Evolution of deformation mechanism and fluid flow in two pre-rift siliciclastic deposits (Pannonian Basin, Hungary)

Distinct stages of deformation and fluid flow-related diagenetic alterations are recorded in Lower Miocene sandstone and conglomerate of the Pannonian Basin, Central Europe. Multiple generations of structural elements (deformation bands, calcite dominoes and veins) as well as host rocks were investigated using petrographical, elemental and stable isotope geochemical methods together with fluid inclusion petrography and microthermometry. The integration of acquired structural and diagenetic data into a subsidence model constrains the spatial and temporal evolution of deformation mechanisms and diagenetic processes. The six investigated sites represent central and marginal areas during both the pre-rift and syn-rift phases of basin evolution. The elements of pre-rift phases were preserved at the eastern margin, where eogenetic calcite precipitated in the host rocks and in the early types of deformation bands. Their δ18OPDB isotope values from −4.3‰ to −1.9‰ paired with δ13CPDB isotope values from −3.8‰ to 1.8‰ refer to precipitation from connate marine and mixed marine and meteoric pore water. The syn-rift phases are represented by eogenetic calcite and start of shallow mesogenetic diagenetic alterations in marginal position as well as in the basin centre. The δ18OPDB and δ13CPDB isotope ratios in these calcites yielded values from −15.2‰ to −5.7‰ and from to −18.5‰ to -–1.6‰, respectively. The gradual depletion in heavy isotopes shows positive covariance as the deformation progressed in time. This trend is attributed to an increasing proportion of deeply circulating meteoric fluid. The sporadic fluid inclusion data confirm meteoric fluid contribution to certain carbonate cement phases. The distinguished calcite generations in pre-rift and syn-rift structural elements and host rocks were mainly related to the phases of intense subsidence that, together with the increased rift-related heat flow, warmed up the circulating fluids. The isotope values deviating from the general trend allow the recognition of local source of light carbon contribution to basin-wide fluid-flow evolution, and cannot be tied to the switch in tectonic settings from pre-rift compression to syn-rift extension.