Characteristics of Shear Wave Velocity Structures Beneath the Gulf of St. Lawrence, Eastern Canada from Ambient Seismic Noise Tomography 1
AYODEJI KUPONIYI , HONN KAO , STAN DOSSO , JOHN CASSIDY , GEORGE SPENCE
1—School of Earth and Ocean Sciences, University of Victoria, BC Canada 2—Geological Survey of Canada – Pacific, Sidney, BC Canada INTERPRETATION
ABSTRACT We study the 3-D shear wave velocity (Vs) structure in the region of the Gulf of St. Lawrence (GSL) to 20 km depth by inverting Rayleigh-wave dispersion extracted from the vertical components of continuous ambient seismic noise waveforms. The GSL and the adjacent region can be divided into three broad zones based on their Vs characteristics. In the north the southernmost portion of the Grenville Province (i.e., the edge of Proterozoic units) is dominated by high-Vs, except for well-known anorthosite sites mostly located both within and at the eastern boundary the Grenville Province, which are characterized by relatively lower velocities. In contrast, the central segment of the GSL region corresponds to a weak belt with generally low-Vs. In the southernmost edge of the GSL region, prominent low velocities are found to coincide with locations of graben structures, while higher Vs characterize the southernmost terranes. The deepest prominent low velocity structures correspond to displaced/deformed Humber zone sediments buried deep beneath a Dunnage-zone Ordovician crust. Prominent high-Vs are found at the boundaries of the OrdovicianSilurian Anticosti Basin and the Carboniferous Magdalen Basin, near the mid-crust. The depth to the top of the basement as well as the geometry of the major sedimentary basins are well imaged across several cross-sections, particularly within the Magdalen Basin where the depth to basement top exceeds 13 km. The top volcanic layers that overlie parts of the Canadian Maritime Basins correspond to high shear velocities with thickness up to 3 km.
Fig 3 Group velocity maps for selected periods (a) 2 s (b) 8 s (c) 20 s and example of (d) checkerboard resolution analyses for 8 s (e) recovered structure from checkerboard test for 8 s
Fig 7 (a) Vs distribution at depths at 0 km – boxes with broken and solid lines mark approximate distribution of sedimentary and volcanic rocks respectively; (b) 10 km – Open circles are locations marking the ASF estimated from this study, broken and solid red lines are the northernmost and southernmost limits of the ASF from Haworth  (and references therein), blue dashed lines mark the southernmost limits of the Grenville basement from this study (c) (Top row) - Co-linear cross-sections drawn from the 3D model from this study overlain on vintage deep seismic reflection lines 86-1 and 86-2 (marked “e” and “f” respectively on Figure 1b). (Middle row) - Tomographic crosssections displayed in top row. Bottom row: Vintage Seismic lines compared in the top row. The base of the low-Vs layer on our tomography cross-sections corresponds to strong reflections on the seismic profile lines at ~7 km on both lines.
To provide new insights about the upper crustal/basin structures of the Gulf of St. Lawrence (Fig. 1) by: 1)
Computing the cross-correlation functions of the vertical wave components which corresponds to an estimate of the Green’s Function (EGF) — Fig. 2
Extracting Rayleigh wave group velocities for periods sensitive to structures of the upper crust — Fig. 2
Generating Rayleigh wave group velocity maps (Fig. 3) and obtaining 1-D shear wave velocity models on o o 0.5 x0.5 grids for the region using trans-dimensional Bayesian inversion — Fig. 4
Constructing pseudo 3D velocity models with corresponding uncertainty estimates for the region to properly identify structure variations — Fig. 5
Fig 4 (a) Interface probability and probability marginals of Vs near station MADG within Magdalen Basin, red-blue colors mark high-low probabilities while dashed black line represents the 1-D representative median model. (b) Data fit for the inversion results: the x’s are dispersion data, black continuous line is the mean of model-derived dispersion data and the magenta broken lines represent the standard deviation of the data.
