the upper mantle and parts of the transition zone. Calculated mantle temperatures at 410 and 660 km depth lie in the range 1250-1650 °C and 1500-1750 °C, respectively, and generally agree with the experimentally-determined temperatures at which the measured phase reactions olivine → β-spinel and γ-spinel → ferropericlase + perovskite occur. The retrieved conductivity structures beneath the various stations tend to follow trends observed for temperature with the strongest lateral variations in the uppermost mantle; for depths > 300 km conductivities appear to depend less on the particular conductivity database. Conductivities at 410 km and at 660 km depth are found to agree overall with purely geophysically-derived global and semi-global one-dimensional conductivity models. Both electrical conductivity databases point to < 0.01 wt.% H2O in the upper mantle. For transition zone minerals results from the laboratory database of Yoshino (2010) suggest that a much higher water content (up to 2 wt.% H2O) is required than in the other database (Karato, 2011), which favors a relatively "dry" transition zone (< 0.01 wt.% H2O). Incorporating laboratory measurements of hydrous silicate melting relations and available conductivity data allows us to consider the possibility of hydration melting and a high-conductivity melt layer above the 410-km discontinuity. The latter appears to be 1) regionally localized and 2) principally a feature from the Yoshino (2010) database. Further, there is evidence of lateral heterogeneity: The mantle beneath southwestern North America and central China appears "wetter" than that beneath central Europe or Australia.

Lithospheric mantle structure beneath Northern Scotland: Pre-plume remnant or syn-plume signature?

NASA Astrophysics Data System (ADS)

Knapp, J.

2003-04-01

Upper mantle reflectors (Flannan and W) beneath the northwestern British Isles are some of the best-known and most-studied examples of preserved structure within the continental mantle lithosphere, and are spatially coincident with the surface location of early Iceland plume volcanism in the British Tertiary Province. First observed on BIRPS (British Institutions Reflection Profiling Syndicate) marine deep seismic reflection profiles in the early 1980's, these reflectors have subsequently been imaged and correlated on additional reflection and refraction profiles in the offshore area of northern and western Scotland. The age and tectonic significance of these reflectors remains a subject of wide debate, due in part to the absence of robust characterization of the upper mantle velocity structure in this tectonically complex area. Interpretations advanced over the past two decades for the dipping Flannan reflector range from fossilized subduction complex to large-scale extensional shear zone, and span ages from Proterozoic to early Mesozoic. Crustal geology of the region records early Paleozoic continental collision and late Paleozoic to Mesozoic extension. Significant modification of the British lithosphere in early Tertiary time, including dramatic thinning and extensive basaltic intrusion associated with initiation and development of the Iceland plume, suggests either (1) an early Tertiary age for the Flannan reflector or (2) preservation of ancient features within the mantle lithosphere despite such pervasive modification. Exisitng constraints are consistent with a model for early Tertiary origin of the Flannan reflector as the downdip continuation of the Rockall Trough extensional system of latest Cretaceous to earliest Tertiary age during opening of the northern Atlantic Ocean and initiation of the Iceland plume. Lithopsheric thinning beneath present-day northern Scotland could have served to focus the early expression of plume volcanism (British Tertiary Province), despite the inferred distant locus of the initial plume head. Alternatively, preservation of large-scale pre-plume fabric in the Scottish mantle would imply long-lived tectonic heredity in the continental lithospheric mantle, and place important constraints on the plume-related effects (or lack thereof) in the mantle lithosphere.

Northeastern Pacific mantle conductivity profile from long-period magnetotelluric sounding using Hawaii-to-California submarine cable data

NASA Astrophysics Data System (ADS)

Lizarralde, Daniel; Chave, Alan; Hirth, Greg; Schultz, Adam

1995-09-01

We present results of a long-period magnetotelluric (MT) investigation of the electrical structure beneath the eastern North Pacific. The electric field data consist of ˜2 years of continuously recorded voltages across an unpowered, ˜4000-km-long submarine telephone cable (HAW-1) extending from Point Arena, California, to Oahu, Hawaii. The electric field measurements are coherent to some degree with magnetic field measurements from Honolulu Observatory at periods of 0.1 to 45 days. This coherence is enhanced at long periods over that observed with point electric field sensors due to horizontal averaging of the motional electric fields of spatial scale smaller than the cable length, significantly diminishing their effect. Robust, controlled leverage MT response estimates and their jacknife confidence limits are computed for the HAW-1 to Honolulu data. An equivalent scalar MT response obtained from Honolulu magnetic variations data is used to correct the HAW-1 MT response for static shift and to extend the MT response estimate to periods of 100 days. The composite response function satisfies necessary and sufficient conditions for consistency with a one-dimensional conductivity structure and is most sensitive to structure between 150 and 1000 km. Inversion of the MT response reveals a conductive zone (0.05-0.1 S/m) between 150 and 400 km depth and a positive gradient below 500 km; these observations are consistent with previous MT studies in the North Pacific. This upper mantle conductivity is too high to be explained by solid-state conduction in dry olivine using reasonable mantle geotherms. Calculations based on measurements of hydrogen solubility and diffusivity in olivine indicate that H+ dissolved in olivine, possibly combined with a lattice preferred orientation consistent with measured seismic anisotropy, provide sufficient conductivity enhancement to explain the inversion results. The high conductivity may also be explained by the presence of gravitationally stable partial melt. Comparison of the HAW-1 results with long-period MT studies conducted on land reveals differences in upper mantle conductivity between different tectonic regimes. In particular, the upper mantle beneath the Pacific Ocean is considerably more conductive than that beneath the Canadian shield and similar in conductivity to that beneath the Basin and Range.

Seismic characteristics of central Brazil crust and upper mantle: A deep seismic refraction study

USGS Publications Warehouse

Soares, J.E.; Berrocal, J.; Fuck, R.A.; Mooney, W.D.; Ventura, D.B.R.

2006-01-01

A two-dimensional model of the Brazilian central crust and upper mantle was obtained from the traveltime interpretation of deep seismic refraction data from the Porangatu and Cavalcante lines, each approximately 300 km long. When the lines were deployed, they overlapped by 50 km, forming an E-W transect approximately 530 km long across the Tocantins Province and western Sa??o Francisco Craton. The Tocantins Province formed during the Neoproterozoic when the Sa??o Francisco, the Paranapanema, and the Amazon cratons collided, following the subduction of the former Goia??s ocean basin. Average crustal VP and VP/VS ratios, Moho topography, and lateral discontinuities within crustal layers suggest that the crust beneath central Brazil can be associated with major geological domains recognized at the surface. The Moho is an irregular interface, between 36 and 44 km deep, that shows evidences of first-order tectonic structures. The 8.05 and 8.23 km s-1 P wave velocities identify the upper mantle beneath the Porangatu and Cavalcante lines, respectively. The observed seismic features allow for the identification of (1) the crust has largely felsic composition in the studied region, (2) the absence of the mafic-ultramafic root beneath the Goia??s magmatic arc, and (3) block tectonics in the foreland fold-and-thrust belt of the northern Brasi??lia Belt during the Neoproterozoic. Seismic data also suggested that the Bouguer gravimetric discontinuities are mainly compensated by differences in mass distribution within the lithospheric mantle. Finally, the Goia??s-Tocantins seismic belt can be interpreted as a natural seismic alignment related to the Neoproterozoic mantle domain. Copyright 2006 by the American Geophysical Union.

Enhanced and asymmetric melting beneath the southern Mariana back-arc spreading ridge under the influence of the Pacific plate subduction

NASA Astrophysics Data System (ADS)

Matsuno, T.; Seama, N.; Shindo, H.; Nogi, Y.; Okino, K.

2017-12-01

Back-arc spreading ridges in the southern Mariana Trough are slow-spreading ridges but have features suggesting enhanced melting beneath the ridges and influences on seafloor spreading processes by fluid derived from the subducted Pacific slab underlying the ridges. To reveal melting and dehydration processes and dynamics in the upper mantle in the southern Mariana Trough, we conducted a marine magnetotelluric (MT) experiment along a 120 km-length transect across a ridge segment at 13°N. We obtained electromagnetic field data at 9 stations along the transect, and analyzed them for estimating MT responses, striping seafloor topographic distortion from the responses, and imaging a 2-D electrical resistivity structure by 2-D inversion of TM-mode responses. A resultant 2-D inversion model showed 1) a conductive area at 10-20 km depth beneath the ridge center, the center of which slightly offsets to the trench side, 2) a moderately conductive area expanding asymmetrically around and under the conductor of 1), 3) a resistive area thickening from the ridge center up to about 40 km on the remnant arc side, and 4) a resistive area with a constant thickness of about 150 km on the trench side. These model features suggest 1) a melt body beneath the ridge center, possibly containing slab-derived water 2) water- and melt-retained mantle area produced by hydration of the back-arc mantle wedge and asymmetric passive decompression melting in the hydrous mantle wedge, 3) cooled and residual lithospheric mantle off the ridge center, and 4) mantle wedge and subducted Pacific lithospheric mantle that are both cold and depleted. The electrical resistivity structure obtained in the southern Mariana Trough, which clearly contrasts with the structure of the central Mariana Trough at 18°N in that this lacks a conductor beneath the ridge center, provides insights on the mantle dynamics and its relation to the characteristic tectonics and many kinds of observational results in the southern Mariana Trough.

Cenozoic volcanism in the Bohemian Massif in the context of P- and S-velocity high-resolution teleseismic tomography of the upper mantle

NASA Astrophysics Data System (ADS)

Plomerová, Jaroslava; Munzarová, Helena; Vecsey, Luděk.; Kissling, Eduard; Achauer, Ulrich; Babuška, Vladislav

2016-08-01

New high-resolution tomographic models of P- and S-wave isotropic-velocity perturbations for the Bohemian upper mantle are estimated from carefully preprocessed travel-time residuals of teleseismic P, PKP and S waves recorded during the BOHEMA passive seismic experiment. The new data resolve anomalies with scale lengths 30-50 km. The models address whether a small mantle plume in the western Bohemian Massif is responsible for this geodynamically active region in central Europe, as expressed in recurrent earthquake swarms. Velocity-perturbations of the P- and S-wave models show similar features, though their resolutions are different. No model resolves a narrow subvertical low-velocity anomaly, which would validate the "baby-plume" concept. The new tomographic inferences complement previous studies of the upper mantle beneath the Bohemian Massif, in a broader context of the European Cenozoic Rift System (ECRIS) and of other Variscan Massifs in Europe. The low-velocity perturbations beneath the Eger Rift, observed in about 200km-broad zone, agree with shear-velocity models from full-waveform inversion, which also did not identify a mantle plume beneath the ECRIS. Boundaries between mantle domains of three tectonic units that comprise the region, determined from studies of seismic anisotropy, represent weak zones in the otherwise rigid continental mantle lithosphere. In the past, such zones could have channeled upwelling of hot mantle material, which on its way could have modified the mantle domain boundaries and locally thinned the lithosphere.

Tomographic evidence for recent extension in the Bentley Subglacial Trench and a hotspot beneath Marie Byrd Land

NASA Astrophysics Data System (ADS)

Lloyd, A. J.; Wiens, D. A.; Nyblade, A.; Anandakrishnan, S.; Aster, R. C.; Huerta, A. D.; Wilson, T. J.; Shore, P.

2013-12-01

Here we present the first regional P and S wave relative velocity models of the upper mantle beneath much of West Antarctica using P and S wave relative travel time residuals from teleseismic events recorded by seismographs from the POLENET/ANET project. 21 of the seismographs form a sparse backbone network co-located with continuously recording GPS stations at rock sites throughout West Antarctica, and 17 stations formed a seismic transect extending from the Whitmore Mountains across the West Antarctic Rift System (WARS) and into Marie Byrd Land (MBL) with a station spacing of 90-100 km. Corrections for heterogeneities above the Moho, including the ice sheet, are applied to the relative travel time residuals using the receiver function models of Chaput et al., [submitted, 2013]. Both P and S wave velocity models indicate velocities faster than the mean of the model beneath the Whitmore Mountains that may be interpreted as thicker, colder lithosphere relative to the rest of West Antarctica. Slow velocity anomalies are observed beneath the Bentley Subglacial Trench (BST) and MBL. Slow velocities extending from the Moho to the transition zone beneath MBL are centered beneath the Mt Sidley volcano and coincide with high topography that is not isostatically supported by the crust [Chaput et al., submitted, 2013]. The slowest velocities occur at 200-300 km depth and are consistent with warm, low viscosity mantle that provides topographic support for the elevated MBL volcanic dome. Poor vertical resolution, typical of body wave tomography, hampers the models ability to resolve whether the anomaly beneath MBL is strictly an upper mantle hotspot or a classic mantle plume that extends into the lower mantle. The shallow (≤ 100 km depth) slow anomaly beneath the BST coincides with a region of thin crust and likely reflects a localized region of Cenozoic extension in the WARS that may have undergone a last phase of extension in the Neogene [Garnot et al., 2013]. Anomalously high heat flow reported by Fudge et al.[2012] at the WAIS divide ice core is also consistent with recent Neogene extension and a thermal perturbation suggested by both P and S tomography models. In general, the strong heterogeneities in our models are predominantly interpreted as reflecting upper mantle temperature variations in addition to possible mantle partial melting beneath MBL.

Seismic anisotropy and mantle creep in young orogens

USGS Publications Warehouse

Meissner, R.; Mooney, W.D.; Artemieva, I.

2002-01-01

Seismic anisotropy provides evidence for the physical state and tectonic evolution of the lithosphere. We discuss the origin of anisotropy at various depths, and relate it to tectonic stress, geotherms and rheology. The anisotropy of the uppermost mantle is controlled by the orthorhombic mineral olivine, and may result from ductile deformation, dynamic recrystallization or annealing. Anisotropy beneath young orogens has been measured for the seismic phase Pn that propagates in the uppermost mantle. This anisotropy is interpreted as being caused by deformation during the most recent thermotectonic event, and thus provides information on the process of mountain building. Whereas tectonic stress and many structural features in the upper crust are usually orientated perpendicular to the structural axis of mountain belts, Pn anisotropy is aligned parallel to the structural axis. We interpret this to indicate mountain-parallel ductile (i.e. creeping) deformation in the uppermost mantle that is a consequence of mountain-perpendicular compressive stresses. The preferred orientation of the fast axes of some anisotropic minerals, such as olivine, is known to be in the creep direction, a consequence of the anisotropy of strength and viscosity of orientated minerals. In order to explain the anisotropy of the mantle beneath young orogens we extend the concept of crustal 'escape' (or 'extrusion') tectonics to the uppermost mantle. We present rheological model calculations to support this hypothesis. Mountain-perpendicular horizontal stress (determined in the upper crust) and mountain-parallel seismic anisotropy (in the uppermost mantle) require a zone of ductile decoupling in the middle or lower crust of young mountain belts. Examples for stress and mountain-parallel Pn anisotropy are given for Tibet, the Alpine chains, and young mountain ranges in the Americas. Finally, we suggest a simple model for initiating mountain parallel creep.

What drives the Tibetan crust to the South East Asia? Role of upper mantle density discontinuities as inferred from the continental geoid anomalies

NASA Astrophysics Data System (ADS)

Rajesh, S.

2012-04-01

The Himalaya-Tibet orogen formed as a result of the northward convergence of India into the Asia over the past 55 Ma had caused the north south crustal shortening and Cenozoic upliftment of the Tibetan plateau, which significantly affected the tectonic and climatic framework of the Asia. Geodetic measurements have also shown eastward crustal extrusion of Tibet, especially along major east-southeast strike slip faults at a slip rate of 15-20 mm a-1 and around 40 mm a-1. Such continental scale deformations have been modeled as block rotation by fault boundary stresses developed due to the India-Eurasia collision. However, the Thin Sheet model explained the crustal deformation mechanism by considering varying gravitational potential energy arise out of varying crustal thickness of the viscous lithosphere. The Channel Flow model, which also suggests extrusion is a boundary fault guided flow along the shallow crustal brittle-ductile regime. Although many models have proposed, but no consensus in these models to explain the dynamics of measured surface geodetic deformation of the Tibetan plateau. But what remains conspicuous is the origin of driving forces that cause the observed Tibetan crustal flow towards the South East Asia. Is the crustal flow originated only because of the differential stresses that developed in the shallow crustal brittle-ductile regime? Or should the stress transfer to the shallow crustal layers as a result of gravitational potential energy gradient driven upper mantle flow also to be accounted. In this work, I examine the role of latter in the light of depth distribution of continental geoid anomalies beneath the Himalaya-Tibet across major upper mantle density discontinuities. These discontinuity surfaces in the upper mantle are susceptible to hold the plastic deformation that may occur as a result of the density gradient driven flow. The distribution of geoid anomalies across these density discontinuities at 220, 410 and 660 km depth in the upper mantle beneath the Himalaya-Tibet has been studied by analyzing the geoid undulation data obtained from various satellite geodetic missions along with the recent and old (EGM2008 and EGM2006) Earth Gravity models. Results show that the net geoid anomaly varies from -65 m to -20 m, which signify a density stratified upper mantle beneath the Himalaya-Tibet and the same has been confirmed from the results of regional seismic tomography studies. The density anomaly distribution beneath Tibet from 163 km depth to its upper mantle thickness of 1063 km show a strong NW-SE elliptically oriented positive geoid anomalies of magnitude around 40 meter. Asymmetric density anomaly gradient have been observed along the Himalayan arc from west to east as well as across the arc from north to south. This caused differential gravitational potential gradient and hence an elliptical flow structure of the Tibetan continental mantle along the resultant NW-SE direction, which is in concurrence with the observed present day direction of the Tibetan crustal flow. Thus the geoid anomalies distributed at various depth ranges show how the gradient in the upper mantle gravitational potential energy, especially across the deformed discontinuity surface, is significant in determining the transfer of deviatoric stresses and providing traction to the flow of crustal layers of the Tibetan Plateau. This suggests the viscous flow model could be a preferable choice, which could better accommodate the dynamics of the upper mantle, in explaining the crustal extrusion processes of the Tibetan Plateau.

Imaging the seismic structure beneath oceanic spreading centers using ocean bottom geophysical techniques

NASA Astrophysics Data System (ADS)

Zha, Yang

This dissertation focuses on imaging the crustal and upper mantle seismic velocity structure beneath oceanic spreading centers. The goals are to provide a better understanding of the crustal magmatic system and the relationship between mantle melting processes, crustal architecture and ridge characteristics. To address these questions I have analyzed ocean bottom geophysical data collected from the fast-spreading East Pacific Rise and the back-arc Eastern Lau Spreading Center using a combination of ambient noise tomography and seafloor compliance analysis. To characterize the crustal melt distribution at fast spreading ridges, I analyze seafloor compliance - the deformation under long period ocean wave forcing - measured during multiple expeditions between 1994 and 2007 at the East Pacific Rise 9º - 10ºN segment. A 3D numerical modeling technique is developed and used to estimate the effects of low shear velocity zones on compliance measurements. The forward modeling suggests strong variations of lower crustal shear velocity along the ridge axis, with zones of possible high melt fractions beneath certain segments. Analysis of repeated compliance measurements at 9º48'N indicates a decrease of crustal melt fraction following the 2005 - 2006 eruption. This temporal variability provides direct evidence for short-term variations of the magmatic system at a fast spreading ridge. To understand the relationship between mantle melting processes and crustal properties, I apply ambient noise tomography of ocean bottom seismograph (OBS) data to image the upper mantle seismic structure beneath the Eastern Lau Spreading Center (ELSC). The seismic images reveal an asymmetric upper mantle low velocity zone (LVZ) beneath the ELSC, representing a zone of partial melt. As the ridge migrates away from the volcanic arc, the LVZ becomes increasingly offset and separated from the sub-arc low velocity zone. The separation of the ridge and arc low velocity zones is spatially coincident with the abrupt transition in crustal composition and ridge morphology. Therefore these results confirm a previous prediction that the changing interaction between the arc and back-arc magmatic systems is responsible for the abrupt change in crustal properties along the ELSC. I further investigate the crustal structure along and across the ELSC using seafloor compliance. Compliance measurements are inverted for local crustal shear velocity structure as well as sediment thickness at 30 OBS locations using a Monte Carlo method. Sediment increases asymmetrically with seafloor age, with much a higher rate to the east of the ridge. Along the ELSC, upper crustal velocities increase from south to north as the ridge migrates away from the volcanic arc front, consistent with a less porous upper crust with possibly less subduction input. Furthermore, average upper crust shear velocities for crust produced at past ELSC when it was near the volcanic arc are considerably slower than crust produced at present day northern ELSC. I show that the implications of previous active seismic studies in the axial ELSC can be extended much farther off-axis and back in time. I also address a challenge of ocean bottom seismology and develop a new method for determining OBS horizontal orientations using multi-component ambient noise correlation. I demonstrate that the OBS orientations can be robustly estimated through maximizing the correlation between the diagonal and cross terms of the noise correlation function. This method is applied to the ELSC OBS experiment dataset and the obtained orientations are consistent with results from a conventional teleseismic method. The new method is promising for a wide range of applications.

SH wave structure of the crust and upper mantle in southeastern margin of the Tibetan Plateau from teleseismic Love wave tomography

NASA Astrophysics Data System (ADS)

Fu, Yuanyuan V.; Jia, Ruizhi; Han, Fengqin; Chen, Anguo

2018-06-01

The deep structure of southeastern Tibet is important for determining lateral plateau expansion mechanisms, such as movement of rigid crustal blocks along large strike-slip faults, continuous deformation or the eastward crustal channel flow. We invert for 3-D isotropic SH wave velocity model of the crust and upper mantle to the depth of 110 km from Love wave phase velocity data using a best fitting average model as the starting model. The 3-D SH velocity model presented here is the first SH wave velocity structure in the study area. In the model, the Tibetan Plateau is characterized by prominent slow SH wave velocity with channel-like geometry along strike-slip faults in the upper crust and as broad zones in the lower crust, indicating block-like and distributed deformation at different depth. Positive radial anisotropy (VSH > VSV) is suggested by a high SH wave and low SV wave anomaly at the depths of 70-110 km beneath the northern Indochina block. This positive radial anisotropy could result from the horizontal alignment of anisotropic minerals caused by lithospheric extensional deformation due to the slab rollback of the Australian plate beneath the Sumatra trench.

Lateral variations of thermo-rheological structure in SE Tibet

NASA Astrophysics Data System (ADS)

Jiang, X.; Gong, W.

2017-12-01

The structure and geodynamics in SE Tibet is important to developing a full understanding of tectonic evolution of the Tibetan plateau. To investigate the lithospheric structure and deformation, we present thermo-rheological models for two transects across SE Tibet. The thermal models are determined by the heat flow and P-wave velocity models. Based on thermal models, the rheological models are constructed in the weak and strong cases where the lower crust is felsic or mafic granulite and the lithospheric mantle is wet or dry peridotite. The thermal models show an obvious high-temperature anomaly within the lithosphere beneath the Chuandian block. Strong lateral heterogeneity is present in the rheological modeling and corresponds to variations of thermal models. The Chuandian block demonstrates a lower level of lithospheric strength than its neighboring regions, which is in accord with the seismogenic layer distribution. Combining with a joint analysis of SKS splitting and GPS data, the crust and mantle is decoupled at a depth below the topmost mantle in SE Tibet. The strong crust beneath the South China plate and Indochina block has two brittle load-bearing layers in the crust, indicating the system is mechanically coupled. The crust beneath the Emeishan igneous province also has two brittle load-bearing layers, but the brittle deformation is restricted to the topmost 10 km of the upper and lower crust. In contrast, only one brittle load-bearing layer resides in the upper crust with the lower crust contributing little to the lithospheric strength at the location where low-velocity-high-conductivity zones have been recognized within the crust in the Chuandian block. This indicates that the crust beneath the Chuandian block becomes decoupled, as evidenced by the crustal anisotropy pattern.

Upper Mantle Texture Patterns In Eastern North America From Seismic Anisotropy And Global Mantle Flow Calculations

NASA Astrophysics Data System (ADS)

Levin, V. L.; Moucha, R.; Yuan, H.

2013-12-01

Global seismic models show gradual and systematic changes in upper mantle seismic properties beneath North America. Faster and thicker lithosphere of the interior thins eastward. Upper mantle rock fabric reflected in observations of seismic anisotropy also varies. Near the coast apparent fast directions of split shear waves are nearly east-west, with considerable scatter. Further inland they are more uniform and align SW-NE, close to the absolute plate motion direction of North America. Mantle convection simulations driven by density inferred from global joint seismic-geodynamic tomography models exhibit complex flow beneath the eastern edge of the North American continent due to the ongoing descent of the Farallon slab deep beneath it (figure 1). Flow predicted beneath the coast is nearly horizontal with a small, though dynamically important, vertical component, while west of the Appalachians it turns downward. Long records of teleseismic observations accumulated at permanent seismic stations HRV, PAL and SSPA (figure 2) are inverted for vertical distribution of anisotropic parameters. We find preference for more than one layer of anisotropy beneath all sites, with significantly different parameters that could reflect either lateral variations in the lithospheric thickness, variations in the asthenospheric flow field, or both. Since we find considerable consistency in directional patterns of P-to-S mode converted waves associated with the lower part of the lithosphere, variations of asthenospheric flow seem to be a more plausible explanation. We explore the links between predicted flow and inferences from seismic data with additional observations of anisotropy and calculations of flow-induced rock fabric.

Mapping the subducted Nazca plate in the lower mantle beneath South America

NASA Astrophysics Data System (ADS)

Contenti, S. M.; Gu, Y. J.; Okeler, A.

2009-12-01

Recent improvements in data coverage have enabled high-resolution imaging of the morphology of subduction zones and mantle plumes. In this study, we migrate the SS precursors from over 5000 seismograms to obtain a detailed map of mid- and upper-mantle reflectors beneath the northern portion of the South American subduction zone, where the oceanic Nazca plate is descending below the South American plate. In addition to an elevated 410 and depressed 660 (as expected for a subduction zone), strong mid-mantle reflectors at 800-1100 km depth are also apparent. The amplitudes of these steeply dipping reflectors are comparable to that of the 660-kilometer discontinuity. This anomaly outlines a high-velocity (therefore presumably cold) region present in recent finite-frequency based mantle velocity models, suggesting the extension of slab material into the lower mantle. The strength of the reflection is interpreted to be caused by a relatively sharp velocity change, likely due to a strong temperature gradient in combination with mineral phase transitions, the presence of water, or other chemical heterogeneities. Significant mass and heat exchange is therefore expected between the upper- and lower-mantle beneath the study region.

Subduction Related Crustal and Mantle Deformations and Their Implications for Plate Dynamics

NASA Astrophysics Data System (ADS)

Okeler, Ahmet

Ocean-continent convergence and subsequent continental collision are responsible for continental growth, mountain building, and severe tectonic events including volcanic eruptions and earthquake activity. They are also key driving forces behind the extensive thermal and compositional heterogeneities at crustal and mantle depths. Active subduction along the Calabrian Arc in southern Italy and the Hellenic Arc are examples of such collisional tectonics. The first part of this thesis examines the subduction related deformations within the crust beneath the southern Apennines. By modeling regional surface wave recordings of the largest temporary deployment in the southern Apennines, a lower-crustal/upper-mantle low-velocity volume extending down to 50 km beneath the mountain chain is identified. The magnitude (˜ 0.4 km/s slower) and anisotropic nature (˜ 10%) of the anomaly suggest the presence of hot and partially molten emplacement that may extend into the upper-crust towards Mt. Vulture, a once active volcano. Since the Apulian basement units are deformed during the compressional and consequent extensional events, our observations favor the "thick-skin" tectonic growth model for the region. In the deeper mantle, active processes are thermodynamically imprinted on the depth and strength of the phase transitions. This thesis examines more than 15000 SS precursors and provides the present-day reflectivity structure and topography associated with these phase transitions. Through case studies I present ample evidence for both slab penetration into the lower mantle (beneath the Hellenic Arc, Kurile Island and South America) and slab stagnation at the bottom of the Mantle Transition Zone (beneath the Tyrrhenian Sea and eastern China). Key findings include (1) thermal anomalies (˜ 200 K) at the base of the MTZ, which represent the deep source for Cenozoic European Rift Zone, Mount Etna and Mount Cameroon volcanism, (2) significant depressions (by 20-40 km) at the bottom of the Mantle Transition Zone beneath subducting slabs, (3) a strong 520-km reflector near subducting slabs, (4) a weak and elevated (15-25 km) 410-km reflector within active deformation zones, (5) strong lower mantle reflectors (˜ 900 km) while slabs penetrate into the lower mantle, and (6) consistency between the topography of a 300-km reflector and an exothermic phase transformation.

Mantle structure beneath eastern Africa: Evidence for a through going-mantle anomaly and its implications for the origin of Cenozoic tectonism in eastern Africa

NASA Astrophysics Data System (ADS)

Mulibo, G.; Tugume, F.; Julia, J.

2012-12-01

In this study, teleseismic earthquakes recorded on over 60 temporary AfricaArray seismic stations deployed in Uganda, Kenya, Tanzania and Zambia between 2007 and 2011 are used to invert P and S travel time residuals, together with travel time residuals from previous deployments, for a 3D image of mantle wave speeds and for examining relief on transition zone discontinuities using receiver function stacks. Tomographic images reveal a low wave speed anomaly (LWA) that dips to the SW beneath northern Zambia, extending to a depth of at least 900 km. The anomaly appears to be continuous across the transition zone, extending into the lower mantle. Receiver function stacks reveal an average transition zone thickness (TZT) across a wide region extending from central Zambia to the NE through Tanzania and into Kenya, which is ~30-40 km thinner than the global average. These results are not easily explained by models for the origin of the Cenozoic tectonism in eastern Africa that invoke a plume head or small scale convection either by edge flow or passive stretching of the lithosphere. However, the depth extent of the LWA coincident with a thin transition zone is consistent with a model invoking a through-going mantle anomaly beneath eastern Africa that links anomalous upper mantle to the African Superplume anomaly in the lower mantle beneath southern Africa. This finding indicates that geodynamic processes deep in the lower mantle are influencing surface dynamics across the Afro-Arabian rift system.

Crustal Footprint of the Hainan Plume beneath Southeast China

NASA Astrophysics Data System (ADS)

Liu, H.; Chen, F.; Leng, W.; Zhang, H.

2016-12-01

A hotspot track is an age-progressive line of volcanos that is connected to a hotspot that may have resulted from interactions between the lithosphere and a deep-seated mantle plume [Campbell and Griffiths, 1990; Richards et al., 1989]. Although global and regional seismic tomography results have revealed the presence of a mantle plume beneath Hainan Island [Lebedev et al., 2003; Lei et al., 2009; Huang, 2014], there is little evidence for a hotspot track associated with the Hainan plume. Here, a joint inversion of seismology and gravity data was performed with the receiver function method, and the results show that a linear corridor of seismic velocity anomalies at the base of the crust is located northeast of Hainan Island beneath southeast China. Geodynamic modeling demonstrates that this corridor could have formed by the interactions between a mantle plume and the continental lithosphere with a weak lower crust. Volcanic age distributions further suggest that this track likely formed in the Cenozoic, which constrains the average plate velocities of the South China Block during the Cenozoic to 2-6 cm/yr to the northeast. These results provide an independent reference frame for the motion history of the Eurasia plate in the Cenozoic. References 1. Campbell I H, Griffiths R W. Implications of mantle plume structure for the evolution of flood basalts [J]. Earth and Planetary Science Letters, 1990, 99(1): 79-93. 2. Richards M A, Duncan R A, Courtillot V E. Flood basalts and hot-spot tracks: plume heads and tails [J]. Science, 1989, 246(4926): 103-107. 3. Lebedev S, Nolet G. Upper mantle beneath Southeast Asia from S velocity tomography [J]. Journal of Geophysical Research: Solid Earth (1978-2012), 2003, 108(B1). 4. Lei J, Zhao D, Steinberger B, et al. New seismic constraints on the upper mantle structure of the Hainan plume [J]. Physics of the Earth and Planetary Interiors, 2009, 173(1): 33-50. 5. Huang J. P-and S-wave tomography of the Hainan and surrounding regions: Insight into the Hainan plume [J]. Tectonophysics, 2014, 633: 176-192.

Southern Africa seismic structure and source studies

NASA Astrophysics Data System (ADS)

Zhao, Ming

1998-09-01

The upper mantle seismic velocity structure beneath southern Africa is investigated using travel time and waveform data. Waveform and travel time data used in this study come mainly from a large mine tremor in South Africa (msb{b} 5.6) recorded on stations of the southern Africa and the Tanzania Broadband Seismic Experiment. Auxiliary data along similar profiles are obtained from other moderate events within eastern and southern Africa. The waveform data from the large tremor show upper mantle triplications for both the 400 and 670-km discontinuities between 18sp° and 27sp° distance. The most notable feature of the data is a large, late P phase that propagates to at least 27sp°. This phase is striking because of its late arrival time (as much as 15 seconds after direct P at 27sp°) and high amplitude relative to the first arrival. Travel times from all available stations are used to invert for the P wave velocity structure down to 800 km depth and S wave velocity structure down to 200 km using the Wiechert-Herglotz (W-H) inversion technique. The P wave velocities from the uppermost mantle down to 300 km are as much as 3% higher than the global average and are slightly slower than the global average between 300 and 400 km depths. The velocity gradient between 300 and 400 km is 0.0015 1/s. The S wave travel time data yield fast velocities above 200-km depth. The S wave velocity structure appears inconsistent with the P wave structure model indicating varying Poisson's ratio in the upper mantle. Little evidence is found for a pronounced upper mantle low velocity zone. Both sharp and gradual-change 400-km discontinuities are favored by the waveform data. The 670-km discontinuity appears as a gradual-change zone. The source mechanism of the mb 5.6 mining tremor itself is important for seismic discrimination and insight into mining tremor sources. Source parameters for this event as well as some other large mining tremors from the South African gold mines are studied using detailed waveform modeling. All these events (mb > 4.8) indicate normal-faulting slip with P wave nodal planes striking approximately NS. Tectonic stress is essential to control the mining seismicity of large magnitude. Mining geometry also plays an important role in influencing the seismicity. The crustal velocity structure at the study area is investigated in detail using teleseismic receiver function and regional surface wave dispersion data. The results indicate some lateral variation in the shallow crust. The thickness of the crust beneath the GSN station BOSA is 33-36 km. Gradually increasing velocities with depth in the crust are preferred. A thin layer with rather low velocity at the top of the crust beneath BOSA is important for generating the regional waveforms. The crust beneath LBTB is a few kilometers thicker than at BOSA and the Moho there is likely to be dipping. (Abstract shortened by UMI.)

Electrical conductivity anomaly beneath Mare Serenitatis detected by Lunokhod 2 and Apollo 16 magnetometers

NASA Technical Reports Server (NTRS)

Vanian, L. L.; Vnuchkova, T. A.; Egorov, I. V.; Basilevskii, A. T.; Eroshenko, E. G.; Fainberg, E. B.; Dyal, P.; Daily, W. D.

1979-01-01

Magnetic fluctuations measured by the Lunokhod 2 magnetometer in the Bay Le Monnier are distinctly anisotropic when compared to simultaneous Apollo 16 magnetometer data measured 1100 km away in the Descartes highlands. This anisotropy can be explained by an anomalous electrical conductivity of the upper mantle beneath Mare Serenitatis. A model is presented of anomalously lower electrical conductivity beneath Serenitatis and the simultaneous magnetic data from the Lunokhod 2 site at the mare edge and the Apollo 16 site are compared to the numerically calculated model solutions. This comparison indicates that the anisotropic fluctuations can be modeled by a nonconducting layer in the lunar lithosphere which is 150 km thick beneath the highlands and 300 km thick beneath Mare Serenitatis. A decreased electrical conductivity in the upper mantle beneath the mare may be due to a lower temperature resulting from heat carried out the magma source regions to the surface during mare flooding.

Upper mantle and crustal structure of southwestern Scandinavia: Results of the TopoScandiaDeep project

NASA Astrophysics Data System (ADS)

Köhler, A.; Balling, N.; Ebbing, J.; England, R.; Frassetto, A.; Gradmann, S.; Jacobsen, B. H.; Kvarven, T.; Maupin, V.; Medhus, A. Bondo; Mjelde, R.; Ritter, J.; Schweizer, J.; Stratford, W.; Thybo, H.; Wawerzinek, B.; Weidle, C.

2012-04-01

The origin of the Scandinavian mountains, located far away from any presently active plate margin, is still not well understood. In particular, it is not clear if the mountains are sustained isostatically either by crustal thickening or by light upper mantle material. Within the TopoScandiaDeep project (a collaborative research project within the ESF TOPO-EUROPE programme), we therefore analyse recently collected passive seismological and active seismic data in the southern Scandes and surrounding regions. We infer crustal and upper mantle (velocity) structures and relate them to results of gravity and temperature-composition modelling. The Moho under the high topography of southern Norway appears from controlled source seismic refraction and Receiver Functions as relatively shallow (<= 45 km) compared to the deeper conversion (>55 km) imaged beneath the low topography in Sweden and elsewhere in the Baltic Shield area outside Norway. The Receiver Function modeling as well as the active seismic results suggest that the differences in the observed Moho response may represent the transition between tectonically reworked Moho under southern Norway and an intact, cratonic crust-mantle boundary beneath the Baltic Shield. Furthermore, the 410km-discontinuity and the LAB is imaged, the latter one suggesting a lithospheric thickening in NE direction. Upper mantle P-wave and S-wave velocities in southern Sweden and southern Norway east of the Oslo Graben are correspondingly relatively high while lower velocities are observed in the southwestern part of Norway and northern Denmark. The lateral velocity gradient, interpreted as the southwestern boundary of thick Baltic Shield lithosphere, is remarkably sharp. Differences in upper mantle velocities are found at depths of 100-400 km and amount to ± 2-3%. S-to-P wave conversions, interpreted to originate from the lithosphere-asthenosphere boundary, are preliminary estimated to 90-120 km depth. Inversion of Rayleigh and Love surface wave phase velocity dispersion curves from observations of ambient noise and earthquakes yield another independent model of the crust and upper mantle structure below southern Norway. Inverted crustal velocities and Moho depths are consistent with the results of seismic refraction and receiver functions. Additionally, indications for radial crustal anisotropy of up to about 3% are found. The inferred upper mantle S-wave velocities show that the lithosphere under southern Norway has characteristics usually found under continental platforms and changes towards a cratonic-like velocity structure in the East, in agreement with the body wave tomography. All in all, these separate investigations give a very consistent and stable picture of the crust and upper mantle configuration. Integrated geophysical modeling of the results shows that a lateral transition from thinner, warmer lithosphere under southern Norway towards thicker, colder lithosphere under Sweden results in a density distribution that significantly adds to the isostatic support of Norway's high topography.

Inferring global upper-mantle shear attenuation structure by waveform tomography using the spectral element method

NASA Astrophysics Data System (ADS)

Karaoǧlu, Haydar; Romanowicz, Barbara

2018-06-01

We present a global upper-mantle shear wave attenuation model that is built through a hybrid full-waveform inversion algorithm applied to long-period waveforms, using the spectral element method for wavefield computations. Our inversion strategy is based on an iterative approach that involves the inversion for successive updates in the attenuation parameter (δ Q^{-1}_μ) and elastic parameters (isotropic velocity VS, and radial anisotropy parameter ξ) through a Gauss-Newton-type optimization scheme that employs envelope- and waveform-type misfit functionals for the two steps, respectively. We also include source and receiver terms in the inversion steps for attenuation structure. We conducted a total of eight iterations (six for attenuation and two for elastic structure), and one inversion for updates to source parameters. The starting model included the elastic part of the relatively high-resolution 3-D whole mantle seismic velocity model, SEMUCB-WM1, which served to account for elastic focusing effects. The data set is a subset of the three-component surface waveform data set, filtered between 400 and 60 s, that contributed to the construction of the whole-mantle tomographic model SEMUCB-WM1. We applied strict selection criteria to this data set for the attenuation iteration steps, and investigated the effect of attenuation crustal structure on the retrieved mantle attenuation structure. While a constant 1-D Qμ model with a constant value of 165 throughout the upper mantle was used as starting model for attenuation inversion, we were able to recover, in depth extent and strength, the high-attenuation zone present in the depth range 80-200 km. The final 3-D model, SEMUCB-UMQ, shows strong correlation with tectonic features down to 200-250 km depth, with low attenuation beneath the cratons, stable parts of continents and regions of old oceanic crust, and high attenuation along mid-ocean ridges and backarcs. Below 250 km, we observe strong attenuation in the southwestern Pacific and eastern Africa, while low attenuation zones fade beneath most of the cratons. The strong negative correlation of Q^{-1}_μ and VS anomalies at shallow upper-mantle depths points to a common dominant origin for the two, likely due to variations in thermal structure. A comparison with two other global upper-mantle attenuation models shows promising consistency. As we updated the elastic 3-D model in alternate iterations, we found that the VS part of the model was stable, while the ξ structure evolution was more pronounced, indicating that it may be important to include 3-D attenuation effects when inverting for ξ, possibly due to the influence of dispersion corrections on this less well-constrained parameter.

Seismic structure of the uppermost mantle beneath the Kenya rift

USGS Publications Warehouse

Keller, Gordon R.; Mechie, J.; Braile, L.W.; Mooney, W.D.; Prodehl, C.

1994-01-01

A major goal of the Kenya Rift International Seismic Project (KRISP) 1990 experiment was the determination of deep lithospheric structure. In the refraction/wide-angle reflection part of the KRISP effort, the experiment was designed to obtain arrivals to distances in excess of 400 km. Phases from interfaces within the mantle were recorded from many shotpoints, and by design, the best data were obtained along the axial profile. Reflected arrivals from two thin (< 10 km), high-velocity layers were observed along this profile and a refracted arrival was observed from the upper high-velocity layer. These mantle phases were observed on record sections from four axial profile shotpoints so overlapping and reversed coverage was obtained. Both high-velocity layers are deepest beneath Lake Turkana and become more shallow southward as the apex of the Kenya dome is approached. The first layer has a velocity of 8.05-8.15 km/s, is at a depth of about 45 km beneath Lake Turkana, and is observed at depths of about 40 km to the south before it disappears near the base of the crust. The deeper layer has velocities ranging from 7.7 to 7.8 km/s in the south to about 8.3 km/s in the north, has a similar dip as the upper one, and is found at depths of 60-65 km. Mantle arrivals outside the rift valley appear to correlate with this layer. The large amounts of extrusive volcanics associated with the rift suggest compositional anomalies as an explanation for the observed velocity structure. However, the effects of the large heat anomaly associated with the rift indicate that composition alone cannot explain the high-velocity layers observed. These layers require some anisotropy probably due to the preferred orientation of olivine crystals. The seismic model is consistent with hot mantle material rising beneath the Kenya dome in the southern Kenya rift and north-dipping shearing along the rift axis near the base of the lithosphere beneath the northern Kenya rift. This implies lithosphere thickening towards the north and is consistent with a thermal thinning of the lithosphere from below in the south changing to thinning of the lithosphere due to stretching in the north. ?? 1994.

Multiple mantle upwellings in the transition zone beneath the northern East-African Rift system from relative P-wave travel-time tomography

NASA Astrophysics Data System (ADS)

Civiero, Chiara; Hammond, James O. S.; Goes, Saskia; Fishwick, Stewart; Ahmed, Abdulhakim; Ayele, Atalay; Doubre, Cecile; Goitom, Berhe; Keir, Derek; Kendall, J.-Michael; Leroy, Sylvie; Ogubazghi, Ghebrebrhan; Rümpker, Georg; Stuart, Graham W.

2015-09-01

Mantle plumes and consequent plate extension have been invoked as the likely cause of East African Rift volcanism. However, the nature of mantle upwelling is debated, with proposed configurations ranging from a single broad plume connected to the large low-shear-velocity province beneath Southern Africa, the so-called African Superplume, to multiple lower-mantle sources along the rift. We present a new P-wave travel-time tomography model below the northern East-African, Red Sea, and Gulf of Aden rifts and surrounding areas. Data are from stations that span an area from Madagascar to Saudi Arabia. The aperture of the integrated data set allows us to image structures of ˜100 km length-scale down to depths of 700-800 km beneath the study region. Our images provide evidence of two clusters of low-velocity structures consisting of features with diameter of 100-200 km that extend through the transition zone, the first beneath Afar and a second just west of the Main Ethiopian Rift, a region with off-rift volcanism. Considering seismic sensitivity to temperature, we interpret these features as upwellings with excess temperatures of 100 ± 50 K. The scale of the upwellings is smaller than expected for lower mantle plume sources. This, together with the change in pattern of the low-velocity anomalies across the base of the transition zone, suggests that ponding or flow of deep-plume material below the transition zone may be spawning these upper mantle upwellings. This article was corrected on 28 SEP 2015. See the end of the full text for details.

Seismic anisotropy in the upper mantle beneath the MAGIC array, mid-Atlantic Appalachians: Constraints from SKS splitting and quasi-Love wave propagation

NASA Astrophysics Data System (ADS)

Aragon, J. C.; Long, M. D.; Benoit, M. H.; Servali, A.

2016-12-01

North America's eastern passive continental margin has been modified by several cycles of supercontinent assembly. Its complex surface geology and distinct topography provide evidence of these events, while also raising questions about the extent of deformation in the continental crust, lithosphere, and mantle during past episodes of rifting and mountain building. The Mid-Atlantic Geophysical Integrative Collaboration (MAGIC) is an EarthScope and GeoPRISMS-funded project that involves a collaborative effort among seismologists, geodynamicists, and geomorphologists. One component of the project is a broadband seismic array consisting of 28 instruments in a linear path from coastal Virginia to western Ohio, which operated between October 2013 and October 2016. A key science question addressed by the MAGIC project is the geometry of past lithospheric deformation and present-day mantle flow beneath the Appalachians, which can be probed using observations of seismic anisotropy Here we present observations of SKS splitting and quasi-Love wave arrivals from stations of the MAGIC array, which together constrain seismic anisotropy in the upper mantle. SKS splitting along the array reveals distinct regions of upper mantle anisotropy, with stations in and to the west of the range exhibiting fast directions parallel to the strike of the mountains. In contrast, weak splitting and null SKS arrivals dominate eastern stations in the coastal plain. Documented Love-to-Rayleigh wave scattering for surface waves originating the magnitude 8.3 Illapel, Chile earthquakes in September 2015 provides complementary constraints on anisotropy. These quasi-Love wave arrivals suggest a pronounced change in upper mantle anisotropy at the eastern edge of present-day Appalachian topography. Together, these observations increase our understanding of the extent of lithospheric deformation beneath North America associated with Appalachian orogenesis, as well as the pattern of present-day mantle flow beneath the passive margin.

Flow, melt and fossil seismic anisotropy beneath Ethiopia

NASA Astrophysics Data System (ADS)

Hammond, James; Kendall, J.-Michael; Wookey, James; Stuart, Graham; Keir, Derek; Ayele, Atalay

2014-05-01

Ethiopia is a region where continental rifting gives way to oceanic spreading. Yet the role that pre-existing lithospheric structure, melt, mantle flow or active upwellings may play in this process is debated. Measurements of seismic anisotropy are often used to attempt to understand the contribution that these mechanisms may play. In this study we use new data in Afar, Ethiopia along with legacy data across Ethiopia, Djibouti and Yemen to obtain estimates of mantle anisotropy using SKS-wave splitting. We show that two layers of anisotropy exist, and use shear-wave splitting tomography to invert for these. We show that fossil anisotropy with fast directions oriented northeast-southwest may be preserved in the lithosphere away from the rift. Beneath the Main Ethiopian Rift and parts of Afar, anisotropy due aligned melt due to sharp changes in lithospheric thickness dominate the shear-wave splitting signal in the mantle. Beneath Afar, away from lithospheric topography, melt pockets associated with the crustal magma storage dominate the signal and little anisotropy is seen in the uppermost mantle suggesting melt retains no preferential alignment, possibly due to a lack of mantle lithosphere. These results show the important role melt plays in weakening the lithosphere and imply that as rifting evolves passive upwelling sustains extension. A dominant northeast-southwest anisotropic fast direction is observed in a deeper layer across all of Ethiopia. This suggests that a conduit like plume is absent beneath Afar today, rather a broad flow from the southwest dominates in the upper mantle.

Seismic Imaging of Mantle Plumes

NASA Astrophysics Data System (ADS)

Nataf, Henri-Claude

The mantle plume hypothesis was proposed thirty years ago by Jason Morgan to explain hotspot volcanoes such as Hawaii. A thermal diapir (or plume) rises from the thermal boundary layer at the base of the mantle and produces a chain of volcanoes as a plate moves on top of it. The idea is very attractive, but direct evidence for actual plumes is weak, and many questions remain unanswered. With the great improvement of seismic imagery in the past ten years, new prospects have arisen. Mantle plumes are expected to be rather narrow, and their detection by seismic techniques requires specific developments as well as dedicated field experiments. Regional travel-time tomography has provided good evidence for plumes in the upper mantle beneath a few hotspots (Yellowstone, Massif Central, Iceland). Beneath Hawaii and Iceland, the plume can be detected in the transition zone because it deflects the seismic discontinuities at 410 and 660 km depths. In the lower mantle, plumes are very difficult to detect, so specific methods have been worked out for this purpose. There are hints of a plume beneath the weak Bowie hotspot, as well as intriguing observations for Hawaii. Beneath Iceland, high-resolution tomography has just revealed a wide and meandering plume-like structure extending from the core-mantle boundary up to the surface. Among the many phenomena that seem to take place in the lowermost mantle (or D''), there are also signs there of the presence of plumes. In this article I review the main results obtained so far from these studies and discuss their implications for plume dynamics. Seismic imaging of mantle plumes is still in its infancy but should soon become a turbulent teenager.

Continent-arc collision in the Banda Arc imaged by ambient noise tomography

NASA Astrophysics Data System (ADS)

Porritt, Robert W.; Miller, Meghan S.; O'Driscoll, Leland J.; Harris, Cooper W.; Roosmawati, Nova; Teofilo da Costa, Luis

2016-09-01

The tectonic configuration of the Banda region in southeast Asia captures the spatial transition from subduction of Indian Ocean lithosphere to subduction and collision of the Australian continental lithosphere beneath the Banda Arc. An ongoing broadband seismic deployment funded by NSF is aimed at better understanding the mantle and lithospheric structure in the region and the relationship of the arc-continent collision to orogenesis. Here, we present results from ambient noise tomography in the region utilizing this temporary deployment of 30 broadband instruments and 39 permanent stations in Indonesia, Timor Leste, and Australia. We measure dispersion curves for over 21,000 inter-station paths resulting in good recovery of the velocity structure of the crust and upper mantle beneath the Savu Sea, Timor Leste, and the Nusa Tenggara Timur (NTT) region of Indonesia. The resulting three dimensional model indicates up to ∼25% variation in shear velocity throughout the plate boundary region; first-order velocity anomalies are associated with the subducting oceanic lithosphere, subducted Australian continental lithosphere, obducted oceanic sediments forming the core of the island of Timor, and high velocity anomalies in the Savu Sea and Sumba. The structure in Sumba and the Savu Sea is consistent with an uplifting forearc sliver. Beneath the island of Timor, we confirm earlier inferences of pervasive crustal duplexing from surface mapping, and establish a link to underlying structural features in the lowermost crust and uppermost mantle that drive upper crustal shortening. Finally, our images of the volcanic arc under Flores, Wetar, and Alor show high velocity structures of the Banda Terrane, but also a clear low velocity anomaly at the transition between subduction of oceanic and continental lithosphere. Given that the footprint of the Banda Terrane has previously been poorly defined, this model provides important constraints on tectonic reconstructions that formerly have lacked information on the lower crust and uppermost mantle.

Mantle Upwellings Below the Ibero-Maghrebian Region with a Common Deep Source from P Travel-time Tomography

NASA Astrophysics Data System (ADS)

Civiero, C.; Custodio, S.; Silveira, G. M.; Rawlinson, N.; Arroucau, P.

2017-12-01

The processes responsible for the geodynamical evolution of the Ibero-Maghrebian domain are still enigmatic. Several geophysical studies have improved our understanding of the region, but no single model has been accepted yet. This study takes advantage of the dense station networks deployed from France in the north to Canary Islands and Morocco in the south to provide a new high-resolution P-wave velocity model of the structure of the upper-mantle and top of the lower mantle. These images show subvertical small-scale upwellings below Atlas Range, Canary Islands and Central Iberia that seem to cross the transition zone. The results, together with geochemical evidence and a comparison with previous global tomographic models, reveal the ponding or flow of deep-plume material beneath the transition zone, which seems to feed upper-mantle "secondary" pulses. In the upper mantle the plumes, in conjunction with the subduction-related upwellings, allow the hot mantle to rise in the surrounding zones. During its rising, the mantle interacts with horizontal SW slab-driven flow which skirts the Alboran slab and connects with the mantle upwelling below Massif Central through the Valencia Trough rift.

Density structure of the lithosphere in the southwestern United States and its tectonic significance

USGS Publications Warehouse

Kaban, M.K.; Mooney, W.D.

2001-01-01

We calculate a density model of the lithosphere of the southwestern United States through an integrated analysis of gravity, seismic refraction, drill hole, and geological data. Deviations from the average upper mantle density are as much as ?? 3%. A comparison with tomographic images of seismic velocities indicates that a substantial part (>50%) of these density variations is due to changes in composition rather than temperature. Pronounced mass deficits are found in the upper mantle under the Basin and Range Province and the northern part of the California Coast Ranges and adjacent ocean. The density structure of the northern and central/southern Sierra Nevada is remarkably different. The central/southern part is anomalous and is characterized by a relatively light crust underlain by a higher-density upper mantle that may be associated with a cold, stalled subducted plate. High densities are also determined within the uppermost mantle beneath the central Transverse Ranges and adjoining continental slope. The average density of the crystalline crust under the Great Valley and western Sierra Nevada is estimated to be up to 200 kg m~3 higher than the regional average, consistent with tectonic models for the obduction of oceanic crust and uppermost mantle in this region.

Forearc structure beneath southwestern British Columbia: A three-dimensional tomographic velocity model

USGS Publications Warehouse

Ramachandran, K.; Dosso, S.E.; Spence, G.D.; Hyndman, R.D.; Brocher, T.M.

2005-01-01

This paper presents a three-dimensional compressional wave velocity model of the forearc crust and upper mantle and the subducting Juan de Fuca plate beneath southwestern British Columbia and the adjoining straits of Georgia and Juan de Fuca. The velocity model was constructed through joint tomographic inversion of 50,000 first-arrival times from earthquakes and active seismic sources. Wrangellia rocks of the accreted Paleozoic and Mesozoic island arc assemblage underlying southern Vancouver Island in the Cascadia forearc are imaged at some locations with higher than average lower crustal velocities of 6.5-7.2 km/s, similar to observations at other island arc terranes. The mafic Eocene Crescent terrane, thrust landward beneath southern Vancouver Island, exhibits crustal velocities in the range of 6.0-6.7 km/s and is inferred to extend to a depth of more than 20 km. The Cenozoic Olympic Subduction Complex, an accretionary prism thrust beneath the Crescent terrane in the Olympic Peninsula, is imaged as a low-velocity wedge to depths of at least 20 km. Three zones with velocities of 7.0-7.5 km/s, inferred to be mafic and/or ultramafic units, lie above the subducting Juan de Fuca plate at depths of 25-35 km. The forearc upper mantle wedge beneath southeastern Vancouver Island and the Strait of Georgia exhibits low velocities of 7.2-7.5 km/s, inferred to correspond to ???20% serpentinization of mantle peridotites, and consistent with similar observations in other warm subduction zones. Estimated dip of the Juan de Fuca plate beneath southern Vancouver Island is ???11??, 16??, and 27?? at depths of 30, 40, and 50 km, respectively. Copyright 2005 by the American Geophysical Union.

Tomography images of the Alpine roots and surrounding upper mantle

NASA Astrophysics Data System (ADS)

Plomerova, Jaroslava; Babuska, Vladislav

2017-04-01

Teleseismic body-wave tomography represents powerful tool to study regional velocity structure of the upper mantle and to image velocity anomalies, such as subducted lithosphere plates in collisional zones. In this contribution, we recapitulate 3D models of the upper mantle beneath the Alps, which developed at a collision zone of the Eurasian and African plates. Seismic tomography studies indicate a leading role of the rigid mantle lithosphere that functioned as a major stress guide during the plate collisions. Interactions of the European lithosphere with several micro-plates in the south resulted in an arcuate shape of this mountain range on the surface and in a complicated geometry of the Alpine subductions in the mantle. Early models with one bended lithosphere root have been replaced with more advanced models showing two separate lithosphere roots beneath the Western and Eastern Alps (Babuska et al., Tectonophysics 1990; Lippitsch et al., JGR 2003). The standard isotropic velocity tomography, based on pre-AlpArray data (the currently performed passive seismic experiment in the Alps and surroundings) images the south-eastward dipping curved slab of the Eurasian lithosphere in the Western Alps. On the contrary, beneath the Eastern Alps the results indicate a very steep northward dipping root that resulted from the collision of the European plate with the Adriatic microplate. Dando et al. (2011) interpret high-velocity heterogeneities at the bottom of their regional tomographic model as a graveyard of old subducted lithospheres. High density of stations, large amount of rays and dense ray-coverage of the volume studied are not the only essential pre-requisites for reliable tomography results. A compromise between the amount of pre-processed data and the high-quality of the tomography input (travel-time residuals) is of the high importance as well. For the first time, the existence of two separate roots beneath the Alps has been revealed from carefully pre-processed, mostly the ISC-bulletin data (Babuska et al., Tectonophysics 1990). Calculated relative travel-time residuals have been assigned to source clusters and filtered relative to the residual mean of each cluster of events. We expect that future 3D studies of the mantle velocities and mantle fabrics with the use of body-wave anisotropic parameters from the AlpArray data will shed a new light on tectonic development of the complex Alpine region and its surroundings.

Imaging the continental mantle beneath Iberia and Northern Morocco: the contribution of the Topo-Iberia project.

NASA Astrophysics Data System (ADS)

Diaz, Jordi

2013-04-01

One of the key assets of the Topo-Iberia research program, aiming to unravel the complex structure and mantle processes in the area of interaction between the African and European continental plates, has been the deploying of a technological observational platform, named IberArray, to provide new seismological, magnetotelluric and geodetical data with unprecedented resolution and coverage. Topo-Iberia has also benefited from the interaction with subsequent projects investigating the same area, as the USA Picasso, the French Pyrope or the Portuguese Wilas. This interaction includes sharing the available data to better assess the key geological questions. This contribution aims to present the current state of the most significant scientific investigations concerning the lithosphere-asthenosphere system beneath Iberia and Northern Morocco which are arising from the data acquired using the Iberarray platform. The area so far investigated extends from the Variscan Central Iberian Massif in the North to the border of the Sahara Platform in the South and includes areas of complex and still not completely understood geodynamics, as the Alboran domain or the Atlas Mountains. SKS splitting analysis clearly image this complexity; the fast polarization directions (FPD) beneath the Betics-Alboran show a spectacular rotation along the Gibraltar arc following the curvature of the Rif-Betic chain. Beneath the High Atlas and SW Iberia, there are a very significant number of high quality events without evidence for anisotropy, suggesting the presence of a large vertical component of flow in the upper mantle. These observations allow inferring a model of mantle flow at regional scale. New body wave tomographic images have confirmed the presence of a high-velocity slab beneath the Gibraltar Arc and allowed to define more precisely its geometry, appearing as a near-vertical feature extending from 50-75 km to about 600 km. Magnetotelluric profiles acquired using broad-band and long-period instrumentation along different N-S profiles from North Iberia to the Atlas have also provided relevant information along a 1500 km long N-S lithospheric transect. Receiver functions have revealed large crustal thickness variations, including a crustal root beneath the Rif not clearly documented previously. Beneath Iberia, the Variscan domain shows a quite uniform Moho depth, but the areas affected by the Alpine orogeny show significant variations, consistent with the results arising from ambient noise interferometry. Moving to the base of the upper mantle, the geometry of the 410-km and 660-km upper mantle discontinuities have been investigated using novel cross-correlation/stacking techniques, which have allowed to obtain a detailed map of the transition zone thickness variations.

Upper Mantle Velocity Structure beneath the Northeastern Philippine Sea Constrained by Waveform Modeling of P Triplicated Phases

NASA Astrophysics Data System (ADS)

Cho, S.; Rhie, J.; Lee, S. H.; Kim, S.; Kang, T. S.

2017-12-01

A study on the detailed velocity structures of the stagnant Pacific slab is important to understand the complex processes happening in the upper mantle. Although waveform modeling of P triplicated phases can reveal the detailed velocity structures especially for the discontinuities, the regions where the method can be applied are limited due to uneven distribution of earthquakes and stations. In this study, we used waveforms generated by two deep earthquakes near Izu-Bonin Trench and recorded by stations in South Korea. These event-station pairs are appropriate to study the upper mantle structures beneath the northeastern Philippine Sea, where no previous results by triplicated waveform modeling have been reported. In this region, the subducting Pacific slab seems to hit the 660 km discontinuity and become stagnant. We applied the reflectivity method to calculate waveforms and found the best fitting model by trial-and-error and manual inspection. In general, our best model is similar to M3.11, which is widely accepted 1D model for the regions where the stagnant slab exists and the 660 km discontinuity is depressed by the slab. The most noticeable feature of our model is that P wave velocities of inside and above the slab are considerably higher and lower than ones for M3.11, respectively. This specific velocity model is necessary to explain arrivals of two distinct phases identified in observed waveforms; one refracts inside the slab and the other reflects on the upper boundary of the slab. To understand the cause of the differences between our model and M3.11, further studies including thermal and mechanical modelling of the slab in this region will be recommended.

Tomography of the upper mantle beneath the African/Iberian collision zone

NASA Astrophysics Data System (ADS)

Mickael, B.; Nolet, G.; Villasenor, A.; Josep, G.; Thomas, C.

2013-12-01

During Cenozoic, geodynamics of the western Mediterranean domain has been characterized by a complex history of subduction of Mesozoic oceanic lithosphere. The final stage of these processes is proposed to have led to the development of the Calabria and Gibraltar arcs, whose formation is still under debate. In this study we take advantage of the dense broadband-station networks now available in Alborán Sea region, to develop a high-resolution 3D tomographic P velocity model of the upper mantle beneath the African/Iberian collision zone that will bring new constraints on the past dynamics of this zone. The model is based on 13200 teleseismic arrival times recorded between 2008 and 2012 at 279 stations for which cross-correlation delays are measured with a new technique in different frequency bands centered between 0.03 and 1.0 Hz, and interpreted using multiple frequency tomography. Our model shows, beneath Alborán Sea, a strong (~ 4%) fast vertically dipping anomaly observed to at least 650 km depth. The arched shape of this anomaly and its extent at depth are coherent with a lithospheric slab, thus favoring the hypothesis of a westward consumption of the Ligurian ocean slab by roll-back during Cenozoic. In addition to this fast anomaly in the deep upper-mantle, several high intensity slow anomalies are widely observed in the lithosphere and asthenosphere beneath Morocco and southern Spain. These anomalies are correlated at surface with the position of the orogens (Rif and Atlas) and with Cenozoic volcanic fields. We thus confirm the presence, beneath Morocco, of an anomalous (hot) upper mantle, with piece of evidence for a lateral connection with the Canary volcanic islands, likely indicating a lateral spreading of the Canary plume to the east.

The upper mantle shear wave velocity structure of East Africa derived from Rayleigh wave tomography

NASA Astrophysics Data System (ADS)

O'Donnell, J.; Nyblade, A.; Adams, A. N.; Weeraratne, D. S.; Mulibo, G.; Tugume, F.

2012-12-01

An expanded model of the three-dimensional shear wave velocity structure of the upper mantle beneath East Africa has been developed using data from the latest phases of the AfricaArray East African Seismic Experiment in conjunction with data from preceding studies. The combined dataset consists of 331 events recorded on a total of 95 seismic stations spanning Kenya, Uganda, Tanzania, Zambia and Malawi. In this latest study, 149 events were used to determine fundamental mode Rayleigh wave phase velocities at periods ranging from 20 to 182 seconds using the two-plane-wave method. These were subsequently combined with the similarly processed published measurements and inverted for an updated upper mantle three-dimensional shear wave velocity model. Newly imaged features include a substantial fast anomaly in eastern Zambia that may have exerted a controlling influence on the evolution of the Western Rift Branch. Furthermore, there is a suggestion that the Eastern Rift Branch trends southeastward offshore eastern Tanzania.

Geophysical and petrological modelling of the structure and composition of the crust and upper mantle in complex geodynamic settings: The Tyrrhenian Sea and surroundings

NASA Astrophysics Data System (ADS)

Panza, G. F.; Peccerillo, A.; Aoudia, A.; Farina, B.

2007-01-01

Information on the physical and chemical properties of the lithosphere-asthenosphere system (LAS) can be obtained by geophysical investigation and by studies of petrology-geochemistry of magmatic rocks and entrained xenoliths. Integration of petrological and geophysical studies is particularly useful in geodynamically complex areas characterised by abundant and compositionally variable young magmatism, such as in the Tyrrhenian Sea and surroundings. A thin crust, less than 10 km, overlying a soft mantle (where partial melting can reach about 10%) is observed for Magnaghi, Vavilov and Marsili, which belong to the Central Tyrrhenian Sea backarc volcanism where subalkaline rocks dominate. Similar characteristics are seen for the uppermost crust of Ischia. A crust about 20 km thick is observed for the majority of the continental volcanoes, including Amiata-Vulsini, Roccamonfina, Phlegraean Fields-Vesuvius, Vulture, Stromboli, Vulcano-Lipari, Etna and Ustica. A thicker crust is present at Albani - about 25 km - and at Cimino-Vico-Sabatini — about 30 km. The structure of the upper mantle, in contrast, shows striking differences among various volcanic provinces. Volcanoes of the Roman region (Vulsini-Sabatini-Alban Hills) sit over an upper mantle characterised by Vs mostly ranging from about 4.2 to 4.4 km/s. At the Alban Hills, however, slightly lower Vs values of about 4.1 km/s are detected between 60 and 120 km of depth. This parallels the similar and rather homogeneous compositional features of the Roman volcanoes, whereas the lower Vs values detected at the Alban Hills may reflect the occurrence of small amounts of melts within the mantle, in agreement with the younger age of this volcano. The axial zone of the Apennines, where ultrapotassic kamafugitic volcanoes are present, has a mantle structure with high-velocity lid ( Vs ˜ 4.5 km/s) occurring at the base of a 40-km-thick crust. Beneath the Campanian volcanoes of Vesuvius and Phlegraean Fields, the mantle structure shows a rigid body dipping westward, a feature that continues southward, up to the eastern Aeolian arc. In contrast, at Ischia the upper mantle contains a shallow low-velocity layer ( Vs = 3.5-4.0 km/s) just beneath a thin but complex crust. The western Aeolian arc and Ustica sit over an upper mantle with Vs ˜ 4.2-4.4 km/s, although a rigid layer ( Vs = 4.55 km/s) from about 80 to 150 km occurs beneath the western Aeolian arc. In Sardinia, no significant differences in the LAS structure are detected from north to south. The petrological-geochemical signatures of Italian volcanoes show strong variations that allow us to distinguish several magmatic provinces. These often coincide with mantle sectors identified by Vs tomography. For instance, the Roman volcanoes show remarkable similar petrological and geochemical characteristics, mirroring similar structure of the LAS. The structure and geochemical-isotopic composition of the upper mantle change significantly when we move to the Stromboli-Campanian volcanoes. The geochemical signatures of Ischia and Procida volcanoes are similar to other Campanian centres, but Sr-Pb isotopic ratios are lower marking a transition to the backarc mantle of the Central Tyrrhenian Sea. The structural variations from Stromboli to the central (Vulcano and Lipari) and western Aeolian arc are accompanied by strong variations of geochemical signatures, such as a decrease of Sr-isotope ratios and an increase of Nd-, Pb-isotope and LILE/HFSE ratios. The dominance of mafic subalkaline magmatism in the Tyrrhenian Sea basin denotes large degrees of partial melting, well in agreement with the soft characteristics of the uppermost mantle in this area. In contrast, striking isotopic differences of Plio-Quaternary volcanic rocks from southern to northern Sardinia does not find a match in the LAS geophysical characteristics. The combination of petrological and geophysical constraints allows us to propose a 3D schematic geodynamic model of the Tyrrhenian basin and bordering volcanic areas, including the subduction of the Ionian-Adria lithosphere in the southern Tyrrhenian Sea, and to place constraints on the geodynamic evolution of the whole region.

VP and VS structure of the Yellowstone hot spot from teleseismic tomography: Evidence for an upper mantle plume

USGS Publications Warehouse

Waite, Gregory P.; Smith, Robert B.; Allen, Richard M.

2006-01-01

The movement of the lithosphere over a stationary mantle magmatic source, often thought to be a mantle plume, explains key features of the 16 Ma Yellowstone–Snake River Plain volcanic system. However, the seismic signature of a Yellowstone plume has remained elusive because of the lack of adequate data. We employ new teleseismic P and S wave traveltime data to develop tomographic images of the Yellowstone hot spot upper mantle. The teleseismic data were recorded with two temporary seismograph arrays deployed in a 500 km by 600 km area centered on Yellowstone. Additional data from nearby regional seismic networks were incorporated into the data set. The VP and VS models reveal a strong low-velocity anomaly from ∼50 to 200 km directly beneath the Yellowstone caldera and eastern Snake River Plain, as has been imaged in previous studies. Peak anomalies are −2.3% for VP and −5.5% for VS. A weaker, anomaly with a velocity perturbation of up to −1.0% VP and −2.5% VS continues to at least 400 km depth. This anomaly dips 30° from vertical, west-northwest to a location beneath the northern Rocky Mountains. We interpret the low-velocity body as a plume of upwelling hot, and possibly wet rock, from the mantle transition zone that promotes small-scale convection in the upper ∼200 km of the mantle and long-lived volcanism. A high-velocity anomaly, 1.2%VP and 1.9% VS, is located at ∼100 to 250 km depth southeast of Yellowstone and may represent a downwelling of colder, denser mantle material.

Waveform Tomography of the South Atlantic Region

NASA Astrophysics Data System (ADS)

Celli, N. L.; Lebedev, S.; Schaeffer, A. J.; Gaina, C.

2016-12-01

The rapid growth in broadband seismic data, along with developments in waveform tomography techniques, allow us to greatly improve the data sampling in the southern hemisphere and resolve the upper-mantle structure beneath the South Atlantic region at a new level of detail. We have gathered a very large waveform dataset, including all publicly available data from permanent and temporary networks. Our S-velocity tomographic model is constrained by vertical-component waveform fits, computed using the Automated Multimode Inversion of surface, S and multiple S waves. Each seismogram fit provides a set of linear equations describing 1D average velocity perturbations within approximate sensitivity volumes, with respect to a 3D reference model. All the equations are then combined into a large linear system and inverted jointly for a model of shear- and compressional-wave speeds and azimuthal anisotropy within the lithosphere and underlying mantle. The isotropic-average shear speeds are proxies for temperature and composition at depth, while azimuthal anisotropy provides evidence on the past and present deformation in the lithosphere and asthenosphere beneath the region. We resolve the complex boundaries of the mantle roots of South America's and Africa's cratons and the deep low-velocity anomalies beneath volcanic areas in South America. Pronounced lithospheric high seismic velocity anomalies beneath the Argentine Basin suggest that its anomalously deep seafloor, previously attributed to dynamic topography, is mainly due to anomalously cold, thick lithosphere. Major hotspots show low-velocity anomalies extending substantially deeper than those beneath the mid-ocean ridge. The Vema Hotspot shows a major, hot asthenospheric anomaly beneath thick, cold oceanic lithosphere. The mantle lithosphere beneath the Walvis Ridge—a hotspot track—shows normal cooling. The volcanic Cameroon Line, in contrast, is characterized by thin lithosphere beneath the locations of recent volcanism.

A shear-wave velocity model of the European upper mantle from automated inversion of seismic shear and surface waveforms

NASA Astrophysics Data System (ADS)

Legendre, C.; Meier, T.; Lebedev, S.; Friederich, W.; Viereck-Götte, L.

2012-04-01

Broadband waveforms recorded at stations in Europe and surrounding regions were inverted for shear-wave velocity of the European upper mantle. For events between 1995 and 2007 seismograms were collected from all permanent stations for which data are available via the data centers ORFEUS, GEOFON, ReNaSs and IRIS. In addition, we incorporated data from temporary experiments, including SVEKALAPKO, TOR, Eifel Plume, EGELADOS and other projects. Automated Multimode Inversion of surface and S-wave forms was applied to extract structural information from the seismograms, in the form of linear equations with uncorrelated uncertainties. Successful waveform fits for about 70,000 seismograms yielded over 300,000 independent linear equations that were solved together for a three-dimensional tomographic model. Resolution of the imaging is particularly high in the mantle lithosphere and asthenosphere. The highest velocities in the mantle lithosphere of the East European Craton are found at about 150 km depth. There are no indications for a large scale deep cratonic root below about 330 km depth. Lateral variations within the cratonic mantle lithosphere are resolved by our model as well. The locations of diamond bearing kimberlites correlate with reduced S-wave velocities in the cratonic mantle lithosphere. This anomaly is present in regions of both Proterozoic and Archean crust, pointing to an alteration of the mantle lithosphere after the formation of the craton. Strong lateral changes in S-wave velocity are found at the western margin of the East European Craton and hint to erosion of cratonic mantle lithosphere beneath the Scandes by hot asthenosphere. The mantle lithosphere beneath Western Europe and between the Tornquist-Teyissere Zone and the Elbe Line shows moderately high velocities and is of an intermediate character, between cratonic lithosphere and the thin lithosphere of central Europe. In central Europe, Caledonian and Variscian sutures are not associated with strong lateral changes in the lithosphere-asthenosphere system. Cenozoic anorogenic intraplate volcanism in central Europe and the Circum Mediterranean is found in regions of shallow asthenosphere and close to sharp gradients in the depth of the lithosphere-asthenosphere boundary. Low-velocity anomalies extending vertically from shallow upper mantle down to the transition zone are found beneath the Massive Central, Sinai, Canary Islands and Iceland.

Rayleigh and S wave tomography constraints on subduction termination and lithospheric foundering in central California

USGS Publications Warehouse

Jiang, Chengxin; Schmandt, Brandon; Hansen, Steven M.; Dougherty, Sara L.; Clayton, Robert W.; Farrell, Jamie; Lin, Fan-Chi

2018-01-01

The crust and upper mantle structure of central California have been modified by subduction termination, growth of the San Andreas plate boundary fault system, and small-scale upper mantle convection since the early Miocene. Here we investigate the contributions of these processes to the creation of the Isabella Anomaly, which is a high seismic velocity volume in the upper mantle. There are two types of hypotheses for its origin. One is that it is the foundered mafic lower crust and mantle lithosphere of the southern Sierra Nevada batholith. The alternative suggests that it is a fossil slab connected to the Monterey microplate. A dense broadband seismic transect was deployed from the coast to the western Sierra Nevada to fill in the least sampled areas above the Isabella Anomaly, and regional-scale Rayleigh and S wave tomography are used to evaluate the two hypotheses. New shear velocity (Vs) tomography images a high-velocity anomaly beneath coastal California that is sub-horizontal at depths of ∼40–80 km. East of the San Andreas Fault a continuous extension of the high-velocity anomaly dips east and is located beneath the Sierra Nevada at ∼150–200 km depth. The western position of the Isabella Anomaly in the uppermost mantle is inconsistent with earlier interpretations that the Isabella Anomaly is connected to actively foundering foothills lower crust. Based on the new Vs images, we interpret that the Isabella Anomaly is not the dense destabilized root of the Sierra Nevada, but rather a remnant of Miocene subduction termination that is translating north beneath the central San Andreas Fault. Our results support the occurrence of localized lithospheric foundering beneath the high elevation eastern Sierra Nevada, where we find a lower crustal low Vs layer consistent with a small amount of partial melt. The high elevations relative to crust thickness and lower crustal low Vs zone are consistent with geological inferences that lithospheric foundering drove uplift and a ∼3–4 Ma pulse of basaltic magmatism.

Rayleigh and S wave tomography constraints on subduction termination and lithospheric foundering in central California

NASA Astrophysics Data System (ADS)

Jiang, Chengxin; Schmandt, Brandon; Hansen, Steven M.; Dougherty, Sara L.; Clayton, Robert W.; Farrell, Jamie; Lin, Fan-Chi

2018-04-01

The crust and upper mantle structure of central California have been modified by subduction termination, growth of the San Andreas plate boundary fault system, and small-scale upper mantle convection since the early Miocene. Here we investigate the contributions of these processes to the creation of the Isabella Anomaly, which is a high seismic velocity volume in the upper mantle. There are two types of hypotheses for its origin. One is that it is the foundered mafic lower crust and mantle lithosphere of the southern Sierra Nevada batholith. The alternative suggests that it is a fossil slab connected to the Monterey microplate. A dense broadband seismic transect was deployed from the coast to the western Sierra Nevada to fill in the least sampled areas above the Isabella Anomaly, and regional-scale Rayleigh and S wave tomography are used to evaluate the two hypotheses. New shear velocity (Vs) tomography images a high-velocity anomaly beneath coastal California that is sub-horizontal at depths of ∼40-80 km. East of the San Andreas Fault a continuous extension of the high-velocity anomaly dips east and is located beneath the Sierra Nevada at ∼150-200 km depth. The western position of the Isabella Anomaly in the uppermost mantle is inconsistent with earlier interpretations that the Isabella Anomaly is connected to actively foundering foothills lower crust. Based on the new Vs images, we interpret that the Isabella Anomaly is not the dense destabilized root of the Sierra Nevada, but rather a remnant of Miocene subduction termination that is translating north beneath the central San Andreas Fault. Our results support the occurrence of localized lithospheric foundering beneath the high elevation eastern Sierra Nevada, where we find a lower crustal low Vs layer consistent with a small amount of partial melt. The high elevations relative to crust thickness and lower crustal low Vs zone are consistent with geological inferences that lithospheric foundering drove uplift and a ∼3-4 Ma pulse of basaltic magmatism.

3D Integrated geophysical-petrological modelling of the Iranian lithosphere

NASA Astrophysics Data System (ADS)

Mousavi, Naeim; Ardestani, Vahid E.; Ebbing, Jörg; Fullea, Javier

2016-04-01

The present-day Iranian Plateau is the result of complex tectonic processes associated with the Arabia-Eurasia Plate convergence at a lithospheric scale. In spite of previous mostly 2D geophysical studies, fundamental questions regarding the deep lithospheric and sub-lithospheric structure beneath Iran remain open. A robust 3D model of the thermochemical lithospheric structure in Iran is an important step toward a better understanding of the geological history and tectonic events in the area. Here, we apply a combined geophysical-petrological methodology (LitMod3D) to investigate the present-day thermal and compositional structure in the crust and upper mantle beneath the Arabia-Eurasia collision zone using a comprehensive variety of constraining data: elevation, surface heat flow, gravity potential fields, satellite gravity gradients, xenoliths and seismic tomography. Different mantle compositions were tested in our model based on local xenolith samples and global data base averages for different tectonothermal ages. A uniform mantle composition fails to explain the observed gravity field, gravity gradients and surface topography. A tectonically regionalized lithospheric mantle compositional model is able to explain all data sets including seismic tomography models. Our preliminary thermochemical lithospheric study constrains the depth to Moho discontinuity and intra crustal geometries including depth to sediments. We also determine the depth to Curie isotherm which is known as the base of magnetized crustal/uppermost mantle bodies. Discrepancies with respect to previous studies include mantle composition and the geometry of Moho and Lithosphere-Asthenosphere Boundary (LAB). Synthetic seismic Vs and Vp velocities match existing seismic tomography models in the area. In this study, depleted mantle compositions are modelled beneath cold and thick lithosphere in Arabian and Turan platforms. A more fertile mantle composition is found in collision zones. Based on our 3D thermochemical model we propose a new scenario to interpret the geodynamical history of area. In this context the present-day central Iran block would be as remain of the older and larger Iranian block present before the onset of Turan platform subduction beneath the Iranian Plateau. Further analysis of sub-lithospheric density anomalies (e.g., subducted slabs) is required to fully understand the geodynamics of the area.

Geochemical and Rheological Constraints on the Dynamics of the Oceanic Upper Mantle

DTIC Science & Technology

2007-09-01

405-412. 201 Dick, H. J. B., Natland, J., Party, L. . S., 1994. Melt transport and evolution in the shallow mantle beneath the East Pacific Rise ...in the shallow mantle beneath the East Pacific Rise . In: M6vel, C., Gillis, K. M., Allan, J. F., Meyer, P. S. (Eds.), Proceedings of the Ocean...according to ridge location: East Pacific Rise (EPR), Mid-Cayman Rise (MCR), Central Indian Ridge/Carlsberg Ridge (CIR/Carl), Mid-Atlantic Ridge (MAR

The depth range of azimuthal anisotropy beneath Southern California via analyses of long-period Rayleigh-waves

NASA Astrophysics Data System (ADS)

Tsang, Stephanie Doris

The motion of the mantle beneath the tectonic plates is still unknown. Mantle shears associated with flow generate anisotropy. In order to investigate the anisotropic properties within the Earth to a range of depths within the crust and upper mantle (and perhaps beyond), long-period Rayleigh waves (periods of 51:282 ≤ T ≤ 333:33 seconds) are used in this study. One model suggests that the fast axis orientation, arising from the preferential alignment of olivine crystal grains in the upper mantle, coincides with the direction of absolute plate motion of the North-American plate. Other models suggest it is aligned with the direction of relative plate motion of the Pacific and North-American plates. A third suggests that an eastward mantle flow occurs beneath the North-American plate. There is also controversy as to the depth to which anisotropy is generated. In this thesis, the Rayleigh-wave phase velocities' dependence on a seismic event's back-azimuth angle is explored within the Southern California Seismic Network (SCSN). Because surface wave velocities vary depending on the range of depth into the Earth sampled by each period, an observed deviation of the resulting phase velocity calculations for a wide range of back-azimuth angles, (6° to 349°) with respect to a reference dispersion curve of the area, provides information on the anisotropy of the subsurface structure. Further work on the fast-axis orientation and its tectonic implications is carried out here. I find that the 2 directions 270° = 90° and 290° = 110° are possible fast-axes orientations. I also find that the amplitude of azimuthal anisotropy is insufficient to explain birefringence of S-body waves, also known as SKS splitting, suggesting that it occurs much deeper than previously thought, perhaps all the way to the transition zone. Future work might involve array analysis of Love-wave components using the beamforming approach. This approach should prove effective in yielding further insight into the heterogeneity of the subsurface structure beneath Southern California.

Rayleigh Wave Phase Velocity in the Upper Mantle Beneath the Indian Ocean

NASA Astrophysics Data System (ADS)

Godfrey, K. E.; Dalton, C. A.; Ritsema, J.

2016-12-01

Most of what is currently understood about the seismic properties of oceanic upper mantle is based on either global studies or regional studies of the upper mantle beneath the Pacific Ocean. However, global seismic models and geochemical studies of mid-ocean ridge basalts indicate differences in the properties of the upper mantle beneath the Pacific, Atlantic, and Indian oceans. Though the Indian Ocean is not as well studied seismically, it is host to a number of geologically interesting features including 16,000 km of mid-ocean ridge with a range of spreading rates from 14 mm/yr along the Southwest Indian Ridge to 55-75 mm/yr along the Southeast Indian Ridge. The Indian Ocean also contains multiple volcanic hotspots, the Australian-Antarctic Discordance, and a low geoid anomaly south of India, and it overlies a portion of a large low-shear-velocity province. We are using Rayleigh waves to construct a high-resolution seismic velocity model of the Indian Ocean upper mantle. We utilize a global dataset of phase delays measured at 20 periods, between 37 and 375 seconds; the dataset includes between 700 and 20,000 that traverse our study region exclusively, with a larger number of paths at shorter periods. We explore variations in phase velocity using two separate approaches. One, we allow phase velocity to vary only as a function of seafloor age. Two, we perform a damped least-squares inversion to solve for 2-D phase velocity maps at each period. Preliminary results indicate low velocities along the Southeast Indian Ridge and Central Indian Ridge, but the expected low velocities are less apparent along the slow-spreading Southwest Indian Ridge. We observe a region of fast velocities extending from Antarctica northward between the Kerguelen and Crozet hotspots, and lower than expected velocities beneath the Reunion hotspot. Additionally, we find low velocities associated with a region of extinct seafloor spreading in the Wharton basin.

