Tectonophysics 479 (2009) 203–213

Contents lists available at ScienceDirect

Tectonophysics

j ourna l homepage: www.e lsev ie r.com/ locate / tecto

Secondary magnetizations in shear and fault zones in southern Finland

Ulla Preeden a,⁎, Satu Mertanen b, Tuija Elminen b, Jüri Plado a

a Department of Geology, Institute of Ecology and Earth Sciences, University of Tartu, Vanemuise 46, 51014, Tartu, Estoniab Geological Survey of Finland, Betonimiehenkuja 4, FIN-02151, Espoo, Finland

⁎ Corresponding author. Fax: +372 7375834.E-mail address: ulla.preeden@ut.ee (U. Preeden).

0040-1951/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.tecto.2009.08.011

a b s t r a c t

a r t i c l e i n f o

Article history:Received 10 March 2009Received in revised form 6 August 2009Accepted 11 August 2009Available online 20 August 2009

Keywords:PaleomagnetismFault zonePrecambrianLate PaleozoicBalticaFinland

Studies on remanent magnetization of shear and fault zones of crystalline rocks in southern Finland havebeen carried out to test juvenile fluid activity that can be dated paleomagnetically. Locating in the centralpart of Baltica, the area has been tectonically relatively quiet since the Precambrian. However, tectonism atthe margins of the craton during the Caledonian, Hercynian and Uralian orogenies probably reactivated faultsand resulted in low-temperature hydrothermal activity also within the craton. We identified two consistentremanent magnetization components in four (out of five) studied localities. One remanence component hasan intermediate to high coercivity NNW directed magnetization (D=334.7°; I=35.2°; k=92.1; α95=9.6°)and unblocking temperatures suggest that it resides in (titano)magnetite. The direction is similar to theremanence that is commonly observed in Svecofennian age (ca. ~1.9 Ga) rocks all over the PrecambrianFennoscandian Shield, and probably represents the primary Svecofennian magnetization preserved in thehost rocks. Another common component is directed NE (I=30.8°; D=35.3°; k=264.5; α95=5.7°) and iscarried by hematite and maghemite. Its resulting paleomagnetic pole compares well with Phanerozoicpaleopoles of Baltica, and is suggested to represent a Permian remagnetization. We interpret that thisremanence reflects the reactivation of the Svecofennian crust in the late Paleozoic due to tectonic eventsrelated to the formation of the supercontinent Pangea. In addition, remagnetizations due to Mesoproterozoicrapakivi magmatism and younger (Mesozoic to present) events were identified.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Shear zones have recorded post-formational remagnetizationprocesses in the Paleozoic Caledonides of Norway (Torsvik et al.,1992; Eide et al., 1997; Andersen et al., 1999). Paleomagnetic studieson fault and shear zone rocks cutting the ~1.88 Ga Svecofennian crusthave demonstrated also in southern Finland the presence of secondarymagnetizations that reflect successive geologic processes (Mertanenet al., 2004, 2008). These components were shown to reside indifferent remanence carrying minerals, possibly implying multiplereactivations of the zones at 1.63–1.58 Ga during the emplacement ofrapakivi granites and at ~410 Ma, possibly due to collisional eventsduring the Caledonian orogeny. In addition, a third component ofabout 250 Ma age was obtained sporadically in some locations. Thestudies gave evidence that the geological evolution of the seeminglysolid bedrock is complicated.

Using mineralogic and paleomagnetic methods, we have investi-gated rocks at five locations at two major shear and fault zones and attwo less obtrusive fault structures (Fig. 1) to find evidence for laterreactivation. Earlier studies have showed that the major Porkkala–Mäntsälä and Vuosaari–Korso fault zones have experienced reactiva-

l rights reserved.

tions at different times, based onmineralogical observations (Elminen,1999) and paleomagnetic studies (Mertanen et al., 2004, 2008). Thestudy area, at the southern margin of the Fennoscandian shield, in theSouthern Finland Granitoid Zone, was formed in different stagesduring the Svecofennian orogeny at ~1.9–1.8 Ga (e.g. Pajunen et al.,2002; Väisänen, 2002; Väisänen and Mänttäri, 2002; Pajunen et al.,2008). The evolution of the Svecofennian orogeny is delineated by anoblique collision of a growing Svecofennian island arc system againstthe Archean continent and by subsequent continental deformation,metamorphism and crust-forming magmatism. The bedrock consistsofmostly felsic plutonic rocks andmetamorphic rocks that are stronglymigmatized. The NE–SW trending Porkkala–Mäntsälä and N–S-trending Vuosaari–Korso shear and fault zones (Fig. 1) were formedin the late stages of the orogeny and theybend and crosscut the generalfoliation structures (Elminen et al., 2008). The Svecofennian crustwas emplaced by rapakivi granites at 1.63–1.58 Ga. Alteration pro-ducts and joint fillings characterize fault zones, including zeolites,epidote, chlorite, carbonate, quartz, albite and hematite (Elminen,1999; Elminen et al., 2008).

