Magnetic properties of pseudotachylytes, Jämtland, central Sweden

Magnetic properties of pseudotachylytes, Jämtland, central Sweden Hagen Bender, Bjarne S.G. Almqvist, Amanda Bergman, Uwe Ring Department of Geological Sciences, Stockholm University , 106 91 Stockholm, Sweden Department of Earth Sciences, Geophysics, Villavägen 16, 752 36 Uppsala, Sweden Ramböll Sverige AB, Box 17009 , Krukmakargatan 21, 104 62 Stockholm, Sweden 5 Correspondence to: Hagen Bender (hagen.bender@geo.su.se)

perpendicular to S hr . Due to small spatial distribution of host/fault rock, specimen cubes are unconventionally small (side length 5.3 ± 1.2 mm, volume 0.17 ± 0.13 cm 3 ; uncertainty levels here and throughout the manuscript are 1σ) compared to standard-size specimens (7 to 11 cm 3 ) used in paleomagnetism (Table S1). Therefore, shape and size of cube dimensions were compared to properties of the AMS ellipsoid and uncertainties related to the cube dimension were also investigated. 95 Despite expending particular care in avoiding to cut specimens with different types of host/fault rock, specimens with mixed rock type occur. Approximate modal proportions for each rock type per specimen is presented in Table S2 in the electronic supplementary material.

4.2
Magnetic properties

Anisotropy of magnetic susceptibility 100
Anisotropy of magnetic susceptibility (AMS) was measured using a MFK1-FA susceptibility bridge (Agico, Inc.) operated at 200 A/m alternating current field and 976 Hz frequency. Orientation parameters used for data acquisition with Safyr4W software were P1 = P3 = 6 and P2 = P4 = 0 so that specimen x-axes plunge parallel to L hr and specimen z-axes point upward perpendicular to S hr ("Safyr4W User Manual," 2011). The AMS is expressed by the orientation and magnitude of the principal axes of susceptibility k 1 ≥ k 2 ≥ k 3 . Further parameters describing AMS data include the mean susceptibility 105 k m = (k 1 + k 2 + k 3 ) / 3, , magnetic foliation F m = k 2 / k 3 , magnetic lineation L m = k 1 / k 2 and Jelinek's parameter for the degree of anisotropy P j (Jelinek, 1981). The shape of the susceptibility ellipsoid is described by . Only at T = +1 and T = −1 is the AMS ellipsoid oblate and prolate, respectively. For 0 < T < +1, the AMS ellipsoid is more oblate than prolate; for 0 > T > −1, it is in contrast more prolate (Jelinek, 1981). Mean susceptibility k m has been normalized for specimen volume. 110 For data visualization, specimens containing more than one host/fault rock type were plotted based on their modal composition. The specimens were accounted to the dominant host/fault rock type composing the specimen. One specimen containing three rock types and 12 specimens composed of two rock types with each 50% mode were considered as mixed analyses. These data are therefore only presented in the data tables but were excluded from orientation and parameter analysis of AMS data. 115

Frequency dependence of susceptibility
Frequency-dependent magnetic susceptibility was measured using a MFK1-FA susceptibility bridge (Agico, Inc.) operated at 200 A/m AC field and frequencies of F 1 = 976 Hz, F 2 = 3904 Hz, F 3 = 15 616 Hz. In order to minimize the effect of anisotropy, all measurements were performed with the sample cubes oriented in the same position with their positive x-axes horizontally pointing toward the operator (POS. 1 in "Safyr4W User Manual," 2011). Frequency dependence of 120 susceptibility is characterized by the parameter X FD = 100(X LF − X HF ) / X LF (%) (Dearing et al., 1996). Frequency dependence is used to identify superparamagnetic magnetite (grain sizes typically <20 nm), as this phase generally shows variation in bulk susceptibility as a function of field frequency. The method was here used to answer the question if very fine-grained magnetite formed during partial melting and recrystallization associated with the fault-slip that formed the pseudotachylyte. 125