Upper-Mantel Earthquakes in the Australia-Pacific Plate Boundary Zone and the Roots of the Alpine Fault

NASA Astrophysics Data System (ADS)

Boese, C. M.; Warren-Smith, E.; Townend, J.; Stern, T. A.; Lamb, S. H.

2016-12-01

Seismicity in the upper mantle in continental collision zones is relatively rare, but observed around the world. Temporary seismometer deployments have repeatedly detected mantle earthquakes at depths of 40-100 km within the Australia-Pacific plate boundary zone beneath the South Island of New Zealand. Here, the transpressive Alpine Fault constitutes the primary plate boundary structure linking subduction zones of opposite polarity farther north and south. The Southern Alps Microearthquake Borehole Array (SAMBA) has been operating continuously since November 2008 along a 50 km-long section of the central Alpine Fault, where the rate of uplift of the Southern Alps is highest. To date it has detected more than 40 small to moderate-sized mantle events (1≤ML≤3.9). The Central Otago Seismic Array (COSA) has been in operation since late 2012 and detected 15 upper mantle events along the sub-vertical southern Alpine Fault. Various mechanisms have been proposed to explain the occurrence of upper mantle seismicity in the South Island, including intra-continental subduction (Reyners 1987, Geology); high shear-strain gradients due to depressed geotherms and viscous deformation of mantle lithosphere (Kohler and Eberhart-Phillips 2003, BSSA); high strain rates resulting from plate bending (Boese et al. 2013, EPSL), and underthrusting of the Australian plate (Lamb et al. 2015, G3). Focal mechanism analysis reveals a variety of mechanisms for the upper mantle events but predominantly strike-slip and reverse faulting. In this study, we apply spectral analysis to better constrain source parameters for these mantle events. These results are interpreted in conjunction with new information about crustal structure and low-frequency earthquakes near the Moho and in light of existing velocity, attenuation and resistivity models.

Mantle wedge anisotropy beneath the Western Alps: insights from Receiver Function analysis

NASA Astrophysics Data System (ADS)

Piana Agostinetti, Nicola; Salimbeni, Simone; Pondrelli, Silvia; Malusa', Marco; Zhao, Liang; Eva, Elena; Solarino, Stefano; Paul, Anne; Guillot, Stéphane; Schwartz, Stéphane; Dumont, Thierry; Aubert, Coralie; Wang, Qingchen; Zhu, Rixiang

2017-04-01

Orogens and subductions zones are the locus where crustal materials are recycled into the upper mantle. Such rocks undergo to several metamorphic reactions during which their seismic properties vary due to the changes in P-T conditions. Metamorphic reactions can imply: (a) the formation of schist-like materials, and (b) a pronounced water flux from the subducted crust. Both these processes should generate highly anisotropic volumes at upper mantle depths. Thus, unveiling the presence of seismic anisotropy at such depth level can put constraints on the metamorphic reactions and the P-T conditions of the subducted materials. The Alpine orogen is composed of three main regions where different geodynamic processes shaped a highly heterogeneous mountain chain. Beneath the Alps, a high velocity body has been imaged sinking in the upper mantle, indicating the presence of a relict of subduction. Such subduction process has been probably terminated with the closure of the Piemont-Liguria Ocean, but evidence of continental subduction has been found beneath the Western Alps. Seismic anisotropy is likely to develop both in the subducted materials and in the mantle wedge, where serpentinized materials could be found due to the low T conditions. We analysed P receiver function (RF) from 46 seismic stations deployed along a linear array crossing the Western Alps, where previous studies revealed the presence of the subducted European lower crust to 80 km depth. RF is a widely used tool for reconstructing subsurface seismic structures, based on the recognition of P-to-S converted phases in teleseismic P-wave coda. The RF data-set is migrated at depth and decomposed into azimuthal harmonics. Computing the first, k=0, and the second, k=1, harmonics allows to separate the "isotropic" contribution, due to the change of the isotropic properties of the sampled materials (recorded on the k=0 harmonics), from the "anisotropic" contribution, where the energy is related to the propagation of the P-wave through anisotropic materials (recorded on the k=1 harmonics). Preliminary results show the presence of a Ps phase on the k=0 harmonics along the western portion of the profile, with increasing time-delay toward East. This phase is interpreted as the European Moho Ps, confirming the geometry of the European Moho beneath the Western Alps. Beneath the internal portion of the orogen, the k=1 harmonics display energetic pulses between 3-7 s, indicating the development of anisotropy within a broad volume of rocks, at lower crustal and upper mantle depths. The presence of anisotropic materials is jointly interpreted with the depicted geometry of the main seismic discontinuities and the location of the intermediate-depth seismicity recorded in the region.

Upper Mantle Dynamics of Bangladesh by Splitting Analyzes of Core Refracted SKS and SKKS Waves

NASA Astrophysics Data System (ADS)

Tiwari, A. K.; Bhushan, K.; Eken, T.; Singh, A.

2017-12-01

New shear wave splitting measurements are obtained from hitherto less studied Bengal Basin using core refracted SKS and SKKS phases. Splitting parameters, time delays (δt) and fast polarization directions (Φ) were estimated through analysis of 64 high-quality waveforms (≥ 2.5 signal to noise ratio) from 29 earthquakes with magnitude ≥5.5 recorded at eight seismic stations deployed over Bangladesh. We found no evidence of splitting which indicates azimuthal isotropy beneath the region. Null measurements can be explained by near vertical axis of anisotropy or by the presence of multiple anisotropic layers with different fast polarization directions, where combined effect results in null. We consider that the presence of partial melts within the upper mantle due to Kerguelen mantle plume activities may be the potential geodynamic cause for observed null measurements. It locally perturbed mantle convection flow beneath the region and reoriented the lattice preferred orientation of the upper mantle mineral mainly olivine as this disabled the core refracted SKS and SKKS phases to scan the anisotropic characteristics of the region, and hence null measurements are obtained.

Sub-Moho Reflectors, Mantle Faults and Lithospheric Rheology

NASA Astrophysics Data System (ADS)

Brown, L. D.

2013-12-01

One of the most unexpected and dramatic observations from the early years of deep reflection profiling of the continents using multichannel CMP techniques was the existing of prominent reflections from the upper mantle. The first of these, the Flannan thrust/fault/feature, was traced by marine profiling of the continental margin offshore Britain by the BIRPS program, which soon found them to be but one of several clear sub-crustal discontinuities in that area. Subsequently, similar mantle reflectors have been observed in many areas around the world, most commonly beneath Precambrian cratonic areas. Many, but not all, of these mantle reflections appear to arise from near the overlying Moho or within the lower crust before dipping well into the mantle. Others occur as subhorizontal events at various depths with the mantle, with one suite seeming to cluster at a depth of about 75 km. The dipping events have been variously interpreted as mantle roots of crustal normal faults or the deep extension of crustal thrust faults. The most common interpretation, however, is that these dipping events are the relicts of ancient subduction zones, the stumps of now detached Benioff zones long since reclaimed by the deeper mantle. In addition to the BIRPS reflectors, the best known examples include those beneath Fennoscandia in northern Europe, the Abitibi-Grenville of eastern Canada, and the Slave Province of northwestern Canada (e.g. on the SNORCLE profile). The most recently reported example is from beneath the Sichuan Basin of central China. The preservation of these coherent, and relatively delicate appearing, features beneath older continental crust and presumably within equally old (of not older) mantle lithosphere, has profound implications for the history and rheology of the lithosphere in these areas. If they represent, as widely believe, some form of faulting with the lithosphere, they provide corollary constraints on the nature of faulting in both the lower crust and upper mantle. The SNORCLE mantle reflectors, which can be traced deep within the early Precambrian (?) mantle by both surface (controlled source) reflection profiles and passive (receiver function) images most clearly illustrates the rheological implications of such feature. The SNORCLE events appear to root upwards into the lower crust and extend to depths approaching 200 km into the mantle. This would seem to require the preservation of undeformed mantle lithosphere for almost 2.5 billion years in this area. This preservation is clearly inconsistent with the interpretation of nearby shallower mantle interfaces as marking the modern lithosphere-asthenosphere boundary. In summary, dipping mantle reflections imply preservation of substantial thicknesses of mantle lithosphere for very long periods of time, and localization of mantle deformation during the formation of these structures along relatively narrow, discrete interfaces rather than across broad zones of diffuse deformation. .

Upper mantle seismic structure beneath southwest Africa from finite-frequency P- and S-wave tomography

NASA Astrophysics Data System (ADS)

Youssof, Mohammad; Yuan, Xiaohui; Tilmann, Frederik; Heit, Benjamin; Weber, Michael; Jokat, Wilfried; Geissler, Wolfram; Laske, Gabi; Eken, Tuna; Lushetile, Bufelo

2015-04-01

We present a 3D high-resolution seismic model of the southwestern Africa region from teleseismic tomographic inversion of the P- and S- wave data recorded by the amphibious WALPASS network. We used 40 temporary stations in southwestern Africa with records for a period of 2 years (the OBS operated for 1 year), between November 2010 and November 2012. The array covers a surface area of approximately 600 by 1200 km and is located at the intersection of the Walvis Ridge, the continental margin of northern Namibia, and extends into the Congo craton. Major questions that need to be understood are related to the impact of asthenosphere-lithosphere interaction, (plume-related features), on the continental areas and the evolution of the continent-ocean transition that followed the break-up of Gondwana. This process is supposed to leave its imprint as distinct seismic signature in the upper mantle. Utilizing 3D sensitivity kernels, we invert traveltime residuals to image velocity perturbations in the upper mantle down to 1000 km depth. To test the robustness of our tomographic image we employed various resolution tests which allow us to evaluate the extent of smearing effects and help defining the optimum inversion parameters (i.e., damping and smoothness) used during the regularization of inversion process. Resolution assessment procedure includes also a detailed investigation of the effect of the crustal corrections on the final images, which strongly influenced the resolution for the mantle structures. We present detailed tomographic images of the oceanic and continental lithosphere beneath the study area. The fast lithospheric keel of the Congo Craton reaches a depth of ~250 km. Relatively low velocity perturbations have been imaged within the orogenic Damara Belt down to a depth of ~150 km, probably related to surficial suture zones and the presence of fertile material. A shallower depth extent of the lithospheric plate of ~100 km was observed beneath the ocean, consistent with plate-cooling models. In addition to tomographic images, the seismic anisotropy measurements within the upper mantle inferred from teleseismic shear waves indicate a predominant NE-SW orientation for most of the land stations. Current results indicate no evidence for a consistent signature of fossil plume.

Seismic imaging of the upper mantle beneath the northern Central Andean Plateau: Implications for surface topography

NASA Astrophysics Data System (ADS)

Ward, K. M.; Zandt, G.; Beck, S. L.; Wagner, L. S.

2015-12-01

Extending over 1,800 km along the active South American Cordilleran margin, the Central Andean Plateau (CAP) as defined by the 3 km elevation contour is second only to the Tibetan Plateau in geographic extent. The uplift history of the 4 km high Plateau remains uncertain with paleoelevation studies along the CAP suggesting a complex, non-uniform uplift history. As part of the Central Andean Uplift and the Geodynamics of High Topography (CAUGHT) project, we use surface waves measured from ambient noise and two-plane wave tomography to image the S-wave velocity structure of the crust and upper mantle to investigate the upper mantle component of plateau uplift. We observe three main features in our S-wave velocity model including (1), a high velocity slab (2), a low velocity anomaly above the slab where the slab changes dip from near horizontal to a normal dip, and (3), a high-velocity feature in the mantle above the slab that extends along the length of the Altiplano from the base of the Moho to a depth of ~120 km with the highest velocities observed under Lake Titicaca. A strong spatial correlation exists between the lateral extent of this high-velocity feature beneath the Altiplano and the lower elevations of the Altiplano basin suggesting a potential relationship. Non-uniqueness in our seismic models preclude uniquely constraining this feature as an uppermost mantle feature bellow the Moho or as a connected eastward dipping feature extending up to 300 km in the mantle as seen in deeper mantle tomography studies. Determining if the high velocity feature represents a small lithospheric root or a delaminating lithospheric root extending ~300 km into the mantle requires more integration of observations, but either interpretation shows a strong geodynamic connection with the uppermost mantle and the current topography of the northern CAP.

Cenozoic extension, volcanism and plateau uplift in eastern Africa and the African Superplume

NASA Astrophysics Data System (ADS)

Nyblade, A.; O'Donnell, J.; Mulibo, G. D.; Adams, A. N.

2013-12-01

Recent body and surface wave studies combine to image mantle velocity structure to a depth of 1200 km beneath eastern Africa using teleseismic earthquake data recorded by the AfricaArray East African Seismic Experiment in conjunction with permanent stations and previously deployed temporary stations. The combined network spans Kenya, Uganda, Tanzania, Zambia and Malawi. The 3-D shear wave velocity structure of the uppermost mantle was imaged using fundamental-mode Rayleigh wave phase velocities measured at periods ranging from 20 to 182 s, subsequently inverted for shear velocity structure. When considered in conjunction with mapped seismicity, the shear velocity model supports a secondary western rift branch striking southwestwards from Lake Tanganyika, likely exploiting the relatively weak lithosphere of the southern Kibaran Belt between the Bangweulu Block and the Congo Craton. In eastern Tanzania a low-velocity region suggests that the eastern rift branch trends southeastwards offshore eastern Tanzania coincident with the purported location of the northern margin of the proposed Ruvuma microplate. The results suggest that existing lithospheric structures exert a significant governing influence on rift development. Sub-lithospheric mantle wave speed variations extending to a depth of 1200 km were tomographically imaged from the inversion of P and S wave relative arrival time residuals. The images shows a low wave speed anomaly (LWA) well developed at shallow depths (100-200 km) beneath the Eastern and Western branches of the rift system and northwestern Zambia, and a fast wave speed anomaly at depths greater than 350 km beneath the central and northern parts of the East African Plateau and the eastern and central parts of Zambia. At depths below 350 km the LWA is most prominent under the central and southern parts of the East African Plateau and dips to the southwest beneath northern Zambia, extending to a depth of at least 900 km. The amplitude of the LWA is consistent with a 150-300 K thermal perturbation, and its depth extent indicates that the African superplume, originally identified as a lower mantle anomaly, is likely a whole mantle structure. A mantle transition zone about 30-40 km thinner than the global average in a region 200-400 km wide extending in a SW-NE direction from central Zambia, across Tanzania and into Kenya was inferred from P to S conversions from the 410 and 660 km discontinuities observed in receiver function stacks. The thinning of the transition zone indicates a 190-300 K thermal anomaly in the same location where the P and S wave tomography models suggest that the lower mantle African superplume structure connects to thermally perturbed upper mantle beneath eastern Africa. These findings provide compelling evidence for the existence of a continuous thermal structure extending from the core-mantle boundary to the surface associated with the African superplume, implying an origin for the Cenozoic extension, volcanism and plateau uplift in eastern Africa rooted in the dynamics of the lower mantle.

Upper mantle structure across the Trans-European Suture Zone imaged by S-receiver functions

NASA Astrophysics Data System (ADS)

Knapmeyer-Endrun, Brigitte; Krüger, Frank; Geissler, Wolfram H.; Passeq Working Group

2017-01-01

We present a high-resolution study of the upper mantle structure of Central Europe, including the western part of the East European Platform, based on S-receiver functions of 345 stations. A distinct contrast is found between Phanerozoic Europe and the East European Craton across the Trans-European Suture Zone. To the west, a pronounced velocity reduction with depth interpreted as lithosphere-asthenosphere boundary (LAB) is found at an average depth of 90 km. Beneath the craton, no strong and continuous LAB conversion is observed. Instead we find a distinct velocity reduction within the lithosphere, at 80-120 km depth. This mid-lithospheric discontinuity (MLD) is attributed to a compositional boundary between depleted and more fertile lithosphere created by late Proterozoic metasomatism. A potential LAB phase beneath the craton is very weak and varies in depth between 180 and 250 km, consistent with a reduced velocity contrast between the lower lithosphere and the asthenosphere. Within the Trans-European Suture Zone, lithospheric structure is characterized by strong heterogeneity. A dipping or step-wise increase to LAB depth of 150 km is imaged from Phanerozoic Europe to 20-22° E, whereas no direct connection to the cratonic LAB or MLD to the east is apparent. At larger depths, a positive conversion associated with the lower boundary of the asthenosphere is imaged at 210-250 km depth beneath Phanerozoic Europe, continuing down to 300 km depth beneath the craton. Conversions from both 410 km and 660 km discontinuities are found at their nominal depth beneath Phanerozoic Europe, and the discontinuity at 410 km depth can also be traced into the craton. A potential negative conversion on top of the 410 km discontinuity found in migrated images is analyzed by modeling and attributed to interference with other converted phases.

Upper Mantle Anisotropy Under Fast Spreading Mid-ocean Ridges: 2-D Whole Mantle Convection Model With Subduction

NASA Astrophysics Data System (ADS)

Lee, C.; Zhou, Y.; King, S. D.

2008-12-01

Analyses of seismic anisotropy caused by spatial alignments of anisotropic minerals (e.g., olivine) have been widely used to infer mantle flow directions in the upper mantle. Deep seismic anisotropy beneath fast spreading mid-ocean ridges (e.g., East Pacific Rise) has been recently observed at depths of 200-300 km and even down to the transition zone, with polarization changes in radial anisotropy from VSH < VSV (shallow) to VSH < VSV (deep). We investigate the origin of the observed deep seismic anisotropy and polarization changes beneath the EPR in 2-D Cartesian numerical models using both kinematically (prescribed velocity) and dynamically (negative buoyancy) driven ridge spreading. Because subduction is thought to be an important controlling factor in the style of ridge spreading and mantle convection, we consider a subduction zone developing at the prescribed weak zone. A whole mantle domain expressed by a one by four box (2890 by 11560 km) is used to minimize the boundary effects on the subducting slab. For the upper mantle rheology, we consider composite viscosity of diffusion and dislocation creep for dry olivine to evaluate the effects of lateral variation of mantle viscosity and the rheological changes from dislocation to diffusion creep under the mid-ocean ridge. For the lower mantle rheology, we use diffusion creep for dry olivine by increasing grain size to match relevant lower mantle viscosity. We also consider the 660 km phase transition with density and viscosity jump as well as Clapeyron slope. Anisotropy is evaluated using finite-strain ellipses based on the assumption that a-axes of olivine crystals are parallel to the major axes of the finite-strain ellipses. Our preliminary results show 1) in general, the development of VSH < VSV anisotropy is confined only in a narrow region under the ridge axis at depths of 200- 300 km; 2) strong VSH > VSV anisotropy can be found in the 'asthenosphere' beneath the entire spreading oceanic lithosphere; and 3) the dominate creep mechanism changes from dislocation creep to diffusion creep at depths of 300-400 km; indicating a more isotropic lower upper mantle. We conclude that our geodynamical modeling in a passive ridge spreading system does not produce the deep seismic anisotropy recently observed beneath the EPR. However, we do not consider partial melting, dynamic recrystallization and anisotropic viscosity which would change seismic interpretation and mantle flow, and thus further study is required.

Lithospheric Structure Beneath the Hangay Dome, Central Mongolia

NASA Astrophysics Data System (ADS)

Stachnik, J. C.; Meltzer, A.; Souza, S.; Munkhuu, U.; Tsaagan, B.; Russo, R. M.

2014-12-01

The Mongolian Plateau is a broad regional uplift positioned between the Siberian Craton to the north and the far northern edge of the India-Asia collision to the south. Within this intracontinental setting of high topography, the Hangay Dome in central Mongolia reaches elevations of 4 km and contains intermittent basaltic magmatism over the last 30 Ma. The relationship between high topography, magmatism, and geodynamic processes remains largely unsolved although processes ranging from lithospheric delamination to mantle plume effects have been proposed. A temporary array of seismic stations was deployed around the Hangay Dome to determine lithospheric structure. Preliminary results are shown from receiver function analysis, ambient noise tomography, and teleseismic P-wave tomography. Crustal thickness measurements from H-k stacking of receiver functions range from 42 km to 57 km across the array, with thicker crust beneath the highest topography. The bulk crustal Vp/Vs ratio ranges from 1.71 to 1.9 with a median value for the array of 1.77, perhaps indicating a variable crustal composition with some regions having a more mafic crust. The stacked receiver functions are also combined with ambient noise phase velocity dispersion measurements in a joint inversion for shear velocity profiles at each station which reveals crustal thickness estimates consistent with the H-k stacks while also determining the shear velocity step at the Moho. Teleseismic P-wave travel time residuals ranging between +/-1 second are inverted for a 3D P-wave velocity model using finite-frequency kernels. Notable features include 1) a low velocity anomaly (-3%) in the upper 200 km beneath the eastern part of the Hangay Dome near the Orkhon River Valley, , 2) a steeply dipping low velocity anomaly to the north of the Hangay Dome, perhaps related to the nearby Baikal Rift, and 3) generally higher velocities in the upper 200 km surrounding the high topography. To first order, the high topography of the Hangay Dome appears to be largely supported by thickened crust. However, lower P-wave velocities in the upper mantle beneath the dome are observed. The relative contributions of crustal thickness and upper mantle structure for support of topography and their relationship to magmatism will be determined with further refinement of the models.

Upper mantle and transition zone structure beneath Ethiopia: Regional evidence for the African Superplume

NASA Astrophysics Data System (ADS)

Benoit, M. H.; Nyblade, A. A.; Pasyanos, M.; Owens, T. J.

2005-12-01

Throughout much of the Cenozoic, Ethiopia has undergone extensive tectonism, including rifting, volcanism and uplift, and the origin of this tectonism remains enigmatic. While the cause of the tectonism has often been attributed to one or more mantle plumes, recent global tomographic studies suggest that the African Superplume, a broad, through-going mantle upwelling, may be related to the tectonism. To further understand the origin of the tectonism in Ethiopia, we employ a variety of methods, including an S wave travel time body wave tomography, receiver function analysis of the 410 and 660 km discontinuities, and surface wave tomography. Using data from the Ethiopia Broadband Seismic Experiment [2000-2002], we computed new S wave models of the upper mantle seismic velocity structure from 150 - 400 km depth. The S wave model revealed an elongated low wave speed region that is deep (> 300 km) and wide (> 500 km). The location of the low wave speed anomaly aligns with the Afar Depression and Main Ethiopian Rift in the uppermost mantle, but the center of the anomaly shifts to the west with depth. Results from receiver function stacking of the 410 and 660 km discontinuities show a shallow 660 beneath most of Ethiopia, implying that the low wave speed anomaly found in the S wave model likely extends to at least 660 km depth. This result suggests that the low velocity anomaly may be related to the African Superplume. A group velocity surface wave tomographic study of East Africa was also computed using data from permanent and temporary stations from Africa and Arabia. Results of this study reveal low Sn velocities beneath much of the region, and suggest that low elevations found in the region between the Ethiopian and East African Plateaus likely reflect an isostatic response to crustal thinning. If the crust in this region had not been thinned by approximately 10 - 15 km, then it is likely that the high elevation of the Ethiopian and East African Plateaus would be continuous and that these plateaus would not be viewed as separate, distinct regions of uplift. This finding further suggests that a large scale, buoyant feature, such as the African Superplume, exists in the mantle beneath the Ethiopia and East African Plateaus that contributes to the uplift of the region.

Evidence for an upper mantle low velocity zone beneath the southern Basin and Range-Colorado Plateau transition zone

USGS Publications Warehouse

Benz, H.M.; McCarthy, J.

1994-01-01

A 370-km-long seismic refraction/wide-angle reflection profile recorded during the Pacific to Arizona Crustal Experiment (PACE) detected an upper mantle P-wave low-velocity zone (LVZ) in the depth range 40 to 55 km beneath the Basin and Range in southern Arizona. Interpretation of seismic data places constraints on the sub-crustal lithosphere of the southern Basin and Range Province, which is important in light of the active tectonics of the region and the unknown role of the sub-crustal lithosphere in the development of the western United States. Forward travel time and synthetic seismogram techniques are used to model this shallow upper mantle LVZ. Modeling results show that the LVZ is defined by a 5% velocity decrease relative to a Pn velocity of 7.95 km s−1, suggesting either a ∼3–5% mafic partial melt or high-temperature, sub-solidus peridotite.

Crust and Mantle Deformation Revealed from High-Resolution Radially Anisotropic Velocity Models

NASA Astrophysics Data System (ADS)

Li, A.; Dave, R.; Yao, Y.

2017-12-01

Love wave tomography, which can achieve a similar model resolution as Rayleigh wave, so far has limited applications to the USArray data. Recently, we have developed high-resolution Love wave phase velocity maps in the Wyoming craton and Texas using data at the Transportable Array stations. 3-D, radially anisotropic velocity models are obtained by jointly inverting Love and Rayleigh wave phase velocities. A high-velocity anomaly extending to about 200 km depth beneath central Wyoming correlates with negative radial anisotropy (Vsv>Vsh), suggesting that mantle downwelling develops under the cratonic lithosphere. Surprisingly, the significantly low velocity beneath the Yellowstone hotspot, which has been interpreted as partial melting and asthenospheric upwelling, is associated with the largest radial anisotropy (Vsh>Vsv) in the area. This observation does not support mantle upwelling. Instead, it indicates that the upper mantle beneath the hotspot has experienced strong shear deformation probably by the plate motion and large-scale mantle flow. In Texas, positive radial anisotropy in the lower crust extends from the coast to the Ouachita belt, which is characterized by high velocity and negative radial anisotropy. In the upper mantle, large variations of velocity and anisotropy exit under the coastal plain. A common feature in these anisotropic models is that high-velocity anomalies in the upper mantle often correlate with negative anisotropy (Vsv>Vsh) while low-velocity anomalies are associated with positive anisotropy (Vsh>Vsv). The manifestation of mantle downweling as negative radial anisotropy is largely due to the relatively high viscosity of the high-velocity mantle block, which is less affected by the surrounding large-scale horizontal flow. However, mantle upwelling, which is often associated with low-velocity anomalies, presumably low-viscosity mantle blocks, is invisible in radial anisotropy models. Such upwelling may happen too quickly to make last effects or too slow to alter the dominant shear deformation in the asthenosphere.

Seismic Velocity Structure and Depth-Dependence of Anisotropy in the Red Sea and Arabian Shield from Surface Wave Analysis

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hansen, S; Gaherty, J; Schwartz, S

2007-07-25

We investigate the lithospheric and upper mantle structure as well as the depth-dependence of anisotropy along the Red Sea and beneath the Arabian Peninsula using receiver function constraints and phase velocities of surface waves traversing two transects of stations from the Saudi Arabian National Digital Seismic Network. Frequency-dependent phase delays of fundamental-mode Love and Rayleigh waves, measured using a cross-correlation procedure, require very slow shear velocities and the presence of anisotropy throughout the upper mantle. Linearized inversion of these data produce path-averaged 1D radially anisotropic models with about 4% anisotropy in the lithosphere, increasing to about 4.8% anisotropy across themore » lithosphere-asthenosphere boundary (LAB). Models with reasonable crustal velocities in which the mantle lithosphere is isotropic cannot satisfy the data. The lithospheric lid, which ranges in thickness from about 70 km near the Red Sea coast to about 90 km beneath the Arabian Shield, is underlain by a pronounced low-velocity zone with shear velocities as low as 4.1 km/s. Forward models, which are constructed from previously determined shear-wave splitting estimates, can reconcile surface and body wave observations of anisotropy. The low shear velocity values are similar to many other continental rift and oceanic ridge environments. These low velocities combined with the sharp velocity contrast across the LAB may indicate the presence of partial melt beneath Arabia. The anisotropic signature primarily reflects a combination of plate- and density-driven flow associated with active rifting processes in the Red Sea.« less

Piecewise delamination of Moroccan lithosphere from beneath the Atlas Mountains

NASA Astrophysics Data System (ADS)

Bezada, M. J.; Humphreys, E. D.; Davila, J. M.; Carbonell, R.; Harnafi, M.; Palomeras, I.; Levander, A.

2014-04-01

The elevation of the intracontinental Atlas Mountains of Morocco and surrounding regions requires a mantle component of buoyancy, and there is consensus that this buoyancy results from an abnormally thin lithosphere. Lithospheric delamination under the Atlas Mountains and thermal erosion caused by upwelling mantle have each been suggested as thinning mechanisms. We use seismic tomography to image the upper mantle of Morocco. Our imaging resolves the location and shape of lithospheric cavities and of delaminated lithosphere ˜400 km beneath the Middle Atlas. We propose discontinuous delamination of an intrinsically unstable Atlas lithosphere, enabled by the presence of anomalously hot mantle, as a mechanism for producing the imaged structures. The Atlas lithosphere was made unstable by a combination of tectonic shortening and eclogite loading during Mesozoic rifting and Cenozoic magmatism. The presence of hot mantle sourced from regional upwellings in northern Africa or the Canary Islands enhanced the instability of this lithosphere. Flow around the retreating Alboran slab focused upwelling mantle under the Middle Atlas, which we infer to be the site of the most recent delamination. The Atlas Mountains of Morocco stand as an example of large-scale lithospheric loss in a mildly contractional orogen.

The mantle flow field beneath western North America.

PubMed

Silver, P G; Holt, W E

2002-02-08

Although motions at the surface of tectonic plates are well determined, the accompanying horizontal mantle flow is not. We have combined observations of surface deformation and upper mantle seismic anisotropy to estimate this flow field for western North America. We find that the mantle velocity is 5.5 +/- 1.5 centimeters per year due east in a hot spot reference frame, nearly opposite to the direction of North American plate motion (west-southwest). The flow is only weakly coupled to the motion of the surface plate, producing a small drag force. This flow field is probably due to heterogeneity in mantle density associated with the former Farallon oceanic plate beneath North America.

Experimental constraints on the damp peridotite solidus and oceanic mantle potential temperature

NASA Astrophysics Data System (ADS)

Sarafian, Emily; Gaetani, Glenn A.; Hauri, Erik H.; Sarafian, Adam R.

2017-03-01

Decompression of hot mantle rock upwelling beneath oceanic spreading centers causes it to exceed the melting point (solidus), producing magmas that ascend to form basaltic crust ~6 to 7 kilometers thick. The oceanic upper mantle contains ~50 to 200 micrograms per gram of water (H2O) dissolved in nominally anhydrous minerals, which—relative to its low concentration—has a disproportionate effect on the solidus that has not been quantified experimentally. Here, we present results from an experimental determination of the peridotite solidus containing known amounts of dissolved hydrogen. Our data reveal that the H2O-undersaturated peridotite solidus is hotter than previously thought. Reconciling geophysical observations of the melting regime beneath the East Pacific Rise with our experimental results requires that existing estimates for the oceanic upper mantle potential temperature be adjusted upward by about 60°C.

Seismic structure of the European upper mantle based on adjoint tomography

NASA Astrophysics Data System (ADS)

Zhu, Hejun; Bozdağ, Ebru; Tromp, Jeroen

2015-04-01

We use adjoint tomography to iteratively determine seismic models of the crust and upper mantle beneath the European continent and the North Atlantic Ocean. Three-component seismograms from 190 earthquakes recorded by 745 seismographic stations are employed in the inversion. Crustal model EPcrust combined with mantle model S362ANI comprise the 3-D starting model, EU00. Before the structural inversion, earthquake source parameters, for example, centroid moment tensors and locations, are reinverted based on global 3-D Green's functions and Fréchet derivatives. This study consists of three stages. In stage one, frequency-dependent phase differences between observed and simulated seismograms are used to constrain radially anisotropic wave speed variations. In stage two, frequency-dependent phase and amplitude measurements are combined to simultaneously constrain elastic wave speeds and anelastic attenuation. In these two stages, long-period surface waves and short-period body waves are combined to simultaneously constrain shallow and deep structures. In stage three, frequency-dependent phase and amplitude anomalies of three-component surface waves are used to simultaneously constrain radial and azimuthal anisotropy. After this three-stage inversion, we obtain a new seismic model of the European curst and upper mantle, named EU60. Improvements in misfits and histograms in both phase and amplitude help us to validate this three-stage inversion strategy. Long-wavelength elastic wave speed variations in model EU60 compare favourably with previous body- and surface wave tomographic models. Some hitherto unidentified features, such as the Adria microplate, naturally emerge from the smooth starting model. Subducting slabs, slab detachments, ancient suture zones, continental rifts and backarc basins are well resolved in model EU60. We find an anticorrelation between shear wave speed and anelastic attenuation at depths < 100 km. At greater depths, this anticorrelation becomes relatively weak, in agreement with previous global attenuation studies. Furthermore, enhanced attenuation is observed within the mantle transition zone beneath the North Atlantic Ocean. Consistent with typical radial anisotropy in 1-D reference models, the European continent is dominated by features with a radially anisotropic parameter ξ > 1, indicating predominantly horizontal flow within the upper mantle. In addition, subduction zones, such as the Apennines and Hellenic arcs, are characterized by vertical flow with ξ < 1 at depths greater than 150 km. We find that the direction of the fast anisotropic axis is closely tied to the tectonic evolution of the region. Averaged radial peak-to-peak anisotropic strength profiles identify distinct brittle-ductile deformation in lithospheric strength beneath oceans and continents. Finally, we use the `point-spread function' to assess image quality and analyse trade-offs between different model parameters.

Mountain Building in Central and Western Tien Shan Orogen: Insight from Joint Inversion of Surface Wave Phase Velocities and Body Wave Travel Times

NASA Astrophysics Data System (ADS)

Wu, S.; Yang, Y.; Wang, K.

2017-12-01

The Tien Shan orogeny, situated in central Asia about 2000 km away from the collision boundary between Indian plate and Eurasian plate, is one of the highest, youngest, and most active intracontinental mountain belts on the earth. It first formed during the Paleozoic times and became reactivated at about 20Ma. Although many studies on the dynamic processes of the Tien Shan orogeny have been carried out before, its tectonic rejuvenation and uplift mechanism are still being debated. A high-resolution model of crust and mantle beneath Tien Shan is critical to discern among competing models for the mountain building. In this study, we collect and process seismic data recorded by several seismic arrays in the central and western Tien Shan region to generate surface wave dispersion curves at 6-140 s period using ambient noise tomography (ANT) and two-plane surface wave tomography (TPWT) methods. Using these dispersion curves, we construct a high-resolution 3-D image of shear wave velocity (Vs) in the crust and upper mantle up to 300 km depth. Our current model constrained only by surface waves shows that, under the Tien Shan orogenic belt, a strong low S-wave velocity anomaly exists in the uppermost mantle down to the depth of 200km, supporting the model that the hot upper mantle is upwelling under the Tien Shan orogenic belt, which may be responsible for the mountain building. To the west of central Tien Shan across the Talas-Fergana fault, low S-wave velocity anomalies in the upper mantle become much weaker and finally disappear beneath the Fergana basin. Because surface waves are insensitive to the structures below 300 km, body wave arrival times will be included for a joint inversion with surface waves to generate S-wave velocity structure from the surface down to the mantle transition zone. The joint inversion of both body and surface waves provide complementary constraints on structures at different depths and helps to achieve a more realistic model compared with body wave or surface wave tomography alone. The joint inversion model will be presented.

Seismically imaging the Afar plume

NASA Astrophysics Data System (ADS)

Hammond, J. O.; Kendall, J. M.; Bastow, I. D.; Stuart, G. W.; Keir, D.; Ayele, A.; Ogubazghi, G.; Ebinger, C. J.; Belachew, M.

2011-12-01

Plume related flood basalt volcanism in Ethiopia has long been cited to have instigated continental breakup in northeast Africa. However, to date seismic images of the mantle beneath the region have not produced conclusive evidence of a plume-like structure. As a result the nature and even existence of a plume in the region and its role in rift initiation and continental rupture are debated. Previous seismic studies using regional deployments of sensors in East-Africa show that low seismic velocities underlie northeast Africa, but their resolution is limited to the top 200-300km of the Earth. Thus, the connection between the low velocities in the uppermost mantle and those imaged in global studies in the lower mantle is unclear. We have combined new data from Afar, Ethiopia with 6 other regional experiments and global network stations across Ethiopia, Eritrea, Djibouti and Yemen, to produce high-resolution models of upper mantle P- and S- wave velocities to the base of the transition zone. Relative travel time tomographic inversions show that the top 100km is dominated by focussed low velocity zones, likely associated with melt in the lithosphere/uppermost asthenosphere. Below these depths a broad SW-NE oriented sheet like upwelling extends down to the top of the transition zone. Within the transition zone two focussed sharp-sided low velocity regions exist: one beneath the Western Ethiopian plateau outside the rift valley, and the other beneath the Afar depression. The nature of the transition zone anomalies suggests that small upwellings may rise from a broader low velocity plume-like feature in the lower mantle. This interpretation is supported by numerical and analogue experiments that suggest the 660km phase change and viscosity jump may impede flow from the lower to upper mantle creating a thermal boundary layer at the base of the transition zone. This allows smaller, secondary upwellings to initiate and rise to the surface. Our images of secondary upwellings suggest that there is no evidence for a plume in the classical sense (i.e. a narrow conduit). Instead, we propose that secondary upwellings rise from the base of the transition zone and connect in the upper mantle. This coupled with measurements of seismic anisotropy suggest that mantle material flows northeast towards Arabia, and may be responsible for the dramatic dynamic topography observed in northeast Africa and western Arabia.

Upper mantle beneath foothills of the western Himalaya: subducted lithospheric slab or a keel of the Indian shield?

NASA Astrophysics Data System (ADS)

Vinnik, L.; Singh, A.; Kiselev, S.; Kumar, M. Ravi

2007-12-01

The fate of the mantle lithosphere of the Indian Plate in the India-Eurasia collision zone is not well understood. Tomographic studies reveal high P velocity in the uppermost mantle to the south of the western Himalaya, and these high velocities are sometimes interpreted as an image of subducting Indian lithosphere. We suggest that these high velocities are unrelated to the ongoing subduction but correspond to a near-horizontal mantle keel of the Indian shield. In the south of the Indian shield upper-mantle velocities are anomalously low, and relatively high velocities may signify a recovery of the normal shield structure in the north. Our analysis is based on the recordings of seismograph station NIL in the foothills of the western Himalaya. The T component of the P receiver functions is weak relative to the Q component, which is indicative of a subhorizontally layered structure. Joint inversion of the P and S receiver functions favours high uppermost mantle velocities, typical of the lithosphere of Archean cratons. The arrival of the Ps converted phase from 410 km discontinuity at NIL is 2.2 s earlier than in IASP91 global model. This can be an effect of remnants of Tethys subduction in the mantle transition zone and of high velocities in the keel of the Indian shield. Joint inversion of SKS particle motions and P receiver functions reveals a change in the fast direction of seismic azimuthal anisotropy from 60° at 80-160 km depths to 150° at 160-220 km. The fast direction in the lower layer is parallel to the trend of the Himalaya. The change of deformation regimes at a depth of 160 km suggests that this is the base of the lithosphere of the Indian shield. A similar boundary was found with similar techniques in central Europe and the Tien Shan region, but the base of the lithosphere in these regions is relatively shallow, in agreement with the higher upper-mantle temperatures. The ongoing continental collision is expressed in crustal structure: the crust beneath NIL is very thick (58 +/- 2 km), and the S velocity in the intermediate and lower crust is around 4.0 km s-1. This anomalously large velocity and thickness can be explained by scraping off the lower crust, when the Indian lithosphere underthrusts the Himalaya.

Crust and Upper Mantle Structure from Joint Inversion of Body Wave and Gravity Data (Postprint). Annual Report 1

DTIC Science & Technology

2012-05-10

Basin, China , the crust and subduction zone beneath western Colombia, and a thermally active region within Utah in the central United States...Burlacu, R., Rowe, C., and Y. Yang (2009). Joint geophysical imaging of the geothermal sites in the Utah area using seismic body waves, surface waves and

Origin and Evolution of the Yellowstone Hotspot from Seismic-GPS Imaging and Geodynamic Modeling

NASA Astrophysics Data System (ADS)

Smith, R. B.; Jordan, M.; Puskas, C. M.; Farrell, J.; Waite, G. P.

2006-12-01

The Yellowstone hotspot resulted from interaction of a mantle plume with the overriding North America plate. This feature and related processes have influenced a large part of the western U.S., producing the 16 Ma Yellowstone-Snake River Plain-Newberry silicic-basalt volcanic field (YSRPN). We integrate results from a multi-institution experiment that deployed 80 seismic stations and 160 campaign and 21 permanent GPS stations for 1999-2003. Crust and mantle velocity models were derived from inversion of teleseismic and local earthquake data. Kinematic and dynamic models were derived from inversion of GPS velocities constrained by stresses associated the topography and the +15 m geoid anomaly. Tomography revealed a P- and S-wave low-velocity body at depths of 8-16 km beneath the caldera that is interpreted as partial melt of 8-15% that feeds the youthful Yellowstone volcanic field. Volume changes in the magma chamber are responsible for GPS-measured episodes of uplift and subsidence of the caldera at decadal scales with average rates of ~20 mm/yr but much higher short-term rates of up to 80 mm/yr. An upper-mantle low-velocity body was imaged by inverting teleseismic data constrained by the geoid structure, crustal structure, and the upper mantle discontinuities. This low P and S velocity body extends from 80 km to ~250 km directly beneath Yellowstone and then continues to 650 km with unexpected tilt to the west at ~60°. The tilt is consistent with the ascent of the buoyant magma entrained in eastward return-flow of the upper mantle. We estimate this body has an excess temperature from 85K to 120K, depending on the water content and with up to 1.5% melt. Using the inclined plume-geometry and plate motion history, we extrapolate the Yellowstone mantle source southwestward ~800 km as a plume-head in oceanic lithosphere centered beneath the Columbia Plateau basalt field at 16 Ma. Magma ascent was truncated there by the passage of thicker continental lithosphere over the plume beginning at 12 Ma, reducing the rate of large-scale volcanic eruptions in the YSRP. The decapitated plume head beneath Oregon underwent mantle return flow above the subducting Juan de Fuca plate and was responsible for the NW transgressive magmatism of the Newberry system. We then model the overall kinematics of the western U.S. from GPS data as SW motion for the YSRP, ~2 mm/yr, rotating into E-W motion in the Basin-Range, with a cumulative rate of ~4 mm/yr, and rotating to the northwest at rates of up to ~5 mm/yr in the Pacific Northwest, totaling ~10 mm/yr. Geodynamic models employing the GPS data and geometry of the crust-mantle structure suggests that southwest motion of the YSRP is dominated by stresses produced by the high potential energy of the Yellowstone hotspot while westward motion of the Basin-Range is driven by stress differences associated with the high topography of the Rocky Mountains.

D" Anisotropy Beneath the Caribbean, Central America and the East Pacific

NASA Astrophysics Data System (ADS)

Nowacki, A.; Wookey, J.; Kendall, J.

2009-05-01

Whilst the majority of the Earth's lower mantle appears to be relatively homogeneous, by contrast the few hundred kilometres above the core-mantle boundary (CMB) are host to a region of probable large chemical and thermal heterogeneity. Seismic observations of this region---known as D"---include a large increase in S-wave velocity that can vary in depth laterally over distances of <~100~km and significant seismic anisotropy (the variation of wavespeed with direction). The most recent candidate to explain these features in D" (including its anisotropy and bounding discontinuity) is the experimentally observed transformation of MgSiO3-perovskite to a post-perovskite structure at near CMB pressures and temperatures. As the phase change has a positive Clapeyron slope, regions where the geotherm is colder than average at the CMB---such as areas beneath long-term subduction---should show evidence of such a discontinuity and, depending on the alignment of mantle minerals or other structure, should also exhibit seismic anisotropy. We study the D" region beneath the Caribbean, Central America and the east Pacific using S and ScS phases mainly from deep-focus earthquakes with magnitude >~Mw~5.5 and depths >~550 km. Our method allows the incorporation of previous estimates of source-side upper mantle anisotropy, and by comparing the splitting parameters of the two phases (thus correcting for anisotropy in the upper mantle below the receiver), we obtain measurements of splitting in ScS alone; hence measuring the anisotropy in the lowermost mantle. The S and ScS phases are detected on around 450 seismic stations in Canada and the US (including Hawaii), yielding over 270 measurements of anisotropy in D". The measurements cover an area ˜4,000~km by ˜2,000~km centred on the CMB beneath Central America, and exhibit ˜1% S-wave anisotropy. In the Caribbean, they show a small but detectable departure from the first-order transverse isotropy with a vertical axis of symmetry (VTI) which can be explained as the same but with a tilted axis of symmetry (TTI). Here this dips a few degrees to the west; beneath Central America it dips to the east. Previous waveform studies agree with our results (e.g., Maupin et al., JGR, 2005). Beneath the east Pacific, where global S-wave models show a much less positive shear velocity anomaly, measurements show a significant degree of TTI, probably dipping by ˜30° to the east or southeast. Our interpretation (similarly to previous studies) of these features proposes that this is a result of the dynamics of the interaction of slab material with that already present at the base of the mantle, leading to deformation into 'ridges' aligned roughly perpendicular to the direction of palaeo-subduction over short scales (˜100 km and less) and the subsequent alignment of the crystals, melt pockets or other features which give rise to the TTI.

Seismic structure beneath the Tengchong volcanic area (southwest China) from receiver function analysis

NASA Astrophysics Data System (ADS)

Xu, Yi; Li, Xuelei; Wang, Sheng

2018-05-01

Tengchong is a young volcanic area on the collision boundary between the Indian and Euro-Asian plates of the southeastern Tibetan margin. Holocene volcanoes are concentrated in the Tengchong basin, where they align an N-S trending string-like cluster. To study the magma activity and its relation with the volcanoes, we deployed a passive seismic observation across the volcanic area in northern Tengchong. Using tele-seismic data and receiver function technique, we determined the S-wave velocity structure beneath nine temporary stations. Results show that the Tengchong basin is underlain by prominent low-velocity zones that are associated with the magma chambers of the volcanoes. In the north, a small and less pronounced magma chamber lies beneath two crater lakes, with a depth range of 9-16 km and a lateral width of <8 km. To the south, an interconnected magma chamber is found between the Dayingshan (DYS) volcano and the Dakongshan (DKS) volcanic cluster, with a depth range of 6-15 km and a lateral width of <12 km. In the south, the Laoguipo (LGP) volcano is characterized by anomalous low velocities throughout the upper-mid crust. Combined with other studies, we infer that the DYS volcano shares the same magma chamber with the DKS volcanic cluster, whereas the heat flow beneath the LGP volcano belongs to another thermal system, probably relating to the magma activity beneath the Rehai geothermal field in the south or affected by the intersection between the Tengchong volcanic fault zone and the Dayingjiang fault zone. In addition, mantle intrusion has resulted in the Moho elevation beneath the DKS volcanic cluster, and the thick transition zones on the crust-mantle boundary imply a possible penetration of the heat flow from the uppermost mantle into the lower crust.

Crustal and Upper Mantle Structure of the Taupo Volcanic Zone, North Island, New Zealand.

NASA Astrophysics Data System (ADS)

Harrison, A. J.; White, R. S.

2003-12-01

The Taupo Volcanic Zone (TVZ) is a major Pliocene-Quaternary NNE-SSW orientated,volcano-tectonic complex, about 250 km long and up to 60 km wide in the central North Island of New Zealand. The TVZ is one of the largest and most frequently active rhyolitic magmatic systems on Earth, characterised by intense shallow seismic activity, high natural heat flow (some 12-20 times the continental norm) and active NW-SE extension. To the north of the TVZ, subduction of the Pacific Plate beneath the oceanic lithosphere of the Australian Plate is accompanied by a region of back-arc extension (the Havre Trough). The TVZ marks the southern continuation of this back-arc extension into continental lithosphere.The TVZ therefore represents an ideal opportunity to study the onset of back-arc spreading onshore. Here we present forward and inverse models of the crustal structure beneath the TVZ. These models incorporate both active and passive source data acquired from the NIGHT (North Island GeopHysical Transect) project. Common to both models is a 2-3km deep basin of low velocity sediments which we interpret to be ignimbrite deposits. Typical basement velocities of ˜6km/s are observed beneath and to either side of the TVZ, where they correlate well with mapped outcrops of basement rocks. Velocities of around 7.3 km/s are observed at depths greater than 16 km beneath the TVZ. Such velocities may be interpreted as anomalously low velocity upper manlte or heavly intruded lower crust. Having constrained the crustal structure we then use earthquake events from the subducting Pacific Plate to yield information on the velocity structure of the upper mantle beneath the TVZ. NIGHT Working Group A. Harrison, J. Haines, R. White (University of Cambridge,United Kingdom); S. Henrys, S. Bannister, I. Pecher, F. Davey (Inst. Geological and Nuclear Sciences, Lower Hutt, New Zealand); T. Stern, W. Stratford (Victoria University of Wellington, New Zealand); H. Shimamura, Y. Nishimura, and A. Yamada (Hokkaido University, Sapporo, Japan).

Deformation Mechanism of the Northern Tibetan Plateau as Revealed by Magnetotelluric Data

NASA Astrophysics Data System (ADS)

Zhang, Letian; Wei, Wenbo; Jin, Sheng; Ye, Gaofeng; Xie, Chengliang

2017-04-01

As a unique geologic unit on the northern margin of the Tibetan Plateau, the Qaidam Basin plays a significant role in constraining the vertical uplift and horizontal expansion of the northern and northeastern Tibetan Plateau. However, due to its complex evolution history and difficult logistic condition, deformation mechanism of the lithosphere beneath the Qaidam Basin is still highly debated. To better understand the lithospheric electrical structure and deformation mechanism of the Qaidam Basin, A 250 km long, NE-SW directed Magnetotelluric (MT) profile was finished in the northern portion of the Basin, which is roughly perpendicular to the thrust fault systems on the western and eastern margins of the Basin, as well as anticlinorium systems within the Basin. The profile consists of 20 broad-band MT stations and 5 long-period MT stations. Original time series data is processed with regular robust routines. Dimensionality and regional strike direction are determined for the dataset through data analysis. Based on the analysis results, 2D inversions were performed to produce a preferred model of the lithospheric electrical structure beneath the northern Qaidam Basin. Uncertainty analysis of the 2D inversion model was also conducted based on a data resampling approach. The outcome 2D electrical model was further used to estimate the distribution of temperature and melt fraction in the upper mantle based on laboratory-determined relationships between the electrical conductivity and temperature of nominally anhydrous minerals and basaltic melt by using the mixing law of Hashin-Shtrikman's bounds. All these results suggest that: (1) the crust-mantle boundary is imaged as a conductive layer beneath the western Qaidam Basin, with its temperature estimated to be 1200-1300 °C and melt fraction 5-8%, indicating decoupling deformation of the crust and upper mantle. (2) A large-scale east-dipping conductor is imaged beneath the eastern Qaidam Basin. This conductor extends from the upper crust to the upper mantle, implying vertical coherent deformation of the lithosphere. Melt fraction of this conductive region is estimated to be as high as 10%, which might accommodates a major portion of the thrust deformation on the boundary between the Qaidam Basin and the Qilian Block. (3) Two different end-member deformation mechanisms, namely the decoupling deformation and vertical coherent deformation are both active on the northern margin of the Tibetan Plateau, and both play a significant role in controlling the uplift and expansion of the northern Tibetan Plateau. *This work was funded by National Natural Science Foundation of China (41404060, 41404059).

The origin of shear wave splitting beneath Iceland

NASA Astrophysics Data System (ADS)

Ito, Garrett; Dunn, Robert; Li, Aibing

2015-06-01

The origin of shear wave splitting (SWS) in the mantle beneath Iceland is examined using numerical models that simulate 3-D mantle flow and the development of seismic anisotropy due to lattice-preferred orientation (LPO). Using the simulated anisotropy structure, we compute synthetic SKS waveforms, invert them for fast polarization directions and split times, and then compare the predictions with the results from three observational studies of Iceland. Models that simulate a mantle plume interacting with the Mid-Atlantic Ridge in which the shallow-most mantle has a high viscosity due to the extraction of water with partial melting, or in which C-type olivine LPO fabric is present due to high water content in the plume, produce the largest chi-squared misfits to the SWS observations and are thus rejected. Models of a low-viscosity mantle plume with A-type olivine fabric everywhere, or with the added effects of E-type fabric in the plume below the solidus produce lower misfits. The lowest misfits are produced by models that include a rapid (˜50 km Myr-1) northward regional flow (NRF) in the mid-upper mantle, either with or without a plume. NRF was previously indicated by a receiver function study and a regional tomography study, and is shown here to be a major cause of the azimuthal anisotropy beneath Iceland. The smallest misfits for the models with both a plume and NRF are produced when LPO forms above depths of 300-400 km, which, by implication, also mark the depths above which dislocation creep dominates over diffusion creep. This depth of transition between dislocation and diffusion creep is greater than expected beneath normal oceanic seafloor, and is attributed to the unusually rapid strain rates associated with an Iceland plume and the NRF.

Rheological structure of the lithosphere in plate boundary strike-slip fault zones

NASA Astrophysics Data System (ADS)

Chatzaras, Vasileios; Tikoff, Basil; Kruckenberg, Seth C.; Newman, Julie; Titus, Sarah J.; Withers, Anthony C.; Drury, Martyn R.

2016-04-01

How well constrained is the rheological structure of the lithosphere in plate boundary strike-slip fault systems? Further, how do lithospheric layers, with rheologically distinct behaviors, interact within the strike-slip fault zones? To address these questions, we present rheological observations from the mantle sections of two lithospheric-scale, strike-slip fault zones. Xenoliths from ˜40 km depth (970-1100 ° C) beneath the San Andreas fault system (SAF) provide critical constraints on the mechanical stratification of the lithosphere in this continental transform fault. Samples from the Bogota Peninsula shear zone (BPSZ, New Caledonia), which is an exhumed oceanic transform fault, provide insights on lateral variations in mantle strength and viscosity across the fault zone at a depth corresponding to deformation temperatures of ˜900 ° C. Olivine recrystallized grain size piezometry suggests that the shear stress in the SAF upper mantle is 5-9 MPa and in the BPSZ is 4-10 MPa. Thus, the mantle strength in both fault zones is comparable to the crustal strength (˜10 MPa) of seismogenic strike-slip faults in the SAF system. Across the BPSZ, shear stress increases from 4 MPa in the surrounding rocks to 10 MPa in the mylonites, which comprise the core of the shear zone. Further, the BPSZ is characterized by at least one order of magnitude difference in the viscosity between the mylonites (1018 Paṡs) and the surrounding rocks (1019 Paṡs). Mantle viscosity in both the BPSZ mylonites and the SAF (7.0ṡ1018-3.1ṡ1020 Paṡs) is relatively low. To explain our observations from these two strike-slip fault zones, we propose the "lithospheric feedback" model in which the upper crust and lithospheric mantle act together as an integrated system. Mantle flow controls displacement and the upper crust controls the stress magnitude in the system. Our stress data combined with data that are now available for the middle and lower crustal sections of other transcurrent fault systems support the prediction for constant shear strength (˜10 MPa) throughout the lithosphere; the stress magnitude is controlled by the shear strength of the upper crustal faults. Fault rupture in the upper crust induces displacement rate loading of the upper mantle, which in turn, causes strain localization in the mantle shear zone beneath the strike-slip fault. Such forced localization leads to higher stresses and strain rates in the shear zone compared to the surrounding rocks. Low mantle viscosity within the shear zone is critical for facilitating mantle flow, which induces widespread crustal deformation and displacement loading. The lithospheric feedback model suggests that strike-slip fault zones are not mechanically stratified in terms of shear stress, and that it is the time-dependent interaction of the different lithospheric layers - rather than their relative strengths - that governs the rheological behavior of the plate boundary, strike-slip fault zones.

The Role of Oceanic Transform Faults in Seafloor Spreading: A Global Perspective From Seismic Anisotropy

NASA Astrophysics Data System (ADS)

Eakin, Caroline M.; Rychert, Catherine A.; Harmon, Nicholas

2018-02-01

Mantle anisotropy beneath mid-ocean ridges and oceanic transforms is key to our understanding of seafloor spreading and underlying dynamics of divergent plate boundaries. Observations are sparse, however, given the remoteness of the oceans and the difficulties of seismic instrumentation. To overcome this, we utilize the global distribution of seismicity along transform faults to measure shear wave splitting of over 550 direct S phases recorded at 56 carefully selected seismic stations worldwide. Applying this source-side splitting technique allows for characterization of the upper mantle seismic anisotropy, and therefore the pattern of mantle flow, directly beneath seismically active transform faults. The majority of the results (60%) return nulls (no splitting), while the non-null measurements display clear azimuthal dependency. This is best simply explained by anisotropy with a near vertical symmetry axis, consistent with mantle upwelling beneath oceanic transforms as suggested by numerical models. It appears therefore that the long-term stability of seafloor spreading may be associated with widespread mantle upwelling beneath the transforms creating warm and weak faults that localize strain to the plate boundary.

New constraints on the textural and geochemical evolution of the upper mantle beneath the Styrian basin

NASA Astrophysics Data System (ADS)

Aradi, Laszlo; Hidas, Károly; Zanetti, Alberto; János Kovács, István; Patkó, Levente; Szabó, Csaba

2016-04-01

Plio-Pleistocene alkali basaltic volcanism sampled sporadically the upper mantle beneath the Carpathian-Pannonian Region (CPR, e.g. [1]). Lavas and pyroclasts often contain mantle derived xenoliths, and the majority of them have been extensively studied [1], except the westernmost Styrian Basin Volcanic Field (SBVF, Eastern Austria and Slovenia). In the SBVF only a few volcanic centers have been studied in details (e.g. Kapfenstein & Tobaj). Based on these studies, the upper mantle beneath the SBVF is consists of dominantly high temperature, texturally and geochemically homogeneous protogranular spinel lherzolite. New major and trace element data from rock-forming minerals of ultramafic xenoliths, coupled with texture and deformation analysis from 12 volcanic outcrops across the SBVF, suggest that the lithospheric roots of the region are more heterogeneous than described previously. The studied xenoliths are predominantly lherzolite, amphibole is a common phase that replaces pyroxenes and spinels and proves modal metasomatism. Phlogopite coupled with apatite is also present in amphibole-rich samples. The texture of the xenoliths is usually coarse-grained and annealed with low abundance of subgrain boundaries in both olivine and pyroxenes. Olivine crystal preferred orientation (CPO) varies between the three most abundant one: [010]-fiber, orthogonal and [100]-fiber symmetry [2]. The CPO of pyroxenes is usually coherent with coeval deformation with olivine, however the CPO of amphibole is suggesting postkinematic epitaxial overgrowth on the precursor pyroxenes. According to equilibrium temperatures, the studied xenolith suite samples a broader temperature range (850-1100 °C) than the literature data, corresponding to mantle depths between 30 and 60 km, which indicates that the xenolith suite only represents the shallower part of the recent 100 km thick lithospheric mantle beneath the SBVF. The equilibrium temperatures show correlation with the varying CPO symmetries, with [100]-fiber and orthorhombic symmetry appear in the high temperature (>1000 °C) xenoliths, which are thought to have an asthenospheric origin [3]. Based on our study, the subcontinental lithospheric mantle beneath the western part of the CPR is not as homogeneous as it was reported before. The shallower part of the mantle lithosphere contains peridotites, where the pervasive deformation and subsequent thermal recovery of the upper mantle was followed by melt percolation events causing extensive metasomatism. This research was granted by the Hungarian Science Foundation (OTKA, 78425 to Cs. Szabó). K. Hidas' research leading to these results was funded by the European Union Framework Programme 7 (EU-FP7) Marie Curie postdoctoral grant PIEF-GA-2012- 327226. References: [1]Szabó, C. et al. 2004. Tectonophysics, 393(1), 119-137. [2] Tommasi, A., Vauchez, A. 2015. Tectonophysics, 661, 11-37. [3] Kovács, I. et al. 2012. Tectonophysics, 514, 168-179.

Anisotropy in the lowermost mantle beneath the Indian Ocean Geoid Low from ScS splitting measurements

NASA Astrophysics Data System (ADS)

Padma Rao, B.; Ravi Kumar, M.; Singh, Arun

2017-02-01

The Indian Ocean Geoid Low (IOGL) to the south of Indian subcontinent is the world's largest geoid anomaly. In this study, we investigate the seismic anisotropy of the lowermost mantle beneath the IOGL by analyzing splitting of high-quality ScS phases corrected for source and receiver side upper mantle anisotropy. Results reveal significant anisotropy (˜1.01%) in the D'' layer. The observed fast axis polarization azimuths in the ray coordinate system indicate a TTI (transverse isotropy with a tilted axis of symmetry) style of anisotropy. Lattice Preferred Orientation (LPO) deformation of the palaeo-subducted slabs experiencing high shear strain is a plausible explanation for the observed anisotropy beneath the IOGL.

Mantle structure and tectonic history of SE Asia

NASA Astrophysics Data System (ADS)

Hall, Robert; Spakman, Wim

2015-09-01

Seismic travel-time tomography of the mantle under SE Asia reveals patterns of subduction-related seismic P-wave velocity anomalies that are of great value in helping to understand the region's tectonic development. We discuss tomography and tectonic interpretations of an area centred on Indonesia and including Malaysia, parts of the Philippines, New Guinea and northern Australia. We begin with an explanation of seismic tomography and causes of velocity anomalies in the mantle, and discuss assessment of model quality for tomographic models created from P-wave travel times. We then introduce the global P-wave velocity anomaly model UU-P07 and the tectonic model used in this paper and give an overview of previous interpretations of mantle structure. The slab-related velocity anomalies we identify in the upper and lower mantle based on the UU-P07 model are interpreted in terms of the tectonic model and illustrated with figures and movies. Finally, we discuss where tomographic and tectonic models for SE Asia converge or diverge, and identify the most important conclusions concerning the history of the region. The tomographic images of the mantle record subduction beneath the SE Asian region to depths of approximately 1600 km. In the upper mantle anomalies mainly record subduction during the last 10 to 25 Ma, depending on the region considered. We interpret a vertical slab tear crossing the entire upper mantle north of west Sumatra where there is a strong lateral kink in slab morphology, slab holes between c.200-400 km below East Java and Sumbawa, and offer a new three-slab explanation for subduction in the North Sulawesi region. There is a different structure in the lower mantle compared to the upper mantle and the deep structure changes from west to east. What was imaged in earlier models as a broad and deep anomaly below SE Asia has a clear internal structure and we argue that many features can be identified as older subduction zones. We identify remnants of slabs that detached in the Early Miocene such as the Sula slab, now found in the lower mantle north of Lombok, and the Proto-South China Sea slab now at depths below 700 km curving from northern Borneo to the Philippines. Based on our tectonic model we interpret virtually all features seen in upper mantle and lower mantle to depths of at least 1200 km to be the result of Cenozoic subduction.

Seismic evidence for mantle suture and a collisional origin for the Canadian Cordillera

NASA Astrophysics Data System (ADS)

Chen, Y.; Gu, Y. J.; Currie, C. A.; Johnston, S. T.; Hung, S. H.; Schaeffer, A. J.; Audet, P.

2017-12-01

The North American Cordillera is a Phanerozoic orogenic belt that extends from Mexico to Alaska. Its eastern boundary is marked by pronounced changes in geophysical observations (e.g., mantle seismic velocity, surface heat flow, and effective elastic thickness) indicating a steep structural gradient beneath the Cordilleran foreland and the adjacent North American Craton. Seismological constraints on this boundary zone have been highly uneven: on the one hand, the knowledge of subsurface structures of the US Cordillera has been greatly enhanced by the USArray; on the other hand, detailed surveys of the northern counterpart, the Canadian Cordillera, are limited due to relatively sparse broadband data coverage. Questions pertaining to where and how Cordillera-Craton transition occurs in the upper mantle remain debated. Here, we utilize new teleseismic travel-time data from recently deployed networks in the Alberta foreland basin and nearby USArray stations and invert for mantle seismic velocities using finite-frequency tomography. The resulting high-resolution 3D model shows a dramatic increase in lithosphere thickness (>200 km) from the Cordillera to Craton. Additionally, independent calculations of mantle temperature from P (4.3%) and S (7.0%) velocity contrasts yield a consistent eastward 200-300 °C decrease at 150 km depth. We attribute the sharp structural and temperature/velocity gradients to the Cordillera-Craton boundary (CCB) established since at least the Late Cretaceous (ca. 100 Ma). The CCB dips steeply to the west beneath a carbonate belt that delineates a cryptic orogenic suture near the southern Rocky Mountain Trench, which provides strong evidence for an upper mantle suture between North America and an allochthonous Cordillera. The westward-dipping CCB may be a preserved structure associated with partial subduction of the leading edge of the North American Craton during its terminal collision with a microcontinent (Cordillera); this would require a strong craton lithosphere in order to preserve the observed geometry for >100 Ma. In this case, the earlier interpretation of the Cordillera as an accretionary orogen characterized by thin-skinned thrusting of exotic terranes over an intact North American basement and oceanward-tapering lithospheric mantle, is deemed unlikely.

Mantle thermal history during supercontinent assembly and breakup

NASA Astrophysics Data System (ADS)

Rudolph, M. L.; Zhong, S.

2013-12-01

We use mantle convection simulations driven by plate motion boundary conditions to investigate changes in mantle temperature through time. It has been suggested that circum-Pangean subduction prevented convective thermal mixing between sub-continental and sub-oceanic regions. We performed thermo-chemical simulations of mantle convection with velocity boundary conditions based on plate motions for the past 450 Myr using Earth-like Rayleigh number and ~60% internal heating using three different plate motion models for the last 200 Myr [Lithgow-Bertelloni and Richards 1998; Gurnis et al. 2012; Seton et al. 2012; Zhang et al. 2010]. We quantified changes in upper-mantle temperature between 200-1000 km depth beneath continents (defined as the oldest 30% of Earth's surface) and beneath oceans. Sub-continental upper mantle temperature was relatively stable and high between 330 and 220 Ma, coincident with the existence of the supercontinent Pangea. The average sub-continental temperature during this period was, however, only ~10 K greater than during the preceding 100 Myr. In the ~200 Myr since the breakup of Pangea, sub-continental temperatures have decreased only ~15 K in excess of the 0.02 K/Myr secular cooling present in our models. Sub-oceanic upper mantle temperatures did not vary more than 5 K between 400 and 200 Ma and the cooling trend following Pangea breakup is less pronounced. Recent geochemical observations imply rapid upper mantle cooling of O(10^2) K during continental breakup; our models do not produce warming of this magnitude beneath Pangea or cooling of similar magnitude associated with the breakup of Pangea. Our models differ from those that produce strong sub-continental heating in that the circum-Pangean subduction curtain does not completely inhibit mixing between the sub-continental and sub-oceanic regions and we include significant internal heating, which limits the rate of temperature increase. Heat transport in our simulations is controlled to first order by plate motions. Most of the temporal variability in surface heat flow is driven by variations in seafloor spreading rate and the accompanying changes in slab velocities dominate variations in buoyancy flux at all mantle depths. Variations in plume buoyancy flux are small but are correlated with the slab buoyancy flux variations.

P-wave velocity structure beneath the northern Antarctic Peninsula

NASA Astrophysics Data System (ADS)

Park, Y.; Kim, K.; Jin, Y.

2010-12-01

We have imaged tomographically the tree-dimensional velocity structure of the upper mantle beneath the northern Antarctic Peninsula using teleseismic P waves. The data came from the seven land stations of the Seismic Experiment in Patagonia and Antarctica (SEPA) campaigned during 1997-1999, a permanent IRIS/GSN station (PMSA), and 3 seismic stations installed at scientific bases, Esperanza (ESPZ), Jubany (JUBA), and King Sejong (KSJ), in South Shetland Islands. All of the seismic stations are located in coast area, and the signal to noise ratios (SNR) are very low. The P-wave model was inverted from 95 earthquakes resulting in 347 ray paths with P- and PKP-wave arrivals. The inverted model shows a strong low velocity anmaly beneath the Bransfield Strait, and a fast anomaly beneath the South Shetland Islands. The low velocity anomaly beneath the Bransfield might be due to a back arc extension, and the fast velocity anomaly beneath the South Shetland Islands could indicates the cold subducted slab.

Three Dimensional Viscoelastic Postseismic Deformation of the 2013 Mw8.3 Okhotsk Deep-Focus Earthquake

NASA Astrophysics Data System (ADS)

Hu, Y.; Burgmann, R.; Shestakov, N.; Titkov, N. N.; Serovetnikov, S.; Prytkov, A.; Vasilenko, N. F.; Wang, K.

2016-12-01

The upper mantle rheology at depths within a few hundred kilometers has been well studied through shallow great megathrust earthquakes. However, understanding of the mantle rheology at greater depths, such as in the vicinity of the transition zone, has been limited by the lack of direct or indirect measurements. The largest well-recorded deep earthquake with magnitude Mw 8.3 occurred within the subducting Pacific plate at 600 km depth beneath the Okhotsk Sea on May 24, 2013. Twenty-seven continuous GPS stations in this region recorded coseismic displacements of up to 15 mm in the horizontal direction and up to 20 mm in the vertical direction. Within three years after the earthquake seventeen continuous GPS stations underwent transient westward motion of up to 8 mm/yr and vertical motion of up to 10 mm/yr. The geodetically delineated postseismic crustal deformation thus provides a unique opportunity to study the three dimensional heterogeneity of the mantle rheology and properties of the subducting slab at great depths. We have developed three-dimensional viscoelastic finite element models of the 2013 Okhotsk earthquake to explore these questions. Our initial model includes an elastic lithosphere including the subducting slab, a viscoelastic continental upper mantle and a viscoelastic oceanic upper mantle. We assume that the upper mantle is characterized by a bi-viscous Burgers rheology. For simplicity, we assume that the transient Kelvin viscosity is one order of magnitude lower than that of the steady-state Maxwell viscosity. Our preliminary models indicate that the viscosity of the upper mantle beneath the transition zone has to be at least one order of magnitude lower than that of the upper mantle at shallower depths. A viscoelastic subducting slab at depths >400 km with viscosities of 2-3 orders of magnitude higher than that of the mantle wedge provides a better fit to the observed surface velocities.

Travel-time Tomography of the Upper Mantle using Amphibious Array Seismic Data from the Cascadia Initiative and EarthScope

NASA Astrophysics Data System (ADS)

Cafferky, S.; Schmandt, B.

2013-12-01

Offshore and onshore broadband seismic data from the Cascadia Initiative and EarthScope provide a unique opportunity to image 3-D mantle structure continuously from a spreading ridge across a subduction zone and into continental back-arc provinces. Year one data from the Cascadia Initiative primarily covers the northern half of the Juan de Fuca plate and the Cascadia forearc and arc provinces. These new data are used in concert with previously collected onshore data for a travel-time tomography investigation of mantle structure. Measurement of relative teleseismic P travel times for land-based and ocean-bottom stations operating during year one was completed for 16 events using waveform cross-correlation, after bandpass filtering the data from 0.05 - 0.1 Hz with a second order Butterworth filter. Maps of travel-time delays show changing patterns with event azimuth suggesting that structural variations exist beneath the oceanic plate. The data from year one and prior onshore travel time measurements were used in a tomographic inversion for 3-D mantle P-velocity structure. Inversions conducted to date use ray paths determined by a 1-D velocity model. By meeting time we plan to present models using ray paths that are iteratively updated to account for 3-D structure. Additionally, we are testing the importance of corrections for sediment and crust thickness on imaging of mantle structure near the subduction zone. Low-velocities beneath the Juan de Fuca slab that were previously suggested by onshore data are further supported by our preliminary tomographic inversions using the amphibious array data.

Lithospheric structure of the Northern Ordos and adjacent regions from surface wave tomography: implications to the tectonics of the North China Craton

NASA Astrophysics Data System (ADS)

LI, S.; Guo, Z.; Chen, Y. J.

2017-12-01

We present a high-resolution upper mantle S velocity model of the northern Ordos block using ambient noise tomography and two-plane-wave tomography between 8 and 143 s. The Ordos block, regarded as the nuclei of the Archean craton of North China Craton, is underlain by high velocity down to 200 km, indicating the preservation of cratonic root at the interior. However, thick lithospheric keel (≥ 200 km) is not observed outside the Ordos, suggesting craton reworking around the Ordos. The most important findings is the prominent low velocity shown beneath the Datong volcano that migrates westward with depth. At 200 km depth, the low velocity locates almost 500 km west to the leading edge of the flat-lying Pacific slab in the mantle transition zone. This observation is in conflict with the previous interpretation that the Datong volcano is fed by the deep upwelling related to the subduction of the Pacific plate. The westward tilted low velocity beneath the Datong volcano, however, is in agreement with the predominant NW-SE trending alignment of fast direction revealed by SKS splitting in this area, suggesting the Datong volcano is likely due to the asthenospheric mantle flow from west. Two possible scenarios could be related to this mantle process. First, the low velocity beneath the Datong volcano may link to the large-scale, deep-rooted mantle upwelling beneath the Mongolia, northwest to the Datong volcano at deeper depth revealed by Zhang et al. (2016). We postulate that when the raising mantle materials reaches the shallow depth, it would be forced bent by the thick lithosphere beneath the Gobi in Mongolia and flow southeastward to Datong volcano. Second, it is also worth noting that the low velocity beneath the Datong volcano connects to the low velocity zone (LVZ) beneath the Ordos block below 200km, which further links the LVZ beneath the northeastern Tibet to the west. Therefore, the Datong volcano could be fed by the mantle flow from northeastern Tibet. The continuous slab-retreating of the western Pacific since the Cenozoic would have created void spaces in eastern Asia which could in turn suck new asthenospheric materials from the Mongolia and northeast Tibet through the northern TNCO. The upward mantle flow along the rapid thinning lithosphere to the northeast of Ordos had generated partial melting to supply the Datong volcano.

Continental lithosphere of the Arabian Plate: A geologic, petrologic, and geophysical synthesis

NASA Astrophysics Data System (ADS)

Stern, Robert J.; Johnson, Peter

2010-07-01

The Arabian Plate originated ˜ 25 Ma ago by rifting of NE Africa to form the Gulf of Aden and Red Sea. It is one of the smaller and younger of the Earth's lithospheric plates. The upper part of its crust consists of crystalline Precambrian basement, Phanerozoic sedimentary cover as much as 10 km thick, and Cenozoic flood basalt (harrat). The distribution of these rocks and variations in elevation across the Plate cause a pronounced geologic and topographic asymmetry, with extensive basement exposures (the Arabian Shield) and elevations of as much as 3000 m in the west, and a Phanerozoic succession (Arabian Platform) that thickens, and a surface that descends to sea level, eastward between the Shield and the northeastern margin of the Plate. This tilt in the Plate is partly the result of marginal uplift during rifting in the south and west, and loading during collision with, and subduction beneath, the Eurasian Plate in the northeast. But a variety of evidence suggests that the asymmetry also reflects a fundamental crustal and mantle heterogeneity in the Plate that dates from Neoproterozoic time when the crust formed. The bulk of the Plate's upper crystalline crust is Neoproterozoic in age (1000-540 Ma) reflecting, in the west, a 300-million year process of continental crustal growth between ˜ 850 and 550 Ma represented by amalgamated juvenile magmatic arcs, post-amalgamation sedimentary and volcanic basins, and granitoid intrusions that make up as much as 50% of the Shield's surface. Locally, Archean and Paleoproterozoic rocks are structurally intercalated with the juvenile Neoproterozoic rocks in the southern and eastern parts of the Shield. The geologic dataset for the age, composition, and origin of the upper crust of the Plate in the east is smaller than the database for the Shield, and conclusions made about the crust in the east are correspondingly less definitive. In the absence of exposures, furthermore, nothing is known by direct observation about the composition of the crust north of the Shield. Nonetheless, available data indicate a geologic history for eastern Arabian crust different to that in the west. The Neoproterozic crust (˜ 815-785 Ma) is somewhat older than in the bulk of the Arabian Shield, and igneous and metamorphic activity was largely finished by 750 Ma. Thereafter, the eastern part of the Plate became the site of virtually continuous sedimentation from 725 Ma on and into the Phanerozoic. This implies that a relatively strong lithosphere was in place beneath eastern Arabia by 700 Ma in contrast to a lithospheric instability that persisted to ˜ 550 Ma in the west. Lithospheric differentiation is further indicated by the Phanerozoic depositional history with steady subsidence and accumulation of a sedimentary succession 5-14 km thick in the east and a consistent high-stand and thin to no Phanerozoic accumulation over the Shield. Geophysical data likewise indicate east-west lithospheric differentiation. Overall, the crustal thickness of the Plate (depth to the Moho) is ˜ 40 km, but there is a tendency for the crust to thicken eastward by as much as 10% from 35-40 km beneath the Shield to 40-45 km beneath eastern Arabia. The crust also becomes structurally more complex with as many as 5 seismically recognized layers in the east compared to 3 layers in the west. A coincident increase in velocity is noted in the upper-crust layers. Complementary changes are evidenced in some models of the Arabian Plate continental upper mantle, indicating eastward thickening of the lithospheric mantle from ˜ 80 km beneath the Shield to ˜ 120 km beneath the Platform, which corresponds to an overall lithospheric thickening (crust and upper mantle) from ˜ 120 km to ˜ 160 km eastward. The locus of these changes coincides with a prominent magnetic anomaly (Central Arabian Magnetic Anomaly, CAMA) in the extreme eastern part of the Arabian Shield that extends north across the north-central part of the Arabian Plate. The CAMA also coincides with a major structural boundary separating a region of northerly and northwesterly basement trends in the west from a region of northerly and northeasterly trends in the northeastern part of the Plate, and with the transition from high-stand buoyant Shield to subsided Platform. Its coincidence with geophysically indicated changes in the lower crust and mantle structure suggests that a fundamental lithospheric boundary is present in the central part of the Arabian Plate. The ages and isotopic characteristics of xenoliths brought to the surface in Cenozoic basalt eruptions indicate that the lower crust and upper mantle are largely juvenile Neoproterozoic additions, meaning that the lower crust and upper mantle formed about the same time as the upper crust. This implies that the lithospheric boundary in the central part of the Arabian Plate dates from Neoproterozoic time. We conclude that lithospheric differentiation across the Arabian Plate is long lived and has controlled much of the Phanerozoic sedimentary history of the Plate.

Teleseismic P-wave tomography and mantle dynamics beneath Eastern Tibet

NASA Astrophysics Data System (ADS)

Lei, Jianshe; Zhao, Dapeng

2016-05-01

We determined a new 3-D P-wave velocity model of the upper mantle beneath eastern Tibet using 112,613 high-quality arrival-time data collected from teleseismic seismograms recorded by a new portable seismic array in Yunnan and permanent networks in southwestern China. Our results provide new insights into the mantle structure and dynamics of eastern Tibet. High-velocity (high-V) anomalies are revealed down to 200 km depth under the Sichuan basin and the Ordos and Alashan blocks. Low-velocity (low-V) anomalies are imaged in the upper mantle under the Kunlun-Qilian and Qinling fold zones, and the Songpan-Ganzi, Qiangtang, Lhasa and Chuan-Dian diamond blocks, suggesting that eastward moving low-V materials are extruded to eastern China after the obstruction by the Sichuan basin, and the Ordos and Alashan blocks. Furthermore, the extent and thickness of these low-V anomalies are correlated with the surface topography, suggesting that the uplift of eastern Tibet could be partially related to these low-V materials having a higher temperature and strong positive buoyancy. In the mantle transition zone (MTZ), broad high-V anomalies are visible from the Burma arc northward to the Kunlun fault and eastward to the Xiaojiang fault, and they are connected upward with the Wadati-Benioff seismic zone. These results suggest that the subducted Indian slab has traveled horizontally for a long distance after it descended into the MTZ, and return corner flow and deep slab dehydration have contributed to forming the low-V anomalies in the big mantle wedge. Our results shed new light on the dynamics of the eastern Tibetan plateau.

Crust and upper mantle shear wave structure of Northeast Algeria from Rayleigh wave dispersion analysis

NASA Astrophysics Data System (ADS)

Radi, Zohir; Yelles-Chaouche, Abdelkrim; Corchete, Victor; Guettouche, Salim

2017-09-01

We resolve the crust and upper mantle structure beneath Northeast Algeria at depths of 0-400 km, using inversion of fundamental mode Rayleigh wave. Our data set consists of 490 earthquakes recorded between 2007 and 2014 by five permanent broadband seismic stations in the study area. Applying a combination of different filtering technics and inversion method shear wave velocities structure were determined as functions of depth. The resolved changes in Vs at 50 km depth are in perfect agreement with crustal thickness estimates, which reflect the study area's orogenic setting, partly overlying the collision zone between the African and Eurasian plates. The inferred Moho discontinuity depths are close to those estimated for other convergent areas. In addition, there is good agreement between our results and variations in orientations of regional seismic anisotropy. At depths of 80-180 km, negative Vs anomalies at station CBBR suggest the existence of a failed subduction slab.

The upper mantle structure of the central Rio Grande rift region from teleseismic P and S wave travel time delays and attenuation

USGS Publications Warehouse

Slack, P.D.; Davis, P.M.; Baldridge, W.S.; Olsen, K.H.; Glahn, A.; Achauer, U.; Spence, W.

1996-01-01

The lithosphere beneath a continental rift should be significantly modified due to extension. To image the lithosphere beneath the Rio Grande rift (RGR), we analyzed teleseismic travel time delays of both P and S wave arrivals and solved for the attenuation of P and S waves for four seismic experiments spanning the Rio Grande rift. Two tomographic inversions of the P wave travel time data are given: an Aki-Christofferson-Husebye (ACH) block model inversion and a downward projection inversion. The tomographic inversions reveal a NE-SW to NNE-SSW trending feature at depths of 35 to 145 km with a velocity reduction of 7 to 8% relative to mantle velocities beneath the Great Plains. This region correlates with the transition zone between the Colorado Plateau and the Rio Grande rift and is bounded on the NW by the Jemez lineament, a N52??E trending zone of late Miocene to Holocene volcanism. S wave delays plotted against P wave delays are fit with a straight line giving a slope of 3.0??0.4. This correlation and the absolute velocity reduction imply that temperatures in the lithosphere are close to the solidus, consistent with, but not requiring, the presence of partial melt in the mantle beneath the Rio Grande rift. The attenuation data could imply the presence of partial melt. We compare our results with other geophysical and geologic data. We propose that any north-south trending thermal (velocity) anomaly that may have existed in the upper mantle during earlier (Oligocene to late Miocene) phases of rifting and that may have correlated with the axis of the rift has diminished with time and has been overprinted with more recent structure. The anomalously low-velocity body presently underlying the transition zone between the core of the Colorado Plateau and the rift may reflect processes resulting from the modern (Pliocene to present) regional stress field (oriented WNW-ESE), possibly heralding future extension across the Jemez lineament and transition zone.

Mantle plumes and associated flow beneath Arabia and East Africa

NASA Astrophysics Data System (ADS)

Chang, Sung-Joon; Van der Lee, Suzan

2011-02-01

We investigate mantle plumes and associated flow beneath the lithosphere by imaging the three-dimensional S-velocity structure beneath Arabia and East Africa. This image shows elongated vertical and horizontal low-velocity anomalies down to at least mid mantle depths. This three-dimensional S-velocity model is obtained through the joint inversion of teleseismic S- and SKS-arrival times, regional S- and Rayleigh waveform fits, fundamental-mode Rayleigh-wave group velocities, and independent Moho constraints from receiver functions, reflection/refraction profiles, and gravity measurements. In the resolved parts of our S-velocity model we find that the Afar plume is distinctly separate from the Kenya plume, showing the Afar plume's origin in the lower mantle beneath southwestern Arabia. We identify another quasi-vertical low-velocity anomaly beneath Jordan and northern Arabia which extends into the lower mantle and may be related to volcanism in Jordan, northern Arabia, and possibly southern Turkey. Comparing locations of mantle plumes from the joint inversion with fast axes of shear-wave splitting, we confirm horizontal mantle flow radially away from Afar. Low-velocity channels in our model support southwestward flow beneath Ethiopia, eastward flow beneath the Gulf of Aden, but not northwestwards beneath the entire Red Sea. Instead, northward mantle flow from Afar appears to be channeled beneath Arabia.

