Relationship between fluid flow and faulting in the Alpine realm (Austria, Germany, Italy)

Abstract

Abstract Five carbonate cement generations in faults and associated fractures, which precipitated during two major cementation phases, were studied in Permian to Tertiary sedimentary rocks in the Drau Range, in the Southern Alps and in different nappes of the Northern Calcareous Alps (NCA). The first major cementation phase is represented by clear saddle dolomite (CSD) that can be further subdivided by cathodoluminescence (CL) into five CL-zones. A non-carbonate zone, consisting of chlinochlorite (chlorite zone), is intercalated between CL-zones II and III. CL-zones I–IV occur in rocks of Permian to Early Jurassic age, while CL-zone V is also present in Upper Jurassic strata. The second major cementation phase, characterized by another saddle dolomite and three generations of blocky calcite, is of Miocene age. Dedolomitized CSD, which occurs in a synsedimentary normal fault of Early Jurassic age contains a zone of pyrite. The trace element composition of this pyrite zone and of an Upper Hettangian Fe–Mn crust (Marmorea crust) were compared with the trace element composition of the chlorite zone. Similar trace element patterns within all three non-carbonate zones suggest the same fluid source for these zones during Upper Hettangian time. This fluid could be of hydrothermal origin, as indicated by the geochemical characteristics of the Fe–Mn crust. Hydrothermal activities started during the beginning of rifting in the Alpine realm in the Late Triassic/Early Jurassic. Fluid inclusion data reveal variable homogenization temperatures for the five CL-zones in the clear saddle dolomite. Formation temperatures between 146 and 201°C were calculated, considering the overburden pressure during the formation of the CL-zones. Calculated equilibrium oxygen isotopic fluid compositions indicate δ 18 O values between 4.0 and 9.0‰ SMOW for the fluid from which the different saddle dolomite zones precipitated. The origin of the fluid can be related to brines present in the hinterland (Vindelician high, Bohemian massif). The stratigraphic occurrence of CSD and its paragenetic relationship to the non-carbonate zones enables a more exact dating of the CL-zones. CL-zones I and II precipitated between Norian and Late Hettangian time, and CL-zones III and IV are of Early Sinemurian age. The formation period of CL-zone V is Late Jurassic to Early Cretaceous. The distribution of the carbonate cements was investigated within different fault sets and in associated subparallel fractures in the NCA, i.e. the Allgau-, Lechtal- and Inntal-nappe, which show a polyphase deformation history. CSD occurs in normal faults, N–S trending high angular faults and WNW-trending strike-slip faults, indicating minimum an Early Jurassic age for these faults. The only carbonate cements that are present within NE-trending sinistral strike-slip faults are of the second major cementation phase, implicating a Miocene age for these faults. Fibrous shaped crystals of the first calcite cement of the second major cementation phase preferentially occur within WNW-trending strike-slip faults. Reactivation of these faults as conjugated shear pairs in connection with the formation of NE-trending strike-slip faults, is assumed.