Geophysical Imaging of the Continental Lithosphere - Global Change

2917-CH15.pdf 11/20/03 5:19 PM Page 368C H A P T E R F I F T E E NGeophysical Imagingof the Continental LithosphereAn essay by Frederick A. Cook15.1 Introduction 36815.2 What Is Seismic Imaging? 36815.3 How Are Data Interpreted? 37015.4 Some Examples 37015.5 The Crust–Mantle Transition 37215.6 The Importance of Regional Profiles—Longer,Deeper, More Detailed 37415.7 An Example from Northwestern Canada 37515.8 Other Geophysical Techniques 37915.9 Closing Remarks 381Additional Reading 38115.1 INTRODUCTIONThe lithosphere beneath the continents is a vast andlargely unexplored region of the Earth. Because it isinaccessible to normal geologic observations, variousgeophysical tools, including measurements of magnetism,gravity, electricity, subsurface temperatures, andearthquake waves, have been used to gather informationabout it for more than 100 years. Since about1975, however, application of controlled-source seismicreflection methods has produced images of thedeep subsurface that are visually similar to geologiccross sections and are therefore readily accessible tomost earth scientists. When analyzed in conjunctionwith other geophysical imaging methods, as well aswith the known geology, these data provide the mostdetailed information on subsurface structure presentlyavailable.In many areas of the continents, preliminary reconnaissancehas been accomplished and a few major featureshave been discovered: some of these are extensionsof the surface geologic context; others resemblefeatures we know but cannot be directly linked to ourgeologic base; still others are unusual and outside ourframe of reference. Overall, we have only begun thesearch.15.2 WHAT IS SEISMIC IMAGING?Seismic imaging methods use vibrational (elastic)energy generated on or near the Earth’s surface as asource of waves that propagate into the subsurface,reflect from or refract through interfaces at depth, andthen return to the surface where they are digitallyrecorded for subsequent data enhancement. Refractedwaves are valuable to delineate regional characteristicsof the crust and upper mantle, including the base of thecrust, whereas reflected waves are useful for mappingstructural detail. The source of energy may be producedartificially, as with an explosion or vibratorysignal, or it may be natural, as with an earthquake. Ingeneral, artificial sources do not have as much energyas earthquakes do, and hence do not penetrate asdeeply, but the results derived from artificial sourceshave much finer detail and are easier to relate to knowngeologic features. Seismic reflection profiling is notnew. It was first employed by the petroleum industryabout 75 years ago in oil exploration. However, manyrefinements, particularly following the advent of digitaltechnology in the 1960s and 1970s, have made itpossible to obtain images from greater depths and withgreater precision than was considered feasible evenjust a few years ago.368

2917-CH15.pdf 11/20/03 5:19 PM Page 370the transit time by a factor of 6.0. In this conversion atransit time of 5.0 s corresponds to a depth of about15.0 km. Below the Mohorovičić discontinuity, theseismic wave velocity increases to about 8.0 km/s.15.3 HOW ARE DATAINTERPRETED?Great strides have been made in using seismic reflectionprofiling for mapping the internal structure of the lithosphere.Reflections from the deep crust (to a depth ofabout 30–40 km) were considered curiosities until the1970s; reflections from the subcrustal lithosphere totwice this depth, or even more, are not unusual now.However, with technological advancements and theresulting improvements in signal quality and imagedepth, the challenges to interpretations have been great,because, as we obtain images from structures that are farremoved from our geologic reference on the surface, theability to relate them to what we know is increasinglylimited. In many cases, ancillary geophysical methodsthat respond to different physical properties may behelpful in limiting possible interpretations, because theseismic images provide a geometric framework of structuresthat are found at many scales in the lithosphere.Interpretation of the resultant seismic sections is inmany ways analogous to interpreting a geologic crosssection; indeed, the goal of the data processing is toprovide an image that closely approximates a geologiccross section. However, careful interpretations requireextensive knowledge of seismic wave propagation inrocks, geologic principles, and the regional geologicand geophysical context. Interpretation is often an iterativeprocess: new images of subsurface geometryfrom the reflection profiles commonly spawn new geologic(or other geophysical) projects to test some of theideas. Data resulting from these projects can then beused to review and often