ISOTOPIC SIGNATURES OF CLAYS REVEAL THE SOURCE AND TIMING OF PALEOFLUIDS, ILLUSTRATED BY CLAY GOUGE ALONG THE LONG-LIVED NORTH ANATOLIAN FAULT, NORTHERN TURKEY

We introduce a new method of determining the fluid source from clays in fault rocks using hydrogen isotopic analysis of multiple grain-size fractions of clay minerals. Complementary dating of these newly-formed clays also constrains the age of fluid infiltration and mineralization. We demonstrate this method, in conjunction with new illite age analysis (IAA), on a suite of regionally-distributed samples along the long-lived tectonic boundary zone of Northern Turkey that is currently occupied by the North Anatolian Fault (NAF). Measured δD values of illitic clays of three samples are reported relative to VSMOW. Size fractions in samples G1 and G2 range from -64 to -94‰; sample RES4-1 ranges from -66 to -83‰; sample KSL values range from -68 to -88‰. End-member δD values based on quantified ratios of clay polytypes for authigenic illite clays (1M/1Md) are -95, -85, and -88‰, for G1+G2, RES4-1 and KSL, respectively, corresponding to fluid compositions of -59 to -83‰ for a temperature of 125°C ± 25°C (based on smectite stability). This calculated fluid composition closely matches the range of modern values of surface water in this region, which have been recently analyzed. End-member δD values of detrital illitic clays (2M1) are -63, -65, and -47‰, respectively, and correspond to fluid compositions of -27 to -45‰ at 300°C. These values closely match reported values of metamorphic fluids in wall rocks adjacent to the fault zone. We conclude that fluids associated with authigenic clay formation in all analyzed samples are of meteoric origin, while detrital clays preserve a metamorphic fluid signature. New IAA gives ages that are 41.4 ± 3.4 Ma for sample G2 in the west and 24.6 ±1.6 Ma for sample RES4-1 in the east. These ages demonstrate that authigenic clay mineralization preceded modern NAF displacements. Mineralization is likely associated with the earlier contractional history along this zone, with meteoric fluid minimally penetrating the upper crustal zone 3-5 km. The present-day location of the NAF reactivated a zone of preexisting weakness that involved newly-formed (low shear strength) clay. The modern NAF is responsible for the surface exposure of relict gouge of various ages that were incorporated in the NAF, which retain the distinct signature of meteorically-derived fluid circulation in the upper crust.