crustal velocity structure of the omineca belt southeastern canadian cordillera travel time inversion and amplitude modeling of a 350 km lithoprobe seismic refraction wide angle reflection profile determined the velocity structure of the crust and upper mantle along strike in the omineca belt of the canadian cordillera the upper crust to 12 18 km depth has velocities from 5 6 to 6 2 km s 1 and two shear zones the monashee decollement and gwillim creek shear zone are imaged by the wide angle reflections and velocity trends minor velocity differences on either side of the monashee decollement may be related to separate rock origins prominent reflections define the boundaries of a low velocity midcrustal layer from 10 15 km to 20 25 km depth with velocities less than 6 1 km s 1 the low velocities of the midcrust associated with high electrical conductivities and high heat flow may be considered as support for the hypothesis of fluids in the cordilleran crust though other possibilities such as the effect of high temperatures on rock velocities are possible in the lower crust velocities range from 6 4 6 5 km s 1 at the top of the lower crust to 6 6 6 8 km s 1 at its base the moho is very clearly defined by the refraction wide angle reflection data and has a gentle southerly dip crustal thicknesses are 35 37 km a thin crust mantle transition zone of 1 2 km thickness in which velocities vary between 7 6 and 7 7 km s 1 is consistent with coincident reflection data upper mantle velocities range from 7 9 to 8 1 km s 1 with indications from the data of upper mantle layering in comparison with neighboring regions the omineca belt has an anomalously thin crust low crustal velocities and a low velocity upper mantle similar only to the basin and range province the velocity structure may partly mirror the temperature profile which has overprinted the geological signature of the region as measured by the seismic refraction method the characteristics of a thin crust and lithosphere along with low velocities from midcrust to mantle suggests that both the basin and range and the southern canadian cordillera are currently being heated from a source within the mantle

seismic tomography a window into deep earth abstract the goal of this paper is to provide an overview of the current state of the art in seismic tomography and trace its origins from pioneering work in the early 1970s to its present status as the pre eminent tool for imaging the earth s interior at a variety of scales due to length limitations we cannot hope to cover every aspect of this diverse topic or include mathematical derivations of the underlying principles rather we will provide a largely descriptive coverage of the methodology that is targeted at readers not intimately familiar with the topic the relative merits of local versus global parameterization ray tracing versus wavefront tracking backprojection versus gradient based inversion and synthetic testing versus model covariance are explored a variety of key application areas are also discussed including body wave traveltime tomography surface wave tomography attenuation tomography and ambient noise tomography established and emerging trends many of which are driven by the ongoing rapid increases in available computing power will also be examined including finite frequency tomography full waveform tomography and joint tomography using multiple datasets several practical applications of seismic tomography including body wave traveltime attenuation and surface waveform are presented in order to reinforce prior discussion of theory

crustal structure and tectonics of the southeastern canadian cordillera the crustal structure of the omineca ob and foreland fb belts of the southeastern canadian cordillera are interpreted from the inversion of seismic refraction reflection travel times and amplitudes and modeling of the bouguer gravity data from a 350 km east west wide angle profile the main features of the resultant velocity and density models include 1 a low average crustal velocity of 6 2 km s 2 variable upper crustal velocities 5 6 6 3 km s within the fb and purcell anticlinorium as compared to farther west in the ob 6 1 6 2 km s 3 a midcrustal 20 km depth 0 4 0 5 km s velocity increase within the ob 4 eastward crustal thickening from 35 to 42 km over 80 km distance beneath the ob fb boundary 5 the slocan lake fault slf dipping east at 15 20 to at least 35 km depth 6 decreased lower crustal velocities 6 7 6 4 km s across the slf from the ob into the fb 7 increased uppermost mantle velocities 7 9 8 0 km s from the ob into the fb and 8 an average crustal density increase of 40 kg m3 from the ob into the fb lower velocities higher conductances and increased densities within the upper crust 0 10 km depth of the purcell anticlinorium appear to be associated with middle and upper proterozoic rift related metasediments and gabbroic dikes 35 55 by volume in contrast to the mainly mesozoic felsic intrusives and metamorphic rocks that dominate farther west in the ob elevated crustal temperatures within the ob a high heat flow province are responsible for low velocity gradients to 20 km depth a more mafic lower crust beneath the ob inferred from higher velocities and the westward decrease in crustal thickness are interpreted as resulting primarily from middle and or late proterozoic extension and rifting of the craton penetration of the slf into the lower crust may be controlled by lateral strength contrasts at the edge of the craton farther west a midcrustal strength contrast a velocity boundary at 20 25 km depth may have acted as a regional detachment zone during compressional and extensional tectonic episodes

evolving geographic patterns of cenozoic magmatism in the north american cordillera the temporal and spatial association of magmatism and metamorphic core complexes four maps are presented here that show the location and extent of magmatic fields between eastern alaska and northern mexico during the successive time intervals of 55 40 40 25 25 10 and 10 0 ma and four others show the distribution of metamorphic core complexes during the same cenozoic time intervals the maps are based on u s geological survey and canadian cordilleran data bases contining about 6000 isotopic dates and extensive literature review for nearly 60 ma the development of metamorphic core complexes has coincided with the locus of a really extensive and voluminous intermediate composition magmatic fields the association is suggestive of a close link between magmatism and core complex formation namely that magma directly and indirectly lowers the strength of the crust magmatism thus controls the location and timing of core complex formation the stresses responsible may be inherited from mesozoic crustal thickening locally created by uplift and magmatic thickening of the crust and imposed by the global pattern of plate motions and driving forces since the miocene rates of magmatism extension and core complex formation have declined the modern basin and range province is not a suitable model for the situation that existed during major magmatic culminations the singular event of early miocene time the merging of two large magmatic fields extinguishing the laramide magmatic gap explains several disconnected observations the hyperextension episode of the colorado river corridor rapid reorientation of stress patterns across much of western north america and subsequent rapid tectonic movements in california magma triggered breakup of western north america lithosphere coincided with development of the san andreas transform system thermal destruction of the laramide magmatic gap created a california microplate about 22 ma ago that moved rapidly away from north america thus two plate tectonic processes thermal destruction of the lithosphere bridge and northward growth of a transform system interacted to produce miocene and later tectonic patterns and events

