Depth Of Lithosphere-asthenosphere Boundary Research Articles

Nature of the seismic lithosphere‐asthenosphere boundary within normal oceanic mantle from high‐resolution receiver functions

AbstractReceiver function observations in the oceanic upper mantle can test causal mechanisms for the depth, sharpness, and age dependence of the seismic wave speed decrease thought to mark the lithosphere‐asthenosphere boundary (LAB). We use a combination of frequency‐dependent harmonic decomposition of receiver functions and synthetic forward modeling to provide new seismological constraints on this “seismic LAB” from 17 ocean‐bottom stations and 2 borehole stations in the Philippine Sea and northwest Pacific Ocean. Underneath young oceanic crust, the seismic LAB depth follows the ∼1300 K isotherm but a lower isotherm (∼1000 K) is suggested in the Daito ridge, the Izu‐Bonin‐Mariana trench, and the northern Shikoku basin. Underneath old oceanic crust, the seismic LAB lies at a constant depth ∼70 km. The age dependence of the seismic LAB depth is consistent with either a transition to partial‐melt conditions or a subsolidus rheological change as the causative factor. The age dependence of interface sharpness provides critical information to distinguish these two models. Underneath young oceanic crust, the velocity gradient is gradational, while for old oceanic crust, a sharper velocity gradient is suggested by the receiver functions. This behavior is consistent with the prediction of the subsolidus model invoking anelastic relaxation mediated by temperature and water content, but is not readily explained by a partial‐melt model. The Ps conversions display negligible two‐lobed or four‐lobed back azimuth dependence in harmonic stacks, suggesting that a sharp change in azimuthal anisotropy with depth is not responsible for them. We conclude that these ocean‐bottom observations indicate a subsolidus elastically accommodated grain‐boundary sliding (EAGBS) model for the seismic LAB. Because EAGBS does not facilitate long‐term ductile deformation, the seismic LAB may not coincide with the conventional transition from lithosphere to asthenosphere corresponding to a change in the long‐term rheological properties.

Thickness of the lithosphere beneath Turkey and surroundings from S-receiver functions

Abstract. We analyze S-receiver functions to investigate variations of lithospheric thickness below the entire region of Turkey and surrounding areas. The teleseismic data used here have been compiled combining all permanent seismic stations which are open to public access. We obtained almost 12 000 S-receiver function traces characterizing the seismic discontinuities between the Moho and the discontinuity at 410 km depth. Common-conversion-point stacks yield well-constrained images of the Moho and of the lithosphere–asthenosphere boundary (LAB). Results from previous studies suggesting shallow LAB depths between 80 and 100 km are confirmed in the entire region outside the subduction zones. We did not observe changes in LAB depths across the North and East Anatolian faults. To the east of Cyprus, we see indications of the Arabian LAB. The African plate is observed down to about 150 km depth subducting to the north and east between the Aegean and Cyprus with a tear at Cyprus. We also observed the discontinuity at 410 km depth and a negative discontinuity above the 410, which might indicate a zone of partial melt above this discontinuity.

From the North-Iberian Margin to the Alboran Basin: A lithosphere geo-transect across the Iberian Plate

A ~1000-km-long lithospheric transect running from the North-Iberian Margin to the Alboran Basin (W-Mediterranean) is investigated. The main goal is to image the changes in the crustal and upper mantle structure occurring in: i) the North-Iberian margin, whose deformation in Alpine times gave rise to the uplift of the Cantabrian Mountains related to Iberia–Eurasia incipient subduction; ii) the Spanish Meseta, characterized by the presence of Cenozoic basins on top of a Variscan basement with weak Alpine deformation in the Central System, and localized Neogene–Quaternary deep volcanism; and iii) the Betic–Alboran system related to Africa–Iberia collision and the roll-back of the Ligurian–Tethyan domain. The modeling approach, combines potential fields, elevation, thermal, seismic, and petrological data under a self-consistent scheme. The crustal structure is mainly constrained by seismic data whereas the upper mantle is constrained by tomographic models. The results highlight the lateral variations in the topography of the lithosphere–asthenosphere boundary (LAB), suggesting a strong lithospheric mantle strain below the Cantabrian and Betic mountain belts. The LAB depth ranges from 180km beneath the Cantabrian Mountains to 135–110km beneath Iberia Meseta deepening again to values of 160km beneath the Betic Cordillera. The Central System, with a mean elevation of 1300m, has a negligible signature on the LAB depth. We have considered four lithospheric mantle compositions: a predominantly average Phanerozoic in the continental mainland, two more fertile compositions in the Alboran Sea and in the Calatrava Volcanic Province, and a hydrated uppermost mantle in the North-Iberian Margin. These compositional differences allowed us to reproduce the main trends of the geophysical observables as well as the inferred P- and S-wave seismic velocities from tomography models and seismic experiments available in the study transect. The high mean topography of Iberia can be partly consistent with a low-velocity/high-temperature/low-density layer in the sublithospheric mantle.