Present study examines the geologic history of the shear and faultzones using remanent magnetization data that can record changes intemperature, fluid activity or other processes. This is done by iden-tifying remanence carrying minerals with rock magnetic tests andoptical investigations, and by specifying the age of their formation. Theother purpose of the study is to assess if secondary magnetizations

Fig. 1. Location and generalized geological map of the area. Studied sites are marked with black circles (RM — Rajamäki; JR — Järvenpää; KE — Kerava; ST — Sotunki; PO — Porvoo).

204 U. Preeden et al. / Tectonophysics 479 (2009) 203–213

found in previous studies in southern Finland (Mertanen et al., 2008)and in Estonia (Plado et al., 2008; Preeden et al., 2008) are observablealso in the presently studied localities.

2. Geology and sampling

In southern Finland, at the Uusimaa area, five localities weresampled (Fig. 1). The samples include granites, gneisses, mylonites,and cataclasite rocks. The Rajamäki (RM) outcrop consists of bandedpyroxene gneiss with shallow foliations (Fig. 2A–B). Long upright andsteep north-trending joints and brittle faults crosscut the rock. Someof the faulted joints are filled with quartz and laumontite and somelow temperature alteration has occurred. The faults are associatedwith larger (5–15 km long) north-trending fault structures inter-preted from topographic and geophysical data of the area (Elminen etal., 2008). At Järvenpää (JR), the outcrop exposes granite (Fig. 2C)with folded amphibolite or mafic dyke inclusions. A 10 m wideexposure is altered to a so called helsinkite type rock, which hastypical cataclastic features and light feldspar grains in hematite-bearing red matrix. The strike of a possible fault cannot be observedbut the site is close to the Porkkala–Mäntsälä fault valley, which isabout 80 km in length. The Kerava (KE) outcrop exposes granodiorite,pegmatite and amphibolite (Fig. 2D–E) and is a road cut at the marginto a ~30 km long fault valley, the Vuosaari–Korso fault. This faultstrikes N–S but it is related to the NE trending Porkkala–Mäntsäläfault. The site is delineated by different steep sub-parallel fault rocks,which vary from ductile mylonites to brittle cataclasites. In Porvoo

(PO), the outcrop exposes gneisses and pegmatites that strike WNW(Fig. 2F–G). There are no fault rocks present, but strong red colorationof gneisses due to hematite is observed. The contact with the Onasrapakivi intrusion is located 4 km away from the site. The Sotunki (ST)outcrop consists of altered granite, granodiorite and migmatite. Therock has mainly helsinkite-type texture with white feldspar clasts inhematite-bearing red matrix (Fig. 2H). Epidote, quartz and prehnitefilled cataclasite veins and joints have different strikes. In thin section,mylonitic textures are partly overprinted by hydrothermal epidoteand quartz. The outcrop is close to the NNE and NWN trending faultsassociated to the Vuosaari–Korso fault zone.

Samples were either drilled in the field as 2.5 cm diameter cores ortaken as oriented blocks and drilled in the laboratory. Orientation ofthe cores or block samples was obtained by sun and/or magneticcompass. Up to 27 independently oriented cores/hand-samples (2–3cores per sample) were taken from each site and 1–3 specimens of thelength of 2.2 cm were cut from each core in the laboratory.

3. Methods

Before demagnetizing specimens, density and magnetic suscepti-bility were measured. To study the remanence components, alternat-ing field (AF) up to peak field of 160mT, and thermal demagnetizationtreatment up to temperature of 680 °C, were used. In cases when AFwas insufficient to demagnetize the sample, the process wascontinued thermally. However, sometimes at ~400 °C changes inthe magnetic mineralogy took place, as demonstrated by an abrupt

Fig. 2. (A) Pyroxene gneiss that is locally reddish and (B) lighter fault filling from Rajamäki. (C) Helsinkite type granite from Järvenpää. (D and E) granodiorite and myloniticamphibolite, respectively, from Kerava. (F) Pegmatite dike and (G) reddish gneiss from Porvoo and (H) helsinkite type granite from Sotunki. Photos (A, D, H) taken by S. Mertanenand (B, C, E, F, G) by U. Preeden.

205U. Preeden et al. / Tectonophysics 479 (2009) 203–213

206 U. Preeden et al. / Tectonophysics 479 (2009) 203–213

raise in susceptibility and/or intensity of magnetization. After eachstep the intensity and direction of the natural remanent magnetiza-tion was measured with a 2G Enterprises superconducting (SQUID)magnetometer in the Paleomagnetism laboratory of the GeologicalSurvey of Finland. Individual NRMmeasurements were subjected to ajoint analysis of stereographic plots, demagnetization decay curvesand orthogonal demagnetization diagrams (Zijderveld, 1967). Theremanence directions were separated by principal componentanalysis (Kirschvink, 1980). Fisher (1953) statistics was used tocalculate the mean remanence directions. Virtual geomagnetic poles(VGP`s) were calculated for each remanence component and plottedon an APW path by using GMAP program of Torsvik and Smethurst(http://www.geophysics.ngu.no).