reinterpret the subsurfaceimages. It is not unusual to rework data that may be 10or 15 years old as new data processing techniques andnew geologic information become available.Although this essay is not intended to provide acomplete review of image types or all major discoveries,it is helpful to consider images of a variety ofcommon geologic features.15.4 SOME EXAMPLESMany of the criteria that are commonly used to identifyfaults in layered sedimentary rocks are not applicablein most crystalline rocks. For example, in layeredstrata, faults are usually delineated by offset layers,whereas reflections from a fault plane are rare. In generallyunlayered crystalline rocks, however, offsets aredifficult to observe (because it is not easy to correlatefrom one side of a fault to the other) and reflectionsfrom fault planes or fault zones are common.In Figure 15.2a, for example, prominent layeredreflections from Precambrian sills outline an anticline,the right (east) side of which is faulted by a westdippingnormal fault, the Rocky Mountain Trench insouthwestern Canada. In this case prominent layeringcan be correlated across the fault to provide some estimateof the type (listric normal) and amount of displacement(about 10 km) along the fault. On the otherhand, in Figure 15.2b reflections are visible from theWind River fault zone in western Wyoming. Near thesurface (to about 4 s travel time, or about 12 kmdepth), Precambrian crystalline rocks on the east arethrust westward over sedimentary strata of the GreenRiver Basin on the west. Accordingly, the seismicvelocity contrast between the crystalline rocks and thesediments is quite large, and a prominent reflectionfrom the boundary is produced. The Wind River Thrustcan then be followed as a series of subparallel reflectionsthat downdip eastward to more than 7.0 s traveltime (about 21 km depth).Here then is a dilemma. If the contrast between thesedimentary rocks of the Green River Basin and theoverlying crystalline rocks of the Wind River uplift iswhat produces the reflection along the shallow portionof the fault, then what is the cause of the reflection atgreater depths where crystalline rocks are juxtaposedwith crystalline rocks? Although this is discussed laterin more detail, the answer in this case appears to bethat the reflections are cause by mylonitic rocks thatwere formed as a result of the faulting. The process ofmylonitization causes a very strong preferred orientationof crystals that, in turn, produces a contrast withthe more randomly oriented crystals above and below.Thus, even where crystalline rocks are seismicallyhomogeneous, the faulting process may produce surfacesthat appear as subparallel reflections, particularlyif faulting occurs below the brittle–plastic transition.Seismic reflections from relatively homogeneousigneous rocks (e.g., plutons, basalt flows) are generallynot easily observed. Exceptions are rocks that aredeformed so that reflections may arise from the deformationsurfaces (as described above for the Wind Riveruplift), and igneous intrusions (e.g., sills) that are sufficientlythin that there is a measurable contrast withsurrounding rocks such as sediments or other crystallinerocks (Figure 15.2c).370 GEOPHYSICAL IMAGING OF THE CONTINENTAL LITHOSPHERE

2917-CH15.pdf 11/20/03 5:19 PM Page 371(a)(b)(c)FIGURE 15.2 (a) Seismic reflection profile across the southern Rocky Mountain Trench near the Canada-U.S. border. Note that theprominent layering, which is drilled on the west and is known to be dominantly Proterozoic sills, is offset along a west-dipping listricnormal fault that has about 10 km of dip-slip displacement. Data were recorded by Duncan Energy of Denver, Colorado. (b) Seismicprofile from the Wind River Mountains in Wyoming (USA). The Wind River fault juxtaposes crystalline rocks of the Wind River Mountainswith sedimentary rocks of the Green River Basin along a moderately east-dipping fault, and this provides a simple explanation for theprominent reflection. Below a travel time of about 3.5–4.0 s, however, the fault zone places crystalline rocks onto crystalline rocks andthe reflections must be caused by other mechanisms. Data recorded by COCORP (Consortium for Continental Reflection Profiling) in1977. (c) Seismic profile from the Proterozoic Trans-Hudson Orogen in northern Saskatchewan (Canada) illustrating prominentsubhorizontal reflections that have been interpreted as intrusive rocks. Note that the reflector appears to cross cut several dippingreflections. Note also the prominent Moho on these data.15.4 SOME EXAMPLES371

