The influence of geomechanical properties on rock strength in the ICDP COSC-2 borehole, at Are, Sweden

How can rocks obtained by scientific drilling increase our understanding of deformation, stress, and strength in one of Earth’s classic collisional orogenic belts? By integrating scientific drilling data with high-resolution laboratory measurements, this study presents a combined structural, mineralogical and geomechanical characterization of the Scandinavian Caledonides, based on data from the COSC-2 borehole acquired during the ICDP logging campaign in 2022 . From the surface to approximately 1200 m depth, the borehole intersected an extensive Early Phanerozoic sedimentary succession, dominated primarily by wacke, shale and siltstone. Beneath this succession, extending to 2276 m, lies a crystalline basement comprising a volcanic sequence, including porphyry, gabbro and gabbroid rocks, intruded by dolerite dykes. The contact between the sedimentary succession and the crystalline basement is relatively undisturbed, with a thin regolith covering the altered top of the porphyry.

A key objective of our study is to investigate the physical properties and in-situ stress state of the COSC-2 rocks using laboratory tests on selected core samples. Specifically, we examine how stress magnitudes vary with depth, which stress regime dominates the area, how rock stiffness varies with lithology, mineralogy, and depth, and whether laboratory-derived elastic properties are consistent with downhole sonic log measurements (Vp and Vs). To address these questions, a suite of laboratory measurements was conducted on 19 core samples, including Brazilian tensile strength (BTS), uniaxial compressive strength (UCS), P- and S-wave velocities, Poisson’s ratio, Young’s, bulk, and shear moduli, grain and bulk density, and quantitative mineralogical analyses using X-ray diffraction (XRD) and electron microprobe analysis (EPMA). Our findings show that crystalline rocks exhibit in general a higher stiffness and compactness, reflected in elevated wave velocities and elastic moduli, combined with greater densities and lower porosity resulting in greater mechanical strength, both in compression and tension loading. This behaviour is reflected in specific samples, which record some of the highest BTS and UCS values. In contrast, three samples in doleritic and gabbroic rocks display unexpectedly low BTS values (19–20 MPa) and UCS values (180–211 MPa) compared to the other crystalline basement samples. Analysing the mineralogical composition, we found the presence of primary and secondary phyllosilicates in these rocks, which likely weaken the rock fabric and can be responsible for the reduced strength. In contrast, the overlying sedimentary rocks exhibit lower stiffness and strength but greater variability largely controlled by porosity and internal heterogeneity.

Of course, such geomechanical properties are also controlled by the presence of microcracks, open and cemented veins, mineral alignment and the precipitation of secondary minerals reflecting enhanced fluid flow and fluid-rock interaction processes. Especially the occurrence of secondary mineral phases identified through XRD and EMPA further reveal a complex tectono-metamorphic history. Together, these findings provide a solid framework for geomechanical modelling and advance our understanding of the evolution of collisional orogens.