New Insights Into the Structure of Sudbury

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New insights into the structure of the Sudbury

Igneous Complex from downhole seismic studies1

David Snyder, Gervais Perron, Karen Pflug, and Kevin Stevens

Abstract: New vertical seismic profiles from the northwest margin of the Sudbury impact structure provide details of

structural geometries within the lower impact melt sheet (usually called the Sudbury Igneous Complex) and the sublayer

norite layer. Vertical seismic profile sections and common depth point transformation images display several continuous

reflections that correlate with faults and stratigraphic boundaries logged from drill cores. Of four possible mechanisms

that explain repeated rock units, late-stage flow or normal faulting that occurred within the last layers to cool and crystallize

might best explain the observations, especially the most prominent reflectors observed in the seismic data. These results

reaffirm previously proposed two-stage cooling and deformation models for the impact melt sheet.

Rsum : De nouveaux profils sismiques verticaux de la bordure nord-ouest de la structure dimpact de Sudbury dtaillent

les gomtries structurales dans la couche infrieure de roche fondue par impact (habituellement appele le complexe

ign de Sudbury) et de la couche de norite sous-jacente. Des sections verticales de profils sismiques et des images de

transformation de points profondeur commune montrent plusieurs rflexions continues qui concordent avec des failles

et des limites stratigraphiques tires de carottes de forage. Des quatre mcanismes possibles qui expliqueraient les units

rocheuses rptes, une coule tardive ou des failles normales, qui ont eu lieu dans les derniers tages refroidir et

cristalliser, pourraient le mieux expliquer les observations, surtout les rflecteurs les plus prominents observs dans les

donnes sismiques. Ces rsultats raffirment les modles antrieurement proposs de refroidissement et de dformation

en deux tapes pour la couche de roche fondue par impact.

[Traduit par la Rdaction] S nyder et al. 95 1

Introduction

Little observational information is available about the detailedstructures associated with the margin of the excavated material,the so-called transient cavity, of impact craters 200 km indiameter or larger. Three craters are presently known, anddeep levels of erosion (Vredefort) or burial (Chicxulub) limitaccess to information (Melosh and Ivanov 1999; Grieve andTherriault 2000). Only the Sudbury impact structure(Fig. 1a) provides the appropriate exposure level to examinesome features of large impact crater margins in detail. TheSudbury structure appears unusual because of the relativelylarge volume of melt sheet produced by the impact and thelong time it took to cool and crystallize (Naldrett andHewins 1984; Ivanov and Deutsch 1999). Recenthigh-resolution seismic surveys conducted by the GeologicalSurvey of Canada Downhole Seismic Imaging (DSI)Consortium (see the Acknowledgments) within rocks of theSudbury Igneous Complex (SIC) have provided new insightinto how the igneous complex deformed as it cooled followingthe Sudbury impact 1850 Ma. Here we report new vertical

seismic profiling (VSP) results acquired with receivers locatedin relatively deep (18002000 m) exploration drill holes(Fig. 1b).

After a century of study (Pye et al. 1984) the generalgeology of the Sudbury basin is well documented, butalthough its origin as an impact structure is now widelyaccepted, questions remain about the origin of specificfeatures of the Sudbury structure (Dressler and Sharpton1999). The Sudbury structure forms an elongate basin, witha long axis oriented east-northeastwest-southwest, superposedon primarily granites and gneisses of the Archean SuperiorProvince (Dressler et al. 1992; Fig. 1a). The structure is definedprimarily by the SIC, an apparently differentiated sequenceof felsic norite and granophyre. The complex is overlain bythe Whitewater Group, which includes the heterolithic brecciasof the Onaping Formation, interpreted as airfall deposits ofthe impact event, and metasedimentary wackes of theChelmsford Formation that are thought to be not directlyrelated to the impact event.

The lowermost unit of the SIC is the sublayer norite.This gabbro to quartz diorite lies along the contact between

Can. J. Earth Sci. 39: 943951 (2002) DOI: 10.1139/E02-013 2002 NRC Canada

94 3

Received 3 July 2001. Accepted 11 March 2002. Published on the NRC Research Press Web site at http://cjes.nrc.ca on26 June 2002.

Paper handled by Associate Editor F. Cook.

