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We analyzed receiver-function data recorded by a temporary broadband array deployed as part of the BOLIVAR project and the permanent seismic network of Venezuela to study the mantle transition zone structure beneath the Caribbean-South American plate boundary and Venezuela. Significant topography on both the 410-km and the 660-km discontinuities was clearly imaged in the CCP (common-conversion-point) stacked images. Beneath the southeastern Caribbean, the 410-km is featured by a narrow (∼ 200 km EW) ∼ 25-km uplift extending in the NS direction around 63° west, while the 660-km is depressed by ∼ 20 km in a narrow region slightly west to the uplift, a scenario that is more consistent with westward descent of the oceanic South American plate rather than a break-off of NNW dipping proto-Caribbean oceanic lithosphere along the El Pilar Fault. We also found a thick transition zone beneath the Falcon region in northwestern Venezuela, possibly associated with the subducted Nazca plate. A flat 410-km was observed beneath the Guayana shield, suggesting that the shield has a stable and moderately deep keel, which has little effect on the underlying transition zone structure.

The Caribbean–South America plate boundary zone is a complex zone of plate interactions, forming thrust belts and foreland basins in northern Venezuela. Within the framework of the BOLIVAR and GEODINOS projects, the geodynamics of plate interactions is being investigated using interdisciplinary geological and geophysical methods. Here, we focus on the results of the land based active seismic observations done in 2004 along four deep seismic wide angle profiles, acquired perpendicular to the Caribbean–South America plate boundary in northern Venezuela between longitudes 63° W and 70° W, and ranging from about latitudes 12 °N to about 9 °N. The mostly unreversed profiles provide information on the crustal structure from the oceanic-transitional crust on the southern border of the Caribbean plate to the continental crust of the Caribbean Mountain System and their associated foreland basins, which are bordered to the south by the Guayana Shield, which corresponds to stable South America plate. The derived crustal thickness oscillates around 35 km along the coastline, corresponding to the Caribbean Mountain System, and decreases only slightly towards the Leeward Antilles. To the south, in the area of the Eastern Venezuela Basin, crustal thickness reaches 40 km, increasing towards the Guayana Shield to 45 km. Nevertheless, there are two regions of anomalous crustal thickness, proven by arrivals from the lower crust and the Moho discontinuity. In the eastern part of the Eastern Venezuela Basin, crustal thickness reaches up to 50 km, with high velocity anomalies within the lower crust, which are interpreted as reworked lower crustal and upper mantle material, associated to the plate interactions of the South American and the Caribbean plates. The second anomalous zone is a remarkable crustal thinning from 35 km to 27 km in the Falcon Basin in western Venezuela, which extends eastwards into the Bonaire Basin, as documented by PmP reflections derived from land shots, and observations of the air gun blasts on the stations of the Venezuelan seismological network.

A suite of reflection seismology investigations of the shallow subsurface was conducted at Hill Air Force Base, Ogden, Utah, at a groundwater contamination site with very large subsurface mechanical heterogeneity. The investigations were designed to expand and improve understanding of the subsurface environment developed from well data and previous seismic investigations. The goal of the investigations was to image the sides and bottom of a paleochannel eroded in a clay layer [Formula: see text] below the surface. The paleochannel is filled with a mix of sands, clays, and gravels of different compaction and water saturation. The paleochannel acts as a contaminant trap for dense nonaqueous-phase liquids (DNAPLs). Therefore, a detailed map of its geometry, lateral boundaries, and depth is crucial to remediation efforts. The results of processing a 3D reflection data set over the channel showed that it is possible to image and delineate the geometry and depth of a very shallow structure. Seismic results agreed well with the available well data in the deepest part of the buried paleochannel. The map of the channel derived from seismic data replaces the need for many wells.

We have applied acoustic-waveform tomography to 45 2D seismic profiles to image the 3D geometry of a buried paleochannel at a groundwater-contamination site at Hill Air Force Base in Utah. The paleochannel, which is incised into an alluvium-covered clay aquitard, acts as a trap for dense nonaqueous-phase liquids (DNAPLs) that contaminate the shallowest groundwater system in the study area. The 2D profiles were extracted from a 3D surface reflection data set. First-arrival traveltime tomography provided initial velocity models for the waveform tomography. We inverted for six frequency components in the band [Formula: see text] of the direct and refracted waves to produce 45 2D velocity models. The flanks and bottom of a channel with a maximum depth of about [Formula: see text] were well modeled in most of the 45 parallel 2D slices, which allowed us to construct a 3D image of the channel by combining and interpolating between the 45 image slices. The 3D model of the channel will be useful for siting extraction wells within the site remediation program. The alluvium that fills the channel showed marked vertical and lateral velocity heterogeneity. Traveltime tomography and waveform tomography can be complementary approaches. Used together, they can provide high-resolution images of complicated shallow structures.

We have applied traveltime tomography to 3D seismic refraction data collected at Hill Air Force Base, Utah, in an approximately [Formula: see text] area over a shallow [Formula: see text] groundwater contamination site. The purpose of this study is to test the ability of 3D first-arrival-time data to characterize the shallow environment and aid remediation efforts. The aquifer is bounded below by a clay aquiclude, into which a paleochannel has been incised and acts as a trap for dense nonaqueous phase liquid (DNAPL) contaminants. A regularized nonlinear tomographic approach was applied to [Formula: see text] first-arrival traveltimes to obtain the smoothest minimum-structure 3D velocity model. The resulting velocity model contains a velocity increase from less than [Formula: see text] in the upper [Formula: see text]. The model also contains a north-south-trending low-velocity feature interpreted to be the paleochannel, based on more than 100 wells in the area. Checkerboard tests show [Formula: see text] lateral resolution throughout most of the model. The preferred final model was chosen after a systematic test of the free parameters involved in the tomographic approach, including the starting model. The final velocity model compares favorably with a 3D poststack depth migration and 2D waveform inversion of coincident reflection data. While the long-wavelength features of the model reveal the primary target of the survey, the paleochannel, the velocity model is likely a very smooth characterization of the true velocity structure, particularly in the vertical direction, given the size of the first Fresnel zone for these data.

This paper is primarily concerned with seismically imaging details in the mantle at an intermediate scale length between the large scales of regional and global tomography and the small scales of reflection profiles and outcrops. This range is roughly 0.1–1 km We also examine the seismic velocity and density contrasts expected between various upper mantle constituents, including fertile upper mantle, depleted upper mantle, normal and eclogitized oceanic crust, and fertile mantle with and without partial melt. This leads directly to predicting the size of seismic signals that can be produced by specular conversion, and scattering from layers and objects with these contrasts. We introduce an imaging scheme that makes use of scattered waves in teleseismic receiver functions to make a depth migrated image of a pseudo-scattering coefficient. Image resolution is theoretically at least an order of magnitude better than traveltime tomography. We apply the imaging scheme to three data sets from 1) the Kaapvaal craton, 2) the Cheyenne Belt, a Paleoproterozoic suture between a protocontinent and an island arc, and 3) the Jemez Lineament, a series of aligned modern volcanic structures at the site of a Proterozoic suture zone. The Kaapvaal image, although not defining a unique base of the tectosphere, shows complicated “layered” events in the region defined as the base of the tectosphere in tomography images. The image of the transition zone discontinuities beneath the Kaapvaal craton is remarkable for clarity. The migrated receiver function image of the upper mantle beneath the Cheyenne belt is complicated, more so than the tomography image, and may indicate limitations in the receiver function imaging system. In contrast the Jemez Lineament image shows large-amplitude negative-polarity layered events beneath the Moho to depths of ∼120 km, that we interpret as melt-containing sills in the upper mantle. These sills presumably feed the Quaternary–Neogene regional basaltic volcanic field.