the crustal structure of the northwestern basin and range province nevada from wide angle seismic data i present a model of lithospheric p wave velocity structure in the northwestern basin and range province nevada based on wide angle seismic data collected in 1986 under the auspices of the program for array seismic studies of the continental lithosphere passcal the lithosphere in the basin and range consists of 1 2 to 3 km thick low velocity 2 5 4 0 km s sedimentary basins and high velocity 4 5 km s ranges overlying a basement of velocity 5 5 6 1 km s 2 a mid crust 6 1 6 2 km s which consists of rocks of probable granitic to granodioritic composition in greenschist to amphibolite facies and which includes two local low velocity zones 5 5 5 7 km s 3 a laterally varying lower crust which consists of rocks of probable gabbroic composition in upper amphibolite to granulite fades and which comprises layers of 6 6 and 7 0 7 2 km s average velocity 4 a complex crust mantle transition best modeled as a velocity gradient zone 1 2 km thick and 5 an upper mantle with velocities of 7 7 7 9 km s which includes a local low velocity zone 7 5 km s crustal thickness in the region varies laterally from 28 to 37 km significantly thicker than several previous estimates the upper mantle velocities and velocity gradients differ slightly on the two perpendicular profiles suggesting possible azimuthal anisotropy a wedge shaped eastward thickening lower crust of high seismic velocity 7 0 7 2 km s on the eastern pan of the survey may represent the westward edge of rift stage precambrien continental crust or alternatively mafic material underplated and intruded into the crust during cenozoic extension comparison of the velocity model with coincident seismic reflection data shows that the reflection and refraction mohos are coincident across the entire profile thus indicating that deep reflections 4 10 sec two way travel time seen on cocorp and passcal piggyback reflection profiles originate entirely within the crust the top of the reflective zone however lies consistently above the mid crustal increase in average velocity indicating that the reflectivity is not restricted to the mafic part of the crust on passcal piggyback profiles in fact the middle silicic crust appears more reflective than the lower mafic crust this can be explained as the result of either mafic sill like intrusions or ductile strain fabrics both of these processes are likely to have accompanied cenozoic extension in the region

the agulhas plateau structure and evolution of a large igneous province summary large igneous provinces lip are of great interest due to their role in crustal generation magmatic processes and environmental impact the agulhas plateau in the southwest indian ocean off south africa has played a controversial role in this discussion due to unclear evidence for its continental or oceanic crustal affinity with new geophysical data from seismic refraction and reflection profiling we are able to present improved evidence for its crustal structure and composition the velocity depth model reveals a mean crustal thickness of 20 km with a maximum of 24 km where three major units can be identified in the crust in our seismic reflection records evidence for volcanic flows on the agulhas plateau can be observed the middle crust is thickened by magmatic intrusions the up to 10 km thick lower crustal body is characterized by high seismic velocities of 7 0 7 6 km s 1 the velocity depth distribution suggests that the plateau consists of overthickened oceanic crust similar to other oceanic lips such as the ontong java plateau or the northern kerguelen plateau the total volume of the agulhas plateau was estimated to be 4 10 6 km 3 of which about 10 per cent consists of extruded igneous material we use this information to obtain a first estimate on carbon dioxide and sulphur dioxide emission caused by degassing from this material the agulhas plateau was formed as part of a larger lip consisting of the agulhas plateau itself northeast georgia rise and maud rise the formation time of this lip can be estimated between 100 and 94 5 ma

lithoprobe seismic reflection structure of the southeastern canadian cordillera initial results nearly 270 km of crustal seismic reflection data obtained by lithoprobe in the southern canadian cordillera provide a geometric link between the rocky mountain foreland thrust and fold belt the purcell anticlinorium and the extensional regime superimposed on the crystalline core zone autochthonous north american basement and its overlying deformed and transported cover can be traced from the thrust and fold belt beneath the rocky mountain trench to 20 km depth beneath the central part of the purcell anticlinorium the purcell anticlinorium is cored by foreshortened proterozoic supracrustal rocks that were carried northeastward on a series of west dipping imbricate thrust faults these faults crop out within and east of the anticlinorium and converge downward with subhorizontal detachments above the autochthonous north american basement beneath the western purcell anticlinorium and kootenay arc reflections associated with the purcell stratigraphy and its underlying crystalline basement terminate at about 20 km depth and may be truncated against the east dipping eocene slocan lake fault zone the slocan lake fault zone is clearly imaged from the surface to about 12 km depth and can probably be followed discontinuously to about 25 km depth a west dipping high amplitude reflection from beneath the valhalla gneiss complex outlines the domal geometry of the complex and is probably related to an east verging compressional shear zone

magnetotelluric observations along the lithoprobe southeastern canadian cordilleran transect as part of the lithoprobe southern cordilleran transect investigations twenty seven wideband magnetotelluric mt responses were obtained along 150 km of the reflection seismic lines the mt profile begins to the east on the rocky mountain trench above autochthonous north american basement traverses across the purcell anticlinorium the kootenay arc the nelson batholith and ends on the east side of the valhalla gneiss complex sites near kootenay lake respond to a major near surface conductivity anomaly that is probably due to either graphitic schists or to an extension of the silver bearing mineralization found further north the short period 1 s data are highly complex and exhibit 3d regional features inversions of the data from the two sites on the valhalla complex yield 1d models with interfaces in electrical conductivity at depths that agree with an increase in the number of reflectors at 9 km the base of layered reflections at 22 km and the mono at 35 km of major significance to our understanding of the current composition and state of the lower continental crust is that although for the valhalla complex there are few reflections below 22 km the mt responses indicate that the zone is conducting this is the only location known to the authors where the lower crust is seismically transparent but electrically conductive