D. Snyder,2 G. Perron,3 and K. Pflug. Geological Survey of Canada, 615 Booth Street, Ottawa, ON K1A 0E9, Canada.K. Stevens. Falconbridge Exploration, Ltd., P.O. Box 40, Falconbridge, ON P0M 1S0, Canada.

1Geological Survey of Canada Contribution 2001066.2Corresponding author (e-mail: [email protected]).3Present address: MIRA Geoscience, 310 Victoria Avenue, Westmount, QC H3Z 2M9, Canada.


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the SIC and Superior Province footwall rocks, and also formsseveral dikes in the footwall. These dikes are known as theOffset Dikes, and some extend radially tens of kilometres

into the footwall rocks (Fig. 1a). The sublayer and offsetshost many of the nickelcopper ore bodies of the area, butore bodies also occur immediately below the sublayer in thefootwall rocks and footwall breccias. Because the sublayer isthe lowest rock layer associated with the impact, it effectivelylines the excavated cavity. Impacts are extremely highenergy events, therefore the sublayer melt would be

expected to have great lateral variation in shape, thickness,and composition. The footwall breccia units include the dis-continuous Late Granite Breccia immediately underlyingthe sublayer norite and the Sudbury breccia, a pseudotachyliteconsisting dominantly of locally derived rock fragments in afine-grained, generally dark colored matrix. In the area ofour survey, the footwall consists of granite, felsic gneiss,mafic gneiss, migmatites, mafic volcanics, gabbro, andyounger Sudbury swarm diabase dykes (Fig. 1a).

A number of important questions about the formation ofthe Sudbury structure, and its igneous complex (the SIC) inparticular, remain unresolved (Naldrett 1999). The proportionof granophyre in the SIC (Fig. 2) is too great to have evolvedby differentiation of a single melt sheet, despite isotopic

evidence that all the material came from crustal rocks ofsimilar age. The granophyre and felsic norite also displaydistinct deformational histories (Cowan et al. 1999) that suggesteither two stages of cooling or the injection of lower crustaland upper mantle melt after the main impact-related thermalevent. Naldrett (1999) suggests that the sublayer norite consistsprimarily of initial melt, but it was enriched by sulfides andmafic inclusions that originated in the target (footwall) rocksand that gravitationally settled out of the melt sheet. Thissublayer melt was not greatly involved with convective mixingwithin the main body of the SIC; it preserved its distinctgeochemical signature and only intermixed with the felsicnorite melt near their mutual contact. This model of stratifiedconvection, mixing, and cooling predicts specific types of

structures along this contact and excludes others. Many ofthese have scale lengths that are resolvable with the downholeseismic technique described here.

Downhole seismic data

AcquisitionThe downhole seismic data were acquired by the Geological

Survey of Canada DSI Consortium in the autumn of 1998and 1999 within the Norman West property of Falconbridge,on the northeast range (North Lobe) of the Sudbury impactstructure (Fig. 1a). In the first year, three-component receiversat 1001855 and 1001915 m depths in holes N26 and N33,respectively, recorded energy from five nearby shot points(called SP1SP5). The resulting 10 overlapping VSPs had amaximum horizontal sourcereceiver offset of 350 m. In thesecond year, again using energy from five shot points (SPN,SPS, SPE, SPW, and SPC in Fig. 1b), three-componentreceivers recorded data at 3501419 and 3501705 m depthsin holes N40 and N43, respectively. These 10 offset VSPshad maximum offsets from 400 to 2500 m (Fig. 1b). Shotpoint SP4 from the first year was reused as shot point SPE inthe second year.

In both years very similar acquisition parameters wereused. The receiver interval was 5 m and the sample raterecorded on a 24-bit OYO DAS-1 was 0.25 ms. Shot records

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Fig. 1. (a) Location map showing the northeast part of the

Sudbury impact structure. The rectangle shows the area of the

downhole seismic survey (Fig. 1b); it lies in an embayment of

the larger structure called the North Lobe by some workers. The

main geological units of interest here are the granophyrenorite

of the Sudbury Igneous Complex (SIC), the sublayer norite layer,

and the wall rocks to the impact (Superior Province granites).