2917-CH15.pdf 11/20/03 5:20 PM Page 37215.5 THE CRUST–MANTLETRANSITIONThe transition from the crust to the mantle is generallyconsidered to be a relatively simple surface that hasmafic rocks such as gabbro or mafic granulites above,and ultramafic rocks below (see Chapter 14). Indeed,much research in the 1950s to 1970s attempted toaddress the question of whether the transition is a compositionalchange (i.e., gabbro or granulite in the crustto peridotite in the mantle) or whether it could be achange in phase (as from mafic granulites in the crustto eclogites in the mantle). Central to the discussionwas the observation from regional seismic refraction(a)FIGURE 15.3 Some reflection characteristics of the crust–mantle transition. (a) Profile from south-central portion of theCanadian Cordillera illustrating a relatively simple, single reflection from near the transition. On the right side of the figure, thenumbers 6.0 and 7.0 represent the positions of the Moho, as identified from adjacent seismic refraction data, for average velocitiesof 6.0 and 7.0 km/s, respectively. RM represents the preferred position of the Moho using the crustal velocity structure determinedfrom the refraction profile.372 GEOPHYSICAL IMAGING OF THE CONTINENTAL LITHOSPHERE(b)

2917-CH15.pdf 11/20/03 5:20 PM Page 373and earthquake data that there is almost always aprominent seismic velocity increase at 40–50 kmbeneath the continents and about 10 km beneath theoceans. On these grounds, the boundary appears to berelatively simple and globally significant, and, as such,is known as the Mohorovičić discontinuity, or Moho.As the information obtained from reflection profilingbecomes increasingly detailed, however, we see that thecrust–mantle transition is clearly not a uniform boundarybecause lateral variations in its geometry and reflectioncharacteristics are common (Figure 15.3). It can bestructurally complex or simple, multilayered or single(c)(d)FIGURE 15.3 (Continued) Note that the Moho appears to be located at the base of crustal reflectivity, and that the underlying mantlehas fewer reflections (e.g., MR). Data were recorded by LITHOPROBE in 1988. (b) Portion of a seismic profile that illustrates listricstructures into the crust–mantle transition. Data were recorded by LITHOPROBE in 1996. This segment is from beneath the Great Beararc region on the regional profile (Figure 15.5). (c) Portion of a seismic profile that illustrates many lower crustal layers that are parallelto the Moho as well as a possible truncation (T?). Data were recorded by LITHOPROBE in 1996. (d) Portion of a seismic profile fromnorthern Saskatchewan (Canada) that illustrates a local deepening of the crust–mantle transition (Moho keel). Note that althoughthere is not a prominent reflection near the transition, the reflectivity does diminish near it. In this figure, two locations for estimates oftravel time to the reflection Moho are indicated from adjacent refraction profiles.15.5 THE CRUST–MANTLE TRANSITION373

2917-CH15.pdf 11/20/03 5:20 PM Page 374surface, flat or dipping; and any of these variations maybe present along a single profile, sometimes changingover distances of only a few kilometers.On the other hand, one of the most obvious characteristicsof many reflection profiles is the transitionfrom reflective crust to relatively nonreflective mantle(Figure 15.3). Indeed, this effect is so pervasive on aglobal scale that it is commonly used as a means toidentify the crust–mantle transition; the “reflectionMoho” is generally interpreted to be at the base ofprominent crustal reflectivity. Thus, on one hand thedetailed structural and reflection characteristics of thetransition are complex and variable (Figure 15.3),while on the other there are regional, large-scale differencesin the reflectivity of the crust and the uppermantle. Until the advent of crustal reflection profiling,and particularly high-quality detailed images of thelower crust and upper mantle, these characteristicswere not observable. As a result, any future interpretationof the crust–mantle transition must account forgeometric complexities at relatively small scales (kilometersto tens of kilometers), and relative uniformitywhen viewed at larger scale with lower resolution. Thismay ultimately be one of the most fundamental resultsof these kinds of data as it will lead to new concepts ofhow the crust and mantle interact.One of the more heated debates in the interpretationof deep-crustal reflection profiles has been the cause(or causes) of reflectivity. Some aspects are wellunderstood. For example, a reflection must resultfrom a change in seismic velocity and/or rock densityand the magnitude of the reflection (amplitude) isrelated to the magnitude of the contrast. Hence, a contrastbetween a rock with relatively low seismic velocity,such as a sandstone, and another with a relativelyhigh seismic velocity, such as a gneiss, will produce asubstantial reflection. At great depth, however, seismicvelocities tend to be somewhat more homogenizedthan they are near the surface, because microcracksand pores are closed within a few kilometers