Crustal thickness and VP/VS variations in the Grenville orogen (Ontario, Canada) from analysis of teleseismic receiver functions
Introduction
The 1.2–0.98 Ga Grenville orogeny was one of the most extensive mountain-building episodes in Earth history. The orogen is exposed primarily in a 2000 × 500 km2 region along the southeastern edge of the Canadian shield, but correlative crustal units underlie a large portion of the U.S. mid-continent, and also occur in parts of Scandinavia, Australia, Africa and Antarctica (Karlstrom et al., 1999). The scale and tectonic architecture of the Grenville orogen resemble the modern Alpine–Himalayan orogen, suggesting that it formed as the result of broadly similar continent–continent collisional processes (Carr et al., 2000). Several recent tectonic models also emphasize the pre-collisional history of the orogen, during which an Andean-style convergent margin persisted for almost 800 Ma along the southeastern margin of the North-American protocontinent, Laurentia (Rivers, 1997).
The Moho is a first-order geological discontinuity that separates crustal rocks from denser underlying lithospheric mantle material. The depth to the Moho is highly variable beneath modern mountain belts, with crustal thicknesses as great as 80 km beneath high orogenic plateaus such as Tibet and the Altiplano, adjacent to areas of more normal continental crustal thickness (∼35–40 km; Mooney et al., 1998). In the Grenville orogen, few vestiges of mountainous relief are preserved at the surface, but significant variations in crustal thickness have been documented by a number of large-scale refraction experiments that have been carried out across the region since the 1950s (Table 1). In the first experiment in this region, Hodgson (1953) placed recording instruments along a line in northern Ontario and derived values of 35.5 km for crustal thickness, 6.25 km/s for Pg velocity and 8.18 km/s for Pn velocity. These measurements stand to this day as representative values for the Canadian Shield.
In 1992, Lithoprobe carried out a major seismic refraction experiment that crossed the Grenville Front on two transects. Over 40 drilled shot points were used, each recorded by over 400 instruments from the USGS and GSC. The large number of shots and receivers enabled researchers to analyze the data using tomographic techniques. For example, Winardhi and Mereu (1997) obtained a velocity model containing lateral velocity variations of up to 0.2 km/s in the upper crust between the Grenville and Superior Provinces. Their study confirmed that there is a major Moho trough to the south of the Grenville Front, as was found in previous studies of profiles across the Front (Mereu and Jobidon, 1971, Mereu et al., 1986). In a subsequent analysis of this data-set, Mereu (2000) used a combination of both P- and S-wave arrivals to compute Poisson's ratio along all of the profiles from measurements of the compressional-to-shear velocity ratio (VP/VS), by taking the ratio of the Pg arrival time to the Sg time (or PmP to SmS times for offsets greater than ∼150 km) for each trace. It can be easily shown that this approach yields an accurate measurement of average VP/VS along the ray path, even for variable crustal velocity field.
Zhu and Kanamori (2000) have developed a simple technique to estimate average crustal thickness and VP/VS directly from teleseismic receiver functions. Their method provides robust “point” estimates of Moho depth and VP/VS by exploiting strong Moho phases that typically dominate the initial part (∼25 s) of a receiver-function record. By stacking several modes of crustal reverberations, their method is largely insensitive to uncertainties in crustal velocity. Since the receiver-function technique involves passive recording of distant earthquakes, it provides a relatively inexpensive way to obtain crustal thickness and VP/VS information over a large area.
The advent of the POLARIS broadband seismograph network since 2001 (Atkinson et al., 2003) has provided unprecedented spatial coverage of the Grenville orogen in Ontario. In this paper, we use a modified form of Zhu and Kanamori's method to map Moho depth and crustal VP/VS ratio. Our study area (Fig. 1) covers the southwestern part of the Grenville Province and adjacent parts of the orogen where the Precambrian crust is buried beneath 0–2 km of Paleozoic sedimentary rocks. Taking into consideration the full extent of Grenvillian crust in North America, our investigation can be considered as centrally located with respect to the orogen as a whole.
Section snippets
Method
Teleseismic receiver functions are waveforms constructed using three-component recordings of P-arrivals from distant earthquakes, usually by deconvolving vertical ground-motion from radial and transverse horizontal ground motions (Langston, 1977, Owens et al., 1984, Bostock and Cassidy, 1995, Eaton and Hope, 2003). Analysis of receiver functions is a well-established method to image the structure of the crust and upper mantle beneath a recording station. Receiver function waveforms are
Data and observations
For this study, we made use of teleseismic waveforms from 71 M > 6 earthquakes (Table 3) that occurred between 19 February, 2003 and 9 October, 2004. The data were recorded by 32 broadband seismic stations (Fig. 1) of the POLARIS network and Canadian National Seismograph network (CNSN). Installation of the semi-portable POLARIS stations commenced in 2002 and continued until August, 2004. These stations are equipped with three-component Guralp CMG-ESP sensors with a flat velocity response from 100 s
Discussion
The maps of crustal thickness and VP/VS ratio are divisible into 3 distinct regions that appear to correlate with surface geology. Referring to Fig. 12, Fig. 13, region 1 is an area of relatively thick crust (40–44 km) and relatively high κ value (> 1.78). This area falls within the Central Gneiss Belt (CGB) of the Grenville Province, a terrane where high-grade (generally granulite facies) gneisses and migmatites are exposed, including areas where relict eclogite-facies assemblages occur within
Conclusions
The use of semblance weighting in the receiver-function stacking method developed by Zhu and Kanamori (2000) leads to reduced uncertainties in point estimates of crustal thickness (H) and VP/VS ratio (κ). We have applied this technique to 32 broadband seismograph stations, most have which have been installed since 2001 as part of the POLARIS project. This broadband seismograph network covers a ∼700 × 400 km2 segment of the central Grenville orogen, in the lower Great Lakes region of eastern North
Acknowledgments
This study would not have been possible without the exceptional work of Isa Asudeh, Bernie Dunn and Kadircan Aktas to install, maintain and operate the POLARIS network. Funding for the POLARIS network in Ontario has been provided by Ontario Power Generation, Bruce Power, the Canada Foundation for Innovation and the Ontario Innovation Trust fund. Operational support has been provided by the Natural Sciences and Engineering Research Council of Canada and the Ontario Research and Development Trust
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2015, TectonophysicsCitation Excerpt :Since early 1900s, the base of the crust has been imaged by seismic experiments. These include, e.g., the analysis of broad-band passive seismic records (Bostock and Rondenay, 1999; Dueker and Sheehan, 1997; Frederiksen and Revenaugh, 2004; Kosarev et al., 1999; Langston, 1979; Vinnik, 1977), specially aimed refraction and wide-angle seismic reflection studies (Carbonell et al., 1998; Clowes et al., 2005; Palomeras et al., 2009; Prodhel & Mooney, 2012) and normal incidence seismic reflection transects (Carbonell et al., 2000, 2002; Cook, 2002; Cook et al., 2010; Eaton et al., 2006; Hammer and Clowes, 1997; Klemperer et al., 1986; Mooney and Meissner, 1992; Valasek and Muller, 1997; Valasek et al., 1991). These data have provided a series of different interpretations about the origin, structure, nature, depth, and topography of the Moho discontinuity (e.g., Carbonell et al., 1998, 2002, 2013; Cook, 2002; Hammer and Clowes, 1997).