The mineralogy of several samples was examined in thin sectionsunder transmitted light with optical microscope at the Department ofGeology, University of Tartu, and at the Geological Survey of Finland.Scanning electron microscopy and electron microprobe studies wereperformed at the Geolaboratory of Geological Survey of Finland toidentify the nature of the magnetic carriers. Acquisition of isothermalremanent magnetization (IRM) and 3-axes thermal demagnetizationof IRM (Lowrie tests; Lowrie, 1990) were studied for seven samples.For determination of domain states of the magnetic carriers,hysteresis properties were measured (one to two samples per site)using a Princeton Measurement Corporation's MicroMagTM3900model Vibrating Sample Magnetometer (VSM) in the Geophysicslaboratory of the University of Helsinki.

4. Results

4.1. Mineralogy

The composition and grain-size distribution of magnetic mineralsdetermine the overall magnetic properties of a rock and the geologic

Fig. 3. Backscattered electron image by scanning electron microscope of studied thin sect(B) alteration of epidote and iron oxide in Sotunki and (C) in Porvoo. (D) Hematite format

stability of its natural remanent magnetization (NRM). Inspection ofeight petrographic thin sections representing all the examined rocktypes indicates that the rocks consist mainly of quartz, potassiumfeldspar, plagioclase, epidote, biotite and opaqueminerals that in SEManalyses were identified as magnetite, titanomagnetite, hematite,ilmenite and rutile. Examples of titanomagnetite are shown in Fig. 3A(Rajamäki), although most iron oxides are usually found as secondaryalteration products— fillings of veins and voids (Fig. 3B–D). Alterationof studied rocks is most clearly seen regarding to epidote (Fig. 3B andC) that is a typical secondary mineral and it has changed, as the shapeof the well developed prismatic crystals is mostly lost and iron oxidesare formed (Fig. 3C). Alteration of micas (Fig. 3D) is also present.Biotite, which largely defines the fabric in these rocks, is partiallyreplaced by specular hematite (Fig. 3D).

The SEM observations show potential remanence carrying mag-netic material phases as a product of new mineral nucleation plusgrowth and oxidation of existing mineral phases. These featuresdemonstrate that the rocks have experienced pervasive alteration dueto oxidizing fluid flow.

4.2. IRM acquisition and Lowrie test

The acquisition curves of isothermal remanent magnetization(IRM; Fig. 4A) reveal gradual increase of intensity with no saturationreached at 1.5 T. This trend is similar in all samples, indicating thedominance of a high coercivity mineral, like hematite or goethite.Thermal demagnetization curves of 3-axes IRM show some variationsbetween different coercivity fractions. However, all samples showrelatively smooth decay in high coercivity fraction and a drop ofintensity at 640–680 °C, further indicating the dominance of hematite(Fig. 4B–F) and excluding the presence of goethite. A slightly steepergradient in high and intermediate coercivity fraction is observedbetween 350 and 400 °C in the sample from Kerava (Fig. 4C), and a

ions. (A) Symplectitic intergrowth of titanomagnetite and altered biotite in Rajamäki,ion on account of biotite in Järvenpää.

Fig. 4. (A) Progressive acquisition of IRM. (B–F) thermal demagnetization of a three-component IRM produced by magnetizing the sample in 1.5 T along the z-axis, followed by 0.4 Talong the y-axis and finally 0.12 T along the x-axis for studied crystalline rocks. The soft (<0.12 T), medium (0.12–0.4 T) and hard (0.4–1.5 T) components are shown during thermaldemagnetization.

207U. Preeden et al. / Tectonophysics 479 (2009) 203–213

smoother gradient in the same temperatures in the high andintermediate coercivity fractions in the samples from Rajamäki andSotunki (Fig. 4E and F). The decrease in remanence at ~350 °C may becaused by inversion of maghemite to hematite (Dankers, 1978). Thesame temperature range could indicate also the presence of pyrrhotite(max unblocking temperature 325 °C; Dekkers, 1989), but electronmicroscopic studies did not reveal any sulphides. In the samples fromPorvoo and Sotunki, a drop of intensity between 560 and 580 °C(Fig. 4D and F) implies the presence of magnetite.

4.3. Hysteresis

Hysteresis measurements of four studied samples show differentcoercivities (Fig. 5). The magnitude of magnetization and the shape ofhysteresis curve are an additive function of mineral concentration and

domain states. The samples from Porvoo (Fig. 5A) and Rajamäki(Fig. 5B) contain abundant high coercivity hematite as the magneti-zationdoes not reach saturation in the highest availablefield of 1 T. Thesample from Sotunki (Fig. 5C) and to a lesser degree from Järvenpää(Fig. 5D), showmore low coercivityminerals. For all samples, a typical“wasp waisted” shape is observed, which reflects a bi-modal nature ofcoercivity distribution (Jackson et al., 1990). Such curves show, either(i) a mixture of domain states as numerical simulations have shownthat wasp-waisted loops can be generated from populations of SD(single domain) and SP (superparamagnetic) grains (Tauxe et al.,1996) or (ii) amixture of twomagnetic phases,whenmagnetically softand magnetically hard minerals contribute equally to the magnetiza-tion (Dunlop and Özdemir, 1997). Other mineralogic and rockmagnetic data suggest that the second option, with (titano)magnetiteand hematite contributing in to the hysteresis curves, is more realistic.