ofthe surface, so that differences in seismic velocityfrom one rock type to another tend to be diminished.Coupled with the fact that boundaries are not ofteneasily traced to known interfaces at the surface or indrill holes, the causes of deep reflections are notalways clear. They may be from metasedimentaryrocks, mylonite zones, layered intrusions, fluids, orcombinations of these.Where such boundaries can be related to knownfeatures, it has been found that any feature in the precedinglist may explain the reflections, so that withoutsome additional information, it is difficult touniquely identify the lithology of specific reflectors.Nevertheless, whether or not geologic causes of specificreflectors can be determined, the patterns ofreflectivity provide first-order geometric frameworksfor interpretation.15.6 THE IMPORTANCE OFREGIONAL PROFILES—LONGER, DEEPER, MOREDETAILEDIn order to provide valuable information on theregional structure of the lithosphere, seismic profilesmust be hundreds of kilometers long. Imagine tryingto look into a dark room through a small hole with theillumination on your side of the hole. As the hole isincreased in size, more of the light can penetratethrough the aperture, and more of the reflected lightfrom the objects inside the room is then visible fromthe vantage point outside the hole. Furthermore, ashigher energy (brighter) light is used, the featureswithin the darkened room become more visible. Thesituation is similar with seismic profiling: as longerprofiles (apertures) are used, large-scale features aremore likely to be seen, and as more energy (truckvibrator sources) is used, the input signal is larger, andmore reflections are usually visible. By analogy,therefore, long seismic lines with many truck vibratorswill be the most beneficial for mapping largestructures of the lithosphere. Of course, these kinds ofsurveys require efforts that are correspondingly moreexpensive.In addition to long profiles with large energysources, it is desirable to obtain the most detailed geologicinformation possible. In order to accomplish this,the signal must include the widest possible range offrequencies. When much of the seismic profiling wasinitiated on regional scales across North America bythe COCORP (COnsortium for COntinental ReflectionProfiling) project from 1975–1980, technological limitations(long travel time for signals from the uppermantle) precluded acquisition of deep data with sufficientlyhigh frequencies to provide much detail. Nowthere are close to 20,000 km that have been recorded inNorth America alone, and over the past 20 years technologicaldevelopments have allowed acquisition ofsuch extensive and detailed data, often with remarkableresults. A data set from the LITHOPROBE programin Canada serves as an example to illustrate theapproach to interpretation as well as some of the informationthat can be obtained.374 GEOPHYSICAL IMAGING OF THE CONTINENTAL LITHOSPHERE

2917-CH15.pdf 11/20/03 5:20 PM Page 37515.7 AN EXAMPLE FROMNORTHWESTERN CANADAA nearly 700-km long profile of the lithosphere innorthwestern Canada recorded in 1996 and processed in1996–1997 was acquired in an effort to map the deepstructure of the western portion of the Canadian Shield,both where it is exposed on the east end of the profileand then where it projects beneath younger sedimentaryrocks of the Western Canada Sedimentary Basin to thewest (Figure 15.4). In this region, the Canadian Shieldconsists of the Archean Slave Province on the east, andyounger, Proterozoic rocks on the west. The Proterozoicrocks are primarily associated with an orogen, the WopmayOrogen, that has been interpreted from surface geologicmeasurements to represent remnants of tectonicaccretion associated with subduction on the west marginof the Slave craton at about 1.85–2.1 Ga. On the west,the profile ended east of the Cordillera, although threemore profiles that cross the Cordillera have since beenrecorded to provide nearly 3,000 km of data that extendfrom some of the oldest rocks in the world (SlaveProvince) to the modern active margin near Alaska.The most dramatic features of this profile are thereflectivity throughout the crustal section, the regionallysubhorizontal Moho, and the extensive, but comparativelysparse, upper mantle reflections (Figure 15.5). It iscommonly observed that the crust is more reflective thanthe mantle and this profile is a nice illustration. Thismakes sense geologically because the crust is lithologicallyheterogeneous at many scales, including scales oftens to hundreds of meters, in which the seismic wavesare most responsive. In contrast, the mantle tends to bemore lithologically, and thus seismically, homogeneous.The difficulties of interpreting the causes of crustalreflectivity are, however, amplified for mantle reflectionsbecause (1) mantle reflections