The Onaping and Chelmsford Formations overlie the SIC. P,Parkin dike. (b) Map view of the Norman West surveys showing

drill holes used for receivers (N40, N43, N33, N26), selected

shot points (SP3, SPW, SPS, SPC, SPN, SPE = SP4), and contours

to the top surface of the sublayer norite (solid lines) and isopach

contours of its thickness (broken lines) as compiled from numerous

drill holes in the area. Also shown is the footprint or surface

projection of the reflection points for VSPCDP transforms for

holeshot combinations N33SP3, N40SPC, and N40SPN; all

assumed a strike of 150 and southwest dip of 30. The sections

related to the first two footprints are shown in Fig. 5; the latter

footprint illustrates an extreme geometry. Grid north and grid

east values are shown at the left and bottom, respectively.


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Fig. 2. Density, P-wave velocity (Vp), and acoustic impedance with lithology and fracture logs for borehole N33 (Pflug et al. 2000). A

synthetic seismogram generated using these impedance contrasts and a Ricker wavelet centred at 70 Hz is shown at the right; all five

traces are equivalent (t1, t2, and b1 locate where the top and bottom of the subnorite layer intersects the borehole; f, fault zone). The

density and acoustic logs were acquired in the same hole used for the downhole seismic receivers. Positive density inflections and negative

P-wave velocity inflections are clearly associated with the sublayer norite layer and its contact with the wall rocks, here mostly Sudbury

and granite breccia or felsic gneiss. The opposite senses of inflection produce a relatively modest change in their product, impedance.

Short wavelength variations remain small throughout the Sudbury Igneous Complex rocks to a depth of 1450 m. This provides nearly

ideal conditions for seismic exploration of the impact contact zone (sublayer norite), as the overlying SIC rocks are very good transmittersof seismic waves.


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were 2.0 s long. An eight-level three-component downholetool was used in holes N26, N40, and N43. A four-level toolwas used in hole N33. Three-component seismometers atindividual levels in these Vibrometric XYZ 8/24 and XYZ4/12 instruments are identical.

The survey design is called a multi-azimuth VSP. Shotholes are located at varying azimuths and distances (e.g.,Fig. 1b) to orient each receiver level during post-surveyprocessing by maximizing direct P-wave energy on the radialhorizontal component. Large horizontal offsets provide moreenergy on horizontal components to implement these so-calledrotations, but near offsets typically provide clearer and simpler

record sections.Shot holes were drilled 35 m into bedrock. Holes were

reused, but filled each time with water to provide better couplingwith the ground. The charge of high-velocity pentholitedynamite boosters was increased from 90 to 227 g whenreceivers reached depths over 1000 m. Shots times andrecorder initiation were synchronized on minute marks usinga set of six blasting boxes, each in turn synchronized toGlobal Positioning System (GPS) time every morning andevening. The clock drift of each box was recorded eachnight and removed during processing (Fig. 2). Drifts weretypically 530 ms.

Processing of VSP sectionsVertical seismic profiles do not resemble geological cross

sections, in part because wireline depth is traditionally plottedacross the horizontal axis and traveltime increases down thevertical axis (Fig. 2). This convention makes prominent, di-rectly arriving, downgoing seismic waves appear to propagatefrom upper left to lower right. Upgoing, reflected wavesappear to propagate from upper right to lower left. Toobserve the reflected waves of interest to exploration, thehigher amplitude direct waves must be attenuated or removed.Table 1 summarizes the processing steps used to produce theVSP sections used in this study (Figs. 3, 4); a few key steps

will be discussed in more detail here. The processing soft-ware used is DSIsoft, developed by the DSI Consortium.

First breaks, the onset of direct P waves, should showsmooth trends that increase in time with increasing depth.Jagged offsets (Fig. 3) result from timing breaks due to driftsin both the blasting box and trigger pulse timing clocks. Clockdrifts were reconstructed in three steps: (i) drifts measured eachevening were subtracted; (ii) offsets recorded in first breaksby a surface geophone, when observable above ambientnoise levels, were subtracted; and (iii) static corrections wereapplied by hand. The first breaks were also used to rotate thehorizontal components so that H1 points toward the source.

This technique attempts to maximize the energy on H1, butconsistently high noise levels on H2 made this analysisunreliable on data from N26 and N33, so only the vertical(Z) component was used in subsequent analysis. Much ofthis noise has a limited frequency range and appears asringing. Predictive deconvolution helped reduce the amplitudeof this noise. The difference in the VSP sections for N40C,as illustrated between Figs. 3 and 4, shows the success of theprocessing in enhancing reflected (downgoing) seismic energy.

