Shear-wave
splitting
beneath
the
southern
Western
Canada
Sedimentary Basin: A snapshot of the interaction between cratons and
terrane?
Yu Jeffrey Gu1, Kenny Kocon1, Ahmet Okeler1, William Menke2
1. Department of Physics, University of Alberta, Edmonton, AB, T6G2G7, Canada.
2. Lamont-Doherty Earth Observatory, Columbia University, 61 Route 9W, Palisades,
NY10964, USA.
Abstract
Western Canadian Sedimentary Basin (WCSB) marks the transition from the old North
American continental lithosphere (east) to young accreted ‘terranes’ (west). Geologic,
seismic and magnetic data in this region have suggested various crustal domains and
strong conductive anomalies and seismic velocity gradients in the mantle. Extensive
deformation is further evidenced by seismic anisotropy in the southern WCSB based on
teleseismic earthquake data recorded by the Canadian Rockies and Alberta Network
(CRANE). Our shear-wave splitting measurements show 1-1.5 sec splitting times and a
fast propagation direction along the northeast-southwest orientation near the Rocky
Mountain foothills, approximately parallel to the absolute plate motion of the North
American continent. This single-layer anisotropic pattern suggests the alignment of the
crystalographic axis of olivine due to shear deformation at the base of the lithosphere. On
the other hand, the spatial distribution of the SKS orientations is significantly more
complex east of the Rocky Mountain. Several stations display two-layer anisotropic
variations and hight the contrasting mantle structures and histories between the Rockies
and its adjacent domains.
Disrupted mantle flow near the edge of the migrating
continental root east of the province may be largely responsible for the complex shearwave splitting fast directions in this region.
1. Introduction
The Western Canada Sedimentary Basin (for brevity, WCSB) is a relatively thin,
northeastward-trending wedge of supracrustal rocks tapering on, or juxtaposed with,
Precambrian crystalline rocks (Bally et al., 1966; Price, 1981; Baumont, 1981; Hoffman,
1988; Mossop, G.D. and Shetsen, 1994). This elongated geological structure began its
formation during the tectonic development of western Laurentia, and continued its
evolution through recent interactions between the North American craton and Cordilleran
orogen (Hoffman, 1989; Ross et al., 1991, 2002; Clowes et al., 2002). Today, this
diverse geological framework consists of Archaen craton(s), Proterozoic orogens and
associated accretionary margins (also known as ‘terranes’) (e.g., Price 1981; Hoffman,
1998; Ross et al., 1991, 2002).
An astute example of geological diversity is the region surrounding the Alberta Basin,
located in the southern half of WCSB.
Beneath the sedimentary cover are juxtaposed
tectonic domains (Figure 2) (Hoffman, 1988; Ross et al., 1991; Clowes et al., 2002;
Shragge et al., 2002) that likely have undergone substantial thermal and tectonic
overprinting (Ross and Eaton, 2002; Aubach et al. 2004; Mahan and Williams, 2005;
Beaumount et al., 2010).
This basin is bounded in the east by the Trans-Hudson
Orogenic Belt, a controversial geological structure that extends well into eastern Canada
(Hoffman, 1988; Banks et al., 1998; Zelt and Ellis, 1999). Directly west of this basin is
the northern Rockies, a section of the western Cordillera presumably originated from the
Laramide orogeny (Livaccari et al., 1981; Bird, 1998; Maxson and Tickoff, 1996; Cook
et al., 2002; English and Johnson, 2010; Liu et al., 2010). Strong seismic velocity
gradients (van der Lee and Frederiksen, 2005; Nettles and Dziewonski, 2008; Mercier et
al., 2010) and anisotropy (e.g., Shragge et al. 2002; Marone and Romanowicz, 2007;
Courtier et al., 2010; Yuan and Romanowicz, 2010) have been proposed for the mantle
beneath this region, which accentuates the sharp transition from the stable continental
mantle east of the Alberta Basin to the accreted terranes west of it.
