Wide-angle reflectivity across the Torngat Orogen, NE Canada

Abstract

Data from a Lithoprobe refraction/wide-angle reflection seismic transect across the Torngat Orogen in NE Canada were depth-migrated to produce a record section similar to near-vertical incidence seismic data. Densely spaced airgun shots and a station spacing between 9 and 25 km were able to produce a low-frequency image of the lower crustal reflectivity for most of the transect. Over the western end of the profile, a series of east-dipping lower crustal reflectors continue down to the Moho, which is clearly defined at a depth of 36 to 38 km. The lower crustal reflectors can be correlated laterally for up to 80 km. This reflection fabric is interpreted as evidence for thick-skinned deformation caused by convergence of the Nain and Superior provinces. Over the eastern end of the line, reflectors in the Nain Province are less continuous and may indicate steeply dipping faults, which have not been modified during formation of the orogen.

Full text

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Prestack depth migration of wide-angle seismic data across the Torngat
Orogen, NE Canada
Thomas Funck
Danish Lithosphere Centre, Øster Voldgade 10, L, 1350 Copenhagen K, Denmark, phone +45 –
38142652, fax +45 – 33110878, email tf@dlc.ku.dk
Keith E. Louden
Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, B3H 4J1, Canada,
phone (902) 494-3452, fax (902) 494-3877, email Keith.Louden@dal.ca
Jeremy Hall
Department of Earth Sciences, Memorial University of Newfoundland, St. John’s,
Newfoundland, A1B 3X5, Canada, phone (709) 737-7569, fax (709) 737-2589, email
jhall@waves.esd.mun.ca
Abstract
Data from a Lithoprobe refraction/wide-angle reflection seismic transect across the Torngat
Orogen in NE Canada were depth-migrated to produce a record section similar to multi-channel
seismic data. Dense airgun shots and a station spacing between 9 and 25 km were able to
produce a low-frequency image of the lower crustal reflectivity for most of the transect. Over the
western end of the profile, a series of east-dipping lower crustal reflectors continue down to the
Moho, which is clearly defined at a depth of 36 to 38 km. The lower crustal reflectors can be

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correlated laterally for up to 80 km. This reflection fabric is interpreted as evidence for thick-
skinned deformation caused by convergence of the Nain and Superior. Over the eastern end of
the line, reflectors in the Nain Province are less continuous and may indicate steeply dipping
faults, which have not been modified during formation of the orogen.
1. Introduction
Results from standard refraction/wide-angle reflection (R/WAR) seismic surveys are
commonly presented as velocity models derived from inverse and/or forward modeling of travel
times of refracted and reflected phases. Crustal fabric is normally not imaged because reflections
within major crustal layers are not very distinctive and are often difficult to correlate between
neighboring record sections. To overcome these limitations, migration of R/WAR seismic data
can be performed to generate a record section similar to multi-channel seismic (MCS) data.
Several authors have applied migration techniques to R/WAR data [e.g., Milkereit et al., 1990;
Lafond and Levander, 1995; Pilipenko et al., 1999]. In a test with synthetic data, Zelt et al.
[1998] show that the quality of the migration greatly depends on a dense receiver and shot
spacing as well as on an accurate velocity model. Zelt et al. [1998] suggest a station spacing of 2
km for marine seismic experiments in order to obtain a good image at both shallow and deep
crustal levels. At present, such a dense receiver spacing is hardly feasible along profiles with a
typical length of >150 km. The experiment presented in this study (Figure 1) uses a station
spacing of >9 km, which is more typical for present R/WAR seismic experiments. The data
across the Torngat Orogen was collected as part of Lithoprobe’s Eastern Canadian Shield
Onshore-Offshore Transect (ECSOOT) and the velocity model (Figure 2) reveals a preserved
crustal root beneath the orogen [Funck and Louden, 1999]. Using the same migration technique