Fig. 5. Hysteresis loops after correction of paramagnetic contribution for four samples from Porvoo (A), Rajamäki (B), Sotunki (C) and Järvenpää (D).

208 U. Preeden et al. / Tectonophysics 479 (2009) 203–213

4.4. Paleomagnetic results

In general, studied fault zone rocks are weakly magnetized. Theaverage intensity of the NRM is between 0.8 and 20 mA/m andsusceptibilities (volume normalized) range from 74 to 753×10−6 SI(Table 1).Despite thegenerally lowNRMintensities, themagnetizationsare above the precision (0.03 mA/m) of the SQUID magnetometer. Twocommon remanence components (called A and B) were isolated byprogressive demagnetization in four of the studied locations (Table 2).Component A is directed NNWand component B towardsNE, bothwithshallow to moderate positive inclination. Other components weresporadically identified as well. In the following, the paleomagneticresults are discussed for each locality.

The NRM intensities of pyroxene gneiss samples from Rajamäkirange from 0.08 mA/m to 47.6 mA/m, with an average of 4.8 mA/m(Table 1). These samples have the highestQ-values (Table 1) of all rocksstudied, ranging from 0.02 to 25.8. Both remanence components, A andB, were identified, independently of the Q-value. Component A isisolated over unblocking temperatures of 520–580 °C (Fig. 6A) and AFvalues up to 160 mT (Fig. 6B). Based on unblocking temperatures dataand SEMobservations (Fig. 3A)we suggest that (titano)magnetite is the

Table 1Physical properties (site mean values and standard deviations).

Site n ρA χ NRM Q(kg/m−3) (×10−6SI) (mAm−1) (−)

Rajamäki 30 2603±84 96±62 4.8±10.1 1.9±6.8Kerava 23 2663±73 203±146 0.8±0.8 0.2±0.3Järvenpää 42 2614±50 74±45 1.2±0.9 0.4±0.2Porvoo 42 2684±89 753±632 20.0±20.3 0.6±0.2Sotunki 33 2694±62 93±57 0.3±0.3 0.3±0.6

N —number of specimens, ρA — density; χ — magnetic susceptibility, normalized byvolume; NRM — intensity of the natural remanent magnetization; Q — Koenigsbergerratio.

principal carrier of this magnetization. Component B was isolated intemperatures between 640 and 680 °C (Fig. 6C), consistent with he-matite as the principal remanence carrier. The dominance of highcoercivity minerals is seen in IRM studies (Fig. 4A) and the Lowrie testconfirms the presence of hematite (Fig. 4E). Component B occurstypically in fault filling material (Fig. 2B).

NRM intensities of the granodiorite and mylonitic amphibolitefrom Kerava range from 0.2 to 3.4 mA/m, with an average of 0.8 mA/m(Table 1). Only one consistent NE directed magnetization componentwith moderate positive inclination was obtained in 10 samples in AFfields of 2.5–40 mT (Fig. 6D). Based on rock magnetic studies andthermal demagnetizations (unblocking temperature 150–400 °C), thecomponent is probably carried by maghemite.

In the Järvenpää granite (helsinkite), three differentmagnetizationcomponents (A, B and JR) are isolated. The NRM intensities of theserocks range from 0.36 to 4.7 mA/m, with an average of 1.2 mA/m(Table 1). Component A was isolated in 8 samples in intermediatecoercivities and in a temperature range of 300–600 °C (Fig. 7A) sug-gesting (titano)magnetite as the remanence carrier, consistent withthe hysteresis properties (Fig. 5D). Component B, with NE declinationand moderate inclination, has typically high coercivities (Fig. 7B).Referring to SEM and IRM studies, it is most probably carried byhematite (Figs. 3D and 4B). The most common magnetization JR(found in 10 samples) at this locality is northeast directed and has veryshallow inclination (Table 2). It is also a high coercivity component, asAF demagnetization up to 160 mT will not demagnetize it (Fig. 7C).Further thermal treatment unblocked the remanence between 200and 660 °C (Fig. 7D), suggesting hematite as the remanence carrier.

The samples from hematite-rich gneisses and pegmatite fromPorvoo, close to the Onas and Wyborg rapakivi intrusions (Fig. 1),yield three remanence components (components A and B and aviscous component). NRM intensities range from 0.75 to 98.6 mA/m,with an average of 20 mA/m. A viscous magnetization, isolated overwide temperature range of 250 to 600 °C and over low coercivities(2.5–20 mT), was mainly found in pegmatite (Fig. 2F). In 10 samples,

Table 2Paleomagnetic results.