Temperature dependence of susceptibility
Temperature dependence of magnetic susceptibility was measured using a MFK1-FA, equipped with a CS4 furnace. Six sample cubes were analyzed individually; two sample cubes were analyzed together (AB15-13 and AB15-61) because of their small volumes. The samples were ground to a powder with an agate mortar, being careful not to contaminate the sample with outside iron particles or magnetic phases from other materials. Magnetic susceptibility measurements at 200 A/m AC 130 field and 976 Hz frequency were conducted from room temperature up to 700°C, and subsequently cooled back to room temperature, with a heating/cooling rate of 11.8 °C/min. Specimen AB15-67 was measured in air; all other specimens in argon atmosphere. Thermomagnetic data of the empty furnace were smoothed (5-point running mean) and subtracted from the sample thermomagnetic data using the Cureval8 software (Agico, Inc.).

Hysteresis 135
Hysteresis loops were performed with a LakeShore vibrating sample magnetometer with a maximum applied field of 1 T.
Data processing was performed with the MATLAB toolbox HystLab (Paterson et al., 2018), using a linear high-field slope correction and automatic drift correction. The hysteresis data was normalized by the mass of the specimen. The extracted hysteresis parameters included saturation magnetization (M s ), saturation remanent magnetization (M rs ) and coercivity (H c ).

Shear sense determination using AMS 140
Obliquity between shear plane and magnetic fabric may be used to determine the sense of slip. Progressive shearing rotates maximum and intermediate principal axes of strain and AMS toward the shear plane (Borradaile and Henry, 1997).
Kinematics are indicated in a plane perpendicular to the shear plane (i.e., fault vein margins) that contains the minimum and maximum AMS axes (cf. Figure 26 in Borradaile and Henry, 1997;and Figure 3 in Ferré et al., 2015). In this case, magnetic foliations are inclined toward the slip direction, which gives the sense of shear.  (Figure 4a, Figure 5b). Ilmenite breakdown to Ti-oxide is observed at grain boundaries with biotite ( Figure 5b).
Boundaries between the brittlely undeformed and fractured host rock or fault or injection veins are sharp (Figure 4b). In the fractured host rock, alteration of biotite is more pronounced.

5.2
Fault rock microstructure and petrography 155 Cataclastic fault rock appears bright in thin section and consists of granular lithic and mineral fragments (Figure 4c). It forms bulky to drawn-out patches that grade into compositional flow banding in fault veins mainly composed of pseudotachylyte Only fault rock with microstructural evidence for frictional melting is considered as pseudotachylyte. Such structural 160 evidence includes microcrystallites, sulfide/oxide droplets and spaced survivor clasts, which may display embayed edges witnessing their partial melting (Magloughlin and Spray, 1992;Kirkpatrick and Rowe, 2013). All of these features are Rock magnetism results

Anisotropy of magnetic susceptibility
AMS data for all specimens are summarized in Table 1 and graphically presented in Figure 6. Magnetic anisotropy in host rock and fault rock specimens displays consistent orientations of principal axes. Maximum principal axes (k 1 ) trend E-W and 185 are subparallel the host rock lineation for all rock types ( Figure 6). Generally, all rock types show prolate AMS symmetry as indicated by distribution of intermediate (k 2 ) and minimum principal axes (k 3 ) in a girdle perpendicular to k 1 . Furthermore, shapes and orientations of the 95% confidence regions for mean k 2 and k 3 axes reflect the prolate AMS shape (Figure 6a, cf). Symmetry of these confidence regions indicates homogeneous AMS fabrics for the analyzed specimen groups (Borradaile and Jackson, 2010). However, intermediate and minimum principal axes for host rock specimens occur in two clusters 190 ( Figure 6a). One cluster has k 3 axes perpendicular to the host rock foliation and k 2 axes lying within the foliation plane ( Figure 6b). The corresponding sub-fabric AMS ellipsoid approaches oblate shape (T = 0.21 ± 0.19). The magnetic foliation expressed by these specimens is subparallel to the schistosity S hr . In the second cluster, k 2 and k 3 axes are inversely oriented.
Measurements of anisotropy (P j , T) scatter over similar ranges for all rock types ( Figure 7a). The anisotropy degree P j shows highest variation for host rock specimens (1.02 < P j < 1.45); lowest for altered pseudotachylyte (1.06 < P j < 1.25). However, 195 the median P j values are similar in all rock types (1.1 < P j < 1.2) and the middle 50% of these data overlap, when shown in box-and-whisker plots ( Figure 7b). The symmetry of the magnetic fabric shows no co-variation with the degree of anisotropy ( Figure 7a). Shapes of AMS ellipsoids for individual specimens of all rock types range from oblate to prolate ( Figure 7c).
Overall, neither degree nor shape of the AMS ellipsoid define a magnetic fabric distinctive for one rock type or a group of several rock types. Nevertheless, the volume-normalized mean susceptibility of altered pseudotachylyte specimens is 200 approximately twice as high (median k m = 4.7 × 10 −3 [SI]) as that of all other rock types (median k m = 2.7 × 10 −3 [SI]; Figure   8).