Upper mantle seismic anisotropy beneath Northern Peru from shear wave splitting analysis.

NASA Astrophysics Data System (ADS)

Franca, G. S.; Condori, C.; Tavera, H.; Eakin, C. M.; Beck, S. L.

2017-12-01

Beneath much of Peru lies the largest region of flat-slab subduction in the world today. The origins and dynamics of the Peruvian flat-slab however remain elusive, particularly in the north away from the Nazca Ridge. Studies of seismic anisotropy can potentially provide us with insight into the dynamics of recent and past deformational processes in the upper mantle. In this study, we conduct shear wave splitting to investigate seismic anisotropy across the northern extent of the Peruvian flat-slab for the first time. For the analysis, we used arrivals of SKS, SKKS and PKS phases from teleseismic events (88° > Δ < 150°) recorded at 30 broadband seismic stations from the Peruvian permanent and portable seismic networks, and international networks (CTBTO and RSBR-Brazil). The preliminary results reveal a complex anisotropy pattern with variations along strike. In the northernmost region, the average delay times range between 1.0 s and 1.2 s, with fast directions predominantly ENE-WSW oriented in a direction approximately perpendicular to the trench, parallel with subduction of the Nazca plate. Meanwhile towards the central region of Peru, the predominant fast direction changes to SE-NW oblique with the trench, but consistent with the pattern seen previously over the southern extent of the flat-slab by Eakin et al. (2013, 2015). These characteristics suggest a fundamental difference between the anisotropic structures, and therefore underlying mantle processes, beneath the northern and central portions of the Peruvian flat-slab.

Gravity field over northern Eurasia and variations in the strength of the upper mantle

NASA Technical Reports Server (NTRS)

Kogan, Mikhail G.; Mcnutt, Marcia K.

1993-01-01

The correlation of long-wavelength gravity anomalies in northern Eurasia with seismic velocity anomalies in the upper mantle reverses in sign between western and eastern Eurasia. The difference between western and eastern Eurasia can be explained by the presence of a low-viscosity zone in the uppermost mantle beneath eastern Eurasia that is absent to the west. The location of the lateral change in viscosity corresponds with the geologic boundary between the older shields and platforms of the Baltics, Russia, and Siberia and the younger, geologically active mountain belts of eastern Asia. This relation provides evidence that differences in the strength of the upper mantle control the locus of intracontinental deformation.

Regional P wave velocity structure of the Northern Cascadia Subduction Zone

USGS Publications Warehouse

Ramachandran, K.; Hyndman, R.D.; Brocher, T.M.

2006-01-01

This paper presents the first regional three-dimensional, P wave velocity model for the Northern Cascadia Subduction. Zone (SW British Columbia and NW Washington State) constructed through tomographic inversion of first-arrival traveltime data from active source experiments together with earthquake traveltime data recorded at permanent stations. The velocity model images the structure of the subducting Juan de Fuca plate, megathrust, and the fore-arc crust and upper mantle. Beneath southern Vancouver Island the megathrust above the Juan de Fuca plate is characterized by a broad zone (25-35 km depth) having relatively low velocities of 6.4-6.6 km/s. This relative low velocity zone coincides with the location of most of the episodic tremors recently mapped beneath Vancouver Island, and its low velocity may also partially reflect the presence of trapped fluids and sheared lower crustal rocks. The rocks of the Olympic Subduction Complex are inferred to deform aseismically as evidenced by the lack of earthquakes withi the low-velocity rocks. The fore-arc upper mantle beneath the Strait of Georgia and Puget Sound is characterized by velocities of 7.2-7.6 km/s. Such low velocities represent regional serpentinization of the upper fore-arc mantle and provide evidence for slab dewatering and densification. Tertiary sedimentary basins in the Strait of Georgia and Puget Lowland imaged by the velocity model lie above the inferred region of slab dewatering and densification and may therefore partly result from a higher rate of slab sinking. In contrast, sedimentary basins in the Strait of Juan de Fuca lie in a synclinal depression in the Crescent Terrane. The correlation of in-slab earthquake hypocenters M>4 with P wave velocities greater than 7.8 km/s at the hypocenters suggests that they originate near the oceanic Moho of the subducting Juan de Fuca plate. Copyright 2006 by the American Geophysical Union.

Structure and seismicity of the upper mantle using deployments of broadband seismographs in Antarctica and the Mariana Islands

NASA Astrophysics Data System (ADS)

Barklage, Mitchell

We determine shear wave splitting parameters of teleseismic SKS and SKKS phases recorded at 43 broadband seismometers deployed in South Victoria Land as part of the Transantarctic Mountains seismic experiment (TAMSEIS) from 2000-2003. We use an eigenvalue technique to linearize the rotated and shifted shear wave particle motions and determine the best splitting parameters. The data show a fairly consistent fast direction of azimuthal anisotropy oriented approximately N60°E with splitting times of about 1 second. Based on a previous study of the azimuthal variations of Rayleigh wave phase velocities which show a similar fast direction, we suggest the anisotropy is localized in the uppermost mantle, with a best estimate of 3% anisotropy in a layer of about 150 km thickness. We suggest that the observed anisotropy near the Ross Sea coast, a region underlain by thin lithosphere, results either from upper mantle flow related to Cenozoic Ross Sea extension or to edge-driven convection associated with a sharp change in lithospheric thickness between East and West Antarctica. Both hypotheses are consistent with the more E-W fast axis orientation for stations on Ross Island and along the coast, sub-parallel to the extension direction and the lithospheric boundary. Anisotropy in East Antarctica, which is underlain by cold thick continental lithosphere, must be localized within the lithospheric upper mantle and reflect a relict tectonic fabric from past deformation events. Fast axes for the most remote stations in the Vostok Highlands are rotated by 20° and are parallel to splitting measurements at South Pole. These observations seem to delineate a distinct domain of lithospheric fabric, which may represent the extension of the Darling Mobile Belt or Pinjarra Orogen into the interior of East Antarctica. Seismic tomography imaging provides an opportunity to constrain mantle wedge processes associated with subduction, volatile transport, arc volcanism, and back-arc spreading. We investigate seismic velocity structure of the upper mantle across the Central Mariana subduction system using data from the 2003-2004 Mariana Subduction Factory Imaging Experiment. This 11-month experiment consisted of 20 broadband seismic stations deployed on islands and 58 semi-broadband ocean bottom seismographs deployed across the forearc, island arc, and back-arc spreading center. We determine Vp and Vp/Vs structure on a three dimensional grid using over 25,000 local travel time observations as well as over 2000 teleseismic arrival times determined by waveform cross correlation. The mantle wedge is characterized by a region of low velocity and high Vp/Vs beneath the forearc, an inclined zone of low velocity underlying the volcanic front, and a broad region of low velocity beneath the back-arc spreading center. The slow velocity anomalies are strongest at roughly 20-30 km depth in the forearc, 60-70 km depth beneath the volcanic arc, and 20-30 km beneath the back-arc spreading center. The slow velocity anomalies beneath the arc and back-arc appear as separate and distinct features in our images, with a small channel of connectivity occurring at approximately 75 km depth. The subducting Pacific plate is characterized by high seismic velocities. An exception occurs in the forearc beneath the big blue seamount and at the top of the slab at roughly 80 km depth where slow velocities are observed. We interpret the forearc anomalies to represent a region of large scale serpentinization of the mantle whereas the arc and back-arc anomalies represent regions of high temperature with a small amount of increased water content and/or melt and constrain the source regions in the mantle for arc and back-arc lavas. We investigate the double seismic zone (dsz) beneath the Central Mariana Arc using data from a land-sea array of 58 ocean bottom seismographs and 20 land seismographs deployed during 2003-2004. Nearly 600 well-recorded earthquakes were located using a P and S wave arrival times and a double difference relocation technique. The double seismic zone is well imaged from the forearc region to a depth of nearly 200 km. The width of the dsz is approximately 30 km at shallow depths and gradually becomes narrower with depth until it is now longer resolvable at depths greater than 180-200 km. Focal mechanisms determined from P and S wave polarities and amplitudes indicate that events from 70-150 km depth show along strike extension, whereas events greater than 150 km show downdip extension. Both the upper and lower zones of the dsz show similar focal mechanisms, demonstrating that the dsz is not caused by bending or unbending stresses. Along-strike tension may result from stresses related to the increasing curvature of the Mariana slab over the past few million years, as indicated by plate reconstructions. Downdip extension may result from slab pull forces consistent with the strong density anomaly of an old, cold plate relative to the surrounding mantle.

Deformation Mechanism on the Northern Margin of the Tibetan Plateau Inferred from Magnetotelluric Data

NASA Astrophysics Data System (ADS)

Zhang, L.; Jin, S.; Wei, W.; Ye, G.; Xie, C.

2017-12-01

As a unique geologic unit on the northern margin of the Tibetan Plateau, the Qaidam Basin plays a significant role in constraining the vertical uplift and horizontal expansion of the plateau. However, deformation mechanism of the lithosphere beneath the Qaidam Basin is still highly debated. To better understand the lithospheric electrical structure and deformation mechanism of the Qaidam Basin, A 250 km long, NE-SW directed Magnetotelluric (MT) profile was finished in the northern portion of the Basin, which is roughly perpendicular to the thrust fault systems on the western and eastern margins of the Basin. The profile consists of 20 broad-band MT stations and 5 long-period MT stations. Original time series data is processed with regular robust routines. Dimensionality and regional strike direction are determined for the dataset through data analysis. 2D inversions were performed to produce a preferred model of the lithospheric electrical structure. Uncertainty analysis of the 2D inversion model was also conducted based on a data resampling approach. The outcome 2D electrical model was further used to estimate the distribution of temperature and melt fraction in the upper mantle based on laboratory-determined relationships between the electrical conductivity and temperature of nominally anhydrous minerals and basaltic melt by using the mixing law of Hashin-Shtrikman's bounds. These results suggest that: (1) the crust-mantle boundary is imaged as a conductive layer beneath the western Qaidam Basin, with its temperature estimated to be 1200-1300 ° and melt fraction 5-8%, indicating decoupling deformation of the crust and upper mantle. (2) A large-scale east-dipping conductor is imaged beneath the eastern Qaidam Basin extending from the upper crust to upper mantle, implying vertical coherent deformation of the lithosphere. Melt fraction of this conductive region is estimated to be as high as 10%, which might accommodates a major portion of the thrust deformation on the basin boundary. (3) Decoupling deformation and vertical coherent deformation are both active on the northern margin of the Tibetan Plateau, and both play significant roles in controlling the uplift and expansion of the northern Tibetan Plateau. *This work is funded by National Natural Science Foundation of China (41404060, 41404059).

Teleseismic P-wave Tomography and Mantle Dynamics beneath Eastern Tibet: Insight into Tengchong Volcano and Large Earthquakes

NASA Astrophysics Data System (ADS)

Lei, J., Sr.; Zhao, D.

2016-12-01

We determined a new 3-D P-wave velocity model of the upper mantle beneath eastern Tibet using 112,613 high-quality arrival-time data collected from teleseismic seismograms recorded by a new portable seismic array in Yunnan and permanent networks in southwestern China. Our results provide new insights into the mantle structure and dynamics of eastern Tibet. High-velocity (high-V) anomalies are revealed down to 200 km depth under the Sichuan basin and the Ordos and Alashan blocks. Low-velocity (low-V) anomalies are imaged in the upper mantle under the Kunlun-Qilian and Qinling fold zones, and the Songpan-Ganzi, Qiangtang, Lhasa and Chuan-Dian diamond blocks, suggesting that eastward moving low-V materials are extruded to eastern China after the obstruction by the Sichuan basin, and the Ordos and Alashan blocks. Furthermore, the extent and thickness of these low-V anomalies are correlated with the surface topography, suggesting that the uplift of eastern Tibet could be partially related to these low-V materials having a higher temperature and strong positive buoyancy. In the mantle transition zone (MTZ), broad high-V anomalies are visible from the Burma arc northward to the Kunlun fault and eastward to the Xiaojiang fault, and they are connected upward with the Wadati-Benioff seismic zone. These results suggest that the subducted Indian slab has traveled horizontally for a long distance after it descended into the MTZ, and return corner flow and deep slab dehydration have contributed to forming the low-V anomalies in the big mantle wedge. Our results shed new light on the deep origin of Tengchong volcano and large crustal earthquakes as well as the mantle dynamics of the eastern Tibetan plateau.

P and S velocity structure of the crust and the upper mantle beneath central Java from local tomography inversion

NASA Astrophysics Data System (ADS)

Koulakov, I.; Bohm, M.; Asch, G.; Lühr, B.-G.; Manzanares, A.; Brotopuspito, K. S.; Fauzi, Pak; Purbawinata, M. A.; Puspito, N. T.; Ratdomopurbo, A.; Kopp, H.; Rabbel, W.; Shevkunova, E.

2007-08-01

Here we present the results of local source tomographic inversion beneath central Java. The data set was collected by a temporary seismic network. More than 100 stations were operated for almost half a year. About 13,000 P and S arrival times from 292 events were used to obtain three-dimensional (3-D) Vp, Vs, and Vp/Vs models of the crust and the mantle wedge beneath central Java. Source location and determination of the 3-D velocity models were performed simultaneously based on a new iterative tomographic algorithm, LOTOS-06. Final event locations clearly image the shape of the subduction zone beneath central Java. The dipping angle of the slab increases gradually from almost horizontal to about 70°. A double seismic zone is observed in the slab between 80 and 150 km depth. The most striking feature of the resulting P and S models is a pronounced low-velocity anomaly in the crust, just north of the volcanic arc (Merapi-Lawu anomaly (MLA)). An algorithm for estimation of the amplitude value, which is presented in the paper, shows that the difference between the fore arc and MLA velocities at a depth of 10 km reaches 30% and 36% in P and S models, respectively. The value of the Vp/Vs ratio inside the MLA is more than 1.9. This shows a probable high content of fluids and partial melts within the crust. In the upper mantle we observe an inclined low-velocity anomaly which links the cluster of seismicity at 100 km depth with MLA. This anomaly might reflect ascending paths of fluids released from the slab. The reliability of all these patterns was tested thoroughly.

Radial anisotropy of the North American upper mantle based on adjoint tomography with USArray

NASA Astrophysics Data System (ADS)

Zhu, Hejun; Komatitsch, Dimitri; Tromp, Jeroen

2017-10-01

We use seismic data from USArray to image the upper mantle underneath the United States based on a so-called `adjoint tomography', an iterative full waveform inversion technique. The inversion uses data from 180 regional earthquakes recorded by 4516 seismographic stations, resulting in 586 185 frequency-dependent measurements. Three-component short-period body waves and long-period surface waves are combined to simultaneously constrain deep and shallow structures. The transversely isotropic model US22 is the result of 22 pre-conditioned conjugate-gradient iterations. Approximate Hessian maps and point-spread function tests demonstrate good illumination of the study region and limited trade-offs among different model parameters. We observe a distinct wave-speed contrast between the stable eastern US and the tectonically active western US. This boundary is well correlated with the Rocky Mountain Front. Stable cratonic regions are characterized by fast anomalies down to 250-300 km, reflecting the thickness of the North American lithosphere. Several fast anomalies are observed beneath the North American lithosphere, suggesting the possibility of lithospheric delamination. Slow wave-speed channels are imaged beneath the lithosphere, which might indicate weak asthenosphere. Beneath the mantle transition zone of the central US, an elongated north-south fast anomaly is observed, which might be the ancient subducted Farallon slab. The tectonically active western US is dominated by prominent slow anomalies with magnitudes greater than -6 per cent down to approximately 250 km. No continuous lower to upper mantle upwellings are observed beneath Yellowstone. In addition, our results confirm previously observed differences between oceans and continents in the anisotropic parameter ξ = (βh/βv)2. A slow wave-speed channel with ξ > 1 is imaged beneath the eastern Pacific at depths from 100 to 200 km, reflecting horizontal shear within the asthenosphere. Underneath continental areas, regions with ξ > 1 are imaged at shallower depths around 100 km. They are characterized by fast shear wave speeds, suggesting different origins of anisotropy underneath oceans and continents. The wave speed and anisotropic signatures of the western Atlantic are similar to continental areas in comparison with the eastern Pacific. Furthermore, we observe regions with ξ < 1 beneath the tectonically active western US at depths between 300 and 400 km, which might reflect vertical flows induced by subduction of the Farallon and Juan de Fuca Plates. Comparing US22 with several previous tomographic models, we observe relatively good correlations for long-wavelength features. However, there are still large discrepancies for small-scale features.

Attenuation tomography in the western central Andes: A detailed insight into the structure of a magmatic arc

NASA Astrophysics Data System (ADS)

Haberland, Christian; Rietbrock, Andreas

2001-06-01

High-quality data from 1498 local earthquakes recorded by the PISCO '94 (Proyecto de Investigatión Sismológica de la Cordillera Occidental, 1994) and ANCORP '96 (Andean Continental Research Project, 1996) temporary seismological networks allowed the detailed determination of the three-dimensional (3-D) attenuation structure (Qp-1) beneath the recent magmatic arc in the western central Andes (20° to 24°S). Assuming a frequency-independent Qp-1 in a frequency band between 1 and 30 Hz, whole path attenuation (t*) was estimated from the amplitude spectra of the P waves using spectral ratios and a spectral inversion technique. The damped least squares inversion (tomography) of the data reveals a complex attenuation structure. Crust and mantle of the forearc and subducting slab are generally characterized by low attenuation (Qp > 1000). Crust and mantle beneath the magmatic arc show elevated attenuation. The strongest anomaly of extremely low Qp is found in the crust between 22° and 23°S beneath the recent volcanic arc (Qp < 100). N-S variations can be observed: The western flank of the crustal attenuation anomaly follows the curved course of the volcanic front. North of 21°S the attenuation is less developed. In the northern part of the study area the low-Qp zone penetrates in the forearc mantle down to the subducting slab. In the south a deeper zone of high attenuation is resolved between 23° and 24°S directly above the subducting slab. Low Qp in the mantle correlates with earthquake clusters. The strong crustal attenuation is confined to the distribution of young ignimbrites and silicic volcanism and is interpreted as a thermally weakened zone with partial melts. The attenuation pattern in the upper mantle might reflect the variable extent of the asthenosphere and maps variations of subduction-related hydration processes in the mantle wedge from slab-derived fluids.

Structure of the crust and upper mantle beneath the Balearic Islands (Western Mediterranean)

NASA Astrophysics Data System (ADS)

Banda, E.; Ansorge, J.; Boloix, M.; Córdoba, D.

1980-09-01

Data are presented from deep seismic sounding along the strike of the Balearic Islands carried out in 1976. The interpretation of the data gives the following results: A sedimentary cover of 4 km around Ibiza to 7 km under Mallorca overlies the crystalline basement. This basement with a P-wave velocity of 6.0 km/s at the top reaches a depth of at least 15 km under Ibiza and 17 km under Mallorca with an increase to 6.1 km/s at these depths. The crust-mantle boundary lies at a depth of 20 km and 25 km, respectively. A well documented upper-mantle velocity of 7.7 km/s is found along the entire profile. The Moho rises to a depth of 20 km about 30 km north of Mallorca and probably continues rising towards the center of the North Balearic Sea. The newly deduced crustal structure together with previously determined velocity-depth sections in the North Balearic Sea as well as heat flow and aeromagnetic data can be interpreted as an extended rift structure caused by large-scale tensional processes in the upper mantle. The available data suggest that the entire zone from the eastern Alboran Sea to the area north of the Balearic Islands represents the southeastern flank of this rift system. In this model the provinces of Spain along the east coast would represent the northwestern rift flank.

Os and HSE of the hot upper mantle beneath southern Tibet: Indian mantle affinity?

NASA Astrophysics Data System (ADS)

Zhao, Z.; Dale, C. W.; Pearson, D. G.; Niu, Y.; Zhu, D.; Mo, X.

2011-12-01

The subduction of the Indian plate (including cratonic continental crust and/or upper mantle) beneath southern Tibet is widely accepted from both geological and geophysical studies. Mantle-derived xenoliths from this region provide a means of directly investigating the mantle underlying the southern part of the plateau. Studies of xenoliths hosted in the Sailipu ultrapotassic volcanic rocks, erupted at ~17 Ma, have indicated that the subcontinental mantle of southern Tibetan Plateau is hot and strongly influenced by metasomatism (Zhao et al., 2008a, b; Liu et al., 2011). Here we report comprehensive EPMA and LA-ICP-MS major and trace element data for the Sailipu xenoliths and also whole rock Os isotope and HSE data in order to constrain the depletion history of the mantle and to identify the presence of any potential Indian cratonic mantle. The xenoliths, ranging in size from 0.5cm to 1.5cm in diameter, are mostly peridotites. The calculated temperatures are 1010-1230°C at the given pressures of ~1.6-2.0 GPa (n=47). These P-T conditions are similar to rift-related upper mantle regimes (e.g. Kenya), indicating the influence of regional extension beneath southern Tibet in the Miocene. A series of compositional discriminations for minerals (Cpx, Opx, Ol, and Phl), e.g. Fo<90, suggest that the xenoliths are non-cratonic spinel-peridotite (cratonic peridotite olivine Fo> ~91), with a clear metasomatic signature We obtained Os isotope data and abundances of highly siderophile elements (HSE, including Os, Ir, Ru, Pt, Pd and Re) on a set of six olivine-dominated peridotite samples from Sailipu volcanics, less than 1 cm in dimension. They allow us to further constrain the nature and state of the upper mantle beneath the southern Tibet. Sailipu samples display low total HSE abundances (Os+Ir+Ru+Pt+Pd+Re) ranging from 8.7 to 25 ppb, with nearly constant Os, Ir , and Ru, but rather varied Pt (2-13), Pd (0.4-5.2), and Re (0.01-0.5). Chondrite-normalised Pd/Ir ratios range from 0.2 to 2.4 reflecting significant metasomatism of some samples. The xenoliths exhibit 187Os/188Os ratios of 0.12213-0.12696, corresponding to γOs ranging from -4.2 to -0.4 - much higher than ancient cratonic mantle. Thus, on the basis of mineral chemistry and whole rock Os isotopes, Indian cratonic mantle is absent from our suite of xenoliths. Therefore, assuming the presence of cratonic mantle, it seems likely that the xenoliths do not sample the deep basal section of the lithosphere where cratonic Indian lithosphere is thought to be present under southern Tibet. In which case, testing of the seismic and tectonic models may not be possible without garnet-facies peridotites. More work need to be done to further reveal the mantle compostion and mantle dynamics beneath Tibet. [Financially supported by the National Key Project for Basic Research of China (Project 2011CB403102 and 2009CB421002)]. [1] Zhao Z, et al., 2008a. Acta Petrologica Sinica, 24 (2): 193-202 [2] Zhao Z, et al., 2008b. Geochimica et Cosmochimica Acta, 72, 12 (Supp.): A1095 [3] Liu C-Z, et al., 2011, Geology, in press

The shear-wave splitting in the crust and the upper mantle around the Bohai Sea, North China

NASA Astrophysics Data System (ADS)

Yutao, Shi; Yuan, Gao; Lingxue, Tai; Yuanyuan, Fu

2015-11-01

In order to infer the distribution of local stress and the deep geodynamic process in North China, this study detects seismic anisotropy in the crust and upper mantle beneath the Bohai Sea area. A total of 535 local shear-wave and 721 XKS (including SKS, PKS and SKKS phases) splitting measurements were obtained from stations in permanent regional seismograph networks and a temporary seismic network called ZBnet-E. The dominant fast polarization orientation of local shear-waves in the crust is nearly East-West, suggesting an East-West direction of local maximum compressive stress in the area. Nearly North-South fast orientation was obtained at some stations in the Tan-Lu fault belt and the Zhang-Bo seismic belt. The average fast orientation from XKS splitting analysis is 87.4° measured clockwise from the North. The average time-delays of XKS splitting are range from 0.54 s to 1.92 s, corresponding to a 60-210 km thick layer of anisotropy. The measured results indicate that upper mantle anisotropy beneath Bohai Sea area, even the eastern part of North China, is mainly from asthenospheric mantle flow from the subduction of the Pacific plate. From the complicated anisotropic characteristics in this study, we infer that there might be multiple mechanisms in the crust and upper mantle around the Bohai Sea area that led to the observed anisotropy.

Subduction of the Indian lithosphere beneath Tibet and deformation of the Tibetan crust and mantle, imaged with broad-band surface waves

NASA Astrophysics Data System (ADS)

Agius, Matthew R.; Lebedev, Sergei

2013-04-01

Seismic deployments over the last two decades have produced dense broadband data coverage across the Tibetan Plateau. Yet, the lithospheric dynamics of Tibet remains enigmatic, with even its basic features debated and with very different end-member models still advocated today. Most body-wave tomographic models do not resolve any high-velocity anomalies in the upper mantle beneath central and northern Tibet, which motivated the inference that the Indian lithosphere may sink into deep mantle beneath the Himalayas in the south, with parts of it possibly extruded laterally eastward. In contrast, surface-wave tomographic models all show pronounced high-velocity anomalies beneath much of Tibet at depths around 200 km. Uncertainties over the shapes and amplitudes of the anomalies, however, contribute to the uncertainty of their interpretations, ranging from the subduction of India or Asia to the extreme viscous thickening of the Tibetan lithosphere. Within the lithosphere itself, a low-viscosity layer in the mid-lower crust is evidenced by many observations. It is still unclear, however, whether this layer accommodates a large-scale channel flow (which may have uplifted eastern Tibet, according to one model) or if, instead, deformation within it is similar to that observed at the surface. Broad-band surface waves provide resolving power from the upper crust down to the asthenosphere, for both the isotropic-average shear-wave speeds (characterising the composition and thermal state of the lithosphere) and the radial and azimuthal shear-wave anisotropy (indicative, in an actively deforming region, of the current and recent flow). We measured highly accurate Love- and Rayleigh-wave phase-velocity curves in broad period ranges (up to 5-200 s) for a few tens of pairs and groups of stations across Tibet, combining, in each case, hundreds to thousands of inter-station measurements made with cross-correlation and waveform-inversion methods. Robust shear-velocity profiles were then determined by extensive series of non-linear inversions, designed to constrain the depth-dependent ranges of isotropic-average shear speeds and radial anisotropy consistent with the data. Temperature anomalies in the upper mantle were estimated from shear-velocity using pre-computed petro-physical relationships. Azimuthal anisotropy in the crust and upper mantle was determined by surface-wave tomography and, also, by sub-array analysis targeting the anisotropy amplitude. Our results show that the prominent high-velocity anomalies in the upper mantle are most consistent with the presence of subducted Indian lithosphere beneath much of Tibet. The large estimated thermal anomalies within the high-velocity features match those to be expected within subducted India. The morphology of India's subduction beneath Tibet is complex and shows pronounced west-east variations. Beneath eastern and northeastern Tibet, in particular, the subducted Indian lithosphere appears to have subducted, at a shallow angle, hundreds of km NNE-wards. Azimuthal anisotropy beneath Tibet is distributed in multiple layers with different fast-propagations directions, which accounts for the complexity of published shear-wave splitting observations. The fast directions within the mid-lower crust are parallel to the extensional components of the current strain rate field at the surface, consistent with similar deformation through the entire ­crust, rather than channel flow. Anisotropy within the asthenosphere beneath northeastern Tibet (sandwiched between the Tibetan lithosphere above and the subducted Indian lithosphere below) indicates SSW-NNE flow, parallel to the direction of motion of the Indian Plate, including its subducted leading edge.

Comparison of the Lithospheric Structure Beneath Kenya and Ethiopia From Joint Inversion of Receiver Functions and Rayleigh Wave Dispersion Velocities

NASA Astrophysics Data System (ADS)

Dugda, M. T.; Nyblade, A. A.; Julia, J.

2007-12-01

Shear-wave velocity structure of the crust and upper mantle beneath Kenya has been investigated using joint inversion of receiver functions, and Rayleigh wave group and phase velocities. Most of the data for this study come from the Kenya broadband seismic experiment, conducted between 2001 and 2002. Shear velocity models obtained from the joint inversion show crustal thicknesses of 37 to 42 km beneath the East African Plateau in Kenya and near the edge of the Kenya Rift, and a crustal thickness of about 30 km beneath the Kenya Rift. These crustal parameters are consistent with crustal thicknesses published previously by different authors. A comparison has been made between the lithosphere under Kenya and other parts of the East African Plateau in Tanzania. A comparison between the lithosphere under Kenya and that under Ethiopia has also been made, specifically between the lithosphere under the Ethiopian Plateau and the Kenya Plateau, and between the lithosphere beneath the Main Ethiopian Rift (MER) and the Kenya (Gregory) Rift. The lithospheric mantle beneath the East African Plateau in Kenya has a maximum shear wave velocity of about 4.6 km/s, similar to the value obtained under the East African Plateau in Tanzania. Beneath the Kenya Rift, the lithosphere extends to a depth of at most ~75 km. The average velocity of the mantle lithosphere under the East African Plateau in Kenya appears to be similar to the lithosphere under Tanzania away from the East African Rift System. The lithosphere under the Kenya Plateau is not perturbed as compared to the highly perturbed lithosphere beneath the Ethiopian Plateau. The lithosphere under the Kenya Rift is perturbed as compared to the rest of the region but is not as perturbed as that under the Main Ethiopian Rift or the Afar. Though Kenya and Ethiopia have similar uplift, volcanism and rifting at the surface, they have different lithospheric structures at the bottom. The Afar Flood Basalt Volcanism (AFB) may be the cause of this striking difference in the two lithosphere.

Mantle transition zone discontinuities beneath the Tien Shan

NASA Astrophysics Data System (ADS)

Yu, Youqiang; Zhao, Dapeng; Lei, Jianshe

2017-10-01

To better understand geodynamic processes of intracontinental mountain building, we conduct a systematic investigation of the mantle transition zone (MTZ) beneath the Tien Shan and its surrounding areas using a receiver function method under non-plane wave front assumption. The resulting apparent depths of the 410 km (d410) and 660 km (d660) discontinuities and the MTZ thickness display significant lateral variations. Both the central Tien Shan and the Pamir Plateau are characterized by a thick MTZ, which can be well explained by the existence of lithospheric segments resulted from possible break-off of the subducted slab or lithosphere delamination. A thin MTZ and an obviously depressed d410, which may be induced by asthenosphere upwelling associated with the dropping lithospheric segment, are revealed beneath the Kazakh Shield. Seismic evidence is obtained for the potential existence of lower mantle upwelling beneath the Tarim Basin based on the observed thin MTZ and relatively significant uplift of d660. The subduction of the Kazakh Shield and Tarim lithosphere driven by the India-Eurasia collision possibly plays an essential role in the formation and evolution of the Tien Shan orogenic belt, and the lower mantle upwelling revealed beneath the Tarim Basin may promote the uplift of the Tien Shan by softening the upper mantle.

Global Discontinuity Structure of the Mantle Transition Zone from Finite-Frequency Tomography of SS Precursors

NASA Astrophysics Data System (ADS)

Guo, Z.; Zhou, Y.

2017-12-01

We report global structure of the 410-km and 660-km discontinuities from finite-frequency tomography using frequency-dependent traveltime measurements of SS precursors recorded at the Global Seismological Network (GSN). Finite-frequency sensitivity kernels for discontinuity depth perturbations are calculated in the framework of traveling-wave mode coupling. We parametrize the global discontinuities using a set of spherical triangular grid points and solve the tomographic inverse problem based on singular value decomposition. Our global 410-km and 660-km discontinuity models reveal distinctly different characteristics beneath the oceans and subduction zones. In general, oceanic regions are associated with a thinner mantle transition zone and depth perturbations of the 410-km and 660-km discontinuities are anti-correlated, in agreement with a thermal origin and an overall warm and dry mantle beneath the oceans. The perturbations are not uniform throughout the oceans but show strong small-scale variations, indicating complex processes in the mantle transition zone. In major subduction zones (except for South America where data coverage is sparse), depth perturbations of the 410-km and 660-km discontinuities are correlated, with both the 410-km and the 660-km discontinuities occurring at greater depths. The distributions of the anomalies are consistent with cold stagnant slabs just above the 660-km discontinuity and ascending return flows in a superadiabatic upper mantle.

Preliminary study of lateral variation in crustal structure of Northeast China from teleseismic receiver functions

NASA Astrophysics Data System (ADS)

Chen, Youlin; Liu, Ruifeng; Huang, Zhibin; Sun, Li

2011-02-01

We conducted comprehensive receiver function analyses for a large amount of high-quality broadband teleseismic waveforms data recorded at 19 China National Digital Seismic Network (CNDSN) stations deployed in Northeast China. An advanced H- κ domain search method was adopted to accurately estimate the crustal thickness and ν P/ ν S ratio. The crust has an average thickness of about 34.4 km. The thinnest crust occurs in the central region of Northeast China, while the thickest crust is beneath the Yanshan belt. The ν P/ ν S ratio is relatively uniform with an average of about 1.733. The highest ν P/ ν S ratio is found beneath the Changbaishan, likely associated with its volcanic activities. We found significant lateral heterogeneity beneath three stations CN2, MDJ, and MIH located along the Suolon suture from the back-zimuthal dependence of Moho depth. The velocity modeling from receiver functions indicated complicated Earth structure beneath these stations with large crust-mantle transition zone, noticeable velocity jump in upper mantle, and low velocity zone in middle crust. Dipping velocity interface in the crust with strike approximately parallel to the Suolon suture and down-dip to the south or southeast might explain the observed lateral heterogeneity.

Seismic Imaging of the crust and upper mantle beneath Afar, Ethiopia

NASA Astrophysics Data System (ADS)

Hammond, J. O.; Kendall, J. M.; Stuart, G. W.; Ebinger, C. J.

2009-12-01

In March 2007 41 seismic stations were deployed in north east Ethiopia. These stations recorded until October 2009, whereupon the array was condensed to 13 stations. Here we show estimates of crustal structure derived from receiver functions and upper mantle velocity structure, derived from tomography and shear-wave splitting using the first 2.5 years of data. Bulk crustal structure has been determined by H-k stacking receiver functions. Crustal Thickness varies from ~45km on the rift margins to ~16km beneath the northeastern Afar stations. Estimates of Vp/Vs show normal continental crust values (1.7-1.8) on the rift margins, and very high values (2.0-2.2) in Afar, similar to results for the Main Ethiopian Rift (MER). This supports ideas of high levels of melt in the crust beneath the Ethiopian Rift. Additionally, we use a common conversion point migration technique to obtain high resolution images of crustal structure beneath the region. Both techniques show a linear region of thin crust (~16km) trending north-south, the same trend as the Red Sea rift. SKS-wave splitting results show a general north east-south west fast direction in the MER, systematically rotating to a more north-south fast direction towards the Red Sea. Additionally, stations close to the recent Dabbahu diking episode show sharp lateral changes over small lateral distances (40° over <30km), with fast directions overlying the Dabbahu segment aligning parallel with the recent diking. This supports ideas of melt dominated anisotropy beneath the Ethiopian rift. The magnitude of splitting in this region is smaller than that seen at the MER, suggesting a thinner region of melt, or less focused melt is causing the anisotropy. Seismic tomography inversions show that in the top 150km low velocities highlight plate boundaries. The low velocity anomalies extend from the main Ethiopian rift NE, towards Djibouti, and from Djibouti NW towards the Dabbahu segment The lowest velocities exist on the rift margins, supporting ideas of preferential melt generation at these regions of high strain. This includes a region of low velocity close to the edge of the proposed location of the Danakil microplate. Outside of these focused regions the velocities are relatively fast. Below ~250km the anomaly broadens to cover most of the Afar region with only the rift margins remaining fast. At transition zone depths little anomaly is seen beneath Afar, but some low velocities remain present beneath the MER. These studies suggest that in northern Ethiopia the Red Sea rift is dominant. The presence of thin crust beneath northern Afar suggests that the Red Sea rift is creating oceanic like crust in this region. The lack of deep mantle low velocity anomalies beneath Afar suggest that a typical narrow conduit plume does not exist in this region, rather the velocity models seem more similar to passive upwelling of material beneath Afar.

Deep structure of the Alborz Mountains by joint inversion of P receiver functions and dispersion curves

NASA Astrophysics Data System (ADS)

Rastgoo, Mehdi; Rahimi, Habib; Motaghi, Khalil; Shabanian, Esmaeil; Romanelli, Fabio; Panza, Giuliano F.

2018-04-01

The Alborz Mountains represent a tectonically and seismically active convergent boundary in the Arabia - Eurasia collision zone, in western Asia. The orogenic belt has undergone a long-lasted tectono-magmatic history since the Cretaceous. The relationship between shallow and deep structures in this complex tectonic domain is not straightforward. We present a 2D velocity model constructed by the assemblage of 1D shear wave velocity (Vs) models from 26 seismic stations, mainly distributed along the southern flank of the Alborz Mountains. The shear wave velocity structure has been estimated beneath each station using joint inversion of P-waves receiver functions and Rayleigh wave dispersion curves. A substantiation of the Vs inversion results sits on the modeling of Bouguer gravity anomaly data. Our velocity and density models show low velocity/density anomalies in uppermost mantle of western and central Alborz at a depth range of ∼50-100 km. In deeper parts of the uppermost mantle (depth range of 100-150 km), a high velocity/density anomaly is located beneath most of the Mountain range. The spatial pattern of these low and high velocity/density structures in the upper mantle is interpreted as the result of post collisional delamination of lower part of the western and central Alborz lithosphere.

Crustal evolution derived from the Izu-Bonin-Mariana arc velocity images

NASA Astrophysics Data System (ADS)

Takahashi, N.; Kodaira, S.; Tatsumi, Y.; Miura, S.; Sato, T.; Yamashita, M.; No, T.; Takahashi, T.; Noguchi, N.; Takizawa, K.; Kaiho, Y.; Kaneda, Y.

2010-12-01

The Izu-Bonin-Mariana arc is known as one of typical oceanic island arcs, which has developed by subduction between oceanic crusts producing continental materials. Japan Agency for Marine-Earth Science and Technology has carried out seismic surveys using a multi-channel reflection survey system (MCS) and ocean bottom seismographs (OBSs) in the Izu-Bonin-Mariana (IBM) arc since 2002, and reported these crustal images. As the results, we identified the structural characteristics of whole Izu-Bonin-Mariana arc. Rough structural characteristics are, 1) middle crust with Vp of 6 km/s, 2) upper part of the lower crust with Vp of 6.5-6.8 km/s, 3) lower part of the lower crust with Vp of 6.8-7.5 km/s, and 4) lower mantle velocity beneath the arc crusts. In addition, structural variation along the volcanic front, for example, thickness variation of andesitic layers was imaged and the distributions is consistent with those of rhyolite volcanoes, that is, it suggested that the cause the structural variation is various degree of crustal growth (Kodaira et al., 2007). Moreover, crustal thinning with high velocity lower crust across arc was also imaged, and it is interpreted that such crust has been influenced backarc opening (Takahashi et al., 2009). According to Tatsumi et al. (2008), andesitic middle crust is produced by differentiation of basaltic lower crust and a part of the restites are transformed to the upper mantle. This means that region showing much crustal differentiation has large volume of transformation of dense crustal materials to the mantle. We calculated volume profiles of the lower crust along all seismic lines based on the petrologic model, and compared them with observed real volumes obtained by seismic images. If the real volume of the lower crust is large, it means that the underplating of dense materials to the crustal bottom is dominant rather than transformation of dense materials to the upper mantle. According to obtained profiles to judge if the region is the transformation dominant or underplating, the transformation dominant regions are located along the volcanic front, the remnant arc for the incipient rifting like the Sumisu Rift just behind the volcanic front, rear arc regions, and fore-arc basins. Beneath the fore-arc basins, multiple rows showing transformation dominant distribute, and it extends from north to south around the Ogasawara Trough. On the other hand, the underplating dominant regions distribute between the volcanic front and the rear arc region, beneath the incipient rift, and between the multiple rows beneath the fore-arc basins. These locations showing underplating dominant are consistent with those with high velocity lower crust.

Composition of the crust beneath the Kenya rift

USGS Publications Warehouse

Mooney, W.D.; Christensen, N.I.

1994-01-01

We infer the composition of the crust beneath and on the flanks of the Kenya rift based on a comparison of the KRISP-90 crustal velocity structure with laboratory measurements of compressional-wave velocities of rock samples from Kenya. The rock samples studied, which are representative of the major lithologies exposed in Kenya, include volcanic tuffs and flows (primarily basalts and phonolites), and felsic to intermediate composition gneisses. This comparison indicates that the upper crust (5-12 km depth) consists primarily of quartzo-feldspathic gneisses and schists similar to rocks exposed on the flanks of the rift, whereas the middle crust (12-22 km depth) consists of more mafic, hornblende-rich metamorphic rocks, probably intruded by mafic rocks beneath the rift axis. The lower crust on the flanks of the rift may consist of mafic granulite facies rocks. Along the rift axis, the lower crust varies in thickness from 9 km in the southern rift to only 2-3 km in the north, and has a seismic velocity substantially higher than the samples investigated in this study. The lower crust of the rift probably consists of a crust/mantle mix of high-grade metamorphic rocks, mafic intrusives, and an igneous mafic residuum accreted to the base of the crust during differentiation of a melt derived from the upper mantle. ?? 1994.

Lithospheric shear velocity structure of South Island, New Zealand, from amphibious Rayleigh wave tomography

NASA Astrophysics Data System (ADS)

Ball, Justin S.; Sheehan, Anne F.; Stachnik, Joshua C.; Lin, Fan-Chi; Yeck, William L.; Collins, John A.

2016-05-01

We present a crust and mantle 3-D shear velocity model extending well offshore of New Zealand's South Island, imaging the lithosphere beneath the South Island as well as the Campbell and Challenger Plateaus. Our model is constructed via linearized inversion of both teleseismic (18-70 s period) and ambient noise-based (8-25 s period) Rayleigh wave dispersion measurements. We augment an array of 4 land-based and 29 ocean bottom instruments deployed off the South Island's east and west coasts in 2009-2010 by the Marine Observations of Anisotropy Near Aotearoa experiment with 28 land-based seismometers from New Zealand's permanent GeoNet array. Major features of our shear wave velocity (Vs) model include a low-velocity (Vs < 4.4 km/s) body extending from near surface to greater than 75 km depth beneath the Banks and Otago Peninsulas and high-velocity (Vs~4.7 km/s) mantle anomalies underlying the Southern Alps and off the northwest coast of the South Island. Using the 4.5 km/s contour as a proxy for the lithosphere-asthenosphere boundary, our model suggests that the lithospheric thickness of Challenger Plateau and central South Island is substantially greater than that of the inner Campbell Plateau. The high-velocity anomaly we resolve at subcrustal depths (>50 km) beneath the central South Island exhibits strong spatial correlation with upper mantle earthquake hypocenters beneath the Alpine Fault. The ~400 km long low-velocity zone we image beneath eastern South Island and the inner Bounty Trough underlies Cenozoic volcanics and the locations of mantle-derived helium measurements, consistent with asthenospheric upwelling in the region.

Transient rheology of the uppermost mantle beneath the Mojave Desert, California

USGS Publications Warehouse

Pollitz, F.F.

2003-01-01

Geodetic data indicate that the M7.1 Hector Mine, California, earthquake was followed by a brief period (a few weeks) of rapid deformation preceding a prolonged phase of slower deformation. We find that the signal contained in continuous and campaign global positioning system data for 2.5 years after the earthquake may be explained with a transient rheology. Quantitative modeling of these data with allowance for transient (linear biviscous) rheology in the lower crust and upper mantle demonstrates that transient rheology in the upper mantle is dominant, its material properties being typified by two characteristic relaxation times ???0.07 and ???2 years. The inferred mantle rheology is a Jeffreys solid in which the transient and steady-state shear moduli are equal. Consideration of a simpler viscoelastic model with a linear univiscous rheology (2 fewer parameters than a biviscous model) shows that it consistently underpredicts the amplitude of the first ???3 months signal, and allowance for a biviscous rheology is significant at the 99.0% confidence level. Another alternative model - deep postseismic afterslip beneath the coseismic rupture - predicts a vertical velocity pattern opposite to the observed pattern at all time periods considered. Despite its plausibility, the advocated biviscous rheology model is non-unique and should be regarded as a viable alternative to the non-linear mantle rheology model for governing postseismic flow beneath the Mojave Desert. Published by Elsevier B.V.

Variations in the mantle transition zone beneath the Ethiopian Rift and Afar

NASA Astrophysics Data System (ADS)

Cornwell, D. G.; Hetenyi, G.; Blanchard, T.; Stuart, G. W.

2010-12-01

We use receiver functions calculated on broadband seismological data across Ethiopia to identify and map 3-D changes in the mantle transition zone (MTZ) thickness beneath the Ethiopian rift, Afar and the uplifted Ethiopian Plateau. The MTZ that divides the upper and lower mantle in the Earth is marked by discontinuities whose position and nature is controlled by local temperature and composition. It is commonly assumed that positive temperature anomalies cause an overall thinning of the MTZ by deepening the mineral phase transition of olivine (α-spinel) to wadsleyite (β-spinel) at around 410 km depth and shallowing the mineral phase transition of ringwoodite (γ-spinel) to magnesiowustite-perovskite at around 660 km depth. Such regions of anomalously hot mantle have been interpreted to extend from the core-mantle boundary (e.g. the African Superplume) to the Earth's surface from global tomographic models. Previous studies in Ethiopia or Afar that invoke receiver functions are mainly restricted to illuminating the MTZ beneath permanent seismological stations and, together with a regional receiver function study, all have found difficulty in imaging the discontinuities. They were unable to provide conclusive evidence for a thinned transition zone and could not constrain lateral changes in MTZ thickness that are required to assess whether the African Superplume intersects the MTZ beneath Ethiopia. We use seismological data from permanent stations as well as from four temporary arrays to compute receiver functions. We perform time-to-depth migration using the common conversion point (CCP) method with a regional velocity model that includes the slow mantle anomalies to estimate the depth-to-discontinuties and produce an MTZ thickness map. The signature of both the 410 and the 660 km discontinuities is clearly identified across ~500x500 km2. The 410 is relatively flat at 444±10 km depth throughout the region. The 660 is more perturbed with steep topographic changes and lies at 685±20 km depth. The mean depth of both interfaces being deeper than the respective nominal depths can be related to the low resolution of the global velocity model. However, the 410 is deepened more than the 660, resulting in a regionally thinned MTZ in the area of study by up to 25 km (equivalent of +150°C anomaly in the MTZ). A locally thickened (+13 km) MTZ is observed beneath part of the rift where the Main Ethiopian Rift opens into Afar. We interpret that elevated temperatures caused by the lower mantle African Superplume interacting with the MTZ in this region explains the thinned MTZ. Furthermore, the very slow upper mantle above the MTZ is a result of heat transfer from lower to upper mantle. This raised the mantle temperature, which facilitated the onset of rifting in Ethiopia.

A seismic transect across West Antarctica: Evidence for mantle thermal anomalies beneath the Bentley Subglacial Trench and the Marie Byrd Land Dome

NASA Astrophysics Data System (ADS)

Lloyd, Andrew J.; Wiens, Douglas A.; Nyblade, Andrew A.; Anandakrishnan, Sridhar; Aster, Richard C.; Huerta, Audrey D.; Wilson, Terry J.; Dalziel, Ian W. D.; Shore, Patrick J.; Zhao, Dapeng

2015-12-01

West Antarctica consists of several tectonically diverse terranes, including the West Antarctic Rift System, a topographic low region of extended continental crust. In contrast, the adjacent Marie Byrd Land and Ellsworth-Whitmore mountains crustal blocks are on average over 1 km higher, with the former dominated by polygenetic shield and stratovolcanoes protruding through the West Antarctic ice sheet and the latter having a Precambrian basement. The upper mantle structure of these regions is important for inferring the geologic history and tectonic processes, as well as the influence of the solid earth on ice sheet dynamics. Yet this structure is poorly constrained due to a lack of seismological data. As part of the Polar Earth Observing Network, 13 temporary broadband seismic stations were deployed from January 2010 to January 2012 that extended from the Whitmore Mountains, across the West Antarctic Rift System, and into Marie Byrd Land with a mean station spacing of ~90 km. Relative P and S wave travel time residuals were obtained from these stations as well as five other nearby stations by cross correlation. The relative residuals, corrected for both ice and crustal structure using previously published receiver function models of crustal velocity, were inverted to image the relative P and S wave velocity structure of the West Antarctic upper mantle. Some of the fastest relative P and S wave velocities are observed beneath the Ellsworth-Whitmore mountains crustal block and extend to the southern flank of the Bentley Subglacial Trench. However, the velocities in this region are not fast enough to be compatible with a Precambrian lithospheric root, suggesting some combination of thermal, chemical, and structural modification of the lithosphere. The West Antarctic Rift System consists largely of relative fast uppermost mantle seismic velocities consistent with Late Cretaceous/early Cenozoic extension that at present likely has negligible rift related heat flow. In contrast, the Bentley Subglacial Trench, a narrow deep basin within the West Antarctic Rift System, has relative P and S wave velocities in the uppermost mantle that are ~1% and ~2% slower, respectively, and suggest a thermal anomaly of ~75 K. Models for the thermal evolution of a rift basin suggest that such a thermal anomaly is consistent with Neogene extension within the Bentley Subglacial Trench and may, at least in part, account for elevated heat flow reported at the nearby West Antarctic Ice Sheet Divide Ice Core and at Subglacial Lake Whillans. The slowest relative P and S wave velocity anomaly is observed extending to at least 200 km depth beneath the Executive Committee Range in Marie Byrd Land, which is consistent with warm possibly plume-related, upper mantle. The imaged low-velocity anomaly and inferred thermal perturbation (~150 K) are sufficient to support isostatically the anomalous long-wavelength topography of Marie Byrd Land, relative to the adjacent West Antarctic Rift System.

Lithospheric density structure beneath the Tarim basin and surroundings, northwestern China, from the joint inversion of gravity and topography

USGS Publications Warehouse

Deng, Yangfan; Levandowski, William Brower; Kusky, Tim

2017-01-01

Intraplate strain generally focuses in discrete zones, but despite the profound impact of this partitioning on global tectonics, geodynamics, and seismic hazard, the processes by which deformation becomes localized are not well understood. Such heterogeneous intraplate strain is exemplified in central Asia, where the Indo-Eurasian collision has caused widespread deformation while the Tarim block has experienced minimal Cenozoic shortening. The apparent stability of Tarim may arise either because strain is dominantly accommodated by pre-existing faults in the continental suture zones that bound it—essentially discretizing Eurasia into microplates—or because the lithospheric-scale strength (i.e., viscosity) of the Tarim block is greater than its surroundings. Here, we jointly analyze seismic velocity, gravity, topography, and temperature to develop a 3-D density model of the crust and upper mantle in this region. The Tarim crust is characterized by high density, vs, vp, and vp/vs, consistent with a dominantly mafic composition and with the presence of an oceanic plateau beneath Tarim. Low-density but high-velocity mantle lithosphere beneath southern (southwestern) Tarim underlies a suite of Permian plume-related mafic intrusions and A-type granites sourced in previously depleted mantle lithosphere; we posit that this region was further depleted, dehydrated, and strengthened by Permian plume magmatism. The actively deforming western and southern margins of Tarim—the Tien Shan, Kunlun Shan, and Altyn Tagh fault—are underlain by buoyant upper mantle with low velocity; we hypothesize that this material has been hydrated by mantle-derived fluids that have preferentially migrated along Paleozoic continental sutures. Such hydrous material should be weak, and herein strain focuses there because of lithospheric-scale variations in rheology rather than the pre-existence of faults in the brittle crust. Thus this world-class example of strain partitioning arises not simply from the pre-existence of brittle faults but from the thermo-chemical and therefore rheological variations inherited from prior tectonism.

Constraints on radial anisotropy in the central Pacific upper mantle from the NoMelt OBS array

NASA Astrophysics Data System (ADS)

Russell, J. B.; Gaherty, J. B.; Lin, P. P.; Zebker, M.

2016-12-01

Observations of seismic anisotropy in ocean basins are important for constraining deformation and melting processes in the upper mantle. The NoMelt OBS array was deployed on relatively pristine, 70-Ma seafloor in the central Pacific with the aim of constraining upper-mantle circulation and the evolution of the lithosphere-asthenosphere system. Azimuthal variations in Rayleigh-wave velocity suggest strong anisotropic fabric both in the lithosphere and deep in the asthenosphere, and we aim to evaluate whether radial anisotropy shows a similar pattern. We use a combination of Love waves from earthquakes (20-100 s) as well as high-frequency ambient noise (5-10 s) to estimate VSH in the upper 300 km beneath the NoMelt array. Waveform fitting of the ambient-noise cross spectra provide phase-velocity estimates that are sensitive to the upper 50 km of the mantle. To constrain structure beneath the lid, we employ an array-based approach to measure Love-wave phase velocities across the array using seven shallow-focus events (< 25 km) with high signal-to-noise ratio and diverse azimuthal coverage. The Love wave phase-velocity measurements suggest strong interference of the first overtone for intermediate periods (20-50 s), while longer periods (>60 s) are mostly dominated by fundamental mode energy. Through forward modeling of Love wave Fréchet kernels, we find an extremely strong nonlinearity in individual mode-branch sensitivity that is dependent on the relative velocity difference between the low-velocity zone (LVZ) and the overlying Pacific lid. For the fundamental mode in the presence of a strong LVZ, intermediate periods (20-50 s) have little sensitivity within the lithospheric mantle with peak sensitivity pushed to the base of the low-velocity zone. This peak sensitivity migrates to much shallower depth as the lid/LVZ contrast is reduced. Therefore, we use a Monte Carlo approach to systematically explore the model space and identify the most robust model features required to minimize phase-velocity misfit of the full multimode Love wave arrivals. The resulting VSH model is combined with the NoMelt VSV model to obtain estimates of radial anisotropy for the top 300km of the central Pacific upper-mantle.

Yellowstone Hotspot Geodynamics

NASA Astrophysics Data System (ADS)

Smith, R. B.; Farrell, J.; Massin, F.; Chang, W.; Puskas, C. M.; Steinberger, B. M.; Husen, S.

2012-12-01

The Yellowstone hotspot results from the interaction of a mantle plume with the overriding N. America plate producing a ~300-m high topographic swell centered on the Late Quaternary Yellowstone volcanic field. The Yellowstone area is dominated by earthquake swarms including a deadly M7.3 earthquake, extraordinary high heat flow up to ~40,000 mWm-2, and unprecedented episodes of crustal deformation. Seismic tomography and gravity data reveal a crustal magma reservoir, 6 to 15 km deep beneath the Yellowstone caldera but extending laterally ~20 km NE of the caldera and is ~30% larger than previously hypothesized. Kinematically, deformation of Yellowstone is dominated by regional crustal extension at up to ~0.4 cm/yr but with superimposed decadal-scale uplift and subsidence episodes, averaging ~2 cm/yr from 1923. From 2004 to 2009 Yellowstone experienced an accelerated uplift episode of up to 7 cm/yr whose source is modeled as magmatic recharge of a sill at the top of the crustal magma reservoir at 8-10-km depth. New mantle tomography suggest that Yellowstone volcanism is fed by an upper-mantle plume-shaped low velocity body that is composed of melt "blobs", extending from 80 km to 650 km in depth, tilting 60° NW, but then reversing tilt to ~60° SE to a depth of ~1500 km. Moreover, images of upper mantle conductivity from inversion of MT data reveal a high conductivity annulus around the north side of the plume in the upper mantle to resolved depths of ~300 km. On a larger scale, upper mantle flow beneath the western U.S. is characterized by eastward flow beneath Yellowstone at 5 cm/yr that deflects the plume to the west, and is underlain by a deeper zone of westerly return flow in the lower mantle reversing the deflection of the plume body to the SE. Dynamic modeling of the Yellowstone plume including a +15 m geoid anomaly reveals low excess plume temperatures, up to 150°K, consistent with a weak buoyancy flux of ~0.25 Mg/s. Integrated kinematic modeling of GPS, Quaternary fault slip, and seismic data suggest that the gravitational potential of the Yellowstone swell creates a regional extension affecting much of the western U.S. Overall, the Yellowstone hotspot swell is the vertex of tensional stress axes rotation from E-W in the Basin-Range to NE-SW at the Yellowstone Plateau as well as the cause of edge faulting, nucleating the nearby Teton and Centennial faults. We extrapolate the original location of the Yellowstone mantle-source southwestward 800 km to an initial position at 17 million years ago beneath eastern Oregon and Washington suggesting a common origin for the YSRP and Columbia Plateau volcanism. We propose that the original plume head ascended vertically behind the subducting Juan de Fuca plate, but was entrained ~12 Ma ago in a faster mantle flow beneath the continental lithosphere and tilted into its present configuration.

Crustal-scale shear zones and heterogeneous structure beneath the North Anatolian Fault Zone, Turkey, revealed by a high-density seismometer array

NASA Astrophysics Data System (ADS)

Kahraman, Metin; Cornwell, David G.; Thompson, David A.; Rost, Sebastian; Houseman, Gregory A.; Türkelli, Niyazi; Teoman, Uğur; Altuncu Poyraz, Selda; Utkucu, Murat; Gülen, Levent

2015-11-01

Continental scale deformation is often localised along strike-slip faults constituting considerable seismic hazard in many locations. Nonetheless, the depth extent and precise geometry of such faults, key factors in how strain is accumulated in the earthquake cycle and the assessment of seismic hazard, are poorly constrained in the mid to lower crust. Using a dense broadband network of 71 seismic stations with a nominal station spacing of 7 km in the vicinity of the 1999 Izmit earthquake we map previously unknown small-scale structure in the crust and upper mantle along this part of the North Anatolian Fault Zone (NAFZ). We show that lithological and structural variations exist in the upper, mid and lower crust on length scales of less than 10 km and less than 20 km in the upper mantle. The surface expression of the NAFZ in this region comprises two major branches; both are shown to continue at depth with differences in dip, depth extent and (possibly) width. We interpret a <10 km wide northern branch that passes downward into a shear zone that traverses the entire crust and penetrates the upper mantle to a depth of at least 50 km. The dip of this structure appears to decrease west-east from ∼90° to ∼65° to the north over a distance of 30 to 40 km. Deformation along the southern branch may be accommodated over a wider (>10 km) zone in the crust with a similar variation of dip but there is no clear evidence that this shear zone penetrates the Moho. Layers of anomalously low velocity in the mid crust (20-25 km depth) and high velocity in the lower crust (extending from depths of 28-30 km to the Moho) are best developed in the Armutlu-Almacik block between the two shear zones. A mafic lower crust, possibly resulting from ophiolitic obduction or magmatic intrusion, can best explain the coherent lower crustal structure of this block. Our images show that strain has developed in the lower crust beneath both northern and southern strands of the North Anatolian Fault. Our new high resolution images provide new insights into the structure and evolution of the NAFZ and show that a small and dense passive seismic network is able to image previously undetectable crust and upper mantle heterogeneity on lateral length scales of less than 10 km.

Lithospheric Structure of Antarctica and Implications for Geological and Cryospheric Evolution

NASA Astrophysics Data System (ADS)

Wiens, Douglas; Heeszel, David; Sun, Xinlei; Lloyd, Andrew; Nyblade, Andrew; Anandakrishnan, Sridhar; Aster, Richard; Chaput, Julien; Huerta, Audrey; Hansen, Samantha; Wilson, Terry

2013-04-01

Recent broadband seismic deployments, including the AGAP/GAMSEIS array of 24 broadband seismographs over the Gamburtsev Subglacial Mountains (GSM) in East Antarctica and the POLENET/ANET deployment of 33 seismographs across much of West Antarctica, reveal the detailed crust and upper mantle structure of Antarctica for the first time. The seismographs operate year-around even in the coldest parts of Antarctica, due to novel insulated boxes, power systems, and modified instrumentation developed in collaboration with the IRIS PASSCAL Instrument Center. We analyze the data using several different techniques to develop high-resolution models of Antarctic seismic structure. We use Rayleigh wave phase velocities at periods of 20-180 s determined using a modified two-plane wave decomposition of teleseismic Rayleigh waves to invert for the three dimensional shear velocity structure. In addition, Rayleigh wave group and phase velocities obtained by ambient seismic noise correlation methods provide constraints at shorter periods and shallower depths. Receiver functions provide precise estimates of crustal structure beneath the stations, and P and S wave tomography provides models of upper mantle structure down to ~ 500 km depth along transects of greater seismic station density. The new seismic results show that the high elevations of the GSM are supported by thick crust (~ 55 km), and are underlain by thick Precambrian continental lithosphere that initially formed during Archean to mid-Proterozoic times. The absence of lithospheric thermal anomalies suggests that the mountains were formed by a compressional orogeny during the Paleozoic, thus providing a locus for ice sheet nucleation throughout a long period of geological time. Within West Antarctica, the crust and lithosphere are extremely thin near the Transantarctic Mountain Front and topographic lows such as the Bentley Trench and Byrd Basin, which represent currently inactive Cenozoic rift systems. Slow seismic velocities beneath Marie Byrd Land at asthenospheric depths suggest a major thermal anomaly, possibly due to a mantle plume. Volcanic earthquakes detected in this region indicate the presence of currently active magma systems. The results suggest large lateral changes in parameters needed for glaciological models, including lithospheric thickness, mantle viscosity, and heat flow. Extremely high heat flow is predicted for much of West Antarctica, consistent with recent results from the WAIS ice drilling. Using the seismic results to estimate mantle viscosity, we find several orders of magnitude difference in viscosity between East and West Antarctica, with lowest viscosities found beneath Marie Byrd Land and the West Antarctic Rift System. Realistic glacial isostatic adjustment models must take these large lateral variations into account.

Lateral variations in upper-mantle seismic anisotropy in the Pacific from inversion of a surface-wave dispersion dataset

NASA Astrophysics Data System (ADS)

Eddy, C. L.; Ekstrom, G.; Nettles, M.; Gaherty, J. B.

2017-12-01

We present a three-dimensional model of the anisotropic velocity structure of the Pacific lithosphere and asthenosphere. The presence of seismic anisotropy in the oceanic upper mantle provides information about the geometry of flow in the mantle, the nature of the lithosphere-asthenosphere boundary, and the possible presence of partial melt in the asthenosphere. Our dataset consists of fundamental-mode dispersion for Rayleigh and Love waves measured between 25-250 s with paths crossing the Pacific Ocean. We invert the phase anomaly measurements directly for three-dimensional anisotropic velocity structure. Our models are radially anisotropic and include the full set of elastic parameters that describe azimuthal variations in velocity (e.g. Gc, Gs). We investigate the age dependence of seismic velocity and radial anisotropy and find that there are significant deviations from the velocities predicted by a simple oceanic plate cooling model. We observe strong radial anisotropy with vsh > vsv in the asthenosphere of the central Pacific. We investigate the radial anisotropy in the shallow lithosphere, where previous models have reported conflicting results. There is a contrast in both upper-mantle isotropic velocities and radial anisotropy between the Pacific and Nazca plates, across the East Pacific Rise. We also investigate lateral variations in azimuthal anisotropy throughout the Pacific upper mantle and find that there are large areas over which the anisotropy fast axis does not align with absolute plate motion, suggesting the presence of small-scale convection or pressure-driven flow beneath the base of the oceanic plate.

Garnet Signatures in Geophysical and Geochemical Observations: Insights into the Thermo-Petrological Structure of Oceanic Upper Mantle

NASA Astrophysics Data System (ADS)

Grose, C. J.; Afonso, J. C.

2013-12-01

We have developed new physically comprehensive thermal plate models of the oceanic lithosphere which incorporate temperature- and pressure-dependent heat transport properties and thermal expansivity, melting beneath ridges, hydrothermal circulation near ridge axes, and insulating oceanic crust. These models provide good fits to global databases of seafloor topography and heat flow, and seismic evidence of thermal structure near ridge axes. We couple these thermal plate models with thermodynamic models to predict the petrology of oceanic lithosphere. Geoid height predictions from our models suggest that there is a strong anomaly in geoid slope (over age) above ~25 Ma lithosphere due to the topography of garnet-field mantle. A similar anomaly is also present in geoid data over fracture zones. In addition, we show that a new assessment of a large database of ocean island basalt Sm/Yb systematics indicates that there is an unmistakable step-like increase in Sm/Yb values around 15-20 Ma, indicating the presence of garnet. To explain this feature, we have attempted to couple our thermo-petrological models of oceanic upper mantle with an open system, non-modal, dynamic melting model with diffusion kinetics to investigate trace element partitioning in an ascending mantle column.

Synthesis of regional crust and upper-mantle structure from seismic and gravity data

NASA Technical Reports Server (NTRS)

Alexander, S. S.; Lavin, P. M. (Principal Investigator)

1982-01-01

Analyses of regional gravity and magnetic patterns, LANDSAT images and geological information revealed two major lineaments crossing western Pennsylvania and parts of surrounding states. These lineaments are inferred to be expressions of fracture zones which penetrare deeply into the crust and possibly the upper mantle. The extensions of the Tyron-Mt. Union and the Pittsburgh-Washington lineaments bound a distinct crustal block (Lake Erie-Maryland block) over 100 km wide and probably more than 600 km in length. Evidence exists for the lateral displacement of this block at least 60 km northwestward during late Precambrian to Lower Ordovician time. Subsequent movements have been mainly vertical with respect to neighboring blocks. A possible crustal block that passes through eastern Kentucky, proposed by a TVA study on tectonics in the southern Appalachians, was also investigated. Finally, the use of regional gravity and magnetic data in identifying major crustal structures beneath western Pennsylvania is discussed.

Radial Anisotropy in the Mantle Transition Zone and Its Implications

NASA Astrophysics Data System (ADS)

Chang, S. J.; Ferreira, A. M.

2016-12-01

Seismic anisotropy is a useful tool to investigate mantle flow, mantle convection, and the presence of melts in mantle, since it provides information on the direction of mantle flow or the orientation of melts by combining it with laboratory results in mineral physics. Although the uppermost and lowermost mantle with strong anisotropy have been well studied, anisotropic properties of the mantle transition zone is still enigmatic. We use a recent global radially anisotropic model, SGLOBE-rani, to examine the patterns of radial anisotropy in the mantle transition zone. Strong faster SV velocity anomalies are found in the upper transition zone beneath subduction zones in the western Pacific, which decrease with depth, thereby nearly isotropic in the lower transition zone. This may imply that the origin for the anisotropy is the lattice-preferred orientation of wadsleyite, the dominant anisotropic mineral in the upper transition zone. The water content in the upper transition zone may be inferred from radial anisotropy because of the report that anisotropic intensity depends on the water content in wadsleyite.

A Receiver Function Study of Mantle Transition Zone Discontinuities beneath Egypt and Saudi Arabia

NASA Astrophysics Data System (ADS)

Liu, K. H.; Mohamed, A. A.; Gao, S. S.; Elsheikh, A. A.; Yu, Y.; Fat-Helbary, R. E.

2014-12-01

The dramatic asymmetry in terms of surface elevation, Cenozoic volcanisms, and earthquake activity across the Red Sea is an enigmatic issue in global tectonics, partially due to the unavailability of broadband seismic data on the African plate adjacent to the Red Sea. Here we report the first results from a receiver function study of the mantle transition zone (MTZ) discontinuities using data from the Egyptian National Seismic Network, and compare the resulting depths of the 410 and 660 km discontinuities (d410 and d660) with those observed on the Arabian side. Results using more than 6000 P-to-S receiver functions recorded at 49 broadband seismic stations in Egypt, Saudi Arabia and adjacent areas show that when the IASP91 Earth model is used for time-to-depth conversion, the resulting depth of the discontinuities increases systematically toward the axis of the Afro-Arabian Dome (AAD) from both the west and east. Relative to the westernmost area, the maximum depression of the 410-km discontinuity is about 30 km, and that of the 660-km discontinuity is about 45 km. Highly correlated d410 and d660 depths suggest that the observed apparent depth variations are mostly caused by lateral velocity anomalies in the upper mantle, while the 15 km additional depression of the d660 relative to the d410 requires either a colder-than-normal MTZ or the presence of water in the MTZ. We tested several models involving upper mantle and MTZ velocity anomalies and undulations of the MTZ discontinuities due to temperature anomalies and water content, and found that the observed systematic variations can best be explained by a model involving a hydrated MTZ and an upper-mantle low-velocity zone beneath the AAD (Mohamed et al., 2014, doi: 10.1093/gji/ggu284). Models invoking one or more mantle plumes originated from the MTZ or the lower-mantle beneath the study area are not consistent with the observations.

Relationship between the upper mantle high velocity seismic lid and the continental lithosphere

NASA Astrophysics Data System (ADS)

Priestley, Keith; Tilmann, Frederik

2009-04-01

The lithosphere-asthenosphere boundary corresponds to the base of the "rigid" plates - the depth at which heat transport changes from advection in the convecting deeper upper mantle to conduction in the shallow upper mantle. Although this boundary is a fundamental feature of the Earth, mapping it has been difficult because it does not correspond to a sharp change in temperature or composition. Various definitions of the lithosphere and asthenosphere are based on the analysis of different types of geophysical and geological observations. The depth to the lithosphere-asthenosphere boundary determined from these different observations often shows little agreement when they are applied to the same region because the geophysical and geological observations (i.e., seismic velocity, strain rate, electrical resistivity, chemical depletion, etc.) are proxies for the change in rheological properties rather than a direct measure of the rheological properties. In this paper, we focus on the seismic mapping of the upper mantle high velocity lid and low velocity zone and its relationship to the lithosphere and asthenosphere. We have two goals: (a) to examine the differences in how teleseismic body-wave travel-time tomography and surface-wave tomography image upper mantle seismic structure; and (b) to summarise how upper mantle seismic velocity structure can be related to the structure of the lithosphere and asthenosphere. Surface-wave tomography provides reasonably good depth resolution, especially when higher modes are included in the analysis, but lateral resolution is limited by the horizontal wavelength of the long-period surface waves used to constrain upper mantle velocity structure. Teleseismic body-wave tomography has poor depth resolution in the upper mantle, particularly when no strong lateral contrasts are present. If station terms are used, features with large lateral extent and gradual boundaries are attenuated in the tomographic image. Body-wave models are not useful in mapping the thickness of the high velocity upper mantle lid because this type of analysis often determines wave speed perturbations from an unknown horizontal average and not absolute velocities. Thus, any feature which extends laterally across the whole region beneath a seismic network becomes invisible in the teleseismic body-wave tomographic image. We compare surface-wave and body-wave tomographic results using southern Africa as an example. Surface-wave tomographic images for southern Africa show a strong, high velocity upper mantle lid confined to depths shallower than ~ 200 km, whereas body-wave tomographic images show weak high velocity in the upper mantle extending to depths of ~ 300 km or more. However, synthetic tests show that these results are not contradictory. The absolute seismic velocity structure of the upper mantle provided by surface wave analysis can be used to map the thermal lithosphere. Priestley and McKenzie (Priestley, K., McKenzie, D., 2006. The thermal structure of the lithosphere from shear wave velocities. Earth and Planetary Science Letters 244, 285-301.) derive an empirical relationship between shear wave velocity and temperature. This relationship is used to obtain temperature profiles from the surface-wave tomographic models of the continental mantle. The base of the lithosphere is shown by a change in the gradient of the temperature profiles indicative of the depth where the mode of heat transport changes from conduction to advection. Comparisons of the geotherms determined from the conversion of surface-wave wave speeds to temperatures with upper mantle nodule-derived geotherms demonstrate that estimates of lithospheric thickness from Vs and from the nodule mineralogy agree to within about 25 km. The lithospheric thickness map for Africa derived from the surface-wave tomographic results shows that thick lithosphere underlies most of the Archean crust in Africa. The distribution of diamondiferous kimberlites provides an independent estimate of where thick lithosphere exists. Diamondiferous kimberlites generally occur where the lower part of the thermal lithosphere as indicated by seismology is in the diamond stability field.

Seismic imaging beneath southwest Africa based on finite-frequency body wave tomography

NASA Astrophysics Data System (ADS)

Youssof, Mohammad; Yuan, Xiaohui; Tilmann, Frederik; Heit, Benjamin; Weber, Michael; Jokat, Wilfried; Geissler, Wolfram; Laske, Gabi

2016-04-01

We present a seismic model of southwest Africa from teleseismic tomographic inversion of the P- and S- wave data recorded by an amphibious temporary seismic network. The area of study is located at the intersection of the Walvis Ridge with the continental margin of northern Namibia, and extends into the Congo craton. Utilizing 3D finite-frequency sensitivity kernels, we invert traveltime residuals of the teleseismic body waves to image seismic structures in the upper mantle. To test the robustness of our tomographic imaging, we employed various resolution assessments that allow us to inspect the extent of smearing effects and to evaluate the optimum regularization weights (i.e., damping and smoothness). These tests include applying different (ir)regular parameterizations, classical checkerboard and anomaly tests and squeezing modeling. Furthermore, we performed different kinds of weighing schemes for the traveltime dataset. These schemes account for balancing between the picks data amount with their corresponding events directions. Our assessment procedure involves also a detailed investigation of the effect of the crustal correction on the final velocity image, which strongly influenced the image resolution for the mantle structures. Our model can resolve horizontal structures of 1° x 1° below the array down to 300-350 km depth. The resulting model is mainly dominated by the difference in the oceanic and continental mantle lithosphere beneath the study area, with second-order features related to their respective internal structures. The fast lithospheric keel of the Congo Craton reaches a depth of ~250 km. The orogenic Damara Belt and continental flood basalt areas are characterized by low velocity perturbations down to a depth of ~150 km, indicating a normal fertile mantle. High velocities in the oceanic lithosphere beneath the Walvis Ridge appear to show signatures of chemical depletion. A pronounced anomaly of fast velocity is imaged underneath continental NW Namibia and is separated from the high velocity anomaly of the Congo Craton. We interpret this positive perturbation as depleted mantle materials. The depletion event is most probably related to the emplacement of the Parana-Etendeka flood basalts at about 132 Ma triggered by a mantle plume, which has left traces on the Walvis Ridge as well.

Symplectite in spinel lherzolite xenoliths from the Little Hungarian Plain, Western Hungary: A key for understanding the complex history of the upper mantle of the Pannonian Basin

NASA Astrophysics Data System (ADS)

Falus, György; Szabó, Csaba; Kovács, István; Zajacz, Zoltán; Halter, Werner

2007-03-01

Two spinel lherzolite xenoliths from Hungary that contain pyroxene-spinel symplectites have been studied using EPMA, Laser ablation ICP-MS and universal stage. Based on their geochemical and structural characteristics, the xenoliths represent two different domains of the shallow subcontinental lithospheric mantle beneath the Pannonian Basin. The occurrence of symplectites is attributed to the former presence and subsequent breakdown of garnets due to significant pressure decrease related to lithospheric thinning. This implies that both mantle domains were once part of the garnet lherzolitic upper mantle and had a similar history during the major extension that formed the Pannonian Basin. Garnet breakdown resulted in distinct geochemical characteristics in the adjacent clinopyroxene crystals in both xenoliths. This is manifested by enrichment in HREE, Y, Zr and Hf towards the clinopyroxene porphyroclast rims and also in the neoblasts with respect to porphyroclast core compositions. This geochemical feature, together with the development and preservation of the texturally very sensitive symplectites, enables us to determine the relative timing of mantle processes. Our results indicate that garnets had been metastable in the spinel lherzolite environment and their breakdown to pyroxene and spinel is one of the latest processes that took place within the upper mantle before the xenoliths were brought to the surface.

High resolution P-wave velocity structure beneath Northeastern Tibet from multiscale seismic tomography

NASA Astrophysics Data System (ADS)

Guo, B.; Gao, X.; Chen, J.; Liu, Q.; Li, S.

2016-12-01

The continuing collision of the northward advancing Indian continent with the Eurasia results in the high elevations and thickened Tibetan Plateau. Numerous geologic and geophysical studies engaged in the mechanics of the Tibetan Plateau deformation and uplift. Many seismic experiments were deployed in south and central Tibet, such as INDEPTH and Hi-climb, but very few in northeastern Tibet. Between 2013 and 2015, The China Seismic Array-experiment operated 670 broadband seismic stations with an average station spacing of 35km. This seismic array located in northeastern Tibet and covered the Qilian Mountains, Qaidam Basin, and part of Songpan-Ganzi, Gobi-Alashan, Yangzi, and Ordos. A new multiscale seismic traveltime tomography technique with sparsity constrains were used to map the upper mantle P-wave velocity structure beneath northeastern Tibet. The seismic tomography algorithm employs sparsity constrains on the wavelet representation velocity model via the L1-norm regularization. This algorithm can efficiently deal with the uneven-sampled volume, and give multiscale images of the model. Our preliminary results can be summarized as follows: 1) in the upper mantle down to 200km, significate low-velocity anomalies exist beneath the northeastern Tibet, and slight high-velocity anomalies beneath the Qaidam basin; 2) under Gobi-Alashan, Yangzi, and Ordos, high-velocity anomalies appear to extend to a depth of 250km, this high-velocity may correspond to the lithosphere; 3) there exist relative high-velocity anomalies at depth of 250km-350km underneath north Tibet, which suggests lithospheric delamination; 4) the strong velocity contrast between north Tibet and Yangzi, Gabi-Alashan is visible down to 200km, which implies the north Tibet boundary.

P-wave and S-wave traveltime residuals in Caledonian and adjacent units of Northern Europe and Greenland

NASA Astrophysics Data System (ADS)

Hejrani, Babak; Balling, Niels; Holm Jacobsen, Bo; Kind, Rainer; Tilmann, Frederik; England, Richard; Bom Nielsen, Søren

2014-05-01

This work combines P-wave and S-wave travel time residuals from in total 477 temporary and 56 permanent stations deployed across Caledonian and adjacent units in Northern Europe and Greenland (Tor, Gregersen et al. 2002; SVEKALAPKO, Sandoval et al., 2003; CALAS, Medhus et al, 2012a; MAGNUS, Weidle et al. 2010; SCANLIPS south, England & Ebbing 2012; SCANLIPS north, Hejrani et al. 2012; JULS Hejrani et al. 2013; plus permanent stations in the region). We picked data from 2002 to 2012 (1221 events) using a cross correlation technique on all waveforms recorded for each event. In this way we achieve maximum consistency of relative residuals over the whole region (Medhus et al. 2012b). On the European side 18362 P-wave travel time residuals was delivered. In East Greenland 1735 P-wave residuals were recovered at the Central Fjord array (13 stations) and 2294 residuals from the sparse GLISN-array (23 stations). Likewise, we picked a total of 6034 residuals of the SV phase (For the Tor and SVEKALAPKO projects we used data from Amaru et al. 2008). Relative residuals within the region are mainly due to sub-crustal uppermost mantle velocity anomalies. A dominant subvertical boundary was detected by Medhus et al. (2012), running along the Tornquist zone, east of the Oslo Graben and crossing under high topography of the southern Scandes. We delineated this boundary in more detail, tracking it towards the Atlantic margin north of Trondheim. Further north (Scanlips north), a similar subvertical upper mantle boundary seems to be present close to the coast, coinciding with the edge of the stretched crust. The North German Caledonides were probed by the new JULS (JUtland Lower Saxony) profile which closes the gap between Tor and CALAS arrays. Mantle structure found by the Tor project was confirmed, and modelling was extended to the eastern edge of the North Sea. References: Amaru, M. L., Spakman, W., Villaseñor, A., Sandoval, S., Kissling, E., 2008, A new absolute arrival time data set for Europe.Geophysical Journal International, 173, 465-472. England, R. W.; Ebbing, J., 2012, Crustal structure of central Norway and Sweden from integrated modelling of teleseismic receiver functions and the gravity anomaly.GEOPHYSICAL JOURNAL INTERNATIONAL, 191, 1-11. Gregersen S., Voss P., TOR Working Group, 2002. Summary of project TOR: delineation of a stepwise, sharp, deep lithosphere transition across Germany-Denmark-Sweden, Tectonophysics, 360, 61-73. Hejrani, B., Jacobsen, B. H., Balling,N. and England, R. W.. 2012, A seismic tomography study of lithospheric structure under the Norwegian Caledonides.Geophysical Research Abstracts, 14, 4334. Hejrani, B.; Jacobsen, B.H.; Balling, N.;Tilmann, F.; Kind, R., 2013, Upper-mantle velocity structure beneathJutland, Denmark and northern Germany:Preliminary results. Joint Assembly Gothenburg Abstract S401S2.01, Medhus, A. B., Balling, N., Jacobsen, B. H., Weidle, C., England, R. W., Kind, R., Thybo, H., Voss, P. (2012a): Upper-mantle structure beneath the Southern Scandes Mountains and the Northern Tornquist Zone revealed by P-wave traveltime tomography. Geophysical Journal International, 189, 3, 1315-1334. Medhus, Jacobsen, B. H.,A. B., Balling, N., 2012b, Bias Problems in Existing Teleseismic Travel Time Databases: Ignore or Repair? Seismological Research Letters, 83, 1030-1037. Sandoval, S., Kissling, E. &Ansorge, J., 2003.High-resolution body wave tomography beneath the SVEKALAPKO array: I. A priori three-dimensional crustal model and associated traveltime effects on teleseismic wave fronts, Geophys. J. Int., 153, 75-87. Weidle, C., Maupin, V., Ritter, J.,Kværna, T., Schweitzer J., Balling, N.,Thybo, H.,Faleide, J. I.,and,Wenzel, F., 2010, MAGNUS-A Seismological Broadband Experiment to Resolve Crustal and Upper Mantle Structure beneath the Southern Scandes Mountains in Norway. SEISMOLOGICAL RESEARCH LETTERS, 81, 76-84.

Piecewise Delamination Drives Uplift in the Atlas Mountains Region of Morocco

NASA Astrophysics Data System (ADS)

Bezada, M. J.; Humphreys, E.; Martin Davila, J.; mimoun, H.; Josep, G.; Palomeras, I.

2013-12-01

The elevation of the intra-continental Atlas Mountains of Morocco and surrounding regions requires a mantle component of buoyancy, and there is consensus that this buoyancy results from an abnormally thin lithosphere. Lithospheric delamination under the Atlas Mountains and thermal erosion caused by upwelling mantle have each been suggested as thinning mechanisms. We use seismic tomography to image the upper mantle of Morocco by inverting teleseimic p-wave delay times, complemented with local delays, recorded on a dense array of stations in the Iberian peninsula and Morocco. A surface wave model provides constraint on the shallower layers. We determine the geometry of lithospheric cavities and mantle upwelling beneath the Middle Atlas and central High Atlas, and image delaminated lithosphere at ~400 km beneath the Middle Atlas. We propose discontinuous delamination of an intrinsically unstable Atlas lithosphere, enabled by the presence of anomalously hot mantle, as a mechanism for producing the imaged structures. The Atlas lithosphere was made unstable by a combination of tectonic shortening and eclogite loading during Mesozoic rifting and Cenozoic magmatism. The presence of hot mantle, sourced from regional upwellings in northern Africa or the Canary Islands, enabled the mobilization of this lithosphere. Flow around the retreating Alboran slab focused upwelling mantle under the Middle Atlas, where we image the most recent delamination. The Atlas Mountains of Morocco stand as an example of mantle-generated uplift and large-scale lithospheric loss in a mildly contractional orogen.

Velocity variations and uncertainty from transdimensional P-wave tomography of North America

NASA Astrophysics Data System (ADS)

Burdick, Scott; Lekić, Vedran

2017-05-01

High-resolution models of seismic velocity variations constructed using body-wave tomography inform the study of the origin, fate and thermochemical state of mantle domains. In order to reliably relate these variations to material properties including temperature, composition and volatile content, we must accurately retrieve both the patterns and amplitudes of variations and quantify the uncertainty associated with the estimates of each. For these reasons, we image the mantle beneath North America with P-wave traveltimes from USArray using a novel method for 3-D probabilistic body-wave tomography. The method uses a Transdimensional Hierarchical Bayesian framework with a reversible-jump Markov Chain Monte Carlo algorithm in order to generate an ensemble of possible velocity models. We analyse this ensemble solution to obtain the posterior probability distribution of velocities, thereby yielding error bars and enabling rigorous hypothesis testing. Overall, we determine that the average uncertainty (1σ) of compressional wave velocity estimates beneath North America is ∼0.25 per cent dVP/VP, increasing with proximity to complex structure and decreasing with depth. The addition of USArray data reduces the uncertainty beneath the Eastern US by over 50 per cent in the upper mantle and 25-40 per cent below the transition zone and ∼30 per cent throughout the mantle beneath the Western US. In the absence of damping and smoothing, we recover amplitudes of variations 10-80 per cent higher than a standard inversion approach. Accounting for differences in data coverage, we infer that the length scale of heterogeneity is ∼50 per cent longer at shallow depths beneath the continental platform than beneath tectonically active regions. We illustrate the model trade-off analysis for the Cascadia slab and the New Madrid Seismic Zone, where we find that smearing due to the limitations of the illumination is relatively minor.