Common depth point (CDP) transformsImages of the subsurface were created using a

data-mapping procedure called the common depth point(CDP) transform or stacked VSPCDP display (Hardage 2000,p. 266). This procedure takes every sample from atimedepth VSP profile and relocates them in three-dimensional(3D) space. The end result is a slice through the subsurfacecomparable to a normal surface seismic CDP profile. The CDPtransform was applied to the 20 VSPs (four holes, five shotsinto each). A CDP line was picked for each profile and 8 m 8 m 8 m bins were used to map the data from spacetimedomain to XYZdomain. The data were then projected ontothe CDP line creating two-dimensional (2D) profiles with8 m CDP spacing (Fig. 1b). Because of the strike and dipassumed, the profile sections shown here correctly relocatethose events which dip 2535 towards the southwest (Fig. 5).

1. Geometry Covert SEG2 to DSIsoft format

2. Sort to wireline depth

3. Drift (timing) corrections

4. Monofrequency noise removal by adaptive filter 60, 180, 300, 360, 420, 540, 660, 780 Hz

5. Residual statics

6. Energy balancing Average channel energy7. Rotation of horizontal components

8. Lowpass filtering Ramp from 400 to 1000 Hz

9. Predictive deconvolution Lag 2.75 ms (Z), 7.50 ms (H); operator length 3 ms; window 02 s (H),

50 to +150 ms relative to first break (Z)

10. Resample to 0.5 ms

11. Energy balancing 00.75 s window

12. Trace editing

13. Removal of downgoing P wave Median velocity filter (13 points)

14. Removal of downgoing S wave Median velocity filter (23 points)

15. Energy balancing 00.60 s window

16. FK filtering of other downgoing energy

17. FK filtering of tube wave energy

18. Bandpass filter 3585 to 225325 Hz

Table 1. Processing parameters.


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These dips are those expected within the North Lobe forlithological contacts of the major SIC units (granophyre, felsicnorite, sublayer norite). Shallower dips (e.g., 15) and otherstrikes (e.g., 100) were also used for comparisons and studiesof the sensitivity to these parameter choices. The choice ofstrike within 45 had little effect on the geometries ofreflections; changes in dip of 10 produced significantchanges in the geometries of reflections and also in the areaimaged. Note that the dips chosen provide an image of thesubsurface as far as 10001200 m away from the boreholes(Fig. 5).

Rock properties

Seismic surveys can complement surface studies to revealimportant 3D geometries of structures by providing depthinformation, if an appropriate velocity and density contrastexists between important rock units. Well logs and core samplesfrom within the Sudbury impact provide such informationabout rock properties (Fig. 2; White et al. 1994). Previousstudies concluded that both the granophyrenorite transitionand the sublayer norite layer have a sufficiently high andconsistent impedance contrast to be mapped regionally using

seismic reflection methods (White et al. 1994). Those studiesand our new studies both indicate that velocities and densitiesvary gradually with depth within the SIC, but variations inboth properties increase in amplitude markedly within thesublayer norite and at deeper contacts with the footwall rocks(Fig. 2).

The transition from granophyre to felsic norite within theSIC has sufficient compositional variations over a shortdepth interval to produce a distinct reflection at wavelengths(100400 m or 1560 Hz) typical of deep surface reflectionprofiling (Milkereit et al. 1992; White et al. 1994). This

transition is too gradual to produce reflections at the shorterwavelengths (20100 m or 60300 Hz) used in our downholeseismic surveys with explosive seismic sources (see syntheticseismogram in Fig. 2). Instead, the entire SIC is a relativelyloss free medium to high-frequency seismic waves, as theyare neither strongly scattered nor reflected by this homogeneousmedium.