Locality n/N D (°) I (°) k/K α95/A95

(°)Plat(°)

Plon(°)

dp(°)

dm(°)

Component ARajamäki 16/6 331.4 38.6 43.3 10.3 46.3 244.8 7.3 12.2Järvenpää 26/8 338.9 38.5 29.4 10.4 48.5 235.4 7.3 12.3Porvoo 23/10 333.7 23.1 17.2 12.0 37.9 239.0 6.8 12.8Sotunki 19/11 335.0 40.4 24.8 9.4 48.9 241.4 6.8 11.3Mean 4 334.7 35.2 92.1/194.5 9.6/6.6 45.3 240.1 6.4 11.1

Component BRajamäki 16/5 29.8 34.6 31.2 13.9 43.4 164.4 9.2 16.0Järvenpää 26/9 27.9 35.8 23.6 10.8 44.8 166.8 7.3 12.5Porvoo 23/16 33.4 29.9 15.4 9.7 39.5 162.3 6.0 10.7Sotunki 19/6 32.0 41.0 39.0 10.9 47.1 159.7 8.0 13.2Mean 4 30.8 35.3 264.5/444.3 5.7/4.4 43.7 163.3 3.8 6.5

Other componentsJärvenpää (JR) 26/10 17.5 5.0 26.9 9.5 30.5 184.6 4.8 9.5Kerava (KE) 17/10 52.6 45.1 15.4 12.7 41.2 134.7 10.2 16.1

n − number of samples analysed, N – number of samples revealing the component, D – declination, I – inclination, k – Fisher's (1953) precision parameter for a remanencecomponent and K for themean pole,α95 – the radius of 95% confidence about themean, A95 is the radius of the circle of 95% confidence of themean pole, Plat and Plong – latitude andlongitude of the virtual geomagnetic poles, dp and dm – semi axes of the oval of 95% confidence of the pole.

Fig. 6. AF and TH demagnetization behaviour (stereographic projection, decay curves and orthogonal plots) from specimens RM11-1B, RM11-1A and RM16-1T from Rajamäki andKR26-1A from Kerava. Closed and open symbols in the orthogonal plots denote data projected onto the horizontal–vertical plane and on stereographic plots downward–upward,respectively. Gray line denotes the interval, where component was found (more details in text).

209U. Preeden et al. / Tectonophysics 479 (2009) 203–213

Fig. 7. AF and TH demagnetization behaviour from specimens JR23-1A, JR13-1B, JR22-1A and JR22-1T from Järvenpää.

210 U. Preeden et al. / Tectonophysics 479 (2009) 203–213

component A was isolated at intermediate coercivities (10–30 mT,Fig. 8A). Thermal demagnetization usually did not give interpretativeresults, but rock magnetic and mineralogical studies hint that the Acomponent resides in magnetite. Component B was isolated in 16

Fig. 8. AF and TH demagnetization behaviour from specimens (A) PO7-1A and

samples in AF fields from 30 to 160 mT (Fig. 8A) or over unblockingtemperature between 200 and 400 °C (Fig. 8B). The carriers of thisoverprint could be both hematite and maghemite, in accord with rockmagnetic (Fig. 4D) and mineralogic studies (Fig. 3C).

(B) PO21-1T from Porvoo and (C) ST13-1A and (D) ST13-1T from Sotunki.

Fig. 9. Site mean paleomagnetic directions. Black triangles denote component A andsquares B. Grey squares mark the separate components JR and KE found in Järvenpääand Kerava, respectively. Circles indicate cones of α95 confidence about the means.

211U. Preeden et al. / Tectonophysics 479 (2009) 203–213

The Sotunki helsinkite-type granites have the lowest NRM intensi-ties (0.14–1.6 mA/m) among the rocks examined (Table 1). Like inPorvoo, a low coercivity NE pointing intermediate to high inclinationviscous component was registered here as well. Component A wasisolated in AF fields of 60–140 mT in 11 samples (Table 2; Fig. 8C).Component B was revealed in intermediate coercivity fields andunblocking temperatures around 350 °C (Fig. 8D). We presume thatcomponent B resides in maghemite as mineralogical studies show noindications of pyrrhotite.

5. Discussion

The Järvenpää, Sotunki and Kerava sites where we have obtainedpaleomagnetic data are located along the major NE–SW orientedPorkkala–Mäntsälä and N–S oriented Vuosaari–Korso shear and faultzones that crosscut the Svecofennian bedrock. The Rajamäki and Porvoolocalities are related to less significant fault structures. Aeromagneticmaps show that the shear and fault zones are most often weakly mag-netized, which is interpreted to be due to hydrothermal alteration ofmagnetic minerals to silicates or other less magnetic Fe-oxides (Airoet al., 2008). Also the presently studied faults can be traced as weaklymagnetic anomalies.