Lithospheric structure of the North China Craton: Integrated gravity, geoid and topography data

The lithospheric structure of ancient cratons provides important constraints on models relating to tectonic evolution and mantle dynamics. Here we present the 3D lithospheric structure of the North China Craton (NCC) from a joint inversion of gravity, geoid and topography data. The NCC records a prolonged history of Archean and Paleoproterozoic accretion of crustal blocks through subduction and collision building the cratonic architecture, which was subsequently differentially destroyed during Mesozoic through extensive magmatism. The thermal structure obtained in our study is considered to define the lithosphere-asthenosphere boundary (LAB) of the NCC, and reflects the density variations within the mantle lithosphere. Employing the Moho depths from deep seismic sounding profiles for the inversion, and based on repeated computations using different parameters, we estimate the Moho depth, LAB depth and average crustal density of the craton. The Moho depth varies from 28 to 50km and the LAB depth varies from 105 to 205km. The LAB and Moho show concordant thinning from West to East of the NCC. The average crustal density is 2870kgm−3 in the western part of the NCC, higher than that in the eastern part (2750kgm−3). The results of joint inversion in our study yielded LAB depth and lithospheric thinning features similar to those estimated from thermal and seismic studies, although our results show different depth and variations in the thickness. The lithosphere gently thins from 145 to 105km in the eastern NCC, where as the thinning is much less pronounced in the western NCC with average depth of about 175km. The joint inversion results in this study provide another perspective on the lithospheric structure from the density properties and corresponding geophysical responses in an ancient craton.

Study on Crustal and Lithosphere Thicknesses of Tengchong Volcanic Area in Yunnan

AbstractTeleseismic waveform data recorded by 9 broadband seismic stations of Tengchong volcanic seismic monitoring network were used to study crustal thickness, Poisson's ratio and the depth of the lithosphere‐asthenosphere boundary (LAB) using the P and S receiver functions method. The results are as follows: ➀ The Moho depth ranges from 33.5 to 38.0km beneath Tengchong volcanic area; ➁ The Poisson's ratios in Tengchong volcanic area mainly range from 0.26 to 0.32, and the Poisson's ratios beneath 6 stations are greater than 0.29, which may be caused by the increased mafic rocks in crust where may exist 2 magma chambers; ➂ The LAB depth beneath Tengchong volcanic area ranges from 78.2 to 88.0 km. There is an obvious rise of the LAB and lateral variations are very large. The obvious dome of lithosphere in Tengchong volcanic area is caused by asthenosphere upwelling (upsurge of thermal mantle material) as well as lithosphere extension and thinning.

Surface heat flow and lithosphere thermal structure of the Rhenohercynian Zone in the greater Luxembourg region

Comprehensive knowledge of surface heat flow and subsurface temperature distribution is indispensable for the interpretation and quantification of crustal/mantle processes as well as for the evaluation of the geothermal potential of an area. In cases where subsurface temperature data are sparse, thermal modeling may be used as a tool for inferring the geothermal resource at depth but requires profound structural, geological, and petrophysical input data. The study area encompasses the Trier–Luxembourg Basin and the western realm of the Rhenish Massif, itself subdivided into the Ardennes region in the west as well as the Eifel and Hunsrück regions in the east. For the study area, 2-D steady-state and conductive thermal models were established based on geological models of lithosphere-scale which were parameterized using thermal rock properties including thermal conductivity, radiogenic heat production, and density. The thermal models are constrained by surface heat flow (qs) and the geophysically-estimated depth of the lithosphere–asthenosphere boundary (LAB). A qs of 75±7 (2σ)mWm−2 was determined in the area. A LAB depth of 100km, as seismically derived for the Ardennes, provides the best fit with the measured qs. Modeled temperatures are in the range of 120–125°C at 5km depth and of 600–650°C at the Moho, respectively. The mantle heat flow amounts to ∼40mWm−2. Possible thermal consequences of the 10–20Ma old Eifel plume, which caused elevation of the LAB to 50–60km depth, were modeled in a steady-state thermal scenario resulting in a qs of 91mWm−2 in the Eifel region. Available qs values (65–80mWm−2) are significantly lower and do indicate that the plume-related heating has not yet reached the surface in its entirety.

