Crystallographic preferred orientations of exhumed subduction channel rocks from the Eclogite Zone of the Tauern Window (Eastern Alps, Austria), and implications on rock elastic anisotropies at great depths

Textures of eclogites and blueschists from Syros island, Greece: Inferences for elastic anisotropy of subducted oceanic crust

“…This is an observation that has also been reported by studies from other subduction complexes [e.g., Mauler et al ., ; Keppler et al ., ]. Velocity and V p / V s signature of the Syros samples are also very similar to those of other eclogites [e.g., Keppler et al ., ]. Apparently, V p / V s can be changed to higher values by retrograde transformation of eclogites to amphibolites [e.g., Gao et al ., ; Keppler et al ., ], probably owing to the growth of lower pressure hornblende, but this is not an issue in the samples of this study.…”

“…Velocity and V p / V s signature of the Syros samples are also very similar to those of other eclogites [e.g., Keppler et al ., ]. Apparently, V p / V s can be changed to higher values by retrograde transformation of eclogites to amphibolites [e.g., Gao et al ., ; Keppler et al ., ], probably owing to the growth of lower pressure hornblende, but this is not an issue in the samples of this study. Moreover, this is a process unlikely to occur in subducted oceanic crust at great depth but may, of course, alter the seismic signature of ophiolite complexes as they are exhumed and tectonically emplaced at higher levels in the crust.…”

“…Following a discussion of Keppler et al . [], the V p / V s values of the blueschists and eclogites are lower than those of peridotites of the lithospheric mantle of a downgoing slab in global Earth models [e.g., Kennett et al ., ] and experimental work [e.g., Christensen , , ; Kern , ; Ullemeyer et al ., ]. In a tomographic study of subducting slabs off Northern Honshu Zhang et al .…”

“…Some influence of pressure and temperature can be expected beyond pressures of 10 kbar. Comparison of modeled and experimentally measured data frequently shows higher values for the measured elastic anisotropies [e.g., Kern et al ., ; Keppler et al ., ]. The velocity patterns (i.e., location of minima and maxima), however, usually are similar in measured and modeled results.…”

“…Knowledge of the orientation and magnitude of this anisotropy is, therefore, essential for the correct construction of seismic velocity models and an improved structural interpretation of subducted slabs. One approach is to investigate the anisotropy of rocks by experimental techniques at elevated pressures [e.g., Christensen, 1965Christensen, , 1966Christensen and Fountain, 1975;Kern, 1978Kern, , 1993Fountain et al, 1994;Mauler et al, 2000;Ullemeyer et al, 2006Ullemeyer et al, , 2010Kern et al, 2008], or by the recalculation of the elastic velocity using rock texture and single-crystal elastic data [e.g., Bascou et al, 2001;Ji et al, 2003b;Ábalos et al, 2011;Keppler et al, 2015]. The major issue we address in this study is how anisotropy is expected to vary between blueschist-grade and eclogite-grade subducted oceanic crust and how it would compare with the surrounding mantle rocks.…”