the strength of the continental crust detachment zones and the development of plastic instabilities abstract the maximum strength of the continental crust is constrained by the geothermal gradient the lithological make up of the crust and whether or not the byerlee relation holds to the base of the continental crust if this linear byerlee relation holds then maximum shear stress levels sustained at low geothermal gradients 10 20 c km 1 could be as high as 300 mpa in the upper part of the mantle for strike slip regimes and up to 700 mpa in thrust regimes towards the base of the crust however if byerlee s relation breaks down at moderate temperatures and pressures and if such breakdown is associated with the transition from unstable to stable sliding on pre existing faults then the maximum stress levels in the crust being set by the shear stress at the breakdown depth are much lower ca 300 mpa for low geothermal gradients and thrust regimes the lithosphere is quite weak below the crust for geothermal gradients greater than 20 c km 1 so attempts to treat the lithosphere as an elastic slab for time scales for which plastic flow contributes significantly to the deformation should be questioned for a geothermal gradient of 10 c km 1 the entire continental crust and part of the upper mantle behave as an elastic brittle slab at higher geothermal gradients the distribution of strength is variable vertically according to the magnitude of the geothermal gradient and the lithological make up large contrasts in strength occur across rheologically defined boundaries as well as across lithological boundaries at these higher geothermal gradients such contrasts in strength could act as nuclei for detachment zones within the continental crust and upper mantle detachment at or near the moho is to be expected only for low geothermal gradients lateral variations in the geothermal gradient produce shallowly dipping zones of highly contrasting strength throughout the continental crust and uppermost mantle possibly resulting in flat lying thrusts and normal faults which may ramp upwards through the crust instabilities leading to seismic events which occur solely during plastic shearing should be common in thrust terranes where the stresses are relatively high ca 100 200 mpa for geothermal gradients ranging from 10 to 50 c km 1 and for quartz rich felsic mafic and peridotitic rock types thrust terranes are therefore predicted to be characterized by coseismic plastic instabilities on all detachment zones throughout the crust and into the uppermost mantle but embryonic rift zones should be aseismic at depth ca 10 km

eocene structural development of the valhalla complex southeastern british columbia the valhalla complex a cordilleran metamorphic core complex is a 100 km by 30 km structural culmination within the omineca belt of southeastern british columbia it comprises sheets of granitic orthogneiss ranging in age from 100 to 59 ma with intervening paragneiss of uncertain age and stratigraphic correlation the complex is roofed by the ductile valkyr shear zone and the ductile brittle slocan lake fault zone the upper plate comprises lower grade metasedimentary rocks intruded by middle jurassic plutons the valkyr shear zone and the slocan lake fault zone deform 62 and 59 ma granitic sheets in their footwalls the easterly directed valkyr shear zone is a 2 to 3 km thick zone of distributed ductile strain which is arched over the complex and is exposed around the periphery on the northern western and southern margins the shear zone was active between 59 and 54 ma under amphibolite facies conditions the juxtaposition of upper and lower plates with different structural and metamorphic histories indicates that the valkyr shear zone is a significant structure with large displacement there is evidence to support an easterly rooting direction consistent with an extensional origin its surface breakaway is suggested to be west of the valhalla complex the slocan lake fault zone on the eastern side of the complex is a gently 30 easterly dipping ductile brittle normal fault which roots to the east it was active between 54 and approximately 45 ma and truncates the valkyr shear zone timing and structural relationships indicate that the valkyr shear zone and the slocan lake fault zone are genetically related movement on the ductile valkyr shear zone arching of the complex and displacement on the slocan lake fault zone occurred as a continuum in early to middle eocene time this paper documents the presence of significant eocene ductile strain in the valhalla complex and suggests that the role of extension in this region is more profound than had been previously recognized

crustal structure of eastern nevada from cocorp deep seismic reflection data the western nevada segments of the cocorp 40 n deep seismic reflection survey of the north american cordillera reveal the geometry of structures of cenozoic and possibly earlier ages to travel times of 10 s corresponding to depths of 30 km the most striking feature of the data is a band of prominent reflections typically at traveltimes of 9 5 to 10 5 s that are present discontinuously across the entire data set few reflections are observed from beneath the base of this reflective zone which is interpreted as the crust mantle transition this reflection mono is inferred to be continuous across the survey area varying gradually in depth but without resolvable offsets it appears to have taken its present form or position during basin range crustal extension the middle to lower crust in much of the survey area is characterized by discontinuous reflections that are typically subhorizontal and locally dip gently west these reflections may represent intrusions or shear zones related to basin range or pre basin range extension but some are likely to be inherited from earlier compressional deformation reflections from the upper crust are interpreted as images of basin fill strata basin range normal faults and mesozoic and paleozoic thrusts related to back arc thrusting and accretion of oceanic and arc related rocks

electromagnetic images of regional structure in the southern canadian cordillera as part of lithoprobe s southern cordilleran transect investigations magnetotelluric mt soundings were made at 160 sites providing unprecedented coverage from the rockies to the west coast striking lateral variation which spatially correlates with the morphogeological belt boundaries is apparent at periods sensing the lower crust 10 s for the rockies mt phases are around 35 indicative of a moderately resistive 100 s 1000 s m north american basement foreland belt phases are transitional and increase from 60 in the east to 70 in the west omineca and coast belt phases are high 75 implying a conductive 10 30 m lower crust whereas intermontane belt phases are more than 10 lower equivalent to 150 m the regional variation in conductivity correlates to first order with surface heat flow changes along the profile and is also correlative with coincident seismic reflection sections in some aspects

three dimensional crustal structure of the eastern cordillera southwestern canada and northwestern united states lithoprobe and cocorp crustal seismic reflection profiles across the southwestern foreland and omineca belts of southern british columbia and the adjacent united states provide regional three dimensional coverage of orogen parallel structural variations in this area north american cratonic basement and cover rocks are deformed into east verging folds and thrusts which increase in scale and depth to the west this simple pattern may be complicated by out of sequence faulting in the upper and middle crust of the kootenay arc and purcell anticlinorium in southern british columbia the southern omineca belt is the structural and metamorphic hinterland of the foreland belt and contains the monashee priest river crustal scale tectonic ramp which records a protracted history of east directed contraction it is overlain by the eagle river vernon colville crustal scale antiform of probable late cretaceous to paleocene age which locally may have formed above blind thrusts an embayment in the autochthonous craton between the monashee and priest river ramps corresponds spatially with the kootenay arc structural salient and eastward deflection of the lower paleozoic shelf to basin facies transition the south flank of the monashee and valhalla complexes and the north flank of the purcell anticlinorium near the international border correspond with the lateral margins of this east trending embayment on an even larger scale the increasing northward tectonic overlap of accreted terranes onto thick cratonic crust from the northern united states into british columbia and associated basement thickening coincide spatially with the northward increased width and depth of the early tertiary foreland basin in this part of the cordillera eocene crustal extension did not completely mask older contractional fabrics and appears to have been accomplished by two mechanisms 1 in the east by crust penetrating shear zones that separate the omineca and foreland belts and 2 in the west by crustal scale boudinage within and west of the omineca belt