h. Fig 8 Schematic interpretation of tomography results. (a) and (b) show the spatial distribution of important features of the tomography results for the uppermost (0 -3 km)- and mid- crust (9-15 km) respectively. (c), (d) and (e) show the structure at depths for profiles taken along the middle, west and east of GSL respectively. BVBL – Baie Verte – Brompton Line, AI – Anticosti Island, ASF – Appalachian Structural Front, QC - Quebec, NF - Newfoundland, NS – Nova Scotia, NB – New Brunswick, MI – Magdalen Island, PEI – Prince Edward Island, OG – Orpheus Graben, NL – Northern Limits, SL – Southern Limits.
Fig 1 (a) Map of the GSL region. Lines: BVBL – Baie Verte-Brompton Line, ASF – Appalachian Structural Front. Terranes: HB – Humber Zone, DN – Dunnage Zone, GD – Gander Zone AV – Avalon Zone, MG – Meguma Zone. Basins: CanMB – Canadian Maritimes Basin, MAGD – Magdalen Basin, SYDB – Sydney Basin, DLB – Deer Lake Basin, SAB – St. Anthony Basin; OG – Orpheus Graben. Provinces and Islands: QC – Quebec, NB – New Brunswick, NS – Nova Scotia, NL – Newfoundland and Labrador, PEI – Prince Edward Island, SI – Sable Island, AI – Anticosti Island, MI – Magdalen Island (b) Station distribution, propagation coverage and selected cross-sections for profiles A-A’, B-B’, C-C’, D-D’ and E-E’ on a grid of 0.5ox0.5o. Black circles with letters are labels for vintage wide-angle seismic reflection (yellow) lines 86-1 (“e”) and 86-2 (“f”) Red triangles are station locations.
METHOD: AMBIENT SEISMIC NOISE TOMOGRAPHY a.
Fig 5 (top-row) - Depth slices of Vs distribution for (a) 0 km (b) 5 km (c) 10 (d) 20 km; (bottom-row; e-h) - corresponding standard deviation maps. Gray lines on the Vs maps are contours of Vs. a.
4) 5) b.
REFERENCES (Only a few)
Fig 2 (a) Cross-correlograms for NATG and DRLN (b) Dispersion for station pair BATG-DRLN; gray line is the curve
A comprehensive 3-D velocity model is developed for the Gulf of St. Lawrence region using ambient noise tomography within a trans-dimensional inversion framework. A relatively low velocity zone separates the high shear velocities of the northernmost (Grenville Province) and the southernmost (Avalon/Meguma terranes) structures of the GSL at the mid-crust. The geometry of the major sedimentary basins in the Gulf of St. Lawrence region, as well as deformations associated with the Appalachian orogeny are well constrained. Depth to the top of basement at most parts of the Canadian Maritimes basins is 5 - 8 km, while the maximum depth exceeds 13 km. Also, we observe the Appalachian Structural Front at the mid-crust, with the location and geometry matching the previously proposed. Future geophysical studies such as receiver functions and seismic anisotropy will provide further insights on the deeper tectonic framework.
Fig 6 Vs and uncertainty distribution with depth for cross-sections along: N-S (left panel) – (a) A-A’ along longitude 67 W (b) B-B’ along 61.5o W; W-E (right-panel) – (c) E-E’ along latitude 47o N (d) F-F’ along latitude 48.5o N. Gray lines are contours of Vs; red arrows – mark the Grenville-Appalachian contact, GD - Gander, DN - Dunnage, GP – Grenville Province, MAGB – Madgalen Basin, SYDB – Sydney Basin, HB - Humber, AV - Avalon, ANTB – Anticosti Basin, DLB – Deer Lake Basin
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Funding Support: Natural Sciences and Engineering Research (NSERC) Council of Canada grants to HK, JC and GS. Canadian Society of Exploration Geophysicists Foundation (CSEGF) award to AK. For more information email: [email protected]
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