Constraining the timing of brittle deformation and sedimentation in southern Finland: Implications for Neoproterozoic evolution of the eastern Fennoscandian shield

Abstract

Sheared sedimentary clay was found in a faulted fracture in crystalline bedrock in a tunnelling site at 60 m depth in southern Finland. Brittle faults are numerous in the Fennoscandian Palaeoproterozoic bedrock, but only some of them have relative age constraints, while absolute ages are nearly lacking. Sedimentary rocks altogether are uncommon in Finland and only sparsely dated by micropaleontological studies. This study reports K-Ar data of fresh, non-weathered authigenic illite and constrains a time framework for the local faulting and sedimentation. The Neoproterozoic Tonian to Cryogenian ages derived from the grains in diminishing grain-size order are c. 967, 947, 809 and 697 Ma. Results indicate that the formation of the extension fracture is related to the collapse of Sveconorwegian orogeny c. 1000 Ma; clay and mature quartz sand were deposited in this extension fracture in shallow water in an intracratonic basin followed by early diagenetic processes and neocrystallization of illite around 967-947 Ma. The Neoproterozoic 1000-700 Ma sedimentation documented in this study is rare in the Fennoscandian shield as a whole. Neocrystallization of authigenic illite in the finest 0.4 and <0.1 µm fractions c. 809-697 Ma ago is interpreted as resulting from reactivation in the fault due to the continental break-up on the western side of the craton as documented by arenite crosscutting relationships. The younger ages may also be attributed to a Caledonian thermal overprint ca. 410 Ma ago that would influence the 967 Ma age if sufficient thermal energy had been present.

Early Cambrian small carbonaceous fossils (SCFs) from an impact crater in western Finland

“…Sediments towards the outer rim of the Lappajärvi crater, like those of the Vimpeli‐302 core, however, have thus far remained assigned to a Mesoproterozoic age (Vaarma & Pipping ; Schmieder & Jourdan ; Elminen et al . ).…”

Precambrian faulting episodes and insights into the tectonothermal history of north Australia: microstructural evidence and K–Ar, &lt;sup&gt;40&lt;/sup&gt;Ar–&lt;sup&gt;39&lt;/sup&gt;Ar, and Rb–Sr dating of syntectonic illite from the intracratonic Millungera Basin

“…The combined West and North Australian cratons were joined to the South Australia Craton along the Albany-Fraser Orogen. The Musgravian Orogen in central Australia was responsible for substantial crustal thickening and high-grade metamorphism at ∼ 1200-1150 Ma associated with granite intrusion (Evins et al, 2010;Kirkland et al, 2013). Thereafter, uplift and erosion were followed at ∼ 1080 Ma by the deposition and of post-tectonic volcanism, accompanied by the intrusion of widespread plutons (Giles Complex) during the extension along the former collision zones (Schmidt et al, 2006;Evins et al, 2010;Aitken et al, 2013).…”

“…7) may be related to the regional extension and associated major thermal event that occurred across Australia at 1120-900 Ma (e.g. Musgrave Orogeny and subsequent Giles Event) due to interactions between Australia and other continents during the assembly of the supercontinent Rodinia (De Vries et al, 2008;Li et al, 2008;Evins et al, 2010). This Australia-wide tectonothermal event that largely developed along former (Mesoproterozoic) collision zones led to the emplacement of Figure 12.…”

“…widespread dyke swarms, sills, and associated granite plutons in the Central and North Australia cratons largely in a time frame between ∼ 1040 and ∼ 1090 Ma (Schmidt et al, 2006;Evins et al, 2010;Aitken et al, 2013). The Musgrave Orogeny involving the widespread emplacement of granite and mafic-ultramafic bodies was recorded in central Australia at 1220 and 1120 Ma (Evins et al, 2010;Kirkland et al, 2013). Major swarms of dolerite intrusions in the North Australian Craton are dated at 1116 ± 12 Ma (Lakeview Dolerite; Tanaka and Idnurm, 1994), and associated hydrothermal events were recorded in Mount Isa Province (adjacent to the study area; Fig.…”

“…Although unknown fault systems buried under thick sedimentary basins can be denoted by geophysical techniques such 2D and 3D seismic reflections, cores from boreholes or tunnel sites intersecting fault zones can be used to date fault reactivation episodes (e.g. Viola et al, 2013;Yamasaki et al, 2013;Elminen et al, 2018). The current study investigates fault rocks and the host sandstone intersected in drill cores from the newly discovered Millungera Basin in northern Queensland, north-central Australia (Fig.…”