proterozoic crustal transition beneath the western canada sedimentary basin new deep seismic reflection data outline a major crustal transition from thick craton on the east to thin basinal crust on the west that is buried beneath the phanerozoic rocks of the western canada sedimentary basin the transition formed prior to or during deposition of westward thickening mesoproterozoic sedimentary rocks between about 1 85 and 0 9 ga and appears to be the southward continuation of the fort simpson structural trend which is crossed by reflection data nearly 700 km to the north it may be analogous to crustal ramps that are now commonly observed in many phanerozoic orogens and along with an associated decrease in moho depth has been preserved at this location for more than 1 0 b y

mean crustal velocity a critical parameter for interpreting crustal structure and crustal growth abstract mean crustal velocity is a critical parameter for genesis of continental crystalline crust because it is a function of mean crustal composition and therefore may be used to resolve continental crustal growth in space and time although the best values of mean crustal velocity are determined from wide angle reflection measurements most studied here necessarily come from vertical averages in crustal refraction determinations the mode of 158 values of mean crustal velocity is 6 3 km s a velocity which corresponds to a mean crustal composition of granodiorite to felsic quartz diorite archean crust may be slightly more mafic mean crustal velocities range from 5 8 to 7 0 km s the lowest values invariably are found in thermally disturbed rift zones and the highest values correspond to velocities in gabbro velocities in island arcs may be as low as 6 0 km s but are typically 6 5 6 9 km s which corresponds to andesitic composition estimates of island arc composition are andesitic if values of mean crustal velocity are not biased this observation suggests that continental crust did not grow simply by addition of island arc material possibilities are that crust formed from fusion of island arcs and was later changed to more felsic composition by addition of material from the mantle or that the late archean episode of major crustal growth did not involve processes similar to younger island arcs some crustal blocks might be changed in composition and thickness by such processes as underplating interthrusting necking and sub crustal erosion specially designed experiments are suggested to determine this parameter so critical for understanding genesis of continental crust

seismic structure of the crust and upper mantle in the peace river arch region canada four intersecting 300 km long reversed refraction lines within northwestern alberta and northeastern british columbia have been interpreted for crustal and upper mantle seismic velocity structure in the peace river arch pra region of the western canada sedimentary basin the data have been modeled using a two dimensional ray trace forward modeling approach based on asymptotic ray theory to match travel times and amplitudes of first and coherent later arrivals in addition an inversion of first arrival travel times along a fan shot profile has been performed to constrain crustal thickness northwest of the arch in a region not sampled by the reversed profiles the major features of the structural models are 1 weak to moderate lateral variations in crustal structure with no evidence of significant layering or thick low velocity zones within the crust 2 an average sub basement rms crustal velocity of 6 6 km s average upper mantle velocity of 8 25 km s and average crustal thickness of 40 km 3 regional variations in structure which appear related to the dominant n s trending cratonic structure including crustal thickness upper crustal and upper mantle velocities and pmp character and 4 subtle variations in structure that may be associated with the e w trending devonian axis of the pra including a shallowing of high lower crustal velocities thickening of the crust and an anisotropic pmp character beneath the arch and a weak trend in the dip of intracrustal reflectors away from the center of the pra region evidence from the refraction and other geophysical data for the presence of a local crustal expression of the pra is weak but suggests a thermal as opposed to flexural origin for its anomalous vertical movements

p wave velocity structure under the changbaishan volcanic region ne china from wide angle reflection and refraction data abstract we present velocity models determined by inverting refracted and reflected arrivals along two active source lines in the changbaishan volcanic region ne china we resolve a prominent low velocity zone lvz in the crust with velocities as low as 5 4 km s away from the lvz the velocity gradients in the crust are relatively smooth with average p wave velocities of about 6 0 6 5 km s the moho is at about 35 km depth thickening to about 40 km under the tianchi volcano and thinning to about 30 km under the lvz the lvz is located about 30 60 km to the north of the summit of the tianchi volcano the most recently active volcano in the region is about 30 75 km in north south extent is at most 35 km in east west extent and is in the depth range of about 10 25 km below the surface we use these results to constrain receiver function inversions and show that the receiver functions in the region are compatible with our findings with these data alone the significance of the lvz in non unique although we do not see any evidence to support the presence of partial melt in the crust and favor the interpretation that the lvz indicates a residual crustal magma chamber

lithoprobe crustal reflection cross section of the southern canadian cordillera 1 foreland thrust and fold belt to fraser river fault seismic reflection data from the south central canadian cordillera covering the interval from the easternmost metamorphic core complexes near arrow lakes to the fraser river fault system along the fraser river reveal a highly reflective and complex crust the base of the crustal reflectivity interpreted as the reflection moho is clearly delineated by a continuous sharp boundary that is essentially planar and slopes uniformly over a distance of 250 km from about 12 0 s in the east to about 10 5 s in the west this virtual lack of relief at the base of the crust contrasts sharply with surface structures that involve 25 km or more of structural relief some of these surface structures can be readily correlated to structures that are outlined by the reflection data and that can be followed into the middle and lower crust even though part of this area was subjected to large amounts of eocene extension the crust is not divisible into transparent upper and reflective lower layers as it is in parts of the u s cordillera three structural culminations the monashee complex the vernon antiform and the central nicola horst are interpreted on the basis of the reflection configuration and the surface geological relationships to have formed initially during jurassic to eocene compression and then to have been modified and exposed during early and middle eocene extension an example of a compressional structure observed on the profiles is the monashee decollement which can be traced from the surface westward into the lower crust extension is manifested along a variety of normal faults including the regionally extensive low angle okanagan valley eagle river fault system moderately dipping faults such as the columbia river and slocan lake faults and high angle faults such as the quilchena creek and coldwater faults both jurassic to eocene compressional shear zones and early to middle eocene extensional shear zones are listric into the lower crust or moho under the intermontane belt

seismic reflections and electrical conductivity a case of holmes s curious dog is there any other point to which you wish to draw my attention to the curious incident of the dog in the night time the dog did nothing in the night time that was the curious incident remarked sherlock holmes arthur conan doyle in the purcell anticlinorium of the canadian cordillera prominent upper crustal seismic reflectors and areas of high electrical conductivity coincide over a wide area reflective layers traceable to outcrop and to a drill hole are caused mostly by impedance contrasts between proterozoic gabbroic sills and metasedimentary rocks layers with high electrical conductivity are zones enriched in magnetic sulfide probably pyrrhotite within the sedimentary rocks attempts to interpret reflectivity and high conductivity in terms of a single geologic process although tempting and certainly satisfying by appealing to occam9s razor must be done with care we speculate that some explanations for crustal reflectors and conductors may be unrealistic and propose an alternative paradigm of interlayered sills and meta sedimentary rocks rich in high conductivity minerals to explain both

