Estimating displacement along the Brenner Fault an

Authors: C L Rosenberg S Garcia

Publish Date: 2011/03/10

Volume: 100, Issue: 5, Pages: 1129-1145

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Abstract

The major structure accommodating orogenparallel extension in the Eastern Alps is inferred to be the Brenner Fault which forms the western boundary of the Tauern Window The estimated amount of extension along this fault varies from a minimum of 10–20 km to a maximum of 70 km All investigations that have attempted to constrain this amount of extension have calculated the fault plane parallel displacement required to restore the difference in structural level between footwall and hanging wall as constrained by geobarometry However these calculations neglected the component of exhumation of the footwall resulting from folding and erosion Therefore the total amount of extensional displacement was systematically overestimated In the present study we project a tectonic marker surface from the footwall and hanging wall of the Brenner Fault onto a N–Sstriking cross section This marker surface which is the base of the Patscherkofel unit in the footwall and the base of the Ötztal basement in the hanging wall is inferred to have occupied the same structural level in the hanging wall and footwall of the Brenner Fault before its activity Therefore the difference in height between the marker projected from the footwall and from the hanging wall is a measure of the vertical offset across the Brenner Fault This construction shows that the vertical offset of the marker horizon on both sides of the Brenner Fault varies strongly and continuously along strike of the Brenner Fault attaining a maximum value of 15 km at the hinge of the folded footwall Tauern Dome The alongstrike change of vertical offset is explained by largescale upright folding of the footwall that did not affect the hanging wall of the Brenner Fault Therefore the difference in vertical offset of 10 km between the area of the Brenner Pass and the area immediately south of Innsbruck corresponds to the shortening upright folding component of exhumation of the footwall The remaining 5 km of vertical offset must be attributed to extensional deformation The Brenner Fault itself is barely folded its dip varies between 20 and 70° and it crosscuts the upright folds of the western Tauern Window Given the offset of 5 km the dip of the fault constrains the extensional displacement to be between 2 and 14 km We conclude that the Tauern Window was exhumed primarily by folding and erosion not by extensional unroofingFinancial support by the DFG Ro 2177/5 is acknowledged R Brandner kindly sent us copies of the BBT cross section M Rockenschaub sent us unpublished versions of the Austrian map ‘Blatt Sterzing’ A Bertrand K Hammerschmidt M Handy A Scharf S Schneider S Schmid and Kamil Ustaszewski did not always agree on the interpretations of this paper but helped us clarify many ideas of the manuscript The constructive reviews of M Handy M Rockenschaub and B Lammerer are greatly acknowledged

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