Upper mantle structure around the Trans-European Suture Zone obtained by teleseismic tomography

Abstract. The presented study aims to resolve the upper mantle structure around the Trans-European Suture Zone (TESZ), which is the major tectonic boundary in Europe. The data of 183 temporary and permanent seismic stations operated during the period of the PASsive Seismic Experiment (PASSEQ) 2006–2008 within the study area from Germany to Lithuania was used to compile the data set of manually picked 6008 top-quality arrivals of P waves from teleseismic earthquakes. We used the TELINV nonlinear teleseismic tomography algorithm to perform the inversions. As a result, we obtain a model of P wave velocity variations up to about ±3% with respect to the IASP91 velocity model in the upper mantle around the TESZ. The higher velocities to the east of the TESZ correspond to the older East European Craton (EEC), while the lower velocities to the west of the TESZ correspond to younger western Europe. We find that the seismic lithosphere–asthenosphere boundary (LAB) is more distinct beneath the Phanerozoic part of Europe than beneath the Precambrian part. To the west of the TESZ beneath the eastern part of the Bohemian Massif, the Sudetes Mountains and the Eger Rift, the negative anomalies are observed from a depth of at least 70 km, while under the Variscides the average depth of the seismic LAB is about 100 km. We do not observe the seismic LAB beneath the EEC, but beneath Lithuania we find the thickest lithosphere of about 300 km or more. Beneath the TESZ, the asthenosphere is at a depth of 150–180 km, which is an intermediate value between that of the EEC and western Europe. The results imply that the seismic LAB in the northern part of the TESZ is in the shape of a ramp dipping to the northeasterly direction. In the southern part of the TESZ, the LAB is shallower, most probably due to younger tectonic settings. In the northern part of the TESZ we do not recognize any clear contact between Phanerozoic and Proterozoic Europe, but further to the south we may refer to a sharp and steep contact on the eastern edge of the TESZ. Moreover, beneath Lithuania at depths of 120–150 km, we observe the lower velocity area following the boundary of the proposed paleosubduction zone.

Lithospheric expression of cenozoic subduction, mesozoic rifting and the Precambrian Shield in Venezuela

We have combined surface wave tomography with Ps and Sp receiver-function images based on common-conversion-point (CCP) stacking to study the upper mantle velocity structure, particularly the lithosphere–asthenosphere boundary (LAB), beneath eastern and central Venezuela. Rayleigh phase velocities in the frequency range of 0.01–0.05 Hz (20–100 s in period) were measured using the two-plane-wave method and finite-frequency kernels, and then inverted on a 0.5°×0.5° grid. The phase velocity dispersion data at grid points were inverted for 1D shear velocity profiles using initial crust-mantle velocity models constructed from previous studies. The 3D velocity model and receiver-function images were interpreted jointly to determine the depth of the LAB and other upper mantle features. The tomographic images revealed two high velocity anomalies extending to more than ∼200 km depth. One corresponds to the top of the subducting Atlantic plate beneath the Serrania del Interior. The other anomaly is a highly localized feature beneath the Maturin Basin. The LAB depth varies significantly in the study region: It is located at ∼110 km depth beneath the Guayana Shield, and reaches ∼130 km at the northern edge of the Maturin Basin, which might be related to the downward flexural bending due to thrust loading of the Caribbean plate and pull from the subducting Atlantic plate. Immediately to the west, the lithosphere is thin (∼50–60 km) along the NE-SW trending Espino Graben from the Cariaco basin to the Orinoco River at the northern edge of the craton. The LAB in this region is the top of a pronounced low velocity zone. Westward, the lithosphere deepens to ∼80 km depth beneath the Barinas Apure Basin, and to ∼90 km beneath the Neogene Merida Andes and Maracaibo block. Both upper mantle velocity structure and lithosphere thickness correlate well with surface geology and are consistent with northern South American tectonics.