Fig. 10. (A) Precambrian north “key poles”with ages (crosses with shaded α95 circles; BuchaBaltica (Torsvik and Rehnström, 2003; Torsvik and Cox, 2005). Paleomagnetic poles of this sfour studied localities, separate components JR and KE are from Järvenpää and Kerava, resptriangles and in lower case (Mertanen et al., 2008). Squares denote Permian–Triassic overpr1998 (S). For further explanations and interpretation, see text.

Mineralogic alteration by hydrothermal activity is common infault zones and often characterizes different fault generations. This isconfirmed by mineralogic and kinematic studies also at Porkkala–Mäntsälä and Vuosaari–Korso fault zones (Elminen et al., 2008).Furthermore, the present and previous paleomagnetic studies provideevidence that the reactivation of the zones has occurred several times(Mertanen et al., 2008).

Four localities provide paleomagnetic results that indicate the pre-servationof the original remanentmagnetization, componentA (Table 2,Fig. 9),whichwaspresumably acquired during cooling of the crust in thelate stages of the Svecofennian orogeny. Pole A agrees (Fig. 10A)with theknown key poles of the ages of ca. 1840 Ma and ca. 1880 Ma (Buchanet al., 2000).Mineralogic (Fig. 3A) androckmagnetic studies (Fig. 4D andF) showed that the component is carried by (titano)magnetite and it hasa thermal (TRM) origin. (Titano)magnetite is known to be the principalremanence carrier in igneous and metamorphosed igneous and sup-racrustal rocks (Haggerty, 1976; Dunlop and Özdemir, 1997).

The Järvenpää samples have a characteristic remanence compo-nent JR, and the pole is in close agreementwith the 1.63Ga and 1.54Gakey poles (Fig. 10A). The remanencemay reflect the reactivation of thecrust at the time of rapakivi magmatism, also noted by Elminen et al.(2008). In the previous paleomagnetic study similar component SB1S(Fig. 10A) was obtained from three shear zones in southern Finland,and the age was regarded as ca. 1.63 Ga (Mertanen et al., 2008). Ourresults indicate similar-aged reactivation also in the Järvenpäähelsinkite. This result is in good agreement with the local geology asthe Järvenpää site is located at the Porkkala–Mäntsälä shear zone(Fig. 1) along which the ca. 1.64 Ga Bodom and Obbnäs rapakivigranites (Kosunen, 2004) were emplaced (see also Elminen, 1999).

The most prevalent remanence component (B) is suggested torepresent a late Paleozoic secondary magnetization event (pole B,Fig. 10B). Typically, this component is isolated in intermediate to highcoercivities. Hematite is the main carrier, but at two localities (Porvooand Sotunki) there are indications ofmaghemite contribution aswell. Inprevious paleomagnetic studies, this magnetization has been identifiedall over Finland, but typically it has been ignored or interpreted differ-ently (Mertanen et al., 1989; Mertanen and Pesonen, 1995; Neuvonenet al., 1997, see discussion in Mertanen et al., 2008). In addition, acomparable pole has been obtained in early Paleoproterozoic rocks ofthe Kola Province (Mertanen et al., 1998) where this remagnetization

n et al., 2000 and Pesonen et al., 2003) and (B) south poles of Phanerozoic APW path fortudy are marked with black dots with A95 confidence circles. Poles A and B are means ofectively. Poles from earlier studies of shear zones in southern Finland are marked withints found by Lubnina, 2004 (L), Preeden et al., 2008 (AN and KO) and Smethurst et al.,

212 U. Preeden et al. / Tectonophysics 479 (2009) 203–213

was found in one studied Paleozoic dike, which questioned the Pro-terozoic origin of the remanence. A similar component was sporadicallyobtained also in the shear zones in southern Finland (Mertanen et al.,2008).

The location of pole B does notmatchwith any knownPrecambrianpoles of the Fennoscandian Shield. However, when compared to thePaleozoic APW path of Baltica (Fig. 10B) it fits well with poles ofage ~260 to 270Ma. B component has normal polarity,which indicatesthat it must have been formed after Permo-Carboniferous reversedsuperchron (PCRS) and this ended at the Illawara reversal at ca. 265Maago (Jin et al., 2000).

Geologic and isotopic indications of Paleozoic geologic processes insouthern Finland have been found by Larson et al. (1999), Murrell(2003) and Alm et al. (2005). In Estonia, the ca. 400 Ma sedimentaryrocks are crosscut by NE-trending faults (Puura et al., 1996, 1999) andthepaleomagnetic data fromthese rocks yield secondary components oflate Paleozoic age (Fig. 10B; Preeden et al., 2008). Secondary mag-netization of Permian age has also been observed in the Ordoviciansequences in northern Estonia (Plado and Pesonen, 2004) and in St.Petersburg area (Fig. 10B; Lubnina, 2004). In St. Petersburg area, thecomponent is interpreted tobe related to the tectonic events at theUralsand inWestern Europe (Lubnina, 2004). A significant stage in formationand rejuvenation of faults in western and southern Norway took placein Permian as well (Torsvik et al., 1992; Andersen et al., 1999). Thisactivity is tiedwith the formation of the Oslo rift thatmight have had anoblique influence also to the studied region. In addition, e.g. Blumsteinet al. (2005) and Elmore et al. (2002) have reported fluid alteration ofPermian age along the fault zones in Scotland, and these studies showedthat palaeomagnetism can be used to date multiple fluid migrationevents. As secondary late Paleozoic magnetizations have been observedworld-wide (Zwing, 2003), we suggest that the global process ofremagnetization is related to the formation of supercontinent Pangea.