Application of 2D frequency-domain waveform tomography to a data set from a high-resolution vertical seismic profiling (VSP) experiment at a groundwater contamination site in Hill Air Force Base (HAFB), Utah, reveals a surprisingly complicated shallow substructure with a resolution of approximately 1.5 m. Variance in the waveform misfit function is reduced 69.4% by using an initial velocity model from first-arrival traveltime tomography. The waveform tomography model suggests (1) a low-velocity layer at 1 to 4 m depth, (2) a high-vertical-velocity gradient of 80 m/s/m on average, and (3) severe lateral variations — velocity contrasts as large as about 200 m/s occur in a distance as short as 1.5 m. The model is well correlated with lithologic logs and is interpreted geologically. A Q-value of 20 is estimated for the target area. The extreme lateral and vertical variations of the subsurface compromise many standard seismic processing methods.

We have used 4th root receiver function stacks, and pre-stack receiver function depth migrations to study the transition zone discontinuity structure beneath southwestern Japan. Receiver functions were calculated from the quiet short-period seismograms recorded by a recently deployed borehole network, Hi-net. We found that a relatively broad frequency band can be retrieved from a short-period seismogram by a deconvolution of the instrument response. The quality of the receiver functions formed from large earthquake recordings is comparable to those from broadband instruments. We applied common-conversion-point gathering to the receiver-function data to image the P to S conversion events beneath the network by stacking with a 4th root technique to improve lateral coherence. We found that the topographic anomalies of the 410- and 660-km discontinuities beneath southwest Japan have very different length scales. The former is characterized by a narrow, ∼150–200 km wide, topographic high, while the latter exhibits a broad, > 400 km wide, moderate topographic low together with a small-scale, larger-amplitude depression. A 2.5D pre-stack depth migration of the receiver functions shows the transition zone features clearly, as well as images of a change of slope in the subducting slab at the 410 discontinuity and flattening of the slab onto the 660-km. These observations show that the subducted Pacific slab is deflected when it encounters the upper and lower boundaries of the transition zone, and is flat lying either above or across the 660-km discontinuity. The flat lying slab is, however, restricted to the bottom of the transition zone, and probably experiences much less thickening than is suggested by some global tomographic images in which subhorizontal high velocity anomalies are seen throughout the transition zone between the two discontinuities.

An important feature of continents and oceans is that they are underlain by chemically distinct mantle, made intrinsically buoyant and highly viscous by melt depletion and accompanying dehydration, respectively. Of interest here are the influences of these preexisting chemical boundary layers on small-scale convective processes (as opposed to large-scale processes, which govern the drift of continents and the eventual fate of oceanic thermal boundary layers, e.g., subduction) at the base of the oceanic and continental thermal boundary layers. This manuscript explores the endmember in which dehydrated and melt-depleted boundary layers are assumed to be strong (in the viscous sense) and chemically buoyant enough that they do not partake in any secondary convection, that is, vertical heat transfer through these lids occurs purely by conduction. This assumption implies that the onlypart ofthe thermalboundary layerthat participates insecondary convectionresides beneath the strong chemical boundary layer. For oceans, this leads to the condition that the onset time of convective instability is suppressed until after the thermal boundary layer has cooled through the base of the strong chemical boundary layer, whose thickness is defined at the outset by the depth at which the solid mantle adiabat crosses the anhydrous peridotite solidus. A scaling law is presented that accounts for the presence of a preexisting strong chemical boundary layer and predicts that the onset time of convective instability beneath oceans correlates with the thickness of the chemical boundary layer, which itself correlates with the potential temperature of the mantle at thetimeofmelting.Estimatedpaleo-potentialtemperaturesrequiredtogenerateoldoceaniccrustinthePacificandAtlanticmayin fact be correlated with onset time of seafloor flattening, but more data are needed to confirm these preliminary observations. Finally, for continents, recent numerical models suggest that the thickness of the convective sublayer, hence the total thermal boundary layer thickness, is also controlled by the thickness of a preexisting strong chemical boundary layer. Xenolith data from cratons are shown to be largely consistent with the model-predicted relationship between the thicknesses of the chemical and thermal boundary layers beneath continents. The conclusion of thisstudy isthat the natureof bothoceanic andcontinental thermal boundary layers is likely to be linked to preexisting strong chemical boundary layers. D 2004 Elsevier B.V. All rights reserved.

S–P travel-time residuals and receiver-function images are used to infer the Vp/Vs (compressional to shear wave velocity) ratio of the lithospheric mantle beneath southern African and the topography of the underlying 410-km discontinuity. Low Vp/Vs ratios provide evidence independent of geochemical observations for a highly depleted root (Mg#f92–94) beneath the Kaapvaal craton. The receiver-function images, on the other hand, consistently show a flat 410-km discontinuity beneath the entire array. This observation, after combined with the results of geodynamical modeling, allows us to place limits on the thickness of this chemical boundary layer, which is between f160 and f370 km. D 2004 Elsevier B.V. All rights reserved.

Although seismic tomography is currently the mainstay of global seismic imaging, the large-scale deployment of portable instruments to be conducted as the USArray component of Earthscope over the next decade will allow much higher resolution imaging of the solid earth using migration/inversion methods similar to those used in petroleum exploration. Direct wavefield imaging in solid earth structural seismology has been uncommon in comparison to practices in exploration seismology, generally due to the large separation between seismic stations. With the development of portable observatory quality broadband (0.01–20 Hz) seismographs and acquisition of modest numbers of them for the PASSCAL instrument pool over the past decade, a number of academic field programs have had receiver spacing dense enough for wavefield back-projection and imaging. Recordings from these experiments have been used in several different imaging schemes, all of which are outgrowths of and bear resemblance to either post- or prestack d...

The impetus to write this article stems from a special session at SEG's 2010 Annual Meeting on “Interaction between Academia and Industry.” The session articulated important issues that both industry and academia involved with Earth sciences have to deal with and the panelists and authors of this paper decided to meet at two subsequent meetings in Houston in the spring of 2011 as an ad-hoc group. The group was engaged in the discussion because of the importance of the human resources issue. The group focused on the problem of a severe manpower shortage that is predicted in the Earth science field in the near future. This article summarizes the issue with data and possible solutions.

Wide‐aperture, prestack depth migration requires application of challenging and time‐consuming velocity analysis and depth focusing, collectively referred to here as depth focusing. We present an approach to depth focusing using (1) a detailed starting velocity model obtained by a 1‐D transformation of the first‐arrival times, followed iteratively by (2) interactive analysis of the common‐image gathers, (3) computation of coherency attributes of the wavefield in the depth domain, and (4) 2‐D traveltime tomography to update the background velocity model. We employ two interactive method of migration velocities refinement. In the first method (similar to the common‐midpoint velocity spectrum approach), residual velocity updates are picked directly from the common‐image gathers. In another method (analogous to the common velocity stacks), we pick the velocity updates from the areas of maximum coherency in depth sections that are migrated using rescaled traveltime maps. Both types of migration velocity picks, optionally combined with the first arrivals, are inputs for a 2‐D traveltime inversion scheme that uses either the infinite‐frequency or a finite‐bandwidth approximation. This flexible and versatile depth focusing approach is implemented for several prestack depth migration algorithms and illustrated on an application to a real, ultrashallow seismic data set. The technique resolves overburden velocity variations and facilitates reliable high‐resolution reflection imaging of a paleochannel that was the target of the study.