Shear wave splitting and upper mantle deformation in French Polynesia: Evidence for small-scale heterogeneity related to the Society hotspot

NASA Astrophysics Data System (ADS)

Russo, R. M.; Okal, E. A.

1998-07-01

We determined shear wave splitting parameters at four island sites in French Polynesia: Tiputa (TPT) on Rangiroa in the Tuamotu archipelago; Papeete (PPT) on Tahiti in the Society Islands; Tubuai (TBI) in the Cook-Austral island chain; and Rikitea (RKT) on Mangareva in the Gambier Islands. We also examined splitting at Pitcairn (PTCN) on Pitcairn Island; because of the short time of operation of PTCN, our results there are preliminary. We find substantial differences in splitting, most likely caused by variable upper mantle deformation beneath the five stations. At TPT the fast split shear wave (ϕ) direction is N66°W±4°, parallel to the current Pacific-hotspots relative motion (APM) vector; the delay time between fast and slow waves is 1.3±0.2 s. At PPT, on Tahiti, we could detect no splitting despite many clear SKS observations. At TBI, on Tubuai we detected splitting with a delay time of 1.1±0.1 s and a ϕ direction midway between the local APM direction and the fossil spreading direction (N86°W±2°), as locally indicated by the nearby Austral Fracture Zone. At RKT in the Gambier Islands, ϕ trends N53°W±6°, 16° clockwise of the local APM azimuth, and delay time at RKT is 1.1±0.1 s. Results at PTCN include ϕ near N38°W±9° and a delay time of 1.1±0.3 s. These different results imply variable upper mantle deformation beneath the five sites. We interpret splitting at TPT and, possibly, RKT as indicative of asthenospheric flow or shear in the APM direction beneath the stations. At PPT, azimuthal isotropy indicates deformed upper mantle with a vertical symmetry axis, or absence of strong or consistently oriented mantle deformation fabric beneath Tahiti. Either effect could be related to recent hotspot magmatism on Tahiti. At TBI, splitting may be complicated by juxtaposition of different lithospheric thicknesses along the nearby Austral Fracture Zone, resulting in perturbation of asthenospheric flow. The absence of splitting related to fossil spreading in French Polynesia indicates that upper mantle deformation processes postdating lithosphere formation are important at all four sites within that region. The ϕ azimuth at PTCN does not align with either the fossil direction or the APM direction, but our best individual determination of splitting parameters at this station lies within 10° of the local APM at Pitcairn Island.

Mantle structure beneath the western edge of the Colorado Plateau

USGS Publications Warehouse

Sine, C.R.; Wilson, D.; Gao, W.; Grand, S.P.; Aster, R.; Ni, J.; Baldridge, W.S.

2008-01-01

Teleseismic traveltime data are inverted for mantle Vp and Vs variations beneath a 1400 km long line of broadband seismometers extending from eastern New Mexico to western Utah. The model spans 600 km beneath the moho with resolution of ???50 km. Inversions show a sharp, large-magnitude velocity contrast across the Colorado Plateau-Great Basin transition extending ???200 km below the crust. Also imaged is a fast anomaly 300 to 600 km beneath the NW portion of the array. Very slow velocities beneath the Great Basin imply partial melting and/or anomalously wet mantle. We propose that the sharp contrast in mantle velocities across the western edge of the Plateau corresponds to differential lithospheric modification, during and following Farallon subduction, across a boundary defining the western extent of unmodified Proterozoic mantle lithosphere. The deep fast anomaly corresponds to thickened Farallon plate or detached continental lithosphere at transition zone depths. Copyright 2008 by the American Geophysical Union.

Primordial helium entrained by the hottest mantle plumes

NASA Astrophysics Data System (ADS)

Jackson, M. G.; Konter, J. G.; Becker, T. W.

2017-02-01

Helium isotopes provide an important tool for tracing early-Earth, primordial reservoirs that have survived in the planet’s interior. Volcanic hotspot lavas, like those erupted at Hawaii and Iceland, can host rare, high 3He/4He isotopic ratios (up to 50 times the present atmospheric ratio, Ra) compared to the lower 3He/4He ratios identified in mid-ocean-ridge basalts that form by melting the upper mantle (about 8Ra; ref. 5). A long-standing hypothesis maintains that the high-3He/4He domain resides in the deep mantle, beneath the upper mantle sampled by mid-ocean-ridge basalts, and that buoyantly upwelling plumes from the deep mantle transport high-3He/4He material to the shallow mantle beneath plume-fed hotspots. One problem with this hypothesis is that, while some hotspots have 3He/4He values ranging from low to high, other hotspots exhibit only low 3He/4He ratios. Here we show that, among hotspots suggested to overlie mantle plumes, those with the highest maximum 3He/4He ratios have high hotspot buoyancy fluxes and overlie regions with seismic low-velocity anomalies in the upper mantle, unlike plume-fed hotspots with only low maximum 3He/4He ratios. We interpret the relationships between 3He/4He values, hotspot buoyancy flux, and upper-mantle shear wave velocity to mean that hot plumes—which exhibit seismic low-velocity anomalies at depths of 200 kilometres—are more buoyant and entrain both high-3He/4He and low-3He/4He material. In contrast, cooler, less buoyant plumes do not entrain this high-3He/4He material. This can be explained if the high-3He/4He domain is denser than low-3He/4He mantle components hosted in plumes, and if high-3He/4He material is entrained from the deep mantle only by the hottest, most buoyant plumes. Such a dense, deep-mantle high-3He/4He domain could remain isolated from the convecting mantle, which may help to explain the preservation of early Hadean (>4.5 billion years ago) geochemical anomalies in lavas sampling this reservoir.

Petrochemical and petrophysical characterization of the lower crust and the Moho beneath the West African Craton, based on Xenoliths from Kimberlites

NASA Technical Reports Server (NTRS)

Haggerty, Stephen E.; Toft, Paul B.

1988-01-01

Additional evidence to the composition of the lower crust and uppermost mantle was presented in the form of xenolith data. Xenoliths from the 2.7-Ga West African Craton indicate that the Moho beneath this shield is a chemically and physically gradational boundary, with intercalations of garnet granulite and garnet eclogite. Inclusions in diamonds indicate a depleted upper mantle source, and zenolith barometry and thermometry data suggest a high mantle geotherm with a kink near the Moho. Metallic iron in the xenoliths indicates that the uppermost mantle has a significant magnetization, and that the depth to the Curie isotherm, which is usually considered to be at or above the Moho, may be deeper than the Moho.

A study of upper mantle discontinuities beneath the Korean Peninsula using teleseismic receiver functions

NASA Astrophysics Data System (ADS)

Lee, S.; Park, Y.; Kim, K.; Rhie, J.

2010-12-01

The study on the topography of the upper mantle discontinuities helps us to understand the complex interactions between the subducting slabs and upper mantle discontinuities. To investigate the depth variation of the upper mantle discontinuities beneath the Korean Peninsula and surrounding regions, we applied the common conversion point stacking of the P-to-s receiver functions. The broadband seismic networks in South Korea and Japan were used to produce the high-resolution receiver function images of the region. The 410- and 660-km discontinuities (hereafter referred to as the 410 and the 660) are clearly imaged and their depth variations show interesting features, especially for the 660. In this region, the subducting Pacific slab bends to flatten over the 660 and several tomographic images indicate that the stagnant slab is extending to the west under China. If the depth of the 660 is affected by the temperature, the broad depression of the 660 is expected and several SS precursor studies support this idea. However, our observation shows that the 660 is locally depressed and its pattern is spatially changing. While the depressed 660 due to the Pacific slab is clearly imaged at lower latitudes (< 37°N), there is no evidence of the depressed 660 to the north. It indicates that the effect of the Pacific slab on the depth variation of the 660 is changing significantly in our study area.

Mantle convection pattern and subcrustal stress field under South America

NASA Technical Reports Server (NTRS)

Liu, H.-S.

1980-01-01

The tectonic, igneous and metallogenic features of South America are discussed in terms of the crustal deformation associated with stresses due to mantle convection as inferred from the high degree harmonics in the geopotential field. The application of Runcorn's model for the laminar viscous flows in the upper mantle to satellite and gravity data results in a convection pattern which reveals the ascending flows between the descending Nazca plate and the overlying South American plate as well as segments of the descending Nazca plate beneath South America. The arc volcanism in South America is shown apparently to be related to the upwelling of high-temperature material induced by the subduction of the Nazca plate, with the South American basin systems associated with downwelling mantle flows. The resulting tensional stress fields are shown to be regions of structural kinship characterized by major concentrations of ore deposits and related to the cordillera, shield and igneous systems and the upward Andean movements. It is suggested that the upwelling convection flows in the upper mantle, coupled with crustal tension, have provided an uplift mechanism which has forced the hydrothermal systems in the basement rocks to the surface.

Multi Plumes and Their Flows beneath Arabia and East Africa

NASA Astrophysics Data System (ADS)

Chang, S.; van der Lee, S.

2010-12-01

The three-dimensional S-velocity structure beneath Arabia and East Africa is estimated down to the lower mantle to investigate vertical and horizontal extension of low-velocity anomalies that bear out the presence of mantle plumes and their flows beneath lithosphere. We estimated this model through joint inversion of teleseismic S- and SKS-arrival times, regional S- and Rayleigh waveform fits, fundamental-mode Rayleigh-wave group velocities, and independent Moho constraints from receiver functions, reflection/refraction profiles, and gravity measurements. With the unprecedented resolution in our S-velocity model, we found different flow patterns of hot materials upwelling beneath Afar beneath the Red Sea and the Gulf of Aden. While the low-velocity anomaly from Afar is well confined beneath the Gulf of Aden, inferring mantle flow along the gulf, N-S channel of low velocity is found beneath Arabia, not along the Red Sea. The Afar plume is distinctively separate from the Kenya plume, showing its origin in the lower mantle beneath southwestern Arabia. We identified another low-velocity extension to the lower mantle beneath Jordan and northern Arabia, which is thought to have caused volcanism in Jordan, northern Arabia, and possibly southern Turkey. Comparing locations of mantle plumes from the joint inversion with fast axes of shear-wave splitting, we confirmed horizontal plume flow from Afar in NS direction beneath Arabia and in NE-SW direction beneath Ethiopia as a likely cause of the observed seismic anisotropy.

Seismic imaging of the lithosphere beneath Hudson Bay: Episodic growth of the Laurentian mantle keel

NASA Astrophysics Data System (ADS)

Darbyshire, Fiona A.; Eaton, David W.; Bastow, Ian D.

2013-07-01

The Hudson Bay basin in northern Canada conceals one of the major collisional zones of the Canadian Shield, the Trans-Hudson Orogen (THO), which marks the Paleoproterozoic collision between the Archean Superior and Western Churchill cratons at ˜1.9-1.8Ga. Improved knowledge of upper mantle structure beneath the region is essential to establish the nature of the THO, specifically whether Himalayan-style plate tectonics operated in Paleoproterozoic times. Detailed seismological constraints on lithospheric architecture are also required to advance our understanding of the mechanism and timing of keel formation. We use surface wave tomography to illuminate new details of the lithospheric architecture of the Hudson Bay region, resolving both seismic wavespeed and azimuthal anisotropy. Phase velocity maps are calculated from fundamental-mode Rayleigh wave dispersion curves, then used to construct a 3D model exploring upper mantle structure to depths of ˜300km. Fast shear wavespeeds suggest a lithospheric thickness varying from ˜180km to almost 280 km beneath the Hudson Bay region. The new study confirms previous inferences that there is no correlation between crustal ages and lithospheric thickness. Patterns of shear wavespeed and azimuthal anisotropy indicate a layered lithosphere. In the uppermost mantle, both the highest velocities and the anisotropic fast directions wrap around the Bay. This structure is likely related to the formation processes of the Paleozoic intracratonic basin. At greater depth (˜70-150km) we resolve two high-wavespeed cores separated by a relatively narrow near-vertical lower-velocity curtain. This internal architecture is suggested to result from the terminal phase of a modern-style plate-tectonic collision between the Archean Superior and Churchill cratons during the Trans-Hudson orogeny, entrapping juvenile Proterozoic material. The lower lithosphere (≥160km depth) has a relatively homogeneous wavespeed structure across the region, with distinct patterns of anisotropy closely resembling the subsurface geometry of the THO. We interpret this basal layer as juvenile or reworked material accreted to the base of the existing cratonic lithosphere during or soon after the Trans-Hudson orogeny. The formation of the Laurentian keel thus likely occurred in multiple phases, with a basal layer developing in post-Archean times, during the THO.

Multiple mantle upwellings through the transition zone beneath the Afar Depression?

NASA Astrophysics Data System (ADS)

Hammond, J. O.; Kendall, J. M.; Stuart, G. W.; Thompson, D. A.; Ebinger, C. J.; Keir, D.; Ayele, A.; Goitom, B.; Ogubazghi, G.

2012-12-01

Previous seismic studies using regional deployments of sensors in East-Africa show that low seismic velocities underlie Africa, but their resolution is limited to the top 200-300km of the Earth. Thus, the connection between the low velocities in the uppermost mantle and those imaged in global studies in the lower mantle is unclear. We have combined new data from Afar, Ethiopia with 7 other regional experiments and global network stations across Kenya, Ethiopia, Eritrea, Djibouti and Yemen, to produce high-resolution models of upper mantle P- and S-wave velocities to the base of the transition zone. Relative travel time tomographic inversions show that within the transition zone two focussed sharp-sided low velocity regions exist: one beneath the Western Ethiopian plateau outside the rift valley, and the other beneath the Afar depression. Estimates of transition zone thickness suggest that this is unlikely to be an artefact of mantle discontinuity topography as a transition zone of normal thickness underlies the majority of Afar and surrounding regions. However, a low velocity layer is evident directly above the 410 discontinuity, co-incident with some of the lowest seismic velocities suggesting that smearing of a strong low velocity layer of limited depth extent may contribute to the tomographic models in north-east Afar. The combination of seismic constraints suggests that small low temperature (<50K) upwellings may rise from a broader low velocity plume-like feature in the lower mantle. This interpretation is supported by numerical and analogue experiments that suggest the 660km phase change and viscosity jump may impede flow from the lower to upper mantle creating a thermal boundary layer at the base of the transition zone. This allows smaller, secondary upwellings to initiate and rise to the surface. These, combined with possible evidence of melt above the 410 discontinuity can explain the seismic velocity models. Our images of secondary upwellings suggest that there is no evidence for a plume in the classical sense (i.e. a narrow conduit). Instead, we propose that secondary upwellings rise from the base of the transition zone and connect with the northeast flowing African superswell in the upper mantle.

Three-dimensional crustal structure of the southern Sierra Nevada from seismic fan profiles and gravity modeling

USGS Publications Warehouse

Fliedner, M.M.; Ruppert, S.; Malin, P.E.; Park, S.K.; Jiracek, G.; Phinney, R.A.; Saleeby, J.B.; Wernicke, B.; Clayton, R.; Keller, Rebecca Hylton; Miller, K.; Jones, C.; Luetgert, J.H.; Mooney, W.D.; Oliver, H.; Klemperer, S.L.; Thompson, G.A.

1996-01-01

Traveltime data from the 1993 Southern Sierra Nevada Continental Dynamics seismic refraction experiment reveal low crustal velocities in the southern Sierra Nevada and Basin and Range province of California (6.0 to 6.6 km/s), as well as low upper mantle velocities (7.6 to 7.8 km/s). The crust thickens from southeast to northwest along the axis of the Sierra Nevada from 27 km in the Mojave Desert to 43 km near Fresno, California. A crustal welt is present beneath the Sierra Nevada, but the deepest Moho is found under the western slopes, not beneath the highest topography. A density model directly derived from the crustal velocity model but with constant mantle density satisfies the pronounced negative Bouguer anomaly associated with the Sierra Nevada, but shows large discrepancies of >50 mgal in the Great Valley and in the Basin and Range province. Matching the observed gravity with anomalies in the crust alone is not possible with geologically reasonable densities; we require a contribution from the upper mantle, either by lateral density variations or by a thinning of the lithosphere under the Sierra Nevada and the Basin and Range province. Such a model is consistent with the interpretation that the uplift of the present Sierra Nevada is caused and dynamically supported by asthenospheric upwelling or lithospheric thinning under the Basin and Range province and eastern Sierra Nevada.

A 3-D shear velocity model of the southern North America and the Caribbean plates from ambient noise and earthquake tomography

NASA Astrophysics Data System (ADS)

Gaite, B.; Villaseñor, A.; Iglesias, A.; Herraiz, M.; Jiménez-Munt, I.

2014-10-01

We use group velocities from earthquake tomography together with group and phase velocities from ambient noise tomography (ANT) of Rayleigh-waves to invert for the 3-D shear-wave velocity structure (5-70 km) of the Caribbean (CAR) and southern North American (NAM) plates. The lithospheric model proposed offers a complete image of the crust and uppermost-mantle with imprints of the tectonic evolution. One of the most striking features inferred is the main role of the Ouachita-Marathon-Sonora orogeny front on the crustal seismic structure of NAM plate. A new imaged feature is the low crustal velocities along USA-Mexico border. The model also shows a break of the E-W mantle velocity dichotomy of the NAM and CAR plates beneath the Isthmus of Tehuantepec and Yucatan Block. High upper-mantle velocities along the Mesoamerican Subduction Zone coincide with inactive volcanic areas while the lowest velocities correspond to active volcanic arcs and thin lithospheric mantle regions.

A 3-D shear velocity model of the southern North American and Caribbean plates from ambient noise and earthquake tomography

NASA Astrophysics Data System (ADS)

Gaite, B.; Villaseñor, A.; Iglesias, A.; Herraiz, M.; Jiménez-Munt, I.

2015-02-01

We use group velocities from earthquake tomography together with group and phase velocities from ambient noise tomography (ANT) of Rayleigh waves to invert for the 3-D shear-wave velocity structure (5-70 km) of the Caribbean (CAR) and southern North American (NAM) plates. The lithospheric model proposed offers a complete image of the crust and uppermost-mantle with imprints of the tectonic evolution. One of the most striking features inferred is the main role of the Ouachita-Marathon-Sonora orogeny front on the crustal seismic structure of the NAM plate. A new imaged feature is the low crustal velocities along the USA-Mexico border. The model also shows a break of the east-west mantle velocity dichotomy of the NAM and CAR plates beneath the Isthmus of the Tehuantepec and the Yucatan Block. High upper-mantle velocities along the Mesoamerican Subduction Zone coincide with inactive volcanic areas while the lowest velocities correspond to active volcanic arcs and thin lithospheric mantle regions.

Seismic velocity structure of the crust and shallow mantle of the Central and Eastern United States by seismic surface wave imaging

USGS Publications Warehouse

Pollitz, Fred; Mooney, Walter D.

2016-01-01

Seismic surface waves from the Transportable Array of EarthScope's USArray are used to estimate phase velocity structure of 18 to 125 s Rayleigh waves, then inverted to obtain three-dimensional crust and upper mantle structure of the Central and Eastern United States (CEUS) down to ∼200 km. The obtained lithosphere structure confirms previously imaged CEUS features, e.g., the low seismic-velocity signature of the Cambrian Reelfoot Rift and the very low velocity at >150 km depth below an Eocene volcanic center in northwestern Virginia. New features include high-velocity mantle stretching from the Archean Superior Craton well into the Proterozoic terranes and deep low-velocity zones in central Texas (associated with the late Cretaceous Travis and Uvalde volcanic fields) and beneath the South Georgia Rift (which contains Jurassic basalts). Hot spot tracks may be associated with several imaged low-velocity zones, particularly those close to the former rifted Laurentia margin.

Probability density functions for radial anisotropy: implications for the upper 1200 km of the mantle

NASA Astrophysics Data System (ADS)

Beghein, Caroline; Trampert, Jeannot

2004-01-01

The presence of radial anisotropy in the upper mantle, transition zone and top of the lower mantle is investigated by applying a model space search technique to Rayleigh and Love wave phase velocity models. Probability density functions are obtained independently for S-wave anisotropy, P-wave anisotropy, intermediate parameter η, Vp, Vs and density anomalies. The likelihoods for P-wave and S-wave anisotropy beneath continents cannot be explained by a dry olivine-rich upper mantle at depths larger than 220 km. Indeed, while shear-wave anisotropy tends to disappear below 220 km depth in continental areas, P-wave anisotropy is still present but its sign changes compared to the uppermost mantle. This could be due to an increase with depth of the amount of pyroxene relative to olivine in these regions, although the presence of water, partial melt or a change in the deformation mechanism cannot be ruled out as yet. A similar observation is made for old oceans, but not for young ones where VSH> VSV appears likely down to 670 km depth and VPH> VPV down to 400 km depth. The change of sign in P-wave anisotropy seems to be qualitatively correlated with the presence of the Lehmann discontinuity, generally observed beneath continents and some oceans but not beneath ridges. Parameter η shows a similar age-related depth pattern as shear-wave anisotropy in the uppermost mantle and it undergoes the same change of sign as P-wave anisotropy at 220 km depth. The ratio between dln Vs and dln Vp suggests that a chemical component is needed to explain the anomalies in most places at depths greater than 220 km. More tests are needed to infer the robustness of the results for density, but they do not affect the results for anisotropy.

Mantle Flow Across the Baikal Rift Constrained With Integrated Seismic Measurements

NASA Astrophysics Data System (ADS)

Lebedev, S.; Meier, T.; van der Hilst, R. D.

2005-12-01

The Baikal Rift is located at the boundary of the stable Siberian Craton and deforming central Mongolia. The origin of the late Cenozoic rifting and volcanism are debated, as is the mantle flow beneath the rift zone. Here we combine new evidence from azimuthally-anisotropic upper-mantle tomography and from a radially-anisotropic inversion of interstation surface-wave dispersion curves with previously published shear-wave-splitting measurements of azimuthal anisotropy across the rift (Gao et al. 1994). While our tomographic model maps isotropic and anisotropic shear-velocity heterogeneity globally, the inversion of interstation phase-velocity measurements produces a single, radially-anisotropic, shear-velocity profile that averages from the rift to 500 km SE of it. The precision and the broad band (8-340 s) of the Rayleigh and Love wave curves ensures high accuracy of the profile. Tomography and shear-wave splitting both give a NW-SE fast direction (perpendicular to the rift) in the vicinity of the rift, changing towards W-E a few hundred kilometers from it. Previously, this has been interpreted as evidence for mantle flow similar to that beneath mid-ocean ridges, with deeper vertical flow directly beneath the rift also proposed. Our radially anisotropic profile, however, shows that while strong anisotropy with SH waves faster than SV waves is present in the thin lithosphere and upper asthenosphere beneath and SE of the rift, no anisotropy is required below 110 km. The tomographic model shows thick cratonic lithosphere north of the rift. These observations suggest that instead of a flow diverging from the rift axis in NW and SE directions, the most likely pattern is the asthenospheric flow in SE direction from beneath the Siberian lithosphere and across the rift. Possible driving forces of the flow are large-scale lithospheric deformation in East Asia and the draining of asthenosphere at W-Pacific subduction zones; a plume beneath the Siberian craton also cannot be ruled out. As shown for the model of subcontinental asthenospheric flow by Morgan and Morgan (2005), this mantle flow pattern can explain not only the rifting but also the basaltic volcanism observed in the Lake Baikal region.

Färoe-Iceland Ridge Experiment: 1. Crustal structure of northeastern Iceland

USGS Publications Warehouse

Staples, Robert K.; White, Robert S.; Brandsdottir, Bryndis; Menke, William; Maguire, Peter K.H.; McBride, John H.

1997-01-01

Results from the Färoe-Iceland Ridge Experiment (FIRE) constrain the crustal thickness as 19 km under the Northern Volcanic Zone of Iceland and 35 km under older Tertiary areas of northeastern Iceland. The Moho is defined by strong P wave and S wave reflections. Synthetic seismogram modeling of the Moho reflection indicates mantle velocities of at least 8.0 km/s beneath the Tertiary areas of northeastern Iceland and at least 7.9 km/s beneath the neovolcanic zone. Crustal diving rays resolve the structure of the upper and lower crust. Surface P wave velocities are 1.1–4.0 km/s in Quaternary rocks and are rather higher, 4.4–4.7 km/s, in the Tertiary basalts that outcrop elsewhere. The highest crustal P wave velocities observed directly from diving rays are 7.1 km/s, from rays that turn at 24 km depth. Velocities of 7.35 km/s at the base of the crust are inferred from extrapolation of the lower crustal velocity gradient (0.024 s−1). A Poisson's ratio of approximately 0.27, equivalent to an S wave to P wave travel time ratio of 1.78, is measured throughout the crust east of the neovolcanic zone. The Poisson's ratio and the steep Moho topography (in places up to 30° from the horizontal) indicate that the entire crust outside the neovolcanic zone is cool (<800°C). Gravity data are well matched by a velocity/density conversion of our seismic crustal model and indicate a region of low mantle density beneath the neovolcanic zone, believed to be due to elevated mantle temperatures. The crustal thickness in the neovolcanic zone is consistent with geochemical estimates of the melt generation, placing constraints on the flow within the Iceland mantle plume.

Local Upper Mantle Upwelling beneath New England: Evidence from Seismic Anisotropy.

NASA Astrophysics Data System (ADS)

Levin, V. L.; Long, M. D.; Lopez, I.; Li, Y.; Skryzalin, P. A.

2017-12-01

The upper mantle beneath eastern North America contains regions where seismic wave speed is significantly reduced. As they cut across the trend of the Appalachian terranes, these anomalies likely post-date the Paleozoic assembly of Pangea. Most prominent of them, the North Appalachian Anomaly (NAA), has been alternatively explained by the localized disruption of lithospheric fabric, the passage of the Great Meteor Hot Spot, and the current local upwelling of the asthenosphere. Comprehensive mapping of shear wave splitting identified a local perturbation of an otherwise uniform regional pattern, with no apparent splitting occurring at a site within the NAA. To evaluate the reality of this apparent localized disruption in the anisotropic fabric of the upper mantle beneath northeastern North America we used observations of shear wave splitting from a set of long-running observatories not included in previous studies. Three methods of evaluating shear wave splitting (rotation-correlation, minimization of the transverse component, and the splitting intensity) yield complementary results. We show that splitting of core-refracted shear waves within the outline of the NAA is significantly weaker than towards its edges and beyond them (Figure 1). Average fast orientations are close to the absolute plate motion in the hot-spot reference frame, thus we can attribute a large fraction of this signal to the coherently sheared sub-lithospheric upper mantle. A decrease in average delay we observe, from 1 s outside the NAA to under 0.2 s within it, translates into a reduction of the vertical extent of the sheared layer from 130 km to 16 km (assuming 4% anisotropy), or alternatively into a weakening of the azimuthal anisotropy from 5% to 0.6% (assuming a 100 km thick layer). The splitting reduction within the NAA is consistent with a localized change in anisotropic fabric that would be expected in case of geologically recent sub-vertical flow overprinting the broadly uniform upper mantle fabric detected throughout the region. Figure 1. Splitting intensity (red circles) plotted over best-fitting sinusoidal functions (blue, parameters in upper right) and predictions based on average delays and fast polarizations (green, parameters in upper left). Outlines of the NAA at 200 km depth from tomographic studies using Earthscope data.

Widespread imaging of the lower crust, Moho, and upper mantle from Rayleigh waves: A comparison of the Cascadia and Aleutian-Alaska subduction zones

NASA Astrophysics Data System (ADS)

Haney, M. M.; Tsai, V. C.; Ward, K. M.

2016-12-01

Recently, Haney and Tsai (2015) developed a new approach to Rayleigh-wave inversion based on assumptions that are similar to those used in the formulation of the Dix equation in reflection seismology. Here we apply the Dix technique to Rayleigh-wave phase-velocity maps by Ekstrom (2013) and Ward (2015) of the contiguous US and Alaska, respectively, at periods between 12 and 45 s. We refine the initial Dix result with subsequent nonlinear inversion to estimate Moho depth together with shear-wave velocity of the lower crust and upper mantle. In the contiguous US, the Moho we image agrees well with recent receiver function studies. There is an apparent deepening of the Moho to the west of the Cascades volcanic chain that we interpret as the waveguide interface transitioning to the slab due to the continental Moho becoming transparent above the mantle forearc. This feature abruptly terminates at the southern extent of the Cascadia subduction zone. We compare the depths of this "apparent Moho" with published estimates of the depth to the Juan de Fuca Plate since, owing to the paucity of tectonic earthquakes, the Slab1.0 model is not defined in Cascadia. Our result in Alaska is the first regional Moho map derived explicitly from seismic waves. We find that crustal thickness is generally correlated with topography, with thicker crust beneath mountain ranges in southern Alaska. North of the Denali Fault, the Moho is smoother than to the south and located at typical depths of 30-35 km. There are also indications that the waveguide interface we solve for beneath Prince William Sound is actually the subducting slab instead of the continental Moho. The slab structure beneath Prince William Sound extends further east than the Pacific slab represented in the Slab1.0 model. Using the limited number of broadband seismometers in the Aleutian Islands, we obtain preliminary estimates for the crustal structure beneath the western portion of the Aleutian-Alaska subduction zone.

Deciphering the Tectonic History of the Northern Transantarctic Mountains

NASA Astrophysics Data System (ADS)

Hansen, Samantha; Graw, Jordan; Brenn, Gregory; Kenyon, Lindsey; Park, Yongcheol; DuBay, Brian

2016-04-01

The Transantarctic Mountains (TAMs) are the largest non-compressional mountain range in the world, and their structure plays a key role in the climatic and tectonic development of Antarctica. While numerous uplift mechanisms for the TAMs have been proposed, there is little consensus on their origin. Over the past three years, we have operated a network of 15 broadband seismic stations within a previously unexplored portion of the northern TAMs. Using data collected by this array, we have undertaken numerous studies to further assess the crustal and lithospheric structure beneath the mountain range and to differentiate between competing origin models. Receiver functions indicate crustal thickening inland from the Ross Sea coast but comparable crustal thickness beneath the TAMs and the East Antarctic plateau, indicating little evidence for a substantial crustal root beneath the mountain range. Body and surface wave analyses show a pronounced low-velocity anomaly beneath Terror Rift, adjacent to the TAMs, and extending beneath Victoria Land in the upper mantle. Together, these findings support a thermally-buoyant source of uplift for the northern TAMs and broad flexure of the East Antarctic lithosphere.

Seismological observations at the Northern Andean region of Colombia: Evidence for a shallowly subducting Caribbean Slab and an extensional regime in the upper plate

NASA Astrophysics Data System (ADS)

Monsalve, G.; Cardona, A.; Yarce, J.; Alvira, D.; Poveda, E.

2013-05-01

A number of seismological observations, among which we can mention teleseismic travel time residuals, P to S receiver functions and Pn velocity quantification, suggest a clear distinction between the seismic structure of the crust and uppermost mantle between the plains on the Caribbean coast of Colombia and the mountains at the Northern Andean region. Absolute and relative travel time residuals indicate the presence of a seismically fast material in the upper mantle beneath northern Colombia; preliminary results of Pn studies show a region of relatively slow Pn velocities (between 7.8 and 7.9 km/s) underneath the Caribbean coast, contrasting with values greater than 8 km/s beneath the Central and Western cordilleras of Colombia, and the Pacific coast; receiver functions suggest a significantly thinner crust beneath the Caribbean coast, with a crustal thickness between 25 and 30 km, than beneath the Northern Andean zone at the cordilleras of Colombia, where it exceeds 40 km and reaches about 57 km at the location of Bogota. Besides the obviuos discrepancies that appear in response to different topography, we think that the seismological observations are a consequence of the presence of two very distinct slab segments beneath Colombia and contrasting behaviors of the upper plate, which correspond to Caribbean and Nazca subductions. Our seismic observations can be explained by a shallowly subducting Caribbean Plate, in the absence of an asthenospheric wedge, that steepens at about the location of the Bucaramanga nest, and a thinned continental crust that reflects an extensional component linked to oblique convergence of the Caribbean, which contrasts with the crustal thickening in the Andean Cordillera linked to crustal shortening and Nazca plate subuction. These new data are consistent with the idea of of a relatively warm Nazca slab of Neogene age which seems to have a relatively frontal convergence, and a colder, more buoyant Caribbean slab which represents an oceanic plateau of Cretaceous age that is characterized by an oblique convergence relation that has promoted extensional tectonics in the upper plate.

P and S Body Wave Tomography of the West Antarctic Rift System: Evidence for Recent Cenozoic Rifting

NASA Astrophysics Data System (ADS)

Soto, D. R.; Nyblade, A.; Anandakrishnan, S.; Aster, R. C.; Wiens, D.; Huerta, A. D.; Winberry, J. P.; Wilson, T. J.

2017-12-01

Imaging the upper mantle of West Antarctica can provide valuable information about its deep structure, the source of subglacial volcanism, and the age of rifting in the West Antarctic Rift System (WARS). The WARS extends across West Antarctica and is characterized by low sub-ice sheet topography, with the deepest area being the Bentley Subglacial Trench. Seismic data from POLENET/ANET broadband seismic stations were used to obtain improved body wave images of the upper mantle. The data comes from 34 backbone stations, 13 temporary broadband stations deployed across the WARS from the Whitmore Mountains to Marie Byrd Land from January 2010 to January 2012, 10 stations deployed above the Byrd Subglacial Basin from January 2015 to January 2017, and 5 stations from the UKANET network deployed January 2016 to the present. Using multi-channel cross correlation of P and S body waves from teleseismic earthquakes, travel time residuals have been obtained from 360 events for the P-wave model and 263 events for the S-wave model. The VanDecar's method of linear inversion method has been used to develop a model of relative P and S wave velocity variations in the upper mantle. Preliminary P and S wave models show a low velocity anomaly 150 km beneath Marie Byrd Land and faster wave speeds across much of the WARS, except for beneath the Bentley Subglacial Trench, where a modest low wave speed region is imaged. These results are consistent with previously published tomographic models of West Antarctica.

Crustal and upper-mantle structure beneath ice-covered regions in Antarctica from S-wave receiver functions and implications for heat flow

NASA Astrophysics Data System (ADS)

Ramirez, C.; Nyblade, A.; Hansen, S. E.; Wiens, D. A.; Anandakrishnan, S.; Aster, R. C.; Huerta, A. D.; Shore, P.; Wilson, T.

2016-03-01

S-wave receiver functions (SRFs) are used to investigate crustal and upper-mantle structure beneath several ice-covered areas of Antarctica. Moho S-to-P (Sp) arrivals are observed at ˜6-8 s in SRF stacks for stations in the Gamburtsev Mountains (GAM) and Vostok Highlands (VHIG), ˜5-6 s for stations in the Transantarctic Mountains (TAM) and the Wilkes Basin (WILK), and ˜3-4 s for stations in the West Antarctic Rift System (WARS) and the Marie Byrd Land Dome (MBLD). A grid search is used to model the Moho Sp conversion time with Rayleigh wave phase velocities from 18 to 30 s period to estimate crustal thickness and mean crustal shear wave velocity. The Moho depths obtained are between 43 and 58 km for GAM, 36 and 47 km for VHIG, 39 and 46 km for WILK, 39 and 45 km for TAM, 19 and 29 km for WARS and 20 and 35 km for MBLD. SRF stacks for GAM, VHIG, WILK and TAM show little evidence of Sp arrivals coming from upper-mantle depths. SRF stacks for WARS and MBLD show Sp energy arriving from upper-mantle depths but arrival amplitudes do not rise above bootstrapped uncertainty bounds. The age and thickness of the crust is used as a heat flow proxy through comparison with other similar terrains where heat flow has been measured. Crustal structure in GAM, VHIG and WILK is similar to Precambrian terrains in other continents where heat flow ranges from ˜41 to 58 mW m-2, suggesting that heat flow across those areas of East Antarctica is not elevated. For the WARS, we use the Cretaceous Newfoundland-Iberia rifted margins and the Mesozoic-Tertiary North Sea rift as tectonic analogues. The low-to-moderate heat flow reported for the Newfoundland-Iberia margins (40-65 mW m-2) and North Sea rift (60-85 mW m-2) suggest that heat flow across the WARS also may not be elevated. However, the possibility of high heat flow associated with localized Cenozoic extension or Cenozoic-recent magmatic activity in some parts of the WARS cannot be ruled out.

Shear Wave Velocities in the Pampean Flat Slab Region from Rayleigh Wave Tomography: Implications for Crustal Composition and Upper Mantle Hydration

NASA Astrophysics Data System (ADS)

Porter, R. C.; Gilbert, H. J.; Zandt, G.; Beck, S. L.; Warren, L. M.; Calkins, J. A.; Alvarado, P. M.; Anderson, M. L.

2011-12-01

The Pampean flat slab region, located in Chile and western Argentina between 29° and 34° S, is characterized by the subducting Nazca plate assuming a sub-horizontal geometry for ~300 km laterally before resuming a more "normal" angle of subduction. The onset of flat slab subduction is associated with the cessation of regional arc related volcanism and the migration of deformation inboard from the high Andes into the thin-skinned Precordillera and thick-skinned Sierras Pampeanas. Developing a better understanding of this region's geology is of particular importance, as it is an ideal area to study flat slab subduction and serves as a modern analogue to Laramide flat slab subduction in the western US. To study the crustal and mantle structure in the region, we combine ambient noise tomography and ballistic surface wave tomography to produce a regional 3D shear wave velocity model that encompasses flat slab subduction in the north and normal subduction geometry in the south, allowing for a comparison of the two. Results from this work show that shear velocities within the upper crust are largely determined by composition, with sedimentary basins and areas with active volcanism exhibiting slower velocities than basement cored uplifts and other bedrock exposures. Though surface waves are not particularly sensitive to the depth of sharp velocity contrasts, we observe an eastward increase in shear velocity at depth that correlates with an eastward decrease in crustal thickness. In both the slab and overlying mantle, we observe significant variations in shear wave velocity. North of 32° S, where flat slab subduction is occurring, the Nazca plate contains low-velocity zones (LVZs) beneath the high Andes and Precordillera that are not present in the east beneath the Sierras Pampeanas. An opposite transition is observed in the overlying mantle, which changes from fast in the west to slow in the east. Both of these observations are consistent with an initially hydrated slab dehydrating and releasing water into the overlying mantle. Within this region we also observe a LVZ immediately above the slab as the subduction angle steepens. This zone potentially represents asthenosphere or hydrated lithospheric mantle. South of 32° S, where subduction is occurring at a more normal angle, the slab is visible as a high-velocity body with a low-velocity mantle wedge present beneath the arc and back arc. The variations in slab and upper mantle shear velocities are consistent with a hydrated flat slab and the presence of a LVZ above the flat slab as it steepens suggests that water is being transported to a significant depth or that an asthenospheric wedge is present between the slab and cratonic lithosphere.

Mantle Flow and Dehydration Beneath the Juan de Fuca Plate Revealed by Shear Velocity and Attenuation

NASA Astrophysics Data System (ADS)

Ruan, Y.; Forsyth, D. W.; Bell, S. W.

2017-12-01

At mid-ocean-ridge spreading centers, it is still unclear to what extent the upwelling is purely passive, driven by viscous drag of the separating plates, or dynamically driven by the buoyancy induced by melt retention and depletion of the mantle matrix. The distinct sensitivities of seismic wavespeed and attenuation to temperature, melt porosity, water content and major element composition yield some of the primary constraints on mid-ocean ridge processes and the associated flow pattern, melt distribution, and the interaction of spreading centers with hotspots. Extensive arrays of ocean-bottom seismometers (OBS) with better quality, longer deployment periods, and the application of noise-removal techniques together provided higher quality data in this study than in any previous regional study of velocity and attenuation of the upper mantle beneath a spreading center. Based on the fundamental-mode Rayleigh waves, we imaged shear wave attenuation and velocity models in the vicinity of the Juan de Fuca plate with the best resolution to date of any spreading center. There is strong attenuation centered at depths of 70-80 km, just below the expected dry solidus and somewhat deeper than predicted for a model of passive mantle upwelling beneath the spreading center. The shear velocity structure shows lowest velocities west of the spreading center, particularly near Axial Seamount and high velocities east of the axis extending to a greater depth than predicted by the passive flow model. Together, these observations support a model in which buoyant upwelling west of the spreading center first depletes and dehydrates the mantle above the dry solidus by melt removal and then the associated downwelling carries depleted, melt-free, residual mantle downward beneath the Juan de Fuca plate. This depleted, dehydrated, melt-free layer can explain why the average attenuation is lower than expected and the velocity is higher than expected in the 30 to 70 km depth range. The compositional buoyancy of the depleted mantle may in most places limit downwelling to the vicinity of the spinel peridotite to garnet peridotite transition at a depth of 80 km.

Tomographic Imaging of the Cascadia Subduction Zone and Juan de Fuca Plate System: Improved Methods Eliminate Artifacts and Reveal New Structures

NASA Astrophysics Data System (ADS)

Bodmer, M.; Toomey, D. R.; Hooft, E. E. E.; Bezada, M.; Schmandt, B.; Byrnes, J. S.

2017-12-01

Amphibious studies of subduction zones promise advances in understanding links between incoming plate structure, the subducting slab, and the upper mantle beneath the slab. However, joint onshore/offshore imaging is challenging due to contrasts between continental and oceanic structure. We present P-wave teleseismic tomography results for the Cascadia subduction zone (CSZ) that utilize existing western US datasets, amphibious seismic data from the Cascadia Initiative, and tomographic algorithms that permit 3D starting models, nonlinear ray tracing, and finite frequency kernels. Relative delay times show systematic onshore/offshore trends, which we attribute to structure in the upper 50 km. Shore-crossing CSZ seismic refraction models predict relative delays >1s, with equal contributions from elevation and crustal thickness. We use synthetic data to test methods of accounting for such shallow structure. Synthetic tests using only station static terms produce margin-wide, sub-slab low-velocity artifacts. Using a more realistic a priori 3D model for the upper 50 km better reproduces known input structures. To invert the observed delays, we use data-constrained starting models of the CSZ. Our preferred models utilize regional surface wave studies to construct a starting model, directly account for elevation, and use 3D nonlinear ray tracing. We image well-documented CSZ features, including the subducted slab down to 350 km, along strike slab variations below 150 km, and deep slab fragmentation. Inclusion of offshore data improves resolution of the sub-slab mantle, where we resolve localized low-velocity anomalies near the edges of the CSZ (beneath the Klamath and Olympic mountains). Our new imaging and resolution tests indicate that previously reported margin-wide, sub-slab low-velocity asthenospheric anomalies are an imaging artifact. Offshore, we observe low-velocity anomalies beneath the Gorda plate consistent with regional deformation and broad upwelling resulting from plate stagnation. At the Juan de Fuca Ridge we observe asymmetric low-velocity anomalies consistent with dynamic upwelling. Our results agree with recent offshore tomography studies using S wave data; however, differences in the recovered relative amplitudes are likely due to anisotropy, which we are exploring.

Seismic images of the transition zone: is Hawaiian volcanism produced by a secondary plume from the top of the lower mantle?

NASA Astrophysics Data System (ADS)

Cao, Q.; van der Hilst, R. D.; Shim, S.; De Hoop, M. V.

2011-12-01

The Hawaiian hotspot is often attributed to hot material rising from depth in the mantle, but efforts to detect a thermal plume seismically have been inconclusive. Most tomographic models reveal anomalously low wavespeeds beneath Hawaii, but the depth extent of this structure is not well known. S or P data used in traveltime inversions are associated with steep rays to distant sources, which degrades depth resolution, and surface wave dispersion does not have sufficient sensitivity at the depths of interest. To investigate pertinent thermal anomalies we mapped depth variations of upper mantle discontinuities using precursors of the surface-reflected SS wave. Instead of stacking the data over geographical bins, which leads to averaging of topography and hence loss of spatial resolution, we used a generalized Radon transform (GRT) to detect and map localized elasticity contrasts in the transition zone (Cao et al., PEPI, 2010). We apply the GRT to produce 3D image volumes beneath a large area of the Pacific Ocean, including Hawaii and the Hawaii-Emperor seamount chain (Cao et al., Science, 2011). The 3D image volumes reveal laterally continuous interfaces near 410 and 660 km depths, that is, the traditional boundaries of the transition zone, but also suggest (perhaps intermittent) scatter horizons near 300-350, 520-550, and 800-1000 km depth. The upper mantle appears generally hot beneath Hawaii, but the most conspicuous topographic (and probably thermal) anomalies are found west of Hawaii. The GRT images reveal a 800 km wide uplift of the 660 discontinuity just west of Hawaii, but there is no evidence for a corresponding localized depression of the 410 discontinuity. This expression of the 410 and 660 km topographies is consistent with some existed geodynamical modeling results, in which a deep-rooted mantle plume impinging on the transition zone, creating a broad pond of hot material underneath endothermic phase change at 660 km depth, and with secondary plumes stemming from this hot pool of materials and rising in the upper mantle to create the present-day hotspot at Earth's surface. West of the upwarp that we interpret as the elevated post-spinel the main interface deepens to nearly 700 km depth. Given this position, it is unlikely that this deep structure is due to low temperatures. Instead, it would be consistent with slightly elevated temperatures (compared to transition temperature of post-spinel) and transitions in the garnet phase. This interpretation, if correct, implies that the area of ponded hot material is at least 2,000 km wide. The presence of an 800- to 2,000-kilometer-wide thermal anomaly deep in the transition zone west of Hawaii suggests that hot material does not rise from the lower mantle through a narrow vertical plume but accumulates near the base of the transition zone before being entrained in flow toward Hawaii and, perhaps, other islands. This implies that geochemical trends in Hawaiian lavas cannot constrain lower mantle domains directly. This type of flow may be a better explanation of bathymetric features in the Pacific (including other seamount chains) than the canonical deep mantle plumes.

Fine-scale crustal structure of the Azores Islands from teleseismic receiver functions

NASA Astrophysics Data System (ADS)

Spieker, K.; Rondenay, S.; Ramalho, R. S.; Thomas, C.; Helffrich, G. R.

2016-12-01

The Azores plateau is located near the Mid-Atlantic Ridge (MAR) and consists of nine islands, most of which lie east of the MAR. Various methods including seismic reflection, gravity, and passive seismic imaging have been used to investigate the crustal thickness beneath the islands. They have yielded thickness estimates that range between roughly 10 km and 30 km, but until now models of the fine-scale crustal structure have been lacking. A comparison of the crustal structure beneath the islands that lie west and east of the MAR might give further constraints on the evolution of the islands. For example, geochemical studies carried out across the region predict the existence of volcanic interfaces that should be detected seismically within the shallow crust of some of the islands. In this study, we use data from ten seismic stations located on the Azores Islands to investigate the crustal structure with teleseismic P-wave receiver functions. We query our resulting receiver functions for signals associated with the volcanic edifice, the crust-mantle boundary, and potential underplated layers beneath the various islands. The islands west of the MAR have a crustal structure comprising two discontinuities - an upper one at 1-2 km depth marking the base of the volcanic edifice, and a lower one at 10 km depth that we interpret as crust-mantle boundary. The islands east of the MAR can be subdivided into two groups. The central islands that are closer to the MAR exhibit a crustal structure similar to that of the western islands, with a volcanic edifice reaching a depth of 2 km and an average crust-mantle boundary at around 12 km depth. The easternmost islands, located on the oldest lithosphere, exhibit a more complex crustal structure with evidence for a mid-crustal interface and an underplated layer, yielding an effective crust-mantle boundary at >15 km depth. The difference in structure between proximal and distal islands might be related to the age of the plate at the time of emplacement of the islands, with an older plate providing conditions that are more favourable for basaltic underplating.

Seismic properties of the upper mantle beneath Lanzarote (Canary Islands): Model predictions based on texture measurements by EBSD

NASA Astrophysics Data System (ADS)

Vonlanthen, Pierre; Kunze, Karsten; Burlini, Luigi; Grobety, Bernard

2006-12-01

We present a petrophysical analysis of upper mantle xenoliths, collected in the Quaternary alkali basalt fields (Series III and IV) from the island of Lanzarote. The samples consist of eight harzburgite and four dunite nodules, 5 to 15 cm in size, and exhibit a typical protogranular to porphyroclastic texture. An anomalous foliation resulting from strong recovery processes is observed in half of the specimens. The lattice preferred orientations (LPO) of olivine, orthopyroxene and clinopyroxene were measured using electron backscatter diffraction (EBSD). In most samples, olivine exhibits LPOs intermediate between the typical single crystal texture and the [100] fiber texture. Occasionally, the [010] fiber texture was also observed. Simultaneous occurrence of both types of fiber textures suggests the existence of more than one deformation regime, probably dominated by a simple shear component under low strain rate and moderate to high temperature. Orthopyroxene and clinopyroxene display a weaker but significant texture. The LPO data were used to calculate the seismic properties of the xenoliths at PT conditions obtained from geothermobarometry, and were compared to field geophysical data reported from the literature. The velocity of P-waves (7.9 km/s) obtained for a direction corresponding to the existing seismic transect is in good agreement with the most recent geophysical interpretation. Our results are consistent with a roughly W-E oriented fastest P-wave propagation direction in the uppermost mantle beneath the Canary Islands, and with the lithosphere structure proposed by previous authors involving a crust-mantle boundary at around 18 km in depth, overlaid by intermediate material between 11 and 18 km.

Crustal structure across the Altyn Tagh Range at the northern margin of the Tibetan Plateau and tectonic implications

USGS Publications Warehouse

Zhao, J.; Mooney, W.D.; Zhang, X.; Li, Z.; Jin, Z.; Okaya, N.

2006-01-01

We present new seismic refraction/wide-angle-reflection data across the Altyn Tagh Range and its adjacent basins. We find that the crustal velocity structure, and by inference, the composition of the crust changes abruptly beneath the Cherchen fault, i.e., ???100 km north of the northern margin of the Tibetan plateau. North of the Cherchen fault, beneath the Tarim basin, a platform-type crust is evident. In contrast, south the Cherchen fault the crust is characterized by a missing high-velocity lower-crustal layer. Our seismic model indicates that the high topography (???3 km) of the Altyn Tagh Range is supported by a wedge-shaped region with a seismic velocity of 7.6-7.8 km/s that we interpret as a zone of crust-mantle mix. We infer that the Altyn Tagh Range formed by crustal-scale strike-slip motion along the North Altyn Tagh fault and northeast-southwest contraction over the range. The contraction is accommodated by (1) crustal thickening via upper-crustal thrusting and lower-crustal flow (i.e., creep), and (2) slip-parallel (SW-directed) underthrusting of only the lower crust and mantle of the eastern Tarim basin beneath the Altyn Tagh Range. ?? 2005 Elsevier B.V. All rights reserved.

Tectonic evolution and mantle structure of the Caribbean

NASA Astrophysics Data System (ADS)

van Benthem, Steven; Govers, Rob; Spakman, Wim; Wortel, Rinus

2013-06-01

investigate whether predictions of mantle structure from tectonic reconstructions are in agreement with a detailed tomographic image of seismic P wave velocity structure under the Caribbean region. In the upper mantle, positive seismic anomalies are imaged under the Lesser Antilles and Puerto Rico. These anomalies are interpreted as remnants of Atlantic lithosphere subduction and confirm tectonic reconstructions that suggest at least 1100 km of convergence at the Lesser Antilles island arc during the past 45 Myr. The imaged Lesser Antilles slab consists of a northern and southern anomaly, separated by a low-velocity anomaly across most of the upper mantle, which we interpret as the subducted North America-South America plate boundary. The southern edge of the imaged Lesser Antilles slab agrees with vertical tearing of South America lithosphere. The northern Lesser Antilles slab is continuous with the Puerto Rico slab along the northeastern plate boundary. This results in an amphitheater-shaped slab, and it is interpreted as westward subducting North America lithosphere that remained attached to the surface along the northeastern boundary of the Caribbean plate. At the Muertos Trough, however, material is imaged until a depth of only 100 km, suggesting a small amount of subduction. The location and length of the imaged South Caribbean slab agrees with proposed subduction of Caribbean lithosphere under the northern South America plate. An anomaly related to proposed Oligocene subduction at the Nicaragua rise is absent in the tomographic model. Beneath Panama, a subduction window exists across the upper mantle, which is related to the cessation of subduction of the Nazca plate under Panama since 9.5 Ma and possibly the preceding subduction of the extinct Cocos-Nazca spreading center. In the lower mantle, two large anomaly patterns are imaged. The westernmost anomaly agrees with the subduction of Farallon lithosphere. The second lower mantle anomaly is found east of the Farallon anomaly and is interpreted as a remnant of the late Mesozoic subduction of North and South America oceanic lithosphere at the Greater Antilles, Aves ridge, and Leeward Antilles. The imaged mantle structure does not allow us to discriminate between an "Intra-Americas origin" and a "Pacific origin" of the Caribbean plate.

Tectonic evolution and mantle structure of the Caribbean

NASA Astrophysics Data System (ADS)

Benthem, Steven; Govers, Rob; Spakman, Wim; Wortel, Rinus

2013-06-01

investigate whether predictions of mantle structure from tectonic reconstructions are in agreement with a detailed tomographic image of seismic P wave velocity structure under the Caribbean region. In the upper mantle, positive seismic anomalies are imaged under the Lesser Antilles and Puerto Rico. These anomalies are interpreted as remnants of Atlantic lithosphere subduction and confirm tectonic reconstructions that suggest at least 1100 km of convergence at the Lesser Antilles island arc during the past ~45 Myr. The imaged Lesser Antilles slab consists of a northern and southern anomaly, separated by a low-velocity anomaly across most of the upper mantle, which we interpret as the subducted North America-South America plate boundary. The southern edge of the imaged Lesser Antilles slab agrees with vertical tearing of South America lithosphere. The northern Lesser Antilles slab is continuous with the Puerto Rico slab along the northeastern plate boundary. This results in an amphitheater-shaped slab, and it is interpreted as westward subducting North America lithosphere that remained attached to the surface along the northeastern boundary of the Caribbean plate. At the Muertos Trough, however, material is imaged until a depth of only 100 km, suggesting a small amount of subduction. The location and length of the imaged South Caribbean slab agrees with proposed subduction of Caribbean lithosphere under the northern South America plate. An anomaly related to proposed Oligocene subduction at the Nicaragua rise is absent in the tomographic model. Beneath Panama, a subduction window exists across the upper mantle, which is related to the cessation of subduction of the Nazca plate under Panama since 9.5 Ma and possibly the preceding subduction of the extinct Cocos-Nazca spreading center. In the lower mantle, two large anomaly patterns are imaged. The westernmost anomaly agrees with the subduction of Farallon lithosphere. The second lower mantle anomaly is found east of the Farallon anomaly and is interpreted as a remnant of the late Mesozoic subduction of North and South America oceanic lithosphere at the Greater Antilles, Aves ridge, and Leeward Antilles. The imaged mantle structure does not allow us to discriminate between an "Intra-Americas origin" and a "Pacific origin" of the Caribbean plate.

Topography of upper mantle seismic discontinuities beneath the North Atlantic: The Azores, Canary and Cape Verde plumes

NASA Astrophysics Data System (ADS)

Saki, Morvarid; Thomas, Christine; Nippress, Stuart E. J.; Lessing, Stephan

2015-01-01

We are mapping the topography of upper mantle seismic discontinuities beneath the North Atlantic and surrounding regions by using precursor arrivals to PP and SS seismic waves that reflect off the seismic discontinuities. Numerous source-receiver combinations have been used in order to collect a large dataset of reflection points beneath our investigation area. We analysed over 1700 seismograms from MW > 5.8 events using array seismic methods to enhance the signal to noise ratio. The measured time lag between PP (SS) arrivals and their corresponding precursors on robust stacks are used to measure the depth of the transition zone boundaries. The reflectors' depths show a correlation between the location of known hotspots and a significantly depressed 410 km discontinuity indicating a temperature increase of 50-300 K compared to the surrounding mantle. For the 660 km discontinuity three distinct behaviours are visible: (i) normal depths beneath Greenland and at a distance of a few hundred kilometres away from known hotspots, (ii) shallower 660 km discontinuity compared with the global average value near hotspots closer to the Mid-Atlantic Ridge, and (iii) very few observations of a 660 km discontinuity at the hotspot locations. We interpret our observations as a large upwelling beneath the southern parts of our study region, possibly due to the South Atlantic convection cell. The thermal anomaly may be ponding beneath the endothermic 660 km phase transformation and likely does not extend through the top of the transition zone as a whole, except for those branches which appear as the thinner upwellings of Azores, Canaries and Cape Verde hotspots at the surface.

Large-scale trench-perpendicular mantle flow beneath northern Chile

NASA Astrophysics Data System (ADS)

Reiss, M. C.; Rumpker, G.; Woelbern, I.

2017-12-01

We investigate the anisotropic properties of the forearc region of the central Andean margin by analyzing shear-wave splitting from teleseismic and local earthquakes from the Nazca slab. The data stems from the Integrated Plate boundary Observatory Chile (IPOC) located in northern Chile, covering an approximately 120 km wide coastal strip between 17°-25° S with an average station spacing of 60 km. With partly over ten years of data, this data set is uniquely suited to address the long-standing debate about the mantle flow field at the South American margin and in particular whether the flow field beneath the slab is parallel or perpendicular to the trench. Our measurements yield two distinct anisotropic layers. The teleseismic measurements show a change of fast polarizations directions from North to South along the trench ranging from parallel to subparallel to the absolute plate motion and, given the geometry of absolute plate motion and strike of the trench, mostly perpendicular to the trench. Shear-wave splitting from local earthquakes shows fast polarizations roughly aligned trench-parallel but exhibit short-scale variations which are indicative of a relatively shallow source. Comparisons between fast polarization directions and the strike of the local fault systems yield a good agreement. We use forward modelling to test the influence of the upper layer on the teleseismic measurements. We show that the observed variations of teleseismic measurements along the trench are caused by the anisotropy in the upper layer. Accordingly, the mantle layer is best characterized by an anisotropic fast axes parallel to the absolute plate motion which is roughly trench-perpendicular. This anisotropy is likely caused by a combination of crystallographic preferred orientation of the mantle mineral olivine as fossilized anisotropy in the slab and entrained flow beneath the slab. We interpret the upper anisotropic layer to be confined to the crust of the overriding continental plate. This is explained by the shape-preferred orientation of micro-cracks in relation to local fault zones which are oriented parallel the overall strike of the Andean range. Our results do not provide any evidence for a significant contribution of trench-parallel mantle flow beneath the subducting slab to the measurements.

Crustal structure beneath the Kenya Rift from axial profile data

USGS Publications Warehouse

Mechie, J.; Keller, Gordon R.; Prodehl, C.; Gaciri, S.; Braile, L.W.; Mooney, W.D.; Gajewski, D.; Sandmeier, K.-J.

1994-01-01

Modelling of the KRISP 90 axial line data shows that major crustal thinning occurs along the axis of the Kenya Rift from Moho depths of 35 km in the south beneath the Kenya Dome in the vicinity of Lake Naivasha to 20 km in the north beneath Lake Turkana. Low Pn velocities of 7.5-7.7 km/s are found beneath the whole of the axial line. The results indicate that crustal extension increases to the north and that the low Pn velocities are probably caused by magma (partial melt) rising from below and being trapped in the uppermost kilometres of the mantle. Along the axial line, the rift infill consisting of volcanics and a minor amount of sediments varies in thickness from zero where Precambrian crystalline basement highs occur to 5-6 km beneath the lakes Turkana and Naivasha. Analysis of the Pg phase shows that the upper crystalline crust has velocities of 6.1-6.3 km/s. Bearing in mind the Cainozoic volcanism associated with the rift, these velocities most probably represent Precambrian basement intruded by small amounts of igneous material. The boundary between the upper and lower crusts occurs at about 10 km depth beneath the northern part of the rift and 15 km depth beneath the southern part of the rift. The upper part of the lower crust has velocities of 6.4-6.5 km/s. The basal crustal layer which varies in thickness from a maximum of 2 km in the north to around 9 km in the south has a velocity of about 6.8 km/s. ?? 1994.

Reconciling Long-Wavelength Dynamic Topography, Geoid Anomalies and Mass Distribution on Earth

NASA Astrophysics Data System (ADS)

Hoggard, M.; Richards, F. D.; Ghelichkhan, S.; Austermann, J.; White, N.

2017-12-01

Since the first satellite observations in the late 1950s, we have known that that the Earth's non-hydrostatic geoid is dominated by spherical harmonic degree 2 (wavelengths of 16,000 km). Peak amplitudes are approximately ± 100 m, with highs centred on the Pacific Ocean and Africa, encircled by lows in the vicinity of the Pacific Ring of Fire and at the poles. Initial seismic tomography models revealed that the shear-wave velocity, and therefore presumably the density structure, of the lower mantle is also dominated by degree 2. Anti-correlation of slow, probably low density regions beneath geoid highs indicates that the mantle is affected by large-scale flow. Thus, buoyant features are rising and exert viscous normal stresses that act to deflect the surface and core-mantle boundary (CMB). Pioneering studies in the 1980s showed that a viscosity jump between the upper and lower mantle is required to reconcile these geoid and tomographically inferred density anomalies. These studies also predict 1-2 km of dynamic topography at the surface, dominated by degree 2. In contrast to this prediction, a global observational database of oceanic residual depth measurements indicates that degree 2 dynamic topography has peak amplitudes of only 500 m. Here, we attempt to reconcile observations of dynamic topography, geoid, gravity anomalies and CMB topography using instantaneous flow kernels. We exploit a density structure constructed from blended seismic tomography models, combining deep mantle imaging with higher resolution upper mantle features. Radial viscosity structure is discretised, and we invert for the best-fitting viscosity profile using a conjugate gradient search algorithm, subject to damping. Our results suggest that, due to strong sensitivity to radial viscosity structure, the Earth's geoid seems to be compatible with only ± 500 m of degree 2 dynamic topography.

Crustal structure of the Sunda-Banda arc transition: results from marine geophysical investigations offshore eastern Indonesia

NASA Astrophysics Data System (ADS)

Planert, L.; Shulgin, A.; Kopp, H.; Lueschen, E.; Mueller, C.; Flueh, E.; Djajadihardja, Y.; Engels, M.

2009-04-01

The Sunda-Banda arc transition, the easternmost portion of the Indonesian convergent margin, presents a probably unique natural laboratory to study lower plate variability and related upper plate deformation in the so-called ‘subduction factory' for a deeper understanding of forearc evolution. In neighboring margin segments, we can observe strong changes of the incoming plate (transition from an oceanic to a continental lower plate, increasing plate age to the East, presence/absence of an oceanic plateau, variability in plate roughness) as well as a wide range of corresponding forearc structures, including large sedimentary basins and an accretionary prism/outer arc high of variable size and shape. During RV Sonne cruise SO190 in 2006 (SINDBAD: Seismic and Geoacoustic Investigations along the Sunda-Banda Arc Transition), we acquired a combination of seismic wide-angle OBH/OBS refraction, multichannel streamer and gravity data in order to study the seismic velocity structure of the subducting crust and the overriding island arc along a number of trench normal corridors located between 113°E and 121°E. Additionally, a number of trench parallel profiles were conducted which mainly focus on the internal structure of the large sedimentary basins and which were also intended for further clarifying the type of underlying forearc crust and mantle respectively. We used a tomographic approach for refracted and reflected phases to obtain seismic velocity models which again were used for prestack depth-migration of the MCS data. In turn, we incorporated the highly resolved sedimentary portions as a priori structure in our tomography. The results show the seismic velocity structure of the incoming plate, starting 100 km seaward of the trench, and the adjoining forearc down to depths of 20-28 km, i.e. well into the upper mantle, and at the same time fit the gravity data very well, using simple velocity-density relations. In the Argo abyssal plain, the models show 8.0-8.5 km thick oceanic crust. The velocities in the crust and uppermost mantle are reduced within distances of ~50 km seaward of the trench, which coincides with the onset of normal faulting on the incoming oceanic plate. Anomalously low mantle velocities of 7.5 km/s directly beneath the Moho are possibly due to the intrusion of seawater and subsequent serpentinisation of mantle peridotite. Landward of the trench in the outer arc high, velocities do not exceed 5.5 km/s down to the top of the subducting slab, which can be traced over ~70 km length beneath the forearc down to ~13 km depth. The plate boundary is of irregular shape, obviously imprinted by the complex deformation of the oceanic basement prior to subduction, which is further amplified as response to thrusting/downbending of the dissected oceanic blocks. Offshore Lombok island, our models reveal the geometry of the Lombok basin as well as the forearc Moho in ~16 km depth. Reduced upper mantle velocities suggest a hydrated shallow mantle wedge for this corridor. Further east offshore Sumba island, where the Java trench terminates and the transition to the collisional regime further east occurs, our models show a subducting oceanic plate of similar thickness and structure. But different to the situation offshore Lombok, we find no evidence for a shallow mantle wedge beneath the forearc; crustal-type velocities are found down to depths of ~20 km. The different forearc regime is most likely related to the collision with the Sumba block. Our results give a detailed view into the complex structure in both the deeper and shallower portions of this convergent margin.

Topography of Upper Mantle Seismic Discontinuities Beneath the North Atlantic: The Azores, Canary and Cape Verde Plumes

NASA Astrophysics Data System (ADS)

Thomas, C.; Saki, M.; Nippress, S. E. J.; Lessing, S.

2014-12-01

We are mapping the topography of upper mantle seismic discontinuities beneath the North Atlantic and surrounding regions by using precursor arrivals to PP and SS seismic waves that reflect off the seismic discontinuities. Numerous source-receiver combinations have been used in order to collect a large dataset of reflection points beneath our investigation area. We analysed over 1700 seismograms from MW>5.8 events using array seismic methods to enhance the signal to noise ratio. The measured time lag between PP (SS) arrivals and their corresponding precursors on robust stacks are used to measure the depth of the transition zone boundaries. The reflectors' depths show a correlation between the location of known hotspots and a significantly depressed 410 km discontinuity indicating a temperature increase of 50-300 K compared to the surrounding mantle. For the 660 km discontinuity three distinct behaviours are visible: i) normal depths beneath Greenland and at a distance of a few hundred kilometres away from known hotspots, ii) shallower 660 km discontinuity compared with the global average value near hotspots closer to the Mid-Atlantic Ridge and iii) very few observations of a 660 km discontinuity at the hotspot locations. We interpret our observations as a large upwelling beneath the southern parts of our study region, possibly due to the South Atlantic convection cell. The thermal anomaly may be blocked by endothermic phase transformation and likely does not extend through the top of the transition zone except for those branches which appear as the Azores, Canaries and Cape Verde hotspots at the surface.

Topography of upper mantle seismic discontinuities beneath the North Atlantic: the Azores, Canary and Cape Verde plumes

NASA Astrophysics Data System (ADS)

Saki, Morvarid; Thomas, Christine; Nippress, Stuart E. J.; Lessing, Stephan

2015-04-01

We are mapping the topography of upper mantle seismic discontinuities beneath the North Atlantic and surrounding regions by using precursor arrivals to PP and SS seismic waves that reflect off the seismic discontinuities. Many source-receiver combinations have been used in order to collect a large dataset of reflection points beneath our investigating area. We analyzed over 1700 seismograms from MW>5.8 events using array seismic methods to enhance the signal to noise ratio. The measured time lag between PP (SS) arrivals and their corresponding precursors on robust stacks are used to measure the depth of the transition zone boundaries. The reflectors' depths show a correlation between the location of hotspots and a significantly depressed 410 km discontinuity indicating a temperature increase of 200-300 K compared to the surrounding mantle. For the 660 km discontinuity three distinct behaviours are visible: i) normal depths beneath Greenland and at a distance of a few hundred kilometres away from the hotspots and ii) shallower 660 km discontinuity compared with the global average value near hotspots closer to the Mid-Atlantic Ridge and iii) very few observations of a 660 km discontinuity at the hotspot locations. We interpret our observations as a large upwelling beneath the southern parts of our study region, possibly due to the South Atlantic convection cell. The thermal anomaly may be blocked by endothermic phase transformation and likely does not extend through the top of the transition zone as whole except for those branches which appear as the Azores, Canaries and Cape Verde hotspots at the surface.

Tracking the Progress of EarthScope/USArray: The crust and upper mantle beneath the transition region between tectonic western US and cratonic eastern US

NASA Astrophysics Data System (ADS)

Shen, W.; Lin, F.; Ritzwoller, M. H.

2010-12-01

The transition region between the tectonic western US and the cratonic eastern US contains numerous significant geological regions (e.g., the Rocky Mountains, the Colorado Plateau, and the Rio Grande Rift), and also, unknowns (e.g, the location or extent of the east-west US dichotomy, the compensation of the high topography of the western Great Plains, the extensional mechanics of the Rio Grande Rift, and the structure of the mantle beneath the Colorado Plateau). The answers to these questions and others are critical to an understanding of the tectonics and tectonic history of this region and its impact on the cratonic eastern US. The recent deployments of seismic stations, particularly the EarthScope USArray Transportable Array (TA), provide an opportunity to construct a detailed 3-D structural model of the crust and upper mantle beneath this transition region, and thus allow us to address some of the questions listed above. We present results from ambient noise tomography (ANT) and teleseismic earthquake tomography by using data from TA stations within the western and central US. We processed continuous seismic noise data from ~600 TA stations from August 2008 to March 2010, which after data selection produces a data set with ~100,000 inter-station paths. Rayleigh wave phase speed maps between 6 and 40 sec period and Love wave phase speed maps between 8 and 30 sec with a resolution of ~60 km are constructed using eikonal tomography. In addition, we applied eikonal tomography (ET) to about 300 teleseismic earthquakes to obtain long-period (30 - 100 sec) Rayleigh wave phase speed maps and Love wave phase speeds maps (30 - 60 sec). By jointly inverting Rayleigh and Love phase speeds maps from ANT and earthquake tomography, we constructed a 3-D isotropic and radially anisotropic shear velocity model of the crust and upper mantle to ~150 km depth together with model uncertainties constrained by a Monte-Carlo inversion. The 3-D isotropic model reveals a variety of robust features in this transition region. In the uppermost crust, the main sedimentary basins (e.g., Green River, Uinta, Washakie, Powder River, Denver, Albuquerque, Permian, and Anadarko) are imaged. In the middle and lower crust where the low shear velocities from basins diminish, the Yellowstone hot spot becomes the main slow anomaly. In the uppermost mantle, high velocity anomalies are observed beneath the Colorado Plateau, the Wyoming craton, and the Great Plains. Although the Colorado Plateau shows more or less homogeneous shear velocity in its middle and towards its northern boundary, the other two main fast anomalies reveal inhomogeneous structures at depths deeper than 100 km. Two main low velocity anomalies are observed: one underlying the Snake River Plain which broadens and dips to the northeast and another U-shaped anomaly on the eastern margin of the Colorado Plateau. These velocity anomalies add to complexities at the transition between the tectonic western US and the stable eastern US. The location and uncertainty of the east-west shear velocity dichotomy also is constrained by this model.

Seismic anisotropy of the lithosphere/asthenosphere system beneath the Rwenzori region of the Albertine Rift

NASA Astrophysics Data System (ADS)

Homuth, B.; Löbl, U.; Batte, A. G.; Link, K.; Kasereka, C. M.; Rümpker, G.

2016-09-01

Shear-wave splitting measurements from local and teleseismic earthquakes are used to investigate the seismic anisotropy in the upper mantle beneath the Rwenzori region of the East African Rift system. At most stations, shear-wave splitting parameters obtained from individual earthquakes exhibit only minor variations with backazimuth. We therefore employ a joint inversion of SKS waveforms to derive hypothetical one-layer parameters. The corresponding fast polarizations are generally rift parallel and the average delay time is about 1 s. Shear phases from local events within the crust are characterized by an average delay time of 0.04 s. Delay times from local mantle earthquakes are in the range of 0.2 s. This observation suggests that the dominant source region for seismic anisotropy beneath the rift is located within the mantle. We use finite-frequency waveform modeling to test different models of anisotropy within the lithosphere/asthenosphere system of the rift. The results show that the rift-parallel fast polarizations are consistent with horizontal transverse isotropy (HTI anisotropy) caused by rift-parallel magmatic intrusions or lenses located within the lithospheric mantle—as it would be expected during the early stages of continental rifting. Furthermore, the short-scale spatial variations in the fast polarizations observed in the southern part of the study area can be explained by effects due to sedimentary basins of low isotropic velocity in combination with a shift in the orientation of anisotropic fabrics in the upper mantle. A uniform anisotropic layer in relation to large-scale asthenospheric mantle flow is less consistent with the observed splitting parameters.

Shear wave velocities in the upper mantle of the Western Alps: new constraints using array analysis of seismic surface waves

NASA Astrophysics Data System (ADS)

Lyu, Chao; Pedersen, Helle A.; Paul, Anne; Zhao, Liang; Solarino, Stefano

2017-07-01

It remains challenging to obtain absolute shear wave velocities of heterogeneities of small lateral extension in the uppermost mantle. This study presents a cross-section of Vs across the strongly heterogeneous 3-D structure of the western European Alps, based on array analysis of data from 92 broad-band seismic stations from the CIFALPS experiment and from permanent networks in France and Italy. Half of the stations were located along a dense sublinear array. Using a combination of these stations and off-profile stations, fundamental-mode Rayleigh wave dispersion curves were calculated using a combined frequency-time beamforming approach. We calculated dispersion curves for seven arrays of approximately 100 km aperture and 14 arrays of approximately 50 km aperture, the latter with the aim of obtaining a 2-D vertical cross-section of Vs beneath the western Alps. The dispersion curves were inverted for Vs(z), with crustal interfaces imposed from a previous receiver function study. The array approach proved feasible, as Vs(z) from independent arrays vary smoothly across the profile length. Results from the seven large arrays show that the shear velocity of the upper mantle beneath the European plate is overall low compared to AK135 with the lowest velocities in the internal part of the western Alps, and higher velocities east of the Alps beneath the Po plain. The 2-D Vs model is coherent with (i) a ∼100 km thick eastward-dipping European lithosphere west of the Alps, (ii) very high velocities beneath the Po plain, coherent with the presence of the Alpine (European) slab and (iii) a narrow low-velocity anomaly beneath the core of the western Alps (from the Briançonnais to the Dora Maira massif), and approximately colocated with a similar anomaly observed in a recent teleseismic P-wave tomography. This intriguing anomaly is also supported by traveltime variations of subvertically propagating body waves from two teleseismic events that are approximately located on the profile great circle.

Magnetic properties of the upper mantle beneath the continental United States

NASA Astrophysics Data System (ADS)

Friedman, S. A.; Ferre, E. C.; Demory, F.; Rochette, P.; Martin Hernandez, F.; Conder, J. A.

2012-12-01

The interpretation of long wavelength satellite magnetic data (Magsat, Oersted, CHAMP, SWARM) requires an understanding of magnetic mineralogy in the lithospheric mantle and reliable models of induced and remanent magnetic sources in the lithospheric mantle and the crust. Blakely et al. (2005) proposed the hypothesis of a magnetic lithospheric mantle in subduction zones. This prompted us to reexamine magnetic sources in the lithospheric mantle in different tectonic settings where unaltered mantle xenolith have been reported since the 1990s. Xenoliths from the upper mantle beneath the continental United States show different magnetic properties depending on the tectonic setting in which they equilibrated. Three localities in the South Central United States (San Carlos, AZ; Kilbourne Hole, NM; Knippa, TX) produced lherzolite and harzburgite xenoliths, while the Bearpaw Mountains in Montana (subduction zone) produced dunite and phlogopite-rich dunite xenoliths. Paleomagnetic data on these samples shows the lack of secondary alteration which is commonly caused by post-eruption serpentinization and the lack of basalt contamination. The main magnetic carrier is pure magnetite. The ascent of mantle xenoliths to the surface of the Earth generally takes only a few hours. Numerical modelling shows that nucleation of magnetite during ascent would form superparamagnetic grains and therefore cannot explain the observed magnetic grain sizes. This implies that the ferromagnetic phases present in the studied samples formed at mantle depth. The samples from the South Central United States exhibit a small range in low-field magnetic susceptibility (+/- 0.00003 [SI]), and Natural Remanent Magnetization (NRM) between 0.001 - 0.100 A/m. To the contrary samples from the Bearpaw Mountains exhibit a wider range of low-field susceptibilities (0.00001 to 0.0015 [SI]) and NRM (0.01 and 9.00 A/m). These samples have been serpentinized in-situ by metasomatic fluids related to the Farallon plate (Facer et al., 2009). Hence, the magnetic properties of the lithospheric mantle beneath the continental United States differ significantly depending on tectonic setting. The combination of the low geotherm observed in the Bearpaw Mountains with the stronger induced and remanent magnetization of mantle rocks in this area may produce a detectable LWMA.

Looking at the roots of the highest mountains: the lithospheric structure of the Himalaya-Tibet and the Zagros orogens. Results from a geophysical-petrological study

NASA Astrophysics Data System (ADS)

Tunini, L.; Jimenez-Munt, I.; Fernandez, M.; Villasenor, A.; Afonso, J. C.; Verges, J.

2013-12-01

The Himalaya-Tibet and Zagros orogens are the two most prominent mountain belts built by continental collision. They are part of a huge belt of Cenozoic age which runs from the Pyrenees to Burma. In its central sector, the collision with the southern margin of the Eurasian plate has resulted not only in the building of mountain ranges over the north-eastern edges of the Arabian and Indian plates but also in widespread deformation 1000-3000 km from the suture zones. Zagros and Himalaya-Tibet orogens share many geodynamic processes but at different rates, amount of convergence and stage of development. The study of their present-day structures provides new insights into their quasi coeval collisional event pointing out differences and similarities in the mountain building processes. We present 2D crust and upper mantle cross-sections down to 400 km depth, along four SW-NE trending profiles. Two profiles cross the Zagros Mountains, running from the Mesopotamian Foreland Basin up to the Alborz and Central Iran. Two other profiles run through the Himalaya-Tibetan orogen: the western transect crosses the western Himalaya, Tarim Basin, Tian Shan Mountains and Junggar Basin; the eastern transect runs from the Indian shield to the Beishan Basin, crossing the eastern Himalaya, Tibetan Plateau, Qaidam Basin and Qilian Mountains. We apply the LitMod-2D code which integrates potential fields (gravity and geoid), isostasy (elevation) and thermal (heat flow and temperature distribution) equations, and mantle petrology. The resulting crust and upper mantle structure is constrained by available data on elevation, Bouguer anomaly, geoid height, surface heat flow and seismic data including P- and S-wave tomography models. Our results show distinct deformation patterns between the crust and the lithospheric mantle beneath the Zagros and Himalaya-Tibetan orogens, indicating a strong strain partitioning in both areas. At crustal level, we found a thickening beneath the Zagros and the Alborz ranges, more pronounced in the southern profile. At sub-crustal level, a lithospheric mantle thinning affects the whole area beneath the Zagros range extending to the north through the zone below the Alborz and the central Iran. In the Himalaya-Tibet region our results show stronger strain partitioning in the horizontal (east-west) direction than in the vertical (depth) direction. At crustal level, the Tibetan Plateau extends more than 1000 km in the eastern profile, whereas it is squeezed between the Himalayan Mountains and the Tarim Basin along the western profile (~600 km). At sub-crustal level, the lithospheric mantle is more homogeneous in thickness and mineral composition along the western profile than the eastern one. Finally, our results on mineral composition show that both collisional regions are characterised by a predominant lherzolitic lithospheric mantle, whereas we observe compositional variations around the suture zones, probably related to subduction and mantle delamination processes.

Universal single grain amphibole thermobarometer for mantle rocks - preliminary calibration.

NASA Astrophysics Data System (ADS)

Ashchepkov, Igor

2017-04-01

Calibration of S-Al- K-Na-Ca distribution in the structure of the mantle amphiboles (Cr- hornblende, pargasite, kaersutite) using experimental data (Niida, Green, 1999; Wallace Green, 1991, Conceicao, Green, 2004; Medard et al, 2006; Safonov, Butvina, 2013; 2016; Pirard, Hermann, 2015 etc) allows to obtain an equation for pressure estimates in 0.5 - 4.5 GPa interval. Regression calculated pressures with experimental values (R 0.82) and precision 5 kbar allow to use barometer for a wide range of mantle rocks from peridotite to pyroxenites and megacrystals. For the higher pressures (Cr- pargasite richterite) calibration is carried by the cross- correlations with the estimates calculated for the natural associations obtained using clino- and orthopyroxene. IT was used KD =Si/(8-Al-2.2*Ti)*(Na+K))/Ca for the following equation: P(GPa)=0.0035*(4+K/(Na+K))*2*Mg)/Fe+3.75*(K+Na)/Ca))*KD*ToK**0.75/ (1+3.32*Fe)-ln(1273/ToK*5*(8*Mg-Al*2 +3*Ti+8*Cr+3*K)/10 Th advantage of this barometer comparing with the previous (Ridolfi, Renzulli, 2012) is that is working with all mantle amphibole types. For the calculations of the PT parameters of the natural xenocrysts it was used monomineral version of Gar-Amph termometer (Ravna et al., 2000) in combination with the received barometer. Contents of Ca- Mg and Fe in associated garnets were calculated usinf the regressions obtained from natural and experimental associations. Aplication of the mantle amphibole thermobarometry for the reconstruction of sections of the cratonic mantle lithosphere of Yakutia show that amphibloles are distributed in various parts of mantle sections in deifferent mantle terranes of Yakutia. The most abundant amphoboles from Alakite region are distributed within all mantle section. In the SCLM beneat Yubileyaya pipe thehalf of them belong to the spinel garnet facie refering to the upper pyroxenitic suit and Cr- hornblende - mica viens. The second group reffer to the eclogite pyroxenite layer in the middle part of SCLM and the third group refer to richterites form the depleted manle peridotites. In SCLM beneat the Sytykanskaya they are more frequent and trace through all the mantle layers. In SCLM beneat the Aykhal they mostly are from the lower and in Komsomolskaya from the middle SCLM parts. In Daldyn field rare amdphibles from Dalnaya are Fe- enriched pargasites belonging to the Ilm bearing peridotites in middle SCLM part as well as in SCLM beneath thr Udachnaya. But there are Fe- low amphiboles substitutng the orthopyroxenes. In Zarnitsa the Cr - hornblendes occur in shallow garnet pyroxenites. One deep seated richterite substitute garnet grains. Rare amphiboles were detedted in Mirninsky filed in Internatiolnaya pipe and reffer to the resorbed and deformed granets from the Garnet -Spinel facies and from 4.0 GPa boundary. Amphiboles are frequent in the SCLM from the northern part of Siberian craton. In SCLM beneath the Kharmai the Fe- encriched varietes are from the Moho boundary. Common Cr-pargasite occurs to 3 GPa in Obnazhennay, pipe, Kharamai field In mantle SCLM beneath Obnazhennaya pipe and circum Anabr region friquent Cr- pargasies and horblendes refer to the relatively hot branch of mantle lithosphere and probably corresponds to the Triassic mantle reactivation. Mantle Cr- hornbleneds occurs on most upper part of the mantle column beneath Quaternary mujeritic Bartoy vocanoes in Transbaikal. The pargasites and kaersutites in this locality refer to more heated conditions and could be found to 2.0 GPa. Grant RFBR 16.-05-000860

Three-dimensional imaging of the S-velocity structure for the crust and the upper mantle beneath the Arabian Sea from Rayleigh wave analysis

NASA Astrophysics Data System (ADS)

Corchete, V.