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as in the synthetic tests of Zelt et al. [1998], the potential and limitations of prestack depth
migration of wide-angle data will be documented when applied to real data.
2. Geological Setting
The Paleoproterozoic orogenic development in northeastern Canada (Figure 1) was controlled
by oblique convergence of the Archean Superior and Nain provinces [Wardle and Van
Kranendonk, 1996]. The Core Zone represents a belt of mainly reworked Archean crust that was
trapped between the two converging cratons. The New Quebec Orogen sutures the Core Zone
with the Superior Province, while the Torngat Orogen links the Core Zone with the Nain
Province. The Torngat Orogen is characterized by a crustal root that is up to 100 km wide with a
Moho relief of >12 km [Funck et al., 2000]. To the east, the root is limited by the Abloviak shear
zone, which developed during the main phase of deformation. ECSOOT line 5 extends from
Ungava Bay (Core Zone) across the Torngat Orogen into the Nain Province and to the Labrador
continental margin.
3. Method
3.1 Data Collection
ECSOOT line 5 is a 432-km long onshore-offshore R/WAR seismic transect with airgun shots
along a 140-km long segment in Ungava Bay (line 5W) and the easternmost 157 km in Labrador
Sea (line 5E). Six airguns with a volume of 16.4 L each were used as source and the average shot
spacing was 130 m. All shots were recorded by 16 land stations deployed across the Torngat
Orogen with an average spacing of 9 km. In addition, seven ocean bottom seismographs (OBS)
were deployed in Ungava Bay and four OBS in Labrador Sea to record shots from lines 5W and

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5E, respectively. The average OBS spacing was 23 km on line 5W and 25 km on line 5E. For
this study, only vertical geophone data were used.
One record section is displayed in Figure 3. Next to crustal and mantle refractions (Pg and Pn)
there are reflections from the mid-crustal boundary (PcP) and the Moho (PmP). Between these
two distinct reflections is a series of low-to medium amplitude reflections, which are more
difficult to correlate laterally than the PmP and PcP phases. These low-amplitude lower crustal
reflections were not considered in the velocity model [Funck and Louden, 1999] and were the
major motivation for the prestack depth migration of the data set.
3.2 Migration
Prior to migration, all 43 record sections (23 on line 5W and 20 on line 5E) were coherency
mixed across five traces (method of Chian and Louden, 1992) to improve the signal-to-noise
ratio. Static corrections were applied to compensate for the station elevation. After deconvolution
the data were band-pass filtered (5 to 9 Hz) and spherical divergence compensation was applied.
Global scale factors were applied to individual receiver gathers to obtain similar amplitude
contents. This was especially important for the OBS, which had a lower signal-to-noise ratio than
the land stations. Refracted energy was generally not muted with exception of some high-
amplitude Pn phases at the continental margin on line 5E. The migration of these Pn phases
otherwise would have resulted in a strong nearly horizontal pseudo-reflector at a depth of ~25
km.
Receiver gathers were migrated using the 2-D Kirchhoff prestack depth migration of the
program package Seismic Unix. Travel times for the migration were calculated by ray-tracing
through the velocity model (Figure 2), which was taken from Funck and Louden [1999].
Velocities at the base of the crust were continued downward with zero gradient into the mantle to