Modeling of the P- and S-wave first breaks observed inour downhole data showed velocity variations of 58406140 m/sfor the P waves from the 10 VSPs and 34103800 m/s for Swaves. The borehole velocity logs indicate consistently higherP-wave velocities centred around 6300 m/s within the SIC

Fig. 3. Vertical seismic profile section for unprocessed data from receivers located in bore hole N40 and a source at shot point SPC

(Fig. 1b). P1 indicates the prominent direct P-wave arrivals that show a sharp apparent offset at a depth of 1320 m; this is due to

shooting clock drift and occurs where work ended one day and began again the next morning. The direct Swaves (S1) show a similar

offset. A chevron pattern is also apparent from seismic phases dipping about 30 on this section; these are tube waves generated within

the bore hole at sharp breaks in the wall rocks, typically where faults intersect the bore hole. The arrow indicates a reflection from a

shallowly dipping planar structure. Plots are variable area, with a gain of 5 and automatic gain control (AGC) using a 200 ms window.


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(Fig. 2). Milkereit et al. (2000) also report that a narrowrange of P-wave velocities, 62006400 m/s, generallycharacterize the SIC felsic norite throughout the Sudburystructure. A P-wave velocity of 6300 m/s and an S-wavevelocity of 3600 m/s were adopted for analysis and inmodeling. We note, however, the attenuation and possibleanisotropy suggested by the observed discrepancy betweenborehole logs sampled at approximately 10 000 Hz frequenciesand refraction velocities calculated using approximately100 Hz arrivals.

Although the SIC effectively acts as a clear lens to theseismic waves, physical property variation with depth nearthe sublayer norite is sufficiently sharp to produce observable

reflections in both types of surveys (e.g., Fig. 4). It is generallyassumed that seismic surveys can distinguish vertical variationsin rocks of about one quarter the seismic wavelength. Thedownhole data are thus able to resolve rock layers, providedsufficient impedance contrast exists, if their thicknesses are525 m. The reflectivity apparent in Fig. 4 indicates thatnumerous layers within the sublayer norite and underlyingfootwall rocks have these characteristics. The syntheticseismogram calculated from the density and velocity logslikewise shows increased reflectivity beneath the sublayernorite and, to a lesser extent, within this unit (Fig. 2), but nostrong reflection from its top contact. Except near their base,the overlying SIC rocks show comparatively little reflectivityor structure. A few notable exceptions are likely faults andfracture zones.

Fig. 4. Final processed VSP sections for (a) the same shotreceiver

pair as that in Fig. 3 and (b) receivers in hole N33 and a source

at shot pointSP3. The borehole can be imagined to lie along the

first breaks at the top of the section; t1, t2, and b1 locate where

the top and bottom of the subnorite layer intersects the borehole

(Fig. 2). In hole N33, a continuous reflection and a strong tube

wave originate at one logged fault zone. Plot parameters as in Fig. 2.

Broken lines labeled P and S show modeled traveltimes fordiffractions scattered from an object 1200 m north of borehole

N40 assuming P- and S-wave velocities, respectively.

Fig. 5. CDP transform images for (a) VSP N40SPC and (b)

VSP N33SP3. See Fig. 1b for section locations. This transform

assumes 30 reflector dips, so only those dips are imaged in

their correct position. Both images assumed a strike of 150, so

both profiles trend N60E. The top (t1, t2) and base (b1, b2) of

the sublayer norite are identified as prominent reflectors in both

images (unit is only 13 m thick in N40), as is the base of a

pocket of Late Granite Breccia (g in Fig. 5b). The main, nearlyflat event in Fig. 5b is a fault imaged for more than 800 m of

offset from a wireline depth in N33 of about 1370 m. Additional

dotted lines mark uninterpreted reflectors within the footwall

rocks and at 1000 m depth.


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Observations

The homogeneity and smooth velocity gradients withinthe granophyre and felsic norite in the upper 1500 m of the3D rock volume investigated provide ideal conditions for thepropagation of seismic waves with little scattering or attenuationand little variation in velocity until the sublayer zone isreached. The synthetic seismogram calculated from the welllogs indicates that the top of the sublayer is not a strong reflector(Fig. 2), but that numerous horizons within the footwall doproduce reflections.