Faulting and episodes of magmatism, accretion and subduction (e.g., Hoffman, 1988;
Ross et al., 2000) can inflict permanent deformation at both crustal and mantle depths
beneath the southern WCSB. In particular, olivine’s crystollographic fast axis is known
to preferentially align with the direction of maximum shear or least compression (e.g.,
Vinnik ??; Anderson, 1989; Silver, 1993). The strength of radial and/or azimuthal
anisotropy duo to lattice preferred orientation (LPO) is therefore a direct reflection of
‘order’ in mantle rocks (Anderson, 1989; Silver, 1993; Long and Silver, 2009). In
southern WCSB, strong azimuthal anisotropy has been inferred from 1+ sec SKS wave
splitting times from temporary arrays (Shragge et al., 2002; Courtier et al., 2010),
suggesting extensive deformation in the region. However, the orientations of fast SKS
directions are questionable, as significant complexities have been previously documented
from the regional permanent stations
(Kendall, ??; Currie and Hyndman, 2006).
Undesirable resolution caused by the restrictive linear or semi-linear array geometries,
which limited the effectiveness of existing shear-wave splitting measurements (e.g.,
Shragge et al., 2002; Courtier et al., 2010) and anisotropic tomography (Marone and
Romanowicz, 2007; Nettles and Dziewonski, 2008; Yuan and Romanowicz, 2010),
remains the greatest challenge in determining the seismic anisotropy beneath the southern
WCSB.
Since early 2006, the regional seismic data coverage in the southern WCSB improved
significantly from the establishment of the Canadian Rockies and Alberta Network
(nicknamed CRANE), the first semi-uniform broadband seismic array in Alberta and
parts of Saskatchewan, Canada (see Figure 1). Continuous seismic signals from this
array enabled a detailed examination of both regional seismicity and crust/mantle
structures.
This study uses SKS splitting measurements to constrain the azimuthal
anisotropy beneath CRANE and nearby permanent (EDM, WALA) broadband seismic
stations (see Figure 1). With vastly improved data coverage and measurement accuracies
(based on four different techniques, see Section 2), we provide an updated model of past
and ongoing mantle processes near the western boundary of the North American craton.
2. Data and Method
The study analyzes earthquake records from eleven CRANE stations and two permanent
stations monitored by the Canadian National Seismic Network (CNSN). These thirteen
stations are semi-uniformly distributed in central and southern Alberta with an average
spacing of ~150 km between adjacent ones, (see Figure 1). Most of the stations operated
continuously for 2+ years, accumulating enough large earthquakes for our examination of
SKS splitting (Figure 3), a phenomenon often interpreted as the consequence of
anisotropy similar to optical birefringence of minerals under polarized light (Bowman
and Ando, 1987; Silver and Chan, 1991; Menke and Levin, 2003). The resulting splitting
parameters, which consist of delay time (between the fast and slow shear waves) and fast
polarization azimuth, are sensitive functions of the strength and direction of receiver-side
anisotropy (see Long and Silver, 2009).
We restrict our data set to Mw>6.5 earthquakes with source-receiver distances between
85 and 115 arc deg. The resulting three-component (East-west, North-south, Vertical)
time series are band-pass filtered with corner frequencies of 1sec and 15 sec, and then
subjected to a signal-noise ratio (SNR) test; records with SNR<?? are automatically
rejected (or Visual inspection?? KENNY?). The average number of earthquakes that
survive the above selection process is ~12 per station (Table 1; Figure 4a). We retain
HYLO, the most recently installed station, due to its unique position within the array and
the robustness of the single SKS waveform. The overall distribution of the sourcereceiver pairs is highly non-uniform, displaying a strong northwest–southeast orientation
(Figure 4b).
In this analysis each measurement is made primarily based on the cross-convolution
method for multiple earthquakes (Menke and Levin, 2003). We convolve the observed
radial and tangential component seismograms with the impulse responses predicted by an
isotropic background model, and then introduce anisotropic perturbations to one or
multiple layer(s) to minimize the misfit between the observed and predicted crossconvolution functions (see Menke and Levin, 2003). This multi-layer approach considers
all earthquakes arriving at a single station, which is more flexible than methods based on
a single anisotropic layer assumption (for instance, the rotation correction (Bowman and
Ando, 1987), minimum energy (Silver and Chan, 1991) or eigenvalue (Silver and Chan,
1991) methods (see Long and Silver, 2009 for a review)). For each station we distinguish
one- or two-layer anisotropy based on the average cross-correlation coefficient between
observed and predicted convolution functions for all earthquakes.
