Estimating the effective elastic thickness of the Arctic lithosphere using the wavelet coherence method: Tectonic implications
Graphical abstract
Introduction
Mechanical strength is a significant property of the lithosphere and aids in understanding Earth's surface tectonics and deformation. To quantify this parameter, the effective elastic thickness (Te) is generally employed as a proxy for the long-term strength of the lithosphere. Studies of Te indicate that it depends mainly on the composition, temperature gradients and thickness of the lithosphere and is related to its thermal structure and rheology (Burov and Michel, 1995; Lowry and Smith, 1995). The many comparisons of Te and other lithospheric models (e.g., heat flow and seismic velocity) in different geological provinces (Pérez-Gussinyé et al., 2007; Ji et al., 2017; Lu et al., 2020) demonstrate that Te can provide important insights into the information of lithospheric structure and tectonic processes. The linkage of this parameter with geological information has also been studied by employing different methods. Chen et al. (2017) analyzed the relationship of Te and cratons, subglacial mountains and rifts in Antarctica. Flück et al. (2003) found that the Te gradient could delineate the boundary of eastern craton and western Cordillera when studying the Te in western Canada. Ratheesh-Kumar et al. (2015) reconstructed the initial separation phase of India and Madagascar based on the large Te gradient along the rifted continental margins. Steffen et al. (2018) proposed that the reduced Te in southern and central Greenland could reflect relics of the passage of Iceland hot spot.
To date, various techniques have been developed to recover Te in either the spatial or the frequency domain (Lowry and Smith, 1995; Pérez-Gussinyé et al., 2004; Wienecke et al., 2007; Kirby and Swain, 2011). The computational efficiency has been improved from initial input data of 2-D profiles across a particular geological structure (Watts, 1978) to 2-D grid data covering an entire study region (Kalnins and Watts, 2009), which greatly aids in the interpretation of Te. Additionally, Forsyth (1985) brought up an inversion technique that combined the surface and subsurface loading models and developed coherence technique for Te estimations. Further, Kirby and Swain (2004) extended Forsyth's method and used continuous wave transform to estimate the coherence between gridded topography and gravity spectra to recover Te, which gained wide popularity in continental (Audet and Bürgmann, 2011; Mao et al., 2012; Chen et al., 2017) and oceanic studies (Ratheesh-Kumar and Windley, 2013; Shi et al., 2017; Ji et al., 2020).
A wide range of geological settings can be found in the Arctic region such as stable cratons, collisional orogens, volcanic provinces and oceanic basins (e.g., Lane, 1997; Oxman, 2003; Rosen, 2003; Døssing et al., 2013). Although these geologic settings differ distinctly in age, composition and structure, their physical and chemical properties, e.g., thermal structure and composition, can be well constrained by lithospheric information on rheology and mechanical strength. Thus, Te is a valuable proxy on structure and process that help explain the tectonics in the Arctic. With various geophysical datasets for the Arctic published in recent years (Kenyon et al., 2008; Jakobsson et al., 2012; Gaina et al., 2014; Lebedeva-Ivanova et al., 2019; Ruppel et al., 2019), especially high-resolution topography and gravity data, carrying out a regional-scale investigation of lithospheric strength in the Arctic to study its tectonics and geodynamic evolution has become feasible.