The processed VSP sections display numerous reflectionsdipping down to the left at nearly all wireline depths(Fig. 4). Only a few of the more continuous reflections thatintersect the borehole will be noted here. In N33, a strongtube wave was generated at a depth of 1400 m (labeled faultin Fig. 4b). A continuous and relatively high amplitudereflection, dipping down to the left (to almost 0.3 straveltime) at a slightly steeper angle than neighboringreflections, also originates at this depth. Nearby, three morecontinuous reflections (labeled t1, t2, and b1 in Fig. 4b) arenearly parallel. This band of reflectors intersects the boreholeat depths of 13201650 m. Reflections at greater traveltimes(>0.45 s at 1400 m depth) with similar dips do not intersectthe bore hole. These may be reflections from surfaces deeperthan the borehole or, as indicated by our forward modeling,diffractions from scattering objects several hundred metresdistant from the boreholes. Forward modeling also indicates

that some of the later reflections are S-wave equivalents ofearlier P waves. Some traveltimes indicate P to Sconversionoccurred near the source, and others indicate conversion atthe reflection point.

The example from hole N40 illustrates many of the samefeatures. The tube wave, prominent in the unprocessed section(Fig. 3), was removed to reveal a large number of upgoing

reflections. None of these reflections have high amplitudesor great continuity, although one (labeled t1 in Fig. 4a) canbe traced across most of the section. Again, reflections anddiffractions appear more continuous and prominent at greatertraveltimes, but these features do not intersect the borehole.

Correlations and interpretations

At depths >1500 m, geological structures, rock units, andsulfide deposits form complex structures at the margin betweenthe igneous complex and the footwall rocks of the impactcrater. Numerous large breccia blocks, lenses, and thicknessvariations in the sublayer norite provide potential pointdiffractors and local variability in reflector amplitudes. This

combination of features generates VSP records rich in clearsignals that are useful in exploration only if they can be isolatedfor interpretation. Here only the more prominent and laterallycontinuous reflectors will be discussed. These include severalmain features whose geometries are defined over an areaabout 1000 m by 400 m at depths of 10001900 m (Figs. 5, 6).

The prominent reflector associated with tube waves on theprocessed shot-gather section for hole N33 (Fig. 4b) transformsto the shallowly dipping reflector observed in the CDP transformimages (dashed line labeled fault at 1370 m wireline depthin Fig. 5b). Forward modeling of this same reflector, inwhich synthetic traveltime curves are matched to observedarrivals on the shot gathers, indicated that the planar reflectordips at 1020 to the west with a strike of 150. A fault zone

was noted in the core log (Fig. 2) where the prominentreflection was modeled to intersect hole N33 at a depth of1370 m (950 m elevation). The description from the corelog for the nearby N26 hole over the depth interval14611468 m indicates a fault zone with strong fabric, intensequartz and carbonate alteration, and minor gouge. No faultorientation is indicated.

A second prominent reflector was modeled to strike 150and dip 2432 to the west. This reflector is associated withthe top of the sublayer norite unit (t2 in Figs. 4b, 5) becauseof its near correlation with this rock layer in the core logs(also see contours in Fig. 1b). A general increase in coherentreflectivity is observed from the depth of this reflector to thebottom of the section. A nearly parallel reflector (b

1

inFig. 5) is interpreted as the base of the sublayer, offset bythe fault. The nearby reflectors (t1 and b2) are interpreted asthese same top and bottom surfaces of the sublayer, offset bythe fault. A fourth reflector (g in Fig. 5) also approximatelyparallels the third one so as to define an irregular layerinterpreted as the Late Granite Breccia. This layer issomewhat less reflective except for a few brighter spots at itsinflections that may coincide with semi-massive sulfideaccumulations. The low reflectivity of this layer is expectedgiven its nature as an anatexite unit in which the rock is aclassic breccia that was partly melted and cooled to formleucocratic crystals, as observed in nearby dikes (Murphy

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Fig. 6. Summary cartoon section of the interpretation of the

19981999 multi-VSP data; ornamentation as in Fig. 2 and as

labeled. Lithological ties to the recording borehole N33 were

honored, but contacts were projected as much as 1 km from nearby

bore holes and located using primarily the reflectors shown in the

CDP transform image (Fig. 5b). A major fault is nearly horizontal

(10W) and repeats the sublayer norite unit in holes N33 and N26.

Its sense of displacement could be either sinistral or top-side outof the section shown. Dark bands show the location of sulfide ore

bodies.


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and Wood 2001). Variations within the footwall are attributedto blocks of felsic gneiss juxtaposed with gabbro or diabase.Depending on the velocity and density contrasts producedwith the overlying layers, this variation may also contributeto local bright reflectors or scatterers (Fig. 2).