3. Shear-Wave Splitting Measurements
3.1 Data Fit and Uncertainty
Of the 15 stations analyzed in this study, clear evidence of shear wave splitting is
observed under 14 stations. With the exception of WALA, where reasonable splitting
parameters could not be determined, most of the stations exhibit significant anisotropy
that requires minimization of the differences between the two ‘corrected’ horizontal
components (see Menke and Levin, 2003). The inverted splitting parameters improve the
overall correlation and the linearity of particle motions for SKS phases after correcting
for anisotropy (see Figures 5a and 5b). The best results are obtained for stations CLA,
HON, JOF, LYA and PER (see Figures 5a and 5b), while highly linear particle motions
prior to the inversions (e.g., CZA, EDM, NOR, REC) are generally retained by the crossconvolution analysis.
Stations FMC and DOR remain problematic: in particular, the
visibly nonlinear particle motions and disagreements among four different shear-wave
splitting methods (see latter part of this section) at DOR indicate complexities in the
anisotropic mantle. Furthermore, similar data fits are obtained for EDM and JOF for 1vs. 2-layer anisotropic models.
The delay times and azimuth uncertainties are determined by a bootstrap re-sampling
algorithm (Efron and Tibshirani, 1991). At each station, we randomly select the same
number of earthquakes from the list of observations and follow the same procedure
detailed in Section 2 to obtain a bootstrapped shear-wave splitting measurement. This
sampling process is repeated for 300 times and the standard deviations of distribution of
the bootstrapped measurements can be used as effective indicators of the timing and split
angle uncertainties at each station (see Table 1). Most of the stations exhibit Gaussian
distributions centered near the mean values (Figure 6). The splitting times range from 1.1
to 1.9 sec, with uncertainties of 0.06 - 0.50 sec (see Table 1). The distributions of the fast
splitting directions are generally Gaussian, which suggests self-consistent measurements
from individual earthquakes. Notable exceptions are DOR and FMC, each containing
two distinct angles, and JOF that exhibits greater variations (hence uncertainty) of fast
azimuth than other stations. Among these three stations, the large number of earthquakes
(14) recorded by DOR argues against the possibility of increased error due to data
shortage. Uncertainties associated with splitting times (see Figure 6) closely track those
of fast orientations: for instance, significant timing uncertainties exist beneath the same
three stations under which substantial angular variations are identified.
To verify the stability of the measurements, the splitting parameters are independently
determined using the rotation correction (Bowman and Ando, 1987), minimum energy
(Silver and Chan, 1991) and eigenvalue (Silver and Chan, 1991) methods (see Long and
Silver, 2009 for a review) for the majority of the stations in the array. For all three
approaches, the median of the splitting parameters from individual events is used to
account for multiple earthquakes recorded by a given station.
3.2 Splitting Parameters and Lateral Variation
The split times and orientations vary systematically across the CRANE array (Figure 7).
The majority of the measurements along the Rocky Mountain foothills (e.g., CLA, LYA,
NOR, BRU) show consistently large (~1.5-1.9 sec) split times (between fast and slow
SKS arrivals) with fast directions along a northwest-southeast orientation. These
measurements correlate strongly with those of Courtier et al. (2010) a few degrees
northwest of this region along the Cascadia Deformation Front (for short, CDF; Courtier
et al., 2010; Figure 8). The amount of azimuthal anisotropy decreases from the northern
part of the array (1.5+ sec) to ~1.1 sec US-Canada border, where the orientati with
stations WALA that However, relatively small splitting time delays are observed beneath
eastern-central Alberta where the fast axes. While .
The orientations are, to first order, consistent with the direction of the absolute plate
motion (~deg southwest) of the North American continent. These value
Similar directions have been previously reported by Shragge et al. (2002) using a linear,
approximately north-south trending temporary array in this region (see Figure 1).
Bottom Left: Alternatively, the complex SKS splitting directions (and
times) may also reflect more localized mantle upwelling. The heat flow
map shows enhanced activities near HLO [Blackwell & Richards, 2004]
and the stations around this geographical location appear to track the
geometry of the regional hotspot.