2017-04-01

A 3D imaging of S-velocity for the Arabian Sea crust and upper mantle structure is presented in this paper, determined by means of Rayleigh wave analysis, for depths ranging from zero to 300 km. The crust and upper mantle structure of this region of the earth never has been the subject of a surface wave tomography survey. The Moho map performed in the present study is a new result, in which a crustal thickening beneath the Arabian Fan sediments can be observed. This crustal thickening can be interpreted as a quasi-continental oceanic transitional structure. A crustal thickness of up to 20 km also can be observed for the Murray Ridge system in this Moho map. This crustal thickening can be due to that the Murray Ridge System consists of Indian continental crust. This continental crust is extremely thinned to the southwest of this region, as shown in this Moho map. This area can be interpreted as oceanic in origin. In the depth range from 30 to 60 km, the S-velocity presents its lower values at the Carlsberg Ridge region, because it is the younger region of the study area. In the depth range from 60 to 105 km of depth, the S-velocity pattern is very similar to that shown for the previous depth range, except for the regions in which the asthenosphere is reached, for these regions appear a low S-velocity pattern. The lithosphere-asthenosphere boundary (LAB), or equivalently the lithosphere thickness, determined in the present study is also a new result, in which the lithosphere thickness for the Arabian Fan can be estimated in 60-70 km. The lower lithospheric thickness observed in the LAB map, for the Arabian Fan, shows that this region may be in the transition zone between continental and oceanic structure. Finally, a low-velocity zone (LVZ) has been determined, for the whole study area, located between the LAB and the boundary of the asthenosphere base (or equivalently the lithosphere-asthenosphere system thickness). The asthenosphere-base map calculated in the present study is also a new result.

Can lower mantle slab-like seismic anomalies be explained by thermal coupling between the upper and lower mantles?

NASA Astrophysics Data System (ADS)

Čížková, Hana; Čadek, Ondřej; van den Berg, Arie P.; Vlaar, Nicolaas J.

Below subduction zones, high resolution seismic tomographic models resolve fast anomalies that often extend into the deep lower mantle. These anomalies are generally interpreted as slabs penetrating through the 660-km seismic discontinuity, evidence in support of whole-mantle convection. However, thermal coupling between two flow systems separated by an impermeable interface might provide an alternative explanation of the tomographic results. We have tested this hypothesis within the context of an axisymmetric model of mantle convection in which an impermeable boundary is imposed at a depth of 660 km. When an increase in viscosity alone is imposed across the impermeable interface, our results demonstrate the dominant role of mechanical coupling between shells, producing lower mantle upwellings (downwellings) below upper mantle downwellings (upwellings). However, we find that the effect of mechanical coupling can be significantly weakened if a narrow low viscosity zone exists beneath the 660-km discontinuity. In such a case, both thermally induced ‘slabs’ in the lower mantle and thermally activated plumes that rise from the upper/lower mantle boundary are observed even though mass transfer between the shells does not exist.

Investigating seismic anisotropy beneath the Reykjanes Ridge using models of mantle flow, crystallographic evolution, and surface wave propagation

NASA Astrophysics Data System (ADS)

Gallego, A.; Ito, G.; Dunn, R. A.

2013-08-01

Surface wave studies of the Reykjanes Ridge (RR) and the Iceland hotspot have imaged an unusual and enigmatic pattern of two zones of negative radial anisotropy on each side of the RR. We test previously posed and new hypotheses for the origin of this anisotropy, by considering lattice preferred orientation (LPO) of olivine A-type fabric in simple models with 1-D, layered structures, as well as in 2-D and 3-D geodynamic models with mantle flow and LPO evolution. Synthetic phase velocities of Love and Rayleigh waves traveling parallel to the ridge axis are produced and then inverted to mimic the previous seismic studies. Results of 1-D models show that strong negative radial anisotropy can be produced when olivine a axes are preferentially aligned not only vertically but also subhorizontally in the plane of wave propagation. Geodynamic models show that negative anisotropy on the sides of the RR can occur when plate spreading impels a corner flow, and in turn a subvertical alignment of olivine a axes, on the sides of the ridge axis. Mantle dehydration must be invoked to form a viscous upper layer that minimizes the disturbance of the corner flow by the Iceland mantle plume. While the results are promising, important discrepancies still exist between the observed seismic structure and the predictions of this model, as well as models of a variety of types of mantle flow associated with plume-ridge interaction. Thus, other factors that influence seismic anisotropy, but not considered in this study, such as power-law rheology, water, melt, or time-dependent mantle flow, are probably important beneath the Reykjanes Ridge.

Tectonic evolution and mantle structure of the Caribbean

NASA Astrophysics Data System (ADS)

van Benthem, Steven; Govers, Rob; Spakman, Wim; Wortel, Rinus

2013-04-01

In the broad context of investigating the relationship between deep structure & processes and surface expressions, we study the Caribbean plate and underlying mantle. We investigate whether predictions of mantle structure from tectonic reconstructions are in agreement with a detailed tomographic image of seismic P-wave velocity structure under the Caribbean region. In the upper mantle, positive seismic anomalies are imaged under the Lesser Antilles and Puerto Rico. These anomalies are interpreted as remnants of Atlantic lithosphere subduction and confirm tectonic reconstructions that suggest at least 1100 km of convergence at the Lesser Antilles island arc during the past ~45 Myr. The imaged Lesser-Antilles slab consists of a northern and southern anomaly, separated by a low velocity anomaly across most of the upper mantle, which we interpret as the subducted North-South America plate boundary. The southern edge of the imaged Lesser Antilles slab agrees with vertical tearing of South America lithosphere. The northern Lesser Antilles slab is continuous with the Puerto Rico slab along the northeastern plate boundary. This results in an amphitheater-shaped slab and it is interpreted as westward subducting North America lithosphere that remained attached to the surface along the northern boundary. At the Muertos Trough, however, material is imaged until a depth of only 100 km, suggesting a small amount of subduction. The location and length of the imaged South Caribbean slab agrees with proposed subduction of Caribbean lithosphere under the northern South America plate. An anomaly related to proposed Oligocene subduction at the Nicaragua rise is absent in the tomographic model. Beneath Panama, a subduction window exists across the upper mantle, which is related to the cessation of subduction of the Nazca plate under Panama since 9.5 Ma and possibly the preceding subduction of the extinct Cocos-Nazca spreading center. In the lower mantle two large anomaly patterns are imaged. The westernmost anomaly agrees with the subduction of Farallon lithosphere. The second lower mantle anomaly is found east of the Farallon anomaly and is interpreted as a remnant of the late Mesozoic subduction of North and South America oceanic lithosphere at the Greater Antilles, Aves ridge and Leeward Antilles. The imaged mantle structure does not allow us to discriminate between an 'Intra-Americas' origin and a 'Pacific origin' of the Caribbean plate.

Big mantle wedge, anisotropy, slabs and earthquakes beneath the Japan Sea

NASA Astrophysics Data System (ADS)

Zhao, Dapeng

2017-09-01

The Japan Sea is a part of the western Pacific trench-arc-backarc system and has a complex bathymetry and intense seismic activities in the crust and upper mantle. Local seismic tomography revealed strong lateral heterogeneities in the crust and uppermost mantle beneath the eastern margin of the Japan Sea, which was determined using P and S wave arrival times of suboceanic earthquakes relocated precisely with sP depth phases. Ambient-noise tomography revealed a thin crust and a thin lithosphere beneath the Japan Sea and significant low-velocity (low-V) anomalies in the shallow mantle beneath the western and eastern margins of the Japan Sea. Observations with ocean-bottom seismometers and electromagnetometers revealed low-V and high-conductivity anomalies at depths of 200-300 km in the big mantle wedge (BMW) above the subducting Pacific slab, and the anomalies are connected with the low-V zone in the normal mantle wedge beneath NE Japan, suggesting that both shallow and deep slab dehydrations occur and contribute to the arc and back-arc magmatism. The Pacific slab has a simple geometry beneath the Japan Sea, and earthquakes occur actively in the slab down to a depth of ∼600 km beneath the NE Asian margin. Teleseismic P and S wave tomography has revealed that the Philippine Sea plate has subducted aseismically down to the mantle transition zone (MTZ, 410-660 km) depths beneath the southern Japan Sea and the Tsushima Strait, and a slab window is revealed within the aseismic Philippine Sea slab. Seismic anisotropy tomography revealed a NW-SE fast-velocity direction in the BMW, which reflects corner flows induced by the fast deep subduction of the Pacific slab. Large deep earthquakes (M > 7.0; depth > 500 km) occur frequently beneath the Japan Sea western margin, which may be related to the formation of the Changbai and Ulleung intraplate volcanoes. A metastable olivine wedge is revealed within the cold core of the Pacific slab at the MTZ depth, which may be related to the deep seismicity. However, many of these results are still preliminary, due to the lack of seismic stations in the Japan Sea. The key to resolving these critical geoscientific issues is seismic instrumentation in the Japan Sea, for which international cooperation of geoscience communities in the East Asian countries is necessary.

Receiver Function Imaging of Crust and Uppermost Mantle Structure beneath the Japan Islands -Inclusion of Hi-net Data-

NASA Astrophysics Data System (ADS)

Yamauchi, M.; Hirahara, K.; Shibutani, T.

2001-12-01

We are examining a large number of teleseismic waveforms observed at stations closely distributed over the Japan Islands to construct body-wave waveform tomography data for determining 3-D crust and upper mantle structure including velocity discontinuities. As one of preparatory studies toward this final goal, we are executing array analyses of Receiver Functions (RF). RF analyses of J-array data ( 32 broad band stations and 269 short period stations ) and Freesia data ( 15 broad band stations ), whose stations are closely distributed, have provided us with new information on the structure including velocity discontinuities beneath the Japan Islands (Tada et al, 2001). In their study, for crustal imaging, RFs transformed from time to depth domain after SVD filtering ( Chevrot and Giardin, 2000 ) are projected onto 2-D profiles, which show average values for cells within +/- 50km from each cross section. However, this cell size does not satisfy our demand to draw the detailed image beneath the Japan Islands. In addition, J-array short period RFs available for the analyses are limited because of high frequency noises. In this research, Hi-net data (short period), whose stations are far more closely distributed, are newly included into our data. We make RF image with α =3 of short period J-array data and Hi-net data for events observed during a period from September, 2000 to July, 2001 with the magnitudes larger than 5.5. The total number of the stations with their average spacing of 10km is about 800 (J-array; 270, Hi-net; 500), which enables to reduce the cell size to +/- 20km at most. We show a new 3-D RF image of the crust and the uppermost mantle, whose best spatial resolution is reaching less than 5km. Therefore we can obtain much more detailed 3-D RF image beneath the whole Japan Islands.

Controls on Magmatic and Hydrothermal Processes at Yellowstone Supervolcano: The Wideband Magnetotelluric Component of an Integrated MT/Seismic Investigation

NASA Astrophysics Data System (ADS)

Schultz, A.; Bennington, N. L.; Bowles-martinez, E.; Imamura, N.; Cronin, R. A.; Miller, D. J.; Hart, L.; Gurrola, R. M.; Neal, B. A.; Scholz, K.; Fry, B.; Carbonari, R.

2017-12-01

Previous seismic and magnetotelluric (MT) studies beneath Yellowstone (YS) have provided insight into the origin and migration of magmatic fluids within the volcanic system. However, important questions remain concerning the generation of magmatism at YS, the migration and storage of these magmatic fluids, as well as their relationships to hydrothermal expressions. Analysis of regional-scale EarthScope MT data collected previously suggests a relative absence of continuity in crustal partial melt accumulations directly beneath YS. This is in contrast to some seismic interpretations, although such long-period MT data have limited resolving power in the upper-to-mid crustal section. A wideband MT experiment was designed as a component of an integrated MT/seismic project to examine: the origin and location of magmatic fluids at upper mantle/lower crustal depths, the preferred path of migration for these magmatic fluids into the mid- to upper-crust, the resulting distribution of the magma reservoir, the composition of the magma reservoir, and implications for future volcanism at YS. A high-resolution wideband MT survey was carried out in the YS region in the summer of 2017, with more than forty-five wideband stations installed within and immediately surrounding the YS National Park boundary. These data provided nearly six decades of bandwidth ( 10-3 Hz -to- 103 Hz). Extraordinary permitting restrictions prevented us from using conventional installation methods at many of our sites, and an innovative "no-dig" subaerial method of wideband MT was developed and used successfully. Using these new data along with existing MT datasets, we are inverting for the 3D resistivity structure at upper crustal through upper mantle scales at YS. Complementary to this MT work, a joint inversion for the 3D crustal velocity structure is being carried out using both ambient noise and earthquake travel time data. Taken together, these data should better constrain the crustal velocity structure of this volcanic system and produce enhanced images of magma storage.

S-wave attenuation structure beneath the northern Izu-Bonin arc

NASA Astrophysics Data System (ADS)

Takahashi, Tsutomu; Obana, Koichiro; Kodaira, Shuichi

2016-04-01

To understand temperature structure or magma distribution in the crust and uppermost mantle, it is essential to know their attenuation structure. This study estimated the 3-D S-wave attenuation structure in the crust and uppermost mantle at the northern Izu-Bonin arc, taking into account the apparent attenuation due to multiple forward scattering. In the uppermost mantle, two areas of high seismic attenuation (high Q -1) imaged beneath the volcanic front were mostly colocated with low-velocity anomalies. This coincidence suggests that these high- Q -1 areas in low-velocity zones are the most likely candidates for high-temperature regions beneath volcanoes. The distribution of random inhomogeneities indicated the presence of three anomalies beneath the volcanic front: Two were in high- Q -1 areas but the third was in a moderate- Q -1 area, indicating a low correlation between random inhomogeneities and Q -1. All three anomalies of random inhomogeneities were rich in short-wavelength spectra. The most probable interpretation of such spectra is the presence of volcanic rock, which would be related to accumulated magma intrusion during episodes of volcanic activity. Therefore, the different distributions of Q -1 and random inhomogeneities imply that the positions of hot regions in the uppermost mantle beneath this arc have changed temporally; therefore, they may provide important constraints on the evolutionary processes of arc crust and volcanoes.

Crustal structure in Tengchong Volcano-Geothermal Area, western Yunnan, China

NASA Astrophysics Data System (ADS)

Wang, Chun-Yong; Huangfu, Gang

2004-02-01

Based upon the deep seismic sounding profiles carried out in the Tengchong Volcano-Geothermal Area (TVGA), western Yunnan Province of China, a 2-D crustal P velocity structure is obtained by use of finite-difference inversion and forward travel-time fitting method. The crustal model shows that a low-velocity anomaly zone exists in the upper crust, which is related to geothermal activity. Two faults, the Longling-Ruili Fault and Tengchong Fault, on the profile extend from surface to the lower crust and the Tengchong Fault likely penetrates the Moho. Moreover, based on teleseismic receiver functions on a temporary seismic network, S-wave velocity structures beneath the geothermal field show low S-wave velocity in the upper crust. From results of geophysical survey, the crust of TVGA is characterized by low P-wave and S-wave velocities, low resistivity, high heat-flow value and low Q. The upper mantle P-wave velocity is also low. This suggests presence of magma in the crust derived from the upper mantle. The low-velocity anomaly in upper crust may be related to the magma differentiation. The Tengchong volcanic area is located on the northeast edge of the Indian-Eurasian plate collision zone, away from the eastern boundary of the Indian plate by about 450 km. Based on the results of this paper and related studies, the Tengchong volcanoes can be classified as plate boundary volcanoes.

Source Parameters for Moderate Earthquakes in the Zagros Mountains with Implications for the Depth Extent of Seismicity

DOE Office of Scientific and Technical Information (OSTI.GOV)

Adams, A; Brazier, R; Nyblade, A

2009-02-23

Six earthquakes within the Zagros Mountains with magnitudes between 4.9 and 5.7 have been studied to determine their source parameters. These events were selected for study because they were reported in open catalogs to have lower crustal or upper mantle source depths and because they occurred within an area of the Zagros Mountains where crustal velocity structure has been constrained by previous studies. Moment tensor inversion of regional broadband waveforms have been combined with forward modeling of depth phases on short period teleseismic waveforms to constrain source depths and moment tensors. Our results show that all six events nucleated withinmore » the upper crust (<11 km depth) and have thrust mechanisms. This finding supports other studies that call into question the existence of lower crustal or mantle events beneath the Zagros Mountains.« less

Mantle dynamics in the Mediterranean

NASA Astrophysics Data System (ADS)

Faccenna, Claudio; Becker, Thorsten W.; Auer, Ludwig; Billi, Andrea; Boschi, Lapo; Brun, Jean Pierre; Capitanio, Fabio A.; Funiciello, Francesca; Horvåth, Ferenc; Jolivet, Laurent; Piromallo, Claudia; Royden, Leigh; Rossetti, Federico; Serpelloni, Enrico

2014-09-01

The Mediterranean offers a unique opportunity to study the driving forces of tectonic deformation within a complex mobile belt. Lithospheric dynamics are affected by slab rollback and collision of two large, slowly moving plates, forcing fragments of continental and oceanic lithosphere to interact. This paper reviews the rich and growing set of constraints from geological reconstructions, geodetic data, and crustal and upper mantle heterogeneity imaged by structural seismology. We proceed to discuss a conceptual and quantitative framework for the causes of surface deformation. Exploring existing and newly developed tectonic and numerical geodynamic models, we illustrate the role of mantle convection on surface geology. A coherent picture emerges which can be outlined by two, almost symmetric, upper mantle convection cells. The downwellings are found in the center of the Mediterranean and are associated with the descent of the Tyrrhenian and the Hellenic slabs. During plate convergence, these slabs migrated backward with respect to the Eurasian upper plate, inducing a return flow of the asthenosphere from the back-arc regions toward the subduction zones. This flow can be found at large distance from the subduction zones and is at present expressed in two upwellings beneath Anatolia and eastern Iberia. This convection system provides an explanation for the general pattern of seismic anisotropy in the Mediterranean, first-order Anatolia, and Adria microplate kinematics and may contribute to the high elevation of scarcely deformed areas such as Anatolia and eastern Iberia. More generally, the Mediterranean is an illustration of how upper mantle, small-scale convection leads to intraplate deformation and complex plate boundary reconfiguration at the westernmost terminus of the Tethyan collision.

Interaction Between Downwelling Flow and the Laterally-Varying Thickness of the North American Lithosphere Inferred from Seismic Anisotropy

NASA Astrophysics Data System (ADS)

Behn, M. D.; Conrad, C. P.; Silver, P. G.

2005-12-01

Shear flow in the asthenosphere tends to align olivine crystals in the direction of shear, producing a seismically anisotropic asthenosphere that can be detected using a number of seismic techniques (e.g., shear-wave splitting (SWS) and surface waves). In the ocean basins, where the asthenosphere has a relatively uniform thickness and lithospheric anisotropy appears to be small, observed azimuthal anisotropy is well fit by asthenospheric shear flow in global flow models driven by a combination of plate motions and mantle density heterogeneity. In contrast, beneath the continents both the lithospheric ceiling and asthenospheric thickness may vary considerably across cratonic regions and ocean-continent boundaries. To examine the influence of a continental lithosphere with variable thickness on predictions of continental seismic anisotropy, we impose lateral variations in lithospheric viscosity in global models of mantle flow driven by plate motions and mantle density heterogeneity. For the North American continent, the Farallon slab descends beneath a deep cratonic root, producing downwelling flow in the upper mantle and convergent flow beneath the cratonic lithosphere. We evaluate both the orientation of the predicted azimuthal anisotropy and the depth dependence of radial anisotropy for this downwelling flow and find that the inclusion of a strong continental root provides an improved fit to observed SWS observations beneath the North American craton. Thus, we hypothesize that at least some continental anisotropy is associated with sub-lithospheric viscous shear, although fossil anisotropy in the lithospheric layer may also contribute significantly. Although we do not observe significant variations in the direction of predicted anisotropy with depth, we do find that the inclusion of deep continental roots pushes the depth of the anisotropy layer deeper into the upper mantle. We test several different models of laterally-varying lithosphere and asthenosphere viscosity. These models can be used to separate the contributions of asthenospheric flow and lithospheric fossil fabric in observations of continental anisotropy.

Seismic and thermal structure of the crust and uppermost mantle beneath Antarctica from inversion of multiple seismic datasets

NASA Astrophysics Data System (ADS)

Wiens, D.; Shen, W.; Anandakrishnan, S.; Aster, R. C.; Gerstoft, P.; Bromirski, P. D.; Dalziel, I.; Hansen, S. E.; Heeszel, D.; Huerta, A. D.; Nyblade, A.; Stephen, R. A.; Wilson, T. J.; Winberry, J. P.; Stern, T. A.

2017-12-01

Since the last decade of the 20th century, over 200 broadband seismic stations have been deployed across Antarctica (e.g., temporary networks such as TAMSEIS, AGAP/GAMSEIS, POLENET/ANET, TAMNNET and RIS/DRIS by U.S. geoscientists as well as stations deployed by Japan, Britain, China, Norway, and other countries). In this presentation, we discuss our recent efforts to build reference crustal and uppermost mantle shear velocity (Vs) and thermal models for continental Antarctica based on those seismic arrays. By combing the high resolution Rayleigh wave dispersion maps derived from both ambient noise and teleseismic earthquakes, together with P receiver function waveforms, we develop a 3-D Vs model for the crust and uppermost mantle beneath Central and West Antarctica to a depth of 200 km. Additionally, using this 3-D seismic model to constrain the crustal structure, we re-invert for the upper mantle thermal structure using the surface wave data within a thermodynamic framework and construct a 3-D thermal model for the Antarctic lithosphere. The final product, a high resolution thermal model together with associated uncertainty estimates from the Monte Carlo inversion, allows us to derive lithospheric thickness and surface heat flux maps for much of the continent. West Antarctica shows a much thinner lithosphere ( 50-90 km) than East Antarctica ( 130-230 km), with a sharp transition along the Transantarctic Mountains (TAM). A variety of geological features, including a slower/hotter but highly heterogeneous West Antarctica and a much faster/colder East Antarctic craton, are present in the 3-D seismic/thermal models. Notably, slow seismic velocities observed in the uppermost mantle beneath the southern TAM are interpreted as a signature of lithospheric foundering and replacement with hot asthenosphere. The high resolution image of these features from the 3-D models helps further investigation of the dynamic state of Antarctica's lithosphere and underlying asthenosphere and provides key constraints on the interaction between the solid Earth and the West Antarctic Ice Sheet.

Low crustal velocities and mantle lithospheric variations in southern Tibet from regional Pnl waveforms

NASA Astrophysics Data System (ADS)

Rodgers, Arthur J.; Schwartz, Susan Y.

We report low average crustal P-wave velocities (5.9-6.1 km/s, Poisson's ratio 0.23-0.27, thickness 68-76 km) in southern Tibet from modelling regional Pnl waveforms recorded by the 1991-1992 Tibetan Plateau Experiment. We also find that the mantle lithosphere beneath the Indus-Tsangpo Suture and the Lhasa Terrane is shield-like (Pn velocity 8.20-8.25 km/s, lid thickness 80-140 km, positive velocity gradient 0.0015-0.0025 s-1). Analysis of relative Pn travel time residuals requires a decrease in the mantle velocities beneath the northern Lhasa Terrane, the Banggong-Nujiang Suture and the southern Qiangtang Terrane. Tectonic and petrologic considerations suggest that low bulk crustal velocities could result from a thick (50-60 km) felsic upper crust with vertically limited and laterally pervasive partial melt. These results are consistent with underthrusting of Indian Shield lithosphere beneath the Tibetan Plateau to at least the central Lhasa Terrane.

Unraveling the Origin of the Bermuda Rise Using Receiver Functions: Insights from Mantle Discontinuity Structure

NASA Astrophysics Data System (ADS)

Burky, A.; Irving, J. C. E.; Simons, F.

2017-12-01

The Bermuda Rise is an enigmatic intraplate bathymetric feature which is considered a candidate hotspot in some catalogs, but remains a poor candidate due to the lack of an associated seamount chain and the absence of any present-day volcanism. Tomographic models of the seismic P and S wave velocity structure in the upper mantle and transition zone beneath Bermuda and the surrounding seafloor consistently resolve low velocity structures, but the magnitude, lateral dimensions, and position of these low velocity structures vary considerably between models. Due to these discrepancies, it remains difficult to attribute the observed velocity anomalies to thermal or chemical heterogeneity in this region. In addition to tomographic modeling, previous studies investigated the mantle transition zone structure beneath Bermuda by calculating receiver functions for GSN station BBSR, and suggested thinning of the transition zone as well as depressed discontinuity topography. In this study, we expand upon those studies by including the wealth of newly available data, and by incorporating a suite of three-dimensional velocity models. We calculate radial receiver functions in multiple frequency bands for the highest quality seismograms selected from over 5,000 waveforms recorded at station BBSR between October 2008 and August 2017 using the iterative deconvolution technique. We use various one- and three-dimensional velocity models to depth-convert our receiver functions to find the depths of the mantle transition zone discontinuities responsible for the signals in our receiver functions. The observed discontinuity topography is interpreted in the context of candidate mineralogical phase transitions and mantle temperature. To gain a more comprehensive understanding of our observations, we also calculate synthetic seismograms using AxiSEM, compute radial receiver functions for these synthetic data, and compare the results to the real receiver functions. Lastly, we discuss our results in the context of the geologic and geodynamic history of the Bermuda Rise.

Constraining the Composition of the Subcontinental Lithospheric Mantle Beneath the East African Rift: FTIR Analysis of Water in Spinel Peridotite Mantle Xenoliths

NASA Technical Reports Server (NTRS)

Erickson, Stephanie Gwen; Nelson, Wendy R.; Peslier, Anne H.; Snow, Jonathan E.

2014-01-01

The East African Rift System was initiated by the impingement of the Afar mantle plume on the base of the non-cratonic continental lithosphere (assembled during the Pan-African Orogeny), producing over 300,000 kmof continental flood basalts approx.30 Ma ago. The contribution of the subcontinental lithospheric mantle (SCLM) to this voluminous period of volcanism is implied based on basaltic geochemical and isotopic data. However, the role of percolating melts on the SCLM composition is less clear. Metasomatism is capable of hybridizing or overprinting the geochemical signature of the SCLM. In addition, models suggest that adding fluids to lithospheric mantle affects its stability. We investigated the nature of the SCLM using Fourier transform infrared spectrometry (FTIR) to measure water content in mantle xenoliths entrained in young (1 Ma) basaltic lavas from the Ethiopian volcanic province. The mantle xenoliths consist dominantly of spinel lherzolites and are composed of nominally anhydrous minerals, which can contain trace water as H in mineral defects. Eleven mantle xenoliths come from the Injibara-Gojam region and two from the Mega-Sidamo region. Water abundances of olivines in six samples are 1-5ppm H2O while the rest are below the limit of detection (<0.5 ppm H2O); orthopyroxene and clinopyroxene contain 80-238 and 111-340 ppm wt H2O, respectively. Two xenoliths have higher water contents - a websterite (470 ppm) and dunite (229 ppm), consistent with involvement of ascending melts. The low water content of the upper SCLM beneath Ethiopia is as dry as the oceanic mantle except for small domains represented by percolating melts. Consequently, rifting of the East African lithosphere may not have been facilitated by a hydrated upper mantle.

Seismic tomographic imaging of P- and S-waves velocity perturbations in the upper mantle beneath Iran

NASA Astrophysics Data System (ADS)

Alinaghi, Alireza; Koulakov, Ivan; Thybo, Hans

2007-06-01

The inverse tomography method has been used to study the P- and S-waves velocity structure of the crust and upper mantle underneath Iran. The method, based on the principle of source-receiver reciprocity, allows for tomographic studies of regions with sparse distribution of seismic stations if the region has sufficient seismicity. The arrival times of body waves from earthquakes in the study area as reported in the ISC catalogue (1964-1996) at all available epicentral distances are used for calculation of residual arrival times. Prior to inversion we have relocated hypocentres based on a 1-D spherical earth's model taking into account variable crustal thickness and surface topography. During the inversion seismic sources are further relocated simultaneously with the calculation of velocity perturbations. With a series of synthetic tests we demonstrate the power of the algorithm and the data to reconstruct introduced anomalies using the ray paths of the real data set and taking into account the measurement errors and outliers. The velocity anomalies show that the crust and upper mantle beneath the Iranian Plateau comprises a low velocity domain between the Arabian Plate and the Caspian Block. This is in agreement with global tomographic models, and also tectonic models, in which active Iranian plateau is trapped between the stable Turan plate in the north and the Arabian shield in the south. Our results show clear evidence of the mainly aseismic subduction of the oceanic crust of the Oman Sea underneath the Iranian Plateau. However, along the Zagros suture zone, the subduction pattern is more complex than at Makran where the collision of the two plates is highly seismic.

The P and S wave velocity structure of the mantle beneath eastern Africa and the African superplume anomaly

NASA Astrophysics Data System (ADS)

Mulibo, Gabriel D.; Nyblade, Andrew A.

2013-08-01

P and S relative arrival time residuals from teleseismic earthquakes recorded on over 60 temporary AfricaArray broadband seismic stations deployed in Uganda, Tanzania, and Zambia between 2007 and 2011 have been inverted, together with relative arrival time residuals from earthquakes recorded by previous deployments, for a tomographic image of mantle wave speed variations extending to a depth of 1200 km beneath eastern Africa. The image shows a low-wave speed anomaly (LWA) well developed at shallow depths (100-200 km) beneath the Eastern and Western branches of the Cenozoic East African rift system and northwestern Zambia, and a fast wave speed anomaly at depths ≤ 350 km beneath the central and northern parts of the East African Plateau and the eastern and central parts of Zambia. At depths ≥350 km the LWA is most prominent under the central and southern parts of the East African Plateau and dips to the southwest beneath northern Zambia, extending to a depth of at least 900 km. The amplitude of the LWA is consistent with a ˜150-300 K thermal perturbation, and its depth extent indicates that the African superplume, originally identified as a lower mantle anomaly, is likely a whole mantle structure. A superplume extending from the core-mantle boundary to the surface implies an origin for the Cenozoic extension, volcanism, and plateau uplift in eastern Africa rooted in the dynamics of the lower mantle.

On the potential asthenospheric linkage between Apenninic slab rollback and Alpine topographic uplift: insights from P wave tomography and seismic anisotropy analysis

NASA Astrophysics Data System (ADS)

Malusa', Marco Giovanni; Salimbeni, Simone; Zhao, Liang; Guillot, Stéphane; Pondrelli, Silvia; Margheriti, Lucia; Paul, Anne; Solarino, Stefano; Aubert, Coralie; Dumont, Thierry; Schwartz, Stéphane; Wang, Qingchen; Xu, Xiaobing; Zheng, Tianyu; Zhu, Rixiang

2017-04-01

The role of surface and deep-seated processes in controlling the topography of complex plate-boundary areas is a highly debated issue. In the Western Alps, which include the highest summits in Europe, factors controlling topographic uplift still remain poorly understood. In the absence of active convergence, recent works have suggested a potential linkage between slab breakoff and fast uplift, but this hypothesis is ruled out by the down-dip continuity of the Alpine slab documented by recent tomographic images of the upper mantle beneath the Alpine region (Zhao et al. 2016). In order to shed light on this issue, we use a densely spaced array of temporary broadband seismic stations and previously published observations to analyze the seismic anisotropy pattern along the transition zone between the Alps and the Apennines, within the framework of the upper mantle structure unveiled by P wave tomography. Our results show a continuous trend of anisotropy fast axes near-parallel to the western alpine arc, possibly due to an asthenospheric counterflow triggered by the eastward retreat of the Apenninic slab. This trend is located in correspondence of a low velocity anomaly in the European upper mantle, and beneath the Western Alps region characterized by the highest uplift rates, which may suggest a potential impact of mantle dynamics on Alpine topography. We propose that the progressive rollback of the Apenninic slab induced a suction effect and an asthenospheric counterflow at the rear of the unbroken Alpine slab and around its southern tip, as well as an asthenospheric upwelling, mirrored by low P wave velocities, which may have favored the topographic uplift of the Alpine belt from the Mt Blanc to the Ligurian coast. Zhao L. et al., 2016. Continuity of the Alpine slab unraveled by high-resolution P wave tomography. J. Geophys. Res., doi:10.1002/2016JB013310.

Upper mantle heterogeneity: Comparisons of regions south of Australia with Philippine Basin

NASA Technical Reports Server (NTRS)

1982-01-01

The nature of mass anomalies that occur beneath the regions of negative residual depth anomalies were identified. Residual geoid anomalies with negative residual depth anomalies are identified in the Philippine Basin (negative) and in the region south of Australia (positive and negative). In the latter region the geoid anomalies are eastward and the depth anomaly is northeast. It is suggested that the negative depth anomaly and the compensating mass excess in the uppermost mantle developed in the Eocene as the lithosphere of the west Philippine basin formed. Heating of the deeper upper mantle which causes slow surface wave velocities and negative gravity and geoid anomalies may be a younger phenomenon which is still in progress.

Investigation of upper mantle seismic discontinuities beneath the Indian Ocean using array seismology methods

NASA Astrophysics Data System (ADS)

van Driel, J.; Reiss, A. S.; Thomas, C.

2016-12-01

The topography of upper mantle seismic discontinuities can be used to constrain regional variations in composition and temperature of the Earths mantle. The 410 km discontinuity is caused by the solid-solid phase transition from olivine to wadsleyite. Due to its positive Clapeyron slope, the discontinuity is depressed in hot regimes. The phase transition from ringwoodite to bridgemanite and magnesiowüstite in contrast has a negative Clapeyron slope and therefore is elevated when hot material is present. Cold material is expected to yield an opposing topographic signature, culminating in an elevated 410 km and a depressed 660 km discontinuity. As part of the RHUM-RUM project (Réunion Hotspot and Upper Mantle - Réunions Unterer Mantel) we extract relevant geophysical parameters, by investigating the properties of upper mantle seismic discontinuities beneath the Indian Ocean. The topography of the 410 and 660 km discontinuities, which define the upper and lower bounds of the mantle transition zone, have been mapped using PP and SS underside reflections. This study has utilised over 8500 events with Mw ≥ 5.8, distributed over the entire Indian Ocean. Our robust data set yields a dense coverage of points, which are defined by consistently crossing ray paths. Array seismology methods, such as vespagrams and slowness-backazimuth analysis, are used to enhance the signal-to-noise-ratio and detect and identify weak precursor signals. The differential travel times are corrected for crustal features and converted into depth values of the discontinuities by comparing the measured travel times with theoretical ones derived from ray tracing through the 1D reference Earth model ak135. A `travel-time' stacking method has also been applied for 4° radius bins around each of the bounce points. The addition of a secondary method derives greater stability of our results and allows an enhanced error analysis procedure. In order to better constrain the mineralogical processes taking place within the mantle transition zone, amplitude ratios, polarities and velocity gradients have also been investigated.

Mantle dynamics in the Mediterranean

NASA Astrophysics Data System (ADS)

Faccenna, Claudio; Becker, Thorsten W.

2016-04-01

The Mediterranean offers a unique avenue to study the driving forces of tectonic deformation within a complex mobile belt. Lithospheric dynamics are affected by slab rollback and collision of two large, slowly moving plates, forcing fragments of continental and oceanic lithosphere to interact. Here, we review the rich and growing set of constraints from geological reconstructions, geodetic data, and crustal and upper mantle heterogeneity imaged by structural seismology. We discuss a conceptual and quantitative framework for the causes of surface deformations. Exploring existing and newly developed tectonic and numerical geodynamic models, we illustrate the role of mantle convection on surface geology. A coherent picture emerges which can be outlined by two, almost symmetric, upper mantle convection cells. The down-wellings are found in the centre of the Mediterranean, and are associated with the descent of the Tyrrhenian and the Hellenic slabs. During plate convergence, these slabs migrated, driving return flow of the asthenosphere from the backarc regions. These currents can be found at large distance from the subduction zones, and are at present expressed in two upwellings beneath Anatolia and eastern Iberia. This convection system provides an explanation for the general pattern of seismic anisotropy in the Mediterranean, the first-order Anatolia and Adria microplate kinematics, and the positive dynamic topography of Anatolia and Eastern Iberia. More generally, it is an illustration of upper mantle, small-scale convection leading to intraplate deformation and complex plate boundary reconfiguration at the westernmost terminus of the Tethyan collision.

Continental collision with a sandwiched accreted terrane: Insights into Himalayan-Tibetan lithospheric mantle tectonics?

NASA Astrophysics Data System (ADS)

Kelly, Sean; Butler, Jared P.; Beaumont, Christopher

2016-12-01

Many collisional orogens contain exotic terranes that were accreted to either the subducting or overriding plate prior to terminal continent-continent collision. The ways in which the physical properties of these terranes influence collision remain poorly understood. We use 2D thermomechanical finite element models to examine the effects of prior 'soft' terrane accretion to a continental upper plate (retro-lithosphere) on the ensuing continent-continent collision. The experiments explore how the style of collision changes in response to variations in the density and viscosity of the accreted terrane lithospheric mantle, as well as the density of the pro-lithospheric mantle, which determines its propensity to subduct or compress the accreted terrane and retro-lithosphere. The models evolve self-consistently through several emergent phases: breakoff of subducted oceanic lithosphere; pro-continent subduction; shortening of the retro-lithosphere accreted terrane, sometimes accompanied by lithospheric delamination; and, terminal underthrusting of pro-lithospheric mantle beneath the accreted terrane crust or mantle. The modeled variations in the properties of the accreted terrane lithospheric mantle can be interpreted to reflect metasomatism during earlier oceanic subduction beneath the terrane. Strongly metasomatized (i.e., dense and weak) mantle is easily removed by delamination or entrainment by the subducting pro-lithosphere, and facilitates later flat-slab underthrusting. The models are a prototype representation of the Himalayan-Tibetan orogeny in which there is only one accreted terrane, representing the Lhasa terrane, but they nonetheless exhibit processes like those inferred for the more complex Himalayan-Tibetan system. Present-day underthrusting of the Tibetan Plateau crust by Indian mantle lithosphere requires that the Lhasa terrane lithospheric mantle has been removed. Some of the model results support previous conceptual interpretations that Tibetan lithospheric mantle was removed by convective coupling to the pro-lithosphere. They can also be interpreted to suggest that delamination beneath Tibet was facilitated by densification and weakening of the plateau lithosphere, perhaps owing to long-lived pre- to syn-collisional subduction-related metasomatism beneath the Asian margin.

Lithospheric structure of the Arabian Shield from the joint inversion of receiver functions and surface-wave group velocities

NASA Astrophysics Data System (ADS)

Julià, Jordi; Ammon, Charles J.; Herrmann, Robert B.

2003-08-01

We estimate lithospheric velocity structure for the Arabian Shield by jointly modeling receiver functions and fundamental-mode group velocities from events recorded by the 1995-1997 Saudi Arabian Portable Broadband Deployment. Receiver functions are primarily sensitive to shear-wave velocity contrasts and vertical travel times, and surface-wave dispersion measurements are sensitive to vertical shear-wave velocity averages, so that their combination bridge resolution gaps associated with each individual data set. Our resulting models correlate well with the observed surface geology; the Asir terrane to the West consists of a 10-km-thick upper crust of 3.3 km/s overlying a lower crust of 3.7-3.8 km/s; in the Afif terrane to the East, the upper crust is 20 km thick and has an average velocity of 3.6 km/s, and the lower crust is about 3.8 km/s; separating the terranes, the Nabitah mobile belt is made of a gradational upper crust up to 3.6 km/s at 15 km overlying an also gradational lower crust up to 4.0 km/s. The crust-mantle transition is found to be sharp in terranes of continental affinity (east) and gradual in terranes of oceanic affinity (west). The upper mantle shear velocities range from 4.3 to 4.6 km/s. Temperatures around 1000 °C are obtained from our velocity models for a thin upper mantle lid observed beneath station TAIF, and suggest that the lithosphere could be as thin as 50-60 km under this station.

Joint geophysical and petrological models for the lithosphere structure of the Antarctic Peninsula continental margin

NASA Astrophysics Data System (ADS)

Yegorova, Tamara; Bakhmutov, Vladimir; Janik, Tomasz; Grad, Marek

2011-01-01

The Antarctic Peninsula (AP) is a composite magmatic arc terrane formed at the Pacific margin of Gondwana. Through the late Mesozoic and Cenozoic subduction has stopped progressively from southwest to northeast as a result of a series of ridge trench collisions. Subduction may be active today in the northern part of the AP adjacent to the South Shetland Islands. The subduction system is confined by the Shackleton and Hero fracture zones. The magmatic arc of the AP continental margin is marked by high-amplitude gravity and magnetic anomaly belts reaching highest amplitudes in the region of the South Shetland Islands and trench. The sources for these anomalies are highly magnetic and dense batholiths of mafic bulk composition, which were intruded in the Cretaceous, due to partial melting of upper-mantle and lower-crustal rocks. 2-D gravity and magnetic models provide new insights into crustal and upper-mantle structure of the active and passive margin segments of the northern AP. Our models incorporate seismic refraction constraints and physical property data. This enables us to better constrain both Moho geometry and petrological interpretations in the crust and upper mantle. Model along the DSS-12 profile crosses the AP margin near the Anvers Island and shows typical features of a passive continental margin. The second model along the DSS-17 profile extends from the Drake Passage through the South Shetland Trench/Islands system and Bransfield Strait to the AP and indicates an active continental margin linked to slow subduction and on-going continental rifting in the backarc region. Continental rifting beneath the Bransfield Strait is associated with an upward of hot upper mantle rocks and with extensive magmatic underplating.

Imaging of Upper-Mantle Upwelling Beneath the Salton Trough, Southern California, by Joint Inversion of Ambient Noise Dispersion Curves and Receiver Functions

NASA Astrophysics Data System (ADS)

Klemperer, S. L.; Barak, S.

2016-12-01

We present a new 2D shear-wave velocity model of the crust and upper-mantle across the Salton Trough, southern California, obtained by jointly inverting our new dataset of receiver functions and our previously published Rayleigh-wave group-velocity model (Barak et al., G-cubed, 2015), obtained from ambient-noise tomography. Our results show an upper-mantle low-velocity zone (LVZ) with Vs ≤4.2 km/s extending from the Elsinore Fault to the Sand Hills Fault, that together bracket the full width of major San Andreas dextral motion since its inception 6 Ma b.p., and underlying the full width of low topography of the Imperial Valley and Salton Trough. The lateral extent of the LVZ is coincident with the lateral extent of an upper-mantle anisotropic region interpreted as a zone of SAF-parallel melt pockets (Barak & Klemperer, Geology, 2016). The shallowest part of the LVZ is 40 km depth, coincident with S-receiver function images. The western part of the LVZ, between the Elsinore and San Jacinto faults (the region of greatest modern dextral slip), appears to continue to significantly greater depth; but a puzzling feature of our preliminary models is that the eastern part of the LVZ, from the San Jacinto Fault to the Sand Hills Fault, appears to be underlain by more-normalvelocity upper mantle (Vs ≥ 4.5 km/s) below 75 km depth. We compare our model to the current SCEC community models CVM-H and CVM-S, and to P-wave velocity models obtained by the active-source Salton Sea Imaging Project (SSIP). The hypothesized lower-crustal low-velocity zone beneath the Salton Trough in our previous model (Barak et al., G-cubed, 2015), there interpreted as a region of partial melt, is not supported by our new modeling. Melt may be largely absent from the lower crust of the Salton trough; but appears required in the upper mantle at depths as shallow as 40 km.

Mantle Flow and Melting Processes Beneath Back-Arc Basins

NASA Astrophysics Data System (ADS)

Hall, P. S.

2007-12-01

The chemical systematics of back-arc basin basalts suggest that multiple mechanisms of melt generation and transport operate simultaneously beneath the back-arc, resulting in a continuum of melts ranging from a relatively dry, MORB-like end-member to a wet, slab-influenced end-member [e.g., Kelley et al., 2006; Langmuir et al., 2006]. Potential melting processes at work include adiabatic decompression melting akin to that at mid-ocean ridges, diapiric upwelling of hydrous and/or partially molten mantle from above the subducting lithospheric slab [e.g., Marsh, 1979; Hall and Kincaid, 2001; Gerya and Yuen, 2003], and melting of back-arc mantle due to a continuous flux of slab-derived hydrous fluid [Kelley et al., 2006]. In this study, we examine the potential for each of these melting mechanisms to contribute to the observed distribution of melts in back-arc basins within the context of upper mantle flow (driven by plate motions) beneath back-arcs, which ultimately controls temperatures within the melting region. Mantle velocities and temperatures are derived from numerical geodynamic models of subduction with back-arc spreading that explicitly include adiabatic decompression melting through a Lagrangian particle scheme and a parameterization of hydrous melting. Dynamical feedback from the melting process occurs through latent heating and viscosity increases related to dehydration. A range of parameters, including subduction rate and trench-back-arc separation distances, is explored. The thermal evolution of individual diapirs is modeled numerically as they traverse the mantle, from nucleation above the subducting slab to melting beneath the back-arc spreading center, and a range of diapir sizes and densities and considered.

Pulling the rug out from under California: Seismic images of the Mendocino Triple Junction region

USGS Publications Warehouse

Tréhu, Anne M.

1995-01-01

In 1993 and 1994 a network of large-aperture seismic profiles was collected to image the crustal and upper-mantle structure beneath northern California and the adjacent continental margin. The data include approximately 650 km of onshore seismic refraction/reflection data, 2000 km of off-shore multichannel seismic (MCS) reflection data, and simultaneous onshore and offshore recording of the MCS airgun source to yield large-aperture data. Scientists from more than 12 institutions were involved in data acquisition.

Seismic velocity anisotropy and heterogeneity beneath the Mantle Electromagnetic and Tomography Experiment (MELT) region of the East Pacific Rise from analysis of P and S body waves

USGS Publications Warehouse

Hammond, W.C.; Toomey, D.R.

2003-01-01

We use teleseismic P and S delay times and shear wave splitting measurements to constrain isotropic and anisotropic heterogeneity in the mantle beneath the southern East Pacific Rise (SEPR). The data comprise 462 P and S delay times and 18 shear wave splitting observations recorded during the Mantle Electromagnetic and Tomography (MELT) Experiment. We estimate the mantle melt content (F) and temperature (T) variation from the isotropic velocity variation. Our results indicate that the maximum variation in F beneath our array is between zero and ???1.2%, and maximum variation in T is between zero and ???100 K. We favor an explanation having partial contributions from both T and F. We approximate the seismic anisotropy of the upper mantle with hexagonal symmetry, consistent with the assumption of two dimensionality of mantle flow. Our new tomographic technique uses a nonlinear inversion of P and slow S polarization delay times to simultaneously solve for coupled VP and VS heterogeneity throughout the model and for the magnitude of anisotropy within discrete domains. The domain dimensions and the dip of the anisotropy are fixed for each inversion but are varied in a grid search, obtaining the misfit of the models to the body wave delay data and to split times of vertically propagating S waves. The data misfit and the isotropic heterogeneity are sensitive to domain dimensions and dip of anisotropy. In a region centered beneath the SEPR the best average dip of the hexagonal symmetry axis is horizontal or dipping shallowly (<30??) west. Given the resolution of our data, a subaxial region characterized by vertically aligned symmetry axes may exist but is limited to be <80 km deep. We infer that the mantle flow beneath the SEPR is consistent with shallow asthenospheric return flow from the direction of the South Pacific superswell.

Dehydrogenation of goethite in Earth’s deep lower mantle

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hu, Qingyang; Kim, Duck Young; Liu, Jin

2017-01-31

The cycling of hydrogen influences the structure, composition, and stratification of Earth’s interior. Our recent discovery of pyrite-structured iron peroxide (designated as the P phase) and the formation of the P phase from dehydrogenation of goethite FeO 2H implies the separation of the oxygen and hydrogen cycles in the deep lower mantle beneath 1,800 km. Here we further characterize the residual hydrogen, x, in the P-phase FeO 2Hx. Using a combination of theoretical simulations and high-pressure–temperature experiments, we calibrated the x dependence of molar volume of the P phase. Within the current range of experimental conditions, we observed a compositionalmore » range of P phase of 0.39 < x < 0.81, corresponding to 19–61% dehydrogenation. Increasing temperature and heating time will help release hydrogen and lower x, suggesting that dehydrogenation could be approaching completion at the high-temperature conditions of the lower mantle over extended geological time. Our observations indicate a fundamental change in the mode of hydrogen release from dehydration in the upper mantle to dehydrogenation in the deep lower mantle, thus differentiating the deep hydrogen and hydrous cycles.« less

Seismic imaging of mantle transition zone discontinuities beneath the northern Red Sea and adjacent areas

NASA Astrophysics Data System (ADS)

Mohamed, A. A.; Gao, S. S.; Elsheikh, A. A.; Liu, K. H.; Yu, Y.; Fat-Helbary, R. E.

2014-11-01

The dramatic asymmetry in terms of surface elevation, Cenozoic volcanisms and earthquake activity across the Red Sea is an enigmatic issue in global tectonics, partially due to the unavailability of broad-band seismic data on the African Plate adjacent to the Red Sea. Here, we report the first comprehensive image of the mantle transition zone (MTZ) discontinuities using data from the Egyptian National Seismic Network, and compare the resulting depths of the 410 and 660-km discontinuities with those observed on the Arabian side. Our results show that when a standard earth model is used for time-to-depth conversion, the resulting depth of the discontinuities increases systematically towards the axis of the Afro-Arabian Dome (AAD) from both the west and east. Relative to the westernmost area, the maximum depression of the 410-km discontinuity is about 30 km, and that of the 660-km discontinuity is about 45 km. The observed systematic variations can best be explained by a model involving a hydrated MTZ and an upper-mantle low-velocity zone beneath the AAD. Models invoking one or more mantle plumes originated from the MTZ or the lower-mantle beneath the study area are not consistent with the observations.

One billion year-old Mid-continent Rift leaves virtually no clues in the mantle

NASA Astrophysics Data System (ADS)

Bollmann, T. A.; Frederiksen, A. W.; van der Lee, S.; Wolin, E.; Revenaugh, J.; Wiens, D.; Darbyshire, F. A.; Aleqabi, G. I.; Wysession, M. E.; Stein, S.; Jurdy, D. M.

2017-12-01

We measured the relative arrival times of more than forty-six thousand teleseismic P waves recorded by seismic stations of Earthscope's Superior Province Rifting Earthscope Experiment (SPREE) and combined them with a similar amount of such measurements from other seismic stations in the larger region. SPREE recorded seismic waves for two and a half years around the prominent, one billion year-old Mid-continent Rift structure. The curvilinear Mid-continent Rift (MR) is distinguished by voluminous one billion year-old lava flows, which produce a prominent gravity high along the MR. As for other seismic waves, these lava flows along with their underplated counterpart, slightly slow down the measured teleseismic P waves, on average, compared to P waves that did not traverse structures beneath the Mid-continent Rift. However, the variance in the P wave arrival times in these two groups is nearly ten times higher than their average difference. In a seismic-tomographic inversion, we mapped all measured arrival times into structures deep beneath the crust, in the Earth's mantle. Beneath the crust we generally find relatively high P velocities, indicating relatively cool and undeformable mantle structures. However, the uppermost mantle beneath the MR shows several patches of slightly decreased P velocities. These patches are coincident with where the gravity anomalies peak, in Iowa and along the northern Minnesota/Wisconsin border. We will report on the likelihood that these anomalies are indeed a remaining mantle-lithospheric signature of the MR or whether these patches indirectly reflect the presence of the lava flows and their underplated counterparts at the crust-mantle interface. Other structures of interest and of varying depth extent in our tomographic image locate at 1) the intersection of the Superior Craton with the Penokean Province and the Marshfield Terrane west of the MR in southern Minnesota, 2) the intersection of the Penokean, Yavapai, and Mazatzal Terranes along the eastern edge of the Michigan arm of the MR, and 3) beneath Lake Nipigon, north of Lake Superior. Our tomographic image also reveals an intricate distribution of deep high-velocity anomalies, including in the lower mantle, potentially related to Mesozoic subduction of the Kula and/or Farallon Plates.

Reconstruction of the Exhumed Mantle Across the North Iberian Margin by Crustal-Scale 3-D Gravity Inversion and Geological Cross Section

NASA Astrophysics Data System (ADS)

Pedrera, A.; García-Senz, J.; Ayala, C.; Ruiz-Constán, A.; Rodríguez-Fernández, L. R.; Robador, A.; González Menéndez, L.

2017-12-01

Recent models support the view that the Pyrenees were formed after the inversion of a previously highly extended continental crust that included exhumed upper mantle rocks. Mantle rocks remain near to the surface after compression and mountain building, covered by the latest Cretaceous to Paleogene sequences. 3-D lithospheric-scale gravity inversion demands the presence of a high-density mantle body placed within the crust in order to justify the observed anomalies. Exhumed mantle, having 50 km of maximum width, continuously extends beneath the Basque-Cantabrian Basin and along the northern side of the Pyrenees. The association of this body with rift, postrift, and inversion structural geometries is tested in a balanced cross section across the Basque-Cantabrian Basin that incorporates a major south-dipping ramp-flat-ramp extensional detachment active between Valanginian and early Cenomanian times. Results indicate that horizontal extension progressed 48 km at variable strain rates that increased from 1 to 4 mm/yr in middle Albian times. Low-strength Triassic Keuper evaporites and mudstones above the basement favor the decoupling of the cover with formation of minibasins, expulsion rollovers, and diapirs. The inversion of the extensional system is accommodated by doubly verging basement thrusts due to the reactivation of the former basin bounding faults in Eocene-Oligocene times. Total shortening is estimated in 34 km and produced the partial subduction of the continental lithosphere beneath the two sides of the exhumed mantle. Obtained results help to pinpoint the original architecture of the North Iberian Margin and the evolution of the hyperextended aborted intracontinental basins.

Lithospheric structure of the westernmost Mediterranean inferred from finite frequency Rayleigh wave tomography S-velocity model.

NASA Astrophysics Data System (ADS)

Palomeras, Imma; Villasenor, Antonio; Thurner, Sally; Levander, Alan; Gallart, Josep; Harnafi, Mimoun

2016-04-01

The Iberian Peninsula and Morocco, separated by the Alboran Sea and the Algerian Basin, constitute the westernmost Mediterranean. From north to south this region consists of the Pyrenees, the result of interaction between the Iberian and Eurasian plates; the Iberian Massif, a region that has been undeformed since the end of the Paleozoic; the Central System and Iberian Chain, regions with intracontinental Oligocene-Miocene deformation; the Gibraltar Arc (Betics, Rif and Alboran terranes) and the Atlas Mountains, resulting from post-Oligocene subduction roll-back and Eurasian-Nubian plate convergence. In this study we analyze data from recent broad-band array deployments and permanent stations on the Iberian Peninsula and in Morocco (Spanish IberArray and Siberia arrays, the US PICASSO array, the University of Munster array, and the Spanish, Portuguese, and Moroccan National Networks) to characterize its lithospheric structure. The combined array of 350 stations has an average interstation spacing of ~60 km, comparable to USArray. We have calculated the Rayleigh waves phase velocities from ambient noise for short periods (4 s to 40 s) and teleseismic events for longer periods (20 s to 167 s). We inverted the phase velocities to obtain a shear velocity model for the lithosphere to ~200 km depth. The model shows differences in the crust for the different areas, where the highest shear velocities are mapped in the Iberian Massif crust. The crustal thickness is highly variable ranging from ~25 km beneath the eastern Betics to ~55km beneath the Gibraltar Strait, Internal Betics and Internal Rif. Beneath this region a unique arc shaped anomaly with high upper mantle velocities (>4.6 km/s) at shallow depths (<65 km) is observed. We interpret this body as the subducting Alboran slab that is depressing the crust of the western Gibraltar arc to ~55 km depth. Low upper mantle velocities (<4.2 km/s) are observed beneath the Atlas, the northeastern end of the Betic Mountains and the Late Cenozoic volcanic fields in Iberia and Morocco, indicative of high temperatures at relatively shallow depths, and suggesting that the lithosphere has been removed beneath these areas

Saudi Arabian seismic-refraction profile: A traveltime interpretation of crustal and upper mantle structure

USGS Publications Warehouse

Mooney, W.D.; Gettings, M.E.; Blank, H.R.; Healy, J.H.

1985-01-01

The crustal and upper mantle compressional-wave velocity structure across the southwestern Arabian Shield has been investigated by a 1000-km-long seismic refraction profile. The profile begins in Mesozoic cover rocks near Riyadh on the Arabian Platform, trends southwesterly across three major Precambrian tectonic provinces, traverses Cenozoic rocks of the coastal plain near Jizan, and terminates at the outer edge of the Farasan Bank in the southern Red Sea. More than 500 surveyed recording sites were occupied, and six shot points were used, including one in the Red Sea. Two-dimensional ray-tracing techniques, used to analyze amplitude-normalized record sections indicate that the Arabian Shield is composed, to first order, of two layers, each about 20 km thick, with average velocities of about 6.3 km/s and 7.0 km/s, respectively. West of the Shield-Red Sea margin, the crust thins to a total thickness of less than 20 km, beyond which the Red Sea shelf and coastal plain are interpreted to be underlain by oceanic crust. A major crustal inhomogeneity at the northeast end of the profile probably represents the suture zone between two crustal blocks of different composition. Elsewhere along the profile, several high-velocity anomalies in the upper crust correlate with mapped gneiss domes, the most prominent of which is the Khamis Mushayt gneiss. Based on their velocities, these domes may constitute areas where lower crustal rocks have been raised some 20 km. Two intracrustal reflectors in the center of the Shield at 13 km depth probably represent the tops of mafic intrusives. The Mohorovic??ic?? discontinuity beneath the Shield varies from a depth of 43 km and mantle velocity of 8.2 km/s in the northeast to a depth of 38 km and mantle velocity of 8.0 km/s depth in the southwest near the Shield-Red Sea transition. Two velocity discontinuities occur in the upper mantle, at 59 and 70 km depth. The crustal and upper mantle velocity structure of the Arabian Shield is interpreted as revealing a complex crust derived from the suturing of island arcs in the Precarnbrian. The Shield is currently flanked by the active spreading boundary in the Red Sea. ?? 1985.

Deep structure of the Afro-Arabian hotspot by S receiver functions

NASA Astrophysics Data System (ADS)

Vinnik, L. P.; Farra, V.; Kind, R.

2004-06-01

We investigated deep structure of the Afro-Arabian hotspot by using recordings from Geoscope seismograph station ATD. The records are processed with the S receiver function technique, which allows a detection of Sp converted phases from the upper mantle discontinuities. The seismic data reveal two unusual discontinuities. The discontinuity at a depth of 160 km beneath the Gulf of Aden corresponds to the onset of melting. If the water content in olivine is around 800 H/106Si, melting at this depth requires a temperature close to 1550°C, about 120°C higher than the average. Another remarkable discontinuity is found at a depth of 480 km, where S velocity drops with depth by about 0.2 km/s. This can be the head of another plume which is trapped in the mantle transition zone.

Evolution of the Archean Mohorovičić discontinuity from a synaccretionary 4.5 Ga protocrust

NASA Astrophysics Data System (ADS)

Hamilton, Warren B.

2013-12-01

This review evaluates and rejects the currently dominant dogmas of geodynamics and geochemistry, which are based on 1950s-1970s assumptions of a slowly differentiating Earth. Evidence is presented for evolution of mantle, crust, and early Moho that began with fractionation of most crustal components, synchronously with planetary accretion, into mafic protocrust by ~ 4.5 Ga. We know little about Hadean crustal geology (> 3.9 Ga) except that felsic rocks were then forming, but analogy with Venus, and dating from the Moon, indicate great shallow disruption by large and small impact structures, including huge fractionated impact-melt constructs, throughout that era. The mantle sample and Archean (< 3.9 Ga) crustal geology integrate well. The shallow mantle was extremely depleted by early removal of thick mafic protocrust, which was the primary source of the tonalite, trondhjemite, and granodiorite (TTG) that dominate preserved Archean crust to its base, and of the thick mafic volcanic rocks erupted on that crust. Lower TTG crust, kept mobile by its high radioactivity and by insulating upper crust, rose diapirically into the upper crust as dense volcanic rocks sagged synformally. The mobile lower crust simultaneously flowed laterally to maintain subhorizontal base and surface, and dragged overlying brittler granite-and-greenstone upper crust. Petrologically required garnet-rich residual protocrust incrementally delaminated, sank through low-density high-mantle magnesian dunite, and progressively re-enriched upper mantle, mostly metasomatically. Archean and earliest Proterozoic craton stabilization and development of final Mohos followed regionally complete early delamination of residual protocrust, variously between ~ 2.9 and 2.2 Ga. Where some protocrust remained, Proterozoic basins, filled thickly by sedimentary and volcanic rocks, developed on Archean crust, beneath which delamination of later residual protocrust continued top-down enrichment of upper mantle. That reenrichment enabled modern-style plate tectonics after ~ 600 Ma, with a transition regime beginning ~ 850 Ma.

Lithospheric electrical structure of the middle Lhasa terrane in the south Tibetan plateau

NASA Astrophysics Data System (ADS)

Liang, Hongda; Jin, Sheng; Wei, Wenbo; Gao, Rui; Ye, Gaofeng; Zhang, Letian; Yin, Yaotian; Lu, Zhanwu

2018-04-01

The Lhasa terrane in southern Tibetan plateau is a huge tectono-magmatic belt and an important metallogenic belt. Its formation evolution process and mineralization are affected by the subduction of oceanic plate and subsequent continental collision. However, the evolution of Lhasa terrane has been a subject of much debate for a long time. The Lithospheric structure records the deep processes of the subduction of oceanic plate and continental collision. The magnetotelluric (MT) method can probe the sub-surface electrical conductivity, newly dense broadband and long period magnetotelluric data were collected along a south-north trending profile that across the Lhasa terrane at 88°-89°E. Dimensionality analyses demonstrated that the MT data can be interpreted using two-dimensional approaches, and the regional strike direction was determined as N110°E.Based on data analysis results, a two-dimensional (2-D) resistivity model of crust and upper mantle was derived from inversion of the transverse electric mode, transverse magnetic mode and vertical magnetic field data. Inversion model shows a large north-dipping resistor that extended from the upper crust to upper mantle beneath the Himalaya and the south of Lhasa Terrane, which may represent the subducting Indian continental lithosphere. The 31°N may be an important boundary in the Lhasa Terrane, the south performs a prominent high-conductivity anomaly from the lower crust to upper mantle which indicates the existence of asthenosphere upwelling, while the north performs a higher resistivity and may have a reworking ancient basement. The formation of the ore deposits in the study area may be related to the upwelling of the mantle material triggered by slab tearing and/or breaking off of the Indian lithosphere, and the mantle material input also contributed the total thickness of the present-day Tibetan crust. The results provide helpful constrains to understand the mechanism of the continent-continent collision and the regional exploratory prospect of the deep resources.

The rheological structure of the lithosphere in the Eastern Marmara region, Turkey

NASA Astrophysics Data System (ADS)

Oruç, Bülent; Sönmez, Tuba

2017-05-01

The aim of this work is to propose the geometries of the crustal-lithospheric mantle boundary (Moho) and lithosphere-asthenosphere boundary (LAB) and the 1D thermal structure of the lithosphere, in order to establish a rheological model of the Eastern Marmara region. The average depths of Moho and LAB are respectively 35 km and 51 km from radially averaged amplitude spectra of EGM08 Bouguer anomalies. The geometries of Moho and LAB interfaces are estimated from the Parker-Oldenburg gravity inversion algorithm. Our results show the Moho depth varies from 31 km at the northern part of North Anatolian Fault Zone (NAFZ) to 39 km below the mountain belt in the southern part of the NAFZ. The depth to the LAB beneath the same parts of the region ranges from 45 km to 55 km. Having lithospheric strength and thermal boundary layer structure, we analyzed the conditions of development of lithosphere thinning. A two-dimensional strength profile has been estimated for rheology model of the study area. Thus we suggest that the rheological structure consists of a strong upper crust, a weak lower crust, and a partly molten upper lithospheric mantle.

Rayleigh-wave dispersion reveals crust-mantle decoupling beneath eastern Tibet.

PubMed

Legendre, Cédric P; Deschamps, Frédéric; Zhao, Li; Chen, Qi-Fu

2015-11-09

The Tibetan Plateau results from the collision of the Indian and Eurasian Plates during the Cenozoic, which produced at least 2,000 km of convergence. Its tectonics is dominated by an eastward extrusion of crustal material that has been explained by models implying either a mechanical decoupling between the crust and the lithosphere, or lithospheric deformation. Discriminating between these end-member models requires constraints on crustal and lithospheric mantle deformations. Distribution of seismic anisotropy may be inferred from the mapping of azimuthal anisotropy of surface waves. Here, we use data from the CNSN to map Rayleigh-wave azimuthal anisotropy in the crust and lithospheric mantle beneath eastern Tibet. Beneath Tibet, the anisotropic patterns at periods sampling the crust support an eastward flow up to 100°E in longitude, and a southward bend between 100°E and 104°E. At longer periods, sampling the lithospheric mantle, the anisotropic structures are consistent with the absolute plate motion. By contrast, in the Sino-Korean and Yangtze cratons, the direction of fast propagation remains unchanged throughout the period range sampling the crust and lithospheric mantle. These observations suggest that the crust and lithospheric mantle are mechanically decoupled beneath eastern Tibet, and coupled beneath the Sino-Korean and Yangtze cratons.

High-resolution Body Wave Tomography of the Ross Sea Embayment, Antarctica

NASA Astrophysics Data System (ADS)

Nyblade, A.; White-Gaynor, A.; Wiens, D.; Aster, R. C.; Gerstoft, P.; Bromirski, P. D.; Stephen, R. A.; Winberry, J. P.; Huerta, A. D.; Anandakrishnan, S.; Wilson, T. J.

2016-12-01

The West Antarctic Rift System (WARS) remains the least understood continental rift system on the planet. The WARS is largely composed of the Ross Sea Embayment, which is overlain by the Ross Ice Shelf between Marie Byrd Land and the Transantarctic Mountains. Active volcanism on Ross Island continues to challenge our understanding of the seismically quiescent rift system. Previous regional-scale body wave tomographic investigations have identified areas of low seismic wave speed to about 200 km depth beneath Ross Island. However mantle structure under the Ross Sea Embayment away from Ross Island has not been previously well imaged. For this investigation we utilize teleseismic P waves recorded on the recently deployed RIS/DRIS network, which consists of 34 seismometers deployed across the Ross Ice Shelf, along with data from nearby POLENET stations and TAMSEIS stations. Relative P wave travel time residuals were obtained from 560 events using a multichannel cross correlation method, and have been inverted to obtain a preliminary model of the upper mantle. Initial results suggest that the low wave speed structure under Ross Island does not extend beneath the Ross Sea Embayment portion of the WARS.

A New Structural Model for the Red Sea from Seismic Data

NASA Astrophysics Data System (ADS)

Mooney, W. D.; Yao, Z.; Zahran, H. M.; El-Hadidy, S. Y.

2017-12-01

We present a new structureal model for the Red Sea that shows opening on an east-dipping low-angle detachment fault. We measured phase velocities using Rayleigh-wave data recorded at recently-installed, dense broadband seismic stations in the Arabian shield and determined the shear-wave velocity structure. Our results clearly reveal a 300-km wide upper mantle seismic low-velocity zone (LVZ) beneath the western Arabian shield at a depth of 60 km and with a thickness of 130 km. The LVZ has a north-south trend and follows the late-Cenozoic volcanic areas. The lithosphere beneath the western Arabian shield is remarkably thin (60-90 km). The 130-km thick mantle LVZ does not appear beneath the western Red Sea and the spreading axis. Thus, the Red Sea at 20°- 26° N is an asymmetric rift, with thin lithosphere located east of the Red Sea axis, as predicted by the low-angle detachment model for rift development. Passive rifting at the Red Sea and extensional stresses in the shield are probably driven by slab pull from the Zagros subduction zone. The low shear-wave velocity (4.0-4.2 km/s) and the geometry of LVZ beneath the western shield indicate northward flow of hot asthenosphere from the Afar hot spot. The upwelling of basaltic melt in fractures or zones of localized lithospheric thinning has produced extensive late Cenozoic volcanism on the western edge of the shield, and the buoyant LVZ has caused pronounced topography uplift there. Thus, the evolution of the Red Sea and the Arabian shield is driven by subduction of the Arabian plate along its northeastern boundary, and the Red Sea opened on a east-dipping low-angle detachment fault.

S-Wave Velocity Models Under the Saudi Arabian Portable Broadband Deployment: Evidence for Lithospheric Erosion Beneath the Arabian Shield

NASA Astrophysics Data System (ADS)

Julià, J.; Ammon, C. J.; Herrmann, R. B.

2002-12-01

Models of crustal evolution strongly rely on our knowledge on the mineralogical composition of subsurface rocks, as well as pressure and temperature conditions. Direct sampling of subsurface rocks is often not possible, so that constraints have to be placed from indirect estimates of rock properties. Detailed seismic imaging of subsurface rocks has the potential for providing such constraints, and probe the extent at depth of surface geologic observations. In this study, we provide detailed S-wave velocity profiles for the crust and uppermost mantle beneath the Saudi Arabian Portable Broadband Deployment stations. Seismic velocities have been estimated from the joint inversion of receiver functions and fundamental mode group velocities. Receiver functions are sensitive to S-wave velocity contrasts and vertical travel times, and surface-wave dispersion is sensitive to vertical S-wave velocity averages, so that their combination bridge resolution gaps associated with each individual data set. Our resulting models correlate well with surface geology observations in the Arabian Shield and characterize its terranes at depth: the Asir terrane consists of a 10-km thick upper crust of 3.3~km/s overlying a lower crust with shear-wave velocities of 3.7-3.8 km/s; the Afif terrane is made of a 20-km thick upper crust with average velocity of 3.6 km/s and a lower crust with a shear-velocity of about 3.8~km/s; the Nabitah mobile belt has a gradational, 15-km thick upper crust up to 3.6 km/s overlying a gradational lower crust with velocities up to 4.0 km/s. The crust-mantle transition is sharper in terranes of continental affinity and more gradational beneath terranes of oceanic affinity. In the uppermost mantle, our models suggest a thin lid between up to 50-60 km depth overlying a low velocity zone beneath station TAIF, located close to a region of upwelling mantle material. Temperatures in the lid are estimated to be about 1000 C, which are in good agreement with independent xenolith data, and suggest that the lithosphere could be eroded to a thickness as little as 50~km under this station.

Magnetotelluric Imaging of Lower Crustal Melt and Lithospheric Hydration in the Rocky Mountain Front Transition Zone, Colorado, USA

NASA Astrophysics Data System (ADS)

Feucht, D. W.; Sheehan, A. F.; Bedrosian, P. A.

2017-12-01

We present an electrical resistivity model of the crust and upper mantle from two-dimensional (2-D) anisotropic inversion of magnetotelluric data collected along a 450 km transect of the Rio Grande rift, southern Rocky Mountains, and High Plains in Colorado, USA. Our model provides a window into the modern-day lithosphere beneath the Rocky Mountain Front to depths in excess of 150 km. Two key features of the 2-D resistivity model are (1) a broad zone ( 200 km wide) of enhanced electrical conductivity (<20 Ωm) in the midcrust to lower crust that is centered beneath the highest elevations of the southern Rocky Mountains and (2) hydrated lithospheric mantle beneath the Great Plains with water content in excess of 100 ppm. We interpret the high conductivity region of the lower crust as a zone of partially molten basalt and associated deep-crustal fluids that is the result of recent (less than 10 Ma) tectonic activity in the region. The recent supply of volatiles and/or heat to the base of the crust in the late Cenozoic implies that modern-day tectonic activity in the western United States extends to at least the western margin of the Great Plains. The transition from conductive to resistive upper mantle is caused by a gradient in lithospheric modification, likely including hydration of nominally anhydrous minerals, with maximum hydration occurring beneath the Rocky Mountain Front. This lithospheric "hydration front" has implications for the tectonic evolution of the continental interior and the mechanisms by which water infiltrates the lithosphere.

Tomography of the upper mantle beneath the African/Iberian collision zone

NASA Astrophysics Data System (ADS)

Bonnin, Mickael; Nolet, Guust; Thomas, Christine; Villaseñor, Antonio; Gallart, Josep; Levander, Alan

2013-04-01

In this study we take advantage of the dense broadband-station networks available in western Mediterranean region (IberArray, PICASSO and MOROCCO-MUENSTER networks) to develop a high-resolution 3D tomographic P velocity model of the upper mantle beneath the African/Iberian collision zone. This model is based on teleseismic arrival times recorded between 2008 and 2012 for which cross-correlation delays are measured with a new technique in different frequency bands centered between 0.03 and 1.0 Hz, and interpreted using multiple frequency tomography. Such a tomography is required to scrutinize the nature and extent of the thermal anomalies inferred beneath Northern Africa, especially in the Atlas ranges region and associated to sparse volcanic activities. Tomography is notably needed to help in determining the hypothetical connection between those hot anomalies and the Canary Island hotspot as proposed by geochemistry studies. It also provides new insights on the geometry of the subducting slab previously inferred from tomography, GPS measurements or shear-wave splitting patterns beneath the Alboran Sea and the Betic ranges and is indispensable for deciphering the complex geodynamic history of the Western Mediterranean region. We shall present the overall statistics of the delays, their geographical distribution, as well as the first inversion results.

Lithospheric Shear Velocity Structure of South Island, New Zealand from Rayleigh Wave Tomography of Amphibious Array Data

NASA Astrophysics Data System (ADS)

Ball, J. S.; Sheehan, A. F.; Stachnik, J. C.; Lin, F. C.; Collins, J. A.

2015-12-01

We present the first 3D shear velocity model extending well offshore of New Zealand's South Island, imaging the lithosphere beneath Campbell and Challenger plateaus. Our model is constructed via linearized inversion of both teleseismic (18 -70 s period) and ambient noise-based (8 - 25 s period) Rayleigh wave dispersion measurements. We augment an array of 29 ocean-bottom instruments deployed off the South Island's east and west coasts in 2009-2010 with 28 New Zealand land-based seismometers. The ocean-bottom seismometers and 4 of the land seismometers were part of the Marine Observations of Anisotropy Near Aotearoa (MOANA) experiment, and the remaining land seismometers are from New Zealand's permanent GeoNet array. Major features of our shear wave velocity (Vs) model include a low-velocity (Vs<4.3km/s) body extending to at least 75km depth beneath the Banks and Otago peninsulas, a high-velocity (Vs~4.7km/s) upper mantle anomaly underlying the Southern Alps to a depth of 100km, and discontinuous lithospheric velocity structure between eastern and western Challenger Plateau. Using the 4.5km/s contour as a proxy for the lithosphere-asthenosphere boundary, our model suggests that the lithospheric thickness of Challenger Plateau is substantially greater than that of Campbell Plateau. The high-velocity anomaly we resolve beneath the central South Island exhibits strong spatial correlation with subcrustal earthquake hypocenters along the Alpine Fault (Boese et al., 2013). The ~400km-long low velocity zone we image beneath eastern South Island underlies Cenozoic volcanics and mantle-derived helium observations (Hoke et al., 2000) on the surface. The NE-trending low-velocity zone dividing Challenger Plateau in our model underlies a prominent magnetic discontinuity (Sutherland et al., 1999). The latter feature has been interpreted to represent a pre-Cretaceous crustal boundary, which our results suggest may involve the entire mantle lithosphere.

Gravitational Evidence for a Low-Density Mass beneath the Galapagos Islands.

PubMed

Case, J E; Ryland, S L; Simkin, T; Howard, K A

1973-09-14

A residual negative free-air and Bouguer anomaly of at least 80 milligals, superimposed on a broader high, occurs over the Galápagos Islands The axis of the anomaly trends roughly east and plunges eastward. Thus, a low-density mass in the crust or upper mantle must underlie the archipelago. This anomaly may be caused by thermal expansion over a crust-mantle " hot spot".

Subduction beneath Laurentia modified the eastern North American cratonic edge: Evidence from P wave and S wave tomography

NASA Astrophysics Data System (ADS)

Boyce, A.; Bastow, I. D.; Darbyshire, F. A.; Ellwood, A. G.; Gilligan, A.; Levin, V.; Menke, W.

2016-07-01

The cratonic cores of the continents are remarkably stable and long-lived features. Their ability to resist destructive tectonic processes is associated with their thick (˜250 km), cold, chemically depleted, buoyant lithospheric keels that isolate the cratons from the convecting mantle. The formation mechanism and tectonic stability of cratonic keels remains under debate. To address this issue, we use P wave and S wave relative arrival-time tomography to constrain upper mantle structure beneath southeast Canada and the northeast USA, a region spanning three quarters of Earth's geological history. Our models show three distinct, broad zones: Seismic wave speeds increase systematically from the Phanerozoic coastal domains, through the Proterozoic Grenville Province, and to the Archean Superior craton in central Québec. We also recover the NW-SE trending track of the Great Meteor hot spot that crosscuts the major tectonic domains. The decrease in seismic wave speed from Archean to Proterozoic domains across the Grenville Front is consistent with predictions from models of two-stage keel formation, supporting the idea that keel growth may not have been restricted to Archean times. However, while crustal structure studies suggest that Archean Superior material underlies Grenvillian age rocks up to ˜300 km SE of the Grenville Front, our tomographic models show a near-vertical boundary in mantle wave speed directly beneath the Grenville Front. We interpret this as evidence for subduction-driven metasomatic enrichment of the Laurentian cratonic margin, prior to keel stabilization. Variable chemical depletion levels across Archean-Proterozoic boundaries worldwide may thus be better explained by metasomatic enrichment than inherently less depleted Proterozoic composition at formation.

Oman Ophiolite Structural Constraints Complement Models of Crustal Accretion at the EAST Pacific RISE

NASA Astrophysics Data System (ADS)

Nicolas, A. A.; Jousselin, D.; Boudier, F. I.

2014-12-01

This review documents significant similarities between East Pacific Rise (EPR), especially EPR at 9°-10°N and the Oman ophiolites. Both share comparable fast spreading rates, size and their dominant source of information that is mainly geophysical in EPR and structural in Oman. In these respects, they are remarkably complementary. Mantle upwelling zones at the EPR and mantle diapirs in Oman have a similar size and spacing. They punctually introduce basaltic melt and heat in the accreting crust, thus controlling elementary segments structure and activity. A tent-shaped magma chamber fits onto the diapir head, the top of which is a Mantle Transition Zone (MTZ) that stores, modifies, and injects the modified melt into the upper Axial Melt Lens (AML) beneath the lid. This MTZ-AML connection is central in crustal accretion, as documented in Oman. Heat from the diapir is captured above the Moho by the magma chamber and escapes through its walls, into a thin thermal boundary layer that bounds the chamber. Beyond, seawater at lower temperatures feeds smokers on the seafloor.

Continental lithospheric subduction and intermediate-depth seismicity: Constraints from S-wave velocity structures in the Pamir and Hindu Kush

NASA Astrophysics Data System (ADS)

Li, Wei; Chen, Yun; Yuan, Xiaohui; Schurr, Bernd; Mechie, James; Oimahmadov, Ilhomjon; Fu, Bihong

2018-01-01

The Pamir has experienced more intense deformation and shortening than Tibet, although it has a similar history of terrane accretion. Subduction as a primary way to accommodate lithospheric shortening beneath the Pamir has induced the intermediate-depth seismicity, which is rare in Tibet. Here we construct a 3D S-wave velocity model of the lithosphere beneath the Pamir by surface wave tomography using data of the TIPAGE (Tien Shan-Pamir Geodynamic program) and other seismic networks in the area. We imaged a large-scale low velocity anomaly in the crust at 20-50 km depth in the Pamir overlain by a high velocity anomaly at a depth shallower than 15 km. The high velocity anomalies colocate with exposed gneiss domes, which may imply a similar history of crustal deformation, partial melting and exhumation in the hinterland, as has occurred in the Himalaya/Tibet system. At mantle depths, where the intermediate-depth earthquakes are located, a low velocity zone is clearly observed extending to about 180 km and 150 km depth in the Hindu Kush and eastern Pamir, respectively. Moreover, the geometry of the low-velocity anomaly suggests that lower crustal material has been pulled down into the mantle by the subducting Asian and Indian lithospheric mantle beneath the Pamir and Hindu Kush, respectively. Metamorphic processes in the subducting lower crust may cause the intermediate-depth seismicity down to 150-180 km depth beneath the Pamir and Hindu Kush. We inverted focal mechanisms in the seismic zone for the stress field. Differences in the stress field between the upper and lower parts of the Indian slab imply that subduction and detachment of the Indian lithosphere might cause intense seismicity associated with the thermal shear instability in the deep Hindu Kush.

P-wave tomography of the western United States: Insight into the Yellowstone hotspot and the Juan de Fuca slab

NASA Astrophysics Data System (ADS)

Tian, You; Zhao, Dapeng

2012-06-01

We used 190,947 high-quality P-wave arrival times from 8421 local earthquakes and 1,098,022 precise travel-time residuals from 6470 teleseismic events recorded by the EarthScope/USArray transportable array to determine a detailed three-dimensional P-wave velocity model of the crust and mantle down to 1000 km depth under the western United States (US). Our tomography revealed strong heterogeneities in the crust and upper mantle under the western US. Prominent high-velocity anomalies are imaged beneath Idaho Batholith, central Colorado Plateau, Cascadian subduction zone, stable North American Craton, Transverse Ranges, and Southern Sierra Nevada. Prominent low-velocity anomalies are imaged at depths of 0-200 km beneath Snake River Plain, which may represent a small-scale convection beneath the western US. The low-velocity structure deviates variably from a narrow vertical plume conduit extending down to ˜1000 km depth, suggesting that the Yellowstone hotspot may have a lower-mantle origin. The Juan de Fuca slab is imaged as a dipping high-velocity anomaly under the western US. The slab geometry and its subducted depth vary in the north-south direction. In the southern parts the slab may have subducted down to >600 km depth. A "slab hole" is revealed beneath Oregon, which shows up as a low-velocity anomaly at depths of ˜100 to 300 km. The formation of the slab hole may be related to the Newberry magmatism. The removal of flat subducted Farallon slab may have triggered the vigorous magmatism in the Basin and Range and southern part of Rocky Mountains and also resulted in the uplift of the Colorado Plateau and Rocky Mountains.

The longevity of Archean mantle residues in the convecting upper mantle and their role in young continent formation

NASA Astrophysics Data System (ADS)

Liu, Jingao; Scott, James M.; Martin, Candace E.; Pearson, D. Graham

2015-08-01

The role played by ancient melt-depleted lithospheric mantle in preserving continental crust through time is critical in understanding how continents are built, disrupted and recycled. While it has become clear that much of the extant Archean crust is underpinned by Archean mantle roots, reports of Proterozoic melt depletion ages for peridotites erupted through Phanerozoic terranes raise the possibility that ancient buoyant lithospheric mantle acts as a "life-raft" for much of the Earth's continental crust. Here we report the largest crust-lithospheric mantle age decoupling (∼2.4 Ga) so far observed on Earth and examine the potential cause for such extreme age decoupling. The Phanerozoic (<300 Ma) continental crust of West Otago, New Zealand, is intruded by Cenozoic diatremes that have erupted cratonic mantle-like highly depleted harzburgites and dunites. These peridotites have rhenium depletion Os model ages that vary from 0.5 to 2.7 Ga, firmly establishing the record of an Archean depletion event. However, the vast range in depletion ages does not correlate with melt depletion or metasomatic tracer indices, providing little support for the presence of a significant volume of ancient mantle root beneath this region. Instead, the chemical and isotopic data are best explained by mixing of relict components of Archean depleted peridotitic mantle residues that have cycled through the asthenosphere over Ga timescales along with more fertile convecting mantle. Extensive melt depletion associated with the "docking" of these melt residues beneath the young continental crust of the Zealandia continent explains the decoupled age relationship that we observe today. Hence, the newly formed lithospheric root incorporates a mixture of ancient and modern mantle derived from the convecting mantle, cooled and accreted in recent times. We argue that in this case, the ancient components played no earlier role in continent stabilization, but their highly depleted nature along with that of their younger counterparts now represents a highly viscous, stable continental keel. This model could account for the large spectrum of ages observed in fertile to moderately depleted peridotites sampled from lithospheric mantle beneath SE Australia, W Antarctica and other locations in Zealandia, as well as the oceanic mantle. Our data confirm the longevity and dispersal of ancient depleted mantle domains in the convecting mantle and their re-appearance beneath young continents.

Upper mantle anisotropic attenuation of the Sierra Nevada and surroundings

NASA Astrophysics Data System (ADS)

Bernardino, M. J.; Jones, C. H.; Monsalve, G.

