Predicted velocity and density structure of the exhuming Papua New Guinea ultrahigh-pressure terrane

Abstract

[1] New electron backscatter diffraction measurements show that the Papua New Guinea (PNG) ultrahigh-pressure (UHP) terrane is dominated by rocks with weakly oriented quartz and feldspar and less abundant strongly oriented hornblende, clinopyroxene, and mica. Velocities measured at high pressures (600 MPa) show that V P is 5.8-6.3 km/s for gneiss samples, 6.5-7.7 km/s for amphibolite, and 7.7-8.2 km/s for eclogite and V S is 3.4-3.9 km/s for gneiss, 4.0-4.4 km/s for amphibolite, and 4.5-4.6 km/s for eclogite. Velocities and anisotropies calculated from mineral crystal preferred orientations (CPOs) are equivalent to within 5% of the measured values. The highest seismic anisotropy for the PNG terrane is in amphibolite at 8% and 7% for V P and V S , respectively. Calculations of seismic velocities at depth based on predicted mineral assemblages indicate that the exhuming UHP terrane is of dominantly mafic composition below ∼20 km depth. Anisotropy in the PNG terrane is expected to be quite low and is controlled by the orientation of the foliation. If observable, changes in anisotropy across the exhuming body may be used to differentiate among the different proposed mechanisms of UHP exhumation.

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 average bulk anisotropy values for chlorite schists, mica schists, phyllites, sillimanite‐mica schists, and amphibole schists at 600 MPa are 12.0 ± 4.1%, 12.8 ± 5.9%, 12.8 ± 9.0%, 17.0 ± 4.9%, and 12.9 ± 3.2% respectively, indicating that such schists are much more anisotropic in V p (13.2 ± 6.0%) than other categories of metamorphic rocks such as granitic gneisses, felsic mylonites, granulites, and peridotites [e.g., Ji et al , , ; Mainprice and Silver , ; Saruwatari et al , ], in which the constituent minerals have geometrically complex seismic responses that interfere destructively to produce either low anisotropy or nearly isotropic properties. The characteristically high seismic anisotropy of the schists is attributed to strongly aligned phyllosilicates such as mica (Figure ) [also see Brownlee et al , ; Erdman et al , ; Ji et al , ; Kern and Wenk , ; Mahan , ; Meltzer and Christensen , ; Shapiro et al , ] as shown also by microstructural observations (e.g., Figure ). However, the average V p anisotropy values in the foliation plane or foliation anisotropy are much smaller than the bulk anisotropy values at the same pressure: 2.4 ± 1.9% for phyllites, 3.3 ± 2.7% for mica schists, 4.1 ± 2.6% for chlorite schists, 6.8 ± 3.4% for sillimanite‐bearing mica schists, and 5.2 ± 1.9% for amphibole schists.…”

“…The dominant b ‐slip fabric was reported for muscovite in a quartzite (sample CB‐q consisting of 95% quartzite, 3% muscovite, and 2% kyanite [ Ward et al , ]). The a + b slip‐induced fabric pattern was reported for biotite in quartzofeldspathic gneisses from the exhuming Papua New Guinea ultrahigh‐pressure terrane [ Brownlee et al , ] and for a quartz‐bearing amphibolite (sample GR‐a), and muscovite in a quartzite (sample ISR‐q consisting of 95% quartz, 3% muscovite, and 2% sillimanite) [ Ward et al , ].…”

“…Unlike mica or hornblende in the same rock, plagioclase or K‐feldspar developed nearly random or very weak CPOs with the pfJ index generally lower than 1.50 (Figure S1). Such weak or diffuse CPOs are less straightforward to interpret [e.g., Brownlee et al , ; Erdman et al , ; Ji and Mainprice , ; Ji and Salisbury , ; Ji et al , ]. In samples GLG132J, GLG134, and AM1, the [100] direction of plagioclase developed a concentration near the X direction, and the poles to (001) form a girdle perpendicular to the lineation with some concentrations at Z .…”

“…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.…”

Shear Velocity Structure From Ambient Noise and Teleseismic Surface Wave Tomography in the Cascades Around Mount St. Helens

“…At 550 °C and 0.8 GPa, they show that Vs lies near 3.8 km/s, and never below 3.7 km/s. While temperature variations are poorly known for these compositions, owing to the competing effects of thermal expansion and dehydration‐mediated reactions, most direct calculations on dry felsic rocks are unable to produce Vs < 3.6 km/s (e.g., Abers et al, ; Brownlee et al, ). Across the United States, at 25‐km depth Vs is less than 3.6 km/s in only 3.4% of the area resolved by Shen and Ritzwoller (), largely beneath volcanic areas of the Cascades or Basin and Range.…”