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limit the stretching of Moho reflections by the migration. The velocity was sampled on a 1 km
(horizontal) by 200 m (vertical) grid and travel times were interpolated onto a 250 by 60 m grid
used for the migration. The migrated receiver gathers were summed and the resulting record
section is displayed in Figure 4.
3.3 Results
The migrated record section (Figure 4) shows a low frequency image of the crustal structure
along the transect. One prominent feature is the strong reflector in the area of the crustal root
(215 to 270 km horizontal distance) at a depth of ca. 38 km (Figure 4a). This reflection could be
easily misinterpreted as indication for westward subduction of Nain Province crust beneath the
Core Zone. In contrast, results from seismic tomography [Funck et al., 2000] and other
geological data suggest the opposite subduction direction. Analysis of individual migrated
receiver gathers and synthetic tests showed that this reflection is caused by diffractions at the
westernmost upper limit of the root, which were recorded by the six easternmost landstations for
shots on line 5W. Muting of these phases prior to migration resulted in the cleaned image shown
in Figure 4b. Although the reflector is identified as an artifact, it provides evidence for the
location of the western flank of the root and indicates a rather abrupt crustal thickening.
The image of the Moho reflection varies along the line (Figure 4b). In the west, weak arrivals
first appear east of 35 km. Between 110 and 210 km, the Moho shows up as a high-amplitude
reflection that is in good agreement with the 2-D velocity model. In the area of the root between
215 and 280 km, the Moho is less well defined. This is related to the fact that no PmP phases
were recorded from the western flank of the root. East of 240 km, the coverage with PmP phases
increases but the shape of the root results in complex travel time curves [Funck and Louden,
1999]. These complexities in conjunction with the low fold generate a rather fuzzy image

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without a clearly defined location of the Moho. Between 280 and 340 km, the image of the Moho
becomes more focused again as the fold increases and the crustal geometry flattens. The position
of the Moho at a depth of 41 km is some 2 km deeper than indicated by the velocity model. The
thinning of the crust between 340 and 360 km is mapped not so much by Moho reflections but by
the eastward termination of nearly horizontal lower crustal reflections. The lateral misfit between
the migrated image and the 2-D velocity model is <5 km. East of 360 km, single segments of a
Moho reflection can be identified at a depth of ~17 km. However, the wide OBS spacing in this
area is unable to produce a continuous image of the Moho at this shallow depth.
The lower crust in the Core Zone east of 190 km has a dominantly east-dipping reflectivity
with a dip of ~7 degrees. Similar to the Moho reflection in this area, the amplitudes of the lower
crustal reflectors increase eastward at ~70 km. This is related to the increased contribution of the
landstations to the migrated image, which have a higher signal-to-noise ratio than the OBS.
Between 190 and 270 km, no clear reflectivity can be identified within the lowermost crust other
than elliptical migration artifacts (“smiles”) that continue from the Moho into the lower crust.
East of 220 km, all reflectors within the lower crust are nearly horizontal and are parallel to sub-
parallel to the geometry of the mid-crustal boundary (MCB).
The MCB is the second major reflective boundary mapped by the velocity model (Figure 2).
Between 20 and 100 km, the migrated record section shows some reflectivity close to the
modeled layer boundary. A strong MCB reflection can be seen between 120 and 180 km with
some migration artifacts in the east. The migrated image of the MCB appears here some 2 km
above the position given by the 2-D velocity model. Farther to the east, between 230 and 365
km, there is generally some reflectivity close to the modeled position of the MCB. However, the
MCB is less well defined than in the west. The reflector that continues eastward from the upper

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corner of the step in the MCB at 265 km is an artifact similar to the reflector in the root area
(Figure 4a).
The boundary between the upper and middle crust was described as reflective between 60 and
80 km [Funck and Louden, 1999]. The migrated record section (Figure 4b) shows that this
reflective boundary possibly extends from 15 to 130 km. However, the image of this reflector is
not very continuous and there is interference with elliptical migration artifacts which is related to
the shallow depth of the reflector with respect to the OBS spacing.
4. Geological Interpretation
In Ungava Bay, the migrated record section is a complement to a ECSOOT92 MCS seismic
line [Hall et al., 1995] some 100 km to the north (Figure 1). The MCS record shows a
dominantly east-dipping reflection pattern down to the Moho but lateral correlation of individual
reflectors is a few kilometers only, compared to 80 km on the migrated wide-angle data. In
contrast, the wide-angle data lacks the resolution of the MCS data in the upper and middle crust.
Assuming a constant dip as indicated by the MCS data, the lower crustal reflectors of the wide-
angle data would outcrop west of ~50 km when extrapolated to the surface. That corresponds
onshore to the region of the western Core Zone and the Kuujjuac Terrane. This region is
characterized by dominantly west-verging structures [Wares and Goutier, 1990] compatible with
the seismic results.
The pervasive east-dipping reflection fabric in the Core Zone crust is interpreted as a thick-
skinned deformational pattern caused by east-west compression during the convergence of the
Nain and Superior provinces. Hall et al. [1995] noticed strong reflections at the Moho in the
western part of the Core Zone, which may be due to shearing at the base of the crust. The
migration technique used for the wide-angle data is not able to provide constraints on lateral