2016-12-01

We investigate the contribution of anelasticity in the generation of seismic velocity variations within the upper mantle of the Sierra Nevada and surrounding regions through teleseismic shear-wave attenuation. Given that anelastic effects are most sensitive to temperature and hydration and less to composition and small degrees of partial melt, we aim constrain the thermal structure beneath this region and identify locations where elevated upper mantle temperatures dominate. We also investigate the dependence of shear-wave attenuation on direction by accounting for seismic anisotropy in our measurements. S-wave t* values are determined from teleseismic S- and SKS- phases recorded on permanent and temporary deployments within the California region with particular focus on the Sierra Nevada Earthscope Project (SNEP) and the Sierran Paradox Experiment (SPE) stations. S-waveforms are rotated into the Sierran SFast, N75°E, and SSlow, N15°W, components. Following the method of Stachnik et al., (2004), S-wave spectra for each event are jointly inverted for a single seismic moment, M0k, and corner frequency, fck, for each event, and separate t* for each ray path. The resulting t*Fast and t*Slow measurements are then inverted for three-dimensional variations in (1/QFast) and (1/QSlow). Results are compared with previous magnetotelluric, surface heat flow, and body-wave velocity inversion studies.

Plume-driven plumbing and crustal formation in Iceland

USGS Publications Warehouse

Allen, R.M.; Nolet, G.; Morgan, W.J.; Vogfjord, K.; Nettles, M.; Ekstrom, G.; Bergsson, B.H.; Erlendsson, P.; Foulger, G.R.; Jakobsdottir, S.; Julian, B.R.; Pritchard, M.; Ragnarsson, S.; Stefansson, R.

2002-01-01

Through combination of surface wave and body wave constraints we derive a three-dimensional (3-D) crustal S velocity model and Moho map for Iceland. It reveals a vast plumbing system feeding mantle plume melt into upper crustal magma chambers where crustal formation takes place. The method is based on the partitioned waveform inversion to which we add additional observations. Love waves from six local events recorded on the HOTSPOT-SIL networks are fitted, Sn travel times from the same events measured, previous observations of crustal thickness are added, and all three sets of constraints simultaneously inverted for our 3-D model. In the upper crust (0-15 km) an elongated low-velocity region extends along the length of the Northern, Eastern and Western Neovolcanic Zones. The lowest velocities (-7%) are found at 5-10 km below the two most active volcanic complexes: Hekla and Bardarbunga-Grimsvotn. In the lower crust (>15 km) the low-velocity region can be represented as a vertical cylinder beneath central Iceland. The low-velocity structure is interpreted as the thermal halo of pipe work which connects the region of melt generation in the uppermost mantle beneath central Iceland to active volcanoes along the neovolcanic zones. Crustal thickness in Iceland varies from 15-20 km beneath the Reykjanes Peninsula, Krafla and the extinct Snfellsnes rift zone, to 46 km beneath central Iceland. The average crustal thickness is 29 km. The variations in thickness can be explained in terms of the temporal variation in plume productivity over the last ~20 Myr, the Snfellsnes rift zone being active during a minimum in plume productivity. Variations in crustal thickness do not depart significantly from an isostatically predicted crustal thickness. The best fit linear isostatic relation implies an average density jump of 4% across the Moho. Rare earth element inversions of basalt compositions on Iceland suggest a melt thickness (i.e., crustal thickness) of 15-20 km, given passive upwelling. The observed crustal thickness of up to 46 km implies active fluxing of source material through the melt zone by the mantle plume at up to 3 times the passive rate.

Crust-mantle coupling mechanism in Cameroon, West Africa, revealed by 3D S-wave velocity and azimuthal anisotropy

NASA Astrophysics Data System (ADS)

Ojo, Adebayo Oluwaseun; Ni, Sidao; Chen, Haopeng; Xie, Jun

2018-01-01

To understand the depth variation of deformation beneath Cameroon, West Africa, we developed a new 3D model of S-wave isotropic velocity and azimuthal anisotropy from joint analysis of ambient seismic noise and earthquake surface wave dispersion. We found that the Cameroon Volcanic Line (CVL) is well delineated by slow phase velocities in contrast with the neighboring Congo Craton, in agreement with previous studies. Apart from the Congo Craton and the Oubanguides Belt, the uppermost mantle revealed a relatively slow velocity indicating a thinned or thermally altered lithosphere. The direction of fast axis in the upper crust is mostly NE-SW, but trending approximately N-S around Mt. Oku and the southern CVL. The observed crustal azimuthal anisotropy is attributed to alignment of cracks and crustal deformation related to magmatic activities. A widespread zone of weak-to-zero azimuthal anisotropy in the mid-lower crust shows evidence for vertical mantle flow or isotropic mid-lower crust. In the uppermost mantle, the fast axis direction changed from NE-SW to NW-SE around Mt. Oku and northern Cameroon. This suggests a layered mechanism of deformation and revealed that the mantle lithosphere has been deformed. NE-SW fast azimuths are observed beneath the Congo Craton and are consistent with the absolute motion of the African plate, suggesting a mantle origin for the observed azimuthal anisotropy. Our tomographically derived fast directions are consistent with the local SKS splitting results in some locations and depths, enabling us to constrain the origin of the observed splitting. The different feature of azimuthal anisotropy in the upper crust and the uppermost mantle implies decoupling between deformation of crust and mantle in Cameroon.

The East African rift system in the light of KRISP 90

USGS Publications Warehouse

Keller, Gordon R.; Prodehl, C.; Mechie, J.; Fuchs, K.; Khan, M.A.; Maguire, Peter K.H.; Mooney, W.D.; Achauer, U.; Davis, P.M.; Meyer, R.P.; Braile, L.W.; Nyambok, I.O.; Thompson, G.A.

1994-01-01

On the basis of a test experiment in 1985 (KRISP 85) an integrated seismic-refraction/teleseismic survey (KRISP 90) was undertaken to study the deep structure beneath the Kenya rift down to depths of 100-150 km. This paper summarizes the highlights of KRISP 90 as reported in this volume and discusses their broad implications as well as the structure of the Kenya rift in the general framework of other continental rifts. Major scientific goals of this phase of KRISP were to reveal the detailed crustal and upper mantle structure under the Kenya rift, to study the relationship between mantle updoming and the development of sedimentary basins and other shallow structures within the rift, to understand the role of the Kenya rift within the Afro-Arabian rift system and within a global perspective and to elucidate fundamental questions such as the mode and mechanism of continental rifting. The KRISP results clearly demonstrate that the Kenya rift is associated with sharply defined lithospheric thinning and very low upper mantle velocities down to depths of over 150 km. In the south-central portion of the rift, the lithospheric mantle has been thinned much more than the crust. To the north, high-velocity layers detected in the upper mantle appear to require the presence of anistropy in the form of the alignment of olivine crystals. Major axial variations in structure were also discovered, which correlate very well with variations in the amount of extension, the physiographic width of the rift valley, the regional topography and the regional gravity anomalies. Similar relationships are particularly well documented in the Rio Grande rift. To the extent that truly comparable data sets are available, the Kenya rift shares many features with other rift zones. For example, crustal structure under the Kenya, Rio Grande and Baikal rifts and the Rhine Graben is generally symmetrically centered on the rift valleys. However, the Kenya rift is distinctive, but not unique, in terms of the amount of volcanism. This volcanic activity would suggest large-scale modification of the crust by magmatism. Although there is evidence of underplating in the form of a relatively high-velocity lower crustal layer, there are no major seismic velocity anomalies in the middle and upper crust which would suggest pervasive magmatism. This apparent lack of major modification is an enigma which requires further study. ?? 1994.

Mountain building at northeastern boundary of Tibetan Plateau and craton reworking at Ordos block from joint inversion of ambient noise tomography and receiver functions

NASA Astrophysics Data System (ADS)

Guo, Zhen; Chen, Yongshun John

2017-04-01

We have obtained a high resolution 3-D crustal and uppermost mantle velocity model of the Ordos block and its surrounding areas by joint inversion of ambient noise tomography and receiver functions using seismic recordings from 320 stations. The resulting model shows wide-spread low velocity zone (Vs ≤ 3.4 km/s) in the mid-to-lower crust beneath northeastern Tibet Plateau, which may favor crustal ductile flow within the plateau. However, our model argues against the eastward crustal ductile flow beneath the Qinling belt from the Tibetan Plateau. We find high velocities in the middle part of Qinling belt which separate the low velocities in the mid-to-lower crust of the eastern Qinling belt from the low velocity zone in eastern Tibetan Plateau. More importantly, we observe significant low velocities and thickened lower crust at the Liupanshan thrust belt as the evidence for strong crustal shortening at this boundary between the northeastern Tibetan Plateau and Ordos block. The most important finding of our model is the upper mantle low velocity anomalies surrounding the Ordos block, particularly the one beneath the Trans North China Craton (TNCO) that is penetrating into the southern margin of the Ordos block for ∼100 km horizontally in the depth range of ∼70 km and at least 100 km. We propose an on-going lithospheric mantle reworking at the southernmost boundary of the Ordos block due to complicated mantle flow surrounding the Ordos block, that is, the eastward asthenospheric flow from the Tibet Plateau proposed by recent SKS study and mantle upwelling beneath the TNCO from mantle transition zone induced by the stagnant slabs of the subducted Pacific plate.

Gravity and geoid anomalies of the Philippine Sea: Evidence on the depth of compensation for the negative residual water depth anomaly

NASA Technical Reports Server (NTRS)

Bowin, C.

1982-01-01

A negative free-air gravity anomaly which occurs in the central part of the Philippine Sea was examined to determine the distribution and nature of possible regional mass excesses or deficiencies. Geoid anomalies from GEOS-3 observation were positive. A negative residual geoid anomaly consistent with the area of negative free-air gravity anomalies were found. Theoretical gravity-topography and geoid-topography admittance functions indicated that high density mantle at about 60 km dept could account for the magnitudes of the gravity and residual geoid anomaly and the 1 km residual water depth anomaly in the Philippine Sea. The negative residual depth anomaly may be compensated for by excess density in the uppermost mantle, but the residual geoid and regional free-air gravity anomalies and a slow surface wave velocity structure might result from low-density warm upper mantle material lying beneath the zone of high-density uppermost mantle. From a horizontal disk approximation, the depth of the low-density warm mantle was estimated to be on the order of 200 km.

The initiation of segmented buoyancy-driven melting during continental breakup

PubMed Central

Gallacher, Ryan J.; Keir, Derek; Harmon, Nicholas; Stuart, Graham; Leroy, Sylvie; Hammond, James O. S.; Kendall, J-Michael; Ayele, Atalay; Goitom, Berhe; Ogubazghi, Ghebrebrhan; Ahmed, Abdulhakim

2016-01-01

Melting of the mantle during continental breakup leads to magmatic intrusion and volcanism, yet our understanding of the location and dominant mechanisms of melt generation in rifting environments is impeded by a paucity of direct observations of mantle melting. It is unclear when during the rifting process the segmented nature of magma supply typical of seafloor spreading initiates. Here, we use Rayleigh-wave tomography to construct a high-resolution absolute three-dimensional shear-wave velocity model of the upper 250 km beneath the Afar triple junction, imaging the mantle response during progressive continental breakup. Our model suggests melt production is highest and melting depths deepest early during continental breakup. Elevated melt production during continental rifting is likely due to localized thinning and melt focusing when the rift is narrow. In addition, we interpret segmented zones of melt supply beneath the rift, suggesting that buoyancy-driven active upwelling of the mantle initiates early during continental rifting. PMID:27752044

Lithospheric structure of Iberia and Morocco using finite-frequency Rayleigh wave tomography from earthquakes and seismic ambient noise

NASA Astrophysics Data System (ADS)

Palomeras, I.; Villaseñor, A.; Thurner, S.; Levander, A.; Gallart, J.; Harnafi, M.

2017-05-01

We present a new 3-D shear velocity model of the western Mediterranean from the Pyrenees, Spain, to the Atlas Mountains, Morocco, and the estimated crustal and lithospheric thickness. The velocity model shows different crustal and lithospheric velocities for the Variscan provinces, those which have been affected by Alpine deformation, and those which are actively deforming. The Iberian Massif has detectable differences in crustal thickness that can be related to the evolution of the Variscan orogen in Iberia. Areas affected by Alpine deformation have generally lower velocities in the upper and lower crust than the Iberian Massif. Beneath the Gibraltar Strait and surrounding areas, the crustal thickness is greater than 50 km, below which a high-velocity anomaly (>4.5 km/s) is mapped to depths greater than 200 km. We identify this as a subducted remnant of the NeoTethys plate referred to as the Alboran and western Mediterranean slab. Beneath the adjacent Betic and Rif Mountains, the Alboran slab is still attached to the base of the crust, depressing it, and ultimately delaminating the lower crust and mantle lithosphere as the slab sinks. Under the adjacent continents, the Alboran slab is surrounded by low upper mantle shear wave velocities (Vs < 4.3) that we interpret as asthenosphere that has replaced the continental margin lithosphere which was viscously removed by Alboran plate subduction. The southernmost part of the model features an anomalously thin lithosphere beneath the Atlas Mountains that could be related to lateral flow induced by the Alboran slab.

Evidence for fluid and melt generation in response to an asthenospheric upwelling beneath the Hangai Dome, Mongolia

NASA Astrophysics Data System (ADS)

Comeau, Matthew J.; Käufl, Johannes S.; Becken, Michael; Kuvshinov, Alexey; Grayver, Alexander V.; Kamm, Jochen; Demberel, Sodnomsambuu; Sukhbaatar, Usnikh; Batmagnai, Erdenechimeg

2018-04-01

The Hangai Dome, Mongolia, is an unusual high-elevation, intra-continental plateau characterized by dispersed, low-volume, intraplate volcanism. Its subsurface structure and its origin remains unexplained, due in part to a lack of high-resolution geophysical data. Magnetotelluric data along a ∼610 km profile crossing the Hangai Dome were used to generate electrical resistivity models of the crust and upper mantle. The crust is found to be unexpectedly heterogeneous. The upper crust is highly resistive but contains several features interpreted as ancient fluid pathways and fault zones, including the South Hangai fault system and ophiolite belt that is revealed to be a major crustal boundary. South of the Hangai Dome a clear transition in crustal properties is observed which reflects the rheological differences across accreted terranes. The lower crust contains discrete zones of low-resistivity material that indicate the presence of fluids and a weakened lower crust. The upper mantle contains a large low-resistivity zone that is consistent with the presence of partial melt within an asthenospheric upwelling, believed to be driving intraplate volcanism and supporting uplift.

Inversion of gravity and bathymetry in oceanic regions for long-wavelength variations in upper mantle temperature and composition

NASA Technical Reports Server (NTRS)

Solomon, Sean C.; Jordan, Thomas H.

1993-01-01

Long-wavelength variations in geoid height, bathymetry, and SS-S travel times are all relatable to lateral variations in the characteristic temperature and bulk composition of the upper mantle. The temperature and composition are in turn relatable to mantle convection and the degree of melt extraction from the upper mantle residuum. Thus the combined inversion of the geoid or gravity field, residual bathymetry, and seismic velocity information offers the promise of resolving fundamental aspects of the pattern of mantle dynamics. The use of differential body wave travel times as a measure of seismic velocity information, in particular, permits resolution of lateral variations at scales not resolvable by conventional global or regional-scale seismic tomography with long-period surface waves. These intermediate scale lengths, well resolved in global gravity field models, are crucial for understanding the details of any chemical or physical layering in the mantle and of the characteristics of so-called 'small-scale' convection beneath oceanic lithosphere. In 1991 a three-year project to the NASA Geophysics Program was proposed to carry out a systematic inversion of long-wavelength geoid anomalies, residual bathymetric anomalies, and differential SS-S travel time delays for the lateral variation in characteristic temperature and bulk composition of the oceanic upper mantle. The project was funded as a three-year award, beginning on 1 Jan. 1992.

Origin and evolution of the deep thermochemical structure beneath Eurasia.

PubMed

Flament, N; Williams, S; Müller, R D; Gurnis, M; Bower, D J

2017-01-18

A unique structure in the Earth's lowermost mantle, the Perm Anomaly, was recently identified beneath Eurasia. It seismologically resembles the large low-shear velocity provinces (LLSVPs) under Africa and the Pacific, but is much smaller. This challenges the current understanding of the evolution of the plate-mantle system in which plumes rise from the edges of the two LLSVPs, spatially fixed in time. New models of mantle flow over the last 230 million years reproduce the present-day structure of the lower mantle, and show a Perm-like anomaly. The anomaly formed in isolation within a closed subduction network ∼22,000 km in circumference prior to 150 million years ago before migrating ∼1,500 km westward at an average rate of 1 cm year -1 , indicating a greater mobility of deep mantle structures than previously recognized. We hypothesize that the mobile Perm Anomaly could be linked to the Emeishan volcanics, in contrast to the previously proposed Siberian Traps.

Seismic and Thermal Structure of the Arctic Lithosphere, From Waveform Tomography and Thermodynamic Modelling

NASA Astrophysics Data System (ADS)

Lebedev, S.; Schaeffer, A. J.; Fullea, J.; Pease, V.

2015-12-01

Thermal structure of the lithosphere is reflected in the values of seismic velocities within it. Our new tomographic models of the crust and upper mantle of the Arctic are constrained by an unprecedentedly large global waveform dataset and provide substantially improved resolution, compared to previous models. The new tomography reveals lateral variations in the temperature and thickness of the lithosphere and defines deep boundaries between tectonic blocks with different lithospheric properties and age. The shape and evolution of the geotherm beneath a tectonic unit depends on both crustal and mantle-lithosphere structure beneath it: the lithospheric thickness and its changes with time (these determine the supply of heat from the deep Earth), the crustal thickness and heat production (the supply of heat from within the crust), and the thickness and thermal conductivity of the sedimentary cover (the insulation). Detailed thermal structure of the basins can be modelled by combining seismic velocities from tomography with data on the crustal structure and heat production, in the framework of computational petrological modelling. The most prominent lateral contrasts across the Arctic are between the cold, thick lithospheres of the cratons (in North America, Greenland and Eurasia) and the warmer, non-cratonic blocks. The lithosphere of the Canada Basin is cold and thick, similar to old oceanic lithosphere elsewhere around the world; its thermal structure offers evidence on its lithospheric age and formation mechanism. At 150-250 km depth, the central Arctic region shows a moderate low-velocity anomaly, cooler than that beneath Iceland and N Atlantic. An extension of N Atlantic low-velocity anomaly into the Arctic through the Fram Strait may indicate an influx of N Atlantic asthenosphere under the currently opening Eurasia Basin.

Stratification of Seismic Anisotropy Beneath Hudson Bay

NASA Astrophysics Data System (ADS)

Darbyshire, F. A.; Eaton, D. W.; Bastow, I. D.

2012-12-01

The Hudson Bay region has a complex tectonic history spanning ~4 Ga of Earth's evolution. During the ~1.8 Ga Trans-Hudson orogeny, the Archean Superior and Western Churchill cratons collided following the subduction of a Pacific-scale ocean. It is thought that a significant amount of juvenile material is preserved in the Trans-Hudson Orogen, in part due to the complex double-indentor geometry of the Superior-Churchill collision. In the region of interest, the orogen lies beneath a large but shallow Paleozoic intra-cratonic basin. Studies of the crust and upper mantle beneath this region have been enabled through the HuBLE (Hudson Bay Lithospheric Experiment) project, through the deployment of broadband seismographs around the Bay and across the islands to the north. A surface-wave tomography study has taken advantage of the data coverage, providing new information on phase velocity heterogeneity and anisotropy for wave periods of 25-200 seconds (equivalent to depths from the lower crust to ~300 km). On a large scale, our results show that the entire region is underlain by a seismically fast lithospheric lid corresponding to the continental keel. The lithospheric thickness ranges from ~180km in the northeast, beneath a zone of Paleozoic rifting, to ~280km beneath central Hudson Bay. Within the lithosphere, seismic velocities vary laterally, including high-velocity material wrapping around the Bay in the uppermost mantle. In the mid-lithosphere, two high-velocity cores are imaged, with a zone of lower velocity between them beneath the Bay. We interpret these high-velocity structures to represent the strongest central cores of the Superior and Churchill cratons, with more-juvenile material preserved between them. The near-vertical geometry of the lower-velocity zone suggests that it is only the effects of terminal collision of the cratonic cores, rather than any preceding subduction, that is preserved today. The lowermost lithosphere has a more uniform velocity, and may represent a pervasive zone of metasomatism or underplating. Anisotropy patterns across the region also vary with depth, suggesting ~3 layers of stratification of lithospheric fabric. At the shallowest depths, anisotropic fast directions wrap around the Bay in a similar fashion to the patterns of isotropic wavespeed. The upper lithospheric mantle below is characterized by relatively weak and incoherent anisotropy; however the mid-to-lower lithosphere shows stronger anisotropy, with a pattern of fast directions broadly consistent with the tectonics of the Superior-Churchill collision as inferred from potential-field data. This may suggest some degree of coherency of deformation between the crust, uppermost mantle and lower lithosphere. These models of seismic wavespeed variation beneath the Hudson Bay region reveal the preservation of a major collision zone during the assembly of the Laurentian continental mass, and also suggest that the Archean cratons can be subdivided into different lithospheric domains that reflect their tectonic history but do not necessarily correspond to surface geological boundaries.

Lateral Variations of the Mantle Transition Zone Structure beneath the Southeastern Tibetan Plateau Revealed by P-wave Receiver Functions

NASA Astrophysics Data System (ADS)

Bai, Y.; Ai, Y.; Jiang, M.; He, Y.; Chen, Q.

2017-12-01

The deep structure of the southeastern Tibetan plateau is of great scientific importance to a better understanding of the India-Eurasia collision as well as the evolution of the magnificent Tibetan plateau. In this study, we collected 566 permanent and temporary seismic stations deployed in SE Tibet, with a total of 77853 high quality P-wave receiver functions been extracted by maximum entropy deconvolution method. On the basis of the Common Conversion Point (CCP) stacking technique, we mapped the topography of the 410km and 660km discontinuities (hereinafter called the `410' and the `660'), and further investigated the lateral variation of the mantle transition zone (MTZ) thickness beneath this region. The background velocity model deduced from H-κ stacking results and a previous body-wave tomographic research was applied for the correction of the crustal and upper mantle heterogeneities beneath SE Tibet for CCP stacking. Our results reveal two significantly thickened MTZ anomalies aligned nearly in the south-north direction. The magnitude of both anomalies are 30km above the global average of 250km. The southern anomaly located beneath the Dianzhong sub-block and the Indo-China block is characterized by a slightly deeper `410' and a greater-than-normal `660', while the northern anomaly beneath western Sichuan has an uplifted `410' and a depressed `660'. Combining with previous studies in the adjacent region, we suggest that slab break-off may occurred during the eastward subduction of the Burma plate, with the lower part of the cold slab penetrated into the MTZ and stagnated at the bottom of the `660' which may cause the southern anomaly in our receiver function images. The origin of the Tengchong volcano is probably connected to the upwelling of the asthenospheric material caused by the slab break-off or to the ascending of the hot and wet material triggered by the dehydration of stagnant slab in the MTZ. The anomaly in the north, on the other hand, might be the consequence of the delamination of the overlying lithosphere sinking into the MTZ. This work is supported by the National Natural Science Foundation of China (grants 41474040, 41125015 and 41274002).

The changing face of the lithosphere-asthenosphere boundary: Imaging continental-scale patterns in upper mantle structure across the contiguous U.S. with Sp converted waves

NASA Astrophysics Data System (ADS)

Hopper, E.; Fischer, K. M.

2017-12-01

The contiguous U.S.A. is a rich tapestry of tectonism spanning over two billion years. On the broadest scale, this complex history can be simplified to three regimes: the tectonically active western U.S., the largely quiescent Archean and Proterozoic cratons of the central U.S., and the Phanerozoic orogen and rifted margin of the eastern U.S. The transitions between these regions can be clearly observed with Sp converted wave images of the uppermost mantle. We use common conversion point stacked Sp waves recorded by EarthScope's Transportable Array and other permanent and temporary broadband stations to image the transition from a strong, sharp velocity decrease in the shallow upper mantle of the western U.S. (the lithosphere-asthenosphere boundary, or LAB) to deeper, more diffuse features moving east that largely lie within the lithosphere. Only sparse, localized, weak phases are seen at LAB depths beneath the cratonic interior. This transition is clearly revealed by cluster analysis, which also shows the eastern U.S. as more similar to the western U.S. than the ancient interior, particularly beneath New England. In the western U.S., the observed strong LAB indicates a large enough velocity decrease to imply that melt has ponded beneath the lithosphere. We compare western U.S. LAB properties to the age distribution of most recent volcanism from NavDat. While LAB properties vary widely within a given age range, their distributions indicate a relationship between age of surface volcanism and LAB phase strength and breadth. LAB depth does not appear to have a clear correlation. In general, the LAB is strongest and broadest beneath zones that have been magmatically active in the last 50 Myr, suggesting an observable fraction of melt that is distributed over a depth range of 10's of kilometers, perhaps due to variations in the degree of thermochemical erosion of the lithosphere even on very local scales. The LAB is strongest and broadest for magmatic ages of 5-10 Ma, but beneath the youngest volcanism (<5 Ma), the LAB is seen as significantly weaker, suggesting more complete destruction of the high velocity lid. The timescale of these changes in LAB character suggests the presence and possibly production of melt in the asthenosphere for many 10's of Myr after surface volcanism ceases.

Array-Based Receiver Function Analysis of the Subducting Juan de Fuca Plate Beneath the Mount St. Helens Region and its Implications for Subduction Geometry and Metamorphism

NASA Astrophysics Data System (ADS)

Mann, M. E.; Abers, G. A.; Creager, K. C.; Ulberg, C. W.; Crosbie, K.

2017-12-01

Mount St. Helens (MSH) is unusual as a prolific arc volcano located 50 km towards the forearc of the main Cascade arc. The iMUSH (imaging Magma Under mount St. Helens) broadband deployment featured 70 seismometers at 10-km spacing in a 50-km radius around MSH, spanning a sufficient width for testing along-strike variation in subsurface geometry as well as deep controls on volcanism in the Cascade arc. Previous estimates of the geometry of the subducting Juan de Fuca (JdF) slab are extrapolated to MSH from several hundred km to the north and south. We analyze both P-to-S receiver functions and 2-D Born migrations of the full data set to locate the upper plate Moho and the dip and depth of the subducting slab. The strongest coherent phase off the subducting slab is the primary reverberation (Ppxs; topside P-to-S reflection) from the Moho of the subducting JdF plate, as indicated by its polarity and spatial pattern. Migration images show a dipping low velocity layer at depths less than 50 km that we interpret as the subducting JdF crust. Its disappearance beyond 50 km depth may indicate dehydration of subducting crust or disruption of high fluid pressures along the megathrust. The lower boundary of the low velocity zone, the JdF Moho, persists in the migration image to depths of at least 90 km and is imaged at 74 km beneath MSH, dipping 23 degrees. The slab surface is 68 km beneath MSH and 85 km beneath Mount Adams volcano to the east. The JdF Moho exhibits 10% velocity contrasts as deep as 85 km, an observation difficult to reconcile with simple models of crustal eclogitization. The geometry and thickness of the JdF crust and upper plate Moho is consistent with similar transects of Cascadia and does not vary along strike beneath iMUSH, indicating a continuous slab with no major disruption. The upper plate Moho is clear on the east side of the array but it disappears west of MSH, a feature we interpret as a result of both serpentinization of the mantle wedge and a westward increase in wavespeed of the continental crust. The seismically-imaged surface of the subducting JdF slab at 68 km beneath MSH is the shallowest yet documented beneath an arc volcano. Combined with the inference of serpentinization in the mantle wedge, this geometry presents a problem in that vertical mantle melt migration seems unfeasible, yet mantle melts contribute to erupted MSH magmas.

Large-scale trench-normal mantle flow beneath central South America

NASA Astrophysics Data System (ADS)

Reiss, M. C.; Rümpker, G.; Wölbern, I.

2018-01-01

We investigate the anisotropic properties of the fore-arc region of the central Andean margin between 17-25°S by analyzing shear-wave splitting from teleseismic and local earthquakes from the Nazca slab. With partly over ten years of recording time, the data set is uniquely suited to address the long-standing debate about the mantle flow field at the South American margin and in particular whether the flow field beneath the slab is parallel or perpendicular to the trench. Our measurements suggest two anisotropic layers located within the crust and mantle beneath the stations, respectively. The teleseismic measurements show a moderate change of fast polarizations from North to South along the trench ranging from parallel to subparallel to the absolute plate motion and, are oriented mostly perpendicular to the trench. Shear-wave splitting measurements from local earthquakes show fast polarizations roughly aligned trench-parallel but exhibit short-scale variations which are indicative of a relatively shallow origin. Comparisons between fast polarization directions from local earthquakes and the strike of the local fault systems yield a good agreement. To infer the parameters of the lower anisotropic layer we employ an inversion of the teleseismic waveforms based on two-layer models, where the anisotropy of the upper (crustal) layer is constrained by the results from the local splitting. The waveform inversion yields a mantle layer that is best characterized by a fast axis parallel to the absolute plate motion which is more-or-less perpendicular to the trench. This orientation is likely caused by a combination of the fossil crystallographic preferred orientation of olivine within the slab and entrained mantle flow beneath the slab. The anisotropy within the crust of the overriding continental plate is explained by the shape-preferred orientation of micro-cracks in relation to local fault zones which are oriented parallel to the overall strike of the Andean range. Our results do not provide any evidence for a significant contribution of trench-parallel mantle flow beneath the subducting slab.

Episodic kinematics in continental rifts modulated by changes in mantle melt fraction.

PubMed

Lamb, Simon; Moore, James D P; Smith, Euan; Stern, Tim

2017-07-05

Oceanic crust is created by the extraction of molten rock from underlying mantle at the seafloor 'spreading centres' found between diverging tectonic plates. Modelling studies have suggested that mantle melting can occur through decompression as the mantle flows upwards beneath spreading centres, but direct observation of this process is difficult beneath the oceans. Continental rifts, however-which are also associated with mantle melt production-are amenable to detailed measurements of their short-term kinematics using geodetic techniques. Here we show that such data can provide evidence for an upwelling mantle flow, as well as information on the dimensions and timescale of mantle melting. For North Island, New Zealand, around ten years of campaign and continuous GPS measurements in the continental rift system known as the Taupo volcanic zone reveal that it is extending at a rate of 6-15 millimetres per year. However, a roughly 70-kilometre-long segment of the rift axis is associated with strong horizontal contraction and rapid subsidence, and is flanked by regions of extension and uplift. These features fit a simple model that involves flexure of an elastic upper crust, which is pulled downwards or pushed upwards along the rift axis by a driving force located at a depth greater than 15 kilometres. We propose that flexure is caused by melt-induced episodic changes in the vertical flow forces that are generated by upwelling mantle beneath the rift axis, triggering a transient lower-crustal flow. A drop in the melt fraction owing to melt extraction raises the mantle flow viscosity and drives subsidence, whereas melt accumulation reduces viscosity and allows uplift-processes that are also likely to occur in oceanic spreading centres.

Elastic and anelastic structure of the lowermost mantle beneath the Western Pacific using waveform inversion

NASA Astrophysics Data System (ADS)

Konishi, K.; Deschamps, F.; Fuji, N.

2015-12-01

We investigate quasi-2D elastic and anelastic structure of the lowermost mantle beneath the Western Pacific by inverting S and ScS waveforms. The transverse component data were obtained from F-net for 32 deep sources beneath Tonga and Fiji, filtered between 12.5 and 200 s. We observe a regional variation of S and ScS arrival times and amplitude ratio, according to which we divide our region of interest into four sub-regions and perform 1D waveform inversion for S-wave velocity and Qμ value simultaneously. We find S-shaped structure of S-wave velocity beneath the whole region with sub-regional variation of S-wave velocity peak depths, which can explain regional difference in travel times. Qμ structure varies with sub-regions as well, but the physical interpretation has not yet done.

Thermal Evolution of Earth's Mantle During the Accretion

NASA Astrophysics Data System (ADS)

Arkani-Hamed, J.; Roberts, J. H.

2017-12-01

Earth is likely formed by accreting Moon to Mars size embryos. The impact heating by an embryo melts the embryo and the upper mantle of the Earth beneath the impact site. The iron core of the embryo sinks and merges with the core of the Earth, while the mantle of the embryo mixes with the upper mantle of the Earth, producing a buoyant molten/partially molten magma pond. Strong but localized mantle dynamics results in fast lithostatic adjustment that pours out a huge amount of molten and partially molten magma which spread on the Earth, and together with impact ejecta creates a globe encircling magma ocean. The lithostatic adjustment diminishes as the magma ocean becomes globe encircling within 104 to 105 yr. The major part of the thermal evolution of Earth's mantle after an impact takes place in the presence of a thick and hot magma ocean, which hampers heat loss from the mantle and suppresses global mantle dynamics. Because the impact velocity of an embryo increases as the Earth grows, a given magma ocean is hotter than the previous ones. We investigated this scenario using 25 Moon to Mars size embryos. Due to random geographic impact sites we considered vertical impacts since no information is available about the impact angles. This may over estimate the impact heating by a factor of 1.4 with respect to the most probable impact angle of 45o. The thermal structure of the Earth at the end of accretion is layered, aside from the localized magma ponds that are distributed randomly due to the random geographic impact sites. We also take into account the impact heating of the solid lower mantle, the heating of the lower mantle by the gravitational energy released through sinking of an embryo's core. We then follow the thermal evolution of the mantle of a growing Earth using a 3D convection model. The Earth grows due to merging of the impactor iron core with the Earth's core, and the accumulating magma ocean on the surface. The growth enhances the lithostatic pressure in the Earth that in turn increase the temperature by compression. Each overlying magma ocean hampers global convection beneath, and the mean temperature gradient at the end of accretion is less steep than the adiabatic gradient, indicating that mantle convection during accretion is mainly localized [JHR1]Is this range because there are multiple models with different numbers of embryos?yes

Shallow Mantle Anisotropy Beneath the Juan de Fuca Plate

NASA Astrophysics Data System (ADS)

VanderBeek, Brandon P.; Toomey, Douglas R.

2017-11-01

The anisotropic fabric of the oceanic mantle lithosphere is often assumed to parallel paleo-relative plate motion (RPM). However, we find evidence that this assumption is invalid beneath the Juan de Fuca (JdF) plate. Using travel times of seismic energy propagating through the topmost mantle, we find that the fast direction of P wave propagation is rotated 18° ± 3° counterclockwise to the paleo-spreading direction and strikes between Pacific-JdF relative and JdF absolute plate motion (APM). The mean mantle velocity is 7.85 ± 0.02 km/s with 4.6% ± 0.4% anisotropy. Synthesis of the plate-averaged Pn anisotropy signal with measurements of Pn anisotropy beneath the JdF Ridge and SKS splits across the JdF plate suggests that the anisotropic structure of the topmost mantle continues to evolve away from the spreading center to more closely align with APM. We infer that the oceanic mantle lithosphere may record the influence of both paleo-RPM and paleo-APM.

Crustal and upper mantle velocity structure of the Hoggar swell (Central Sahara, Algeria)

NASA Astrophysics Data System (ADS)

Ayadi, A.; Dorbath, C.; Lesquer, A.; Bezzeghoud, M.

2000-02-01

The Hoggar region is known as one of the most important swells in the African continent. Its altitude culminates at 2908 m in the Tahat hill (Atakor). The Hoggar and other massifs of central Africa (Aı̈r, Eghei, Tibesti, Darfur, Cameroon mount, …) form a system of domal uplifts with similar scale, morphology and volcanic activity. The knowledge of the structure beneath the Hoggar swell will help us to understand the origin of continental swells. In order to get an image of the lithosphere in this region, we have performed a teleseismic field experiment. The 33 short-period seismic stations have been maintained for 2 1/2 month along a 700-km long NNW-SSW profile. This experiment crossed the Central Hoggar and extended northward into the In-Salah Sahara basin which is characterized by high heat flow values of deep origin. The high quality of the data recorded during this experiment allows us to perform a velocity inversion. The Hoggar appears to be characterized by lower mantle velocities. The anomalous zone extends from the upper lithosphere to the mantle. The weak velocity contrast is interpreted in agreement with gravity, geothermal and petrological data as due to extensive mantle modifications inherited from Cenozoic volcanic activity. It confirms that the Hoggar swell is not due to a large-scale uplift of hot asthenospheric materials but corresponds to a now cooled-off modified mantle. On the contrary, local low-velocity zones associated with the Atakor and Tahalra volcanic districts show that hot materials still exist at depths in relation with recent basaltic volcanism.

Joint inversion of seismic and gravity data for imaging seismic velocity structure of the crust and upper mantle beneath Utah, United States

NASA Astrophysics Data System (ADS)

Syracuse, E. M.; Zhang, H.; Maceira, M.

2017-10-01

We present a method for using any combination of body wave arrival time measurements, surface wave dispersion observations, and gravity data to simultaneously invert for three-dimensional P- and S-wave velocity models. The simultaneous use of disparate data types takes advantage of the differing sensitivities of each data type, resulting in a comprehensive and higher resolution three-dimensional geophysical model. In a case study for Utah, we combine body wave first arrivals mainly from the USArray Transportable Array, Rayleigh wave group and phase velocity dispersion data, and Bouguer gravity anomalies to invert for crustal and upper mantle structure of the region. Results show clear delineations, visible in both P- and S-wave velocities, between the three main tectonic provinces in the region. Without the inclusion of the surface wave and gravity constraints, these delineations are less clear, particularly for S-wave velocities. Indeed, checkerboard tests confirm that the inclusion of the additional datasets dramatically improves S-wave velocity recovery, with more subtle improvements to P-wave velocity recovery, demonstrating the strength of the method in successfully recovering seismic velocity structure from multiple types of constraints.

Tomography-based mantle flow beneath Mongolia-Baikal area

NASA Astrophysics Data System (ADS)

Zhu, Tao

2014-12-01

Recent progress in seismic tomography of Asia allows us to explore and understand more clearly the mantle flow below the Mongolia-Baikal area. We present a tomography-based model of mantle convection that provides a good match to the residual topography. The model provides predictions on the present-day mantle flow and flow-induced asthenospheric deformation which give us new insights on the mantle dynamics in the Mongolia-Baikal area. The predicted mantle flow takes on a very similar pattern at the depths shallower or deeper than 400 km and almost opposite flow directions between the upper (shallower than 400 km) and lower (deeper than 400 km) parts. The flow pattern could be divided into the 'simple' eastern region and the 'complex' western region in the Mongolia. The upwelling originating from about 350 km depth beneath Baikal rift zone is an important possible drive force to the rifting. The seismic anisotropy cannot be simply related with asthenospheric flow and flow-induced deformation in the entire Mongolia-Baikal area, but they could be considered as an important contributor to the seismic anisotropy in the eastern region of Mongolia and around and in Sayan-Baikal orogenic belt.

Thermo-compositional and strength variability of the Australian plate: clues of intraplate deformation

NASA Astrophysics Data System (ADS)

Tesauro, M.; Kaban, M. K.; Aitken, A.

2017-12-01

The Australian plate has a long and complex tectonic history and its crust and upper mantle have been deeply investigated in the last two decades using a variety of geophysical methods. To discern temperature and compositional variations of the Australian upper mantle, we apply an iterative technique, which jointly interprets seismic tomography and gravity data. This technique consists in removing the effect of the crust from the observed gravity field and topography. In the second step, the residual mantle gravity field and residual topography are inverted to obtain a 3-D density model of the upper mantle. The inversion technique accounts for the notion that these fields are controlled by the same factors but in a different way (e.g., depending on depth and horizontal dimension of the heterogeneity.) This enables us to locate the position of principal density anomalies in the upper mantle. Afterwards, the thermal contribution to the density structure is estimated by inverting the seismic tomography model AusREM (http://rses.anu.edu.au/seismology/AuSREM/index.php). In this way, we improve the initial thermal and compositional models iteratively. The final thermal model compared to the initial one shows temperatures higher by 100-150 °C in the Archean and Proterozoic upper mantle. Furthermore, we observe larger iron depletion in the Western Australian craton than in the Proterozoic terranes. At the depths larger than 150 km, the depletion becomes negligible beneath the Proterozoic regions, while persists in the Western Australian craton also below the depth of the lithosphere. We interpret this feature as a result of the leakage of the depleted mantle, possibly caused by the erosion of the thermal boundary layer, which was thicker before than in present-days. Using the final thermo-compositional model, we estimated the strength and effective elastic distribution within the Australian lithosphere. For this purpose, we assumed a stiff rheology, on account of the mafic composition of the Australian crust. The results show large variability of the rigidity of the plate within the cratonic areas, reflecting the long tectonic history of the Australian plate. On the other hand, the younger eastern terranes are uniformly weak, due to the higher temperatures.

Continental margin subsidence from shallow mantle convection: Example from West Africa

NASA Astrophysics Data System (ADS)

Lodhia, Bhavik Harish; Roberts, Gareth G.; Fraser, Alastair J.; Fishwick, Stewart; Goes, Saskia; Jarvis, Jerry

2018-01-01

Spatial and temporal evolution of the uppermost convecting mantle plays an important role in determining histories of magmatism, uplift, subsidence, erosion and deposition of sedimentary rock. Tomographic studies and mantle flow models suggest that changes in lithospheric thickness can focus convection and destabilize plates. Geologic observations that constrain the processes responsible for onset and evolution of shallow mantle convection are sparse. We integrate seismic, well, gravity, magmatic and tomographic information to determine the history of Neogene-Recent (<23 Ma) upper mantle convection from the Cape Verde swell to West Africa. Residual ocean-age depths of +2 km and oceanic heat flow anomalies of +16 ± 4 mW m-2 are centered on Cape Verde. Residual depths decrease eastward to zero at the fringe of the Mauritania basin. Backstripped wells and mapped seismic data show that 0.4-0.8 km of water-loaded subsidence occurred in a ∼500 × 500 km region centered on the Mauritania basin during the last 23 Ma. Conversion of shear wave velocities into temperature and simple isostatic calculations indicate that asthenospheric temperatures determine bathymetry from Cape Verde to West Africa. Calculated average excess temperatures beneath Cape Verde are > + 100 °C providing ∼103 m of support. Beneath the Mauritania basin average excess temperatures are < - 100 °C drawing down the lithosphere by ∼102 to 103 m. Up- and downwelling mantle has generated a bathymetric gradient of ∼1/300 at a wavelength of ∼103 km during the last ∼23 Ma. Our results suggest that asthenospheric flow away from upwelling mantle can generate downwelling beneath continental margins.

Electrical conductivity imaging in the western Pacific subduction zone

NASA Astrophysics Data System (ADS)

Utada, Hisashi; Baba, Kiyoshi; Shimizu, Hisayoshi

2010-05-01

Oceanic plate subduction is an important process for the dynamics and evolution of the Earth's interior, as it is regarded as a typical downward flow of the mantle convection that transports materials from the near surface to the deep mantle. Recent seismological study showed evidence suggesting the transportation of a certain amount of water by subduction of old oceanic plate such as the Pacific plate down to 150-200 km depth into the back arc mantle. However it is not well clarified how deep into the mantle the water can be transported. The electromagnetic induction method to image electrical conductivity distribution is a possible tool to answer this question as it is known to be sensitive to the presence of water. Here we show recent result of observational study from the western Pacific subduction zone to examine the electrical conductivity distribution in the upper mantle and in the mantle transition zone (MTZ), which will provide implications how water distributes in the mantle. We take two kinds of approach for imaging the mantle conductivity, (a) semi-global and (b) regional induction approaches. Result may be summarized as follows: (a) Long (5-30 years) time series records from 8 submarine cables and 13 geomagnetic observatories in the north Pacific region were analyzed and long period magnetotelluric (MT) and geomagnetic deep sounding (GDS) responses were estimated in the period range from 1.7 to 35 days. These frequency dependent response functions were inverted to 3-dimensional conductivity distribution in the depth range between 350 and 850 km. Three major features are suggested in the MTZ depth such as, (1) a high conductivity anomaly beneath the Philippine Sea, (2) a high conductivity anomaly beneath the Hawaiian Islands, and (3) a low conductivity anomaly beneath and in the vicinity of northern Japan. (b) A three-year long deployment of ocean bottom electro-magnetometers (OBEM's) was conducted in the Philippine Sea and west Pacific Ocean from 2005 to 2008. As a preliminary investigation, MT response functions from 20 sites in the Philippine Sea and 4 sites in the west Pacific basin in the period range between 300 and 80000 sec were respectively inverted to one-dimensional (1-D) profile of electrical conductivity by quantitatively considering the effect of the heterogeneous conductivity distribution (ocean and lands) at the surface. The resultant 1-D models show three main features: (1) Strong contrast in the conductivity for the shallower 200 km of the upper mantle depths is recognized between the two regions, which is qualitatively consistent with the difference in lithospheric age. (2) The conductivity at 200-300 km depth is more or less similar to each other at about 0.3 S /m. (3) The conductivity around the MTZ depth is higher for the Philippine Sea mantle than for the Pacific mantle, which is consistent with the implication obtained from a semi-global approach (a). As already suggested in our previous work, the high conductivity in the MTZ below the Philippine Sea can be explained by the excess conduction due to the presence of hydrogen (water) in wadesleyite or in ringwoodite. Therefore, it implies a large scale circulation of water in the back arc mantle not only in the upper mantle but also down to the MTZ depth. However, our interpretation indicates that the high conductivity of the Philippine Sea uppermost upper mantle cannot be explained only by the effect of hydrogen conduction in olivine, but that additional conduction enhancement such as the presence of partial melt is required.

Deformation in D″ Beneath North America From Anisotropy

NASA Astrophysics Data System (ADS)

Nowacki, A. J.; Wookey, J.; Kendall, J. M.

2009-12-01

The lowermost few hundred kilometres of the Earth's mantle—known as D″—form the boundary between it and the core below, control the Earth's convective system, and are the site of probable large thermochemical heterogeneity. Seismic observations of D″ show a strong heterogeneity in seismic wave velocity and significant seismic anisotropy (the variation of wave speed with direction) are present in many parts of the region. On the basis of continuous regions of fast shear velocity (VS) anomalies in global models, it is also proposed as the resting place of subducted slabs, notably the Farallon beneath North America. A phase change of MgSiO3-perovskite (pv) to a post-perovskite (ppv) structure at near-core-mantle boundary (CMB) conditions is a compelling mechanism to explain the seismic features of D″. An outstanding question is how this and other mineral phases may deform to produce anisotropy, with different mechanisms possible. With knowledge either of mantle flow or which slip system is responsible for causing deformation, we can potentially determine the other with observations of the resulting seismic anisotropy. We investigate the dynamics at the CMB beneath North America using differential shear wave splitting in S and ScS phases from earthquakes of magnitude MW>5.5 in South and Central America, Hawaii the Mid-Atlantic Ridge and East Pacific Rise. They are detected on ~500 stations in North America, giving ~700 measurements of anisotropy in D″. We achieve this by correcting for anisotropy in the upper mantle (UM) beneath both the source and receiver. The measurements cover three regions beneath western USA, the Yucatan peninsula and Florida. In each case, two different, crossing ray paths are used, so that the style of anisotropy can be constrained—a single azimuth cannot distinguish differing cases. Our results showing ~1% anisotropy dependent on azimuth are not consistent with transverse isotropy with a vertical symmetry axis (VTI) anywhere. The same but with a tilted axis is possible (TTI) and would be consistent with inclusions of seismically-distinct material such as melt. TTI planes of isotropy dip south beneath Florida, southwest beneath western USA and southeast beneath Yucatan. However we test other slip systems in MgO, pv and ppv to determine if deformation in these phases can account for the observed anisotropy. The systems [100](010) and [1̅10](110) in ppv are consistent everywhere; pv is not beneath Yucatan. If we assume a general downwelling and displacement of mantle material in the seismically fast D″, corresponding to the impingement of slab material, slip along [100](010) seems more likely, with the possibility that slip along (110) as a transformation texture also occurs in the seismically fastest regions (Walte et al, GRL, 2009). With a new breed of detailed mantle deformation models, or experimental evidence of which system dominates, seismic anisotropy may be used to map deformation in D″ and provide greater insight into Earth's convecting interior.

Deep Structure of Northern Apennines Subduction Orogen (Italy) as Revealed by a Joint Interpretation of Passive and Active Seismic Data

NASA Astrophysics Data System (ADS)

Piana Agostinetti, Nicola; Faccenna, Claudio

2018-05-01

The Apennines is a well-studied orogeny formed by the accretion of continental slivers during the subduction of the Adriatic plate, but its deep structure is still a topic of controversy. Here we illuminated the deep structure of the Northern Apennines belt by combining results from the analysis of active seismic (CROP03) and receiver function data. The result from combining these two approaches provides a new robust view of the structure of the deep crust/upper mantle, from the back-arc region to the Adriatic subduction zone. Our analysis confirms the shallow Moho depth beneath the back-arc region and defines the top of the downgoing plate, showing that the two plates separate at depth about 40 km closer to the trench than reported in previous reconstructions. This spatial relationship has profound implications for the geometry of the shallow subduction zone and of the mantle wedge, by the amount of crustal material consumed at trench.

Gravity modeling of the Muertos Trough and tectonic implications (north-eastern Caribbean)

USGS Publications Warehouse

Granja, Bruna J.L.; Muñoz-Martín, A.; ten Brink, Uri S.; Carbó-Gorosabel, Andrés; Llanes, Estrada P.; Martín-Dávila, J.; Cordoba-Barba, D.; Catalan, Morollon M.

2010-01-01

The Muertos Trough in the northeast Caribbean has been interpreted as a subduction zone from seismicity, leading to infer a possible reversal subduction polarity. However, the distribution of the seismicity is very diffuse and makes definition of the plate geometry difficult. In addition, the compressive deformational features observed in the upper crust and sandbox kinematic modeling do not necessarily suggest a subduction process. We tested the hypothesized subduction of the Caribbean plate's interior beneath the eastern Greater Antilles island arc using gravity modeling. Gravity models simulating a subduction process yield a regional mass deficit beneath the island arc independently of the geometry and depth of the subducted slab used in the models. This mass deficit results from sinking of the less dense Caribbean slab beneath the lithospheric mantle replacing denser mantle materials and suggests that there is not a subducted Caribbean plateau beneath the island arc. The geologically more realistic gravity model which would explain the N-S shortening observed in the upper crust requires an overthrusted Caribbean slab extending at least 60 km northward from the deformation front, a progressive increase in the thrusting angle from 8?? to 30?? reaching a maximum depth of 22 km beneath the insular slope. This new tectonic model for the Muertos Margin, defined as a retroarc thrusting, will help to assess the seismic and tsunami hazard in the region. The use of gravity modeling has provided targets for future wide-angle seismic surveys in the Muertos Margin. ?? 2010 Springer Science+Business Media B.V.

Robustness of Global Radial Anisotropy Models of the Upper Mantle

NASA Astrophysics Data System (ADS)

Xing, Z.; Beghein, C.; Yuan, K.

2014-12-01

Radial anisotropy provides important constraints on mantle deformation. While its presence is well accepted in the uppermost mantle, large discrepancies remain among existing models, even at depths well sampled by seismic data, and its presence at greater depths is highly uncertain. Surface wave phase velocity dispersion measurements are routinely used to constrain lateral variations in mantle S-wave velocity (dlnVS) and radial anisotropy (ξ=VSH2/VSV2). Here, we employed the fundamental and higher mode surface wave phase velocity maps of Visser et al. (2008) that have unprecedented sensitivity to structure down to 800-1000km depth, and we adopted a probabilistic forward modeling approach, the Neighbourhood Algorithm, to quantify posterior model uncertainties and parameter trade-offs. We investigated the effect of prior crustal corrections on 3-D ξ and dlnVS models. To avoid mapping crustal structure onto mantle heterogeneities, it is indeed important to accurately account for 3-D crustal anomalies and variations in Moho depth. One approach is to solve the non-linear problem and simultaneously constrain Moho depth and mantle anomalies (Visser et al., 2008). Another approach, taken here, is to calculate non-linear crustal corrections with an a priori crustal model, which are then applied to the phase velocity maps before inverting the remaining signal for mantle structure. In this work, we also determined laterally varying sensitivity kernels to account for lateral changes in the crust. We compare models obtained using CRUST2.0 (Bassin et al., 2000) and the new CRUST1.0 (Laske et al., 2012) models, which mostly differ under continents. Our preliminary results show strong differences (ΔdlnVS>2%) between the two models in continental dlnVS for the upper 150-200km, and strong changes in x amplitudes in the top 200km (Δξ>2%). Some of the differences in ξ persist down to the transition zone, in particular beneath central Asia and South America. Despite these discrepancies, inferences on the depth of continental roots (~200-250km) based on either the extent of the dlnVS>0 anomalies or the depth at which ξ changes sign remain independent of the crustal model employed. We also note that VSV>VSH dominates the deep upper mantle except in central Pacific, which is characterized by VSH>VSV down to the transition zone.

Seismic Anisotropy in Mantle Transition Zone: Constraints from Observations and Synthetic Modeling of SS Precursors

NASA Astrophysics Data System (ADS)

Huang, Q.; Schmerr, N. C.; Waszek, L.; Beghein, C.; Weidner, E. C.

2017-12-01

Mantle transition zone (MTZ) is delineated by the 410 and 660 km discontinuities and plays an important role in mantle convection. Mineral physics experiments predict that wadsleyite and ringwoodite can have 13% and 2% single-crystal anisotropy respectively, indicating that seismic anisotropy is likely to exist in the upper part of the MTZ when MTZ minerals are aligned by mantle flow (e.g. subducting slabs). Here we use the SS precursors to study the topography change and seismic anisotropy in the vicinity of MTZ discontinuities. An up-to-date SS precursor dataset consisting of 45,624 records was collected to investigate MTZ topography and anisotropy. We stacked the whole dataset into 9 geographical caps to obtain the global topography of 410 and 660 km discontinuities. The MTZ is thickened by 15 km beneath subduction zones (e.g. Japan and South America) and also thinned by 15 km beneath mantle plume regions (e.g. Bowie and Iceland hotspots), which is consistent with thermal heterogeneity in the mid-mantle. We identify four locations with sufficient bounce point density and azimuthal coverage of SS precursors to study azimuthal anisotropy in MTZ; the central Pacific, the northwest Pacific, Greenland and the central Atlantic. We stack the data by the azimuth of SS bounce points falling within the range of 2000 km in these four locations. The goal is to detect the azimuthal dependence of travel time and amplitude of SS precursors, thus to constrain azimuthal anisotropy in MTZ. The central Pacific bin has fast direction at 110° for both S410S and S660S azimuthal stacks, which is interpreted as seismic anisotropy in the overlying upper mantle. We also stack data in subduction zones by the relative azimuths of bounce points compared to mantle flow directions to test the hypothesis that subducting slabs can cause azimuthal anisotropy in MTZ. A trench-parallel fast direction is observed for both S410S and S660S travel times and amplitudes, but not for their differential travel times. This indicates that subducting slabs impart azimuthal anisotropy right above 410 discontinuity, but detectable anisotropy does not extend into the MTZ. We will present results from 3D synthetic modeling based on SPECFEM3D software to further interrogate the effects of anisotropic structures on the waveforms of the SS precursors.

Constraining the Composition of the Subcontinental Lithospheric Mantle Beneath the East African Rift: FTIR Analysis of Water in Spinel Peridotite Mantle Xenoliths

NASA Astrophysics Data System (ADS)

Erickson, S. G.; Nelson, W. R.; Peslier, A. H.; Snow, J. E.

2014-12-01

The East African Rift System was initiated by the impingement of the Afar mantle plume on the base of the non-cratonic continental lithosphere (assembled during the Pan-African Orogeny), producing over 300,000 km3 [1] of continental flood basalts ~30 Ma ago. The contribution of the subcontinental lithospheric mantle (SCLM) to this voluminous period of volcanism is implied based on basaltic geochemical and isotopic data. However, the role of percolating melts on the SCLM composition is less clear. Metasomatism is capable of hybridizing or overprinting the geochemical signature of the SCLM. In addition, models suggest that adding fluids to lithospheric mantle affects its stability [e.g. 2, 3]. We investigated the nature of the SCLM using Fourier transform infrared spectrometry (FTIR) to measure water content in mantle xenoliths entrained in young (1 Ma) basaltic lavas from the Ethiopian volcanic province. The mantle xenoliths consist dominantly of spinel lherzolites and are composed of nominally anhydrous minerals, which can contain trace water as H in mineral defects. Eleven mantle xenoliths come from the Injibara-Gojam region and two from the Mega-Sidamo region. Water abundances of olivines in six samples are 1-5ppm H2O while the rest are below the limit of detection (<0.5 ppm H2O); orthopyroxene and clinopyroxene contain 80-238 and 111-340 ppm wt H2O, respectively. Two xenoliths have higher water contents - a websterite (470 ppm) and dunite (229 ppm), consistent with involvement of ascending melts. The low water content of the upper SCLM beneath Ethiopia is as dry as the oceanic mantle [2] except for small domains represented by percolating melts. Consequently, rifting of the East African lithosphere may not have been facilitated by a hydrated upper mantle. [1] Hoffman et al., 1997 Nature 389, 838-841. [2] Peslier et al., 2010 Nature 467, 78-81. [3] Lee et al., 2011 AREPS 39, 59-90.

Slab melting beneath the Cascade Arc driven by dehydration of altered oceanic peridotite

NASA Astrophysics Data System (ADS)

Walowski, K. J.; Wallace, P. J.; Hauri, E. H.; Wada, I.; Clynne, M. A.

2015-05-01

Water is returned to Earth’s interior at subduction zones. However, the processes and pathways by which water leaves the subducting plate and causes melting beneath volcanic arcs are complex; the source of the water--subducting sediment, altered oceanic crust, or hydrated mantle in the downgoing plate--is debated; and the role of slab temperature is unclear. Here we analyse the hydrogen-isotope and trace-element signature of melt inclusions in ash samples from the Cascade Arc, where young, hot lithosphere subducts. Comparing these data with published analyses, we find that fluids in the Cascade magmas are sourced from deeper parts of the subducting slab--hydrated mantle peridotite in the slab interior--compared with fluids in magmas from the Marianas Arc, where older, colder lithosphere subducts. We use geodynamic modelling to show that, in the hotter subduction zone, the upper crust of the subducting slab rapidly dehydrates at shallow depths. With continued subduction, fluids released from the deeper plate interior migrate into the dehydrated parts, causing those to melt. These melts in turn migrate into the overlying mantle wedge, where they trigger further melting. Our results provide a physical model to explain melting of the subducted plate and mass transfer from the slab to the mantle beneath arcs where relatively young oceanic lithosphere is subducted.

Slab melting beneath the Cascades Arc driven by dehydration of altered oceanic peridotite

USGS Publications Warehouse

Walowski, Kristina J; Wallace, Paul J.; Hauri, E.H.; Wada, I.; Clynne, Michael A.

2015-01-01

Water is returned to Earth’s interior at subduction zones. However, the processes and pathways by which water leaves the subducting plate and causes melting beneath volcanic arcs are complex; the source of the water—subducting sediment, altered oceanic crust, or hydrated mantle in the downgoing plate—is debated; and the role of slab temperature is unclear. Here we analyse the hydrogen-isotope and trace-element signature of melt inclusions in ash samples from the Cascade Arc, where young, hot lithosphere subducts. Comparing these data with published analyses, we find that fluids in the Cascade magmas are sourced from deeper parts of the subducting slab—hydrated mantle peridotite in the slab interior—compared with fluids in magmas from the Marianas Arc, where older, colder lithosphere subducts. We use geodynamic modelling to show that, in the hotter subduction zone, the upper crust of the subducting slab rapidly dehydrates at shallow depths. With continued subduction, fluids released from the deeper plate interior migrate into the dehydrated parts, causing those to melt. These melts in turn migrate into the overlying mantle wedge, where they trigger further melting. Our results provide a physical model to explain melting of the subducted plate and mass transfer from the slab to the mantle beneath arcs where relatively young oceanic lithosphere is subducted.

Velocity structure around the 410 km discontinuity beneath the East China Sea based on the waveform modeling method

NASA Astrophysics Data System (ADS)

Li, W.; Cui, Q.; Gao, Y.; Wei, R.; Zhou, Y.; Yu, J.

2017-12-01

The 410 km discontinuity is the upper boundary of the mantle transition zone. Seismic detections on the structure and morphology of the 410 km discontinuity are helpful to understand the compositions of the Earth's interior and the relevant geodynamics. In this paper, we select the broadband P waveforms of an intermediate earthquake that occurred in the Ryukyu subduction zone and retrieved from the China Digital Seismograph Network, and study the fine velocity structure around the 410 km discontinuity by matching the observed triplicated waveforms with the theoretical ones. Our results reveal that (1) the 410 km discontinuity beneath the East China Sea is mostly a sharp boundary with a small-scale uplift of 8-15 km and a gradient boundary up to 20 km in the most southern part, and (2) there exist a low velocity layer atop the 410 km discontinuity with the thickness of 50-62 km and P-wave velocity decrease of 0.5%-1.5%, and (3) a high velocity anomaly with P-wave decrease of 1.0%-3.0% below 440 km. Combining with the previous topographic results in this area, we speculate that the high velocity anomaly is relevant to the stagnancy of the western Pacific slab in the mantle transition zone, the decomposition of phase E in the slab results in the increase of water content, which would cause the uplift of the 410 km discontinuity, and the low velocity layer atop the discontinuity should be related to the partial melting of the mantle peridotite induced by the dehydration of the hydrous minerals.

Crustal and uppermost mantle S-wave velocity structure beneath the Japanese islands from seismic ambient noise tomography

NASA Astrophysics Data System (ADS)

Guo, Zhi; Gao, Xing; Shi, Heng; Wang, Weiming

2013-04-01

In this study, the crustal and uppermost mantle shear wave velocities beneath the Japanese islands have been determined by inversion from seismic ambient noise tomography using data recorded at 75 Full Range Seismograph Network of Japan broad-band seismic stations, which are uniformly distributed across the Japanese islands. By cross-correlating 2 yr of vertical component seismic ambient noise recordings, we are able to extract Rayleigh wave empirical Green's functions, which are subsequently used to measure phase velocity dispersion in the period band of 6-50 s. The dispersion data are then inverted to yield 2-D tomographic phase velocity maps and 3-D shear wave velocity models. Our results show that the velocity variations at short periods (˜10 s), or in the uppermost crust, correlate well with the major known surface geological and tectonic features. In particular, the distribution of low-velocity anomalies shows good spatial correlation with active faults, volcanoes and terrains of sediment exposure, whereas the high-velocity anomalies are mainly associated with the mountain ranges. We also observe that large upper crustal earthquakes (5.0 ≤ M ≤ 8.0, depth ≤ 25 km) mainly occurred in low-velocity anomalies or along the boundary between low- and high-velocity anomalies, suggesting that large upper crustal earthquakes do not strike randomly or uniformly; rather they are inclined to nucleate within or adjacent to low-velocity areas.

Implications for Lithospheric Reheating beneath the African Superswell from P nl Wave Propagation in Central and Southern Africa,

DTIC Science & Technology

1995-08-14

characterized by a lid structure with a constant velocity. There appears to be little evidence for (1) mantle velocity gradients beneath southern Africa that...therefore conclude that there is little evidence for (1) a mantle lid beneath central Africa containing a positive velocity gradient or (2).a low velocity... Fucks , M.A. Khan, P.K.H. Maguire, W.D. Mooney, U. Achauer, P.M. Davis, R.P. Meyer, L.W. Braile, 1.0. Nyambok, and G.A. Thompson, The East African rift

Lower-mantle plume beneath the Yellowstone hotspot revealed by core waves

NASA Astrophysics Data System (ADS)

Nelson, Peter L.; Grand, Stephen P.

2018-04-01

The Yellowstone hotspot, located in North America, is an intraplate source of magmatism the cause of which is hotly debated. Some argue that a deep mantle plume sourced at the base of the mantle supplies the heat beneath Yellowstone, whereas others claim shallower subduction or lithospheric-related processes can explain the anomalous magmatism. Here we present a shear wave tomography model for the deep mantle beneath the western United States that was made using the travel times of core waves recorded by the dense USArray seismic network. The model reveals a single narrow, cylindrically shaped slow anomaly, approximately 350 km in diameter that we interpret as a whole-mantle plume. The anomaly is tilted to the northeast and extends from the core-mantle boundary to the surficial position of the Yellowstone hotspot. The structure gradually decreases in strength from the deepest mantle towards the surface and if it is purely a thermal anomaly this implies an initial excess temperature of 650 to 850 °C. Our results strongly support a deep origin for the Yellowstone hotspot, and also provide evidence for the existence of thin thermal mantle plumes that are currently beyond the resolution of global tomography models.

Local thickening of the Cascadia forearc crust and the origin of seismic reflectors in the uppermost mantle

USGS Publications Warehouse

Calvert, A.J.; Ramachandran, K.; Kao, H.; Fisher, M.A.

2006-01-01

Seismic reflection profiles from three different surveys of the Cascadia forearc are interpreted using P wave velocities and relocated hypocentres, which were both derived from the first arrival travel time inversion of wide-angle seismic data and local earthquakes. The subduction decollement, which is characterized beneath the continental shelf by a reflection of 0.5 s duration, can be traced landward into a large duplex structure in the lower forearc crust near southern Vancouver Island. Beneath Vancouver Island, the roof thrust of the duplex is revealed by a 5–12 km thick zone, identified previously as the E reflectors, and the floor thrust is defined by a short duration reflection from a − 1. We suggest that these relatively low velocities indicate the presence of either crustal rocks from the oceanic plate that have been underplated to the continent or crustal rocks from the forearc that have been transported downward by subduction erosion. The absence of seismicity from within the E reflectors implies that they are significantly weaker than the overlying crust, and the reflectors may be a zone of active ductile shear. In contrast, seismicity in parts of the D reflectors can be interpreted to mean that ductile shearing no longer occurs in the landward part of the duplex. Merging of the D and E reflectors at 42–46 km depth creates reflectivity in the uppermost mantle with a vertical thickness of at least 15 km. We suggest that pervasive reflectivity in the upper mantle elsewhere beneath Puget Sound and the Strait of Georgia arises from similar shear zones.

Teleseismic tomography for imaging Earth's upper mantle

NASA Astrophysics Data System (ADS)

Aktas, Kadircan

Teleseismic tomography is an important imaging tool in earthquake seismology, used to characterize lithospheric structure beneath a region of interest. In this study I investigate three different tomographic techniques applied to real and synthetic teleseismic data, with the aim of imaging the velocity structure of the upper mantle. First, by applying well established traveltime tomographic techniques to teleseismic data from southern Ontario, I obtained high-resolution images of the upper mantle beneath the lower Great Lakes. Two salient features of the 3D models are: (1) a patchy, NNW-trending low-velocity region, and (2) a linear, NE-striking high-velocity anomaly. I interpret the high-velocity anomaly as a possible relict slab associated with ca. 1.25 Ga subduction, whereas the low-velocity anomaly is interpreted as a zone of alteration and metasomatism associated with the ascent of magmas that produced the Late Cretaceous Monteregian plutons. The next part of the thesis is concerned with adaptation of existing full-waveform tomographic techniques for application to teleseismic body-wave observations. The method used here is intended to be complementary to traveltime tomography, and to take advantage of efficient frequency-domain methodologies that have been developed for inverting large controlled-source datasets. Existing full-waveform acoustic modelling and inversion codes have been modified to handle plane waves impinging from the base of the lithospheric model at a known incidence angle. A processing protocol has been developed to prepare teleseismic observations for the inversion algorithm. To assess the validity of the acoustic approximation, the processing procedure and modelling-inversion algorithm were tested using synthetic seismograms computed using an elastic Kirchhoff integral method. These tests were performed to evaluate the ability of the frequency-domain full-waveform inversion algorithm to recover topographic variations of the Moho under a variety of realistic scenarios. Results show that frequency-domain full-waveform tomography is generally successful in recovering both sharp and discontinuous features. Thirdly, I developed a new method for creating an initial background velocity model for the inversion algorithm, which is sufficiently close to the true model so that convergence is likely to be achieved. I adapted a method named Deformable Layer Tomography (DLT), which adjusts interfaces between layers rather than velocities within cells. I applied this method to a simple model comprising a single uniform crustal layer and a constant-velocity mantle, separated by an irregular Moho interface. A series of tests was performed to evaluate the sensitivity of the DLT algorithm; the results show that my algorithm produces useful results within a realistic range of incident-wave obliquity, incidence angle and signal-to-noise level. Keywords. Teleseismic tomography, full waveform tomography, deformable layer tomography, lower Great Lakes, crust and upper mantle.

High-resolution Teleseismic Tomography Reveals a Complex Lithospheric Structure Beneath the North Anatolian Fault

NASA Astrophysics Data System (ADS)

Papaleo, E.; Cornwell, D. G.; Rawlinson, N.

2016-12-01

We present high-resolution tomography images of a major active continental strike slip fault zone, the North Anatolian Fault (NAF) in northern Turkey. Historical seismic records show that the NAF, with a length of 1500 km and a current slip rate of 25 mm/yr, is capable of producing large magnitude earthquakes that have activated different segments of the fault in a westward progression towards the study region, where the devastating Izmit and Düzce events occurred in 1999. The NAF poses a major seismic hazard to the city of Istanbul, situated close to one of the two strands into which the fault splays east of the Sea of Marmara. In order to improve our understanding of the lower crust and upper mantle properties that influence fault dynamics throughout the seismic cycle, we constrain NAF structure across the Moho in unprecedented detail by applying teleseismic tomography to data recorded by an array of 70 temporary seismic stations deployed with 7 km spacing (Dense Array for North Anatolia, DANA). High quality recordings of teleseismic earthquakes combined with the dense nature of the array allow high-resolution (i.e. horizontal and vertical resolution of 8 and 15 km, respectively) 3D seismic imaging of the velocity structure beneath the NAF. The northern branch of the NAF coincides with an abrupt change between opposite polarity velocity anomalies and can be traced to at least Moho depths ( 36 km) with a width of ≤8 km. A similar pattern of antithetic anomalies occurs over a horizontal distance of 30-50 km below the Moho and may indicate a widening shear zone as it passes from the crust into the upper mantle. We find evidence for significant along-strike variation in NAF structure over distances of ≤20 km and interpret an east-to-west narrowing of upper mantle slow velocity anomalies (from 50 to 30 km) to represent laterally variable strain focussing within the lithosphere. Our observations are consistent with the notion that the NAF marks the boundary between compositionally distinct lithospheres with different tectonic histories and reactivates the pre-existing Intra-Pontide suture zone. We discuss our results in terms of the influence of lithosphere heterogeneity on the development and evolution of global continental strike-slip fault zones and assess the applicability of current shear zone deformation models.

Dynamical consequences of mantle heterogeneity in two-phase models of mid-ocean ridges

NASA Astrophysics Data System (ADS)

Katz, R. F.

2010-12-01

The mid-ocean ridge system, over 50,000 km in length, samples the magmatic products of a large swath of the asthenosphere. It provides our best means to assess the heterogeneity structure of the upper mantle. Interpretation of the diverse array of observations of MOR petrology, geochemistry, tomography, etc requires models that can map heterogeneity structure onto predictions testable by comparison with these observations. I report on progress to this end; in particular, I describe numerical models of coupled magma/mantle dynamics at mid-ocean ridges [1,2]. These models incorporate heterogeneity in terms of a simple, two-component thermochemical system with specified amplitude and spatial distribution. They indicate that mantle heterogeneity has significant fluid-dynamical consequences for both mantle and magmatic flow. Models show that the distribution of enrichment can lead to asymmetry in the strength of upwelling across the ridge-axis and channelised magmatic transport to the axis. Furthermore, heterogeneity can cause off-axis upwelling of partially molten diapirs, trapping of enriched melts off-axis, and re-fertilization of the mantle by pooled and refrozen melts. Predicted consequences of geochemical heterogeneity may also be considered. References: [1] Katz, RF, (2008); Magma dynamics with the Enthalpy Method: Benchmark Solutions and Magmatic Focusing at Mid-ocean Ridges. Journal of Petrology, doi: 10.1093/petrology/egn058. [2] Katz RF, (2010); Porosity-driven convection and asymmetry beneath mid-ocean ridges. Submitted to G3.

Comment on `Banana-doughnut kernels and mantle tomography' by van der Hilst and de Hoop

NASA Astrophysics Data System (ADS)

Montelli, R.; Nolet, G.; Dahlen, F. A.

2006-12-01

E debbasi considerare come non è cosa più difficile a trattare, né più dubbia a riuscire, nè più pericolosa a maneggiare, che farsi capo ad introdurre nuovi ordini. Perché lo introduttore ha per nimici tutti quelli che delli ordini vecchi fanno bene, et ha tepidi defensori tutti quelli che delli ordini nuovi farebbono bene.† Machiavelli, Il Principe The claim by van der Hilst and de Hoop that finite-frequency (FF) inversion of seismic traveltimes does not result in measurable improvements in tomographic images is misguided, and based upon a biased selection of images in the upper mantle, where wave front healing effects are indeed small, and where our models are generally poorly resolved because we primarily used teleseismic waves that travel steeply in the upper mantle; and upon an improper application of statistics to the better-resolved anomalies in the lower mantle. If station corrections for long-period P waves are computed using ray theory, as we do, unmodelled FF effects may be responsible for slow anomalies of up to 0.3 per cent beneath very small island stations, but these effects are negligible for larger islands such as Reunion and Kerguelen. The presence of a plume beneath these islands is the most probable explanation for the observed low velocities.

Geophysical constraints on geodynamic processes at convergent margins: A global perspective

NASA Astrophysics Data System (ADS)

Artemieva, Irina; Thybo, Hans; Shulgin, Alexey

2016-04-01

Convergent margins, being the boundaries between colliding lithospheric plates, form the most disastrous areas in the world due to intensive, strong seismicity and volcanism. We review global geophysical data in order to illustrate the effects of the plate tectonic processes at convergent margins on the crustal and upper mantle structure, seismicity, and geometry of subducting slab. We present global maps of free-air and Bouguer gravity anomalies, heat flow, seismicity, seismic Vs anomalies in the upper mantle, and plate convergence rate, as well as 20 profiles across different convergent margins. A global analysis of these data for three types of convergent margins, formed by ocean-ocean, ocean-continent, and continent-continent collisions, allows us to recognize the following patterns. (1) Plate convergence rate depends on the type of convergent margins and it is significantly larger when, at least, one of the plates is oceanic. However, the oldest oceanic plate in the Pacific ocean has the smallest convergence rate. (2) The presence of an oceanic plate is, in general, required for generation of high-magnitude (M N 8.0) earthquakes and for generating intermediate and deep seismicity along the convergent margins. When oceanic slabs subduct beneath a continent, a gap in the seismogenic zone exists at depths between ca. 250 km and 500 km. Given that the seismogenic zone terminates at ca. 200 km depth in case of continent-continent collision, we propose oceanic origin of subducting slabs beneath the Zagros, the Pamir, and the Vrancea zone. (3) Dip angle of the subducting slab in continent-ocean collision does not correlate neither with the age of subducting oceanic slab, nor with the convergence rate. For ocean-ocean subduction, clear trends are recognized: steeply dipping slabs are characteristic of young subducting plates and of oceanic plates with high convergence rate, with slab rotation towards a near-vertical dip angle at depths below ca. 500 km at very high convergence rate. (4) Local isostasy is not satisfied at the convergent margins as evidenced by strong free air gravity anomalies of positive and negative signs. However, near-isostatic equilibrium may exist in broad zones of distributed deformation such as Tibet. (5) No systematic patterns are recognized in heat flow data due to strong heterogeneity of measured values which are strongly affected by hydrothermal circulation, magmatic activity, crustal faulting, horizontal heat transfer, and also due to low number of heat flow measurements across many margins. (6) Low upper mantle Vs seismic velocities beneath the convergent margins are restricted to the upper 150 km and may be related to mantle wedge melting which is confined to shallow mantle levels. Artemieva, I.M., Thybo, H., and Shulgin, A., 2015. Geophysical constraints on geodynamic processes at convergent margins: A global perspective. Gondwana Research, http://dx.doi.org/10.1016/j.gr.2015.06.010

Deep Probe: Investigating the lithosphere of western North America with refraction seismology

NASA Astrophysics Data System (ADS)

Gorman, Andrew Robert

The Laurentian Craton, composed of the exposed Canadian Shield ringed by sediment-covered platforms, is the Precambrian heart of North America. The craton can be divided into several provinces representing ancient Archean blocks and the suture regions which stitched them together. In western Canada, Montana and Wyoming, the general distribution of Precambrian cratonic elements has been established by previous potential field studies combined with the analysis of basement rocks extracted from a small number of exploration drill holes that penetrated the overlying sedimentary basin, and from limited outcrops in southern Montana and Wyoming. The major blocks identified in this region include the Archean Hearne (mostly beneath Alberta) and Wyoming (beneath Montana and Wyoming) Provinces. A third block, the Medicine Hat Block, often interpreted to be the southernmost part of the Hearne Province, is considered independent in this study. The objectives of this thesis are to determine the velocity structure and characteristics of the crust and sub-crustal lithospheric mantle beneath the three Archean domains and the relationships among them to further understanding of the tectonic development of cratonic western North America. These objectives are met through interpretation of data from the Deep Probe/SAREX seismic refraction experiment of 1995, the largest of its type ever undertaken on the continent. Twenty large chemical explosions were detonated along a 3000-km-long profile running from Great Slave Lake to southern New Mexico and recorded at ˜2000 closely spaced seismograph stations between central Alberta and northern New Mexico. Interpretations, of increasing complexity, are based on: (1) the tau-p downward continuation of individual shot records, (2) a ray-theoretical travel-time inversion with Earth curvature considerations, and (3) detailed modelling of specific features with a finite difference wave propagation method. Interpretations of velocities and structures are made to depths as great as 150 km. From features of the crustal structure and their correspondence with two north-dipping relict subduction zones in the upper mantle, the boundaries between the three major Archean blocks are delineated and associated with the Vulcan Structure and Great Falls Tectonic Zone, two poorly understood tectonic features in the region. A prominent 10-to-30 km thick high velocity layer at the base of the Wyoming Province and Medicine Hat Block is interpreted to represent Proterozoic crustal underplating and alteration. The composition and physical properties of the crust-mantle boundary, the relict subduction zones and a heterogeneous upper mantle layer lying between depths of 100 km and 140 km are investigated to further understand lithospheric development in this region. The seismic interpretation is combined with previous work to develop a revised scenario for the tectonic assembly of western Laurentia.

Seismic Tomography

NASA Astrophysics Data System (ADS)

Nowack, Robert L.; Li, Cuiping

The inversion of seismic travel-time data for radially varying media was initially investigated by Herglotz, Wiechert, and Bateman (the HWB method) in the early part of the 20th century [1]. Tomographic inversions for laterally varying media began in seismology starting in the 1970’s. This included early work by Aki, Christoffersson, and Husebye who developed an inversion technique for estimating lithospheric structure beneath a seismic array from distant earthquakes (the ACH method) [2]. Also, Alekseev and others in Russia performed early inversions of refraction data for laterally varying upper mantle structure [3]. Aki and Lee [4] developed an inversion technique using travel-time data from local earthquakes.

Upper Mantle Responses to India-Eurasia Collision in Indochina, Malaysia, and the South China Sea

NASA Astrophysics Data System (ADS)

Hongsresawat, S.; Russo, R. M.

2016-12-01

We present new shear wave splitting and splitting intensity measurements from SK(K)S phases recorded at seismic stations of the Malaysian National Seismic Network. These results, in conjunction with results from Tibet and Yunnan provide a basis for testing the degree to which Indochina and South China Sea upper mantle fabrics are responses to India-Eurasia collision. Upper mantle fabrics derived from shear wave splitting measurements in Yunnan and eastern Tibet parallel geodetic surface motions north of 26°N, requiring transmission of tractions from upper mantle depths to surface, or consistent deformation boundary conditions throughout the upper 200 km of crust and mantle. Shear wave splitting fast trends and surface velocities diverge in eastern Yunnan and south of 26°N, indicating development of an asthenospheric layer that decouples crust and upper mantle, or corner flow above the subducted Indo-Burma slab. E-W fast shear wave splitting trends southwest of 26°N/104°E indicate strong gradients in any asthenospheric infiltration. Possible upper mantle flow regimes beneath Indochina include development of olivine b-axis anisotropic symmetry due to high strain and hydrous conditions in the syntaxis/Indo-Burma mantle wedge (i.e., southward flow), development of strong upper mantle corner flow in the Indo-Burma wedge with olivine a-axis anisotropic symmetry (i.e., westward flow), and simple asthenospheric flow due to eastward motion of Sundaland shearing underlying asthenosphere. Further south, shear-wave splitting delay times at Malaysian stations vary from 0.5 seconds on the Malay Peninsula to over 2 seconds at stations on Borneo. Splitting fast trends at Borneo stations and Singapore trend NE-SW, but in northern Peninsular Malaysia, the splitting fast polarization direction is NW-SE, parallel to the trend of the Peninsula. Thus, there is a sharp transition from low delay time and NW-SE fast polarization to high delay times and fast polarization directions that parallel the strike of the now-inoperative spreading center in the South China Sea. This transition appears to occur in the central portion of Peninsular Malaysia and may mark the boundary between Tethyan upper mantle extruded from the India-Asia collision zone and supra-subduction upper mantle of the Indonesian arc.

Crustal structure of southwestern Saudi Arabia

USGS Publications Warehouse

Gettings, M.E.; Blank, H.R.; Mooney, W.D.; Healy, J.H.

1983-01-01

The southwestern Arabian Shield is composed of uplifted Proterozoic metamorphic and plutonic rocks. The Shield is bordered on the southwest by Cenozoic sedimentary and igneous rocks of the Red Sea paar and on the east by the Arabian Platform, an area of basin sedimentation throughout Phanerozoic time. The Shield appears to have been formed by successive episodes of island arc volcanism and sea-floor spreading, followed by several cycles of compressive tectonism and metamorphism. An interpretation and synthesis of a deep-refraction seismic profile from the Riyadh area to the Farasan Islands, and regional gravity, aeromagnetic, heat flow, and surface geologic data have yielded a self-consistent regional-scale model of the crust and upper mantle for this area. The model consists of two 20 km-thick layers of crust with an average compressional wave velocity in the upper crust of about 6.3 km/s and an average velocity in the lower. crust of about 7.0 km/s. This crust thins abruptly to less than 20 km near the southwestern end of the profile where Precambrian outcrops abut the Cenozoic rocks and to 8 km beneath the Farasan Islands. The data over the coastal plain and Red Sea shelf areas are fit satisfactorily by an oceanic crustal model. A major lateral velocity inhomogeneity in the crust is inferred about 25 km northeast of Sabhah and is supported by surface geologic evidence. The major velocity discontinuities occur at about the same depth across the entire Shield and are interpreted to indicate horizontal metamorphic stratification of the Precambrian crust. Several lateral inhomogenities in both the upper and lower .crust of the . Shield are interpreted, to indicate bulk compositional variations. The subcrustal portion of the model is composed of a hot, low-density lithosphere beneath the Red Sea which is systematically cooler and denser to the northeast. This model provides a mechanism which explains the observed topographic uplift, regional gravity pattern, heat flow, and mantle compressional wave velocities. Such a lithosphere could be produced by upwelling of hot asthenosphere beneath the Red Sea which then flows laterally beneath the lithosphere of the Arabian Plate.

Upper and lower mantle anisotropy inferred from comprehensive SKS and SKKS splitting measurements from India

NASA Astrophysics Data System (ADS)

Roy, Sunil K.; Ravi Kumar, M.; Srinagesh, D.