Seismic upper mantle discontinuities beneath Southeast Tibet and geodynamic implications

In this study, a total of 13,080 P receiver functions and 2800 S receiver functions recorded by 116 stations in Southeast Tibet, are used to determine the depths of the Moho, the lithosphere–asthenosphere boundary (LAB), and the 410km and 660km discontinuities. The results show that the LAB depth increases from 80–100km beneath southern Yunnan to 140–180km beneath Sichuan. Most of the 410km discontinuity is flat and at 410km depth, with a slight depression of 10–20km beneath the Longmenshan (LMS) Fault, the central Sichuan–Yunnan (SY) diamond-shaped block, and southeastern Yunnan. For the 660km discontinuity, the greatest depth occurs beneath the LMS and southern Yunnan, reaching 670–680km. The deepening of 660km discontinuity is attributed to compositional heterogeneity within the MTZ or dynamic pressure on the discontinuity. Additionally, the velocity of movement in the crust and upper mantle beneath SY is different. In Yunnan, crustal movement is southeastward, whereas across the Red-River fault, part of the crustal motion is redirected southwest by the westward rollback of the mantle beneath Myanmar. The interaction between the asthenosphere moving northeastward from Myanmar and southeastward from Tibet near latitude 26°N causes the crust and lithospheric mantle beneath southern Yunnan to have different movement directions.

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

Abstract. 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.

Thermal conditions of the central Sinai Microplate inferred from new surface heat-flow values and continuous borehole temperature logging in central and southern Israel

This paper reports ten new surface heat-flow density (qs) values for central and southern Israel (central Sinai Microplate), whose crystalline crust and lithosphere formed as part of the Neoproterozoic Arabian-Nubian Shield. Heat flow was calculated in Mesozoic sediments using the classical approach of heat-flow determination by implementing in the analysis high-precision continuous temperature logs obtained in air- and/or water-filled boreholes. Thermal conductivity (TC) measured for a large suite of rock samples of lithotypes making up the sequence was assigned to temperature gradients in intervals for which the lithology was known. The heat-flow values obtained for different depth intervals in a borehole as well as the average values for the individual borehole locations cover a narrow range, attesting heat-conduction conditions. A steady-state thermal model along an E–W crustal cross section through the area shows that the observed systematic spatial distribution of the qs values, which range between 50 and 62mWm−2, can primarily be explained by variations in the thickness of the upper crust and in the ratio between sedimentary and crystalline rocks therein. Given the time lapse of thermal heat transfer through the lithosphere, the qs data monitor the crustal thermal conditions prior to rift- and plume-related lithospheric thermal perturbations that have started in the larger area ca. 30Ma ago. Observed and modeled qs display the best fit for a pre-Oligocene lithosphere–asthenosphere boundary (LAB) at ∼150km, which would be at the upper end of LAB depths determined from stable areas of the Arabian Shield (150–120km) not affected by the young, deep-seated thermal processes that have caused a further uprise of the LAB. Our data imply or predict that the surface heat flow of the Sinai Microplate generally tends to increase along N–S and W–E traverses, from ∼45–50mWm−2 to ∼55–60mWm−2. Surface heat flows on the order of 55–60mWm−2 may be common in the northern Arabian Shield, where it exhibits typical lithosphere structure and composition and is unaffected by young heating processes, compared to values of ≤45mWm−2 recently determined in the southern Arabian Plate for the Arabian Platform.