Temperature dependence of magnetic susceptibility
Thermomagnetic curves for heating and cooling of host rock, as well as for pristine and altered pseudotachylyte are presented in Figure 9a-c. With increasing temperature, host rock thermomagnetic data exhibit steadily decreasing magnetic 205 susceptibility, followed first by a rapid increase to about twice the initial value at c. 500 °C, and then followed by a rapid decrease at c. 580 °C (specimens AB15-115, AB15-116; Figure 9d). During cooling, host rock specimens show a prominent rise in susceptibility at temperatures <600 °C and a peak at c. 430 °C. Pseudotachylyte specimens (AB15-12, AB15-13/61, AB15-62) show a small but noticeable drop in susceptibility at 550-590 °C (Figure 9e). During cooling, susceptibility rises sharply for all pseudotachylyte specimens at temperatures <590 °C (Figure 9b). Altered pseudotachylyte exhibits 210 progressively decreasing susceptibility with increasing temperature without any significant drop (specimens AB15-43, AB15-67; Figure 9f). During cooling, susceptibility progressively increases to a peak at c. 300 °C and then gradually https://doi.org/10.5194/se-2019-128 Preprint. Discussion started: 21 August 2019 c Author(s) 2019. CC BY 4.0 License. decreases again. For specimens AB15-43, there is a small sharp increase of susceptibility at 590 °C observed in the cooling curve.

Hysteresis loops 215
Magnetic hysteresis measurements show all rock types respond dominantly paramagnetically to applied high magnetic fields (Table 2, Figure 10a, e, i). Hysteresis results for pseudotachylyte-free specimens show either no or very minor ferromagnetic response. They have saturation magnetizations (M s = 2.3 ± 1.3 × 10 −4 Am 2 kg -1 ) about one order of magnitude below those specimens containing pseudotachylyte (M s = 1.73 ± 0.6 × 10 −3 Am 2 kg -1 ) ( Table 2). Furthermore, pseudotachylyte-free specimens have generally very open slope-corrected hysteresis loops, which do not display branches characteristic of 220 ferromagnetic minerals (Figure 10b, j) (cf. Paterson et al., 2018). Slope-corrected hysteresis curves for these specimens accordingly also display atypical shapes, which may result from an artificial correction to the data (Figure 10c, k).

Specimen size and shape 230
Specimen cube dimensions deviate moderately from neutral shapes. Their long edges are between 4.1% and 20.9% longer than their short edges. Prolate and oblate shapes are equally common ( Figure 11a, Table S1). The shape parameters of specimen dimensions (T d ) and magnetic anisotropy (T) are independent of each other ( Figure 11b, Table S1). The degree of anisotropy of specimen shape and magnetic susceptibility show no significant correlation ( Figure 11c).
Raw measurements of mean susceptibility (k m ) and anisotropy degree (P j ) are inversely proportional ( Figure 12a). 235 Additionally, the standard error of k m decreases with increasing specimen volume ( Figure 12b). Consequently, the AMS data are dependent on specimen size. Small specimen volumes result in larger uncertainties, which in turn causes higher P j values. This observation is further discussed in section 7.5 which also discusses the limitation of specimen size in studies using AMS.