Analysis of the Lithoprobe Deep Probe and Southern Alberta Refraction Experiment data sets, focusing on the region between Deep Probe shots 43 and 55, has resulted in a continental-scale velocity structural model of the lithosphere of platformal western Laurentia reaching depths of ~150 km. Three major lithospheric blocks were investigated: (i) the Hearne Province, a typical continental Archean cratonic province lying beneath the Western Canada Sedimentary Basin; (ii) the Wyoming Province, an even older block of Phanerozoic-modified Archean crust with an enigmatic lower lithosphere; and (iii) the Yavapai–Mazatzal Province, Proterozoic terranes underlying the Colorado Plateau and Southern Rocky Mountains. In this study, the northern two of these regions are investigated with a modified ray-theoretical traveltime inversion routine that respects the spherical geometry of the Earth. The resulting crustal velocity structure, combined with supporting geological and geophysical data, reveals that the Medicine Hat block (MHB), lying between the Hearne and Wyoming provinces, is a third independent Archean crustal block. The subcrustal lithosphere along the profile is homogeneous in velocity structure, but two significant northward-dipping reflectors are apparent and interpreted as relic subduction zones associated with sutures between the three Archean blocks. The Hearne crust is typical of an Archean shield or platform both in its thickness of 34–50 km and its seismic velocity structure. The crust of the Archean MHB and Wyoming Province, which ranges in thickness from 49 to 60 km, includes a 10–30 km thick high-velocity layer, interpreted to be Proterozoic in age. Such a feature is unexpected beneath Archean crustal provinces, but if the region is considered to be the remanent marginal portion of a larger Archean continent, then the interpreted Proterozoic underplating and lack of an Archean lithospheric root can be explained. The variable topography along the reflective upper and lower boundaries of this layer, especially within the MHB, suggests considerable variability in its emplacement and subsequent tectonic history.

The crustal structure and topography of the Moho boundary beneath the Atlas Mountains of Morocco has been constrained by a controlled source, wide-angle seismic reflection transect: the SIMA experiment. This paper presents the first results of this project, consisting of an almost 700 km long, high-resolution seismic profile acquired from the Sahara craton across the High and the Middle Atlas and the Rif Mountains. The interpretation of this seismic data set is based on forward modeling by raytracing, and has resulted in a detailed crustal structure and velocity model for the Atlas Mountains. Results indicate that the High Atlas features a moderate crustal thickness, with the Moho located at a minimum depth of 35 km to the S and at around 31 km to the N, in the Middle Atlas. Upper crustal shortening is resolved at depth through a crustal root where the Saharan crust underthrusts the northern Moroccan crust. This feature defines a lower crust imbrication that, locally, places the Moho boundary at 40-41 km depth in the northern part of the High Atlas. The P-wave velocity model is characterized by relatively low velocities, mostly in the lower crust and upper mantle, when compared to other active orogens and continental regions. These low deep crustal velocities together with other geophysical observables such as conductivity estimates derived from MT measurements, moderate Bouguer gravity anomaly, high heat flow, and surface exposures of recent alkaline volcanism lead to a model where partial melts are currently emplaced at deep crustal levels and in the upper mantle. The resulting model supports the existence of a mantle upwelling as mechanism that would contribute significantly to sustain the High Atlas topography. However, the detailed Moho geometry deduced in this work should lead to a revision of the exact geometry and position of this mantle feature and will require new modeling efforts, This work has been primarily funded by the Spanish MEC project CGL2007–63889. Additional funding was provided by projects CGL2010–15416, CSD2006-00041, and GL2009–09727 (Spain), CGL2008–03474-E, 07-TOPO_EUROPE_FP-006 (ESF Eurocores) and EAR-0808939 (US, NSF).

The western Mediterranean tectonic system consists of the Betic Mountains in southern Spain and the Rif Mountains in northern Morocco curved around the back-arc extensional Alboran basin. Multiple tectonic models have been developed to explain the coeval compressional and extensional tectonic processes that have affected the western Mediterranean since the Oligocene. In order to provide constraints on these evolutionary models, we use Ps teleseismic receiver functions (RF), thermobarometric analyses of post-Oligocene basalts, and previous teleseismic tomography images to investigate the lithospheric structure of the region. Ps RFs were calculated using seismic data from 239 broadband seismic stations in southern Iberia and northern Morocco and thermobarometric analysis was performed on 19 volcanic samples distributed throughout the region. The RF images reveal a highly variable Moho depth (∼25 to ∼55 km), as well as a strong positive, sub-Moho horizon between ∼45 and ∼80 km depth beneath the central Betic and Rif Mountains, which we interpret to be the top of the previously imaged Alboran Sea slab. Thermobarometric constraints from magmas in the eastern Betics and Rif indicate mantle melting depths between 40 and 60 km, typical of melting depths beneath mid-oceanic ridges where little to no lithosphere exists. Together, the RF and thermobarometric data suggest ongoing and recent slab detachment resulting from delamination of the continental lithosphere. Key Points There is a highly variable lithospheric structure throughout the Gibraltar Arc Slab is delaminating continental lithosphere in Gibraltar Arc Slab pull force is exerted on base of lithosphere causing depression of Moho © 2014. American Geophysical Union. All Rights Reserved., This research was funded by the U.S. National Science Foundation EAR-0808939. The deployment of the IberArray broadband seismic network is part of the CONSOLIDER CSD2006-00041 (Geosciences in Iberia: Integrated studies on Topography and 4D Evolution) grant from the Spanish Ministry of Science and Innovation. Additional funding was provided by the Spanish ministry under grants CGL2010–17280 and by Generalitat de Catalunya under grant 2009 SGR 6.

The different geodynamic models proposed since the late 1990s to account for the complex evolution of the Gibraltar Arc System lack definite constraints on the crustal structure of the Rif orogen. Here we present the first well-resolved P-wave velocity crustal models of the Rif Cordillera and its southern continuation toward the Atlas made using controlled-source seismic data. Two 3001 km-long wide-angle reflection profiles crossed the Rif along NS and EW trends. The profiles recorded simultaneously five land explosions of 1Tn each using 850 high frequency seismometers. The crustal structure revealed from 2-D forward modeling delineates a complex, laterally varying crustal structure below the Rif domains. The most surprising feature, seen on both profiles, is a 50 km deep crustal root localized beneath the External Rif. To the east, the crust thins rapidly by 20 km across the Nekkor fault, indicating that the fault is a crustal scale feature. On the NS profile the crust thins more gradually to 40 km thickness beneath Middle Atlas and 42 km beneath the Betics. These new seismic results are in overall agreement with regional trends of Bouguer gravity and are consistent with recent receiver function estimates of crustal thickness. The complex crustal structure of the Rif orogen in the Gibraltar Arc is a consequence of the Miocene collision between the Iberian and African plates. Both the abrupt change in crustal thickness at the Nekkor fault and the unexpectedly deep Rif crustal root can be attributed to interaction of the subducting Alboran slab with the North African passive margin at late Oligocene-early Miocene times., This research was funded by Spanish RIFSIS project CGL2009–09727. AG benefited from a PhD grant associated to this project. Support was provided by Spanish projects CSD2006-00041 ‘‘Topo-Iberia’’ and CGL2008–03474 ‘‘Topo-Med,’’ as well as the NSF Continental Dynamics Program Grant EAR0808939.

In order to elucidate the nature and history of the different geological units in the western USA and correlate geochemical signatures with features identified in seismic models developed from USArray, we compiled from the literature Pb isotope compositions of ores, granitic K-feldspars, and felsic plutonic rocks and complemented the database by analyzing Pb isotopes for 82 felsic xenoliths and plutonic rocks from the Colorado Plateau and its immediate vicinity. The raw Pb isotope abundances for the complete data set were converted into model ages T (time of last U/Pb fractionation), 238 U/ 204 Pb (μ)

Determination of the physical properties and the geometry of features within the subsurface is a common application of active source seismic data. We use the example of unusually bright reflections identified in the lower crust of the Coast Ranges of northern California during the Mendocino Triple Junction Seismic Experiment to investigate the robustness of such determinations. The sources of these reflections are significant because they lie in a region of transition between subduction and strike-slip tectonics, but they also have an overburden of heterogeneous Franciscan terrane rocks. We use finite difference synthetic seismograms including a stochastic overburden representative of Franciscan rocks and a realistic target geometry to examine how typical seismic diagonostics are affected under these conditions. The addition of a realistic overburden to the models replicates variations in the amplitude of the observed Pg arrival and leads to variations in measures of the amplitude of the reflected arrivals that are also similar to those in the data. Some of these variations are due to the measures of amplitude used, some represent real variations at the target, and others are due to distortion of the illumination field by the overburden. Overall reflection amplitudes are reduced due to transmission losses within the overburden, and the P wave seismic velocity within the reflectors may be as low as 2.5 km?s?1 rather than the 3.5 km?s?1 estimated when the overburden was not taken into account. Migrations of simulations, including combinations of overburden, target dip, and uneven spatial sampling, recover somewhat consistent but inaccurate approximations to the true target geometry: Discontinuous migrated images may result from a discontinuous wave field even where reflectors were continuous. The simulations also imply that neither reversed reflection polarity nor converted shear waves are a reliable aspect of the reflected wave field if the overburden or target is complex. Comparison of migrated simulations to the data suggests that the reflections originate from two relatively continuous layers at the top and base of the lower crust, 300 m and 100–150 m in thickness, respectively. We believe that the reflections are from lenses of basaltic melt recently emplaced in the crust and generated by decompression melting in a “slab-free” window.

