In situ Rb-Sr dating of slickenfibres in deep crystalline basement faults

Abstract

Establishing temporal constraints of faulting is of importance for tectonic and seismicity reconstructions and predictions. Conventional fault dating techniques commonly use bulk samples of syn-kinematic illite and other K-bearing minerals in fault gouges, which results in mixed ages of repeatedly reactivated faults as well as grain-size dependent age variations. Here we present a new approach to resolve fault reactivation histories by applying high-spatial resolution Rb-Sr dating to finegrained mineral slickenfibres in faults occurring in Paleoproterozoic crystalline rocks. Slickenfibre illite and/or K-feldspar together with co-genetic calcite and/or albite were targeted with 50 µm laser ablation triple quadrupole inductively coupled plasma mass spectrometry analyses (LA-ICP-MS/MS). The ages obtained disclose slickenfibre growth at several occasions spanning over 1 billion years, from at least 1527 Ma to 349 ± 9 Ma. The timing of these growth phases and the associated structural orientation information of the kinematic indicators on the fracture surfaces are linked to far-field tectonic events, including the Caledonian orogeny. Our approach links faulting to individual regional deformation events by minimizing age mixing through micro-scale analysis of individual grains and narrow crystal zones in common fault mineral assemblages.

Using 87Sr/86Sr LA-MC-ICP-MS Transects within Modern and Ancient Calcite Crystals to Determine Fluid Flow Events in Deep Granite Fractures

“…The events started with Paleoproterozoic mylonitization followed by Mesoproterozoic, Neoproterozoic, Silurian-Carboniferous, Permian, and Jurassic fracture reactivation, featuring successively lower formation temperatures of the fracture coating minerals [23,[34][35][36]39,57,58]. Fracture reactivation was related to far-field orogenic events [57,59], as confirmed by high spatial resolution Rb/Sr and/or U-Pb dating of both slickenfibre minerals [36], veins [34], and bulk sample 40 Ar/ 39 Ar dating [39,60]. Bulk sample 87 Sr/ 86 Sr analysis has shown an overall evolution from low values of Proterozoic calcite veins (from~0.7070) to much higher ratios (up to~0.7175-0.7185) in coatings of calcite in currently open and water-conducting fractures [23,32,61].…”

“…We connect the calcite Sr isotope compositions with those of modern groundwater to determine whether the youngest of these calcite growth zones could be precipitated from the present-day waters. We compare and link the distinguished generations with published geochronological constraints [34,[36][37][38][39] of the same fracture assemblage to assess the overall temporal span of the precipitation history in presently water-conducting fractures. We also investigate the 87 Sr/ 86 Sr variability of modern calcite precipitated from fracture water in a borehole at 415 m depth over the course of 17 years, and compare these 87 Sr/ 86 Sr values with the modern source water.…”

Resolving multiple geological events using in situ Rb-Sr geochronology: implications for metallogenesis at Tropicana, Western Australia

“…Following the work of Zack and Hogmalm (2016) and Hogmalm et al (2017) for assessing the most suitable reaction cell gases, several publications have attempted to solve geological problems using the in situ Rb-Sr technique (Şengün et al, 2019;Tillberg et al, 2017;Tillberg et al, 2020). All these studies, except the one from Tillberg et al (2020), identified only a single age population within individual samples, which could have been resolved (at higher precision) with solution Rb-Sr or 40 Ar/ 39 Ar. Tillberg et al (2020) observed multiple age populations in their samples, but these were from mineral separates and the textural context was not preserved.…”

“…All these studies, except the one from Tillberg et al (2020), identified only a single age population within individual samples, which could have been resolved (at higher precision) with solution Rb-Sr or 40 Ar/ 39 Ar. Tillberg et al (2020) observed multiple age populations in their samples, but these were from mineral separates and the textural context was not preserved. To date, no published study has taken full advantage of the spatial resolving power of the in situ Rb-Sr technique whilst retaining textural context.…”

Syncing fault rock clocks: Direct comparison of U-Pb carbonate and K-Ar illite fault dating methods

“…Fault histories are also important for building regional tectonic frameworks, understanding ore genesis, forming basin evolution models, and studying seismically active faults that pose geohazards and engineering challenges (e.g., Zwingmann et al, 2010). Fault timing is typi-cally constrained through cross-cutting relationships, but is more precisely achieved by direct dating of fault-generated materials such as fault gouge, slip-surface hematite, opal, pseudotachylyte, and slickenfibers (Nuriel et al, 2012(Nuriel et al, , 2019aTagami, 2012;Ault et al, 2015;Tillberg et al, 2020).…”