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amplitude variations. However, record sections (Figure 1) show extremely well defined Moho
reflections in the Core Zone, in support for a sharp crust-mantle boundary consistent with
shearing at the base of the crust. The east-dipping reflectors soling at the base of the crust (Figure
4b) are interpreted as additional evidence for the transfer of strain from upper crustal levels down
to the Moho and the mantle.
The mostly horizontal reflectivity within the lower crust of the Torngat Orogen and the Nain
Province is less continuous than the reflectors within the Core Zone. This is probably related to
steeply dipping faults dissecting the crust.
5. Conclusions
The migration of the wide-angle seismic data set in this study yields important additional
information to the velocity model previously developed [Funck and Louden, 1999]. The east-
dipping reflectivity within the Core Zone is better imaged on this profile than by MCS data from
the area and provides supporting evidence for thick-skinned crustal deformation.
The study also shows the limitations of the migration technique. The OBS spacing of 25 km
in Labrador Sea only allows for a discontinuous image of the Moho at a depth of 17 km and the
resolution in the upper and middle crust is reduced along the entire line. The image of the lower
crust varies laterally. Lack of shots in the Torngat Orogen resulted in reduced data coverage and,
together with the complex root geometry, prevented a detailed image of the reflectivity in the
lower Torngat crust. In the Core Zone and the Nain Province, with a simpler geometry and a
better data coverage, lower crustal structures are better constrained, particularly between 70 and
190 km. The migrated post-critical wide-angle reflections provide a better definition of the Moho
than the MCS data.

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In summary, migration of wide-angle data can be recommended for relatively homogeneous
crustal sections as in continental shields, even if the shot and receiver spacing is lower than
indicated by the study of Zelt et al. [1998]. However, it is crucial to check carefully for migration
artifacts like the one in the crustal root (Figure 4a), which can be easily misinterpreted as real
reflectors.
Acknowledgements
This work was supported by the Lithoprobe program of the Natural Science and Engineering
Research Council of Canada and Geological Survey of Canada. This is Lithoprobe contribution
____.
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Figure captions
Figure 1. Simplified geological map of the study area [after Hall et al., 1995] showing the
location of the seismic line (lines 5W and 5E). Solid lines show the airgun shots, open triangles
and circles indicate the location of land stations and ocean bottom seismographs, respectively.
Solid lines with triangles mark thrust faults, and dashed lines show shear zones with arrows
indicating the sense of displacement.
Figure 2. P-wave velocity model with a contour interval of 0.1 km/s along the line 5E/W
transect [after Funck and Louden, 1999]. Mantle velocities in brackets specify the velocities used
for the migration of the wide-angle seismic data. MCB is mid-crustal boundary.
Figure 3. Record section of a land station for shots in Ungava Bay (line 5W). Horizontal scale is
the shot-receiver distance, and vertical scale is the travel time using a reduction velocity of 7.5
km/s. Display with trace normalization. See text for description of phases.
Figure 4. Depth-migrated record section (a) with display of a migration artifact resembling a
real reflector in the area of the crustal root, and (b) after muting this event. Box on top of (a)
indicates the distribution of shots (shaded area), ocean bottom seismographs (circles), and land
stations (triangles). Box on top of (b) shows the major tectonic units. Grey lines show the layer
boundaries as determined by the velocity model (Figure 2). MCB is mid-crustal boundary, MS is
migration “smile”. Vertical exaggeration is 1.5.