As the Mendocino triple junction (MTJ) moves northwards up the North American margin, the tectonic regime changes from subduction to strike-slip. For the first few million years following triple junction migration, the San Andreas Fault system consists of several strike-slip faults distributing deformation over a region ~ 150 km wide. This same region is expected to be affected by a slab gap beneath North America, created by the northward removal of the subducting Gorda plate, and into which asthenospheric mantle is thought to rise to crustal depths. The onshore and offshore Mendocino Triple Junction Seismic Experiment (MTJSE) provides a continuous seismic velocity-reflectivity cross-section across the deforming zone from the Pacific ocean basin to the eastern edge of the California Coast Ranges. The accretionary complex rocks that make up most of the crustal thickness are underlain by a 5-10 km thick high-velocity(6.4-7.2 km s- 1) layer at the base of the crust that extends from the Pacific to at least 50 km, and probably 90 km east of the San Andreas Fault. The top of the lower crustal layer deepens from 7 km beneath the Pacific ocean basin at the west end of the profile to 23 km at the east end by a gentle (5° -10°) eastward dip punctuated by abrupt offsets at the San Andreas and Maacama fault zones. At each fault the top of the lower crust is offset by up to 4 km, down to the east. The Moho is similarly deformed beneath the faults, although by only 2 km. Such localized deformation of the Moho implies that these two strike-slip faults penetrate through the entire crust to the upper mantle. Good agreement between seismic velocity and seismic reflectivity in the vicinity of the faults gives confidence in these results, although details of the offset beneath the San Andreas Fault are better resolved than those under the Maacama Fault. Seismic velocities in the upper mantle show only a small change along the profile, from 8.1 km s- 1 beneath the Pacific to about 7.9 km s- 1 beneath the Coast Ranges. We infer that upwelling of asthenosphere into the slab gap is limited laterally, or a lithospheric lid is present in the slab gap by 2 Ma. Gravity data and crustal density structure show that most of the margin width is in local Airy isostasy with the changes in crustal thickness near the strike-slip faults corresponding closely to changes in surface topography. The crustal blocks defined by the strike-slip faults appear to be independently in isostatic equilibrium, provided that the mantle beneath the Coast Ranges has a somewhat lower density than that beneath the Pacific plate. The densities in the Coast Range upper mantle are consistent with limited temperature elevation, suggesting that the asthenospheric mantle is present beneath the depth of seismic energy penetration from our survey.

An unprecedented examination of the Earth's deep interior and investigation across a broad range of scales of the structure of the North American continent and the processes that formed it would be among the undertakings of a proposed 10-year Earth Science project called USArray. Now in the planning and development stage, the project would permit a three-dimensional (3-D) systematic investigation of North America, improving the resolution of lithospheric images by an order of magnitude. For the Earth sciences, the project would be the seismological equivalent of the Hubble space telescope. A number of factors suggest that North America is particularly suited for this project, including the states of current knowledge and technology, the availability of a sophisticated infrastructure, organization in the seismological community scientific economy, and widespread scientific interest.

The Rocky Mountain region has undergone a complex tectonic history that includes Proterozoic accretion to form the North American craton, late Paleozoic deformation, Cretaceous to early Tertiary shortening, and Oligocene to Recent extension. Understanding the effects of these events on lithospheric structure was the primary goal of the Deep Probe seismic experiment. This is a lithospheric-scale study of the Rocky Mountain region that attempted to image crust and upper mantle structures up to 500 km depth to provide insights on the effect of various tectonic events on today's continental structure. To accomplish this goal, instruments were deployed along a 2400-km-long transect from New Mexico to Canada to record explosions 10 times more powerful than those employed in conventional crustal studies. The Deep Probe results provide new constraints on the location and geometry of the Archean–Proterozoic boundary near the Colorado–Wyoming border, as well as new information on crustal thickness, and uppermost mantle velocities along the profile. Geophysical modeling of the profile used well log and geologic data to evaluate the composition and structure of the uppermost crust. Seismic refraction and reflection, gravity, and receiver function studies were employed to constrain properties of the lower crust and upper mantle structure. The final model shows that seismic velocities along the Deep Probe profile range from 3.5 km/s in the basins to over 8.2 km/s in the upper mantle. At the southern end of the profile, the model indicates a crustal thickness of about 35 km beneath the Basin and Range province. The crust gradually thickens to about 40 to 45 km going north along the profile into the Colorado Plateau. An area of 50 km-thick crust under northwestern Colorado may reflect Proterozoic tectonism related to the suture zone between the Archean and Proterozoic terranes. Northwestward thinning of the crust to about 40 km under southern Wyoming is interpreted as evidence for a relict (2.0 Ga) passive continental margin. The crust in the Archean Wyoming province thickens to over 50 km going north, and then thins again under southern Canada. This thickening is due to a lowermost crustal layer that is about 20 km thick and is confined to the Archean Wyoming province. This lower crustal layer has velocities ranging from 7.05 to 7.3 km/s, which corresponds to a mafic composition. Thus, this layer is interpreted as mafic material that was probably underplated during the Archean. The uppermost mantle of the Archean Wyoming province has lower velocities (∼8.1 km/s) on average than typical cratonal areas, which is consistent with it being located in and adjacent to the North American Cordillera, which has undergone significant recent tectonism.

In spite of an order of magnitude increase over the past 15 years in spatial sampling of the wavefield, a major uncertainty in the analysis of active source seismic data remains phase identification. This uncertainty results in part from the wide range of spatial scales of velocity heterogeneity in the crust. Smaller scale variations than those which can be deterministically resolved given the design of a particular seismic experiment can be modeled statistically using geologic constraints. Here we present synthetic seismograms generated from several different realizations of a stochastic model describing the velocity heterogeneity of Franciscan terrane rocks. We compare the results to observed data and to synthetic seismograms generated for a model derived from tomographic inversion of the data in order to obtain qualitative insights into the relative importance of large and small scale velocity heterogeneity. Not surprisingly, the synthetic data for the tomographic model best reproduce observed small-scale variations in first arrival time, which only occur for particular realizations of the stochastic model. The synthetic seismograms generated for the stochastic models best reproduce the level of signal-generated noise and suggest that the amplitude of velocity variation locally within the Franciscan is approximately 1 km/s. They also illustrate the effect of a strongly heterogeneous upper and mid-crust on the amplitude-versus-offset pattern of arrivals from the lower crust and upper mantle. These effects may sometimes be interpreted deterministically, leading to biased models or an overly optimistic estimate of lower crustal resolution.

A continuous seismic velocity and reflectivity cross-section of the San Andreas fault system in northern California shows offsets in the lower crust and the Mohorovic̆ić Discontinuity near the San Andreas and Maacama strike-slip faults. These faults may cut through the crust to the upper mantle in a zone less than 10 kilometers wide. The northern California continental margin to the eastern edge of the Coast Ranges is underlain by a high-velocity lowermost crustal layer that may have been emplaced within 2 million years following the removal of the Gorda plate slab. The rapid emplacement and structure within this layer are difficult to reconcile with existing tectonic models.