cannot be linkeddirectly to outcrop, and (2) the relatively homogeneouslithology of the mantle is not usually expected to havesufficient contrasts in properties to produce reflections.Nevertheless, reflections are present from within themantle, and the large lateral extent of them implies thatthey are related to major regional features. Furthermore,even though the large-scale features are visible andmappable along this section, many smaller features,from the size of a sedimentary basin down to a few hundredmeters, can also be delineated and are, indeed,most helpful in interpreting the large-scale structures.It has been suggested that the regional patternsalong this profile are related to Proterozoic subduction,with East-dipping mantle reflectors as images of aremnant subduction zone, and many of the crustalstructures associated with this subduction and accretionprocess. The ability to image structures at variousFIGURE 15.4 Map of northwestern Canada showing the division of major geologic domains and the location of a∼700-km long reflection profile (prominent dark line). Precambrian domains HO, GB, CN, RA, and TA are all defined onthe basis of regional gravity and magnetic anomaly patterns that can be correlated to outcrops in the exposedCanadian Shield to the north and east. Domains NH, FS, KW, KS, CH, and BH are entirely covered by the sedimentaryrocks of the Western Canada Sedimentary Basin (gray), the eastern edge of which is labeled WCSB.15.7 AN EXAMPLE FROM NORTHWESTERN CANADA375

2917-CH15.pdf 11/20/03 5:20 PM Page 376FIGURE 15.5 (upper) Regional seismic profile from ancient (>2.6 Ga) rocks of the SlaveProvince on the east, across the Proterozoic (2.1–1.85 Ga) Wopmay Orogen in the center, and thenthe younger Proterozoic (∼ 1.74–0.55 Ga) Fort Simpson Basin on the west. The data are plotted to32.0 s travel time, or about 120 km depth. Note the prominent crustal reflectivity, thecrust–mantle transition, and sparse, but important reflections from within the upper mantle(M1 and M2). A general interpretation is shown (lower) to illustrate that the accretion of theProterozoic rocks to the Slave Province probably resulted from subduction, the remnants of whichare probably the dipping mantle reflections.scales further allows the large and regionally significantboundaries to be related to more local featuresthat, in turn, can be correlated with geologic observations(e.g., outcrop patterns, drill holes, and so on). Forexample, a key factor in the interpretation of the mantlereflections is that they can be followed to structuresin the crust that can be approximately dated.Consider the relationship between reflections M1(Figure 15.5) and the crustal geometry previously discussed.On the west (left) side of the section, a seriesof layers thickens westward between the surface and30 km depth (Figure 15.6a). These are almost certainlythe expression of a westward thickening Proterozoicbasin (the Fort Simpson Basin). They are overlain byshallow, more or less flat-lying, Paleozoic sedimentaryrocks, and they are underlain by west-dipping surfacesthat can be followed updip eastward to where they subcropat the base of the Paleozoic. Drill holes have intersectedthe crystalline rocks that underlie the east flank ofthe basin and samples dated with radiometric techniquesyield ages of about 1.845 Ga. Thus the basin layersoverlying these crystalline rocks must be younger thanabout 1.845 Ga, but older than the Paleozoic.Within the basin, even finer scale structures andstratigraphy may be discerned (Figure 15.6b). Near thesurface, the unconformity between the base of thePaleozoic and the Proterozoic is evident as a truncationof dipping layers at ∼0.2 s (about 500 m depth). Nearhere, drill holes penetrated from the Paleozoic sedimentaryrocks into Proterozoic argillaceous rocks, thusestablishing that the uppermost layers of the Proterozoicare indeed of sedimentary origin. Note also thatthe lower Paleozoic layers appear to be arched slightlyinto an anticline at the position of the truncation (Figure15.6b). This anticline must have formed after thePaleozoic strata were deposited, and was probablyassociated with the uplift of the Cordillera, the easternfront of which is located about 50 km west of the profile.At greater reflection times, unconformities are visiblewithin the Proterozoic layering, thus indicatingthat these deep layers are indeed also of sedimentaryorigin and filled a deep basin during this time (Figure15.6a). Although the depth of the basin is not certain,the thickness of the layering indicates it may be asmuch as 20 km (Figure 15.6a). A prominent reflectioncrosses the stratified basin layers at a relatively highangle (Figure 15.6b). Although it is not known withcertainty what this is, because it does not outcrop, itscross-cutting geometry is characteristic of dike intrusions,and there are such intrusions known within theProterozoic sedimentary rocks of this region.Thus, even though there are no direct observations(drill holes or outcrops) of the layering to 20 km depth,the large-scale geometry of a basin shape, the geometricrelationships between the layers indicating unconformitiesand stratigraphic thickening (Figure 15.6aand b), and regional relationships that indicate Proterozoicsedimentary rocks are very thick to the west,376 GEOPHYSICAL IMAGING OF THE CONTINENTAL LITHOSPHERE

