Seismic properties and anisotropy of the continental crust: Predictions based on mineral texture and rock microstructure

Abstract

Progress in seismic methodology and ambitious large‐scale seismic projects are enabling high‐resolution imaging of the continental crust. The ability to constrain interpretations of crustal seismic data is based on laboratory measurements on rock samples and calculations of seismic properties. Seismic velocity calculations and their directional dependence are based on the rock microfabric, which consists of mineral aggregate properties including crystallographic preferred orientation (CPO), grain shape and distribution, grain boundary distribution, and misorientation within grains. Single‐mineral elastic constants and density are crucial for predicting seismic velocities, preferably at conditions that span the crust. However, high‐temperature and high‐pressure elastic constant data are not as common as those determined at standard temperature and pressure (STP; atmospheric conditions). Continental crust has a very diverse mineral composition; however, a select number of minerals appear to dominate seismic properties because of their high‐volume fraction contribution. Calculations of microfabric‐based seismic properties and anisotropy are performed with averaging methods that in their simplest form takes into account the CPO and modal mineral composition, and corresponding single crystal elastic constants. More complex methods can take into account other microstructural characteristics, including the grain shape and distribution of mineral grains and cracks and pores. Dynamic or active wave propagation schemes have recently been developed, which offer a complementary method to existing static averaging methods generally based on the use of the Christoffel equation. A challenge for the geophysics and rock physics communities is the separation of intrinsic factors affecting seismic anisotropy, due to properties of crystals within a rock and apparent sources due to extrinsic factors like cracks, fractures, and alteration. This is of particular importance when trying to deduce crustal composition and the state of deformation from seismic parameters.

Modification of the Seismic Properties of Subducting Continental Crust by Eclogitization and Deformation Processes

“…The most apparent difference between velocities from eclogites and granulites is an increase of the P and S wave velocity by ~0.8 and ~0.6 km/s, respectively (Figure a). P and S wave velocities of granulite samples at granulite‐facies conditions (~0.05 km/s slower than those at eclogite‐facies conditions) are in agreement with representative mid to lower crustal velocities (e.g., Almqvist & Mainprice, ; Kern et al, ; Lloyd, Butler, et al, ). This significant velocity contrast between the eclogite‐facies shear zones and the metastable granulite‐facies protolith at eclogite‐facies conditions suggests that depending on scale they should be detectable using geophysical methods such as the receiver function method (Kind et al, ).…”

New Crystal Preferred Orientation of Amphibole Experimentally Found in Simple Shear

“…The samples showing type IV CPO produced P wave anisotropies of 4.0–7.3% and maximum S wave anisotropies of 2.7–4.8% (Figure a and Table ). These seismic anisotropies are much lower than those of type II and mixed type I + II CPOs (Figure a) and many other crustal rocks (e.g., Almqvist & Mainprice, ; Ji et al, ). Our data indicate that the type IV CPO of hornblende, which is produced in a high‐strain zone in the midcrust ( P ~ 0.5 GPa), will induce a weak seismic anisotropy.…”

“…Although plagioclase, alkali feldspar, and quartz are very abundant in the crust (Almqvist & Mainprice, ; McLennan & Taylor, ; Nesbitt & Young, ; Ronov, ), they showed very weak seismic anisotropy in previous studies due to their weak CPO strength (Tatham et al, ). Mica and clay minerals are elastically very anisotropic minerals in the crust (Almqvist & Mainprice, ). Although they are scarce in the middle to lower crust, they can strongly influence seismic anisotropy (Almqvist & Mainprice, ; Lloyd et al, ; Lloyd et al, ; Mahan, ; Meissner et al, ; Shapiro et al, ).…”

Probing crustal anisotropy by receiver functions at the deep continental drilling site KTB in Southern Germany

“…Christensen ; Barruol and Mainprice ; Christensen and Mooney ; Lloyd et al . ; Almqvist and Mainprice ; Okaya et al . ).…”