Oceanic lithosphere-asthenosphere boundary from surface wave dispersion data

Abstract According to different types of observations, the nature of lithosphere-asthenosphere boundary (LAB) is controversial. Using a massive data set of surface wave dispersions in a broad period range (15–300 s), we have developed a three-dimensional upper mantle tomographic model (first-order perturbation theory) at the global scale. This is used to derive maps of the LAB from the resolved elastic parameters. The key effects of shallow layers and anisotropy are taken into account in the inversion process. We investigate LAB distribution primarily below the oceans, according to different kinds of proxies that correspond to the base of the lithosphere from the shear velocity variation at depth, the amplitude radial anisotropy, and the changes in azimuthal anisotropy G orientation. The estimations of the LAB depth based on the shear velocity increase from a thin lithosphere (∼20 km) in the ridges, to a thick old-ocean lithosphere (∼120–130 km). The radial anisotropy proxy shows a very fast increase in the LAB depth from the ridges, from ∼50 km to the older ocean where it reaches a remarkable monotonic subhorizontal profile (∼70–80 km). The LAB depths inferred from the azimuthal anisotropy proxy show deeper values for the increasing oceanic lithosphere (∼130–135 km). The difference between the evolution of the LAB depth with the age of the oceanic lithosphere computed from the shear velocity and azimuthal anisotropy proxies and from the radial anisotropy proxy raises questions about the nature of the LAB in the oceanic regions and of the formation of the oceanic plates

The lithosphere–asthenosphere boundary and the tectonic and magmatic history of the northwestern United States

This study explores the properties of the lithosphere–asthenosphere boundary (LAB) and other shallow mantle discontinuities across the diverse geologic provinces of the northwest United States. Sp phases were used to image three-dimensional discontinuity structure by common conversion point stacking with data from 804 temporary and permanent broadband stations, including 247 from EarthScope's USArray Transportable Array. Substantial variations in mantle discontinuity structure are apparent over a variety of spatial and temporal scales. To the west of the Sevier Thrust Belt, a coherent Sp negative phase coincides with the LAB depth range inferred from tomographic models. To the east, within the stable craton, multiple negative phases typically occur in the high velocity lithospheric layer, although in places a single mid-lithospheric discontinuity is imaged. Sub-cratonic LAB phases are often absent, indicating an LAB velocity gradient that is distributed over >50 km and that is consistent with effects of temperature alone. Where weak and intermittent LAB phases appear, they suggest more vertically localized velocity gradients produced by other factors such as bulk composition, volatile content, or contrasts in grain size or melt. In the tectonically active west, a positive Sp phase at depths consistent with the base of the asthenospheric low velocity zone in tomography models is intermittently observed. Beneath magmatic provinces in the west, the LAB Sp discontinuity deepens by ∼10 km from the High Lava Plains, where magmatism has occurred from 0–10.5 Ma, to the northern region of the Columbia River Basalts, which has been magmatically quiet since 15 Ma. Here we suggest a model in which the negative LAB velocity gradient is created by a layer of partial melt ponding beneath a solidus-defined boundary. This model predicts that higher temperatures associated with more recent magmatism would result in a shallowing of the intersection of the geotherm with the solidus. Beneath the Yellowstone Caldera, the absence of an LAB Sp phase suggests that the contrast in seismic velocity between the lithosphere and asthenosphere has been erased by intrusion of partial melt and heat into the lithosphere.

Structures of the oceanic lithosphere‐asthenosphere boundary: Mineral‐physics modeling and seismological signatures

AbstractWe explore possible models for the seismological signature of the oceanic lithosphere‐asthenosphere boundary (LAB) using the latest mineral‐physics observations. The key features that need to be explained by any viable model include (1) a sharp (<20 km width) and a large (5–10%) velocity drop, (2) LAB depth at ~70 km in the old oceanic upper mantle, and (3) an age‐dependent LAB depth in the young oceanic upper mantle. We examine the plausibility of both partial melt and sub‐solidus models. Because many of the LAB observations in the old oceanic regions are located in areas where temperature is ~1000–1200°K, significant partial melting is difficult. We examine a layered model and a melt‐accumulation model (at the LAB) and show that both models are difficult to reconcile with seismological observations. A sub‐solidus model assuming absorption‐band (AB) physical dispersion is inconsistent with the large velocity drop at the LAB. We explore a new sub‐solidus model, originally proposed by Karato [2012], that depends on grain‐boundary sliding. In contrast to the previous model where only the AB behavior was assumed, the new model predicts an age‐dependent LAB structure including the age‐dependent LAB depth and its sharpness. Strategies to test these models are presented.

Lithospheric mantle thickness gradient focuses seismic activity

Using an array of 556 seismic sensors, Levander and Miller charted two key features of the subsurface structure of the western continental United States, with implications for explaining the locations of seismic and volcanic activity. The solid crust and the solid mantle of the Earth combine to form the lithosphere; together they overlay the plastic mantle of the asthenosphere. Deeper still are the upper and lower mantles and the liquid and solid cores. The boundary between the crust and the solid mantle is known as the Mohorovičić discontinuity (Moho), and the one between the solid and plastic mantle is the lithosphere‐asthenosphere boundary (LAB). Measuring seismic waves produced by 163 earthquakes from 2005 to 2009, the authors charted the depth of Moho and LAB in the western United States and thus the thickness of the solid lithospheric mantle.