Source of magnetic susceptibility and its anisotropy
Thermomagnetic heating curves for host rock specimens show decrease in magnetic susceptibility with increasing temperature until 400 °C, which is characteristic of paramagnetic behavior ( Figure 9d) (Hunt et al., 1995). Formation of new magnetite at temperatures above 400 °C is indicated by the peak and sudden decrease in magnetic susceptibility at 580 °C, the Curie temperature of magnetite (Hunt et al., 1995). These results, together with magnetic hysteresis data (Table 2, Figure  245 10), show that the magnetic susceptibility of the host rock micaschist arises from paramagnetic minerals. It follows that the AMS in the host rock is controlled by the crystallographic orientation of the paramagnetic minerals (Borradaile and Jackson, 2010). An AMS sub-fabric in host rock specimens has parallel magnetic and mineral lineations and sub-parallel magnetic and ductile foliations ( Figure 6b). Shape-preferred orientation of tabular biotite crystals in the host rock ( Figure 4a) implies crystallographic c-axes of biotite are oriented perpendicular to the schistosity. This AMS sub-fabric is therefore inferred to 250 originate from crystallographic preferred orientation of biotite, which in single crystals exhibits k 3 axes subparallel to biotite crystallographic c-axes (Borradaile and Henry, 1997;Martín-Hernández and Hirt, 2003). The mean magnetic susceptibility  (Table   1, Figure 8), which is a common approach to separate AMS sub-fabrics caused by paramagnetic and ferromagnetic minerals (Borradaile and Jackson, 2010). The presence of magnetite does not seem to increase k m to values significantly higher than 265 the (fractured) host rock and/or cataclasite specimens ( Figure 8). The ferromagnetic contribution to the pseudotachylyte AMS is consequently small. The pseudotachylyte AMS is therefore likely controlled by crystallographic preferred orientation of its paramagnetic minerals, that is most probably biotite, with a subordinate contribution from the shape preferred orientation of magnetite (cf. section 5.2).
In altered pseudotachylyte specimens, successive decrease in magnetic susceptibility without significant drop at 580 °C 270 during heating indicates dominant paramagnetic behavior. This behavior suggests that magnetite present in pristine pseudotachylyte has been altered to an unknown phase in chloritized pseudotachylyte (Figure 9f) Figure 8b), the AMS of altered pseudotachylyte apparently has an additional or a different mineral source than the other rock types. Bulk magnetic 275 susceptibility for single-crystal chlorite without high-susceptibility mineral inclusions is about twice that of biotite and muscovite single crystals (Martín-Hernández and Hirt, 2003). These sheet silicates were also argued to collectively cause AMS in host rock specimens, but in altered pseudotachylyte chlorite is much more abundant, making up to c.
replacement of biotite to chlorite in fractured host rock domains indicate that hydrothermal alteration was associated with 305 faulting. The chlorite microstructure suggests that recrystallization was static ( Figure 5). After pseudotachylyte formation, ambient temperature conditions in the fault zone are therefore inferred to be of lower greenschist-facies (cf. Di Toro and Pennacchioni, 2004;Kirkpatrick et al., 2012). We deduce seismic faulting and subsequent alteration of fault rocks in the Finntjärnen fault zone occurred in the brittle-ductile transition zone near the base of the upper crust. Assuming the typical temperature range of 300-350 °C, and depending on the thermal gradient, the faulting occurred at c. 12 ± 4 km depth (cf. 310 Sibson and Toy, 2006).
Brittle faults and fibrous calcite + quartz veins crosscut both the ductile host rock fabric and the fault veins at high angles.
Their orientations relative to the fault vein geometry, together with microscopic and macroscopic observations (Figures 2-5), suggest that these E-W extensional structures formed latest. These structures are consistent with other extensional structures related to the Røragen Detachment west of the Tännforsen Synform ( Figure 1) (Gee et al., 1994;Bergman and Sjöström, 315 1997). In summary, seismic faulting in the Finntjärnen fault zone occurred after the formation of the upper greenschist-/amphibolite-facies schistosity, and prior to late-stage E-W extensional brittle structures. Structural overprinting relations imply transport of thrust sheets in the Köli Nape Complex during exhumation of these nappes from the middle to the upper crust. The sense of faulting can, however, not be deduced from the here presented data. Nevertheless, previous work in the area indicated that thrusting was toward the ESE (Bergman and Sjöström, 1997;Bender et al., 2018). 320