2917-CH15.pdf 11/20/03 5:20 PM Page 377(a)(b)(c)FIGURE 15.6 (a) The regional seismic profile across the Proterozoic basin illustrating the huge thickness of strata on the west andthe associated shallowing of the Moho. (b) Enlargement of the regional profile in the upper part of the Proterozoic Fort Simpson Basin onthe west. Note the sedimentary features such as the unconformity at the base of the Paleozoic sediments, unconformities in theeastward-thinning Proterozoic layers, and the prominent cross-cutting reflection that may be an igneous dike. (c) Enlargement of theregional profile across a feature that has been interpreted as the remnants of an accretionary complex. Note that the mantle reflections,M1, can be followed westward where they correlate with the Moho and that dipping layers above M1 tend to steepen eastward (upperarrows) as is common in accretionary wedges.15.7 AN EXAMPLE FROM NORTHWESTERN CANADA377

2917-CH15.pdf 11/20/03 5:20 PM Page 378all lead to the conclusion that the western 100 km or soof this profile provides an image of a large, deep, andpreviously unknown sedimentary basin.Reflections from layered sediments are well knownin petroleum industry exploration. Thus, mapping suchreflections from a large basin, even though it is Proterozoic,are not surprising. However, most of the deepcontinent includes crystalline metamorphic andigneous rock, and the common belief 20 to 25 yearsago was that the velocity and density contrasts in theserocks were insufficient to produce reflections. If properlyrecorded and processed, however, data from thecrystalline crust can have reflections that are just asprominent as those from a sedimentary basin.To the east of the sedimentary basin discussed above,the crust beneath the thin Paleozoic cover consists ofProterozoic igneous and metamorphic rocks. This isknown because the Paleozoic rocks thin to zero eastward(Figure 15.4), with crystalline rocks exposed onthe surface east of there, and because drill holes intersectPrecambrian crystalline rocks where the Paleozoiccover is present. Throughout this region, however,reflections are visible between the surface and about35 km depth (Figure 15.5), which must all be within thecrystalline basement.The complex reflections from the crystalline crustcan be interpreted by applying the same principles aswith the Proterozoic basin: Drill holes provide directevidence for the lithology and ages of rocks near thesurface; the regional geology is incorporated to theextent possible, and detailed geometric relationships(i.e., truncations) are utilized to establish structuralpatterns and age relationships. Although there are toomany details to address them completely here, threeimportant characteristics stand out when the reflectionsare interpreted:1. The reflectivity pattern delineates a series ofcomplex structures associated with the accretionof middle Proterozoic rocks to an older Archeancraton (Slave craton).2. These structures are for the most part confined tothe crust.3. The base of the layering is remarkably abrupt atabout 10–11 s (about 30–33 km) beneath boththe Proterozoic and the Archean regions.The first of these is significant because it provideskey evidence on how subduction and accretionoccurred during the Proterozoic (about 1.85–2.1 Ga inthis region). From the geometric and geologic information,it appears that the products (what is visible today)of the tectonic process acting at that time are nearlyidentical to structures in modern subduction accretionzones. For example, east of the Fort Simpson Basin andabove the mantle subduction reflections, the crustalgeometry is nearly identical to that of an accretionarywedge and associated structures (Figure 15.6c).The second and third characteristics emphasize theapparent structural (or at least reflection) differencesbetween the crust and the mantle. The base of thecrustal reflectivity (the “reflection Moho”) is nearlyhorizontal along most of the profile east of the FortSimpson Basin and is at a travel time that correspondsto the Moho identified from collocated regional seismicrefraction data. This means that the crust–mantletransition here is either a zone of late intrusives (e.g.,sills) that underlies the crust, or that it is a structuraldetachment zone. Examination of some detailed featureswith travel times near 10–11 