Until now, a few studies on the mechanical strength of the lithosphere in the Arctic region have been undertaken, but most of them focus on a particular tectonic unit or study area. In addition, the Te maps obtained by various methods differ in their amplitudes and lateral resolutions. The variations in Te in the Barents Sea and Kara Sea were calculated based on the computed rheological model from Gac et al. (2016). Steffen et al. (2018) recovered the Te values in Greenland using the wavelet coherence method. The few other studies concentrated on the North American continent. Flück et al. (2003) obtained the Te map in western Canada using the maximum entropy method. Audet and Mareschal, 2004, Audet and Mareschal, 2007 used various spectral method to calculate the coherence method between bouguer gravity and topography and estimate Te of the Canadian Shield. Kirby and Swain (2009) employed the wavelet coherence method to recover spatial variations in Te in North America. Tesauro et al. (2015) utilized inverted thermal models in North America from seismic tomography models NA07 and SL2013sv to predict the yield strength envelope, which allowed quantitative evaluation the integrated strength of the lithosphere by summing the mechanically strong layer thicknesses. These studies all show low Te values of < 20 km in Alaska, and extremely high values >100 km in the eastern North American craton. The different spectral measurements, coherence and admittance, yielded consistent Te (< 40 km) in the Caledonian orogen and Baltic shield, as reported by Pérez-Gussinyé et al. (2004) and Pérez-Gussinyé and Watts (2005). Struijk et al. (2018) first estimated the mechanical strength of the entire Arctic lithosphere based on Tesauro's method (Tesauro et al., 2015), but the estimated Te model exhibits limited spatial resolution and is unable to recover some of the shorter-wavelength features of Te. For example, Greenland underwent the collision of old provinces in the early Proterozoic and was subsequently affected by the passage of the Iceland hotspot in the Paleogene (Henriksen et al., 2000). The Alpha-Mendeleev Ridge in the Amerasia Basin is a part of the High Arctic Large Igneous Province (HALIP), and its crustal structure and evolution differ from those of the adjacent Makarov Basin and Podvodnikov Basin. However, the Te values recovered by Struijk et al. (2018) in Greenland and Amerasia Basin are quite flat, which weakens the relationship between lithospheric strength and its structure and tectonics, and new high-resolution lithospheric strength information needs to be reported.
In this study, we aim to determine the spatial variations in Te in the Arctic region (> 67°N) using the wavelet coherence method (Kirby and Swain, 2011). We first introduce the appropriate method, the geophysical data we applied and the corresponding procedure. Subsequently, our results are presented and interpreted in terms of different geological features. Then, we evaluate the relationships between Te and other proxies for geophysical properties (e.g., seismic velocity and surface heat flow) in this area. Finally, the implications of the Te values for the interpretation of pre-existing lithospheric tectonics and seismicity are discussed.
Section snippets
Geological setting
The Arctic lithosphere (north of 67°N) is made up of a series of lithospheric blocks from Precambrian cratons of Siberia, Greenland and North America to Cenozoic oceanic basins in Arctic Ocean and the tectonic evolution of this region is still highly debated (Pease et al., 2014). Since the breakup of the supercontinent Pangea, in addition to regional lithospheric collisions (e.g., Paleozoic orogeny in the Caledonian), rifting and seafloor spreading (e.g., Amerasia Basin and Eurasia Basin),
Wavelet coherence method
A thin uniform elastic plate above an inviscid fluid produces flexure under applied loads, which is similar to the scenario in which rigid lithosphere bends in response to tectonic loads. Te is expressed as the thickness of the plate if its degree of bending equals that of the lithosphere, so the loads and the corresponding flexure are the keys to solving for Te. The topography and Bouguer gravity anomalies describe the surface and subsurface topographic relief of the plate, respectively, which
Variations in Te
The calculated Te values over the Arctic region are shown in Fig. 4. This parameter varies from 5 to 95 km and can basically delineate the tectonic boundaries, especially the borders of cratons and continental-oceanic transition zones. Ratheesh-Kumar et al. (2015) also found that the distinct reduction in lithospheric strength along the rifted continental margins featured large gradients in Te. Combined with the lithospheric structure and evolution history in the Arctic region, the variations
Volcanic provinces
The Te and F values obtained in this study show that the lithosphere beneath the HALIP is extremely weak and that the corresponding subsurface load which is expressed as the magmatically high-density underplated material in the lower crust (Oakey and Saltus, 2016) is very prominent. Given that seafloor spreading of the North Atlantic Ocean is separating Greenland and Lomonosov Ridge from Eurasia in the Cenozoic era (Lawver et al., 2002) and that these areas with large F correspond to the known
Conclusions
In this study, we constructed high-resolution effective elastic thickness (Te) map of the Arctic lithosphere through the coherency of equivalent topography and complete Bouguer gravity anomaly data. The values obtained for Te in the Arctic region vary significantly in different tectonic units.
We find that low Te values are located in the areas of known HALIP and possibly other undetected HALIP fragments revealed by the expressed significant subsurface loads. We interpret the weak lithosphere as
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We are grateful to the editor Mark Jellinek and two anonymous reviewers for their insightful and constructive comments. The research was funded by the Institute of Crustal Dynamics, CEA (grant ZDJ2019-30), the National Natural Science Foundation of China (grant 41706215), the National Key Research and Development Program of China (Grant 2018YFC1503504), and the Institute of Geomechanics, the Chinese Academy of Geological Sciences (Grants DZLXJK201903 and DD20190579). Most figures were prepared
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