Plagioclase preferred orientation and induced seismic anisotropy in mafic igneous rocks

Abstract

Fractional crystallization and crystal segregation controlled by settling or floating of minerals during the cooling of magma can lead to layered structures in mafic and ultramafic intrusions in continental and oceanic settings in the lower crust. Thus, the seismic properties and fabrics of layered intrusions must be calibrated to gain insight into the origin of seismic reflections and anisotropy in the deep crust. To this end, we have measured P and S wave velocities and anisotropy in 17 plagioclase-rich mafic igneous rocks such as anorthosite and gabbro at hydrostatic pressures up to 650 MPa. Anorthosites and gabbroic anorthosites containing >80 vol% plagioclase and gabbros consisting of nearly equal modal contents of plagioclase and pyroxene display distinctive seismic anisotropy patterns: , respectively, pyroxene and amphibole play a dominant role over plagioclase in the formation of seismic anisotropy. The Poisson's ratio calculated using the average P and S wave velocities from the three principal propagation-polarization directions (X, Y, and Z) of a highly anisotropic anorthosite cannot represent the value of a true isotropic equivalent. The CPO-induced anisotropy enhances and decreases the foliation-normal incidence reflectivity at gabbro-peridotite and anorthosite-peridotite interfaces, respectively.

Constraints on the rheology of the lower crust in a strike-slip plate boundary: evidence from the San Quintín xenoliths, Baja California, Mexico

“…The [100] axes show concentrations within or close to the foliation plane and at a high angle to the lineation. This CPO pattern corresponds to the type-C plagioclase CPO of Ji et al (2014), which is characteristic of plastically deformed mafic rocks. Type-C CPO indicates the dominance of the (010)[001] plagioclase slip T. van der Werf et al: Lower crust rheology in a strike-slip plate boundary Figure 11.…”

“…9a). The poles to the (010) and/or (001) planes tend to be grouped into maxima oriented at a high angle to the foliation (Ji et al, 1988(Ji et al, , 2000(Ji et al, , 2014Xie et al, 2003;Gómez Barreiro et al, 2007;Mehl and Hirth, 2008;Hansen et al, 2013). One of the xenoliths (SQW-75) with a high concentration of [100] subparallel to the lineation is characterized by poles to (001) planes distributed along a girdle oriented at a high angle to the lineation (Fig.…”

Fabric heterogeneity in the Mojave lower crust and lithospheric mantle in Southern California

“…Lower crustal samples include plagioclase A‐ and B‐type LPOs [cf. Ji et al , ] and olivine C‐type LPO. Deformed mantle samples exhibit both A‐ and E‐type olivine LPOs with orthopyroxene LPO commonly mirroring olivine in E‐type samples.…”

Magnitude and symmetry of seismic anisotropy in mica‐ and amphibole‐bearing metamorphic rocks and implications for tectonic interpretation of seismic data from the southeast Tibetan Plateau

“…The CPO of each mineral, which is induced by differential stress and ductile deformation, depends on the prevailing deformation mechanism (e.g., dislocation slip system); the magnitude, geometry, and history of strain (e.g., coaxial or noncoaxial strain); and the conditions during deformation (e.g., temperature, pressure, differential stress, and fluid content) [e.g., Hansen et al , ; Miyazaki et al , ; Raterron et al , ]. As a consequence, the seismic properties of naturally deformed rocks consisting dominantly of trigonal (e.g., α‐quartz and calcite), hexagonal (e.g., β‐quartz) [ Mainprice and Casey , ; Naus‐Thijssen et al , ; Ward et al , ; Zhao et al , ], orthorhombic (e.g., olivine and orthopyroxene [ Ji et al , , ; Jung et al , ; Lee and Jung , ; Park and Jung , ; Saruwatari et al , ]), monoclinic (e.g., amphibolite and clinopyroxene [ Barberini et al , ; Barruol and Mainprice , ; Ji et al , , ; Ko and Jung , ; Tatham et al , ]), and triclinic (e.g., plagioclase [ Ji and Mainprice , ; Ji and Salisbury , ; Ji et al , ; Satsukawa et al , ]) minerals are often of complex geometry.…”

“…The interpretation of seismic data generally uses an assumption that the rock formations interrogated by seismic waves have a simple transverse isotropy (TI) or hexagonal symmetry [e.g., Bostock and Christensen , ; Christensen and Okaya , ; Levin and Park , ; Okaya and Christensen , ; Porter et al , ]. In a TI material, P wave velocities are virtually the same in all radial directions perpendicular to the axis of symmetry, along which the seismic velocity can be either higher (e.g., laminated anorthosite [ Ji et al , ]) or lower (e.g., mica schist [ Brownlee et al , ; Dempsey et al , ; Erdman et al , ; Naus‐Thijssen et al , ; Ward et al , ; Wenk et al , ]) than the velocity normal to the axis. Shear wave splitting is maximum and null for propagation perpendicular and parallel to the axis of symmetry, respectively.…”