Open Session - SWISS GEOSCIENCE MEETINGs

thrusting regime the thrusting of the continental crust over the oceanic plate is associated with deflection but not subduction
of this plate, which remains attached to the continent. Transition from stable margin to either overthrusting or subduction
is mainly dependent on the ductile strength of the lower crust with weak and hot crust favouring its oceanward
movement. On the other hand, transition from overthrusting to subduction is crucially controlled by the ductile strength
and density of the continental mantle lithosphere. Subduction is strongly favoured by rheologically weak (hot, hydrated) and
depleted continental mantle characterized by lowered density compared to the oceanic lithosphere. Moreover, the present
numerical experiments show that the age of the oceanic lithosphere at a passive margin does not play any significant role
for the initiation of subduction.
REFERENCES
Feng, M., van der Lee, S., and Assumpcao, M. 2007: Upper mantle structure of South America from joint inversion of
waveforms and fundamental mode group velocities of Rayleigh waves, Journal of geophysical research, 112, B04312,
doi:10.1029/2006JB004449.
Gerya, T.V., and Yuen, D. 2007: Robust characteristics method for modelling multiphase visco-elasto-plastic thermomechanical
problems, Physics of the Earth and planetary interiors, 163, 83-105.
Tassara, A., Swain, C., Hackney, R., and Kirby, J. 2007: Elastic thickness structure of South America estimated using wavelets
and satellite-derived gravity data, Earth and planetary science letters, 253, 17-36.
1. 2
Structural Mapping along the Meran-Mauls and the Faltleis Faults in the
Sarntaler Alps (South Tyrol, Northern Italy)
Nommensen Lisa*, Stipp M.**, Pomella H. & Fügenschuh B.***
* Geologisches Institut, Albertstr. 23B, D-79114 Freiburg (lisa.nommensen@web.de)
** IFM-GEOMAR, Wischhofstraße 1-3, D-24148 Kiel (mstipp@ifm-geomar.de)
*** Institut für Geologie und Paläontologie, Innrain 52, A-6020 Innsbruck
The Periadriatic Fault System (PFS) separates the Southern Alps from the Western, Central and Eastern Alps to the North. The
Meran Mauls fault (MF) is one segment of the PFS which connects the NNE-SSW-striking Giudicarie fault with the E-W-trending
Pustertal-Gailtal fault. So far, the MF has been described as a NW-dipping thrust with SE-directed transport of the
Austroalpine Meran-Mauls basement on top of the Southalpine late-Variscan Brixen granodiorite (e.g. Mancktelow et al.
2001). Located approximately 2 km NW and parallel to the MF, the Faltleis fault (FF) is a branch or parallel fault within the
Austroalpine Meran-Mauls basement separating the Ky-bearing paragneisses and micaschists of the Punta Cervina unit (PCU),
late Permian granitic to gabbroic intrusives (orthogneisses) with amphibolites, and Permotriassic cover sediments from the
overlying San Leonardo unit (SLU) consisting mainly of banded paragneisses.
Detailed structural mapping was carried out along a part of the MF and the FF in the Sarntaler Alps of South Tyrol near the
village of Weißenbach in order to analyze the kinematics of the two faults and to identify related structures in the underand
overlying rock units. The area of the two faults is characterized by a strong mylonitic foliation (S1) with an orientation
of approximately 330/35 extending from the top of the Brixen granodiorite in the SE through the PCU and into the SLU in
the NW except for minor portions of the granites and amphibolites as well as for the Triassic dolomites of the PCU which
are not pervasively foliated. Two stretching lineations are related to S1, a steep or downdip stretching lineation indicating a
top to the SE transport and an older subhorizontal stretching lineation with a transpressive right-lateral sense of shear. S1
is folded by two major folding phases. The older isoclinal to tight folds (F1) have moderately NNW plunging fold axes, while
the younger tight to open folds (F2) strike NE-SW with subhorizontal axes. Locally, an F1 axial plane cleavage slightly steeper
than S1 can be observed.
The MF itself is marked by a discrete cataclastic zone overprinting the Brixen granodiorite in the footwall. Hence, the MF
displays the classical set-up of a major thrust fault at the frictional-viscous transition with cataclasites in the footwall and
mylonites in the hanging wall. However, the thrust-related deformation overprints an earlier dextral-transpressive shearing
as indicated by subhorizontal stretching lineations and related shear sense indicators within the same foliation (S1). The FF
has a more complicated structural assembly with two cataclastic zones, one at the contact between the SLU in the hanging
wall and the Permotriassic cover sediments of the PCU in the footwall and the other one inside the footwall at the transition
Symposium 1: Structural Geology, Tectonics and Geodynamics
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