SPATIOTEMPORAL EVOLUTION OF BRITTLE FAULT ACTIVITY BASED ON STRAIN ANALYSIS AND IN SITU U-PB AGES OF SYNTECTONIC TWINNED CALCITE

Direct dating of brittle fault activity and inferring the associated strain directions are of fundamental importance to structural and tectonic reconstructions. One approach to address these issues is by constraining the ages of syntectonic calcites in fault zones and obtaining strain directions from them. We present a novel methodology consisting of micro-scale strain analysis (obtaining strain directions from mechanical twins in syntectonic calcite), precise U-Pb dating of the same calcite precipitates, meso-scale analyses (e.g., strain inversion from fault kinematics) and integration of the results. We utilize the latest advances in Laser Ablation techniques (LA) combined with multi-collector inductively coupled plasma mass spectrometry MC-ICPMS to analyze calcite material without chemical separation (in situ). The use of U-Pb dating system allows to constrain early phases of brittle fault activity (>1 Ma). This methodology has been applied to study the tectonic activity of the Dead Sea Fault (DSF), the active plate boundary between the Arabian and African plates. We mainly focus on (1) defining the initiation, localization and propagation of the DSF over time; (2) quantifying variations in the strain field within fault zones and around fault strands; and (3) estimating the amount of “weakness” of the DSF based on the alignment (or misalignment) of the maximum shortening direction with respect to the strike of the studied faults The results indicate that left-lateral motion commenced by 20 Ma along several sub-parallel faults within a ~8 km wide deformation zone in southern Israel. Lateral motion most likely propagated northward and by 15 Ma a ~N-S plate boundary was well-developed. Calcite strain analyses indicate the primary DSF-related strain field with a NNW direction of maximum horizontal shortening and a secondary direction oriented N-S. The two directions may represent the distal DSF-related strain field (NNW-SSE) and the local 'weak' subparallel direction (N-S) that prevailed during the early stages of DSF evolution. We conclude that age-constraint strain analyses are an important aspect in assessing the activity along the DSF, and argue that a similar methodological approach could potentially shed light on the spatiotemporal evolution of activity in other brittle fault zones.