Brittle faulting in the Rawil depression: field observations from the Rezli fault zones, Helvetic nappes, Western Switzerland |
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Authors: | Deta Gasser Neil S Mancktelow |
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Institution: | 1. Department of Earth Sciences, University of Graz, Universit?tsplatz 2, 8010, Graz, Austria 2. Geological Institute, ETH Zürich, 8092, Zürich, Switzerland
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Abstract: | The Helvetic nappes in the Swiss Alps form a classic fold-and-thrust belt related to overall NNW-directed transport. In western
Switzerland, the plunge of nappe fold axes and the regional distribution of units define a broad depression, the Rawil depression,
between the culminations of Aiguilles Rouge massif to the SW and Aar massif to the NE. A compilation of data from the literature
shows that, in addition to thrusts related to nappe stacking, the Rawil depression is cross-cut by four sets of brittle faults:
(1) NE-SW striking normal faults, (2) NW-SE striking normal faults and joints, (3) ENE-WSW striking and (4) WNW-ESE striking
normal plus dextral oblique-slip faults. Fault set 1 was probably initiated during sedimentation and reactivated during nappe
stacking, whereas the other fault sets formed after emplacement of the Helvetic nappes. We studied in detail two well-exposed
parallel fault zones from set 4, the Rezli fault zones (RFZ) in the Wildhorn Nappe. They are SW-dipping oblique-slip faults
with a total displacement across the two fault zones of ~200 m vertically and ~680 m horizontally. The fault zones crosscut
four different lithologies: limestone, intercalated marl and limestone, marl and sandstone. The internal architecture of the
RFZ strongly depends on the lithology in which they developed. In the limestones, they consist of extension veins, stylolites,
cataclasites and cemented gouge, in the intercalated marls and limestones of shear zones, brittle fractures and chaotic folds,
in the marls of anastomosing shear zones, pressure solution seams and veins and in the sandstones of coarse breccia, brittle
faults and extension veins. Sharp, discrete fault planes within the broader fault zones cross-cut all lithologies. Fossil
fault zones in the Rezli area can act as a model for studying processes still occurring at deeper levels in this seismically
active region. |
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