We have interpreted an integrated vertical incidence to wide-angle seismic data set to develop a consistent migrated seismic reflection image and seismic velocity model of the Brooks Range fold and thrust belt in north central Alaska. The common midpoint (CMP) reflection data image the principal structures comprising the Brooks Range: the Endicott Mountains allochthon (EMA), the crustal scale Doonerak duplex, the master detachment, a 1.0–1.5 s thick zone of lower crustal reflectivity just above the crust-mantle boundary, and a complex crustal root. The master detachment separates the crust into units which have been uplifted and deformed in the fold and thrust belt from those which have not. Least squares inversion of both reflection and refraction travel time data produced a velocity model consistent with the CMP image of the Brooks Range as well as with the Bouguer gravity data. The different layers comprising the seismic velocity model correlate well with the principal structural elements identified in the seismic reflection data, and seismic velocities in the model compare favorably to petrophysical data from Brooks Range rock samples. Maximum crustal thickness in the Brooks Range is 49 km, at an asymmetric root located under the EMA. At the root we observe an offset in the lower crustal reflectivity and two deep zones of reflections north of the root. We interpret these as a Moho offset of some 5 km near the range front, which in our favored interpretation resulted from subduction of the Brooks Range lower crust northward beneath the North Slope. A mantle reflective zone which we interpret as the subducted lower crust can be traced to depths as great as 65 km. Above this zone at considerably shallower depth is the original North Slope Moho. Proximity of the continental subduction zone to the crustal scale Doonerak duplex suggests that the development of the fold and thrust belt has been at least partially controlled by the lower crust/mantle subduction.

Stochastic models for the crystalline crust produce synthetic seismograms that compare well with recorded data for a variety of crustal ages and tectonic environments. In this paper, we explore the parameter space describing such stochastic models as a basis for formulating the inverse problem; that is, we wish to estimate the parameters which define a stochastic model from the recorded backscattered wave field. We base the estimation on approximate relations between the primary reflectivity series, which is the ideal wave field response of a medium, and various seismic gathers. A two-dimensional lateral correlation method is used to investigate the sensitivity of synthetic wave fields to horizontal characteristic length. A derived empirical relationship relates the scale length to the half width of the correlation coefficient. A horizontal wavenumber-time domain spectral analysis successfully estimates the horizontal characteristic length (ax) and the fractal dimension (D) of the stochastic medium from which the wave field was backscattered. We employ a Monte Carlo based optimization scheme to fit the observed spectral values to a one-dimensional von Karman function and simultaneously estimate values for ax and the Hurst number ν. The method yields consistent results when tested on synthetic data. Application of this technique to data sorted into different domains shows that zero-offset sections yield the best estimate. We also present results of our estimates using this method on a Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) dynamite source shot gather and a vibroseis-source Consortium for Continental Reflection Profiling (COCORP) data set recorded in the Basin and Range Province.

The seismic signature of the Moho from which geologic and tectonic evolution hypothesis are derived are, to a large degree, a result of the seismic methodology which has been used to obtain the image. Seismic data of diferent types: passive source (earthquake) broad-band recordings; controlled source seismic refraction; controlled source dense wide-angle deep seismic reflection; controlled source normal incidence (Virboseis or explosives); have contributed to delineate the Moho as a relatively complex transition zone. Of critical importance for the quality and resolution of the seismic image are the acquisition parameters, used in the imaging acquisition experiments. A variety of signatures have been obtained for the Moho at diferent scales. This variety prevents the development of a single universally applicable interpretation. In this way source frequency and sensor spacing are mostly responsible for the vertical and lateral resolution of the images, respectively. In most cases the diferent types of data have proven to be complementary in order to provide a full picture of this important structure and its relationship to the crustmantle transition. In regional seismic studies carried out using passive source recordings the Moho is a relatively well de fined structure with marked lateral continuity. The characteristics of this boundary change depending on the geology and tectonic evolution of the targeted area. Controlled source local studies reveal the Moho as a sharp refraction velocity contrast while the Moho in the high quality normal incidence seismic reflection images is interpreted to be the base or abrupt downward decrease in seismic reflectivity. The origin of the Moho and its relation to the crust mantle boundary is most probably better constrained by careful analysis of the structural details of its internal structure, these are complex and varied. Simple conclusions are that: the Moho may be in areas and old feature in others, it can be interpreted as a young structure result of recent tectonic scenarios.

As a high resolution addition to the 1992 Pacific to Arizona Crustal Experiment (PACE), a 45-km-long deep crustal seismic reflection profile was acquired across the Chocolate Mountains in southeastern California to illuminate crustal structure in the transition between the Salton Trough and the Basin and Range province. The complex seismic data are analyzed for both large-scale (deterministic) and fine-scale (stochastic) crustal features. A low-fold near-offset common-midpoint (CMP) stacked section shows the northeastward lateral extent of a high-velocity lower crustal body which is centered beneath the Salton Trough. Off-end shots record a high-amplitude diffraction from the point where the high velocity lower crust pinches out at the Moho. Above the high-velocity lower crust, moderate-amplitude reflections occur at midcrustal levels. These reflections display the coherency and frequency characteristics of reflections backscattered from a heterogeneous velocity field, which we model as horizontal intrusions with a von Karman (fractal) distribution. The effects of upper crustal scattering are included by combining the mapped surface geology and laboratory measurements of exposed rocks within the Chocolate Mountains to reproduce the upper crustal velocity heterogeneity in our crustal velocity model. Viscoelastic finite difference simulations indicate that the volume of mafic material within the reflective zone necessary to produce the observed backscatter is about 5%. The presence of wavelength-scale heterogeneity within the near-surface, upper, and middle crust also produces a 0.5-s-thick zone of discontinuous reflections from a crust-mantle interface which is actually a first-order discontinuity.

Reflection quality in deep crustal seismic sections is often poor due to surface waves, irregularities in the surface layer and variations in topography. In an effort to remove the effects of the variable near-surface layer and topography, we have used wave-equation datuming to downward continue shot gathers to the base of the surface layer, and then redatumed upward to a flat surface using the basement velocity as the replacement velocity. We discuss briefly the theory behind the method, present a simple synthetic example of a datumed shot gather, and conclude with an example of a datumed shot gather from the RISC crustal seismic experiment in southeastern California.

Intermediate range bottom interacting deep ocean acoustic experiments (up to ∼50 km offset in water depths exceeding 1 km using frequencies in the 100–500 Hz range) encompass a variety of different scales: the water column varies smoothly over many wavelengths, whereas the seafloor is rough at scales from 1 m to several kilometers. We present a general hybrid technique to solve the two-dimensional acoustic/viscoelastic equations for problems that include these different wave propagation regimes. This approach makes realistic large-scale computations tractable and assures high accuracy. The technique, which we call Hybrid Adaptive Regime Visco-Elastic Simulation Technique (HARVEST), consists of three parts. A Gaussian beam method is used to propagate the source wave field through a vertically varying water column to the scattering region near the seafloor. This extrapolated source wave field is inserted into a viscoelastic finite difference grid on which the complex acoustic/anelastic interaction of the wave field with the rough seafloor is computed. The backscattered wave field collected on hypothetical vertical and horizontal lines is then extrapolated by means of the Kirchhoff integral to a receiver array distant from the scattering locale. The fidelity of the method is demonstrated by comparison with solutions from a WKBJ approximation method and full finite difference simulations.

