Mesoproterozoic Strike-Slip Faulting within the Åland Rapakivi Batholith, Southwestern Finland

Abstract

Paleostress inversion analysis of outcrop data from brittle fault structures within the Mesoproterozoic 1.58 Ga Åland rapakivi granite, southwestern Finland, revealed two separate strike-slip faulting stages. Stage 1 is dominated by dextral slip along E–W-trending faults under WNW–ESE to NNW–SSE compression, whereas Stage 2 displays less prominent faulting localized in an orthogonal network of N–S and E–W trending faults that developed under NE–SW compression. Relative age constraints indicate that faulting occurred between 1.58 and 0.5 Ga, and further correlation with previously published results indicate a 1.55–1.4 Ga age for Stage 1 faulting, while Stage 2 is compatible with previously described fault reactivations between 1.3–1.2 Ga. To place the results of the fault analyses in a wider framework, we conducted a regional structural interpretation using bathymetric, topographic, and geophysical datasets and reviewed previously published results. Based on the above, we attribute the emplacement of the 1.6–1.5 Ga rapakivi granites and the subsequent development of the Mesoproterozoic sedimentary basins to the reactivation of inherited Paleoproterozoic shear zones during Mesoproterozoic crustal extension. As such, this study contributes towards understanding the relationships between magmatism and strain localisation in continental (failed) rift settings.