s (about 30–33 kmdepth) provides information to distinguish betweenthese possibilities (Figure 15.3b), as many of the layersin the lower crust beneath the Great Bear arc (Figure15.5) of the Wopmay Orogen are listric (flatten)into the horizontal reflections near the Moho. Thus, thecrust–mantle transition at this location is almost certainlya structural detachment rather than a layeredintrusion zone. There are many profiles around theworld that have images of lower crustal structureslistric into the Moho, but the data must be of sufficientlyhigh quality and have sufficiently fine detail forsuch subtle structures to be observed.At some locations, in contrast to the listric structuresjust noted, the crust–mantle transition may havecharacteristics appropriate for an interpretation of silllikeintrusions; an example is visible beneath the SlaveProvince (Figure 15.7). Here, reflections project fromthe lower crust to below the Moho, and horizontalreflections at the Moho appear to cross cut them.Other characteristics of the variable crust–mantletransition on this profile include the following: At twolocations, reflections dip from the lower crust into themantle; one of these corresponds to the interpretedProterozoic subduction zone, and the other is locatedbeneath the Archean craton. In some parts of the profile,the reflectivity of the crust–mantle transition isweak or nonexistent; whereas in others it is flat andprominent. At some locations the reflection Moho isflat, whereas beneath the Proterozoic basin it shallowsby a few kilometers (Figure 15.6b). Thus, many of thevariable characteristics of the crust–mantle transitionthat occur on deep reflection data around the world arevisible along this single profile over relatively shortlateral distances.378 GEOPHYSICAL IMAGING OF THE CONTINENTAL LITHOSPHERE

2917-CH15.pdf 11/20/03 5:20 PM Page 379FIGURE 15.7 Enlargement of a segment of the regional profile from the Slave Province(Figure 15.5). Here, the Moho appears to have a series of dipping surfaces (arrows) that are crosscut by horizontal reflections (RM). One possible interpretation is that these horizontal reflectionsrepresent intrusives.15.8 OTHER GEOPHYSICALTECHNIQUESIntracrustal structures such as the Proterozoic basin aresufficiently large to be compared with other regionaldata. The basin geometry on the west is observed onseveral other seismic profiles that are parallel to thisone but that are located over a lateral distance of morethan 500 km. They can be correlated from one toanother with the application of other geophysical data;seismic profiles provide regional cross sections, but itis often difficult to project far away from the twodimensionalsections without additional information.In many areas, geophysical data such as gravity andmagnetics, can provide such information.The map in Figure 15.8a shows the isostatic gravityvariations in northwestern Canada, and Figure 15.8b isan enlargement of the central portion of the map in thevicinity of the regional seismic profile. To produce thismap, known characteristics of the Earth’s shape andother effects have been estimated and removed fromthe measured values. The residual values, or anomalies,were contoured, and ideally represent variationsdue to rocks in the near subsurface. The contoured valueswere then plotted as a pseudotopographic image.As gravity anomalies result from variations in rockmass, in principle we should be able to determine therelative positions of different masses at depth. In practice,however, there is a fundamental problem underlyingthe interpretations of these results, as well as othergeophysical anomalies such as magnetics. Becauseneither the subsurface structure nor the values of mass(or magnetism in the case of magnetic anomalies) ofthe rocks is known, an anomaly may be caused bysmall regions with large contrasts in properties, or15.8 OTHER GEOPHYSICAL TECHNIQUES379

2917-CH15.pdf 11/20/03 5:21 PM Page 380large regions with small contrasts in properties. Thereare some limits (spatially), of course, because theanomalies are located according to map position (e.g.,anomaly FS in Figure 15.8b must be due to somethingin the subsurface beneath it), but it is difficult to determinemuch more detail without some additional informationfrom other techniques.In the context of regional variations of continentalstructure, however, the patterns of large-scale anomaliescan be extremely valuable in delineating patternsof continental structures. For example, in Figure 15.8a,the gravity patterns exhibit prominent, but relativelysubdued, anomalies in the eastern part of the map (FSin Figure 15.8b); and more random and higherfrequency patterns in the west, which are crossed bysome major northwest oriented features (TT in Figure15.8a). This change occurs where the sedimentaryrocks of the Western Canada