Ficha Volcan Nicholson

Geophys. J. Int. (2002) 149, 1–14

Seismic anisotropy and mantle creep in young orogens
Rolf Meissner,1,2 Walter D. Mooney2 and Irina Artemieva3
1 Institute 2 US

for Geoscience, Kiel University, D-24118 Kiel, Germany Geological Survey, MS 977 Menlo Park, CA 94025, USA 3 Department of Earth Sciences, Uppsala University, S-75236 Uppsala, Sweden

Accepted 2001 August 8. Received 2001 July30; in original form 2000 July 7

SUMMARY Seismic anisotropy provides evidence for the physical state and tectonic evolution of the lithosphere. We discuss the origin of anisotropy at various depths, and relate it to tectonic stress, geotherms and rheology. The anisotropy of the uppermost mantle is controlled by the orthorhombic mineral olivine, and may result from ductile deformation, dynamicrecrystallization or annealing. Anisotropy beneath young orogens has been measured for the seismic phase Pn that propagates in the uppermost mantle. This anisotropy is interpreted as being caused by deformation during the most recent thermotectonic event, and thus provides information on the process of mountain building. Whereas tectonic stress and many structural features in the upper crust areusually orientated perpendicular to the structural axis of mountain belts, Pn anisotropy is aligned parallel to the structural axis. We interpret this to indicate mountainparallel ductile (i.e. creeping) deformation in the uppermost mantle that is a consequence of mountain-perpendicular compressive stresses. The preferred orientation of the fast axes of some anisotropic minerals, such as olivine, isknown to be in the creep direction, a consequence of the anisotropy of strength and viscosity of orientated minerals. In order to explain the anisotropy of the mantle beneath young orogens we extend the concept of crustal ‘escape’ (or ‘extrusion’) tectonics to the uppermost mantle. We present rheological model calculations to support this hypothesis. Mountain-perpendicular horizontal stress(determined in the upper crust) and mountain-parallel seismic anisotropy (in the uppermost mantle) require a zone of ductile decoupling in the middle or lower crust of young mountain belts. Examples for stress and mountain-parallel Pn anisotropy are given for Tibet, the Alpine chains, and young mountain ranges in the Americas. Finally, we suggest a simple model for initiating mountain parallel creep.Key words: anisotropy, coupling, creep, rheology, tectonic escape.

INTRODUCTION Seismic anisotropy is a characteristic property of large parts of the crust and upper mantle. It is caused by several factors, such as: (1) fine-scale layering of isotropic layers with velocity contrasts (Backus 1962; Helbig 1984); (2) cracks and fractures aligned in a preferred direction (Crampin 1984, 1989); and (3)strain-induced orientation of anisotropic minerals, such as biotite and hornblende in the crust, and olivine and pyroxene in the upper mantle (Babuska 1981; Mainprice & Nicolas 1989; Babuska & Cara 1991). There is a wealth of papers on seismic anisotropy, and we summarize the most important results in Table 1. Theoretical studies define anisotropy as direction dependence of a physical property suchas the elastic tensor. For practical purposes we calculate seismic anisotropy according to Kern & Richter (1981) A = (Vmax − Vmin )/Vmax (in per cent),
C

(1)

where Vmax and Vmin are the maximum and minimum seismic velocities, respectively. Most observations of seismic anisotropy distinguish between azimuthal and transverse anisotropy. Azimuthal anisotropy describes wave propagation thatdepends on the azimuth of propagation in the horizontal plane (Babuska & Cara 1991); the term transverse anisotropy was originally introduced by Love (1927) and describes a medium with one axis of cylindrical symmetry, which is equivalent to hexagonal symmetry for seismic wave propagation. Azimuthal anisotropy has been observed on seismic refraction profiles recorded at different azimuths (Bamford...