Local heat-flow anomalies (circled region) are supported
by the reduced values of Bouguer gravity. It is unclear whether the SKS
splitting observations reflect a larger-scale tectonic movement/history
(see Top Left) and/or more localized thermal variations (bottom figures).
3.2 Regional variations in fast splitting directions
4. Interpretation and Discussion
The splitting parameters in the vicinity of the Canadian Rockies suggest strong ‘order’ in
mantle mineralogy (Long and Silver, 2009). The northeast-southwest trending fast
direction is consistent with previously reported values utilizing Lithoprobe data (Shragge
et al., 2002) and the direction of maximum horizontal stress – a proxy for the ‘fossil’
strain field within the lithosphere in response to the past episodes of northwest-southeast
plate convergence and subduction of Farallon and Kula plates (e.g., Helmstaedt and
Schulze, 1989; Ross et al. 2000). The combination of pre-existing fabric within the
mantle lithosphere and the present-day absolute plate-motion that is, coincidentally,
northeast-southwest, offers an attractive explanation for the large SKS delays and 3-6%
azimuthal anisotropy in this region. Without further data constraint it is difficult to
resolve the full history of mantle deformation or the number of anisotropic layers (e.g.,
Shragge et al. 2002; Marone and Romanowicz, 2007), however.
The origin of the complex shear-wave splitting pattern beneath eastern-central Alberta
remains unclear. It could potentially be linked to the adjacent Buffalo Head Terrane, a
region that has attracted national attention in recent years due to the discovery of precious
minerals.
The vicinity of the anisotropic anomaly exhibits enhanced heat-flow
(Blackwell and Richards, 2004) and below-average seismic velocity (van der Lee and
Frederiksen, 2005) and bouguer gravity values. The presence of a divot (Fouch et al.,
2000) or an abandoned plume conduit (Bank et al., 1998) on the continental root offers a
viable explanation. On a local scale, geometrical imperfection associated with past plate
interactions could trap hot asthenospheric material and disrupt the mantle flow around it.
Within a larger tectonic framework, the anomalous shear-wave splitting observations in
eastern-central Alberta could signal a hidden tectonic boundary between stable continents
(east/northeast) and accreted terranes (west).
For instance, streamlined mantle flow
around the edges of moving continental `keels' (e.g., Gaherty and Jordan, 1995; Ben
Ismail and Mainprice, 1998; Bokelmann and Silver, 2002) can induce strong north-south
oriented horizontal strain. In other words, shear deformation base of the lithosphere
(~200 km) and disrupted flow at shallower depths could both be present, hence producing
complex, multi-layered anisotropy in this region. Furthermore, due to the substantial
topographical relief on the base of the lithosphere (Hyndman et al., 2005), both radial and
azimuthal anisotropy would be expected in this transitional region.
5. Conclusions
Our SKS splitting analysis provides first-order evidence for strong mantle anisotropy
beneath the southern WCSB. Large delay times and coherent SKS fast directions are
observed along the Rocky Mountain foothills, which are roughly consistent with the
direction of present-day plate motion but contrast with the highly variable splitting
directions east of the northern Rockies. Our SKS measurements do not correlate strongly
with local Bouguer gravity or heat-flow data, but the general pattern may be explained by
recent models of shear velocity variation and suggest relatively sharp change of mantle
deformation mechanism from simple, plate-motion induced shear in the southwestern
WCSB to complex, disrupted asthenospheric flow east of it. Based on this interpretation,
the spatial distribution of SKS splitting variations across the stations in the southern
WCSB could mark the transition between old, seismically fast cratons east of the WCSB
and the significantly younger accreted terranes in western Canada.
Overall, the
broadband data from CRANE has undoubtedly offered a window of opportunity to probe
the history, present state and geometry of the continental lithosphere. The continued data
acquisition and development of this array in the near future may, ultimately, contribute to
the discussion of the lithosphere evolution from thin convergent margins to thick,
depleted continental roots.
3.3 Measurement uncertainties
The t
To validate the measurements further we compare
Above: Tabulated splitting measurements with the available data constraints.
5. Conclusions
Key Observations:
The SKS wave splitting pattern in the transition region between cratons (east) and the
accreted terranes (west) are more complex than previously documented.
