Elastic anisotropies of deformed upper crustal rocks in the Alps

Abstract

Abstract. The crust within collisional orogens is very heterogeneous both in composition and grade of deformation, leading to highly variable physical properties at small scales. This causes difficulties for seismic investigations of tectonic structures at depth since the diverse and partially strong upper crustal anisotropy might overprint the signal of deeper anisotropic structures in the mantle. In this study, we characterize the range of elastic anisotropies of deformed crustal rocks in the Alps. Furthermore, we model average elastic anisotropies of these rocks and their changes with increasing depth due to the closure of microcracks. For that, pre-Alpine upper crustal rocks of the Adula Nappe in the central Alps, which were intensely deformed during the Alpine orogeny, were sampled. The two major rock types found are orthogneisses and paragneisses; however, small lenses of metabasites and marbles also occur. Crystallographic preferred orientations (CPOs) and volume fractions of minerals in the samples were measured using time-of-flight neutron diffraction. Combined with single crystal elastic anisotropies these were used to model seismic properties of the rocks. The sample set shows a wide range of different seismic velocity patterns even within the same lithology, due to the microstructural heterogeneity of the deformed crustal rocks. To approximate an average for these crustal units, we picked common CPO types of rock forming minerals within gneiss samples representing the most common lithology. These data were used to determine an average elastic anisotropy of a typical crustal rock within the Alps. Average mineral volume percentages within the gneiss samples were used for the calculation. In addition, ultrasonic anisotropy measurements of the samples at increasing confining pressures were performed. These measurements as well as the microcrack patterns determined in thin sections were used to model the closure of microcracks in the average sample at increasing depth. Microcracks are closed at approximately 740 MPa yielding average elastic anisotropies of 4 % for the average gneiss. This value is an approximation, which can be used for seismic models at a lithospheric scale. At a crustal or smaller scale, however, local variations in lithology and deformation as displayed by the range of elastic anisotropies within the sample set need to be considered. In addition, larger-scale structural anisotropies such as layering, intrusions and brittle faults have to be included in any crustal-scale seismic model.

Anisotropic Reversible‐Jump McMC Shear‐Velocity Tomography of the Eastern Alpine Crust

“…This includes E‐W oriented foliation (e.g., Rosenberg et al., 2018, and references therein) as well as folding and thrusting due to N‐S compression causing E‐W oriented structures that lead to SPO anisotropy. The SPO may be enhanced by intrinsic anisotropy due to minerals aligning parallel to the foliation such as highly anisotropic micas (e.g., Fry et al., 2010; Keppler et al., 2021). Nevertheless, also this concept does not provide a fully coherent picture when looking at the curvature of the fast anisotropic axis around the Giudicarie fault.…”

“…Keppler et al. (2021) further find that microcracks which they term type I (mostly oriented along muscovite platelets) have a major influence on the anisotropic amplitude and are fully closed only at pressures of 740 Mpa (∼25 km). They thus imply a strong dependence of this type of anisotropy on depth, with effective anisotropies for an average Alpine crustal rock model ranging from 12% close to the surface to about 4% in the lower crust where microcracks are completely closed.…”

“…(2020) report an expected effective anisotropy of ∼4% for an eclogite facies effective medium. The experimentally determined fast axis are approximately oriented in the foliation plane and in lineation direction, that is, approximately in strain direction, even though there may be a discrepancy between quartz lineation and the orientation of mica crystals depending on the rock type (Keppler et al., 2021). Next to microcracks and LPO, there are more microstructural effects that potentially influence the anisotropy such as shape preferred orientation (SPO) of grains, grain boundaries distribution of mineral grains or deformation lamellae (Almqvist & Mainprice, 2017).…”