Sebastian Garcia

Reply to the comment of Fügenschuh et al. on the paper ‘Estimating displacement along the Brenner Fault and orogen-parallel extension in the Eastern Alps’ by Rosenberg and Garcia, Int J Earth Sci (Geol Rundsch) (2011) 100:1129–1145

Fugenschuh et al. (2011) question several aspects of our paper (Rosenberg and Garcia 2011) on the estimate of extension along the Brenner Fault (BF). First, they emphasize the necessity of a two-phase extensional model to understand exhumation of the footwall of the BF; second, they claim that the vertical offsets and extensional displacement calculated by Rosenberg and Garcia (2011) only refer to the brittle phase of the BF; third, they follow the new interpretations of Tochterle et al. (2011), inferring that the continuation of the Brenner ductile mylonites is located along the north-western border of the Tauern Window (TW); fourth, they present a new estimate of extension and of exhumation by folding and erosion based on the quantification of tectonic omissions. We discuss and reply to these four issues below.

Stress rotations and the long-term weakness of the Median Tectonic Line and the Rokko-Awaji Segment

We used a field analysis of rock deformation microstructures and mesostructures to reconstruct the long-term orientation of stresses around two major active fault systems in Japan, the Median Tectonic Line and the Rokko-Awaji Segment. Our study reveals that the dextral slip of the two fault systems, active since the Plio-Quaternary, was preceded by fault normal extension in the Miocene and sinistral wrenching in the Paleogene. The two fault systems deviated the regional stress field at the kilometer scale in their vicinity during each of the three tectonic regimes. The largest deviation, found in the Plio-Quaternary, is a more fault normal rotation of the maximum horizontal stress to an angle of 79° with the fault strands, suggesting an extremely low shear stress on the Median Tectonic Line and the Rokko-Awaji Segment. Possible causes of this long-term stress perturbation include a nearly total release of shear stress during earthquakes, a low static friction coefficient, or lowelastic properties of the fault zones comparedwith the country rock. Independently of the preferred interpretation, the nearly fault normal orientation of the direction of maximum compression suggests that the mechanical properties of the fault zones are inadequate for the buildup of a pore fluid pressure sufficiently elevated to activate slip. The long-term weakness of the Median Tectonic Line and the Rokko-Awaji Segment may reside in low-friction/low-elasticity materials or dynamic weakening rather than in preearthquake fluid overpressures.