Seismic anisotropy in mid to lower orogenic crust: Insights from laboratory measurements of Vp and Vs in drill core from central Scandinavian Caledonides
Introduction
Linking seismic observation with subsurface geology can most directly be realized through the drilling of deep boreholes. The Swedish Scientific Drilling Program (sponsored by the International Continental Scientific Drilling Program) established a dual-phase drilling program to investigate the geologic structure, composition, and physical properties of the Caledonian orogeny in central Sweden. Drilling of the first borehole, COSC-1, took place between April and September 2014 (Lorenz et al., 2015). An extensive regional reflection seismic survey took place prior to drilling COSC-1, providing a subsurface map to guide drilling and high resolution seismic section for structural interpretation (Hedin et al., 2012). The drill-site was selected in order to penetrate through a highly reflectivity wedge, that extends from shallow depth down to 2.5 km at the drill site, with a west dipping basal plane (Fig. 1). The drill samples a section of the Seve Nappe Complex, an amphibolite grade metamorphic sequence of mid-crustal continental origin. Recently a 3D reflection seismic survey was carried out around the borehole (Hedin et al., 2016), with the aim to map in detail the structures that immediately surround the borehole. Borehole measurements, including sonic logs and vertical seismic profile studies, were performed in the COSC-1 borehole during and after drilling (Lorenz et al., 2015).
The source of the reflectivity in the continental terrain can be explained by several multi-genetic features. Suggestions in the literature include a variety of igneous, metamorphic, and structural explanations. Reflectivity caused by igneous intrusions has been documented, for example in the Siljan region, Sweden, by deep drilling and field mapping studies associated with seismic surveys (Juhlin and Pedersen, 1987). Layered sequences of mafic and felsic rocks, or an interlayering of mafic rocks and metapelites has been often indicated as the source of strong reflectivity in middle crust of different ages and tectonic contexts (e.g., Burke and Fountain, 1990, Goff and Holliger, 2003, Holliger et al., 1993). Warner (2004) explored the possibility that reflectivity could be caused by the existence of free water in the lower crust, with stratified porosity or pore pressure (as suggested for example by Brown et al., 1986, Brown et al., 1980, Hyndman and Shearer, 1989). Anastomosing shear zones and mylonitization have been proposed as a mechanism to enhance the reflection coefficient in the middle to lower crust by strong re-orientation of grains (e.g., Meissner, 1986, Meissner, 1989, Meissner, 1996). Reflectivity originating from layering in the lower crust, in particular, have been found in thin-skinned tectonic areas in extensional tectonic regimes (e.g., Gans, 1987, Le Gall, 1990, Rey, 1993, Warner, 1990). It is suggested that the deep crust, when reflective, must be composed of layers of differing acoustic impedances with thicknesses appropriate to cause constructive interference (e.g., Fuchs, 1969, Meissner, 1973). This implies that layering on a scale of about 150–300 m thick must be prevalent in some regions. Typically the layers extend for few kilometers (3 to 8 km). The model presented by Fountain and Christensen (1989) for lower crusts of different ages, from Archean to Phanerozoic, shows significant lateral and vertical changes in lithologic composition. Structural complexity is pervasive throughout, and at all scales. Salisbury and Fountain (1990) discussed the nature of the lower crust reflectivity both in terms of lithological heterogeneities and ductile deformation. They analyzed many cases where the reflective lower crust is not uniformly layered, but appears to have an internal structure and they speculate that reflectivity is due to ductile deformation, perhaps transposing pre-existing lithological units into a strong sub-planar fabric.
Interpretation of active source seismic reflection surveys and vertical seismic profiles is based on what is observed at the surface and the elastic properties of the rock, including seismic anisotropy (e.g., Almqvist et al., 2013, Rey et al., 1994). Deep scientific boreholes present a unique opportunity to study variations in seismic properties through a continuous geologic section and to directly relate them to seismic imaging. Seismic anisotropy in orogenic belts results from tectonic processes that create structural and compositional layering in rocks. Tectono-metamorphic rocks develop seismic anisotropy due to alignment of grains in a preferential elongation orientation (also known as shape preferred orientation or SPO) and/or isoorientation of crystallographic axis (crystallographic preferred orientation or CPO) of minerals (Mainprice, 2007), and due to anisotropic minerals such as sheet silicates (Hacker et al., 2014, Lloyd et al., 2009, Mahan, 2006, Meissner et al., 2006, Meltzer and Christensen, 2001, Shapiro et al., 2004) and amphiboles (Ji et al., 2013, Ko and Jung, 2015, Moschetti et al., 2010, Tatham et al., 2008). In orogenic belts the compositional and structural layering in the form of schistosity and foliation is often laterally continuous for tens to hundreds of kilometers, which assists identification of tectonostratigrapic units in seismic sections. However, the compositional layering through the section can be very heterogeneous, causing significant variations in elastic properties and seismic anisotropy.