The direction of fast-splitting axis coincide with present-day plate motion near the
Canadian Rockies, but is nearly horizontal or northwest-southeast trending east of the
Rockies.
Enhanced heat flow is observed at ~55N, which may be partially responsible for
complexities in SKS spitting measurements away from the Rockies.
Acknowledgement:
We thank our collaborator, Bill Menke, for technical help and suggestions (see reference
below). The work presented
is supported by the Canadian Foundation for Innovation (CFI), Alberta Ingenuity and the
University of Alberta. Some
of the data used in this analysis was distributed by IRIS and the Canadian National
Seismographic Network (CNSN).
We are also grateful to the hosts of our seismic instruments in the province of Alberta.
Figure Caption:
Top Right: Event distribution for the SKS splitting analysis. All earthquakes have
magnitudes (MB, MS, MW)
greater than 6.5, and all event-station pairs are restricted to distances within the range of
85-115 deg.
Bottom Right: A schematic diagram of SKS waves at two epicentral distances.
Right: Measurement (time, fast-axis orientation) uncertainties obtained using
a bootstrapping approach. Each distribution is computed from an automated
algorithm that considers random subsets of the original data set. Best results
are seen in EDM and LYA, whereas the splitting parameters cannot be accurately
determined at WALA, suggesting considerable complexities in mantle structure.
Above: Sample cross-convolution functions and particle motions from our shear-wave
splitting analysis. For the majority of the CRANE/CNSN stations, inverted splitting
parameters not only improve the agreement between the horizontal-component crossconvolution functions (see Menke and Levin, 2003), but also increase the linearity
of the particle motions.
Above: Tabulated splitting measurements with the available data constraints.
Right: Measurement (time, fast-axis orientation) uncertainties obtained using
a bootstrapping approach. Each distribution is computed from an automated
algorithm that considers random subsets of the original data set. Best results
are seen in EDM and LYA, whereas the splitting parameters cannot be accurately
determined at WALA, suggesting considerable complexities in mantle structure.
With the exception of HYLO (newly installed station with 1 event only) and WALA Top
Left: Splitting vectors super-imposed on the shear velocity map
of NA04. The splitting directions change rapidly with relative distance
from Canadian Rockies, e.g., consistent ~240 deg angle in the North to
more northwest-southeasterly directions in the east. This result implies
three distinct tectonic environment in the mantle: Zone 3 (dotted line)
may have been associated with plate motions where the eastern-most
zone (Zone 1) could be strongly affected by channeled flow around a
migrating continental root. The mantle flow in the transition region
(Zone 2) also appears to track the shear velocity contour lines.
Reference List
Blackwell, D. D. and M. Richards, M.. Geothermal Map of North America. American
Assoc. Petroleum Geologist (AAPG), 1 sheet, scale 1:6,500,000, 2004.
Gu, Y. J., A. Okeler, S. Contenti, K. Kocon, L. Shen, and K. Brzak, Broadband seismic
array deployment and data analysis in Alberta, CSEG Recorder, September,
33-40, 2009.
Gu, Y. J., K. Kocon, A. Okeler, and W. Menke, Mapping the western boundary of the
North America, in preparation, 2009.
Gripp, A. E. and Gordon, R. G., Young tracks of hotspots and current plate velocities,
Geophys. J. Int., 150, 321-361, 2002.
Menke, W. and V. Levin, The Cross-Convolution method for interpreting SKS splitting
observations, with application to one and two layer anisotropic Earth
models, Geophys. J. Int 15, 379-392, 2003.
Marone, F., and B. Romanowicz, The depth distribution of azimuthal anisotropy in the
continental upper mantle, Nature, 447, 7141, 198-201, 2007.
Van der Lee, S. and A. Frederiksen, Surface wave tomography applied to the North
American upper mantle, in AGU Monograph "Seismic Earth: Array Analysis of
Broadband Seismograms", Eds: Levander A., and G. Nolet, 67-80, 2005.
Mossop, G.D. and Shetsen, I (comp.) (1994): Geological atlas of the Western
Canada Sedimentary Basin; Canadian Society of Petroleum Geologists and
Alberta
Research
Council,
Special
Report
4,
URL
<http://www.ags.gov.ab.ca/publications/wcsb_atlas/atlas.html>,
accessed online].
[Date
last