Stochastic models are valuable and sometimes essential tools for investigating the behavior of complex phenomena. In seismology, stochastic models can be used to describe velocity heterogeneities that are too small or too numerous to be described deterministically. Where analytic approaches are often infeasible, synthetic realizations of such models can be used in conjunction with finite difference algorithms to systematically investigate the response of the seismic wave field to complex heterogeneity. This paper represents a continuing effort at formulating a complete and robust stochastic model of lithologic heterogeneity within the crust, and the means of generating synthetic realizations; “complete” implies that the model is flexible enough to describe all types of random heterogeneity within the crust, while “robust” implies sufficiently constrained parameterization that an inversion problem may be well-posed. As a basis for investigation we use geologic maps of crustal exposures and petrophysically inferred velocities. Earlier efforts at stochastic modeling have focused on characterization of the univariate probability density function, which is typically modal (i.e., binary, ternary, etc.), and the covariance function, which is typically fit with a von Karman function. Here we provide a means of characterizing the property of “sinuous connectivity” and for generating realizations that possess this property. Sinuous connectivity is the tendency for individual lithologic units to be continuous over long and highly contorted paths; there is no means in the earlier modeling of either characterizing or synthesizing this property. We generate sinuously connective realizations by mapping regions encompassed by two contours in a Gaussian-distributed surface into the two values of the binary field. This operation is nonunique, as one can choose, in many ways, the values for the contours.

Quantitative modeling of bottom-interacting ocean acoustic waves is complicated by the long propagation ranges and by the complexity of the scattering targets. We employ a two-dimensional (2-D) hybrid technique combining Gaussian beam, finite difference, and Kirchhoff integral solutions of the wave equation to simulate ocean acoustic experiments within half of a convergence zone in the SOFAR channel. The 2-D modeling approach is reasonable due to the one-dimensional (1-D) velocity distribution in the water column and the strong lineation of the seafloor morphology parallel to the mid-ocean ridges. Full-waveform modeling of ocean acoustic data requires that the topography and the material properties of the seafloor are available at scales that are several orders of magnitude smaller than typical bathymetric sampling rates. We have therefore investigated the effects on the ocean acoustic response of a stochastic interpolation scheme used to generate seafloor models. For typical grazing angles of the incident wave field (approximately 5°–20°), we found that different stochastic realizations of the same seafloor segment (sampled at 200 m) yield an intrinsic uncertainty of the order of 3–8 dB in amplitude and 0.1–0.3 s in time for individual prominent events in the reverberant acoustic field. Hybrid simulations are compared to beam-formed ocean acoustic data collected during the Acoustic Reverberation Special Research Program (ARSRP) cruises. Side lobe noise in the observed acoustic data is simulated by adding band-limited white noise at −30 dB relative to the maximum intensity in the synthetic data. Numerical simulations can be limited to the response of only one of the mirror azimuth beams provided that the experimental geometry is suitably chosen. For the 2-D approximation to be valid, the cross-range resolution of the observed data must be smaller than the characteristic scale of seafloor lineations, and the beams of interest must be approximately perpendicular to the dominant structural grain. Under these premises, the arrival time and maximum intensity of the observed backscattered acoustic events can be modeled within 0.3 s and 5–10 dB, respectively. The mismatch in arrival time is interpreted to be due predominantly to intrinsic uncertainties in the stochastic interpolation of the seafloor profiles, whereas the fact that the intensity of the backscattered events is systematically overestimated is attributed to 3-D effects within the “footprint” of the beam and/or to underestimated noise levels.

We conducted a high shot and receiver density, onshore-offshore seismic experiment in 1986 across the central California transform margin as part of the Pacific Gas and Electric / EDGE Deep Crustal Geophysical Survey. The data set consists of land recordings of 406 marine air gun shots fired along a 70-km track across the Santa Maria basin. The seismic vessel's track was collinear with 154 land seismic recorders deployed over 50 km across the Sur Obispo terrane and part of the Salinian block. The source-receiver offsets range from 2.5 km to 120 km. The unique recording geometry, and high shot and receiver density, permit imaging the deeper crustal structure of the California transform margin but pose serious challenges for conventional seismic imaging methods. Consequently we developed a unified approach to wide-angle imaging which incorporates a tomography technique for obtaining initial migration velocity models, and a prestack Kirchhoff depth migration with depth focusing of the main reflectors for velocity model refinement and imaging. This approach retains the velocity and dip information in the data and allows user control over the imaging process. We applied this processing to subsets of the onshore-offshore data to obtain an upper crustal velocity model, and a 32-km-long, wide-angle depth migrated image of the lower crust, in the coastal region under the Santa Maria basin, and the Sur-Obispo terrane. The migration image shows a part of an underplated oceanic crustal layer, interpreted as the Monterey plate (Meltzer and Levander, 1991; Miller et al., 1992), which dips coastward at depths of 18 to 28 km. The plate appears to be kinked or imbricated beneath the coastline. The image correlates well with existing wide-angle seismic interpretations based on ray tracing (Walter and Sharpless, 1987; Putzig, 1988; Howie et al., 1993). The migrated lower crustal image has more structural information than is apparent in conventional ray-based interpretations along the same, crossing, and adjacent profiles. The crustal image demonstrates that automated imaging techniques applied to wide-angle data can provide images of crustal structure comparable in fidelity to conventionally processed and migrated near-vertical incidence data.

The processes which result in backscattered acoustic energy from the seafloor are of primary interest for interpreting sonar signals measured in the deep ocean basins. This complex problem can be approached from a realistic perspective with the employment of modern high‐resolution seafloor bathymetry measurements and viscoelastic wave simulation techniques capable of modeling highly heterogeneous anelastic media. A numerical study of low‐frequency (100–200 Hz) backscattering processes from rough seafloor models is presented and several scattering hypotheses are proposed. In particular the effects that low‐Q sediments have on the backscattered field from a rough seafloor and the effects that perturbations in the different material properties have on the backscattered energy are investigated. The principal conclusions reached are that anelastic sediments on the seafloor can have a significant impact on the backscattered field from low grazing angle scattering experiments, and that small perturbations in the...

[1] 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.

International audience; Mafic volcanism gradually encroaching on the tectonically stable Colorado Plateau in the southwestern United States appears to originate from within Paleoproterozoic lithosphere. New Hf and Nd isotope data strengthen evidence for magma source heterogeneity; other geochemical signatures show these sources to consist dominantly of peridotite. Tomographic and receiver function analyses reveal that young volcanism occurs above or outboard of a pronounced shallowing in the lithosphere-asthenosphere boundary. Melt extraction extends to the base of lithosphere thinned to

SUMMARY We present a stochastic seismic model of gneissic/granitic upper crust by estimating the amplitude probability distribution and the autocorrelation function of the inferred seismic velocity field of the Strona-Ceneri Zone (northern Italy). Unlike previous investigations of the adjacent Ivrea Zone and the Lewisian gneiss complex (Scotland), the lithologies mapped in the Strona-Ceneri Zone cannot be directly correlated with laboratory-measured seismic velocities. The amplitude probability distribution of the extensive petrophysical database and the relatively uniform mineralogical composition of the lithologies indicate that the velocity distribution in the Strona-Ceneri Zone can be approximated as being continuous and Gaussian. Based on this assumption and using the spatial autocorrelation function measured from a digitized geological cross-section, we have modelled the fine-scale structure of the Strona-Ceneri Zone as a self-affine medium with a fractal dimension of 2.50 × 0.15 and horizontal and vertical characteristic scales of 2.6 × 0.4 km and 1.4 × 0.3 km, respectively. On near-offset shot-gather seismic data, Strona-Ceneri-type upper crustal heterogeneity produces a reflected field with the appearance of incoherent scattered noise characteristic of non-reflective, ‘transparent’ upper crust found in extended Phanerozoic provinces. This type of heterogeneity has a considerable effect on seismic imaging of underlying reflective lower crust: individual reflection segments are disrupted and shifted in time by up to several tenths of a second, and the boundaries of the reflective zone become distorted and blurred. At far offsets seismic scattering from heterogeneity of this type reproduces the commonly observed rapid decay and scattered distribution of Pg amplitudes. Comparable amplitude decay patterns result from layered deterministic upper crustal velocity structures with velocity inversions.

