Abstract
Abstract. Fault kinematics can provide information on the relationship and assembly of tectonic units in an orogen. Magnetic fabric studies of faults where pseudotachylytes form have recently been used to determine direction and sense of seismic slip in prehistoric earthquakes. Here we apply this methodology to study magnetic fabrics of pseudotachylytes in field structures of the Köli Nappe Complex (central Swedish Caledonides), with the aim to determine fault kinematics and decipher the role of seismic faulting in the assembly of the Caledonian nappe pile. Because the pseudotachylyte veins are thin, we focused on small (ca. 0.2 to 0.03 cm3) samples for measuring the anisotropy of magnetic susceptibility. The small sample size challenges conventional use of magnetic anisotropy and results acquired from such small specimens demand cautious interpretation. Importantly, we find that magnetic fabric results show inverse proportionality among specimen size, degree of magnetic anisotropy and mean magnetic susceptibility, which is most likely an analytical artifact related to instrument sensitivity and small sample dimensions. In general, however, it is shown that the principal axes of magnetic susceptibility correspond to the orientation of foliation and lineation, where the maximum susceptibility (k1) is parallel to the mineral lineation, and the minimum susceptibility (k3) is dominantly oriented normal to schistosity. Furthermore, the studied pseudotachylytes develop distinct magnetic properties. Pristine pseudotachylytes preserve a signal of ferrimagnetic magnetite that likely formed during faulting. In contrast, portions of the pseudotachylytes have altered, with a tendency of magnetite to break down to form chlorite. Despite magnetite breakdown, the altered pseudotachylyte mean magnetic susceptibility is nearly twice that of altered pseudotachylyte, likely originating from the Fe-rich chlorite, as implied by temperature-dependent susceptibility measurements and thin-section observations. Analysis of structural and magnetic fabric data indicates that seismic faulting occurred during exhumation into the upper crust, but these data yield no kinematic information on the direction and sense of seismic slip. Additionally, the combined structural field and magnetic fabric data suggest that seismic faulting was postdated by brittle E–W extensional deformation along steep normal faults. Although the objective of finding kinematic indicators for the faulting was not fully achieved, we believe that the results from this study may help guide future studies of magnetic anisotropy with small specimens (<1 cm3), as well as in the interpretation of magnetic properties of pseudotachylytes.
Highlights
Pseudotachylytes are fault rocks that represent quenched frictional melts generated during coseismic slip (Magloughlin and Spray, 1992; Sibson, 1975)
The pseudotachylyte data can be compared to kinematic data from late- and post-orogenic extensional faults that crosscut the nappe architecture (Bergman and Sjöström, 1997; Gee et al, 1994), which is important to understand the relationship between the top-W shear sense late-orogenic extensional phase and brittle deformation related to pseudotachylyte formation
The observed anisotropy of magnetic susceptibility (AMS) data raise several questions: (1) If such a kinematic model does not agree with the observed fault rock AMS, what process aligned the maximum principal axes? (2) How is it possible to explain the distribution of intermediate and minimum principal axes in a girdle perpendicular to the k1 axes? (3) Why are AMS fabrics of all rock types compatible? The questions are challenging to answer but they are likely related, given that (1) the magnetic fabrics are coaxial in host rock and pseudotachylytes and (2) the petrofabric and magnetic fabric are coaxial, even though a pronounced magnetic foliation has not developed
Summary
Pseudotachylytes are fault rocks that represent quenched frictional melts generated during coseismic slip (Magloughlin and Spray, 1992; Sibson, 1975). Information about fault kinematics could offer evidence for nappe stacking dynamics along this shear zone within the Köli Nappe Complex (e.g., Bender et al, 2018). The pseudotachylyte data can be compared to kinematic data from late- and post-orogenic extensional faults that crosscut the nappe architecture (Bergman and Sjöström, 1997; Gee et al, 1994), which is important to understand the relationship between the top-W shear sense late-orogenic extensional phase and brittle deformation related to pseudotachylyte formation. It is found that the magnetic fabric reflects the petrofabric, but it does not reveal the direction or sense of seismic slip. Observations made on the magnetic properties of pseudotachylytes reveal differences in bulk susceptibility of altered and pristine pseudotachylytes. An additional insight provided in this work is that magnetic fabric studies that use small-to-very-small sample sizes (i.e., 0.2 to 0.03 cm3) need to be carefully considered given potential measurement-related artifacts
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