Geologic evidences cited above suggest that the overall stabledevelopment of the East European Cratonwas interrupted by a series offluid/thermal events and possibly tectonic reactivation during the last500 Ma. The old fault structures of the basement may have beenrecurrently reactivated during Phanerozoic tectonism and have givenrise to the formation of deep fluid circulation systems. The fluid mi-grationwas fracture controlled and themost intense chemical reactionsare tightly coupled to fracture zones with enhanced permeability. Fur-thermore, in the early Palaeozoic, the shield was buried under marinesediments and subsequently uncovered giving rise to possibility of nearsurfacemeteoricfluid circulation. Permianwidespread regression of theseas (Zharkov and Chumakov, 2001 and references therein) coupledwith the presence of hematite and maghemite hints that the base levelof erosion lowered and the oxidizing fluids could reach the older rocks.

The secondary magnetization of Kerava hints to an even youngerremagnetization event, possibly Triassic, in the beginning of Mesozoic(pole KE, Fig. 10B). As the carrier of the KE component is maghemite,we favor the idea that a similar mechanism of oxidizing fluids, as forcomponent B, is responsible for the acquisition of this secondary mag-netization. A Triassic remanence component has been found also inlimestones fromSt. Petersburg area by Smethurst et al. (1998) (Fig. 10B).

The importance of fluids is further confirmed by the mode ofoccurrence of the alteration products in the studied rocks. The newlyformed or altered minerals are along fluid pathways such as cracks,grain boundaries and interconnected voids,where the fluids can eithertotally destroy minerals that carry pre-existing remanent magnetiza-tion as in Kerava, or recrystallize and formnewmagneticminerals thatcarry stable chemical secondary magnetizations. The high tempera-tures reached during thermal demagnetization (up to 680 °C for manysamples) require high reworking temperatures. There is no evidencethat the rocks were heated to such high temperatures; a thermo-viscous origin, therefore, is not probable. The Permian overprint istherefore interpreted as a chemical remanent magnetization (CRM).The likely source of iron comes either locally from alteration and

dissolution of micas, epidote or from iron-titanium oxides. Additionaliron could also have been transported into the system by externallyderivedfluids.Without further geochemical analysis offluid inclusionsand isotopic composition, the exact origin of fluids remains specula-tive. However, the coeval nature of remagnetizations of the studiedregion (Mertanen et al., 2008) as well as worldwide (Andersen et al.,1999; Elmore, 2001; Geissman and Harlan, 2002; Klootwijk, 2003;Lewchuk et al., 2003; Torsvik and Rehnström, 2003; Cederquist et al.,2006) with different deformation events in the tectonic history,indicates that tectonism could be the major catalyst for fluid flow, andconsequently the underlying cause of remagnetization.

6. Conclusions

The study reveals markedly consistent remanence directions andprovides evidence of multiple reactivation of the Paleoproterozoic crustin southern Finland. Rocks from shear and fault zones have preservedthe original remanentmagnetization (component A), that was acquiredduring cooling of the crust in the late stages of the Svecofennianorogeny. (Titano)magnetite is the carrier of this primary component.The presence of secondarymagnetizations indicates the vulnerability ofshear zones to later reactivation. Hematite and maghemite carry themost prevalent Permian component. Also, reactivation of the Porkkala–Mäntsälä fault zone related to the nearby Mesoproterozoic (~1.63 Ga)rapakivi event is recorded at the Järvenpää locality where the rem-anence resides in hematite.

Thepresent study of crystalline rocks in the shear zones in southernFinland provides a link between oxidizing fluid circulation and theacquisition of a secondary magnetization which is triggered by tec-tonic activity in the late Paleozoic. Determination of the exact mech-anisms of remagnetization, however, remains unresolved and shouldbe the focus of future chemical studies.

Acknowledgements

We are thankful to Bo Johanson andMatti Leino (Geological Surveyof Finland), Juho Kirs (University of Tartu) and Tiiu Elbra (HelsinkiUniversity) for their kind help. We are grateful to the reviewers of ourmanuscript — R.D. Elmore, J. Geissman and C. Langereis, who offeredimportant adjustments and constructive comments to improve thisarticle.

This studywas supported by the Estonian Science Foundation (grant#6613).

References

Airo, M.L., Elminen, T., Mertanen, S., Niemelä, R., Pajunen, M., Wasenius, P., Wennerström,M., 2008. Aerogeophysical approach to ductile and brittle structures in the denselypopulated urban Helsinki area, southern Finland. In: Pajunen, M. (Ed.), TectonicEvolution of the Svecofennian Crust in Southern Finland — A Basis for CharacterizingBedrock Technical Properties, vol. 47. Geological Survey of Finland, Special Paper,Finland, pp. 283–308.