Understanding the variation of elastic properties and anisotropy in tectono-metamorphic rocks in the COSC-1 borehole is crucial to properly interpret regional seismic reflection surveys, vertical seismic profiling studies, and sonic logs. This contribution reports laboratory measurements of seismic properties at ultrasonic frequency, both p- and s- wave, in the most common lithologies encountered in the COSC-1 borehole. Measurements of anisotropic seismic properties are provided for amphibolite, felsic gneiss, calc-silicate gneiss, amphibole-rich gneiss, and micaschist. The presence of cracks, some due to sample preparation, at low confining pressure strongly influences elastic wave propagation in the sample (e.g., Almqvist et al., 2013, Barberini et al., 2007, Birch, 1960, Zappone et al., 2000). Therefore, laboratory elastic wave velocities were measured under hydrostatic confining pressures from to 260 MPa to determine the intrinsic rock velocity. Measurements show that mineral orientation strongly influences the directional variation in seismic velocities. The results are contextualized with respect to compositional and textural differences in lithology, and provide insight for crustal reflectivity, both near the borehole and in the middle crust. Additionally, structural relationships in the seismic images are aided by foliation measurements using image logs from the borehole.
Section snippets
Geologic setting and drilling details
The closure of the Iapetus Ocean followed by subduction of Baltica underneath Laurentia during the mid-Paleozoic Caledonian orogeny is preserved in what is now Scandinavia and Greenland (Gee et al., 2008). The major thrust sheets of the Scandinavian Caledonian orogeny are subdivided into four parts: Lower, Middle, Upper, and Uppermost Allochthons (Gee et al., 1985). Gee et al. (2010) summarizes the regional geology for the area around COSC-1. The allochthons are distinguished from one another
Sample selection, preparation, and characterization
A geologic facies based approach was used to guide the sample selection process (Amendt et al., 2013). Six individual 15–20 cm long core sections, spanning the length of the borehole (0–2495.8 m), were selected for testing seismic anisotropy. The core sections were selected to test the most abundant lithologies encountered in the COSC-1 well: amphibolite, calc-silicate gneiss, felsic gneiss, amphibole-rich gneiss, and micaschists that are part of a shear zone of varying deformation degree.
On the
Characterization
The sample modal compositions are reported in Table 1 and thin sections images are depicted in Fig. 4. Each sample is reported with its corresponding International Geo Sample Number (IGSN). Amphibolites (samples C193-2, IGSN:ICDP5054EXL6601 and C556-2, IGSN:ICDP5054EXU6601) consist predominantly of amphibole and plagioclase, with small amounts of quartz and opaque minerals. The modal composition of C193-2 is 62% amphibole, 28% plagioclase, 9% quartz, and 2% opaques. The modal composition of
General comments on measured velocities
Although most rock forming minerals in the middle to lower crust have significant seismic anisotropy as single crystals, the net effect of polycrystalline bulk composition on seismic anisotropy is largely a function of CPO and SPO. Polycrystalline rocks with high concentrations of quartz, feldspar, and calcite commonly create complex seismic responses due to grain geometry (Lloyd et al., 2011a). While quartz, feldspar, and calcite individually have anisotropic crystal characteristics (AVp , s >
Conclusions
This study focuses on the seismic properties and anisotropy of amphibolites, gneisses, and micaschists derived from middle crustal metamorphic conditions and on their implications for reflectivity in the middle crust. Laboratory measurements on core samples from COSC-1 scientific borehole indicate significant lithological, velocity, and anisotropy differences, which allow us to speculate that the high reflectivity observed in the upper part of the seismic section is due to lithological
Acknowledgements
The Swedish Deep Drilling Program (SDDP) through the Swedish Research Council (VR-Grant 2013-94) and the International Continental Scientific Drilling Program (ICDP) provided funding for the COSC project. The velocity experiments were performed in the Rock Deformation Laboratory at ETH Zurich. B.S.G.A. thanks the Swedish Research Council for financial support (Junior Researcher Grant 2012-4449). Q.C.W. thanks the ETH Zurich Foundation for providing financial support (Excellence Scholarship &
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