assessment of crustal velocity models using seismic refraction and reflection tomography two tomographic methods for assessing velocity models obtained from wide angle seismic traveltime data are presented through four case studies the modelling inversion of wide angle traveltimes usually involves some aspects that are quite subjective for example 1 identifying and including later phases that are often difficult to pick within the seismic coda 2 assigning specific layers to arrivals 3 incorporating pre conceived structure not specifically required by the data and 4 selecting a model parametrization these steps are applied to maximize model constraint and minimize model non uniqueness however these steps may cause the overall approach to appear ad hoc and thereby diminish the credibility of the final model the effect of these subjective choices can largely be addressed by estimating the minimum model structure required by the least subjective portion of the wide angle data set the first arrival times for data sets with moho reflections the tomographic velocity model can be used to invert the pmp times for a minimum structure moho in this way crustal velocity and moho models can be obtained that require the least amount of subjective input and the model structure that is required by the wide angle data with a high degree of certainty can be differentiated from structure that is merely consistent with the data the tomographic models are not intended to supersede the preferred models since the latter model is typically better resolved and more interpretable this form of tomographic assessment is intended to lend credibility to model features common to the tomographic and preferred models four case studies are presented in which a preferred model was derived using one or more of the subjective steps described above this was followed by conventional first arrival and reflection traveltime tomography using a finely gridded model parametrization to derive smooth minimum structure models the case studies are from the se canadian cordillera across the rocky mountain trench central india across the narmada son lineament the iberia margin across the galicia bank and the central chilean margin across the valparaiso basin and a subducting seamount these case studies span the range of modern wide angle experiments and data sets in terms of shot receiver spacing marine and land acquisition lateral heterogeneity of the study area and availability of wide angle reflections and coincident near vertical reflection data the results are surprising given the amount of structure in the smooth tomographically derived models that is consistent with the more subjectively derived models the results show that exploiting the complementary nature of the subjective and tomographic approaches is an effective strategy for the analysis of wide angle traveltime data

geophysical transect across a paleoproterozoic continent continent collision zone the trans hudson orogen a summary and comparison of geophysical data and models for the trans hudson orogen in northern manitoba and saskatchewan are presented magnetic total field and bouguer gravity maps are used to define the along strike extension of geological domains of the orogen exposed on the canadian shield and a two dimensional density model is produced which accounts for the observed variations of the bouguer gravity field across the orogen an 800 km long crustal section across the entire continent continent collision zone including the edges of the bounding cratonic blocks is presented it incorporates seismic reflectivity seismic velocities resistivity and density models key results include 1 evidence for west vergent crustal stacking and exhumation in the eastern trans hudson orogen in the form of preserved moho topography and the presence of higher grade higher velocity rocks in the hanging wall of an east dipping crustal stack 2 definition of the eastward extent of the archean sask craton in the

structure of the lower crust beneath the carolina trough u s atlantic continental margin data from three large offset seismic profiles provide information on the crustal structure beneath the carolina trough the profiles obtained by the u s geological survey the naval oceanographic research development agency and the scripps institution of oceanography in 1985 were oriented parallel to the trough and were located 1 seaward of the east coast magnetic anomaly ecma which is generally thought to represent the boundary between oceanic and continental crust 2 along the axis of the trough between the ecma and the hinge zone which is thought to reflect the landward limit of highly stretched and altered transitional crust and 3 along the carolina platform landward of the basement hinge zone on crust thought to have been thinned only slightly during rifting these data constrain the velocity structure of the lower crust and provide evidence for a thick lens of high velocity 7 1 km s lower crustal material that extends beneath the carolina trough and the adjacent ocean basin this lens reaches a maximum thickness of about 13 km beneath the deepest part of the trough thins to about 5 km seaward of the ecma and is either very thin or absent landward of the hinge zone it is interpreted to represent material that was underplated beneath and or intruded into the crust during the late stage of continental rifting and that led to an anomalously thick plutonic layer during the early seafloor spreading phase these data thus support the recent conclusions of white et al 1987b and mutter et al 1988 that the initiation of seafloor spreading is attended in many if not most cases by the generation of an anomalously large volume of melt

crustal structure across the three gorges area of the yangtze platform central china from seismic refraction wide angle reflection data abstract we present active source seismic data recorded along a 300 km long profile across the three gorges area of the western yangtze platform central china from west to east the profile crosses the zigui basin huangling dome and jianghan basin the derived crustal p wave velocity structure changes significantly across the tongchenghe fault that lies at the transition from the huangling dome to the jianghan basin west of the tongchenghe fault beneath the zigui basin and the huangling dome we observe a 42 km thick crust of relatively low average velocity 6 3 6 4 km s in contrast east of the tongchenghe fault beneath the jianghan basin the crust is only 30 km thick and has a high average velocity 6 6 6 7 km s a west east variation in crustal composition along the tongchenghe fault is also inferred west of the fault p wave velocities suggest a felsic composition with an intermediate layer at the base of the crust whilst east of the fault felsic intermediate and mafic crustal layers are apparent our results suggest that the crust beneath the jianghan basin has been thinned by rifting accompanied by intrusion of the lower crust by mafic dikes and sills the west to east division of the crust in the three gorges area coincides with first order geophysical contrasts in gravity topography crustal and lithospheric thickness

gravity inversion for rifted margin deep structure using extension and isostatic constraints summary we present a gravity inversion method for determining the deep structure of rifted margins it uses a 2 d earth model parametrized as multiple irregularly shaped polygonal bodies each of uniform density the method has three novel features first it links parameters in the shallow parts of the model to those in the deep parts by using a uniform extension model as a constraint in the inversion the shallow structure will typically be known with greater certainty than the deep structure from shallow seismic and borehole data second it provides for variable weighting of prior information on densities shapes the extension model and smoothing to find geologically reasonable models third it estimates densities and shapes simultaneously the first two features are used to compensate for the inherent deficiencies of poor depth resolution and non uniqueness in gravity modelling the last two make the method an efficient way to explore a range of models synthetic tests of sensitivity to noise indicate that the isostatic extension constraint promotes the recovery of the short wavelength moho topography and eliminates spatial undulations in deep structure due to noise in the data synthetic tests of sensitivity to untrue prior information show that the isostatic extension constraint increases the range of acceptable recovered models over no isostatic extension constraint the range of recovered moho positions suggests a vertical resolution of about 2 km although many recovered models fit the data the results imply a methodology for choosing a best set of models and we suggest guidelines for applying the method to real margins