SUMMARY We have developed statistical models of upper and middle crustal seismic velocity heterogeneity based on geologic maps and petrophysical data from the Lewisian gneiss complex, Scotland. the level of heterogeneity we have measured is relevant to seismic exploration of the crystalline crust using conventional reflection and refraction techniques. We digitized two 1:10560 geologic maps of Laxfordian (Archean) age Lewisian rocks on a 26.8m grid. Both maps are believed to be representative of the upper and middle crust in north-western Scotland, and both are believed to provide cross-sectional views of parts of the crust. the digital maps were characterized by the statistics of their lithologic populations and by their 2-D spatial autocorrelation functions. Different lithologies were assigned seismic velocities appropriate for the mid-crust using petrophysical data. Three lithologies are dominant: silicic gneisses (Vp∼6.2 km s−1), mafic amphibolites (Vp— 6.75 kms−1), and intermediate composition schists (Vp—6.5kms−1). Both maps have self-affine spatial fabrics. The first map covers the core of a syncline. Its autocorrelation function defines a medium with a fractal dimension of 2.78, a horizontal characteristic length of about 244m and a vertical correlation of about 133m (aspect ratio is 2:1). It has an essentially trimodal velocity (lithology) population consisting of 37 per cent silicicgneiss, 43 per cent mafic amphibolites, and 20 per cent schists. This map is representative of 30-40 per cent of Laxfordian rocks. the second map is a plan view which can be rotated 90° about an axis perpendicular to strike to give a cross-section. This map is characterized by a fractal dimension of 2.55, with a horizontal correlation length of about 111m and a vertical correlation of about 38m (aspect ratio 3:1). It has a nearly bimodal population consisting of 77 per cent silicic gneisses and 22 per cent mafic amphibotites. It is representative of 60-70 per cent of Laxfordian rocks. Lastly we examine the seismic response of an upper to middle crust based on our statistical models using acoustic and elastic 2-D finite-difference synthetic seismograms. Short-offset shot records demonstrate that a Lewisian upper crust produces scattered waves which significantly disrupt signals reflecting from deeper levels. Measurements of transmission scattering Q, and coda decay rates confirm that seismic scattering in Lewisian-type crust is strong. the migrated CMP response of a Lewisian crustal model shows the characteristic ‘salt and pepper’ pattern often observed in the upper crust, and described, incorrectly, as ‘transparent'. We suggest that ‘translucent’ is a more appropriate descriptor.

We describe results of an integrated seismic reflection/refraction experiment across the Brooks Range and flanking geologic provinces in Arctic Alaska. The seismic acquisition was unusual in that reflection and refraction data were collected simultaneously with a 700 channel seismograph system deployed numerous times along a 315 km profile. Shot records show continuous Moho reflections from 0–180 km offset, as well as numerous upper- and mid-crustal wide-angle events. Single and low-fold near-vertical incidence common midpoint (CMP) reflection images show complex upper- and middle-crustal structure across the range from the unmetamorphosed Endicott Mountains allochthon (EMA) in the north, to the metamorphic belts in the south. Lower-crustal and Moho reflections are visible across the entire reflection profile. Travel-time inversion of PmP arrivals shows that the Moho, at 33 km depth beneath the North Slope foothills, deepens abruptly beneath the EMA to a maximum of 46 km, and then shallows southward to 35 km at the southern edge of the range. Two zones of upper- and middle-crustal reflections underlie the northern Brooks Range above ~ 12–15 km depth. The upper zone, interpreted as the base of the EMA, lies at a maximum depth of 6 km and extends over 50 km from the range front to the north central Brooks Range where the base of the EMA outcrops above the metasedimentary rocks exposed in the Doonerak window. We interpret the base of the lower zone, at ~ 12 km depth, to be from carbonate rocks above the master detachment upon which the Brooks Range formed. The seismic data suggest that the master detachment is connected to the faults in the EMA by several ramps. In the highly metamorphosed terranes south of the Doonerak window, the CMP section shows numerous south-dipping events which we interpret as a crustal scale duplex involving the Doonerak window rocks. The basal detachment reflections can be traced approximately 100 km, and dip southward from about 10–12 km near the range front, to 14–18 km beneath the Doonerak window, to 26–28 km beneath the metamorphic belts in the central Brooks Range. The section documents middle- and lower-crustal involvement in the formation of the Brooks Range.

Based on petrophysical data, geologic maps, and a well log, we present statistical descriptions of likely upper-, middle-, and lower-crustal rocks to characterize the fine-scale heterogeneity observed in crustal exposures and inferred from deep-crustal seismic data. The statistical models, developed for granitic and metamorphic upper crust, and for an extended metamorphic lower crust, are used to construct whole-crustal models of seismic velocity heterogenity. We present finite-difference synthetic CMP data from several models which compare favorably with field data. The statistical models also permit classification of the seismic reflection experiment and the crustal heterogeneity according to scattering regime. The “optical”, or scattering properties of importance for classification are the velocity fluctuation intensity, the horizontal and vertical correlation lengths of the medium, the correlation function of the medium, and the velocity population function. For the crustal properties we measured, the bandwidth of a typical deep crustal experiment overlaps from the weak to the strong scattering regime, with implications for crustal seismic data processing and imaging. Notably, deep-crustal signals are likely to have experienced multiple scattering, making common seismic imaging techniques of questionable value. Moreover, the details of the unmigrated CMP stacked section bears little resemblance to the underlying medium.

Orogenic collapse, lithospheric extension, and associated basaltic magmatism have affected many Phanerozoic regions. This has led to the hypothesis that the high lower crustal reflectivity of these areas originates from horizontal mafic dikes embedded in a more felsic ma. trix. Tectonic extension, however, favors the intrusion of unreflective vertical dikes. Stress distribution in an extending lithosphere varies over several orders of magnitude, which explains the simultaneous intrusion of vertical and horizontal dikes. Vertical dikes penetrate rheologically strong zones and transform into horizontal dikes in weak zones where stress conditions are quasi-isostatic and a subhorizontal strain fabric is likely to exist. The seismic response of a corresponding crustal model faithfully reproduces banded reflectivity patterns typical of extended provinces. Because the loci of mechanical weakness vary with temperature, composition, and fluid content, other common reflectivity patterns can be explained similarly. This combination of two prominent end-member models on the origin of lower crustal reflections (i.e., ductile flow and mafic intrusions) is compatible with pertinent features associated with postorogenic lithospheric extension, such as high heat flow, bimodal volcanism, anatectic granites, granulite-facies metamorphism, and uniform crustal thickness.

We present a layer-stripping method of migration for irregularly layered media in which first-order velocity discontinuities separate regions of constant or smoothly varying velocity. We use the reverse-time method to migrate seismic data layer by layer, from the surface downwards. As part of the migration of a given layer, the bottom boundary of the layer is defined based on power in the migrated signal, and a seismic section is collected along it. This new section serves as the boundary condition for migration in the next layer. This procedure is repeated for each layer, with the final image formed from the individual layer images. Layer-stripping migration consists of three steps: (1) layer definition, (2) wavefield extrapolation and imaging, and (3) boundary determination. The migration scheme when used with reverse-time extrapolation is similar to datuming with an imaging condition. The reverse-time method uses an explicit fourth-order time, tenth-order space, finite-difference approximation to the scalar wave equation. The advantages of layer-stripping reverse-time migration are: (1) it preserves the benefits of the reverse-time method by handling strong velocity contrasts between layers and steeply dipping structures; (2) it reduces computer memory and saves computation time in high-velocity layers, and (3) it allows interpretational control of the image. Post-stack layer-stripping reverse-time migration is illustrated with a synthetic CMP data example. Prestack migration is illustrated with a synthetic data set and with a marine seismic reflection profile across the Santa Maria Basin and the Hosgri Fault in central California.