Global distribution of the lithosphere-asthenosphere boundary: a new look

Abstract. New global maps of the depth to the boundary between the lithosphere and the asthenosphere are presented. The maps are based on updated global databases for heat flow and crustal structure. For continental regions the estimates of lithospheric thickness are based on determinations of subcrustal heat flow, after corrections for contributions of radiogenic heat in crustal layers. For oceanic regions the estimates of lithospheric thickness are based on the newly proposed finite half-space (FHS) model. Unlike the half-space cooling (HSC) and the plate models the FHS model takes into account effects of buffered solidification at the lower boundary of the lithosphere and assumes that the vertical domain for downward growth of the boundary layer have an asymptotic limit. Results of numerical simulations reveal that theoretical values derived from the FHS model provide vastly improved fits to observational data for heat flow and bathymetry than can be achieved with HSC and plate models. Also, the data fits are valid for the entire age range of the oceanic lithosphere. Hence estimates of depths to lithosphere- asthenosphere boundary (LAB) based on the FHS model are believed to provide more reliable estimates than those reported in previous thermal models. The global maps of depths to LAB derived in the present work reveal several features in regional variations of lithosphere thicknesses that have not been identified in earlier studies. For example, regions of ocean floor with ages less than 55 Ma are characterized by relatively rapid thickening of the lithosphere. Also there is better resolution in mapping the transition from oceanic to continental lithosphere, as most of the latter ones are characterized by lithospheric thickness greater than 150 km. As expected the plate spreading centers in oceanic regions as well as areas of recent magmatic activity in continental regions are characterized by relatively thin lithosphere, with LAB depths of less than 50 km. On the other hand, the areas of continental collisions and Precambrian cratonic blocks are found to have lithosphere thicknesses in excess of 250 km. Regional variations of lithosphere thickness in the interiors of continents are found to depend on the magnitude of subcrustal heat flux as well as the tectonic age of crustal blocks.

Evolutionary aspects of lithosphere discontinuity structure in the western U.S.

We have produced common conversion point (CCP) stacked Ps and Sp receiver function image volumes of the Moho and lithosphere‐asthenosphere boundary (LAB) beneath the western United States using Transportable Array data. The large image volumes and the diversity of tectonic environments they encompass allow us to investigate evolution of these structural discontinuities. The Moho is a nearly continuous topographic surface, whereas the LAB is not and the seismic images show a more complex expression. The first order change in LAB depth in the western U.S. occurs along the Cordilleran hingeline, the former Laurasian passive margin along the southwestern Precambrian North American terranes. The LAB is about 50% deeper to the east of the hingeline than to the west, with most of the increase in LAB thickness being in the mantle lithosphere. We infer that the Moho and the LAB are Late Mesozoic or Cenozoic everywhere west of the hingeline, modified during Farallon subduction and its aftermath. Between the hingeline and the Rocky Mountain Front, the LAB, and to a lesser extent the Moho, have been partially reset during the Cenozoic by processes that continue today. Seismicity and recent volcanism in the interior of the western U.S. are concentrated along gradients in crustal and/or lithospheric thickness, for example the hingeline, and the eastern edge of the coastal volcanic‐magmatic terranes. To us this suggests that lateral gradients in integrated lithospheric strength focus deformation. Similarly, areas conjectured to be the sites of convective downwellings and associated volcanism are located along gradients in regional lithosphere thickness.

