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.
Integrated microHall magnetometer to measure the magnetic properties of nanoparticles