Calcite U-Pb ages constrain Dead Sea Fault activity

Time constraint on early phase of brittle fault activity (>1 Ma) are seldom determined or involves very high uncertainties, thereby limiting our understanding of the long-term deformational history of faults and fault systems in the brittle environment. For the long-lived Dead Sea Fault (DSF) system, constrain on the beginning of left-lateral motion are determined indirectly by K-Ar age of ca. 20 Myr old dikes from Sinai that are offset by the DSF as much as Precambrian basement rocks. The use of U-Pb dating system allows us to see beyond the past 500 kyr, and important tectonic questions can be approached for the first time. Traditional U-Pb bulk analyses for carbonate samples require high clean lab standards and precious Pb spike (205Pb) that are rarely available. In this study we utilize 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 in situ approach offers several advantages over the traditional bulk analysis and samples are preserved for additional analyses. We combine U-Pb chronology with calcite strain analyses, measured on twined calcite, to demonstrate how the DSF strain field changes in space and time. The method is applied to fault-related calcite samples such as breccia cement, fault-coating, and vein-fill taken from different sites along the DSF. Preliminary results from the DSF indicate possible propagation and localization of fault activity in space and time. The southern part of the DSF (Gishron Fault, Eilat) was active by 18 Ma while the northern part of the DSF in Israel (Neve Ativ Fault) was active by 15 Ma. In addition, the western Gishron Fault was active before the more eastern Shelomo Fault (18 and 14 Ma, respectively). Calcite strain analyses indicate DSF-related strain field with NNW direction of σ1 and a secondary direction towards N-S (N=49). The two directions may represent the far DSF-related strain field (NNW) and the local "weak" subparallel direction (N-S). Significance and broader impacts of this study lays in future neotectonic studies and its potential to provide (1) age constraints on fault initiation or early phases of activity over million years timescales; (2) knowledge of the deformational history of a specific structure (e.g., multiple events); and ultimately, (3) improved accuracy of dated deformation events by employing a direct approach. In addition, the U-Pb in situ dating method can be applied to other low-U carbonate samples (<1 ppm), from different tectonic settings (e.g. speleothems, hydrothermal, sedimentary) and contribute to paleoclimate and stratigraphy studies.


תאריך 24/01/2016 10:30 12:00
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