The Ivrea Zone in northern Italy is a well studied sliver of extended lower continental crust which was brought to the surface in the course of Alpine lithospheric shortening. Based on two 1:25,000 geological maps from the central Ivrea Zone and petrophysical parameters of the corresponding lithologies we have constructed geologically constrained, stochastic seismic models of Ivrea-type lower crust. A stochastic rather than a deterministic approach is applied due to the limited availability of. detailed geological maps and the high degree of small-scale structural and petrophysical complexity of the Ivrea Zone. The primary characteristic of the resulting model is its “layered” self-affine or fractal structure and its bimodal velocity distribution. While we are not suggesting that Ivrea-type structure is universal for extended lower continental crust, synthetic seismograms may help to constrain some pertinent quantitative aspects observed in deep seismic reflection data: 1. (1) Ivrea-type lower crust can explain the igh observed reflection coefficients and bright reflectivity in the lower crust; 2. (2) observed lower-crustal Q -factors can be largely explained by scattering losses and intrinsic Q -factors of reflective lower crust are likely to be larger than 1000, which is incompatible with the presence of free volatiles; 3. (3) in agreement with recent broad band studies of the lower crust, Ivrea-type lower crust does not create any tuning effects; and 4. (4) for Ivrea-type heterogeneity the lateral correlation of the reflected signal is largely independent of the lateral characteristic scale of the scattering structures, which suggests that multiple scattering may be important in the reflective lower crust.

Geologically and petrophysically constrained synthetic random velocity fields are important tools for exploring (through the application of numerical codes) the seismic response of small-scale lithospheric heterogeneities. Statistical and geophysical analysis of mid- and lower-crustal exposures has demonstrated that the probability density function for some seismic velocity fields is likely to be discrete rather than continuous. We apply the term “modal” fields to describe fields of this sort. This letter details a methodology for generating synthetic modal fields which satisfy the von Karman covariance function. In addition, we explore some of the mathematics of “modality”, and define a modality parameter which quantifies the variation between end members binary and continuous fields.

A new 3-D shear velocity model of the crust and upper mantle beneath the Colorado Plateau and surrounding regions of the southwestern United States was made with finite frequency Rayleigh wave tomography using EarthScope/USArray data. The goal of our study is to examine the Colorado Plateau lithospheric modification that has resulted from Cenozoic tectonism and magmatism. We have inverted for the isotropic Vs model from a grid of Rayleigh wave dispersion curves obtained by a modified two-plane wave method for periods from 20 to 167 s. We map the lithosphere-asthenosphere boundary under the Colorado Plateau by identifying the middle of the shallowest upper mantle negative Vs gradient. The depths of the lithosphere-asthenosphere boundary inferred here agree well with receiver function estimates made independently. The strong lateral heterogeneity of shear velocity can be mainly attributed to 200–400 K variations in temperature together with ∼1% partial melt fraction in the shallow upper mantle. The resulting Vs structures clearly image the upper mantle low-velocity zones under the Colorado Plateau margins that are associated with magmatic encroachment. These upper mantle low-velocity zones resulted from the convective removal of the Colorado Plateau lithosphere that had been rehydrated by subduction-released water, refertilizing and destabilizing it. This convective erosion by the asthenosphere at the low-viscosity part of the lithosphere is driven by the large step in lithospheric thickness and the thermal gradient across the boundary between the plateau and the extended Basin and Range since the Mid-Cenozoic at a rate similar to that of magmatic migration into the plateau from the southeast, south, and northwest. Moreover, the Rayleigh wave tomography model images parts of a high-velocity drip in the western Colorado Plateau and thus provides additional seismic evidence for ongoing convective downwelling of the lithosphere that was initially suggested by receiver functions and body wave tomography. The widespread edge convective erosion, which the regional delamination-style downwelling processes are a 3-D manifestation of, could provide additional buoyancy sources to support the excess uplift at the margins of the plateau.

We present a seismic model of the South-African cratonic region obtained by combining receiver functions and teleseismic P and S traveltime tomography. We determined receiver functions (RFs) for 82 stations by iterative deconvolution. Based on HK analysis (Zhou and Kananmori, JGR,2001), we measure a relatively standard crustal thickness (~38 km) with a flat and sharp Moho discontinuity within the Kaapvaal and Zimbabwe cratons. The lowest Vp/Vs values ~1.69, are found near the locations of diamondiferous kimberlite pipes in the heart of the Kaapvaal craton. We also observe strong crustal anisotropy in the lower crust. Our best-fit RF model has an average fast polarization oriented at 30° to 40° and a crustal fabric that produces, in some cases, around 50% of the total anisotropy inferred from SKS splitting (Paul G. Silver et al., GRL,2001). We also performed finite-frequency tomography in 3 frequency bands (1, 0.5 and 0.25 Hz for P and 0.1, 0.05 and 0.02 Hz for S) to obtain 3-D P- and S-wave perturbation models for the upper mantle. Crustal corrections are based on the RF models. Tests showed that our dataset is able to resolve structure of 3°x3° up to 400 km depth. The high-velocity cratonic roots extend to 300-350 km depth. Lower velocities are detected below the Bushveld complex and the mobile belts. The model also suggests a stratified structure, since we found a low velocity zone (LVZ) at about 170 km depth in the cratonic areas. SdP RFs and surface-wave tomography are in progress and should improve imaging of the LVZ and resolving the unusual structures within the cratonic lithosphere.

Crustal-scale seismic wave equation modeling demonstrates that wavelength-scale velocity fluctuations can dominate the seismic response over a wide range of offsets and that a wide range of velocity distributions qualitatively can explain deep seismic data. This range can be narrowed by studying the detailed petrophysical and structural properties of middle and lower crustal exposures. The Ivrea Zone, in the Southern Alps, is a well-studied sliver of extended lower continental crust brought into its present upright position as a result of late Alpine lithospheric shortening. The high degree of structural and petrophysical complexity and in particular the limited exposure of the Ivrea Zone favor a statistical rather than deterministic approach to develop a two-dimensional crustal-scale seismic model. For this purpose we digitized two 1:25,000 geological maps from the central Ivrea Zone on a 60-m grid. We find that the Ivrea Zone is characterized by a bimodal velocity distribution, and a band-limited self-affine structure having a fractal dimension of 2.7, a characteristic horizontal scale of 500 to 1000 m, and an aspect ratio around 4.0. Assuming statistical stationarity, a realization of this stochastic model was used to develop synthetic seismograms typical of “Ivrea-type” lower continental crust. Our results indicate that (1) Ivrea-type lower crust can explain qualitatively commonly observed lower crustal reflectivity, (2) relatively minor changes in bulk composition and/or metamorphic grade may lead to dramatic changes in reflectivity, which cannot be explained by current generic models on the origin of lower crustal reflections, and (3) travel time interpretation of the synthetic wide-angle data may lead to erroneous large-scale velocity structures.

Seismic reflection data from the 1986 EDGE/Pacific Gas and Electric experiment together with refraction and gravity data are used to construct an integrated north-south geophysical cross section along the offshore Santa Maria Basin and across the offshore extension of the Transverse Ranges. Thrust faults offsetting acoustic basement and folding Miocene sediments root in a midcrustal detachment at 12–13 km depth. The detachment coincides with an increase in reflectivity seen on the seismic reflection section and the maximum depth of earthquakes. A high-velocity oceanic crustal layer at the base of the crust (17–22 km) and Moho (≈ 25 km) is offset by a low-angle thrust fault with an apparent dip ≈ 10° to the north offshore Point Arguello. Gravity models indicate crustal thickening and a change in the depth to Moho from 25 to 30 km in this region. However, the contribution of elastic flexure from loading of the Transverse Ranges is considered to be minor in this part of the borderland. The cross section constrains Neogene shortening to less than 3% based on faulting and folding of the sedimentary section. Shortening of the upper crust is balanced by a combination of ductile flow and subduction of the oceanic crust beneath the midcrustal detachment.