2014-04-01

In this study, we investigate the upper mantle anisotropy beneath India using high quality SKS and SKKS waveforms from 382 teleseismic earthquakes recorded at 119 broadband seismic stations. In addition, we present evidence for anisotropy in the D″ layer beneath southeast Asia using SKS and SKKS splitting discrepancies on the same seismogram. During this exercise, we obtain 200 new splitting measurements from 35 stations recently deployed in the Indo-Gangetic plains (IGP), central India and northeast India. While the delay times between the fast and slow axes of anisotropy (δt) range from 0.3 to 1.7 s, the fast polarization azimuths (Φ) at a majority of stations in the IGP and central India coincide with the absolute plate motion of India implying shear at the base of the lithosphere as the dominant mechanism for forging anisotropy. However, stations in NE India reveal fast polarization azimuths mainly in the ENE-WSW direction suggestive of lithospheric strain induced by the ongoing Indo-Eurasian collision. Our analysis for D″ anisotropy yielded a total of 100 SKS-SKKS pairs, which can be categorized into those exhibiting (I) null measurements for one phase and significant splitting for the other phase, (II) null measurement for both the phases, (III) significant splitting for both the phases. A pair is considered to be anomalous if the splitting difference between SKS and SKKS is ⩾0.5 s and the individual split time is ⩾0.5 s. Using this criterion, we obtain 12 measurements under category III and 9 under category I that show a null measurement for SKS and large splitting for the SKKS phase. Further, we quantify the strength of the lower mantle anisotropy by correcting the SKKS measurement for the upper mantle anisotropy obtained by the SKS phase on the same seismogram. The SKS delay times are found to be consistently less than SKKS times, suggesting that the SKS phases do not capture the lower mantle anisotropy in comparison to their SKKS counterparts. Seven coherent measurements thus obtained reveal measurable D″ anisotropy, with fast polarization azimuths oriented mainly in the ENE-WSW direction. These results suggest presence of a large region of deformation in the lowermost mantle beneath southeast Asia. A possible model for anisotropy in these regions could be the presence of slab material that pounded upon the core mantle boundary (CMB) and is experiencing large shear deformation, resulting in lattice preferred orientation (LPO) of the lower mantle (Van der Hilst and Kárason, 1999; Long, 2009). The other possibility is the phase transformation from MgSiO3 perovskite to a more stable post-perovskite phase under favorable conditions, which results in LPO of the lower mantle.

Complex Anisotropic Structure of the Mantle Wedge Beneath Kamchatka Volcanoes

NASA Astrophysics Data System (ADS)

Levin, V.; Park, J.; Gordeev, E.; Droznin, D.

2002-12-01

A wedge of mantle material above the subducting lithospheric plate at a convergent margin is among the most dynamic environments of the Earth's interior. Deformation and transport of solid and volatile phases within this region control the fundamental process of elemental exchange between the surficial layers and the interior of the planet. A helpful property in the study of material deformation and transport within the upper mantle is seismic anisotropy, which may reflect both microscopic effects of preferentialy aligned crystals of olivine and orthopyroxene and macroscopic effects of systematic cracks, melt lenses, layering etc. Through the mapping of anisotropic properties within the mantle wedge we can establish patterns of deformation. Volatile content affects olivine alignment, so regions of anomalous volatile content may be evident. Indicators of seismic anisotropy commonly employed in upper mantle studies include shear wave birefringence and mode-conversion between compressional and shear body waves. When combined together, these techniques offer complementary constraints on the location and intensity of anisotropic properties. The eastern coast of southern Kamchatka overlies a vigorous convergent margin where the Pacific plate descends at a rate of almost 80 mm/yr towards the northwest. We extracted seismic anisotropy indicators from two data sets sensitive to the anisotropic properties of the uppermost mantle. Firstly, we evaluated teleseismic receiver functions for a number of sites, and found ample evidence for anisotropicaly-influenced P-to-S mode conversion. Secondly, we measured splitting in S waves of earthquakes with sources within the downgoing slab. The first set of observations provides constraints on the depth ranges where strong changes in anisotropic properties take place. The local splitting data provides constraints on the cumulative strength of anisotropic properties along specific pathways through the mantle wedge and possibly parts of the slab. To explain the vertical stratification of anisotropy implied from receiver functions, and the strong lateral dependence of shear-wave splitting observations, we cannot rely on simple models of mantle wedge behaviour e.g., olivine-crystal alignment through subduction-driven corner flow. Diverse mechanisms can contribute to the observed pattern of anisotropic properties, with volatiles likely being a key influence. For instance, we find evidence in favor of a slow-symmetry-axis anisotropy within the uppermost 10-20 km of the mantle wedge, implying either excessive hydration of the mantle or else a presence of systematically aligned volatile-filled cracks or lenses. Also, shear-wave splitting is weak beneath the Avachinsky-Koryaksky volcanic center, suggesting either vertical flow or the influence of volatiles and/or thermally-enhanced diffusion creep.

Seismic character of the crust and upper mantle beneath the Sierra Nevada

NASA Astrophysics Data System (ADS)

Frassetto, A.; Gilbert, H.; Zandt, G.; Owens, T. J.; Jones, C.

2008-12-01

Recent geophysical studies of the Southern Sierra Nevada suggest that the removal of a gravitationally unstable, eclogitic residue links to recent volcanism and uplift in the Eastern Sierra. The Sierra Nevada EarthScope Project (SNEP) investigates the extent of this process beneath Central and Northern Sierra Nevada. We present receiver functions, which provide estimates of crustal thickness and Vp/Vs and image the response of the crust and upper mantle to lithospheric removal. For completeness this study combines data from the 2005-2007 SNEP broadband experiment, EarthScope's BigFoot Array, regional backbone stations, and earlier PASSCAL deployments. We analyze transects of teleseismic receiver functions generated using a common-conversion-point stacking algorithm. These identify a narrow, "bright" conversion from the Moho at depths of ~25-35 km along the crest of the Eastern Sierra and adjacent Basin and Range northward to the Cascade Arc. Trade-off analysis using the primary conversion and reverberations shows a high Vp/Vs (~1.9) throughout the Eastern Sierra, which may relate to partial melt present in the lower crust. To the west the crust-mantle boundary vanishes beneath the western foothills. However, low frequency receiver functions do image the crust-mantle boundary exceeding 50 km depth along the foothills to the west and south of Yosemite National Park. Unusually deep, intraplate earthquakes (Ryan et al., this session) occur in the center of this region. The frequency dependence of the Moho conversion implies a gradational increase from crust to mantle wavespeeds over a significant depth interval. The transition from a sharp to gradational Moho probably relates to the change from a delaminated granitic crust to crust with an intact, dense, eclogitic residue. The spatial correlation and focal mechanisms of the deep earthquakes suggest that a segment of this still intact residue is currently delaminating.

Box Tomography: first application to the imaging of upper-mantle shear velocity and radial anisotropy structure beneath the North American continent

NASA Astrophysics Data System (ADS)

Clouzet, P.; Masson, Y.; Romanowicz, B.

2018-06-01

The EarthScope Transpotable Array (TA) deployment provides dense array coverage throughout the continental United States and with it, the opportunity for high-resolution 3-D seismic velocity imaging of the stable part of the North American (NA) upper mantle. Building upon our previous long-period waveform tomographic modeling, we present a higher resolution 3-D isotropic and radially anisotropic shear wave velocity model of the NA lithosphere and asthenosphere. The model is constructed using a combination of teleseismic and regional waveforms down to 40 s period and wavefield computations are performed using the spectral element method both for regional and teleseismic data. Our study is the first tomographic application of `Box Tomography', which allows us to include teleseismic events in our inversion, while computing the teleseismic wavefield only once, thus significantly reducing the numerical computational cost of several iterations of the regional inversion. We confirm the presence of high-velocity roots beneath the Archean part of the continent, reaching 200-250 km in some areas, however the thickness of these roots is not everywhere correlated to the crustal age of the corresponding cratonic province. In particular, the lithosphere is thick (˜250 km) in the western part of the Superior craton, while it is much thinner (˜150 km) in its eastern part. This may be related to a thermomechanical erosion of the cratonic root due to the passage of the NA plate over the Great Meteor hotspot during the opening of the Atlantic ocean 200-110 Ma. Below the lithosphere, an upper-mantle low-velocity zone (LVZ) is present everywhere under the NA continent, even under the thickest parts of the craton, although it is less developed there. The depth of the minimum in shear velocity has strong lateral variations, whereas the bottom of the LVZ is everywhere relatively flat around 270-300 km depth, with minor undulations of maximum 30 km that show upwarping under the thickest lithosphere and downwarping under tectonic regions, likely reflecting residual temperature anomalies. The radial anisotropy structure is less well resolved, but shows distinct signatures in highly deformed regions of the lithosphere.

Earthquake focal parameters and lithospheric structure of the anatolian plateau from complete regional waveform modeling

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rodgers, A

2000-12-28

This is an informal report on preliminary efforts to investigate earthquake focal mechanisms and earth structure in the Anatolian (Turkish) Plateau. Seismic velocity structure of the crust and upper mantle and earthquake focal parameters for event in the Anatolian Plateau are estimated from complete regional waveforms. Focal mechanisms, depths and seismic moments of moderately large crustal events are inferred from long-period (40-100 seconds) waveforms and compared with focal parameters derived from global teleseismic data. Using shorter periods (10-100 seconds) we estimate the shear and compressional velocity structure of the crust and uppermost mantle. Results are broadly consistent with previous studiesmore » and imply relatively little crustal thickening beneath the central Anatolian Plateau. Crustal thickness is about 35 km in western Anatolia and greater than 40 km in eastern Anatolia, however the long regional paths require considerable averaging and limit resolution. Crustal velocities are lower than typical continental averages, and even lower than typical active orogens. The mantle P-wave velocity was fixed to 7.9 km/s, in accord with tomographic models. A high sub-Moho Poisson's Ratio of 0.29 was required to fit the Sn-Pn differential times. This is suggestive of high sub-Moho temperatures, high shear wave attenuation and possibly partial melt. The combination of relatively thin crust in a region of high topography and high mantle temperatures suggests that the mantle plays a substantial role in maintaining the elevation.« less

3-D lithospheric structure and regional/residual Bouguer anomalies in the Arabia-Eurasia collision (Iran)

NASA Astrophysics Data System (ADS)

Jiménez-Munt, I.; Fernãndez, M.; Saura, E.; Vergés, J.; Garcia-Castellanos, D.

2012-09-01

The aim of this work is to propose a first-order estimate of the crustal and lithospheric mantle geometry of the Arabia-Eurasia collision zone and to separate the measured Bouguer anomaly into its regional and local components. The crustal and lithospheric mantle structure is calculated from the geoid height and elevation data combined with thermal analysis. Our results show that Moho depth varies from ˜42 km at the Mesopotamian-Persian Gulf foreland basin to ˜60 km below the High Zagros. The lithosphere is thicker beneath the foreland basin (˜200 km) and thinner underneath the High Zagros and Central Iran (˜140 km). Most of this lithospheric mantle thinning is accommodated under the Zagros mountain belt coinciding with the suture between two different mantle domains on the Sanandaj-Sirjan Zone. The regional gravity field is obtained by calculating the gravimetric response of the 3-D crustal and lithospheric mantle structure obtained by combining elevation and geoid data. The calculated regional Bouguer anomaly differs noticeably from those obtained by filtering or just isostatic methods. The residual gravity anomaly, obtained by subtraction of the regional components to the measured field, is analyzed in terms of the dominating upper crustal structures. Deep basins and areas with salt deposits are characterized by negative values (˜-20 mGal), whereas the positive values are related to igneous and ophiolite complexes and shallow basement depths (˜20 mGal).

Trench-parallel flow beneath the nazca plate from seismic anisotropy.

PubMed

Russo, R M; Silver, P G

1994-02-25

Shear-wave splitting of S and SKS phases reveals the anisotropy and strain field of the mantle beneath the subducting Nazca plate, Cocos plate, and the Caribbean region. These observations can be used to test models of mantle flow. Two-dimensional entrained mantle flow beneath the subducting Nazca slab is not consistent with the data. Rather, there is evidence for horizontal trench-parallel flow in the mantle beneath the Nazca plate along much of the Andean subduction zone. Trench-parallel flow is attributale utable to retrograde motion of the slab, the decoupling of the slab and underlying mantle, and a partial barrier to flow at depth, resulting in lateral mantle flow beneath the slab. Such flow facilitates the transfer of material from the shrinking mantle reservoir beneath the Pacific basin to the growing mantle reservoir beneath the Atlantic basin. Trenchparallel flow may explain the eastward motions of the Caribbean and Scotia sea plates, the anomalously shallow bathymetry of the eastern Nazca plate, the long-wavelength geoid high over western South America, and it may contribute to the high elevation and intense deformation of the central Andes.

Crustal and Upper Mantle Velocity Structure beneath Northwestern South America revealed by the CARMArray

NASA Astrophysics Data System (ADS)

Miao, W.; Cornthwaite, J.; Levander, A.; Niu, F.; Schmitz, M.; Dionicio, V.; Nader-Nieto, M. F.

2017-12-01

The Caribbean plate (CAR) is a fragment of the Farallon plate heavily modified by igneous processes that created the Caribbean large igneous province (CLIP) between 110 and 80 Ma.The CAR collided with and initiated subduction beneath northwestern South America plate (SA) at about 60-55 Ma as a narrow flat-slab subduction zone with an accretionary prism offshore, but no volcanic arc. Large scale regional tomography suggests that 1000 km of the CAR has been subducted (Van Benthem et al., 2013, JGR). The flat slab has caused Laramide-style basement uplifts of the Merida Andes, Sierra de la Perija, and Santa Marta ranges with elevations >5 km. The details of subduction geometry of the CAR plate beneath northeastern Colombia and northwestern Venezuela are complicated and remain unclear. The region of slab steepening lies below the triangular Maracaibo block (Bezada et al, 2010, JGR), bounded by major strike slip faults and currently escaping to the north over the CAR. Geodetic data suggests the this region has the potential for a magnitude 8+ earthquake (Bilham and Mencin, 2013, AGU Abstract). To better understand the subduction geometry, we deployed 65 broadband (BB) stations across northeastern Colombia and northwestern Venezuela in April of 2016. The 65 stations interweave with the 32 existing Colombian and Venezuelan BB stations, forming a 2-D array (hereafter referred to as CARMArray) with a station spacing of 35-100 km that covers an area of 600 km by 400 km extending from the Caribbean coast in Colombia to the interior plains of Venezuela. With data from the first year of operation, we have measured the Rayleigh wave phase velocities and Z/H ratios in the period range of 8-40 s using both ambient noise and earthquake data recorded by the CARMArray. We also generated Ps receiver functions from waveform data of teleseismic events recorded by the array. We then jointly inverted the three datasets to construct a 3-D S-wave velocity model beneath the array. We will report the initial results of the inversion and discuss the lateral variations of crustal and upper mantle structure and their potential links with surface geology and regional tectonics.

Determining Ocean-Bottom Seismometer Orientations from the RHUM-RUM experiment from P-wave and Rayleigh wave polarizations

NASA Astrophysics Data System (ADS)

Scholz, John-Robert; Barruol, Guilhem; Fontaine, Fabrice R.; Sigloch, Karin

2016-04-01

To image the upper mantle structure beneath La Réunion hotspot, a large-scale seismic network has been deployed on land and at sea in the frame of the RHUM-RUM project (Réunion Hotspot and Upper Mantle - Réunions Unterer Mantel). This French-German passive seismic experiment was designed to investigate and image the deep structure beneath La Réunion, from crust to core, to precise the shape and depth origin of a mantle plume, if any, and to precise the horizontal and vertical mantle flow associated to a possible plume upwelling, to its interaction with the overlying plate and with the neighboring Indian ridges. For this purpose, 57 Ocean-Bottom Seismometers (OBS) were installed around La Réunion and along the Central and Southwest Indian ridges. Broad-band instruments were deployed with the French R/V Marion Dufresne in late 2012 (cruise MD192), and recovered 13 months later by the German R/V Meteor (cruise M101). The pool of OBS was complemented by ~60 terrestrial stations, installed on different islands in the western Indian Ocean, such as La Réunion, Madagascar, Mauritius, Seychelles, Mayotte and the Îles Éparses in the Mozambique channel. The OBS installation is a free-fall down to the seafloor, where they landed in an unknown orientation. Since seismologic investigations of crustal and upper mantle structure (e.g., receiver functions) and azimuthal anisotropy (e.g., SKS-splitting and Rayleigh waves) rely on the knowledge of the correct OBS orientation with respect to the geographic reference frame, it is of importance to determine the orientations of the OBS while recording on the seafloor. In an isotropic, horizontally homogeneous and non-dipping layered globe, the misorientation of each station refers to the offset between theoretical and recorded back-azimuth angle of a passive seismic event. Using large earthquakes (MW > 5.0), it is possible to establish multiple successful measurements per station and thus to determine with good confidence the sensor orientations. In this study, we analyzed particle motions of P-waves (P-pol) and of Rayleigh waves (R-pol) to quantify the orientation of each of the 57 OBS. We performed 213 polarization measurements based on 35 earthquakes for P-pol, and 381 polarization measurements based on 48 earthquakes for R-pol. This allowed us to successfully determine the North/South orientations for 40 out of 57 OBS: 13 stations were devoid of usable data and 4 stations were too noisy. From twice the standard deviation (95% confidence interval), we estimate the errors between 1° and 20°.

3-D S-velocity structure in the lowermost mantle beneath the Northern Pacific

NASA Astrophysics Data System (ADS)

Suzuki, Y.; Kawai, K.; Geller, R. J.; Borgeaud, A. F. E.; Konishi, K.

2017-12-01

We previously (Suzuki et al., EPS, 2016) reported the results of waveform inversion to infer the three-dimensional (3-D) S-velocity structure in the lowermost 400 km of the mantle (the Dʺ region) beneath the Northern Pacific region. Our dataset consists of about 20,000 transverse component broadband body-wave seismograms observed at North American stations (mainly USArray) for 131 intermediate and deep earthquakes which occurred beneath the western Pacific subduction region. Synthetic resolution tests indicate that our methods and dataset can resolve the velocity structure in the target region with a horizontal scale of about 150 km and a vertical scale of about 50 km. The 3-D S-velocity model obtained in that study shows three prominent features: (i) horizontal high-velocity anomalies up to about 3 per cent faster than the Preliminary Reference Earth Model (PREM) with a thickness of a few hundred km and a lower boundary which is at most about 150 km above the core-mantle boundary (CMB), (ii) low-velocity anomalies about 2.5 per cent slower than PREM beneath the high-velocity anomalies at the base of the lower mantle, (iii) a thin (about 150 km) low-velocity structure continuous from the base of the low-velocity zone to at least 400 km above the CMB. We interpret these features respectively as: (i) remnants of slab material where the Mg-perovskite to Mg-post-perovskite phase transition could have occurred within the slab, (ii, iii) large amounts of hot and less dense materials beneath the cold Kula or Pacific slab remnants immediately above the CMB which ascend and form a passive plume upwelling at the edge of the slab remnants. Since our initial work we subsequently conducted waveform inversion using both the transverse- and radial-component horizontal waveform data to infer the isotropic shear velocity structure in the lowermost mantle beneath the Northern Pacific in more detail. We also compute partial derivatives with respect to the 5 independent elastic constants (A, C, F, L, N) of a transversely isotropy (TI) medium, and conduct a synthetic resolution test to examine the ability of our methods and dataset to resolve the anisotropic structure in this region using two-component waveform data.

Whole-mantle convection with tectonic plates preserves long-term global patterns of upper mantle geochemistry.

PubMed

Barry, T L; Davies, J H; Wolstencroft, M; Millar, I L; Zhao, Z; Jian, P; Safonova, I; Price, M

2017-05-12

The evolution of the planetary interior during plate tectonics is controlled by slow convection within the mantle. Global-scale geochemical differences across the upper mantle are known, but how they are preserved during convection has not been adequately explained. We demonstrate that the geographic patterns of chemical variations around the Earth's mantle endure as a direct result of whole-mantle convection within largely isolated cells defined by subducting plates. New 3D spherical numerical models embedded with the latest geological paleo-tectonic reconstructions and ground-truthed with new Hf-Nd isotope data, suggest that uppermost mantle at one location (e.g. under Indian Ocean) circulates down to the core-mantle boundary (CMB), but returns within ≥100 Myrs via large-scale convection to its approximate starting location. Modelled tracers pool at the CMB but do not disperse ubiquitously around it. Similarly, mantle beneath the Pacific does not spread to surrounding regions of the planet. The models fit global patterns of isotope data and may explain features such as the DUPAL anomaly and long-standing differences between Indian and Pacific Ocean crust. Indeed, the geochemical data suggests this mode of convection could have influenced the evolution of mantle composition since 550 Ma and potentially since the onset of plate tectonics.

Asthenosphere rheology inferred from observations of the 2012 Indian Ocean earthquake.

PubMed

Hu, Yan; Bürgmann, Roland; Banerjee, Paramesh; Feng, Lujia; Hill, Emma M; Ito, Takeo; Tabei, Takao; Wang, Kelin

2016-10-20

The concept of a weak asthenospheric layer underlying Earth's mobile tectonic plates is fundamental to our understanding of mantle convection and plate tectonics. However, little is known about the mechanical properties of the asthenosphere (the part of the upper mantle below the lithosphere) underlying the oceanic crust, which covers about 60 per cent of Earth's surface. Great earthquakes cause large coseismic crustal deformation in areas hundreds of kilometres away from and below the rupture area. Subsequent relaxation of the earthquake-induced stresses in the viscoelastic upper mantle leads to prolonged postseismic crustal deformation that may last several decades and can be recorded with geodetic methods. The observed postseismic deformation helps us to understand the rheological properties of the upper mantle, but so far such measurements have been limited to continental-plate boundary zones. Here we consider the postseismic deformation of the very large (moment magnitude 8.6) 2012 Indian Ocean earthquake to provide by far the most direct constraint on the structure of oceanic mantle rheology. In the first three years after the Indian Ocean earthquake, 37 continuous Global Navigation Satellite Systems stations in the region underwent horizontal northeastward displacements of up to 17 centimetres in a direction similar to that of the coseismic offsets. However, a few stations close to the rupture area that had experienced subsidence of up to about 4 centimetres during the earthquake rose by nearly 7 centimetres after the earthquake. Our three-dimensional viscoelastic finite-element models of the post-earthquake deformation show that a thin (30-200 kilometres), low-viscosity (having a steady-state Maxwell viscosity of (0.5-10) × 10 18 pascal seconds) asthenospheric layer beneath the elastic oceanic lithosphere is required to produce the observed postseismic uplift.

Determination of Mantle Discontinuity Depths beneath the South Pacific Superswell As Inferred Using Data From Broadband OBS Array

NASA Astrophysics Data System (ADS)

Suetsugu, D.; Shiobara, H.; Sugioka, H.; Kanazawa, T.; Fukao, Y.

2005-12-01

We determined depths of the mantle discontinuities (the 410-km and 660-km discontinuities) beneath the South Pacific Superswell using waveform data from broadband ocean bottom seismograph (BBOBS) array to image presumed mantle plumes and their temperature anomalies. Seismic structure beneath this region had not previously been well explored in spite of its significance for mantle dynamics. The region is characterized by a topographic high of more than 680 m (Adam and Bonneville, 2005), a concentration of hotspot chains (e.g., Society, Cook-Austral, Marquesas, and Pitcairn) whose volcanic rocks have isotopic characteristics suggesting deep mantle origin, and a broad low velocity anomaly in the lower mantle revealed by seismic tomography. These observations suggest the presence of a whole-mantle scale upwelling beneath the region, which is called a 'superplume' (McNutt, 1998). However, the seismic structure has been only poorly resolved so far and the maximum depth of anomalous material beneath the hotspots has not yet been determined, mainly due to the sparseness of seismic stations in the region. To improve the seismic coverage, we deployed an array of 10 BBOBS over the French Polynesia area from 2003 to 2005. The BBOBS has been developed by Earthquake Research Institute of University of Tokyo and are equipped with the broadband CMG-3T/EBB sensor. The observation was conducted as a Japan-France cooperative project (Suetsugu et al., 2005, submitted to EOS). We computed receiver functions from the BBOBS data to detect Ps waves from the mantle discontinuities. The Velocity Spectrum Stacking method (Gurrola et al., 1994) were employed to enhance the Ps waves for determination of the discontinuity depths, in which receiver functions were stacked in a depth-velocity space. The Ps-waves from the mantle discontinuities were successfully detected at the most of the BBOBS stations, from which the discontinuity depths were determined with the Iasp91 velocity model. The 410-km discontinuity depths were estimated to be 403-431 km over the Superswell region, which are not substantially different from the global average considering the estimation error of 10 km. The 660-km discontinuity depths were also determined to be 654-674 km, close to the global average, at most of the stations. Data from a station near the Society hot spot, however, provide an anomalously shallow depth of 623 km, indicating a presence of a local hot anomaly at the bottom of the mantle transition zone beneath near the Society hot spot. Taking into consideration a possible effect of velocity anomalies on the depth estimation, the shallow anomaly is significant. The present result suggests that the thermal anomalies are not obvious in the Superswell-scale, but present locally beneath the Society hot spot.

Lateral Variations in SKS Splitting Across the MAGIC Array, Central Appalachians

NASA Astrophysics Data System (ADS)

Aragon, John C.; Long, Maureen D.; Benoit, Margaret H.

2017-11-01

The eastern margin of North America has been shaped by several cycles of supercontinent assembly. These past episodes of orogenesis and continental rifting have likely deformed the lithosphere, but the extent, style, and geometry of this deformation remain poorly known. Measurements of seismic anisotropy in the upper mantle can shed light on past lithospheric deformation, but may also reveal contributions from present-day mantle flow in the asthenosphere. Here we examine SKS waveforms and measure splitting of SKS phases recorded by the MAGIC experiment, a dense transect of seismic stations across the central Appalachians. Our measurements constrain small-scale lateral variations in azimuthal anisotropy and reveal distinct regions of upper mantle anisotropy. Stations within the present-day Appalachian Mountains exhibit fast splitting directions roughly parallel to the strike of the mountains and delay times of about 1.0 s. To the west, transverse component waveforms for individual events reveal lateral variability in anisotropic structure. Stations immediately to the east of the mountains exhibit complicated splitting patterns, more null SKS arrivals, and a distinct clockwise rotation of fast directions. The observed variability in splitting behavior argues for contributions from both the lithosphere and the asthenospheric mantle. We infer that the sharp lateral transition in splitting behavior at the eastern edge of the Appalachians is controlled by a change in anisotropy in the lithospheric mantle. We hypothesize that beneath the Appalachians, SKS splitting reflects lithospheric deformation associated with Appalachian orogenesis, while just to the east this anisotropic signature was modified by Mesozoic rifting.

Comparisons of seismic and electromagnetic structures of the MELT area

NASA Astrophysics Data System (ADS)

Evans, R. L.; Hirth, G.; Forsyth, D.; Baba, K.; Chave, A.

2003-04-01

Both seismic and electromagnetic (EM) models from the MELT experiment show similar broad scale features in the mantle beneath the Southern EPR. In all EM models, the conductivity in the upper 50-60˜km is considerably higher to the west of the ridge than to the east. Similarly, seismic models of short period Love waves are asymmetric in velocity structure, with slower velocities to the west of the ridge within the upper 60˜km. Body wave data suggest a similar asymmetry, although the depth extent is not as well defined. West of the ridge, both the higher conductivities and lower velocities have been attributed to the presence of a small melt fraction, although the anomalous regions estimated from different techniques do not entirely agree. To the east, there is a rapid increase in resistivity and S-wave velocity, indicating that within 25˜km of the axis the mantle above 70˜km is both dry and melt-free. Further away from the ridge, the boundary between a conductive asthenospheric mantle and a resistive overlying mantle flattens, at a depth around 60-80˜km. Rayleigh wave inversions also show fairly flat velocity contours with a broad minimum centered at 60-80˜km. Both of these features are consistent with a transition from dry to damp mantle. Also away from the ridge, EM data, shear-wave splitting, and Rayleigh waves all require an azimuthally anisotropic mantle consistent with the a-axis of olivine being preferentially oriented horizontally and perpendicular to the ridge. Anisotropy in EM data suggests damp mantle conditions in the 100-200˜km depth range, with enhanced conduction along the a-axis of olivine. Rayleigh waves are most sensitive to shallower structure and require anisotropy in the upper 70˜km. In the uppermost 40˜km, the most conductive and lowest velocity regions are close to the axis but offset 5-10˜km to the west. Some anisotropic inversions recover a vertically conductive feature that could be interpreted as a few percent melt distributed in vertically aligned channels or tubes. However, modeling of seismic data rule out the presence of a vertical melt bearing channel larger than 5˜km wide with a velocity reduction of 0.5˜kms-1 (3-4% melt fraction). This apparent discrepancy may provide clues as to how melt is distributed.

Seafloor age dependence of Rayleigh wave phase velocities in the Indian Ocean

NASA Astrophysics Data System (ADS)

Godfrey, Karen E.; Dalton, Colleen A.; Ritsema, Jeroen

2017-05-01

Variations in the phase velocity of fundamental-mode Rayleigh waves across the Indian Ocean are determined using two inversion approaches. First, variations in phase velocity as a function of seafloor age are estimated using a pure-path age-dependent inversion method. Second, a two-dimensional parameterization is used to solve for phase velocity within 1.25° × 1.25° grid cells. Rayleigh wave travel time delays have been measured between periods of 38 and 200 s. The number of measurements in the study area ranges between 4139 paths at a period of 200 s and 22,272 paths at a period of 40 s. At periods < 100 s, the phase velocity variations are strongly controlled by seafloor age and shown to be consistent with temperature variations predicted by the half-space-cooling model for a mantle potential temperature of 1400°C. The inferred thermal structure beneath the Indian Ocean is most similar to the structure of the Pacific upper mantle, where phase velocities can also be explained by a half-space-cooling model. The thermal structure is not consistent with that of the Atlantic upper mantle, which is best fit by a plate-cooling model and requires a thin plate. Removing age-dependent phase velocity from the 2-D maps of the Indian Ocean highlights anomalously high velocities at the Rodriguez Triple Junction and the Australian-Antarctic Discordance and anomalously low velocities immediately to the west of the Central Indian Ridge.

Upper Mantle Structure beneath Afar: inferences from surface waves.

NASA Astrophysics Data System (ADS)

Sicilia, D.; Montagner, J.; Debayle, E.; Lepine, J.; Leveque, J.; Cara, M.; Ataley, A.; Sholan, J.

2001-12-01

The Afar hotspot is related to one of the most important plume from a geodynamic point of view. It has been advocated to be the surface expression of the South-West African Superswell. Below the lithosphere, the Afar plume might feed other hotspots in central Africa (Hadiouche et al., 1989; Ebinger & Sleep, 1998). The processes of interaction between crust, lithosphere and plume are not well understood. In order to gain insight into the scientific issue, we have performed a surface-wave tomography covering the Horn of Africa. A data set of 1404 paths for Rayleigh waves and 473 paths for Love waves was selected in the period range 45-200s. They were collected from the permanent IRIS and GEOSCOPE networks and from the PASSCAL experiment, in Tanzania and Saudi Arabia. Other data come from the broadband stations deployed in Ethiopia and Yemen in the framework of the French INSU program ``Horn of Africa''. The results presented here come from a path average phase velocities obtained with a method based on a least-squares minimization (Beucler et al., 2000). The local phase velocity distribution and the azimuthal anisotropy were simultaneously retrieved by using the tomographic technique of Montagner (1986). A correction of the data is applied according to the crustal structure of the 3SMAC model (Nataf & Ricard, 1996). We find low velocities down to 200 km depth beneath the Red Sea, the Gulf of Aden, Afars, the Ethiopian Plateau and southern Arabia. High velocities are present in the eastern Arabia and the Tanzania Craton. The anisotropy beneath Afar seems to be complex, but enables to map the flow pattern at the interface lithosphere-asthenosphere. The results presented here are complementary to those obtained by Debayle et al. (2001) at upper-mantle transition zone depths using waveform inversion of higher Rayle igh modes.

Complex deformation in western Tibet revealed by anisotropic tomography

NASA Astrophysics Data System (ADS)

Zhang, Heng; Zhao, Junmeng; Zhao, Dapeng; Yu, Chunquan; Liu, Hongbing; Hu, Zhaoguo

2016-10-01

The mechanism and pattern of deformation beneath western Tibet are still an issue of debate. In this work we present 3-D P- and S-wave velocity tomography as well as P-wave radial and azimuthal anisotropy along the ANTILOPE-I profile and surrounding areas in western Tibet, which are determined by using a large number of P and S arrival-time data of local earthquakes and teleseismic events. Our results show that low-velocity (low-V) zones exist widely in the middle crust, whereas low-V zones are only visible in the lower crust beneath northwestern Tibet, indicating the existence of significant heterogeneities and complex flow there. In the upper mantle, a distinct low-V gap exists between the Indian and Asian plates. Considering the P- and S-wave tomography and P-wave azimuthal and radial anisotropy results, we interpret the gap to be caused mainly by shear heating. Depth-independent azimuthal anisotropy and high-velocity zones exist beneath the northern part of the study region, suggesting a vertically coherent deformation beneath the Tarim Basin. In contrast, tomographic and anisotropic features change with depth beneath the central and southern parts of the study region, which reflects depth-dependent (or decoupled) deformations there. At the northern edge of the Indian lithospheric mantle (ILM), P-wave azimuthal anisotropy shows a nearly east-west fast-velocity direction, suggesting that the ILM was re-built by mantle materials flowing to the north.

Three-Dimensional Seismic Structure of the Mid-Atlantic Ridge: An Investigation of Tectonic, Magmatic, and Hydrothermal Processes in the Rainbow Area

NASA Astrophysics Data System (ADS)

Dunn, Robert A.; Arai, Ryuta; Eason, Deborah E.; Canales, J. Pablo; Sohn, Robert A.

2017-12-01

To test models of tectonic, magmatic, and hydrothermal processes along slow-spreading mid-ocean ridges, we analyzed seismic refraction data from the Mid-Atlantic Ridge INtegrated Experiments at Rainbow (MARINER) seismic and geophysical mapping experiment. Centered at the Rainbow area of the Mid-Atlantic Ridge (36°14'N), this study examines a section of ridge with volcanically active segments and a relatively amagmatic ridge offset that hosts the ultramafic Rainbow massif and its high-temperature hydrothermal vent field. Tomographic images of the crust and upper mantle show segment-scale variations in crustal structure, thickness, and the crust-mantle transition, which forms a vertical gradient rather than a sharp boundary. There is little definitive evidence for large regions of sustained high temperatures and melt in the lower crust or upper mantle along the ridge axes, suggesting that melts rising from the mantle intrude as small intermittent magma bodies at crustal and subcrustal levels. The images reveal large rotated crustal blocks, which extend to mantle depths in some places, corresponding to off-axis normal fault locations. Low velocities cap the Rainbow massif, suggesting an extensive near-surface alteration zone due to low-temperature fluid-rock reactions. Within the interior of the massif, seismic images suggest a mixture of peridotite and gabbroic intrusions, with little serpentinization. Here diffuse microearthquake activity indicates a brittle deformation regime supporting a broad network of cracks. Beneath the Rainbow hydrothermal vent field, fluid circulation is largely driven by the heat of small cooling melt bodies intruded into the base of the massif and channeled by the crack network and shallow faults.

Paleoproterozoic Collisional Structures in the Hudson Bay Lithosphere Constrained by Multi-Observable Probabilistic Inversion

NASA Astrophysics Data System (ADS)

Darbyshire, F. A.; Afonso, J. C.; Porritt, R. W.

2015-12-01

The Paleozoic Hudson Bay intracratonic basin conceals a Paleoproterozoic Himalayan-scale continental collision, the Trans-Hudson Orogen (THO), which marks an important milestone in the assembly of the Canadian Shield. The geometry of the THO is complex due to the double-indentor geometry of the collision between the Archean Superior and Western Churchill cratons. Seismic observations at regional scale show a thick, seismically fast lithospheric keel beneath the entire region; an intriguing feature of recent models is a 'curtain' of slightly lower wavespeeds trending NE-SW beneath the Bay, which may represent the remnants of more juvenile material trapped between the two Archean continental cores. The seismic models alone, however, cannot constrain the nature of this anomaly. We investigate the thermal and compositional structure of the Hudson Bay lithosphere using a multi-observable probabilistic inversion technique. This joint inversion uses Rayleigh wave phase velocity data from teleseismic earthquakes and ambient noise, geoid anomalies, surface elevation and heat flow to construct a pseudo-3D model of the crust and upper mantle. Initially a wide range of possible mantle compositions is permitted, and tests are carried out to ascertain whether the lithosphere is stratified with depth. Across the entire Hudson Bay region, low temperatures and a high degree of chemical depletion characterise the mantle lithosphere. Temperature anomalies within the lithosphere are modest, as may be expected from a tectonically-stable region. The base of the thermal lithosphere lies at depths of >250 km, reaching to ~300 km depth in the centre of the Bay. Lithospheric stratification, with a more-depleted upper layer, is best able to explain the geophysical data sets and surface observables. Some regions, where intermediate-period phase velocities are high, require stronger mid-lithospheric depletion. In addition, a narrow region of less-depleted material extends NE-SW across the Bay, likely associated with the trace of the THO collision and the entrapment of juvenile material between the highly-depleted Archean cores.

The lithosphere architecture and geodynamic of the Middle and Lower Yangtze metallogenic belt in eastern China: constraints from integrated geophysical data

NASA Astrophysics Data System (ADS)

Lü, Qingtian; Shi, Danian; Jiang, Guoming; Dong, Shuwen

2014-05-01

The lithosphere structure and deep processes are keys to understanding mineral system and ore-forming processes. Lithosphere-scale process could create big footprints or signatures which can be observed by geophysics methods. SinoProbe has conducted an integrated deep exploration across middle and lower reaches of Yangtze Metallogenic Belt (YMB) in Eastern China, these included broadband seismic, reflection seismic, wide-angle reflection and magnetotellurics survey. Seismic reflection profiles and MT survey were also performed in Luzong, Tongling and Ningwu ore districts to construct 3D geological model. The resulting geophysical data provides new information which help to better understanding the lithosphere structure, geodynamic, deformation and heat and mass transportation that lead to the formation of the Metallogenic Belt. The major results are: (1) Lower velocity body at the top of upper mantle and a SE dipping high velocity body were imaged by teleseismic tomography beneath YMB; (2) Shear wave splitting results show NE parallel fast-wave polarization direction which parallel with tectonic lineament; (3) The reflection seismic data support the crustal-detachment model, the lower and upper crust was detached during contraction deformation near Tanlu fault and Ningwu volcanic basin; (4) Broadband and reflection seismic confirm the shallow Moho beneath YMB; (5) Strong correlation of lower crust reflectivity with magmatism; (6) The lower crust below Luzong Volcanics shows obvious reflective anisotropy both at the crust-mantle transition and the brittle-ductile transition in the crust. All these features suggest that introcontinental subduction, lithosphere delamination, mantle sources magmatic underplating, and MASH process are responsible for the formation of this Mesozoic metallogenic belt. Acknowledgment: We acknowledge the financial support of SinoProbe by the Ministry of Finance and Ministry of Land and Resources, P. R. China, under Grant sinoprobe-03, and financial support by National Natural Science Foundation of China under Grant 40930418

Large-scale variation in lithospheric structure along and across the Kenya rift

USGS Publications Warehouse

Prodehl, C.; Mechie, J.; Kaminski, W.; Fuchs, K.; Grosse, C.; Hoffmann, H.; Stangl, R.; Stellrecht, R.; Khan, M.A.; Maguire, Peter K.H.; Kirk, W.; Keller, Gordon R.; Githui, A.; Baker, M.; Mooney, W.; Criley, E.; Luetgert, J.; Jacob, B.; Thybo, H.; Demartin, M.; Scarascia, S.; Hirn, A.; Bowman, J.R.; Nyambok, I.; Gaciri, S.; Patel, J.; Dindi, E.; Griffiths, D.H.; King, R.F.; Mussett, A.E.; Braile, L.W.; Thompson, G.; Olsen, K.; Harder, S.; Vees, R.; Gajewski, D.; Schulte, A.; Obel, J.; Mwango, F.; Mukinya, J.; Riaroh, D.

1991-01-01

The Kenya rift is one of the classic examples of a continental rift zone: models for its evolution range from extension of the lithosphere by pure shear1, through extension by simple shear2, to diapiric upwelling of an asthenolith3. Following a pilot study in 19854, the present work involved the shooting of three seismic refraction and wide-angle reflection profiles along the axis, across the margins, and on the northeastern flank of the rift (Fig. 1). These lines were intended to reconcile the different crustal thickness estimates for the northern and southern parts of the rift4-6 and to reveal the structure across the rift, including that beneath the flanks. The data, presented here, reveal significant lateral variations in structure both along and across the rift. The crust thins along the rift axis from 35 km in the south to 20 km in the north; there are abrupt changes in Mono depth and uppermost-mantle seismic velocity across the rift margins, and crustal thickening across the boundary between the Archaean craton and PanAfrican orogenic belt immediately west of the rift. These results suggest that thickened crust may have controlled the rift's location, that there is a decrease in extension from north to south, and that the upper mantle immediately beneath the rift may contain reservoirs of magma generated at greater depth.

Anomalous mantle transition zone beneath the Yellowstone hotspot track

NASA Astrophysics Data System (ADS)

Zhou, Ying

2018-06-01

The origin of the Yellowstone and Snake River Plain volcanism has been strongly debated. The mantle plume model successfully explains the age-progressive volcanic track, but a deep plume structure has been absent in seismic imaging. Here I apply diffractional tomography to receiver functions recorded at USArray stations to map high-resolution topography of mantle transition-zone discontinuities. The images reveal a trail of anomalies that closely follow the surface hotspot track and correlate well with a seismic wave-speed gap in the subducting Farallon slab. This observation contradicts the plume model, which requires anomalies in the mid mantle to be confined in a narrow region directly beneath the present-day Yellowstone caldera. I propose an alternative interpretation of the Yellowstone volcanism. About 16 million years ago, a section of young slab that had broken off from a subducted spreading centre in the mantle first penetrated the 660 km discontinuity beneath Oregon and Idaho, and pulled down older stagnant slab. Slab tearing occurred along pre-existing fracture zones and propagated northeastward. This reversed-polarity subduction generated passive upwellings from the lower mantle, which ascended through a water-rich mantle transition zone to produce melting and age-progressive volcanism.

Project WILAS: Seismic imaging of crustal and upper mantle structures beneath the western Iberian Peninsula by means of the receiver-function technique

NASA Astrophysics Data System (ADS)

Dündar, Süleyman; Dias, Nuno A.; Silveira, Graça; Vinnik, Lev; Haberland, Christian

2013-04-01

An accurate knowledge of the structure of the earth's interior is of great importance to our understanding of tectonic processes. The WILAS-project (REF: PTDC/CTE-GIX/097946/2008) is a three-year collaborative project developed to study the subsurface structure of the western Iberian Peninsula, putting the main emphases on the lithosphere-asthenosphere system beneath the mainland of Portugal. The tectonic evolution of the target area has been driven by major plate-tectonic processes such as the historical opening of the Central Atlantic and the subsequent African-Eurasian convergence. Still, very little is known about the spatial structure of the continental collision. Within the framework of this research, a temporary network of 30 broadband three-component digital stations was operated between 2010 and 2012 in the target area. To carry out a large-scale structural analysis and facilitate a dense station-coverage for the area under investigation, the permanent Global Seismic Network stations, and temporary broadband stations deployed within the scope of the several seismic experiments (e.g. Doctar Network, Portuguese National Seismic Network), were included in the research analysis. In doing so, an unprecedented volume of high-quality data of a ca. 60X60 km density along with a combined network of 65 temporary and permanent broadband seismic stations are currently available for research purposes. One of the tasks of the WILAS research project has been a study of seismic velocity discontinuities beneath the western Iberian Peninsula region, up to a depth range of 700 km, utilizing the P- and S-receiver function techniques (PRF, SRF). Both techniques are based mainly on mode conversion of the elastic body-waves at an interface dividing the layers with different elastic properties. In the first phase of the project, PRF analysis was conducted in order to image the crust-mantle interface (Moho) and the mantle-transition-zone discontinuities at a depth of 410 km and 660 km beneath the area under investigation. While applying the common data processing steps (e.g., rotation, deconvolution and moveout-correction) to the selected data-set, we were able to create approximately 4.500 PRFs. The signals from the Moho, 410-km and 660-km discontinuities are clearly visible in many PRF stacks. The Moho depth range is from 26 to 34 km, with an average value of 29 km. No significant lateral variations in the depths of the "410-km" and "660-km" discontinuities have been identified so far. In the second phase of this project, the S-receiver-function technique will be applied in order to map the thickness of the underlying mantle lithosphere. Additionally, joint inversion of PRFs and waveforms of SKS will be used to investigate depth-localized azimuthal anisotropy and the related past and present mantle flows.

Does Late Miocene Exhumation Along the Western Slope of the Colorado Rockies Reflect Differential Rock Uplift?

NASA Astrophysics Data System (ADS)

Rosenberg, R. H.; Kirby, E.; Aslan, A.; Karlstrom, K. E.; Heizler, M. T.; Kelley, S. A.; Piotraschke, R. E.; Furlong, K. P.

2011-12-01

It is increasingly recognized that dynamic effects associated with changes in mantle flow and buoyancy can influence the evolution of surface topography. In the Rocky Mountain province of the western United States, recent seismic deployments reveal intriguing correlations between anomalies in the velocity structure of the upper mantle and regions of high topography. Here, we investigate whether regional correlations between upper-mantle structure and topography are associated with the history of Late Cenozoic fluvial incision and exhumation. Major tributaries of the upper Colorado River, including the Gunnison and Dolores Rivers, which drain high topography in central and western Colorado overlie upper mantle with slow seismic wave velocities; these drainages exhibit relatively steep longitudinal profiles (normalized for differences in drainage area and discharge) and are associated with ~1000-1500 m of incision over the past 10 Ma. In contrast, tributaries of the Green River that drain the western slope in northern Colorado (White, Yampa, and Little Snake Rivers) overlie mantle of progressively higher seismic wave velocities. River profiles in northern Colorado are two to three times less steep along reaches with comparable bedrock lithologies. New Ar39/Ar40 ages on ~11 Ma basalt flows capping the Tertiary Brown's Park Formation in northern Colorado indicate that the magnitude of exhumation along these profiles ranges from ~400 - 600 m over this time interval. The correspondence of steep river profiles in regions of greater incision implies that the fluvial systems are dynamically adjusting to an external forcing. New constraints on the exhumation history of the upper Colorado River from apatite fission track ages in boreholes near Rifle, Colorado are best explained by an onset of exhumation at ca. 8-10 Ma. Thus, relative base level fall associated with development of Grand Canyon (ca. 6-5 Ma) does not explain the regional onset of incision along the western slope of the Rockies. Additionally, new cosmogenic burial ages from fan-terrace complexes near Rifle, Colorado show that Colorado River incision occurred at similar rates over both 10 Ma and 2 Ma timescales. Fluvial incision in response to relative base level fall or to changes in regional climate cannot easily explain the history of differential incision along the western slope. Given the correspondence of steep channels, large magnitude incision and regions of low seismic velocity mantle, we suggest that differential rock uplift, driven, in part, by differences in the buoyancy and/or convective flow of the mantle beneath western Colorado is the likely driver for Neogene incision.

Imaging paleoslabs in the D″ layer beneath Central America and the Caribbean using seismic waveform inversion.

PubMed

Borgeaud, Anselme F E; Kawai, Kenji; Konishi, Kensuke; Geller, Robert J

2017-11-01

D″ (Dee double prime), the lowermost layer of the Earth's mantle, is the thermal boundary layer (TBL) of mantle convection immediately above the Earth's liquid outer core. As the origin of upwelling of hot material and the destination of paleoslabs (downwelling cold slab remnants), D″ plays a major role in the Earth's evolution. D″ beneath Central America and the Caribbean is of particular geodynamical interest, because the paleo- and present Pacific plates have been subducting beneath the western margin of Pangaea since ~250 million years ago, which implies that paleoslabs could have reached the lowermost mantle. We conduct waveform inversion using a data set of ~7700 transverse component records to infer the detailed three-dimensional S-velocity structure in the lowermost 400 km of the mantle in the study region so that we can investigate how cold paleoslabs interact with the hot TBL above the core-mantle boundary (CMB). We can obtain high-resolution images because the lowermost mantle here is densely sampled by seismic waves due to the full deployment of the USArray broadband seismic stations during 2004-2015. We find two distinct strong high-velocity anomalies, which we interpret as paleoslabs, just above the CMB beneath Central America and Venezuela, respectively, surrounded by low-velocity regions. Strong low-velocity anomalies concentrated in the lowermost 100 km of the mantle suggest the existence of chemically distinct denser material connected to low-velocity anomalies in the lower mantle inferred by previous studies, suggesting that plate tectonics on the Earth's surface might control the modality of convection in the lower mantle.

Lowermost mantle anisotropy near the eastern edge of the Pacific LLSVP: constraints from SKS-SKKS splitting intensity measurements

NASA Astrophysics Data System (ADS)

Deng, Jie; Long, Maureen D.; Creasy, Neala; Wagner, Lara; Beck, Susan; Zandt, George; Tavera, Hernando; Minaya, Estela

2017-08-01

Seismic anisotropy has been documented in many portions of the lowermost mantle, with particularly strong anisotropy thought to be present along the edges of large low shear velocity provinces (LLSVPs). The region surrounding the Pacific LLSVP, however, has not yet been studied extensively in terms of its anisotropic structure. In this study, we use seismic data from southern Peru, northern Bolivia and Easter Island to probe lowermost mantle anisotropy beneath the eastern Pacific Ocean, mostly relying on data from the Peru Lithosphere and Slab Experiment and Central Andean Uplift and Geodynamics of High Topography experiments. Differential shear wave splitting measurements from phases that have similar ray paths in the upper mantle but different ray paths in the lowermost mantle, such as SKS and SKKS, are used to constrain anisotropy in D″. We measured splitting for 215 same station-event SKS-SKKS pairs that sample the eastern Pacific LLSVP at the base of the mantle. We used measurements of splitting intensity(SI), a measure of the amount of energy on the transverse component, to objectively and quantitatively analyse any discrepancies between SKS and SKKS phases. While the overall splitting signal is dominated by the upper-mantle anisotropy, a minority of SKS-SKKS pairs (∼10 per cent) exhibit strongly discrepant splitting between the phases (i.e. the waveforms require a difference in SI of at least 0.4), indicating a likely contribution from lowermost mantle anisotropy. In order to enhance lower mantle signals, we also stacked waveforms within individual subregions and applied a waveform differencing technique to isolate the signal from the lowermost mantle. Our stacking procedure yields evidence for substantial splitting due to lowermost mantle anisotropy only for a specific region that likely straddles the edge of Pacific LLSVP. Our observations are consistent with the localization of deformation and anisotropy near the eastern boundary of the Pacific LLSVP, similar to previous observations for the African LLSVP.

On the Complicated 410 km Discontinuity beneath Eastern China with the Seismic Triplications

NASA Astrophysics Data System (ADS)

Zhou, Y.; Li, G.; Sui, Y.

2013-12-01

The seismic triplications from the seismograms of mid-deep earthquakes at the Ryuku subduction zone recorded by the Chinese Digital Seismic Network (CDSN) between the epicentral distance between 10°-23° are used to study the upper mantle structure beneath Eastern China. Comparing the observed seismograms with the synthetic ones from different models based on IASP91 earth model and using the ray-tracing method, we found that the 410 km discontinuity is a gradient zone with the thickness of 20 km and there is low velocity layer atop the discontinuity which becomes thin from north to south beneath Eastern China. The complicated 410 km discontinuity with an atop low velocity layer may be caused by the dehydration of the Philippine sea subducting materials which are observed by the seismic tomopgraphy (Qu, et al., 2007; Li and van der Hilst, 2010). The low velocity gradient zone between the depths of 80-200 km is also been observed and may be related to the lithospheric-asthenosphere boundary.

The lithospheric Structure of the Sahara Metacraton From Joint Analysis of Satellite Gravity Gradients and Seismological Data

NASA Astrophysics Data System (ADS)

Sobh, M.; Ebbing, J.; Goetze, H. J.; Abdelsalam, M. G.

2016-12-01

For the Saharan Metacraton in northern Africa only a few geophysical results exists, which can be used to characterize its deep structure. We combine recent seismological models with satellite gravity gradients to build a 3D lithospheric density model of the metacraton and its surrounding regions. Due to the sparse distribution of seismic data, we estimate the Moho boundary by non-linear gravity inversion in spherical coordinates. The model is constrained by some wide angle refraction seismic profiles and receiver function Moho depths. Despite the high topography of the Darfur and Tibisti Cenozoic volcanic provinces, we estimate thin crust which indicates an upper mantle contribution to the isostatic balance. In combination with seismic tomography models, we found that the lithospheric thickness in the western part of the Metacraton is thicker than in the eastern part. This indicates that the western resembles the remnants of the pre-Neoproterozoic Sahara craton (e.g. the Marzuk craton which escaped the metacratonization process). In order to explain the partial loss of the expected cratonic root beneath the Metacraton, we present different petrological-geophysical scenario testing for different upper mantle compositions.

Crustal structure and gravity anomalies beneath the Rif, northern Morocco: implications for the current tectonics of the Alboran region

NASA Astrophysics Data System (ADS)

Petit, Carole; Le Pourhiet, Laetitia; Scalabrino, Bruno; Corsini, Michel; Bonnin, Mickaël; Romagny, Adrien

2015-07-01

We analyse Bouguer anomaly data and previously published Moho depths estimated from receiver functions in order to determine the amount of isostatic compensation or uncompensation of the Rif topography in northern Morocco. We use Moho depth variations extracted from receiver function analyses to predict synthetic Bouguer anomalies that are then compared to observed Bouguer anomaly. We find that Moho depth variations due to isostatic compensation of topographic and/or intracrustal loads do not match Moho depth estimates obtained from receiver function analyses. The isostatic misfit map evidences excess crustal root as large as 10 km in the western part of the study area, whereas a `missing' crustal root of ˜5 km appears east of 4.3°E. This excess root/missing topography correlates with the presence of a dense mantle lid, the noticeable southwestward drift of the Western Rif area, and with a current surface uplift. We propose that a delaminated mantle lid progressively detaching westward or southwestward from the overlying crust is responsible for viscous flow of the ductile lower crust beneath the Rif area. This gives rise to isostatic uplift and westward drift due to viscous coupling at the upper/lower crust boundary. At the same time, the presence of this dense sinking mantle lid causes a negative dynamic topography, which explains why the observed topography is too low compared to the crustal thickness.

An upper bound on the electrical conductivity of hydrated oceanic mantle at the onset of dehydration melting

NASA Astrophysics Data System (ADS)

Naif, Samer

2018-01-01

Electrical conductivity soundings provide important constraints on the thermal and hydration state of the mantle. Recent seafloor magnetotelluric surveys have imaged the electrical conductivity structure of the oceanic upper mantle over a variety of plate ages. All regions show high conductivity (0.02 to 0.2 S/m) at 50 to 150 km depths that cannot be explained with a sub-solidus dry mantle regime without unrealistic temperature gradients. Instead, the conductivity observations require either a small amount of water stored in nominally anhydrous minerals or the presence of interconnected partial melts. This ambiguity leads to dramatically different interpretations on the origin of the asthenosphere. Here, I apply the damp peridotite solidus together with plate cooling models to determine the amount of H2O needed to induce dehydration melting as a function of depth and plate age. Then, I use the temperature and water content estimates to calculate the electrical conductivity of the oceanic mantle with a two-phase mixture of olivine and pyroxene from several competing empirical conductivity models. This represents the maximum potential conductivity of sub-solidus oceanic mantle at the limit of hydration. The results show that partial melt is required to explain the subset of the high conductivity observations beneath young seafloor, irrespective of which empirical model is applied. In contrast, the end-member empirical models predict either nearly dry (<20 wt ppm H2O) or slightly damp (<200 wt ppm H2O) asthenosphere for observations of mature seafloor. Since the former estimate is too dry compared with geochemical constraints from mid-ocean ridge basalts, this suggests the effect of water on mantle conductivity is less pronounced than currently predicted by the conductive end-member empirical model.

Topography of the 410 km and 660 km discontinuities beneath the Japan Sea and adjacent regions by analysis of multiple-ScS waves

NASA Astrophysics Data System (ADS)

Wang, Xin; Li, Juan; Chen, Qi-Fu

2017-02-01

The northwest Pacific subduction region is an ideal location to study the interaction between the subducting slab and upper mantle discontinuities. Due to the sparse distribution of seismic stations in the sea, previous studies mostly focus on mantle transition zone (MTZ) structures beneath continents or island arcs, leaving the vast area of the Japan Sea and Okhotsk Sea untouched. In this study, we analyzed multiple-ScS reverberation waves, and a common-reflection-point stacking technique was applied to enhance consistent signals beneath reflection points. A topographic image of the 410 km and 660 km discontinuities is obtained beneath the Japan Sea and adjacent regions. One-dimensional and 3-D velocity models are adapted to obtain the "apparent" and "true" depth. We observe a systematic pattern of depression ( 10-20 km) and elevation ( 5-10 km) of the 660, with the topography being roughly consistent with the shift of the olivine-phase transition boundary caused by the subducting Pacific plate. The behavior of the 410 is more complex. It is generally 5-15 km shallower at the location where the slab penetrates and deepened by 5-10 km oceanward of the slab where a low-velocity anomaly is observed in tomography images. Moreover, we observe a wide distribution of depressed 410 beneath the southern Okhotsk Sea and western Japan Sea. The hydrous wadsleyite boundary caused by the high water content at the top of the MTZ could explain the depression. The long-history trench rollback motion of Pacific slab might be responsible for the widely distributed depression of the 410 ranging upward and landward from the slab.

Magnetotelluric investigations of the lithosphere beneath the central Rae craton, mainland Nunavut, Canada

NASA Astrophysics Data System (ADS)

Spratt, Jessica E.; Skulski, Thomas; Craven, James A.; Jones, Alan G.; Snyder, David B.; Kiyan, Duygu

2014-03-01

New magnetotelluric soundings at 64 locations throughout the central Rae craton on mainland Nunavut constrain 2-D resistivity models of the crust and lithospheric mantle beneath three regional transects. Responses determined from colocated broadband and long-period magnetotelluric recording instruments enabled resistivity imaging to depths of > 300 km. Strike analysis and distortion decomposition on all data reveal a regional trend of 45-53°, but locally the geoelectric strike angle varies laterally and with depth. The 2-D models reveal a resistive upper crust to depths of 15-35 km that is underlain by a conductive layer that appears to be discontinuous at or near major mapped geological boundaries. Surface projections of the conductive layer coincide with areas of high grade, Archean metasedimentary rocks. Tectonic burial of these rocks and thickening of the crust occurred during the Paleoproterozoic Arrowsmith (2.3 Ga) and Trans-Hudson orogenies (1.85 Ga). Overall, the uppermost mantle of the Rae craton shows resistivity values that range from 3000 Ω m in the northeast (beneath Baffin Island and the Melville Peninsula) to 10,000 Ω m beneath the central Rae craton, to >50,000 Ω m in the south near the Hearne Domain. Near-vertical zones of reduced resistivity are identified within the uppermost mantle lithosphere that may be related to areas affected by mantle melt or metasomatism associated with emplacement of Hudsonian granites. A regional decrease in resistivities to values of 500 Ω m at depths of 180-220 km, increasing to 300 km near the southern margin of the Rae craton, is interpreted as the lithosphere-asthenosphere boundary.

The Origin and Mantle Dynamics of Quaternary Intraplate Volcanism in Northeast China From Joint Inversion of Surface Wave and Body Wave

NASA Astrophysics Data System (ADS)

Guo, Zhen; Wang, Kai; Yang, Yingjie; Tang, Youcai; John Chen, Y.; Hung, Shu-Huei

2018-03-01

We present a 3-D model of NE China by joint inversion of body and surface waves. The joint inversion significantly improves the resolution at shallow depths compared with body wave tomography alone and provides seismic evidence for the origin of Quaternary volcanism in NE China. Our model reveals that the mantle upwelling beneath the Changbaishan volcano originates from the transition zone and extends up to 60 km, and spreads at the base of the lithosphere with the upwelling head 5 times wider than the raising tail in the lower upper mantle. However, low velocities beneath the Halaha and Abaga volcanoes in the Xingmeng belt are confined to depths shallower than 150 km, suggesting that magmatism in the Xingmeng belt is more likely caused by localized asthenospheric upwelling at shallow depths rather than from the common deep source. A small-scale sublithospheric mantle convection may control the spatial and temporal distribution of Quaternary magmatism in NE China; that is, the upwelling beneath the Changbaishan volcano triggers the downwelling beneath the southern Songliao basin, where the high velocity imaged extends to 300 km. The downwelling may further induce localized upwelling in the surrounding areas, such as the Halaha and Abaga volcanoes. Thanks to the joint constraints from both surface and body waves, we can estimate the dimension of the convection cell. The convection cell is located between 42°N and 45°N, spreads around 500 km in the W-E direction measured from the distance between centers of downwelling and upwelling, and extends to 300 km vertically.

Upper mantle seismic anisotropy beneath the West Antarctic Rift System and surrounding region from shear wave splitting analysis

NASA Astrophysics Data System (ADS)

Accardo, Natalie J.; Wiens, Douglas A.; Hernandez, Stephen; Aster, Richard C.; Nyblade, Andrew; Huerta, Audrey; Anandakrishnan, Sridhar; Wilson, Terry; Heeszel, David S.; Dalziel, Ian W. D.

2014-07-01

We constrain azimuthal anisotropy in the West Antarctic upper mantle using shear wave splitting parameters obtained from teleseismic SKS, SKKS and PKS phases recorded at 37 broad-band seismometres deployed by the POLENET/ANET project. We use an eigenvalue technique to linearize the rotated and shifted shear wave horizontal particle motions and determine the fast direction and delay time for each arrival. High-quality measurements are stacked to determine the best fitting splitting parameters for each station. Overall, fast anisotropic directions are oriented at large angles to the direction of Antarctic absolute plate motion in both hotspot and no-net-rotation frameworks, showing that the anisotropy does not result from shear due to plate motion over the mantle. Further, the West Antarctic directions are substantially different from those of East Antarctica, indicating that anisotropy across the continent reflects multiple mantle regimes. We suggest that the observed anisotropy along the central Transantarctic Mountains (TAM) and adjacent West Antarctic Rift System (WARS), one of the largest zones of extended continental crust on Earth, results from asthenospheric mantle strain associated with the final pulse of western WARS extension in the late Miocene. Strong and consistent anisotropy throughout the WARS indicate fast axes subparallel to the inferred extension direction, a result unlike reports from the East African rift system and rifts within the Basin and Range, which show much greater variation. We contend that ductile shearing rather than magmatic intrusion may have been the controlling mechanism for accumulation and retention of such coherent, widespread anisotropic fabric. Splitting beneath the Marie Byrd Land Dome (MBL) is weaker than that observed elsewhere within the WARS, but shows a consistent fast direction, possibly representative of anisotropy that has been `frozen-in' to remnant thicker lithosphere. Fast directions observed inland from the Amundsen Sea appear to be radial to the dome and may indicate radial horizontal mantle flow associated with an MBL plume head and low upper mantle velocities in this region, or alternatively to lithospheric features associated with the complex Cenozoic tectonics at the far-eastern end of the WARS.

Seismic Discontinuities within the Crust and Mantle Beneath Indonesia as Inferred from P Receiver Functions

NASA Astrophysics Data System (ADS)

Woelbern, I.; Rumpker, G.

2015-12-01

Indonesia is situated at the southern margin of SE Asia, which comprises an assemblage of Gondwana-derived continental terranes, suture zones and volcanic arcs. The formation of SE Asia is believed to have started in Early Devonian. Its complex history involves the opening and closure of three distinct Tethys oceans, each accompanied by the rifting of continental fragments. We apply the receiver function technique to data of the temporary MERAMEX network operated in Central Java from May to October 2004 by the GeoForschungsZentrum Potsdam. The network consisted of 112 mobile stations with a spacing of about 10 km covering the full width of the island between the southern and northern coast lines. The tectonic history is reflected in a complex crustal structure of Central Java exhibiting strong topography of the Moho discontinuity related to different tectonic units. A discontinuity of negative impedance contrast is observed throughout the mid-crust interpreted as the top of a low-velocity layer which shows no depth correlation with the Moho interface. Converted phases generated at greater depth beneath Indonesia indicate the existence of multiple seismic discontinuities within the upper mantle and even below. The strongest signal originates from the base of the mantle transition zone, i.e. the 660 km discontinuity. The phase related to the 410 km discontinuity is less pronounced, but clearly identifiable as well. The derived thickness of the mantle-transition zone is in good agreement with the IASP91 velocity model. Additional phases are observed at roughly 33 s and 90 s relative to the P onset, corresponding to about 300 km and 920 km, respectively. A signal of reversed polarity indicates the top of a low velocity layer at about 370 km depth overlying the mantle transition zone.

P-wave tomography of Northeast Asia: Constraints on the western Pacific plate subduction and mantle dynamics

NASA Astrophysics Data System (ADS)

Ma, Jincheng; Tian, You; Liu, Cai; Zhao, Dapeng; Feng, Xuan; Zhu, Hongxiang

2018-01-01

A high-resolution model of 3-D P-wave velocity structure beneath Northeast Asia and adjacent regions is determined by using 244,180 arrival times of 14,163 local and regional earthquakes and 319,857 relative travel-time residuals of 9988 teleseismic events recorded at ∼2100 seismic stations in the study region. Our tomographic results reveal the subducting Pacific slab clearly as a prominent high-velocity anomaly from the Japan Trench to the North-South Gravity lineament (NSGL) in East China. The NSGL is roughly coincident with the western edge of the stagnant Pacific slab in the mantle transition zone (MTZ). The subducting Pacific slab has partly sunk into the lower mantle beneath Northeast China, but under the Sino-Korean Craton the slab lies horizontally in the MTZ. The NSGL, as an important tectonic line in Mainland China, is marked by sharp differences in the surface topography, gravity anomaly, crustal and lithospheric thickness and mantle seismic velocity from the east to the west. These features of the NSGL and large-scale hot and wet upwelling in the big mantle wedge (BMW) in the east of the NSGL are all related to the subduction processes of the Western Pacific plate. The Changbai intraplate volcanic group is underlain by a striking low-velocity anomaly from the upper MTZ and the BMW up to the surface, and deep earthquakes (410-650 km depths) occur actively in the subducting Pacific slab to the east of the Changbai volcano. We propose that the Changbai volcanic group is caused by upwelling of hot and wet asthenospheric materials and active convection in the BMW. The formation of other volcanic groups in the east of the NSGL is also associated with the subduction-driven corner flow in the BMW.

Structure of the lithosphere-asthenosphere system in the vicinity of the Tristan da Cunha hot spot as seen by surface waves

NASA Astrophysics Data System (ADS)

Bonadio, Raffaele; Geissler, Wolfram H.; Ravenna, Matteo; Lebedev, Sergei; Celli, Nicolas L.; Jokat, Wilfried; Jegen, Marion; Sens-Schönfelder, Christoph; Baba, Kiyoshi

2017-04-01

Tristan da Cunha is a volcanic island located above a hotspot in the South Atlantic. The deep mantle plume origin of the hotspot volcanism at the island is supported by anomalous geochemical data (Rohde et al., 2013 [1]) and global seismological evidences (French and Romanovicz, 2015 [2]). However, until recently, due to lack of local geophysical data in the South Atlantic and especially around Tristan da Cunha, the existence of a plume has not yet been confirmed. Therefore, an Ocean Bottom Seismometer experiment was carried out in 2012 and 2013 in the vicinity of the archipelago, with the aim of obtaining geophysical data that may help to get some more detailed insights into the structure of the upper mantle, possibly confirming the existence of a plume. In this work we study the shear wave velocity structure of the lithosphere-asthenosphere system beneath the Island. Rayleigh surface wave phase velocity dispersion curves have been obtained using a recent powerful implementation of the inter-station cross-correlation method (Meier et al., 2004 [3]; Soomro et al., 2016 [4]). The measured dispersion curves are used to invert for the 1D shear wave velocity structure beneath the study area and to obtain phase velocity tomographic maps. Our results show a pronounced low shear wave velocity anomaly between 70 and 120 km depth beneath the area; the lid shows high velocity, suggesting a cold, depleted and dehydrated shallow lithosphere, while the deeper lithosphere shows a velocity structure similar to young or rejuvenated Pacific oceanic lithosphere (Laske et al., 2011 [5]; Goes et al., 2012 [6]). Below the base of the lithosphere, shear wave velocities appear to be low, suggesting thermal effects and partial melting (as confirmed by petrological data). Decreasing velocities within the lithosphere south-westward reflect probably a thermal imprint of an underlying mantle plume. References [1] J.K. Rohde, P. van den Bogaard, K. Hoernle, F. Hauff, R. Werner, Evidence for an age progression along the Tristan-Gough volcanic track from new 40Ar/ 39Ar ages on phenocryst phases, Tectonophysics, Volume 604, p. 60-71 (2013). [2] S. French and B. Romanowicz, Broad plumes rooted at the base of the Earth's mantle beneath major hotspots, Nature, 525(7567), 95-99 (2015). [3] T. Meier, K. Dietrich, B. Stockhert and H. Harjes, One-dimensional models of shear wave velocity for the eastern Mediterranean obtained from the inversion of Rayleigh wave phase velocities and tectonic implications, Earth and Planetary Science Letters, 249(3), 415-424 (2004). [4] R.A. Soomro, C. Weidle, L. Cristiano, S. Lebedev, T. Meier and PASSEQ Working Group, Phase velocities of Rayleigh and Love waves in central and northern Europe from automated, broad-band, interstation measurements, Geophys. J. Int. (2016) 204, 517-534. [5] G. Laske, A. Markee, J.A. Orcutt, C.J. Wolfe, J.A. Collins and S.C. Solomon, R.S. Detrick, D. Bercovici and E.H. Hauri, Asymmetric shallow mantle structure beneath the Hawaiian Swell-evidence from Rayleigh waves recorded by the PLUME network, Geophys. J. Int. (2011) 187, 1725-1742. [6] S. Goes, J. Armitage, N. Harmon, H. Smith and R. Huismans, Low seismic velocities below mid-ocean ridges: Attenuation versus melt retention, Journal of geophysical research, Vol. 117, B12403, (2012).

Late Cretaceous - recent lithosphere scale evolution of Turkey: linking the crustal surface evolution to the structure of the mantle

NASA Astrophysics Data System (ADS)

Bartol, J.; Govers, R. M. A.; Wortel, M. J. R.

2015-12-01

Central Anatolia (Central Turkey) possesses all the characteristics of a plateau. It experienced a period of rapid and substantial uplift (late Miocene, ˜8 Ma) while significant crustal shortening did not occur. Similar to other plateaus, the presence of volcanic ash and tuff within the sediments suggest that uplift was preceded by widespread volcanism (˜14-9Ma). The lithospheric context of these events is, however, unknown. For the Eastern Anatolian plateau, similar events have been attributed to southward retread followed by slab break-off of the northern Neotethys slab. Recent tomographic results indicate that this northern Neotethys slab extended beneath both the Eastern and Central Anatolian plateau prior to late Miocene delamination and possibly even beneath western Anatolia prior to the Eocene (?). We propose a new lithospheric scenario for the regional evolution for the Aegean-Anatolia-Near East region that combines a recent compilation of surface geology data with the structure of the upper mantle imaged with tomography. In our new scenario for the evolution of the Aegean-Anatolia-Near East region, a single continuous subduction zone south of the Pontides (Izmir - Ankara - Erzincan crustal suture zone) accommodated the Africa - Eurasia convergence until the end of the late Cretaceous. In the Late Cretaceous - Eocene the northern Neotethys Ocean closed followed by Anatolide - Taurides (south) and Pontides (north) continental collision along the Izmir - Ankara - Erzincan crustal suture zone. While the trench jumped to the south of Anatolide - Taurides terrane, subduction continued beneath the Izmir-Ankara-Erzincan suture where the northern Neotethys slab continued to sink into the deeper mantle. In the early Miocene (˜20-15Ma), the northern Neotethys slab started to retreat southward towards the trench, resulting in delamination of the lithospheric mantle. The last part of (early Miocene - recent) our scenario is testable. We use a coupled thermal-flexural model of the lithosphere. Model results show that delamination can explain the average present-day long-wavelength topography of the Central Anatolian plateau. For the Eastern Anatolian plateau, delamination explains half the present-day elevation: the other half resulted from crustal thickening.

Multifrequency tomography of the La Reunion mantle plume with P and S waves using ocean bottom seismometers in the Indian Ocean

NASA Astrophysics Data System (ADS)

Tsekhmistrenko, M.; Sigloch, K.; Hosseini, K.

2017-12-01

The RHUM-RUM experiment (Reunion Hotspot Upper Mantle - Reunions Unterer Mantel) investigates the presence or absence of a whole mantle plume beneath the volcanic hotspot island of La Reunion. From 2011 to 2016, RHUM-RUM instrumented a 2000 km x 2000 km area of western Indian Ocean seafloor, islands and Madagascar with broadband seismometers and hydrophones. The central component was a 13-month deployment of 57 German and French Ocean Bottom Seismometers (OBS) in 2300-5600 m depth. This was supplemented by 2-3 years deployments of 37 island stations on Reunion, Mauritius, Rodrigues, the southern Seychelles, the Iles Eparses and southern Madagascar. Two partner projects contributed another 30+ stations on Madagascar. We present results of multifrequency P- and S-waveform tomography of the entire mantle column beneath the Reunion hotspot. We use all frequency passbands that efficiently transmit body waves and rise above the considerable noise floor of OBS measurements. More than 200 teleseismic events during the 13-month long OBS deployment yielded usable measurements, and another 400 events before and after. We present our methods, discuss data yield and quality of ocean-bottom versus island/land seismometers and hydrophones. 150,000 combined cross-correlations measurements were used in multifrequency P-wave tomography, in passbands between 30 s and 2.7 s dominant period. Cross-correlation coefficients at permanent and temporal land stations are generally higher than on OBS, which are more affected by both microseismic and self-noise. Hydrophones worked more reliably, but strong reverberations from the water column mean that they are still less usable than seismograms. All measurements of the RHUM-RUM array are embedded in a global P-wave inversion. Mantle structures obtained from this new, high resolution tomographic model of the La Reunion area are compared to existing tomographies. We also compare to local and global convection models in order to understand the relation between mantle flow and the development of mantle plumes through time.

Finite-frequency P-wave tomography of the Western Canada Sedimentary Basin: Implications for the lithospheric evolution in Western Laurentia

NASA Astrophysics Data System (ADS)

Chen, Yunfeng; Gu, Yu Jeffrey; Hung, Shu-Huei

2017-02-01

The lithosphere beneath the Western Canada Sedimentary Basin has potentially undergone Precambrian subduction and collisional orogenesis, resulting in a complex network of crustal domains. To improve the understanding of its evolutionary history, we combine data from the USArray and three regional networks to invert for P-wave velocities of the upper mantle using finite-frequency tomography. Our model reveals distinct, vertically continuous high (> 1%) velocity perturbations at depths above 200 km beneath the Precambrian Buffalo Head Terrane, Hearne craton and Medicine Hat Block, which sharply contrasts with those beneath the Canadian Rockies (<- 1%) at comparable depths. The P velocity increases from - 0.5% above 70 km depth to 1.5% at 330 km depth beneath southern Alberta, which provides compelling evidence for a deep, structurally complex Hearne craton. In comparison, the lithosphere is substantially thinner beneath the adjacent Buffalo Head Terrane (160 km) and Medicine Hat Block (200 km). These findings are consistent with earlier theories of tectonic assembly in this region, which featured distinct Archean and Proterozoic plate convergences between the Hearne craton and its neighboring domains. The highly variable, bimodally distributed craton thicknesses may also reflect different lithospheric destruction processes beneath the western margin of Laurentia.

Constraints on the viscosity of the continental crust and mantle from GPS measurements and postseismic deformation models in western Mongolia

USGS Publications Warehouse

Vergnolle, M.; Pollitz, F.; Calais, E.

2003-01-01

We use GPS measurements and models of postseismic deformation caused by seven M6.8 to 8.4 earthquakes that occurred in the past 100 years in Mongolia to assess the viscosity of the lower crust and upper mantle. We find an upper mantle viscosity between 1 ?? 1018 and 4 ?? 1018 Pa s. The presence of such a weak mantle is consistent with results from independent seismological and petrological studies that show an abnormally hot upper mantle beneath Mongolia. The viscosity of the lower crust is less well constrained, but a weak lower crust (3 ?? 1016 to 2 ?? 1017 Pa s) is preferred by the data. Using our best fit upper mantle and lower crust viscosities, we find that the postseismic effects of viscoelastic relaxation on present-day horizontal GPS velocities are small (<2 mm yr-1) but still persist 100 years after the 1905, M8.4, Bolnay earthquake. This study shows that the GPS velocity field in the Baikal-Mongolia area can be modeled as the sum of (1) a rigid translation and rotation of the whole network, (2) a 3-5 mm yr-1 simple shear velocity gradient between the Siberian platform to the north and northern China to the south, and (3) the contribution of postseismic deformation, mostly caused by the 1905 Bolnay-Tsetserleg sequence and by the smaller, but more recent, 1957 Bogd earthquake. Copyright 2003 by the American Geophysical Union.

3D modeling of magnetotelluric data unraveling the tectonic setting and sources of magmatism in the northeastern corner of Borborema Province, NE Brazil

NASA Astrophysics Data System (ADS)

Padilha, A. L.; Vitorello, I.; Padua, M. B.; Batista, J. C.; Fuck, R. A.

2017-12-01

The Borborema Province in northeast Brazil is a complex orogenic system formed by crustal blocks of different ages, origin and evolution amalgamated during the West Gondwana convergence in late Neoproterozoic-early Phanerozoic Brasiliano Orogeny. We discuss here new magnetotelluric (MT) data collected along four linear profiles crisscrossing the northeastern corner of the province to assess its deep electrical resistivity structure. Dimensionality analysis showed that a 3D electrical structure predominates in the subsurface and thus the data were modeled by a 3D MT data inversion scheme. The modeling revealed several subvertical discontinuities, with significant lateral contrast in the overall geoelectric structure, down to upper mantle depths. A major conductivity anomaly is registered in the crust beneath Neoproterozoic supracrustal rocks (Serido Group) and this anomaly deepens to upper mantle depths in the northwest direction below a zone of Paleoproterozoic plutons (Caico Complex). It has been suggested that the Serido Group was originally initiated as a sedimentary basin developed upon a Paleoproterozoic basement during a Neoproterozoic extension event related to a collisional foredeep of a south-dipping subduction slab, contrary to our northwest-dipping conductivity vergence. In case of the Caico Complex, because of the petrogenesis of its orthogneisses that indicates partial melting of a metasomatically enriched spinel-to garnet-bearing lherzolite with adakitic features, we also propose a subduction zone environment for its original magmatism. Considering the tenuous evidence indicating that this conductive anomaly could extend down into the upper mantle in the same region where teleseismic tomography register an attenuation of P waves, it can be concluded that this zone could also be the source of the metasomatic fluids and minerals observed along north-south Mesozoic volcanic plugs and flows of alkaline rocks and alkali basalts (Macau-Queimadas belt). In contrast to the general pattern in several parts of the province exhibiting a multitude of resistive and conductive zones marking the crust and upper mantle, an elongated resistive cratonic-like keel in the WSW-ENE direction is observed along the southeastern side of the study area.

Constraints on the rheology of the lower crust in a strike-slip plate boundary: evidence from the San Quintín xenoliths, Baja California, Mexico

NASA Astrophysics Data System (ADS)

van der Werf, Thomas; Chatzaras, Vasileios; Marcel Kriegsman, Leo; Kronenberg, Andreas; Tikoff, Basil; Drury, Martyn R.

2017-12-01

The rheology of lower crust and its transient behavior in active strike-slip plate boundaries remain poorly understood. To address this issue, we analyzed a suite of granulite and lherzolite xenoliths from the upper Pleistocene-Holocene San Quintín volcanic field of northern Baja California, Mexico. The San Quintín volcanic field is located 20 km east of the Baja California shear zone, which accommodates the relative movement between the Pacific plate and Baja California microplate. The development of a strong foliation in both the mafic granulites and lherzolites, suggests that a lithospheric-scale shear zone exists beneath the San Quintín volcanic field. Combining microstructural observations, geothermometry, and phase equilibria modeling, we estimated that crystal-plastic deformation took place at temperatures of 750-890 °C and pressures of 400-560 MPa, corresponding to 15-22 km depth. A hot crustal geotherm of 40 ° C km-1 is required to explain the estimated deformation conditions. Infrared spectroscopy shows that plagioclase in the mafic granulites is relatively dry. Microstructures are interpreted to show that deformation in both the uppermost lower crust and upper mantle was accommodated by a combination of dislocation creep and grain-size-sensitive creep. Recrystallized grain size paleopiezometry yields low differential stresses of 12-33 and 17 MPa for plagioclase and olivine, respectively. The lower range of stresses (12-17 MPa) in the mafic granulite and lherzolite xenoliths is interpreted to be associated with transient deformation under decreasing stress conditions, following an event of stress increase. Using flow laws for dry plagioclase, we estimated a low viscosity of 1.1-1.3×1020 Pa ṡ s for the high temperature conditions (890 °C) in the lower crust. Significantly lower viscosities in the range of 1016-1019 Pa ṡ s, were estimated using flow laws for wet plagioclase. The shallow upper mantle has a low viscosity of 5.7×1019 Pa ṡ s, which indicates the lack of an upper-mantle lid beneath northern Baja California. Our data show that during post-seismic transients, the upper mantle and the lower crust in the Pacific-Baja California plate boundary are characterized by similar and low differential stress. Transient viscosity of the lower crust is similar to the viscosity of the upper mantle.

Joint inversion of seismic and gravity data for imaging seismic velocity structure of the crust and upper mantle beneath Utah, United States

DOE PAGES

Syracuse, Ellen Marie; Zhang, Haijiang; Maceira, Monica

2017-07-11

Here, we present a method for using any combination of body wave arrival time measurements, surface wave dispersion observations, and gravity data to simultaneously invert for three-dimensional P- and S-wave velocity models. The simultaneous use of disparate data types takes advantage of the differing sensitivities of each data type, resulting in a comprehensive and higher resolution three-dimensional geophysical model. In a case study for Utah, we combine body waves first arrivals mainly from the USArray Transportable Array, Rayleigh wave group and phase velocity dispersion data, and Bouguer gravity anomalies to invert for crustal and upper mantle structure of the region.more » Results show clear delineations, visible in both P- and S-wave velocities, between the three main tectonic provinces in the region. In conclusion, without the inclusion of the surface wave and gravity constraints, these delineations are less clear, particularly for S-wave velocities. Indeed, checkerboard tests confirm that the inclusion of the additional datasets dramatically improves S-wave velocity recovery, with more subtle improvements to P-wave velocity recovery, demonstrating the strength of the method in successfully recovering seismic velocity structure from multiple types of constraints.« less

Joint inversion of seismic and gravity data for imaging seismic velocity structure of the crust and upper mantle beneath Utah, United States

DOE Office of Scientific and Technical Information (OSTI.GOV)

Syracuse, Ellen Marie; Zhang, Haijiang; Maceira, Monica

Here, we present a method for using any combination of body wave arrival time measurements, surface wave dispersion observations, and gravity data to simultaneously invert for three-dimensional P- and S-wave velocity models. The simultaneous use of disparate data types takes advantage of the differing sensitivities of each data type, resulting in a comprehensive and higher resolution three-dimensional geophysical model. In a case study for Utah, we combine body waves first arrivals mainly from the USArray Transportable Array, Rayleigh wave group and phase velocity dispersion data, and Bouguer gravity anomalies to invert for crustal and upper mantle structure of the region.more » Results show clear delineations, visible in both P- and S-wave velocities, between the three main tectonic provinces in the region. In conclusion, without the inclusion of the surface wave and gravity constraints, these delineations are less clear, particularly for S-wave velocities. Indeed, checkerboard tests confirm that the inclusion of the additional datasets dramatically improves S-wave velocity recovery, with more subtle improvements to P-wave velocity recovery, demonstrating the strength of the method in successfully recovering seismic velocity structure from multiple types of constraints.« less

Role of mantle dynamics in rebuilding the Tianshan Orogenic Belt in NW China: A seismic tomographic investigation

NASA Astrophysics Data System (ADS)

He, Chuansong; Santosh, M.

