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.
Elastic anisotropies of deformed upper crustal rocks in the Alps