Sedimentary Basin giveway to the complexly deformed rocks of the MackenzieMountains in the northern Cordillera. It is logicalto interpret the change in gravity anomalies as beingrelated to the large-scale geologic transition from thebasin to the Cordillera.On the other hand, the causes of the patterns beneaththe Western Canada Basin are less clear. In this area,the sedimentary rocks are relatively flat and thin (Figure15.6a), hence they should not exhibit major changesin gravity signature. The observed gravity variations(a)(b)FIGURE 15.8 (a) Isostatic gravity map ofnorthwestern Canada plotted with shaded relief(artificial illumination from the west, view towardthe northeast). The position of the regional seismicprofile is shown by the thick white line. TTrepresents the Tintina Fault, a late strike-slip faultwithin the Cordillera, and FS represents the FortSimpson Trend associated with the Fort SimpsonBasin. The gridded digital gravity data were providedby the Canadian Geophysical Data Centre, and theoriginal version of this figure was made by KevinHall. (b) Enlargement of the map in the vicinity ofthe seismic profile to emphasize the relationship ofthe profile to the FS anomaly. The smaller white linenear the bottom right is the location of a secondprofile across the southern portion of the FS trend,and the white circles represent locations of drillholes that penetrated crystalline rocks below theWestern Canada Sedimentary Basin strata.380 GEOPHYSICAL IMAGING OF THE CONTINENTAL LITHOSPHERE

2917-CH15.pdf 11/20/03 5:21 PM Page 381must therefore be associated with structure and lithologybeneath the sedimentary basin, such as the largescalestructures observed on the reflection profile.The interpretation of these observations is facilitatedby the fact that the anomalies can be followed eastwardinto the Canadian Shield, where they are correlatedwith regional structures on the surface in the ancient(1.8–3.5 Ga) rocks. Accordingly, the patterns observedin the basin provide an image of structural patterns thatproject westward from the Canadian Shield, beneaththe basin, to the eastern part of the Cordillera. Thus,while this approach does not necessarily provide uswith much detail on the nature of the causes of individualanomalies, it does provide important information onthe orientation and extent of subsurface structures in aregion where there are no exposures.All of the other seismic profiles that cross anomalyFS (Figure 15.8b) have essentially the same geometry;the Proterozoic basin previously described occurseverywhere to the west of FS. In all locations, the gravitysignature indicates that the rocks at depth (belowthe Paleozoic sediments) are low density west of FSbecause the gravity anomalies are low, and this informationis consistent with a deep, but old, basin withinthe crust. In applications for regional crustal andlithospheric imaging, therefore, one of the most valuablecontributions of potential field maps is to projectinformation over long distances away from the muchhigher resolution seismic cross sections.15.9 CLOSING REMARKSGeophysical imaging techniques have become standardtools for mapping the subsurface structure of thecontinental lithosphere. The most successful resultsderive from seismic reflection data that can be linkedto known geologic features, either in outcrop or in drillholes. Improvements in field acquisition methods andsignal processing have led to remarkable images of thecrust and mantle lithosphere. Thus, as more, longer,and increasingly detailed profiles are acquired, theability to compare, contrast, and link the results withother geologic and geophysical data will continue tofoster new concepts on the origin and tectonic developmentof the continental lithosphere.ADDITIONAL READINGCook, F., van der Velden, A., Hall, K., and Roberts, B,1999. Frozen subduction in Canada’s Northwest Territories:lithoprobe deep lithospheric reflection profilingof the western Canadian Shield. Tectonics, 18, 1–24.Lucas, S., Green, A., Hajnal, Z., White, D., Lewry, J.,Ashton, K., Weber, W., and Clowes, R., 1993. Deepseismic profile of a Proterozoic collision zone: surprisesat depth. Nature, 363, 339–342.Mandler, H., and Clowes, R., 1997. Evidence forextensive tabular intrusions in the Precambrianshield of western Canada: a 160-km long sequenceof bright reflections. Geology, 25, 271–274.Smithson, S., Brewer, J., Kaufman, S., Oliver, J., andHurich, C., 1978. Nature of the Wind River thrust,Wyoming, from COCORP deep reflection data andfrom gravity data. Geology, 6, 648–652.van der Velden, A., and Cook, F., 1996. Structure andtectonic development of the southern Rocky Mountaintrench. Tectonics, 15, 517–544.van der Velden, A., and Cook, F., 1999. Proterozoicand Cenozoic subduction complexes: a comparisonof geometric features. Tectonics, 18, 575–581.ADDITIONAL READING381