Regional tectonic implications
Structural

Methodological remarks on AMS of small specimens 330
There is an apparent inverse relationship between k m and P j , as well as a linear relationship between degree of anisotropy and standard error. This effect is caused by specimen size. The larger specimens (by volume) have in general higher bulk susceptibility, and P j tends towards lower values ranging from 1.01 up to 1.10. Normalization for specimen volume has little impact in removing this bias and it is therefore evident that specimens with very small size are more likely to produce a large scatter in the degree of anisotropy. Although this is an undesired artifact, it demonstrates the limitation of using small sample cubes in the current setup with the MFK1-FA system. The effect is furthermore emphasized by the increase in k m standard error as a function of P j .
Observations of magnetic anisotropy made in this study raises the issue of measuring AMS of specimens with small volume.
Current equipment that exists commercially are not designed for handling small specimen volumes and in most applications the intended volume ranges from 7 to 11 cm 3 (representing standard size cubes and cylinders used in paleomagnetic and 340 AMS studies). However, there is a growing interest for measurements of small specimens, as many AMS studies target geological structures that occur on the cm to sub-cm scale (e.g., Ferré et al., 2015). One of the challenges in using smaller specimens is clearly an increased uncertainty in manufacturing specimens that have appropriate dimensions. However, specimens can be constructed with care to compensate for this effect, and in this study, we have demonstrated that the nonequidimensional effect is secondary in importance to the specimen volume. Furthermore, our AMS data show a consistent 345 magnetic fabric in the different rock types, which suggests that they most likely represent the true rock fabric (although the magnitudes are variable).

Conclusions
Field, microstructural and magnetic fabric data from the Finntjärnen fault zone provide the following constraints on seismic faulting recorded by pseudotachylyte-bearing fault veins: 350 (1) Structural overprinting relations show seismic faulting occurred during exhumation of the Köli Nappe Complex into the upper crust within the seismic zone, and before brittle E-W extension.
(2) Neither the petrofabric nor magnetic fabrics reveal the coseismic slip direction.
(3) Chloritization of pseudotachylyte resulted in higher bulk magnetic susceptibility as compared to pristine pseudotachylyte. The very low amount of magnetite in pseudotachylyte, although detectable by its magnetic 355 behavior based on thermomagnetic curves, hysteresis loops and bulk susceptibility, does not contribute substantially to the pseudotachylyte bulk magnetic behavior. (4) Unconventionally small specimen size increases the degree of anisotropy of magnetic susceptibility measurement data. Magnetic anisotropy results in small specimens demand cautious interpretation, but offers a promising new venue to study detailed geological features. 360

Data availability
All data that led to the conclusions of this paper are presented in the figures, tables and supplementary material.  Please see separate file 'TableS3HysteresisRawData.xlsx'

Author contribution 370
Field work was carried out by HB and AB. HB and BSGA conducted the magnetic experiments, processed and interpreted the results. HB created figures and tables and wrote the initial draft, which was edited by all co-authors.

Competing interests
The authors declare that they have no conflict of interest.     Figure 6. (a-f) Lower-hemisphere, equal-area plots for principal axes of magnetic anisotropy in different rock types. Comments about data presentation: (a) The measurement for specimen AB15-75 was excluded because it was considered as outlier due to its high k m (cf. Table 1). (e) All data for specimens containing ≥50% pseudotachylyte was plotted. (f) All data for specimens with 475 containing ≥50% altered pseudotachylyte was plotted.    /kg]