rheological controls on the geometry of deep faults and the tectonic delamination of the continental crust recent seismic reflection profiles indicate that the deep faults in the continental crust may decouple deep within the crust or at the moho in the contribution it is argued that the decoupling horizons were initially zones across which variations in flow strength occur due to mineralogical differences in the principal rock types these can result in the tectonic delamination of the crust and at the moho calculations reveal marked strength differences across the moho once initial delamination occurs weak planes of mylonite develop along such surfaces and are preferentially reactivated during any subsequent deformation it is postulated that during tectonism the continental crust 1 does not act at depth as a single rigid entity 2 should not be considered as being firmly attached to the upper mantle and 3 is capable of moving independantly of the upper mantle

crust and upper mantle velocity structure of the intermontane belt southern canadian cordillera as part of the lithoprobe southern cordillera transect seismic refraction data were recorded along a 330 km long strike profile in the intermontane belt an iterative combination of two dimensional traveltime inversion and amplitude forward modelling was used to interpret crust and upper mantle p wave velocity structure this region is characterized by i a thin near surface layer with large variations in velocity between 2 8 and 5 4 km s and low velocity regions that correlate well with surface expressions of tertiary sedimentary and volcanic rocks ii an upper and middle crust with low average velocity gradient possibly a weak low velocity zone and lateral velocity variations between 6 0 and 6 4 km s iii a distinctive lower crust characterized by significantly higher average velocities relative to midcrustal values beginning at 23 km depth approximately 8 km thick with average velocities of 6 5 and 6 7 km s at top and base iv a depth to moho as defined by wide angle reflections that aver

formation of non expandable 7 a halloysite during eocene miocene continental weathering at djebel debbagh algeria a geochemical and stable isotope study abstract in north eastern algeria autochthonous limestone units at djebel debbagh varying in age from aptian to cenomanian underwent burial diagenesis followed by deformation during the alpine orogeny karst windows formed during oligocene to miocene times and are now filled with clay minerals which currently help to sustain a local mineral industry the largest karst window at djebel debbagh contains substantial volumes of 7 a halloysite ore mineralogical investigations on the 7 a halloysite indicate a non expandable material with measured water contents varying from 14 1 to 15 5 wt h2o non expandable 7 a halloysite showed limited hydrogen exchange capacity during a 24 month laboratory experiment suggesting that hydrogen and oxygen isotopes had not exchanged since equilibrium at the time of halloysite formation therefore stable isotope data were used to calculate reliable temperature profiles and to determine fluid composition it is suggested on the basis of geochemical and mineralogical data that 7 a halloysite formed between 25 c and 40 c in equilibrium with meteoric water during eocene to miocene times estimates of supergene temperatures and calculated stable isotope values for rainfall correspond to continental climate conditions during miocene times coeval with the closure of the liguro oceanic basin

accumulation and preservation of organic carbon in marine sediments the roles of anoxia vs production organic carbon enrichments in marine sediments and sedimentary rocks commonly are explained by the preferential preservation of the deposited organic matter under anoxic conditions the role of primary organic plankton production is seldom considered a review of the available information shows that modern marine sediment accumulating in oxic and anoxic basins in similar topographic and sedimentary settings have very similar carbon contents on continental slopes carbon maxima are apparently produced by the complex interplay between the supply of carbon to the sea floor the texture of the sediment the dilution of carbon by other sediment components and the decreasing settling flux of carbon in the deeper waters of the open ocean contrary to contemporary thought there is no causal relationship between such maxima and the position of the oxygen minimum the degradation of sedimentary organic matter by aerobes and by sulfate reducers is very similar where the supply of fresh organic matter to the sea floor is similar hence there is no evidence for the preferential preservation of organic matter under anoxic conditions terrestrial organic matter however appears to be degraded to a lesser extent by sulfate reducers the burial of carbon below the surficial oxygenated horizons of a sedimentmore removes the easily oxidized fractions leaving material that may be less susceptible to attack by sulfate reducers sedimentary carbon maxima in pleistocene glacial horizons are due to the increased settling flux of organic matter brought about by climatically induced increases in upwelling in the equatorial and marginal areas of the ocean changes in bottom water oxygen levels during these periods plays a minor role in producing these signals previous work that claimed that anoxic bottom waters were prevalent during the accumulation of organic rich black shales in the geological record should be reevaluated less

a crustal seismic refraction study in west central arizona a reversed seismic refraction profile was recorded in the southern basin and range province between parker and globe arizona interpretation of both refracted and reflected phases indicates a two layered crust 23 25 km thick and a low pn velocity 7 67 km s pmp pn amplitude analysis on individual traces and synthetic seismogram modeling indicate that a positive velocity gradient may exist in the upper mantle the low pn velocity is indicative of partial melting in the upper mantle and implies that the crust is identical with the lithosphere the heat flow data suggest that the lithosphere was about 40 km thick approximately 5 m y ago on the basis of this and a negative free air gravity anomaly we propose that the lithosphere has been thinned from about 40 to 24 km over the past 5 m y and will probably thin and uplift further in the southern basin and range in arizona

seismic images of crustal duplexing and continental subduction in the brooks range 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

density temperature and rheological models for the southeastern canadian cordillera implications for its geodynamic evolution two dimensional density temperature and rheological models are constructed for a 350 km northeast trending transect of the southeastern canadian cordillera all models highlight several major physical differences between foreland and hinterland lithosphere significant features of the density model are the presence of an anomalously low density 3 10 103 kg m 3 layer with a maximum thickness of 12 km beneath the moho in the hinterland the similar densities of the monashee terrane and the cratonic crust of the foreland and an increase in crustal thickness beneath the southern rocky mountain trench the temperature model shows steeper gradients and higher moho temperatures beneath the hinterland than beneath the foreland in the rheological model the hinterland is characterized by a thin brittle upper crust beneath which the entire lithosphere is hot weak and ductile in contrast the foreland is composed of a thick brittle upper crust with an additional brittle zone in the upper mantle the