Significance to impact models

A repeat of the sublayer norite unit was observed in severalholes drilled in this area, including hole N33 (Fig. 6). Twoseparate occurrences of sublayer norite in this geologicalcontext have at least four possible explanations: (1) isoclinalfolding of a single layer, (2) two parts of an intrusive body,(3) normal fault offset, or (4) alongstrike offset of a curvedlayer. This repeat of the sublayer norite unit was originallyattributed to a large-scale (>500 m), overturned, isoclinalfold structure (G. Snyder, personal communication, 1999)presumably related to flow and intermixing with the SICduring the crater collapse phase and before it cooled. The repeatcould also represent an apophysis, a dike shooting off themain sublayer body if the latter was still fluid when the

norite layer had cooled sufficiently to behave plastically(Naldrett 1999).

The identification of the prominent and extensive reflectorwith a fault zone is significant in that it now provides additionalpossible explanations for the repeated section if the overlyingrock layer that includes some sublayer moved toward thesouth or west. The shallow dip to the southwest that wasmodeled for this reflector indicates that the fault has a normalsense of displacement; it repeats the section because the rockunits locally dip at a steeper angle (30) to the southwest. Ifreflectors b and t (Fig. 5) delimit the same rock layer, thenpalinspastic restoration of this layer using the CDP transformsection indicates that greater than 400 m of displacementoccurred on this fault.

Both the stratigraphic units and the fault dip toward thecentre of the Sudbury structure. This might suggest that thefault is somehow related to the crater collapse process, butthis appears unlikely given the brittle nature of the fault zoneand alteration where it was observed in drill core. The faultis thus probably a later deformation feature. The low-anglereflector projects to the surface well east of the study areaand the main Sudbury impact structure near where a faultoffsets the Parkin dike (Fig. 1a). The Parkin dike is one ofthe large pseudotachylite filled intrusions thought by someworkers to represent late-stage impact deformations (e.g.,Scott and Spray 2000). The sense of displacement along thisfault is sinistral, but the dip is unknown. Planar faults andfractures are known in the area, and the Sudbury swarmdiabase dikes (Fig. 1) have roughly this orientation.

The recently recognized two-stage cooling of the SIC presentsanother possibility, a hybrid of options 3 and 4 given earlierin this section. This study area lies just north of the axis ofthe North Lobe of the Sudbury structure (Fig. 1). If measuredfoliations within the felsic norite layer do represent convectiveflow down the margin of the impact structure (Cowan et al.1999; Naldrett 1999), then gravitationally driven magma flowwas locally south toward the North Lobe axis where it wouldturn west. Similarly, brittleplastic deformation driven bygravitational collapse during late stages of cooling wouldalso exhibit hanging-wall rocks displaced to the south or

southwest. In this case the relative motion of the hangingwall observed in the CDP transform section (orientedN60E) would be out of the section or sinistral (Fig. 6). Thischoice of interpretation appears the most consistent with theobservations available, including those from the Parkin dike.

In summary, although synthetic seismograms generatedfrom well logs do not predict strong reflections from main

stratigraphic contacts of the SIC, processed VSP sections doshow continuous reflections intersecting the boreholes at thetop and base of the sublayer norite unit. VSP methods thusappear useful in defining mesoscale structure within theSudbury impact structure. Tube waves and another continuousreflection correlate with a fault logged in the borehole; forwardmodeling predicts a 15 southwest dip for this fault. Becausethe sublayer norite contacts are similarly modeled to dip locallyat 35 to the southwest, this fault will have a normal sense ofoffset to repeat units in boreholes. This fault probably representsvery late stage collapse within the impact structure.

Acknowledgments

The Downhole Seismic Imaging Consortium is funded bythe Geological Survey of Canada, Noranda, Inc., QuantecGeosciences Ltd., and Falconbridge, Ltd. Key advisors forthis work included D.W. Eaton (University of Western Ontario),D. Schmitt (University of Alberta), E. Adam (GeologicalSurvey of Canada), M. Salisbury (Geological Survey ofCanada), and the geologists and staff at the Falconbridge,Ltd. Sudbury offices. Vibrometrics OY provided importanttechnical support during acquisition. I. Kay, G. Bellefleur,and M. Mah processed some of these data.

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New Insights Into the Structure of Sudbury

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