Fluid Flow and Fault Zone Fabrics Associated with Normal Faulting in the Aegean Region

Griet Verhaert^1,2, Philippe Muchez¹ and Manuel Sintubin^2
1 Fysico-chemische geologie, K.U. Leuven, Celestijnenlaan 200C, B-3001 Leuven, Belgium ([email protected])
² Structural geology & tectonics group, K.U. Leuven, Redingenstraat 16, B-3000 Leuven, Belgium

    The study area, located in the SW of Turkey is a seismic active region with a number of large earthquakes (Ms > 6.0) in the previous century. Also during ancient times devastating earthquakes struck the Roman city Sagalassos, situated in the study area. Several normal faults are identified in the limestone substrate in the vicinity of Sagalassos. These normal faults comprise multiple slip planes and zone-parallel layers of fault breccia, typical for “Aegean”-type normal faults (e.g. Stewart & Hancock 1991). The slip planes contain coatings of striated calcite crystals. A structural, petrographical, microthermometric, geochemical and geochronological study of the striated fault-related calcite precipitates and a fabric analysis of the different types of fault breccia have been undertaken to reconstruct the fluid flow associated with normal faulting and to determine the kinematics during normal faulting. This contributes to a better understanding of the interaction between structural development, geochemistry, diagenesis, fluid flow and timing.
    Slickenside lineations on fault planes have been measured for a profound palaeostress analysis (cfr. Angelier 1984). This palaeostress analysis reveals the neotectonic significance of the sampled faults and indicates the presence of two important extensions, i.e. a NW-SE and an ENE-WSW oriented extension. Both extensions are in agreement with the biaxial extensional regime suggested for this area by Bozkurt (2001). The Sarikaya area, near the ancient Roman city of Sagalassos in the Burdur province (SW Turkey) forms a natural laboratory for a detailed study of the interaction between fault kinematics and fluid flow. The fault zones are all composed of multiple slip planes and zone-parallel layers of fault breccia, which indicate several faulting events. The slip planes contain several kinematic indicators, like slickenside lineations, comb fractures and corrugations. A microstructural study of the calcite crystals indicates the presence of micro-slickensteps, fibrous calcite crystals, cleavage planes and pressure solution seams parallel to movement. The calcites underwent intense deformation, shown by calcite twins and dynamic recrystallisation. This characteristic occurrence of calcites along faults indicates that the calcite precipitation was linked with fault activity.
    The fault zones are composed of an alternation between incohesive breccia belts and compact breccia sheets. The incohesive breccia consists of angular limestone fragments, ranging from 2 mm to 2 cm and enveloped by a beige calcite cement, with a volume percentage up to 30%. The compact breccia is also composed of angular limestone fragments with diameters up to 4 cm. The fragments are compacted and recrystallised. Only small quantities of white, transparent calcite cement can be detected. The characterisation of the normal faults allows identifying other surfaces as fault planes, especially when they are less visible due to intense weathering, i.e. fault degradation.
    Staining of the calcites reveals the iron-rich character of these precipitates. The dull to bright luminescence indicates that the calcite cements did not precipitate from near-surface water under oxidising conditions.
    The fluid inclusion study of the fault-related calcites shows the dominant presence of stable single-phase fluid inclusions and few two-phase fluid inclusions ( < 12 µm). The final melting temperature (Tm ice) of the two-phase fluid inclusions ranges between 0°C and -1.7°C. This indicates a meteoric origin of the responsible fluids with some degree of water-rock interaction or mixing with another fluid, which leads to a maximum salinity of 2.9 wt.% NaCl (Bodnar 1993). The homogenisation temperatures (Th) between 42°C and 74°C are in accordance with the presence of the stable single-phase fluid inclusions.
    The oxygen isotope values of the striated calcite precipitates show a broad range between -14.9‰ VPDB and -2.2‰ VPDB (n=17). Several of these fault-related calcites have a ¹⁸O composition that falls outside the range of the meteoric calcites (between -10‰ VPDB and -6‰ VPDB, Verhaert et al 2002). The more negative ¹⁸O values (n=3) could be due to a higher precipitation temperature. The higher ¹⁸O values (n=7) are the result of a higher ¹⁸O value of the ambient fluid. This indicates a fluid composition different from that of meteoric water or an intense interaction of a meteoric fluid with the limestone. The ¹⁸O values of the limestone range between -4.1‰ VPDB and -0.2‰ VPDB (Muchez et al. accepted). The carbon isotopic composition of the fault-related calcites ranges between -0.6‰ VPDB and +3.7‰ VPDB. The ¹³C composition of most fault-related calcites (n=14) fall within the range of the host limestones (between +1.0‰ VPDB and +3.9‰ VPDB), and reflect a rock-buffered system. Higher temperatures and intense water-rock interaction indicate a longer residence time of meteoric fluids in the subsurface. The minimum precipitation temperature of the most negative ¹⁸O values (-14.9‰ VPDB), calculated following the equation of Craig (1965) is ~50°C. So, with a geothermal gradient of 30°C/km, the upwelling of fluids from a depth of ~1100 m can be inferred.
    The fault-related calcite precipitates can also be used as a geochronological tool, eventually to date seismic events. One fault-related calcite sample, belonging to a fault movement caused by the ENE-WSW extension, has been dated by U/Th techniques and gives an age of 200ka. However, dating of several calcite precipitates are needed in order to determine the absolute timing of other important fault activities to obtain a complete geochronological evolution of the neotectonic events.
The combined study of fault-related calcite precipitates leads to the reconstruction of the fluid flow associated with normal faulting. Meteoric water infiltrates in the limestones at least to a depth of 1 km and underwent water-rock interaction or mixing with a residual fluid. Afterwards the fluid became expelled during fault activity.

References

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Verhaert G, Muchez P, Sintubin M, Zeelmaekers E (2002) Calcite cementation of screes: palaeoclimatic implications. Aardkundige Mededelingen, 12, 153-156.

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