three dimensional crustal structure of the purcell anticlinorium in the cordillera of southwestern canada the purcell anticlinorium in southwestern canada and the northwestern united states is situated at the transition from deformed north american rocks of the foreland thrust and fold belt to accreted rocks of the kootenay arc nearly 1000 km of industry seismic reflection data were processed with extended vibroseis correlation and coherency filtering and then combined with lithoprobe reflection data to outline the three dimensional crustal structure of the anticlinorium reflection moho is visible at 12 0 s 36 km beneath the anticlinorium and deepens eastward to 15 0 s 45 km beneath the rocky mountain thrust and fold belt the transition from thick to thin crust coincides with westward deepening of north american basement and with the southern rocky mountain trench a basin or series of basins formed during tertiary extension regionally extensive reflections in the upper crust correlate with ca 1468 ma gabbroic sills in the aldridge formation of the mesoproterozoic belt purcell supergroup features outlined by the sills support an interpretation that the anticlinorium formed during mesozoic contraction when imbricate thrust faults carried up to 15 km of belt purcell and paleozoic margin sedimentary rocks and basement in some areas eastward over a prominent basement ramp

crustal velocity structure of the omineca and intermontane belts southeastern canadian cordillera we provide information on the physical properties across two important morphogeological regions of the southeastern canadian cordillera the omineca and intermontane belts and tie together two previously interpreted wide angle seismic lines which have separately probed each zone along strike the data analyzed here line 7 of lithoprobe s 1990 southern cordillera refraction experiment in southern british columbia is along a 410 km profile from the fraser river to radium hot springs near the rocky mountain trench iterative forward and inverse modeling of the seismic travel times constrained by seismic amplitude characteristics and earlier cross line interpretations defines a thin crust with relatively low velocities the upper crust has an average thickness of 15 3 km and an average p wave velocity of less than 6 1 km s 1 with velocities at the bottom reaching 6 2 6 3 km s 1 it is further characterized by velocity gradients which decrease markedly with depth and by low velocity trends which correlate with the traces of crustal scale shear zones a high velocity region at the west end of the profile is coincident with a major uplift structure the midcrust has an average thickness of 9 2 km with velocities below 6 1 km s 1 except for the extreme west end of the profile this defines a midcrustal low velocity zone lvz in the omineca belt and partly in the intermontane belt the lower crust from 24 km to 34 km depth has velocities averaging 6 6 6 7 km s 1 higher velocities in the deep crust may mark the westward extent of north american cratonic rocks at the okanagan valley the upper mantle velocity has an average 8 0 km 5 increasing to 8 2 km s 1 at the maximum depth of ray penetration at 48 km the model presented here compliments well the earlier interpretations of vertical incidence seismic reflection profiles the velocity structure of the crust especially the lvz closely correlates with the regional isotherms while there seems to be no well defined velocity boundary between the intermontane and omineca belts there are distinct velocity differences between the intermontane and neighboring coast plutonic complex to the west the low velocity characteristics of the omineca and much of the intermontane belts may be a consequence of temperature control of the physical parameters of the crust the lithosphere in the southeastern canadian cordillera is clearly anomalous with respect to much of northwest north america and in terms of crustal thickness and velocity structure it is similar only to the basin and range province of the united states

seismic studies of the crust under the williston basin the seismic refraction method was used in 1981 to study the crust under the northern half of the williston basin in saskatchewan a new method of spatial seismic recording based on a triangular arrangement of receivers was used for the first time to obtain three dimensional structure and velocity information the broadside seismic refraction and wide angle reflection data obtained by the technique were of particular value in defining several faulted blocks these blocks are also characterized by aeromagnetic anomalies trending in a northerly direction the crustal thickness in the southern part of the western provinces shows large variation ranging from 35 to 50 km much of the area is also notable for the presence of one or more low velocity layers and a high velocity lower crust there is good evidence for significant lateral heterogeneity and detailed deep seismic reflection and refraction studies would likely yield information on dips and strikes of beds and faults around the basin as well as defi

a fine structural section through the crust and lower lithosphere along the axial region of the alps summary a unified two dimensional lithospheric model has been derived for the p wave velocity structure beneath the alpine longitudinal profile alp 75 main line by making new phase correlations and using ray tracing and ray theoretical seismogram calculations in laterally inhomogeneous media due essentially to an increase in thickness of the lower crust the total crustal thickness increases from 30 40 km in the peripheral regions to about 50 km in the central region of the alps the thickness change is more abrupt at the eastern end than at the western end of the line the boundary between the upper and lower crust occurs at 18 20 km depth except in the peripheral region at the eastern end of the line where it occurs at 14 16 km depth along the whole line the seismic basement has velocities averaged over some tens of kilometres of 5 85 6 15 km s 1 with individual values having uncertainties of 0 05 0 1 km s 1 at least east of shot point b the lower part of the upper crust has been modelled as a low velocity layer which is most pronounced beneath the peripheral regions and which is generally underlain by a transition zone at the base of which the top of the lower crust occurs the top of the lower crust has been modelled with velocities ranging from 6 25 6 5 km s 1 beneath the peripheral regions to 6 5 6 65 km s 1 beneath the central alps velocity uncertainties at the top of the lower crust are estimated to be around 0 2 km s 1 beneath the central alps a low velocity layer has also been modelled in the lower crust the top 6 km of the mantle has been modelled with velocities of 8 0 8 2 km s 1 with individual estimates having uncertainties of 0 15 km s 1 below this the upper mantle down to almost 100 km depth has been modelled as a series of high velocity layers embedded in material with lower velocities of 8 0 8 2 km s 1 the combined effects of tectonic overthrusting and uplift and erosion have probably led to the lower crust being apparently thicker than the upper crust in the central alps at the present day