The lithosphere-asthenosphere boundary observed with USArray receiver functions

Abstract. The dense deployment of seismic stations so far in the western half of the United States within the USArray project provides the opportunity to study in greater detail the structure of the lithosphere-asthenosphere system. We use the S receiver function technique for this purpose, which has higher resolution than surface wave tomography, is sensitive to seismic discontinuities, and is free from multiples, unlike P receiver functions. Only two major discontinuities are observed in the entire area down to about 300 km depth. These are the crust-mantle boundary (Moho) and a negative boundary, which we correlate with the lithosphere-asthenosphere boundary (LAB), since a low velocity zone is the classical definition of the seismic observation of the asthenosphere by Gutenberg (1926). Our S receiver function LAB is at a depth of 70–80 km in large parts of westernmost North America. East of the Rocky Mountains, its depth is generally between 90 and 110 km. Regions with LAB depths down to about 140 km occur in a stretch from northern Texas, over the Colorado Plateau to the Columbia basalts. These observations agree well with tomography results in the westernmost USA and on the east coast. However, in the central cratonic part of the USA, the tomography LAB is near 200 km depth. At this depth no discontinuity is seen in the S receiver functions. The negative signal near 100 km depth in the central part of the USA is interpreted by Yuan and Romanowicz (2010) and Lekic and Romanowicz (2011) as a recently discovered mid-lithospheric discontinuity (MLD). A solution for the discrepancy between receiver function imaging and surface wave tomography is not yet obvious and requires more high resolution studies at other cratons before a general solution may be found. Our results agree well with petrophysical models of increased water content in the asthenosphere, which predict a sharp and shallow LAB also in continents (Mierdel et al., 2007).

PandSreceiver function analysis of seafloor borehole broadband seismic data

[1] The crustal and lithospheric structure of the normal oceanic plates is investigated using converted wave techniques (P and S receiver functions (RFs) and novel stacking analysis techniques without deconvolution) applied to the data from two seafloor borehole broadband seismic stations located in the central Philippine Sea and in the northwest Pacific ocean. We observe sufficient energy from at least two discontinuities within the error bounds, one from the crust-mantle (Moho) boundary and the other from the seismic lithosphere-asthenosphere boundary (LAB). Synthetic seismograms for seafloor stations show that the water reverberations interfere with the vertical component of seismograms but to a lesser extent with the radial part of P receiver functions. On the other hand, S receiver functions are devoid of such effects since all the multiples and converted waves are separated in time by the primary S wave in time. Waveform modeling of RFs shows that the crustal thicknesses of the western Philippine Sea plate and northwest Pacific plate are ∼7–8 km, and that depths of LAB are 76 ± 1.8 km and 82 ± 4.4 km, respectively, with an abrupt Vs drop at LAB of ∼7%–8%, as reported by Kawakatsu et al. (2009). The LAB depth for the eastern Philippine plate is found to be ∼55 km. To confirm the robustness of this observation, we further analyze vertical and radial components of the data without deconvolution for P wave backscattered reflection phases and P-to-S converted phases. The result indicates that the reflected/converted phases from Moho and LAB are observed at timings consistent with the receiver function results. The effect of seismic anisotropy for observed RFs is also investigated.

Electrical conductivity of continental lithospheric mantle from integrated geophysical and petrological modeling: Application to the Kaapvaal Craton and Rehoboth Terrane, southern Africa

[1] The electrical conductivity of mantle minerals is highly sensitive to parameters that characterize the structure and state of the lithosphere and sublithospheric mantle, and mapping its lateral and vertical variations gives insights into formation and deformation processes. We review state-of-the-art conductivity models based on laboratory studies for the most relevant upper mantle minerals and define a bulk conductivity model for the upper mantle that accounts for temperature, pressure, and compositional variations. The bulk electrical conductivity model has been integrated into the software package LitMod, which allows for petrological and geophysical modeling of the lithosphere and sublithospheric upper mantle within an internally consistent thermodynamic-geophysical framework. We apply our methodology to model the upper mantle thermal structure and hydrous state of the western block of the Archean Kaapvaal Craton and the Proterozoic Rehoboth Terrane, in southern Africa, integrating different geophysical and petrological observables: namely, elevation, surface heat flow, and magnetotelluric and xenolith data. We find that to fit the measured magnetotelluric responses in both the Kaapvaal and Rehoboth terranes, the uppermost depleted part of the lithosphere has to be wetter than the lowermost melt-metasomatized and refertilized lithospheric mantle. We estimate present-day thermal lithosphere-asthenosphere boundary (LAB) depths of 230–260 and 150 ± 10 km for the western block of the Kaapvaal and Rehoboth terranes, respectively. For the Kaapvaal, the depth of the present-day thermal LAB differs significantly from the chemical LAB, as defined by the base of a depleted mantle, which might represent an upper level of melt percolation and accumulation within the lower lithosphere.