2018-05-01

The Tianshan orogenic belt, Junggar terrane and Altai terrane are located at the southwestern part of the Central Asian Orogenic Belt (CAOB). Here, we investigate the velocity structure beneath the Xinjiang region in NW China, which includes the Tarim terrane, Tianshan orogenic belt, Junggar terrane and Altai terrane with a view to evaluate the mantle dynamics based on teleseismic data recorded by 103 seismic stations. Our tomographic results show both high and low velocity perturbations beneath the Tianshan orogenic belt. We suggest that the high velocity perturbations beneath this orogenic belt might represent the northward subducted lithosphere of the Tarim Basin and the southward subducted lithosphere of the Junggar Basin. The low velocity structure beneath the Tianshan orogenic belt might represent asthenosphere upwelling that triggered the extensive magmatism which contributed to rebuilding of the Tianshan orogenic belt.

Upper-mantle deformation beneath the Pyrenean domain inferred from SKS splitting in northern Spain and southern France

NASA Astrophysics Data System (ADS)

Bonnin, Mickaël; Chevrot, Sébastien; Gaudot, Ianis; Haugmard, Méric

2017-08-01

We performed shear wave splitting analysis on 203 permanent (French RLPB, CEA and Catalonian networks) and temporary (PyrOPE and IberArray experiments) broad-band stations around the Pyrenees. These measurements considerably enhance the spatial resolution and coverage of seismic anisotropy in that region. In particular, we characterize with different shear wave splitting analysis methods the small-scale variations of splitting parameters ϕ and δt along three dense transects crossing the western and central Pyrenees with an interstation spacing of about 7 km. While we find a relatively coherent seismic anisotropy pattern in the Pyrenean domain, we observe abrupt changes of splitting parameters in the Aquitaine Basin and delay times along the Pyrenees. We moreover observe coherent fast directions despite complex lithospheric structures in Iberia and the Massif Central. This suggests that two main sources of anisotropy are required to interpret seismic anisotropy in this region: (i) lithospheric fabrics in the Aquitaine Basin (probably frozen-in Hercynian anisotropy) and in the Pyrenees (early and late Pyrenean dynamics); (ii) asthenospheric mantle flow beneath the entire region (imprint of the western Mediterranean dynamics since the Oligocene).

Upper-Mantle Deformation Beneath the Pyrenean Domain Inferred from SKS Splitting in Northern Spain and Southern France

NASA Astrophysics Data System (ADS)

Bonnin, M. J. A.; Chevrot, S.; Gaudot, I.; Haugmard, M.

2017-12-01

We performed shear wave splitting analysis on 203 permanent (French RLPB, CEA and Catalonian networks) and temporary (PYROPE and IberArray experiments) broad-band stations around the Pyrenees. These measurements considerably enhance the spatial resolution and coverage of seismic anisotropy in that region. In particular, we characterize with different shear wave splitting analysis methods the small-scale variations of splitting parameters φ and δt along three dense transects crossing the western and central Pyrenees with an interstation spacing of about 7 km. While we find a relatively coherent seismic anisotropy pattern in the Pyrenean domain, we observe abrupt changes of splitting parameters in the Aquitaine Basin and delay times along the Pyrenees. We moreover observe coherent fast directions despite complex lithospheric structures in Iberia and the Massif Central. This suggests that two main sources of anisotropy are required to interpret seismic anisotropy in this region: (i) lithospheric fabrics in the Aquitaine Basin (probably frozen-in Hercynian anisotropy) and in the Pyrenees (early and late Pyrenean dynamics); (ii) asthenospheric mantle flow beneath the entire region (imprint of the western Mediterranean dynamics since the Oligocene).

Geophysical investigations of the area between the Mid-Atlantic Ridge and the Barents Sea: From water to the lithosphere-asthenosphere system

NASA Astrophysics Data System (ADS)

Grad, Marek; Mjelde, Rolf; Krysiński, Lech; Czuba, Wojciech; Libak, Audun; Guterch, Aleksander

2015-03-01

As a part of the large international panel "IPY Plate Tectonics and Polar Gateways" within the "4th International Polar Year" framework, extensive geophysical studies were performed in the area of southern Svalbard, between the Mid-Atlantic Ridge and the Barents Sea. Seismic investigations were performed along three refraction and wide-angle reflection seismic lines. Integrated with gravity data the seismic data were used to determine the structure of the oceanic crust, the transition between continent and ocean (COT), and the continental structures down to the lithosphere-asthenosphere system (LAB). We demonstrate how modeling of multiple water waves can be used to determine the sound velocity in oceanic water along a seismic refraction profile. Our 2D seismic and density models documents 4-9 km thick oceanic crust formed at the Knipovich Ridge, a distinct and narrow continent-ocean transition (COT), the Caledonian suture zone between Laurentia and Barentsia, and 30-35 km thick continental crust beneath the Barents Sea. The COT west of southern Spitsbergen expresses significant excess density (more than 0.1 g/cm3 in average), which is characteristic for mafic/ultramafic and high-grade metamorphic rocks. The results of the gravity modeling show relatively weak correlation of the density with seismic velocity in the upper mantle, which suggests that the horizontal differences between oceanic and continental mantle are dominated by mineralogical changes, although a thermal effect is also present. The seismic velocity change with depth suggests lherzolite composition of the uppermost oceanic mantle, and dunite composition beneath the continental crust.

Structure of the mantle lithosphere beneath the Siberian kimberlite pipes reconstructed by monomineral thermobarometry

NASA Astrophysics Data System (ADS)

Ashchepkov, I. V.

2009-04-01

The original methods of the monomineral thermobarometry for clinopyroxene, garnet, ilmenite, chromite (Ashchepkov,2008) and orthopyroxene (Brey, Kohler, 1990- McGregor, 1974) thermobarometer allow to reconstruct the mantle columns. TP diagram for Udachnaya pipe suggests creation at least in tree stages of the melt percolation through the mantle column differing in Fe# and other parameters. The most high temperature (HT) (45 mvm-2) and Fe# rich refer to the last HT reactions with the protokimberlite melts formed the megacrystalline associations. Relict low temperature (LT) geotherm (35 45 mvm-2 and lower) is close to the conductive geotherm (Boyd et al., 1997). Most of ТР parameters for the minerals refer to the middle part of the geotherm (40- 45 mvm-2). Monomineral thermobarometry reconstructing the PTX values (Fe#; CrCpx, Cr-Ilm CaGar, TiChr) showing the high overlapping formed by the the melt percolation. The clinopyroxene growth in the mantle lithosphere in Daldyn, Akakite, Nakyn and Upper Muna are produced by the refertilization events under the influence of the protokimberlite melts. Their spreading in the lower part of mantle section of Garnet trend to subcalsic and pyroxenitic types is likely the result of submelting and heating of the mantle peridotites. Similar process for eclogites is responsible for the appearance of LT eclogites tracing subduction gradients and HT branches with the Ti- bearing associations corresponding to advective gradients. . For the larger pipes the scale of the perturbation is much higher then for smaller. The levels of the melt intrusions are reconstructed by the clotting of TP values inflections of TP paths and TiChr, CrIlm and Fe#. Ilmenite trends reveal the polybaric character of the fractionation and high degree interaction with the wall rock peridotites visible by CrIlm increase. The metasomatic associations differ in PTX diagrams by higher Cr and LT conditions the HT megacrystalls. The evident layered nature of the mantle columns (10-13) is reconstructed by the stepped TPX trends formed at first by the combinations of subduction and superplume events coinciding with the Re/Os ages (Spetsius, 2007), overprinted by the reactions with the plume and other percolating melts The Fe# increase near the 60 kbar refer to the last superplume events the previous leave similar rhythmic Fe- dunite horizons at 11-12 levels. The comparison of the compositions of minerals and reconstruction of mantle roots for several phases for Yubileinay, Udachnaya and Nyurbinskaya pipes allow to reveal the evolution of the magmatic sources and their interaction with the mantle lithosphere. Reconstruction of the mantle columns beneath 60 pipes allow to make the transsects of the kimberlite fields and the 3D model of the mantle beneath the dense kimberlite clusters with many close located diatrems Mesozoic mantle columns beneath the Anabar, Olenek, Aldan show the HT -Fe# alteration in 60-40 kbar due to interaction with the PT superplume, but relic and LT and low Fe# associations occurs to 60 kbar also. RBRF 05-05-74718, 06-05-65021, 06-05-64416.

Geodynamic Evolution of Subduction to Collision to Escape in Central Anatolia From Surface to Mantle - Results From the CD-CAT Project

NASA Astrophysics Data System (ADS)

Darin, Michael

2017-04-01

Despite significant progress toward understanding the kinematics of modern tectonic escape in Anatolia, considerable uncertainty remains regarding the dynamics of the transition from collision to escape. Because of the relatively small size of the Anatolia microplate, regional-scale studies spanning the plate margins and interior are well-suited to investigate the driving forces and space-time evolution of this unique tectonic transition in collisional orogens. CD-CAT (Continental Dynamics-Central Anatolia Tectonics) is a five-year (2011-2016) project funded by the National Science Foundation (USA) designed to explore the surface-to-mantle dynamics of Anatolia during the Cenozoic subduction-collision-escape transition in central Anatolia. Our approach integrates results from a diversity of methods including: structural, stratigraphic, and geomorphic analyses; magnetostratigraphy; low-temperature thermochronometry; Ar/Ar geochronology; geochemistry; passive seismic experiments (71 stations over two years); magnetotellurics; and numerical modeling. The principal results from this project include: recognition of a margin-wide magmatic lull from 40-20 Ma, followed by a southwestward migration of the initiation of magmatism toward and within the Central Anatolia Volcanic Province (CAVP); an early Miocene switch from contraction/transpression to extension/transtension in the Kırşehir and Niǧde Massifs, while contraction changed to late Miocene strike-slip deformation east of the Central Anatolian fault zone (CAFZ); rain shadow development due to uplift of the central Taurides 11-5 Ma; thin to absent lithospheric mantle beneath central Anatolia; the lack of an Arabia slab shallower than 800 km depth; and a change in the Cyprus slab from horizontal beneath the central Taurides and apparently fragmented beneath the CAVP, to very steeply dipping beneath the eastern Isparta Angle. The CAFZ lies along part of the Inner Tauride Suture (ITS) and represents a fundamental inherited lithosphere-scale structure that has accommodated contrasting magnitudes and styles of deformation to the east and west since Arabia collision. The coincidence of a similarly NNE-oriented lower plate boundary (Africa COB) or STEP fault between the Cyprus and Arabia slabs may have amplified the role of the CAFZ in controlling differential upper plate deformation. These findings support the following tectonic scenario: the first stage involved late Eocene to early Miocene horizontal subduction of the Afro-Arabia slab from central Anatolia to the Zagros, culminating in the final suturing of the Taurides and Pontides in Anatolia. The second stage occurred during the Miocene and involved the segmentation of the downgoing slab at the longitude of the CAFZ to form the Arabia slab in the east and the Cyprus slab in the west. North of Arabia, early Miocene rollback and foundering of the Arabia slab resulted in widespread volcanism, slab delamination beneath the eastern Taurides and eventual break-off and rapid sinking into the lower mantle starting at 15-10 Ma. North of Cyprus, initial rollback, steepening and breakup of the Cyprus slab are recorded by early Miocene upper plate extension and exhumation, followed by middle Miocene voluminous CAVP magmatism and uplift of the southern Taurides margin. The final stage involved a transition from diffuse to localized strain along transcurrent structures that have facilitated the westward escape of Anatolia since the latest Miocene-Pliocene.

Window into the Caledonian orogen: Structure of the crust beneath the East Shetland platform, United Kingdom

USGS Publications Warehouse

McBride, J.H.; England, R.W.

1999-01-01

Reprocessing and interpretation of commercial and deep seismic reflection data across the East Shetland platform and its North Sea margin provide a new view of crustal subbasement structure beneath a poorly known region of the British Caledonian orogen. The East Shetland platform, east of the Great Glen strike-slip fault system, is one of the few areas of the offshore British Caledonides that remained relatively insulated from the Mesozoic and later rifting that involved much of the area around the British Isles, thus providing an "acoustic window" into the deep structure of the orogen. Interpretation of the reflection data suggests that the crust beneath the platform retains a significant amount of its original Caledonian and older architecture. The upper to middle crust is typically poorly reflective except for individual prominent dipping reflectors with complex orientations that decrease in dip with depth and merge with a lower crustal layer of high reflectivity. The three-dimensional structural orientation of the reflectors beneath the East Shetland platform is at variance with Caledonian reflector trends observed elsewhere in the Caledonian orogen (e.g., north of the Scottish mainland), emphasizing the unique tectonic character of this part of the orogen. Upper to middle crustal reflectors are interpreted as Caledonian or older thrust surfaces that were possibly reactivated by Devonian extension associated with post-Caledonian orogenic collapse. The appearance of two levels of uneven and diffractive (i.e., corrugated) reflectivity in the lower crust, best developed on east-west-oriented profiles, is characteristic of the East Shetland platform. However, a north-south-oriented profile reveals an interpreted south-vergent folded and imbricated thrust structure in the lower crust that appears to be tied to the two levels of corrugated reflectivity on the east-west profiles. A thrust-belt origin for lower crustal reflectivity would explain its corrugated appearance. Regional seismic velocity models derived from refraction data suggest that this reflectivity correlates with a continuous lower crustal layer that has an intermediate seismic velocity. The lower crustal reflectivity is determined to be older than Mesozoic age by the bending down and truncation of the two reflectivity levels at the western margin of the North Sea Viking graben by a major mantle reflector inferred to be associated with Mesozoic rifting. The results of this study are thus in contrast with orthodox interpretations of the reflective layered lower crust as being caused by mantle-derived igneous intrusion or by deformation fabrics associated with stretching in response to continental rifting.

Imaging paleoslabs in the D″ layer beneath Central America and the Caribbean using seismic waveform inversion

PubMed Central

Borgeaud, Anselme F. E.; Kawai, Kenji; Konishi, Kensuke; Geller, Robert J.

2017-01-01

D″ (Dee double prime), the lowermost layer of the Earth’s mantle, is the thermal boundary layer (TBL) of mantle convection immediately above the Earth’s liquid outer core. As the origin of upwelling of hot material and the destination of paleoslabs (downwelling cold slab remnants), D″ plays a major role in the Earth’s evolution. D″ beneath Central America and the Caribbean is of particular geodynamical interest, because the paleo- and present Pacific plates have been subducting beneath the western margin of Pangaea since ~250 million years ago, which implies that paleoslabs could have reached the lowermost mantle. We conduct waveform inversion using a data set of ~7700 transverse component records to infer the detailed three-dimensional S-velocity structure in the lowermost 400 km of the mantle in the study region so that we can investigate how cold paleoslabs interact with the hot TBL above the core-mantle boundary (CMB). We can obtain high-resolution images because the lowermost mantle here is densely sampled by seismic waves due to the full deployment of the USArray broadband seismic stations during 2004–2015. We find two distinct strong high-velocity anomalies, which we interpret as paleoslabs, just above the CMB beneath Central America and Venezuela, respectively, surrounded by low-velocity regions. Strong low-velocity anomalies concentrated in the lowermost 100 km of the mantle suggest the existence of chemically distinct denser material connected to low-velocity anomalies in the lower mantle inferred by previous studies, suggesting that plate tectonics on the Earth’s surface might control the modality of convection in the lower mantle. PMID:29209659

Constraints on mantle melt geometries from body wave attenuation in the Salton Trough and Snake River Plain

NASA Astrophysics Data System (ADS)

Byrnes, J. S.; Bezada, M.

2017-12-01

Melt can be retained in the mantle at triple junctions between grain boundaries, be spread in thin films along two-grain boundaries, or be organized by shear into elongate melt-rich bands. Which of these geometries is most prevalent is unknown. This ambiguity makes the interpretation of anomalous seismic velocities and quality factors difficult, since different geometries would result in different mechanical effects. Here, we compare observations of seismic attenuation beneath the Salton Trough and the Snake River Plain; two regions where the presence of melt has been inferred. The results suggest that seismic attenuation is diagnostic of melt geometry. We measure the relative attenuation of P waves from deep focus earthquakes using a time-domain method. Even though the two regions are underlain by comparably strong low-velocity anomalies, their attenuation signature is very different. The upper mantle beneath the Salton Trough is sufficiently attenuating that the presence of melt must lower Qp, while attenuation beneath the Snake River Plain is not anomalous with respect to surrounding regions. These seemingly contradictory results can be reconciled if different melt geometries characterize each region. SKS splitting from the Salton Trough suggests that melt is organized into melt-rich bands, while this is not the case for the Snake River Plain. We infer that beneath the Snake River Plain melt is retained at triple junctions between grain boundaries, a geometry that is not predicted to cause seismic attenuation. More elongate geometries beneath the Salton Trough may cause seismic attenuation via the melt-squirt mechanism. In light of these results, we conclude that prior observations of low seismic velocities with somewhat high quality factors beneath the East Pacific Rise and Southern California suggest that melt does not organize into elongate bands across much of the asthenosphere.

Deformation of Tibetan Crust and Mantle and the Uplift of the Plateau: Insights from Broadband Surface Waves

NASA Astrophysics Data System (ADS)

Agius, M. R.; Lebedev, S.

2013-12-01

Seismic deployments over the last two decades have produced dense broadband data coverage across the Tibetan Plateau. Yet, the lithospheric dynamics of Tibet is still debated, with very different end-member models advocated to this day. Uncertainties over the anomalies in seismic tomography models contribute to the uncertainty of their interpretations, ranging from the subduction of India as far as northern Tibet to subduction of Asia there and to extreme viscous thickening of the entire Tibetan lithosphere. Within the lithosphere itself, a low-viscosity layer in the mid-lower crust is evidenced by many observations. It is still unclear, however, whether this layer accommodates a large-scale channel flow (which may have uplifted northern and eastern Tibet, according to one model) or if, instead, deformation within it is similar to that observed at the surface (which implies different uplift mechanisms). Broad-band surface waves provide resolving power from the upper crust down to the asthenosphere, for both isotropic-average shear-wave speeds (proxies for composition and temperature) and the radial and azimuthal shear-wave anisotropy (indicative of the patterns of deformation and flow). We measured highly accurate Love- and Rayleigh-wave phase-velocity curves in broad period ranges (up to 5-200 s) for a few tens of pairs and groups of stations across Tibet, combining, in each case, hundreds to thousands of inter-station measurements, made with cross-correlation and waveform-inversion methods. Robust shear-velocity profiles were then determined by series of non-linear inversions, yielding depth-dependent ranges of shear speeds and radial anisotropy consistent with the data. Temperature anomalies in the upper mantle were estimated from shear-velocity ones using accurate petro-physical relationships. Azimuthal anisotropy in the crust and upper mantle was determined by surface-wave tomography and, also, by sub-array analysis targeting the anisotropy amplitude. Our results show that the prominent high-velocity anomalies in the upper mantle are most consistent with the presence of subducted Indian lithosphere beneath large portions of Tibet. Estimated thermal anomalies within the high-velocity features match those expected for subducted India. The morphology of India's subduction beneath Tibet is complex and shows pronounced west-east variations. Beneath eastern and northeastern Tibet, in particular, the subducted Indian lithosphere appears to have subducted, at a shallow angle, hundreds of km NNE-wards. Azimuthal anisotropy beneath Tibet is distributed in multiple layers with different fast-propagations directions, which accounts for the complexity of published shear-wave splitting observations. The fast directions within the mid-lower crust are parallel to the extensional components of the current strain rate field at the surface, consistent with similar deformation through the entire crust, rather than channel flow. Anisotropy within the asthenosphere beneath northeastern Tibet (sandwiched between the Tibetan lithosphere above and the subducted Indian lithosphere below) indicates SSW-NNE flow, parallel to the direction of motion of the Indian Plate, including its subducted leading edge.

Broad plumes rooted at the base of the Earth's mantle beneath major hotspots.

PubMed

French, Scott W; Romanowicz, Barbara

2015-09-03

Plumes of hot upwelling rock rooted in the deep mantle have been proposed as a possible origin of hotspot volcanoes, but this idea is the subject of vigorous debate. On the basis of geodynamic computations, plumes of purely thermal origin should comprise thin tails, only several hundred kilometres wide, and be difficult to detect using standard seismic tomography techniques. Here we describe the use of a whole-mantle seismic imaging technique--combining accurate wavefield computations with information contained in whole seismic waveforms--that reveals the presence of broad (not thin), quasi-vertical conduits beneath many prominent hotspots. These conduits extend from the core-mantle boundary to about 1,000 kilometres below Earth's surface, where some are deflected horizontally, as though entrained into more vigorous upper-mantle circulation. At the base of the mantle, these conduits are rooted in patches of greatly reduced shear velocity that, in the case of Hawaii, Iceland and Samoa, correspond to the locations of known large ultralow-velocity zones. This correspondence clearly establishes a continuous connection between such zones and mantle plumes. We also show that the imaged conduits are robustly broader than classical thermal plume tails, suggesting that they are long-lived, and may have a thermochemical origin. Their vertical orientation suggests very sluggish background circulation below depths of 1,000 kilometres. Our results should provide constraints on studies of viscosity layering of Earth's mantle and guide further research into thermochemical convection.

Upper mantle structure at Walvis Ridge from Pn tomography

NASA Astrophysics Data System (ADS)

Ryberg, Trond; Braeuer, Benjamin; Weber, Michael

2017-10-01

Passive continental margins offer the unique opportunity to study the processes involved in continental extension and break-up. Within the LISPWAL (LIthospheric Structure of the Namibian continental Passive margin at the intersection with the Walvis Ridge from amphibious seismic investigations) project, combined on- and offshore seismic experiments were designed to characterize the Southern African passive margin at the Walvis Ridge in northern Namibia. In addition to extensive analysis of the crustal structures, we carried out seismic investigations targeting the velocity structure of the upper mantle in the landfall region of the Walvis Ridge with the Namibian coast. Upper mantle Pn travel time tomography from controlled source, amphibious seismic data was used to investigate the sub-Moho upper mantle seismic velocity. We succeeded in imaging upper mantle structures potentially associated with continental break-up and/or the Tristan da Cunha hotspot track. We found mostly coast-parallel sub-Moho velocity anomalies, interpreted as structures which were created during Gondwana break-up.

RHUM-RUM investigates La Réunion mantle plume from crust to core

NASA Astrophysics Data System (ADS)

Sigloch, Karin; Barruol, Guilhem

2013-04-01

RHUM-RUM (Réunion Hotspot and Upper Mantle - Réunions Unterer Mantel) is a French-German passive seismic experiment designed to image an oceanic mantle plume - or lack of plume - from crust to core beneath La Réunion Island, and to understand these results in terms of material, heat flow and plume dynamics. La Réunion hotspot is one of the most active volcanoes in the world, and its hotspot track leads unambiguously to the Deccan Traps of India, one of the largest flood basalt provinces on Earth, which erupted 65 Ma ago. The genesis and the origin at depth of the mantle upwelling and of the hotspot are still very controversial. In the RHUM-RUM project, 57 German and French ocean-bottom seismometers (OBS) are deployed over an area of 2000 km x 2000 km2 centered on La Réunion Island, using the "Marion Dufresne" and "Meteor" vessels. The one-year OBS deployment (Oct. 2012 - Oct. 2013) will be augmented by terrestrial deployments in the Iles Eparses in the Mozambique Channel, in Madagascar, Seychelles, Mauritius, Rodrigues and La Réunion islands. A significant number of OBS will be also distributed along the Central and South West Indian Ridges to image the lower-mantle beneath the hotspot, but also to provide independent opportunity for the study of these slow to ultra-slow ridges and of possible plume-ridge interactions. RHUM-RUM aims to characterize the vertically ascending flow in the plume conduit, as well as any lateral flow spreading into the asthenosphere beneath the western Indian Ocean. We want to establish the origin of the heat source that has been fueling this powerful hotspot, by answering the following questions: Is there a direct, isolated conduit into the deepest mantle, which sources its heat and material from the core-mantle boundary? Is there a plume connection to the African superswell at mid-mantle depths? Might the volcanism reflect merely an upper mantle instability? RHUM-RUM also aims at studying the hotspot's interaction with the neighboring ridges of the Indian Ocean. There is in particular a long-standing hypothesis, not yet examined seismically, that channelized plume flow beneath the aseismic Rodrigues Ridge could feed the Central Indian Ridge at 1000 km distance. The RHUM-RUM group (www.rhum-rum.net): * IPG Paris & Géosciences Réunion: G. Barruol, J.P. Montagner, E. Stutzmann, F.R. Fontaine, C. Deplus, M. Cannat, G. Roult, J. Dyment, S. Singh, W. Crawford, C. Farnetani, N. Villeneuve, L. Michon. V. Ferrazzini, Y. Capdeville. * Univ. Munich (LMU): K. Sigloch, H. Igel. AWI Bremerhaven: V. Schlindwein. Univ. Frankfurt: G. Rümpker. Univ. Münster: C. Thomas. Univ. Bonn: S. Miller. * Géosciences Montpellier: C. Tiberi, A. Tommasi, D. Arcay, C. Thoraval. * Mauritius Oceanography Institute: D. Bissessur. Univ. Antananarivo: G. Rambolamanana. SEYPEC Seychelles Petroleum: P. Samson, P. Joseph. * Other institutes: A. Davaille, M. Jegen, M. Maia, G. Nolet, D. Sauter, B. Steinberger.

RHUM-RUM investigates La Réunion mantle plume from crust to core

NASA Astrophysics Data System (ADS)

Sigloch, K.; Barruol, G.

2012-12-01

RHUM-RUM (Réunion Hotspot and Upper Mantle - Réunions Unterer Mantel) is a French-German passive seismic experiment designed to image an oceanic mantle plume - or lack of plume - from crust to core beneath La Réunion Island, and to understand these results in terms of material, heat flow and plume dynamics. La Réunion hotspot is one of the most active volcanoes in the world, and its hotspot track leads unambiguously to the Deccan Traps of India, one of the largest flood basalt provinces on Earth, which erupted 65 Ma ago. The genesis and the origin at depth of the mantle upwelling and of the hotspot are still very controversial. In the RHUM-RUM project, 57 German and French ocean-bottom seismometers (OBS) are deployed over an area of 2000 km x 2000 km2 centered on La Réunion Island, using the "Marion Dufresne" and "Meteor" vessels. The one-year OBS deployment (Oct. 2012 - Oct. 2013) will be augmented by terrestrial deployments in the Iles Eparses in the Mozambique Channel, in Madagascar, Seychelles, Mauritius, Rodrigues and La Réunion islands. A significant number of OBS will be also distributed along the Central and South West Indian Ridges to image the lower-mantle beneath the hotspot, but also to provide independent opportunity for the study of these slow to ultra-slow ridges and of possible plume-ridge interactions. RHUM-RUM aims to characterize the vertically ascending flow in the plume conduit, as well as any lateral flow spreading into the asthenosphere beneath the western Indian Ocean. We want to establish the origin of the heat source that has been fueling this powerful hotspot, by answering the following questions: Is there a direct, isolated conduit into the deepest mantle, which sources its heat and material from the core-mantle boundary? Is there a plume connection to the African superswell at mid-mantle depths? Might the volcanism reflect merely an upper mantle instability? RHUM-RUM also aims at studying the hotspot's interaction with the neighboring ridges of the Indian Ocean. There is in particular a long-standing hypothesis, not yet examined seismically, that channelized plume flow beneath the aseismic Rodrigues Ridge could feed the Central Indian Ridge at 1000 km distance. The RHUM-RUM group (www.rhum-rum.net): * IPG Paris & Géosciences Réunion: G. Barruol, J.P. Montagner, E. Stutzmann, F.R. Fontaine, C. Deplus, M. Cannat, G. Roult, J. Dyment, S. Singh, W. Crawford, C. Farnetani, N. Villeneuve, L. Michon. V. Ferrazzini, Y. Capdeville. * Univ. Munich (LMU): K. Sigloch, H. Igel. AWI Bremerhaven: V. Schlindwein. Univ. Frankfurt: G. Rümpker. Univ. Münster: C. Thomas. Univ. Bonn: S. Miller. * Géosciences Montpellier: C. Tiberi, A. Tommasi, D. Arcay, C. Thoraval. * Mauritius Oceanography Institute: D. Bissessur. Univ. Antananarivo: G. Rambolamanana. SEYPEC Seychelles Petroleum: P. Samson, P. Joseph. * Other institutes: A. Davaille, M. Jegen, M. Maia, G. Nolet, D. Sauter, B. Steinberger.

A review of the regional geophysics of the Arizona Transition Zone

NASA Technical Reports Server (NTRS)

Hendricks, J. D.; Plescia, J. B.

1991-01-01

A review of existing geophysical information and new data presented in this special section indicate that major changes in crustal properties between the Basin and Range and Colorado Plateau occur in, or directly adjacent to, the region defined as the Arizona Transition Zone. Although this region was designated on a physiographic basis, studies indicate that it is also the geophysical transition between adjoining provinces. The Transition Zone displays anomalous crustal and upper mantle seismic properties, shallow Curie isotherms, high heat flow, and steep down-to-the-plateau Bouguer gravity gradients. Seismic and gravity studies suggest that the change in crustal thickness, from thin crust in the Basin and Range to thick crust in the Colorado Plateau, may occur as a series of steps rather than a planar surface. Anomalous P wave velocities, high heat flow, shallow Curie isotherms, and results of gravity modeling suggest that the upper mantle is heterogeneous in this region. A relatively shallow asthenosphere beneath the Basin and Range and Transition Zone contrasted with a thick lithosphere beneath the Colorado Plateau would be one explanation that would satisfy these geophysical observations.

Crustal structure of central Lake Baikal: Insights into intracontinental rifting

USGS Publications Warehouse

ten Brink, Uri S.; Taylor, M.H.

2002-01-01

The Cenozoic rift system of Baikal, located in the interior of the largest continental mass on Earth, is thought to represent a potential analog of the early stage of breakup of supercontinents. We present a detailed P wave velocity structure of the crust and sediments beneath the Central Basin, the deepest basin in the Baikal rift system. The structure is characterized by a Moho depth of 39-42.5 km; an 8-km-thick, laterally continuous high-velocity (7.05-7.4 km/s) lower crust, normal upper mantle velocity (8 km/s), a sedimentary section reaching maximum depths of 9 km, and a gradual increase of sediment velocity with depth. We interpret the high-velocity lower crust to be part of the Siberian Platform that was not thinned or altered significantly during rifting. In comparison to published results from the Siberian Platform, Moho under the basin is elevated by <3 km. On the basis of these results we propose that the basin was formed by upper crustal extension, possibly reactivating structures in an ancient fold-and-thrust belt. The extent and location of upper mantle extension are not revealed by our data, and it may be offset from the rift. We believe that the Baikal rift structure is similar in many respects to the Mesozoic Atlantic rift system, the precursor to the formation of the North Atlantic Ocean. We also propose that the Central Baikal rift evolved by episodic fault propagation and basin enlargement, rather than by two-stage rift evolution as is commonly assumed.

The mantle beneath the Red Sea margin: xenoliths from western Saudi Arabia

NASA Astrophysics Data System (ADS)

McGuire, Anne Vaughan

1988-07-01

Xenoliths from alkali basalts in western Saudi Arabia provide the opportunity to study the composition and rheology of the mantle beneath the Red Sea rift margins. Characteristics of mantle xenolith suites from each of three localities in western Saudi Arabia can be related to locality position relative to the rift axis, and to crustal thickness and heat flow at each locality. Mantle xenoliths from Harrat al Birk, nearest the rift axis, are dominantly websterites (± spinel, plagioclase, amphibole, olivine), garnet clinopyroxenite, and two-pyroxene gabbro (± olivine); peridotite xenoliths, are rare. Garnet clinopyroxenites contain zoned clinopyroxene with Fe-Al-rich rims and reaction rims on garnet formed by breakdown of garnet to orthopyroxene + clinopyroxene + spinel + plagioclase. Zoning and reaction rims are interpreted as forming under conditions of increasing temperature. Thermobarometry on Harrat al Birk garnet-bearing xenoliths yield high temperatures (1015-1040°C) at about 12 kbar. The abundance of plagioclase-bearing assemblages may be related to a relatively shallow upper mantle which extends up into stability fields for plagioclase-bearing pyroxenite and peridotite. Harrat al Kishb and Harrat Hutaymah are farther from the Red Sea axis, on the flanks of the rift. The mantle xenolith suites of al Kishb and Hutaymah are similar, consisting of abundant spinel peridotite and spinel pyroxenite xenoliths and minor garnet pyroxenites; plagioclase-bearing xenoliths are extremely rare. The Harrat Hutaymah suite includes wehrlite and amphibole-bearing peridotite lithologies not found at al Kishb. Variation of peridotite composition may reflect varying degrees of partial melt extraction. Igneous textures of some pyroxenite xenoliths and structural relationships in composite peridotite/pyroxenite nodules suggest that pyroxenites formed by crystallization of magmas within mantle veins. Abundant pyroxenites and fragments of amphibole veins reflect the activity of magmas and hydrous fluids within the mantle. Thermobarometry of al Kishb and Hutaymah garnet-bearing nodules yield temperatures of 1000-1050 °C at pressures of about 13.5-16.0 kbar. Mineral zoning and exsolution features indicate decreasing temperature conditions in the mantle at the flanks of the Red Sea rift.

Deep electrical resistivity structure of northwestern Costa Rica

NASA Astrophysics Data System (ADS)

Brasse, H.; Kapinos, G.; Mütschard, L.; Alvarado, G. E.; Worzewski, T.; Jegen, M.

2009-01-01

First long-period magnetotelluric investigations were conducted in early 2008 in northwestern Costa Rica, along a profile that extends from the coast of the Pacific Ocean, traverses the volcanic arc and ends currently at the Nicaraguan border. The aim of this study is to gain insight into the electrical resistivity structure and thus fluid distribution at the continental margin where the Cocos plate subducts beneath the Caribbean plate. Preliminary two-dimensional models map the only moderately resistive mafic/ultramafic complexes of the Nicoya Peninsula (resistivity of a few hundred Ωm), the conductive forearc and the backarc basins (several Ωm). Beneath the backarc basin the data image a poor conductor in the basement with a clear termination in the south, which may tentatively be interpreted as the Santa Elena Suture. The volcanic arc shows no pronounced anomaly at depth, but a moderate conductor underlies the backarc with a possible connection to the upper mantle. A conductor at deep-crustal levels in the forearc may reflect fluid release from the downgoing slab.

3D Density Structure of Oceanic Lithosphere Affected by A Plume: A Case Study from the Greater Jan Mayen-East Greenland Region (NE Atlantic)

NASA Astrophysics Data System (ADS)

Tan, P.; Sippel, J.; Breivik, A. J.; Scheck-Wenderoth, M.; Meeßen, C.

2017-12-01

Unraveling the density structure of the oceanic lithosphere north of Iceland is key for understanding the effects of the Iceland Plume on the mid-ocean ridges of the greater Jan Mayen-East Greenland Region. We use a data-integrative approach for 3D gravity modeling to develop new insights into the crust and upper mantle density structure of this region. First, we obtain the 3D density structure of the sediments and crust from interpretations of regional reflection and refraction seismic lines. Then, the temperature and density structure of the mantle between 50 and 250 km are derived from a published shear-wave velocity (Vs) tomography model. To assess the density configuration between the Moho and 50 km depth, we follow a combined forward and inverse 3D gravity modeling approach. The Vs tomography and derived density of the deeper mantle (>50 km depth) reveal that the low-density anomaly related to the Iceland plume gets weaker with increasing distance from the plume, i.e. from the strongly influenced Middle Kolbeinsey Ridge (MKR) to the Mohn's Ridge. The West Jan Mayen Fracture Zone is identified as a main mantle density contrast, indicative of differences in the thermal evolution of the ridge systems it separates. Beneath the MKR region, the low-density anomaly at depths of >50 km continues upwards into the uppermost mantle, where its lateral dimensions narrow considerably. This elongated density anomaly is consistent with a basement high and indicates a channelization of the Iceland plume effects. The NE-SW elongated mantle anomaly does not, however, coincide with the topographical NNE-SSW striking ridge axis. Thus, the modelled plume-affected oceanic lithosphere reveals discrepancies with the half-space cooling model. We discuss the 3D density model in terms of such spatial relations between deeper mantle anomalies and the shallow crustal structure.

Anisotropy in subduction zones: Insights from new source side S wave splitting measurements from India

NASA Astrophysics Data System (ADS)

Roy, Sunil K.; Kumar, M. Ravi; Davuluri, Srinagesh

2017-08-01

This study presents 106 splitting and 40 null measurements of source side anisotropy in subduction zones, utilizing direct S waves registered at two stations sited on the Indian continent, which show null shear wave splitting measurements for SKS phases. Our results suggest that trench-parallel anisotropy is dominant beneath the Philippines, Mariana, Izu-Bonin, and edge of the Java slab, while plate motion-parallel anisotropy is observed beneath the Solomon, Aegean, Japan, and Java slabs. Results from Kuril and Aleutian regions reveal trench-oblique anisotropy. We chose to interpret these observations primarily in terms of mantle flow beneath a subduction zone. While the two-dimensional (2-D) slab entrained flow model offers a simple explanation for trench-normal fast polarization azimuths (FPA), the trench-parallel FPA can be reconciled by extension due to slab rollback. The model that invokes age of the subducting lithosphere can explain anisotropy in the subslab, derived from rays recorded at the updip stations. However, when downdip stations are used, contributions from the slab and supraslab need to be considered. In Japan, anisotropy in the subslab mantle shallower than 300 km might be associated with trench-parallel mantle flow resulting in the alignment of FPA in the same direction. Anisotropy in the deeper part, above the transition zone, is probably associated with 2-D flow resulting in trench-normal FPA. Anisotropy in the Mariana Trench might be associated with trench-parallel mantle flow in the supraslab region, with similar deformation in the upper mantle and the transition zone.

Detailed Configuration of the Underthrusting Indian Lithosphere Beneath Western Tibet Revealed by Receiver Function Images

NASA Astrophysics Data System (ADS)

Xu, Qiang; Zhao, Junmeng; Yuan, Xiaohui; Liu, Hongbing; Pei, Shunping

2017-10-01

We analyze the teleseismic waveform data recorded by 42 temporary stations from the Y2 and ANTILOPE-1 arrays using the P and S receiver function techniques to investigate the lithospheric structure beneath western Tibet. The Moho is reliably identified as a prominent feature at depths of 55-82 km in the stacked traces and in depth migrated images. It has a concave shape and reaches the deepest location at about 80 km north of the Indus-Yarlung suture (IYS). An intracrustal discontinuity is observed at 55 km depth below the southern Lhasa terrane, which could represent the upper border of the eclogitized underthrusting Indian lower crust. Underthrusting of the Indian crust has been widely observed beneath the Lhasa terrane and correlates well with the Bouguer gravity low, suggesting that the gravity anomalies in the Lhasa terrane are induced by topography of the Moho. At 20 km depth, a midcrustal low-velocity zone (LVZ) is observed beneath the Tethyan Himalaya and southern Lhasa terrane, suggesting a layer of partial melts that decouples the thrust/fold deformation of the upper crust from the shortening and underthrusting in the lower crust. The Sp conversions at the lithosphere-asthenosphere boundary (LAB) can be recognized at depths of 130-200 km, showing that the Indian lithospheric mantle is underthrusting with a ramp-flat shape beneath southern Tibet and probably is detached from the lower crust immediately under the IYS. Our observations reconstruct the configuration of the underthrusting Indian lithosphere and indicate significant along strike variations.

Understanding the geodynamic setting of São Miguel, Azores: A peculiar bit of mantle in the Central Atlantic

NASA Astrophysics Data System (ADS)

Wilson, M.; Houlie, N.; Khan, A.; Lithgow-Bertelloni, C. R.

2012-12-01

The Azores Plateau and Archipelago in the Central Atlantic Ocean has traditionally been considered as the surface expression of a deep mantle plume or hotspot that has interacted with a mid-ocean ridge. It is geodynamically associated with the triple junction between the North American, African and Eurasian plates. (Yang et al., 2006) used finite frequency seismic tomography to demonstrate the presence of a zone of low P-wave velocities (peak magnitude -1.5%) in the uppermost 200km of the mantle beneath the plateau. The tomographic model is consistent with SW deflection of a mantle plume by regional upper mantle shear flow driven by absolute plate motions. The volcanic island of Sao Miguel is located within the Terceira Rift, believed to represent the boundary between the African and Eurasian plates; magmatic activity has been characterised by abundant basaltic eruptions in the past 30,000 years. The basalts are distinctive within the spectrum of global ocean island basalts for their wide range in isotopic composition, particularly in 87Sr/86Sr. Their Sr-Nd-Pb isotopic compositions show systematic variations from west to east across the island which can be interpreted in terms of melting of a two-component mantle source. The low melting point (enriched) component in the source has been attributed to recycled ancient (~3 Ga) oceanic crust(Elliott et al., 2007). Using the thermo-barometry approach of (Lee et al., 2009) we demonstrate that the pressure and temperature of magma generation below Sao Miguel increase from west (2 GPa, 1425 °C) to east (3.8 GPa, 1575 °C), consistent with partial melting along a mantle geotherm with a potential temperature of ~ 1500 °C. This is consistent with the magnitude of the thermal anomaly beneath the Azores Plateau (ΔT ~ 150-200 °C) inferred on the basis of the seismic tomography study. The site of primary magma generation extends from the base of the local lithosphere (~ 50 km) to ~ 125 km depth. To understand the geodynamic setting of the Sao Miguel magmatism we combine GPS data and mantle convection models with our interpretation of the geochemistry of the basalts. We demonstrate strong south-westerly and downward flow in the asthenospheric mantle above the Transition Zone (410 km seismic discontinuity), consistent with a zone of upper mantle shearing below the base of the lithosphere. The maximum flow velocity is broadly consistent with the depth of magma generation. The advection of the mantle with respect to the oceanic plate "moves" an isotopically distinct mantle source component beneath the active volcanoes of Sao Miguel and carries its previous melting residues to the south-west. We discuss the nature of this mantle source and its contribution to the mantle velocity anomalies determined by seismic tomography. This study opens-up new perspectives for seismic tomography and potentially new connections between the fields of geophysics and geochemistry in oceanic domains.

A two-dimensional hybrid method for modeling seismic waves propagation in laterally-varying anisotropic media and its application to central Tibet

NASA Astrophysics Data System (ADS)

Zhao, L.; Wen, L.

2009-12-01

The shear wave splitting measurements provide important information on mantle flow, deformation and mineralogy. They are now routinely made using the method developed by Silver and Chan (1994). More and more dense regional observations also begin to reveal sharp spatial variations of seismic anisotropy which could not be explained by simplified horizontal homogeneous anisotropic structures. To better constrain the mantle anisotropy beneath those regions, we developed a two-dimensional hybrid method for simulating seismic wave propagation in laterally-varying anisotropic media [Zhao et al., 2008]. In this presentation, we apply the method to study anisotropic structures beneath central Tibet by waveform modeling the teleseismic SKS phases recorded in the International Deep Profiling of Tibet and the Himalayas project (INDEPTH) III. Using data from two events that were selected such that the stations and sources can be approximated as a two-dimensional profile, we derived an optimal model for the anisotropic structures of the upper mantle beneath the study region: a 50-70 km thick anisotropic layer with a fast direction trending N95°E beneath the Qiangtang block, a 150 km thick and 60 km wide anisotropic segment with an axis trending N95°E beneath the northernmost Lhasa block, and a ~30 km wide transition zone in between within which the fast direction trends N45°E and the depth extent of anisotropy decreases northward sharply. Synthetic waveform modeling further suggests that an anisotropic model with a horizontal symmetry axis can explain the observations better than that with a dipping symmetry, and a low velocity zone possibly underlies or mixes with the anisotropic structures in the northern portion of the region. The optimal model yields synthetic seismograms that are in good agreement with the observations in both amplitudes and relative arrival times of SKS phases. Synthetic tests also indicate that different elastic constants, source parameters and depth extents of anisotropy adopted in the calculations do not affect the general conclusions, although trade-offs exist between the model parameters. Our modeling results suggest that, if the complex seismic structures in central Tibet are associated with the underthrusting of the Indian lithosphere beneath the Asian lithosphere, the inferred horizontal symmetry of anisotropy was likely generated during the collision because an inherited anisotropy would have a dipping angle of symmetry axis that is parallel to the underthrusting direction. References Silver, P. G., and M. K. Savage (1994), The interpretation of shear-wave splitting parameters in the presence of two anisotropic layers, Geophys. J. Int., 119, 949-963. Zhao L., L.X. Wen, L. Chen, T.Y. Zheng (2008). A two-dimensional hybrid method for modeling seismic wave propagation in anisotropic media, J. Geophys. Res., 113, B12307, doi:10.1029/2008JB005733.

Thermal structure and geodynamics of subduction zones

NASA Astrophysics Data System (ADS)

Wada, Ikuko

The thermal structure of subduction zones depends on the age-controlled thermal state of the subducting slab and mantle wedge flow. Observations indicate that the shallow part of the forearc mantle wedge is stagnant and the slab-mantle interface is weakened. In this dissertation, the role of the interface strength in controlling mantle wedge flow, thermal structure, and a wide range of subduction zone processes is investigated through two-dimensional finite-element modelling and a global synthesis of geological and geophysical observations. The model reveals that the strong temperature-dependence of the mantle strength always results in full slab-mantle decoupling along the weakened part of the interface and hence complete stagnation of the overlying mantle. The interface immediately downdip of the zone of decoupling is fully coupled, and the overlying mantle is driven to flow at a rate compatible with the subduction rate. The sharpness of the transition from decoupling to coupling depends on the rheology assumed and increases with the nonlinearity of the flow system. This bimodal behaviour of the wedge flow gives rise to a strong thermal contrast between the cold stagnant and hot flowing parts of the mantle wedge. The maximum depth of decoupling (MDD) thus dictates the thermal regime of the forearc. Observed surface heat flow patterns and petrologically and geochemically estimated mantle wedge temperatures beneath the volcanic arc require an MDD of 70--80 km in most, if not all, subduction zones regardless of their thermal regime of the slab. The common MDD of 70--80 km explains the observed systematic variations of the petrologic, seismological, and volcanic processes with the thermal state of the slab and thus explains the rich diversity of subduction zones in a unified fashion. Models for warm-slab subduction zones such as Cascadia and Nankai predict shallow dehydration of the slab beneath the cold stagnant part of the mantle wedge, which provides ample fluid for mantle wedge serpentinization in the forearc but little fluid for melt generation beneath the arc. In contrast, models for colder-slab subduction zones such as NE Japan and Kamchatka predict deeper dehydration, which provides greater fluid supply for melt generation beneath the arc and allows deeper occurrence of intraslab earthquakes but less fluid for forearc mantle wedge serpentinization. The common MDD also explains the intriguing uniform configuration of subduction zones, that is, the volcanic arc always tends to be situated where the slab is at about 100 km depth. The sudden onset of mantle wedge flow downdip of the common MDD overshadows the thermal effect of the slab, and the resultant thermal field and slab dehydration control the location of the volcanic arc. The recognition of the fundamental importance of the MDD has important implications to the study of geodynamics and earthquake hazard in subduction zones.

Uppermost Mantle Deformation and Hydration Beneath the Gorda Plate Inferred from Pn Travel-times

NASA Astrophysics Data System (ADS)

VanderBeek, B. P.; Toomey, D. R.

2017-12-01

Deformation of the uppermost oceanic mantle is thought to occur primarily in response to divergence beneath mid-ocean ridges with little subsequent deformation off-axis. A notable exception to this is the Gorda plate where sinuous magnetic anomalies and numerous intra-plate earthquakes indicate diffuse, plate-wide deformation. Thus, the Gorda region provides a natural laboratory to investigate the non-rigid behavior of tectonic plates. We invert Pn (the seismic head wave refracted below the Moho) arrival times from 770 local earthquakes for epicentral and mantle anisotropic velocity parameters to understand how the surficial pattern of deformation translates into the uppermost 10 km of the mantle. Specifically, we ask does the pattern of seismic anisotropy reflect spreading-induced fabrics or has it been re-worked by extensive deformation of the Gorda plate? If it has been re-worked, does it reflect pervasive faulting of the uppermost mantle or plate-scale ductile deformation? And, are isotropic velocities anomalously slow suggesting significant mantle hydration? Preliminary results show that the average mantle velocity beneath Gorda is 7.55 km/s. Velocities vary azimuthally by 4% and the fast-propagation direction is sub-parallel to Pacific absolute plate motion (APM). In comparison, the uppermost mantle beneath the Juan de Fuca (JdF) plate is characterized by 4.6% anisotropy with a mean velocity of 7.85 km/s [VanderBeek and Toomey, 2017]; the fast propagation direction trends between the paleo-spreading direction and JdF APM. The reduced Gorda velocities may indicate a greater extent of fault-controlled hydration of the shallow mantle compared to the JdF plate. In both regions, the anisotropic structure argues against the notion that shallow mantle deformation ceases away from the ridge. Instead, shearing across Gorda due to differential motion between the Pacific and JdF plates [e.g. Bodmer et al., 2015] may cause broad scale ductile deformation and the realignment of shallow mantle fabrics. Beneath the JdF plate, the anisotropic signal is inferred to track the evolution of mantle flow as it evolves from divergence at the ridge to simple shear that is more closely aligned with APM. We discuss the rheologic implications of these observations and the patterns of mantle flow and deformation in Cascadia.

The Hellenic Subduction Zone: A tomographic image and its geodynamic implications

NASA Astrophysics Data System (ADS)

Spakman, W.; Wortel, M. J. R.; Vlaar, N. J.

1988-01-01

New tomographic images of the Hellenic subduction zone demonstrate slab penetration in the Aegean Upper Mantle to depths of at least 600 km. Beneath Greece the lower part of the slab appears to be detached at a depth of about 200 km whereas it still seems to be unruptured beneath the southern Aegean. Schematically we derive minimum time estimates for the duration of the Hellenic subduction zone that range from 26 to 40 Ma. This is considerably longer than earlier estimates which vary between 5 and about 13 Ma.

Magma plumbing in the Grímsvötn volcanic system, Iceland: an overview

NASA Astrophysics Data System (ADS)

Thordarson, T.

2016-12-01

The basaltic Grímsvötn volcanic system (GVS) consists of Grímsvötn central volcano (GCV) and an immature fissure swarm extending 70 km to the southwest from GCV. The GCV has the highest eruption frequency of all central volcanos in Iceland, or 7 events per 100 years. In contrast, the GVS fissure swarm has only featured two events in postglacial times, the 1783-4 Laki and the prehistoric Lambavatnsgígar fissure eruptions. These two events account for 25% of the total Holocene magma output from the GVS and 80% of the output in historic time (i.e. last 1100 years). Although GVS magma plumbing has been a topic of research for four decades, its general structure, extent and geometry is still deliberated. Is mantle-derived magma delivered straight up beneath the GCV to an upper crustal magma chamber and then vertically to eruptions at the GCV and laterally to eruption on the GVS fissure swarm? Or does the system feature two levels of crustal storage, one in the upper crust beneath GCV and another at mid-crustal depth? Or is the structure of the GVS plumbing more complex? The data that we have so far and is pertinent to GVS magma plumbing is summarised below: Geophysical measurements imply that shallowest magma storage beneath GCV is at 3-4 km. The Zr and Nb concentrations in the tephra from the 1998 and 2004 GCV plus Laki eruptions show that the parent magmas for each was produced by different degrees of partial melting of a similar mantle source. It also demonstrates transport to the surface via separate pathways and that neither magma can be derived by fractional crystallization from a Laki-like magma. Detailed petrological studies on the Laki tephra and lava indicate polybaric magma evolution within the mid-crust (at 6 to 15 km depth), with further evolution at shallower depths induced either by disequilibrium crystal growth during ascent of magma from the mid-crust storage or a brief residence at 3-6 km depths. The Laki magma contains significant abundances of polymineralic glomerocrysts, signifying that erupted magma interacted with preexisting crystal mushes. These data support the notion of a crustal plumbing system with multiple storage level involving polybaric magma evolution and are inconsistent with the idea that all of the magma erupted within the GVS is delivered from a single upper crustal magma chamber beneath GCV.

Xenolith constraints on seismic velocities in the upper mantle beneath southern Africa

NASA Astrophysics Data System (ADS)

James, D. E.; Boyd, F. R.; Schutt, D.; Bell, D. R.; Carlson, R. W.

2004-01-01

We impose geologic constraints on seismic three-dimensional (3-D) images of the upper mantle beneath southern Africa by calculating seismic velocities and rock densities from approximately 120 geothermobarometrically calibrated mantle xenoliths from the Archean Kaapvaal craton and adjacent Proterozoic mobile belts. Velocity and density estimates are based on the elastic and thermal moduli of constituent minerals under equilibrium P-T conditions at the mantle source. The largest sources of error in the velocity estimates derive from inaccurate thermo-barometry and, to a lesser extent, from uncertainties in the elastic constants of the constituent minerals. Results are consistent with tomographic evidence that cratonic mantle is higher in velocity by 0.5-1.5% and lower in density by about 1% relative to off-craton Proterozoic samples at comparable depths. Seismic velocity variations between cratonic and noncratonic xenoliths are controlled dominantly by differences in calculated temperatures, with compositional effects secondary. Different temperature profiles between cratonic and noncratonic regions have a relatively minor influence on density, where composition remains the dominant control. Low-T cratonic xenoliths exhibit a positive velocity-depth curve, rising from about 8.13 km/s at uppermost mantle depths to about 8.25 km/s at 180-km depth. S velocities decrease slightly over the same depth interval, from about 4.7 km/s in the uppermost mantle to 4.65 km/s at 180-km depth. P and S velocities for high-T lherzolites are highly scattered, ranging from highs close to those of the low-T xenoliths to lows of 8.05 km/s and 4.5 km/s at depths in excess of 200 km. These low velocities, while not asthenospheric, are inconsistent with seismic tomographic images that indicate high velocity root material extending to depths of at least 250 km. One plausible explanation is that high temperatures determined for the high-T xenoliths are a nonequilibrium consequence of relatively recent thermal perturbation and compositional modification associated with emplacement of kimberlitic fluids into the deep tectospheric root. Seismic velocities and densities for cratonic xenoliths differ significantly from those predicted for both primitive mantle peridotite and mantle eclogite. A model primitive mantle under cratonic P-T conditions exhibits velocities about 1% lower for P and about 1.5% lower for S, a consequence of a more fertile composition and different modal composition. Primitive mantle is also about 2% more dense at 150-km depth than low-T garnet lherzolite at cratonic P-T conditions. Similar calculations based on an oceanic geotherm are consistent with the isopycnic hypothesis of comparable density columns beneath oceanic and cratonic regions. Calculations for a hypothetical "cratonic" eclogite (50:50 garnet/omphacite) with an assumed cratonic geotherm produce extremely high VP and VS (8.68 km/s and 4.84 km/s, respectively, at 150 km depth) as well as high density (˜3.54 gm/cc). The very high velocity of eclogite should render it seismically conspicuous in the cratonic mantle if present as large volume blocks or slabs. We discuss how the seismic velocity data we have compiled in this paper from both xenoliths and generic petrologic models of the upper mantle differ from commonly used standard earth models IASPEI and PREM.

Lithospheric Structure In Asia Based On Pp-, P-waves Data

NASA Astrophysics Data System (ADS)

Bushenkova, N.

There is the RR-R method of tomographic inversion, which is based on joint use of teleseismic P or S refracted rays and corresponding PP or SS rays with bounce points located within a study region. This scheme allows imaging the deep seismic structure beneath "blank" areas where there are neither recording stations nor earthquakes. Uti- lization of differential travel times makes it possible to avoid the difficulty of source and station corrections, which cause problems in teleseismic tomography. The RR-R scheme has been applied to more than 10000 ray pairs from the ISC database to in- vestigate a large region from the North Arctic Ocean to the northern part of China and Mongolia. Velocity anomalies were computed in grid nodes distributed in the study 3D area according to ray density. Relatively high velocities beneath the Siberian cra- ton (positive velocity anomalies under 3 % ) observed down to the depth of 350 km are consistent with the geothermal model of Artemieva and Mooney (2001) and with global seismic tomography and may correspond to traces of thick lithosphere. A local low-velocity anomaly is imaged in the northern part of the Siberian craton at sub- lithospheric depths below 300 km. Its centre coincides with the swell of the contem- porary Putorana plateau. This isolated anomaly may be accounted for by the presence of a limited volume of hot and light mantle material that, by its buoyancy, provides a dynamic uplift of the surface. Upper mantle beneath West Siberia is dominated by low velocities which can be interpreted as a result of relatively thin lithosphere. The veloc- ity jump at the base of the lithosphere (~150 km) is smooth due to its essentially ther- mal nature. The velocity anomalies in the southern part of the study area have NW-SE trend, which corresponds to the strike of major lithospheric structures formed by suc- cessive accretion of terranes to the Siberian craton through the Paleozoic (Molnar and Tapponnier, 1975; Sengör et al., 1993; Dobretsov et al., 1996). Negative veloc- ity anomalies are related to the thin lithosphere of the Altai Mountains and the Hangai plateau (negative velocity anomalies under 3 % in the depth range 30 to 430 km). The low-velocity seismic anomaly beneath the eastern Hangai plateau can be interpreted as a mantle plume, which is consistent with other geophysical and geological data. The strong negative velocity anomaly beneath Altai reflects the present-day weak rhe- ology most likely related to conductive heating of the thinned continental lithosphere in the Late Cenozoic. The positive velocity anomaly beneath and the western Hangai plateau and Tuva corresponds to the Precambrian Mongolia-Tuva microcontinent.

Constraints on the structure of the crust and lithosphere beneath the Azores Islands from teleseismic receiver functions

NASA Astrophysics Data System (ADS)

Spieker, Kathrin; Rondenay, Stéphane; Ramalho, Ricardo; Thomas, Christine; Helffrich, George

2018-05-01

The Azores Archipelago is located near the Mid-Atlantic Ridge (MAR) and consists of nine islands, resting on both sides of the ridge. Various methods including seismic reflection, gravity and passive seismic imaging have previously been used to investigate the crustal thickness beneath the islands. They have yielded thickness estimates that range between roughly 10 and 30 km, but until now models of the more fine-scale crustal structure have been lacking. Pending questions include the thickness of the volcanic edifice beneath the islands and whether crustal intrusions or even underplating can be observed beneath any island. In this study, we use data from nine seismic stations located on the Azores Islands to investigate the crustal structure with teleseismic P-wave receiver functions. Our results indicate that the base of the volcanic edifice is located approximately 1 to 4 km depth beneath the different islands and that the crust-mantle boundary has an average depth of ˜17 km. There is strong evidence for magmatic underplating beneath the island of São Jorge, and indications that the underplating is also present beneath São Miguel and possibly Santa Maria. Additionally, the seismological lithosphere-asthenosphere boundary, defined as a seismic velocity drop in the uppermost mantle, seems to deepen with increasing distance from the MAR. It has a depth of ˜45 km beneath the islands close to the MAR, compared to depths >70 km beneath the more distal islands.

Seismic evidence for water transport out of the mantle transition zone beneath the European Alps

NASA Astrophysics Data System (ADS)

Liu, Zhen; Park, Jeffrey; Karato, Shun-ichiro

2018-01-01

The mantle transition zone has been considered a major water reservoir in the deep Earth. Mass transfer across the transition-zone boundaries may transport water-rich minerals from the transition zone into the water-poor upper or lower mantle. Water release in the mantle surrounding the transition zone could cause dehydration melting and produce seismic low-velocity anomalies if some conditions are met. Therefore, seismic observations of low-velocity layers surrounding the transition zone could provide clues of water circulation at mid-mantle depths. Below the Alpine orogen, a depressed 660-km discontinuity has been imaged clearly using seismic tomography and receiver functions, suggesting downwellings of materials from the transition zone. Multitaper-correlation receiver functions show prominent ∼0.5-1.5% velocity reductions at ∼750-800-km depths, possibly caused by partial melting in the upper part of lower mantle. The gap between the depressed 660-km discontinuity and the low-velocity layers is consistent with metallic iron as a minor phase in the topmost lower mantle reported by laboratory studies. Velocity drops atop the 410-km discontinuity are observed surrounding the Alpine orogeny, suggesting upwelling of water-rich rock from the transition zone in response to the downwelled materials below the orogeny. Our results provide evidence that convective penetration of the mantle transition zone pushes hydrated minerals both upward and downward to add hydrogen to the surrounding mantle.

Geochemistry of ultramafic xenoliths from Kapfenstein, Austria: evidence for a variety of upper mantle processes

NASA Astrophysics Data System (ADS)

Kurat, G.; Palme, H.; Spettel, B.; Baddenhausen, Hildegard; Hofmeister, H.; Palme, Christl; Wänke, H.

1980-01-01

Major, minor, and trace element contents have been determined in seven ultramafic xenoliths, the host basanite, and some mineral separates from xenoliths from Kapfenstein, Austria. Most of the xenoliths represent residues after extraction of different amounts of basaltic liquid. Within the sequence Iherzolite to harzburgite contents of Al, Ca, Ti, Na, Sc, V, Cr and the HREE decrease systematically with increasing Mg/Fe and decreasing Yb/Sc. Although all samples are depleted in highly incompatible elements, the less depleted end of our suite very closely approaches the chondritic Yb/Sc ratio and consequently the primitive upper mantle composition. Chromium behaved as a non-refractory element. Consequently it should have higher abundances in basalts than observed, suggesting that most basalts experienced Cr fractionation by chromite separation during ascent. Several processes have been active in addition to partial melting within the upper mantle beneath Kapfenstein: (1) a hornblendite has been identified as wet alkali-basaltic mobilisate; (2) an amphibole Iherzolite is the product of alkali-basalt metasomatism of a common depleted Iherzolite; (3) two amphibole Iherzolites contain evidence for rather pure water metasomatism of normal depleted Iherzolites; (4) a garnet-spinel websterite was a tholeiitic liquid trapped within the upper mantle and which suffered a subsequent partial melting event (partial remobilization of a mobilisate). (5) Abundances of highly incompatible elements are generally very irregular, indicating contamination of upper mantle rocks by percolating liquids (in the mantle). Weathering is an important source of contamination: e.g. U mobilization by percolating groundwater. Contamination of the xenoliths by the host basanite liquid can only amount to approximately 5.5 × 10 -4 parts. Distributions of minor and trace elements between different minerals apparently reflect equilibrium and vary with equilibration temperature.

Inferred rheological structure and mantle conditions from postseismic deformation following the 2010 Mw 7.2 El Mayor-Cucapah Earthquake

NASA Astrophysics Data System (ADS)

Dickinson-Lovell, Haylee; Huang, Mong-Han; Freed, Andrew M.; Fielding, Eric; Bürgmann, Roland; Andronicos, Christopher

2018-06-01

The 2010 Mw7.2 El Mayor-Cucapah earthquake provides a unique target of postseismic study as deformation extends across several distinct geological provinces, including the cold Mesozoic arc crust of the Peninsular Ranges and newly formed, hot, extending lithosphere within the Salton Trough. We use five years of global positioning system measurements to invert for afterslip and constrain a 3-D finite-element model that simulates viscoelastic relaxation. We find that afterslip cannot readily explain far-field displacements (more than 50 km from the epicentre). These displacements are best explained by viscoelastic relaxation of a horizontally and vertically heterogeneous lower crust and upper mantle. Lower viscosities beneath the Salton Trough compared to the Peninsular Ranges and other surrounding regions are consistent with inferred differences in the respective geotherms. Our inferred viscosity structure suggests that the depth of the Lithosphere/Asthenosphere Boundary (LAB) is ˜65 km below the Peninsular Ranges and ˜32 km beneath the Salton Trough. These depths are shallower than the corresponding seismic LAB. This suggests that the onset of partial melting in peridotite may control the depth to the base of the mechanical lithosphere. In contrast, the seismic LAB may correspond to an increase in the partial melt percentage associated with the change from a conductive to an adiabatic geotherm.

Geologic Map of The Volcanoes Quadrangle, Bernalillo and Sandoval Counties, New Mexico

USGS Publications Warehouse

Thompson, Ren A.; Shroba, Ralph R.; Menges, Christopher M.; Schmidt, Dwight L.; Personius, Stephen F.; Brandt, Theodore R.

2009-01-01

This geologic map, in support of the U.S. Geological Survey Middle Rio Grande Basin Geologic Mapping Project, shows the spatial distribution of surficial deposits, lava flows, and related sediments of the Albuquerque volcanoes, upper Santa Fe Group sediments, faults, and fault-related structural features. These deposits are on, along, and beneath the Llano de Albuquerque (West Mesa) west of Albuquerque, New Mexico. Some of these deposits are in the western part of Petroglyph National Monument. Artificial fill deposits are mapped chiefly beneath and near the City of Albuquerque Soil Amendment Facility and the Double Eagle II Airport. Alluvial deposits were mapped in and along stream channels, beneath terrace surfaces, and on the Llano de Albuquerque and its adjacent hill slopes. Deposits composed of alluvium and colluvium are also mapped on hill slopes. Wedge-shaped deposits composed chiefly of sandy sheetwash deposits, eolian sand, and intercalated calcic soils have formed on the downthrown-sides of faults. Deposits of active and inactive eolian sand and sandy sheetwash deposits mantle the Llano de Albuquerque. Lava flows and related sediments of the Albuquerque volcanoes were mapped near the southeast corner of the map area. They include eleven young lava flow units and, where discernable, associated vent and near-vent pyroclastic deposits associated with cinder cones. Upper Santa Fe Group sediments are chiefly fluvial in origin, and are well exposed near the western boundary of the map area. From youngest to oldest they include a gravel unit, pebbly sand unit, tan sand and mud unit, tan sand unit, tan sand and clay unit, and silty sand unit. Undivided upper Santa Fe Group sediments are mapped in the eastern part of the map area. Faults were identified on the basis of surface expression determined from field mapping and interpretation of aeromagnetic data where concealed beneath surficial deposits. Fault-related structural features are exposed and were mapped near the western boundary of the map area.

Slab Penetration vs. Slab Stagnation: Mantle Reflectors as an Indicator

NASA Astrophysics Data System (ADS)

Okeler, A.; Gu, Y. J.; Schultz, R.; Contenti, S. M.

2011-12-01

Subducting oceanic lithosphere along convergent margins may stagnate near the base of the upper mantle or penetrate into the lower mantle. These dynamic processes cause extensive thermal and compositional variations, which can be observed in terms of impedance contrast (reflectivity) and topography of mantle transition zone (MTZ) discontinuities, i.e., 410- and 660-km discontinuities. In this study, we utilize ~ 15000 surface-reflected shear waves (SS) and their precursory arrivals (S410S and S660S) to analyze subduction related deformations on mantle reflectivity structure. We apply pre-stack, time-to-depth migration technique to SS precursors, and move weak underside reflections using PREM-predicted travel-time curves. Common Mid-point gathers are formed to investigate structure under the western Pacific, south America, and Mediterranean convergent boundaries. In general, mantle reflectivity structures are consistent with previous seismic tomography models. In regions of slab penetration (e.g., southern Kurile arc, Aegean Sea), our results show 1) a substantial decrease in S660S amplitude, and 2) strong lower mantle reflector(s) at ~ 900 km depth. These reflective structures are supported by zones of high P and S velocities extending into the lower mantle. Our 1-D synthetic simulations suggest that the decreasing S660S amplitudes are, at least partially, associated with shear wave defocusing due to changes in reflector depth (by ±20 km) within averaging bin. Assuming a ~500 km wide averaging area, a dipping reflector with 6-8 % slope can reduce the amplitude of a SS precursor by ~50%. On the other hand, broad depressions with strong impedance contrast at the base of the MTZ characterize the regions of slab stagnation, such as beneath the Tyrrhenian Sea and northeastern China. For the latter region, substantial topography on the 660-km discontinuity west of the Wadati-Benioff zone suggests that the stagnant part of the Pacific plate across Honshu arc is not nearly as flat as previously suggested.

Pathway from subducting slab to surface for melt and fluids beneath Mount Rainier.

PubMed

McGary, R Shane; Evans, Rob L; Wannamaker, Philip E; Elsenbeck, Jimmy; Rondenay, Stéphane

2014-07-17

Convergent margin volcanism originates with partial melting, primarily of the upper mantle, into which the subducting slab descends. Melting of this material can occur in one of two ways. The flow induced in the mantle by the slab can result in upwelling and melting through adiabatic decompression. Alternatively, fluids released from the descending slab through dehydration reactions can migrate into the hot mantle wedge, inducing melting by lowering the solidus temperature. The two mechanisms are not mutually exclusive. In either case, the buoyant melts make their way towards the surface to reside in the crust or to be extruded as lava. Here we use magnetotelluric data collected across the central state of Washington, USA, to image the complete pathway for the fluid-melt phase. By incorporating constraints from a collocated seismic study into the magnetotelluric inversion process, we obtain superior constraints on the fluids and melt in a subduction setting. Specifically, we are able to identify and connect fluid release at or near the top of the slab, migration of fluids into the overlying mantle wedge, melting in the wedge, and transport of the melt/fluid phase to a reservoir in the crust beneath Mt Rainier.

Pathway from subducting slab to surface for melt and fluids beneath Mount Rainier

NASA Astrophysics Data System (ADS)

McGary, R. Shane; Evans, Rob L.; Wannamaker, Philip E.; Elsenbeck, Jimmy; Rondenay, Stéphane

2014-07-01

Convergent margin volcanism originates with partial melting, primarily of the upper mantle, into which the subducting slab descends. Melting of this material can occur in one of two ways. The flow induced in the mantle by the slab can result in upwelling and melting through adiabatic decompression. Alternatively, fluids released from the descending slab through dehydration reactions can migrate into the hot mantle wedge, inducing melting by lowering the solidus temperature. The two mechanisms are not mutually exclusive. In either case, the buoyant melts make their way towards the surface to reside in the crust or to be extruded as lava. Here we use magnetotelluric data collected across the central state of Washington, USA, to image the complete pathway for the fluid-melt phase. By incorporating constraints from a collocated seismic study into the magnetotelluric inversion process, we obtain superior constraints on the fluids and melt in a subduction setting. Specifically, we are able to identify and connect fluid release at or near the top of the slab, migration of fluids into the overlying mantle wedge, melting in the wedge, and transport of the melt/fluid phase to a reservoir in the crust beneath Mt Rainier.

Using Receiver Functions to Image the Montana Crust and Upper Mantle

NASA Astrophysics Data System (ADS)

Sirianni, R. T.; Russo, R. M.

2008-12-01

We determined receiver functions (RFs) at six permanent Advanced National Seismic System (ANSS) stations to examine crust and upper mantle structure of the Wyoming craton (WC) and Medicine Hat block (MHB). The Deep Probe & SAREX projects (Henstock et al., 1998; Clowes et al., 2002; Gorman et al., 2002) used active source seismics to model a high velocity crustal layer (the so-called 7x layer) beneath the WC. This layer exhibits P wave velocities that are high for lower continental crust (~7+ km/s) and extends from 30-55 km below the surface. Interpretations of the active source data indicate that this layer may represent wide scale crustal underplating of the WC, implying post-Archean craton modification with implications for Laurentia assembly. We used 43 earthquakes from a wide azimuthal distribution recorded at the Montana ANSS stations; high signal-to-noise ratios of 25 of these RFs were acceptable for further analysis. Receiver functions constrain crustal velocity structure beneath a seismometer by using P-to-S wave conversions at sharp velocity contrast boundaries. Preliminary results for seismic stations DGMT, EGMT, and LAO, located to the east of the Deep Probe and SAREX seismic line on the Wyoming craton/Medicine Hat block show the influence of sedimentary cover and a strong Ps phase at approximately four seconds after P. At BOZ and MSO, located in the Rocky mountains, the sedimentary cover signal previously noted is absent, and instead we observe a sharp Ps phase at about four and a half seconds after P. RFs at station RLMT (on the WC) are highly anomalous, probably reflecting complex conversions from two differently oriented dipping layers. We will use the RFs to produce suites of acceptable structural models to test for the presence and lateral extent of the 7x layer and other structural features of the Rocky Mountains-craton transition.

Lithospheric strucutre and relationship to seismicity beneath the Southeastern US using reciever functions

NASA Astrophysics Data System (ADS)

Cunningham, E.; Lekic, V.

2017-12-01

Despite being on a passive margin for millions of years, the Southeastern United States (SEUS) contains numerous seismogenic zones with the ability to produce damaging earthquakes. However, mechanisms controlling these intraplate earthquakes are poorly understood. Recently, Biryol et al. 2016 use P-wave tomography suggest that upper mantle structures beneath the SEUS correlate with areas of seismicity and seismic quiescence. Specifically, thick and fast velocity lithosphere beneath North Carolina is stable and indicative of areas of low seismicity. In contrast, thin and slow velocity lithosphere is weak, and the transition between the strong and weak lithosphere may be correlated with seismogenic zones found in the SEUS. (eg. Eastern Tennessee seismic zone and the Central Virginia seismic zone) Therefore, I systematically map the heterogeneity of the mantle lithosphere using converted seismic waves and quantify the spatial correlation between seismicity and lithospheric structure. The extensive network of seismometers that makes up the Earthscope USArray combined with the numerous seismic deployments in the Southeastern United States allows for unprecedented opportunity to map changes in lithospheric structure across seismogenic zones and seismic quiescent regions. To do so, I will use both P-to-s and S-to-p receiver functions (RFS). Since RFs are sensitive to seismic wavespeeds and density discontinuities with depth, they particularly useful for studying lithospheric structure. Ps receiver functions contain high frequency information allowing for high resolution, but can become contaminated by large sediment signals; therefore, I removed sediment multiples and correct for time delays of later phases using the method of Yu et. al 2015 which will allow us to see later arriving phases associated with lithospheric discontinuities. S-to-p receiver functions are not contaminated by shallow layers, making them ideal to study deep lithospheric structures but they can suffer from low signal-to-noise levels. I compensate for this difficulty by using high quality deployments and stacking these data at common conversion points to increase lateral resolution.

Formation and modification of chromitites in the mantle

NASA Astrophysics Data System (ADS)

Arai, Shoji; Miura, Makoto

2016-11-01

Podiform chromitites have long supplied us with unrivaled information on various mantle processes, including the peridotite-magma reaction, deep-seated magmatic evolution, and mantle dynamics. The recent discovery of ultrahigh-pressure (UHP) chromitites not only sheds light on a different aspect of podiform chromitites, but also changes our understanding of the whole picture of podiform chromitite genesis. In addition, new evidence was recently presented for hydrothermal modification/formation chromite/chromitite in the mantle, which is a classical but innovative issue. In this context, we present here an urgently needed comprehensive review of podiform chromitites in the upper mantle. Wall-rock control on podiform chromitite genesis demonstrates that the peridotite-magma reaction at the upper mantle condition is an indispensable process. We may need a large system in the mantle, far larger than the size of outcrops or mining areas, to fulfill the Cr budget requirement for podiform chromitite genesis. The peridotite-magma reaction over a large area may form a melt enriched with Na and other incompatible elements, which mixes with a less evolved magma supplied from the depth to create chromite-oversaturated magma. The incompatible-element-rich magma trapped by the chromite mainly precipitates pargasite and aspidolite (Na analogue of phlogopite), which are stable under upper mantle conditions. Moderately depleted harzburgites, which contain chromite with a moderate Cr# (0.4-0.6) and a small amount of clinopyroxene, are the best reactants for the chromitite-forming reaction, and are the best hosts for podiform chromitites. Arc-type chromitites are dominant in ophiolites, but some are of the mid-ocean ridge type; chromitites may be common beneath the ocean floor, although it has not yet been explored for chromitite. The low-pressure (upper mantle) igneous chromitites were conveyed through mantle convection or subduction down to the mantle transition zone to form ultrahigh-pressure chromitites. Some of these reappear at the shallower mantle, and can coexist with newly formed low-pressure igneous chromitites. High-temperature hydrothermal fluids can dissolve and precipitate chromite, and hydrothermal chromitites (chromitites precipitated from aqueous fluids) are possibly formed within the mantle where the circulation of hydrous fluid is available, e.g., at the mantle wedge.

Large-scale shear velocity structure of the upper mantle beneath Africa and surrounding regions

NASA Astrophysics Data System (ADS)

Legendre, Cédric; Meier, Thomas; Lebedev, Sergei; Friederich, Wolfgang; Egelados Working Group

2010-05-01

The automated multimode waveform inversion technique developed by Lebedev et al. (2005) was applied to available data of broadband stations in Africa and surrounding regions. It performs a fitting of the complete waveform starting from the S-wave onset to the surface wave. Assuming the location and focal mechanism of a considered earthquake as known, the first basic step is to consider each available seismogram separately and to find the velocity perturbations that can explain the filtered seismogram best. In a second step, each velocity perturbations serves as a linear constraint in an inversion for a 3D S-wave velocity model of the upper mantle. We collected data for the years from 1990 to 2006 from all permanent stations for which data were available via the data centers of ORFEUS, GEOFON and IRIS, and from others that build the Virtual European Seismological Network (VEBSN) as well as all available African stations. Just recently we were also able to add the data recorded by the temporary broadband EGELADOS network in the southern Aegean. This represents a huge dataset with all available stations in Africa and surroundings regions. The resulting models exhibit an overwhelming structural detail in relation to the size of the region considered in the inversion. They are to our knowledge the most detailed models of shear wave velocity currently available for the African upper mantle and surroundings. Most prominent features are an extremely sharp demarcation of the Dead Sea Rift System. Narrow high velocity regions follow the Hellenic arc and the Ionian trench toward the north. Low velocity zones are found at depths around 150 km in the Middle East region. The hotspots in North Africa are also clearly imaged.

Evolution of the upper mantle beneath the southern Baikal rift zone: an Sr-Nd isotope study of xenoliths from the Bartoy volcanoes

NASA Astrophysics Data System (ADS)

Ionov, D. A.; Kramm, U.; Stosch, H.-G.

1992-06-01

Anhydrous and amphibole-bearing peridotite xenoliths occur in roughly equal quantitites in the Bartoy volcanic field about 100 km south of the southern tip of Lake Baikal in Siberia (Russia). Whole-rock samples and pure mineral separates from nine xenoliths have been analyzed for Sr and Nd isotopes in order to characterize the upper mantle beneath the southern Baikal rift zone. In an Sr-Nd isotope diagram both dry and hydrous xenoliths from Bartoy plot at the junction between the fields of MORB and ocean island basalts. This contrasts with data available on two other localities around Lake Baikal (Tariat and Vitim) where peridotites typically have Sr-Nd isotope compositions indicative of strong long-term depletion in incompatible elements. Our data indicate significant chemical and isotopic heterogeneity in the mantle beneath Bartoy that may be attributed to its position close to an ancient suture zone separating the Siberian Platform from the Mongol-Okhotsk mobile belt and occupied now by the Baikal rift. Two peridotites have clinopyroxenes depleted in light rare earth elements (LREE) with Sr and Nd model ages of about 2 Ga and seem to retain the trace element and isotopic signatures of old depleted lithospheric mantle, while all other xenoliths show different degrees of LREE-enrichment. Amphiboles and clinopyroxenes in the hydrous peridotites are in Sr-Nd isotopic disequilibrium. If this reflects in situ decay of 147Sm and 87Rb rather than heterogeneities produced by recent metasomatic formation of amphiboles then 300 400 Ma have passed since the minerals were last in equilibrium. This age range then indicates an old enrichment episode or repeated events during the Paleozoic in the lithospheric mantle initially depleted maybe ˜2 Ga ago. The Bartoy hydrous and enriched dry peridotites, therefore, are unlikely to represent fragments of a young asthenospheric bulge which, according to seismic reflection studies, reached the Moho at the axis of the Baikal rift zone a few Ma ago. By contrast, hydrous veins in peridotites may be associated with rift formation processes.

Viscoelastic lower crust and mantle relaxation following the 14-16 April 2016 Kumamoto, Japan, earthquake sequence

NASA Astrophysics Data System (ADS)

Pollitz, Fred F.; Kobayashi, Tomokazu; Yarai, Hiroshi; Shibazaki, Bunichiro; Matsumoto, Takumi

2017-09-01

The 2016 Kumamoto, Japan, earthquake sequence, culminating in the Mw=7.0 16 April 2016 main shock, occurred within an active tectonic belt of central Kyushu. GPS data from GEONET reveal transient crustal motions from several millimeters per year up to ˜3 cm/yr during the first 8.5 months following the sequence. The spatial pattern of horizontal postseismic motions is shaped by both shallow afterslip and viscoelastic relaxation of the lower crust and upper mantle. We construct a suite of 2-D regional viscoelastic structures in order to derive an optimal joint afterslip and viscoelastic relaxation model using forward modeling of the viscoelastic relaxation. We find that afterslip dominates the postseismic relaxation in the near field (within 30 km of the main shock epicenter), while viscoelastic relaxation dominates at greater distance. The viscoelastic modeling strongly favors a very weak lower crust below a ˜65 km wide zone coinciding with the Beppu-Shimabara graben and the locus of central Kyushu volcanism. Inferred uppermost mantle viscosity is relatively low beneath southern Kyushu, consistent with independent inferences of a hydrated mantle wedge within the Nankai trough fore -arc.

Viscoelastic lower crust and mantle relaxation following the 14–16 April 2016 Kumamoto, Japan, earthquake sequence

USGS Publications Warehouse

Pollitz, Fred; Kobayashi, Tomokazu; Yarai, Hiroshi; Shibazaki, Bunichiro; Matsumoto, Takumi

2017-01-01

The 2016 Kumamoto, Japan, earthquake sequence, culminating in the Mw=7.0 16 April 2016 main shock, occurred within an active tectonic belt of central Kyushu. GPS data from GEONET reveal transient crustal motions from several millimeters per year up to ∼3 cm/yr during the first 8.5 months following the sequence. The spatial pattern of horizontal postseismic motions is shaped by both shallow afterslip and viscoelastic relaxation of the lower crust and upper mantle. We construct a suite of 2-D regional viscoelastic structures in order to derive an optimal joint afterslip and viscoelastic relaxation model using forward modeling of the viscoelastic relaxation. We find that afterslip dominates the postseismic relaxation in the near field (within 30 km of the main shock epicenter), while viscoelastic relaxation dominates at greater distance. The viscoelastic modeling strongly favors a very weak lower crust below a ∼65 km wide zone coinciding with the Beppu-Shimabara graben and the locus of central Kyushu volcanism. Inferred uppermost mantle viscosity is relatively low beneath southern Kyushu, consistent with independent inferences of a hydrated mantle wedge within the Nankai trough fore -arc.

Seismic Constraints on the Lithosphere-Asthenosphere Boundary Beneath the Izu-Bonin Area: Implications for the Oceanic Lithospheric Thinning

NASA Astrophysics Data System (ADS)

Cui, Qinghui; Wei, Rongqiang; Zhou, Yuanze; Gao, Yajian; Li, Wenlan

2018-01-01

The lithosphere-asthenosphere boundary (LAB) is the seismic discontinuity with negative velocity contrasts in the upper mantle. Seismic detections on the LAB are of great significance in understanding the plate tectonics, mantle convection and lithospheric evolution. In this paper, we study the LAB in the Izu-Bonin subduction zone using four deep earthquakes recorded by the permanent and temporary seismic networks of the USArray. The LAB is clearly revealed with sP precursors (sdP) through the linear slant stacking. As illustrated by reflected points of the identified sdP phases, the depth of LAB beneath the Izu-Bonin Arc (IBA) is about 65 km with a range of 60-68 km. The identified sdP phases with opposite polarities relative to sP phases have the average relative amplitude of 0.21, which means a 3.7% velocity drop and implies partial melting in the asthenosphere. On the basis of the crustal age data, the lithosphere beneath the IBA is located at the 1100 °C isotherm calculated with the GDH1 model. Compared to tectonically stable areas, such as the West Philippine Basin (WPB) and Parece Vela Basin (PVB) in the Philippine Sea, the lithosphere beneath the Izu-Bonin area shows the obvious lithospheric thinning. According to the geodynamic and petrological studies, the oceanic lithospheric thinning phenomenon can be attributed to the strong erosion of the small-scale convection in the mantle wedge enriched in volatiles and melts.