regionalization of crustal coda q in the continental united states a contour map of crustal q sub 0 values for the entire united states is presented based on a scattering model to explain the coda waves of local and near regional earthquakes these coda q sub 0 values are in good agreement with q of lg waves data are obtained from over 250 local earthquakes with most magnitudes between 3 0 and 5 0 recorded by 25 wwssn and 27 lrsm stations throughout the continental united states these two sets of data provide a range of frequencies from 0 5 to 3 5 hz a frequency dependence of q is observed in the range of frequencies considered a power law dependence of the form q q sub 0 f f sub 0 sup eta is assumed the value of frequency dependence eta is found to be maximum in the tectonically active western united states and least in the stable regions of the central and south central united states the lowest q sub 0 values are obtained in the western united states with values ranging from 140 to 200 in the coastal plains average crustal q sub 0 values for the basin and range province vary from 200 to 300 increasing gradually to 400 in the colorado plateau region q sub 0 valuesmore in the columbia plateau vary from 200 to 400 q sub 0 values increase very rapidly along the central and southern rocky mountains from 400 to 800 east of the rocky mountains q sub 0 values increase gradually in the interior plains to a maximum value of around 1300 in the mississippi embayment region farther east q sub 0 values decrease gradually to an almost constant value of 1000 along the appalachian mountains coastal regions of eastern and northeastern united states have a crustal q sub 0 value between 900 to 700 regions of north central and gulf coastal plains show low q sub 0 values ranging from 600 to 400 and less the source factor b f sub p a measure of the intensity of scattering was obtained for several different regions of the continental united states regions of high q sub 0 values exhibit low intensity of scattering less

modeling wide angle seismic data for crustal structure southeastern grenville province a modeling methodology for obtaining two dimensional 2 d crustal structure from wide angle seismic data is applied to data from the southeastern grenville province pre modeling steps include 1 assignment of arrival pick uncertainties for appropriate data fitting and weighting using an empirical relationship based on signal to noise ratio 2 using a modified form of travel time reciprocity to avoid unreasonable levels of model heterogeneity and 3 identifying data unsuitable for 2 d modeling the goal of the travel time inversion amplitude modeling approach is to obtain a minimum structure and minimum parameter model that takes into account both horizontal and vertical variations in the resolution of typical wide angle data each step of a layer stripping procedure involves a series of inversions in which a one dimensional or simple starting model is improved with additional velocity and or interface nodes until a satisfactory trade off between travel time fit parameter resolution and complete ray coverage of all source receiver pairs is achieved using zero vertical velocity gradient layers and head waves during preliminary first arrival inversion can 1 decrease the number of intermediate models 2 allow greater lateral heterogeneity to be imaged and 3 simplify incorporation of amplitude modeling constraints into the final model using amplitude distance curves allows quantitative modeling of the relative amplitude and offset variations of phases discrepancies between observed and calculated reflection amplitudes are used to infer fundamental non step like velocity changes at layer boundaries later arrivals due to unresolved velocity anomalies are modeled using reflecting segments that float within the model without an associated velocity structure these reflectors provide a spatial image like that obtained from vertical incidence reflection data as opposed to a velocity image the model of grenville crustal structure is more detailed than a model obtained from a previous interpretation of the data and includes elements analogous to those imaged in nearby deep reflection data a crustal scale zone of wide angle reflectors with an average easterly apparent dip of 13 defines a major grenvillian terrane boundary

strong lateral strength contrasts in the mantle lithosphere of continents a case study from the hot sw canadian cordillera abstract this study aims at quantifying the 3 d variability in lithosphere strength of the south eastern canadian cordillera and adjacent craton to the east strength is calculated in a forward manner starting from rheological laws of brittle and ductile deformation the work flow calculates a temperature model based on a multi layer compositional model and subsequently estimates the strength distribution from both the compositional and temperature models the temperature modeling involves numerical inversion and cubic spline algorithms which enable to impose boundary conditions that account better for strong lateral variations in lithosphere thickness and lateral heat flow this addresses the lithosphere structure between the canadian cordillera and craton the canadian cordillera is marked by a hot and thin lithosphere of 60 km thickness that stands in contrast to a cold and thick craton of 170 km thickness to the east which consequently results in pronounced changes in bulk lithosphere strength the high surface heat flow of the cordillera interior and its contrast with the foreland basin can be reproduced by temperature models that combine elevated mantle heat flow due to the thin lithosphere and higher crustal heat production from magmatic intrusions a series of rheological models which examines the role of different temperature input models composition and strain rate shows that the first order strength pattern is persistent for the hot canadian cordillera strength resides for 80 in the upper crust and with integrated lithosphere strength of 2 0 3 5 106 mpa m for the cold craton the upper mantle provides 75 of the integrated strength with values of 4 0 8 0 106 mpa m for the crust and 20 65 106 mpa m for the lithosphere effective elastic thickness is estimated between 5 and 15 km for the cordillera and 40 80 km for the craton this illustrates the cordillera to craton transition as a prominent rheological feature for upper mantle flow dynamics and spatial seismicity trends

crustal velocity structure from sarex the southern alberta refraction experiment lithoprobe s southern alberta refraction experiment sarex extends 800 km from east central alberta to central montana it was designed to investigate crustal velocity structure of the archean domains underlying the western canada sedimentary basin from north to south sarex crosses the loverna domain of the hearne province the vulcan structure the medicine hat block previously considered part of the hearne province the great falls tectonic zone and the northern wyoming province ten shot points along the profile in canada were recorded on 521 seismographs deployed at 1 km intervals to extend the line an additional 140 seismographs were deployed at intervals of 1 25 2 50 km in montana data interpretation used an iterative application of damped least squares inversion of traveltime picks and forward modeling results show different velocity structures for the major blocks loverna medicine hat and wyoming indicating that each is distinct wavy undulations in the velocity structure of the lov

lith5 0 a revised crustal model for canada based on lithoprobe results summary during the last 15 yr numerous seismic refraction experiments have been conducted overthe canadian landmass most as part of the lithoprobe initiative the new results providethe basis for revising existing global models of crustal structure e g crust5 1 mooney et al 1998 which do not incorporate the new lithoprobe data the crustal model discussedhere lith5 0 combines the lithoprobe and crust5 1 data to provide better constraints oncrustal structure in canada the objectives of the study are to 1 compile available new datainto a single consistent database of crustal thickness and velocity including the uppermostmantle pn velocity and 2 to construct a revised 5 5 crustal model of the canadianlandmass in a format that is generally compatible with existing global models the lithoproberesults suggest a 5 5 km average root mean square rms crustal thickness difference betweenlith5 0andcrust5 1 evenwiththeincorporationofthenewdata thegeographiccoveragein north and central canada remains sparse the new model does not explicitly incorporategeological constraints but instead relies on spherical spline interpolation for extrapolatingvalues into regions that lack control to illustrate the implications of this new crustal model we use lith5 0 to correct for the isostatically compensated crustal heterogeneity in order toobtain improved estimates of the dynamic surface topography the latter provides importantconstraints on the density structure of the uppermost mantle