Alm, E., Huhma, H., Sundblad, K., 2005. Preliminary Paleozoic Sm–Nd ages of fluorite-calcite-galena veins in the south eastern part of the Fennoscandian Shield. SvenskKärnbränslehantering AB, R-04-27.

Andersen, T.B., Torsvik, T.H., Eide, E.A., Osmundsen, P.T., Faleide, J.I., 1999. Permian andMesozoic extensional faulting within the Caledonides of central south Norway.J. Geol. Soc. 156, 1073–1080.

Blumstein, R.D., Elmore, R.D., Engel, M.H., Parnell, J., Baron, M., 2005. Date and origin ofmultiple fluid flow events along the Moine Thrust Zone, Scotland. J. Geol. Soc. Lond.162, 1031–1045.

Buchan, K., Mertanen, S., Elming, S.Å., Park, R.G., Pesonen, L.J., Bylund, Abrahamsen, N.,2000. The drift of Laurentia nad Baltica in the Proterozoic: a comparison based onkey paleomagnetic poles. Tectonophysics 319, 167–198.

Cederquist, D.P., Van Der Voo, R., Pluijm, B.A., 2006. Syn-folding remagnetization ofCambro-Ordovician carbonates from the Pennsylvania Salient post-dates oroclinalrotation. Tectonophysics 422, 41–54.

Dankers, P.H.M., 1978. Magnetic properties of dispersed natural iron-oxides of knowngrain-size. PhD thesis, University of Utrecht.

Dekkers, M., 1989.Magnetic properties of natural pyrrhotite. II. High- and low-temperaturebehaviour of Jrs anTRMas a functionof grain size. Phys. Earth Planet. Inter. 57, 266–283.

213U. Preeden et al. / Tectonophysics 479 (2009) 203–213

Dunlop, D.J., Özdemir, Ö., 1997. RockMagnetism: Fundamentals and Frontiers. CambridgeUniversity Press, New York.

Eide, E.A., Torsvik, T.H., Andersen, T.B., 1997. Absolute dating of brittle fault movements:Late Permian and Late Jurassic extensional fault breccias in western Norway. TerraNova 9, 135–139.

Elminen, T., 1999. Porkkala–Mäntsälä — shear zone description and study. MSc thesis,University of Helsinki, Finland (in Finnish).

Elminen, T., Airo, M.L., Niemelä, R., Pajunen, M., Vaarma, M., Wasenius, P., Wennerström,M., 2008. Fault structures in the Helsinki area, southern Finland. In: Pajunen,M. (Ed.),Tectonic Evolution of the Svecofennian Crust in Southern Finland — A Basis forCharacterizing Bedrock Technical Properties, vol. 47. Geological Survey of Finland,Special Paper, Finland, pp. 185–213.

Elmore, R.D., 2001. A review of paleomagnetic data on the timing and origin ofmultiple fluid-flow events in the Arbuckle Mountains, southern Oklahoma. Pet.Geosci. 7–3, 223–229.

Elmore, R.D., Parnell, J., Engel, M.H., Baron, M., Woods, S., Abraham, M., Davidson, M.,2002. Paleomagnetic dating of fluid-flow events in dolomitized rocks along theHighland Boundary Fault, central Scotland. Geofluids 2, 299–314.

Fisher, R., 1953. Dispersion of a sphere. Proc. R. Soc. Lond. 217, 295–305.Geissman, J.W., Harlan, S.S., 2002. Late Paleozoic remagnetization of Precambrian

crystalline rocks along the Precambrian/Carboniferous nonconformity, Rocky Moun-tains: a relationship among deformation, remagnetization, and fluid migration. EarthPlanet. Sci. Lett. 203, 905–924.

Haggerty, S.E., 1976. Opaque mineral oxides in terrestrial igneous rocks. In: RumbleIII, D. (Ed.), Oxide Minerals, vol. 3. Mineralogical Society of America, Washington,pp. Hg–101–HG-277.

Jackson, M., Worm, H.U., Banarjee, S.K., 1990. Fourier analysis of digital hysteresis data:rock magnetic applications. Phys. Earth Planet. Inter. 65, 78–87.

Jin, Y.G., Shang, Q.H., Cao, C.Q., 2000. Late Permian magnetostratigraphy and its globalcorrelation. Chin. Sci. Bull. 45, 698–704.

Kirschvink, J.L., 1980. The least square line and plane and the analysis of paleomagneticdata. Geophys. J. R. Astron. Soc. 62, 699–718.

Klootwijk, C.T., 2003. Carboniferous paleomagnetism of theWerrie Block, northwesternTamworth Belt, and the New England pole path. Aust. J. Earth Sci. 50-6, 865–902.

Kosunen, P., 2004. Petrogenesis of mid-Proterozoic A-type granites: Case studies fromFennoscandia (Finland) and Laurentia (New Mexico). PhD thesis, University ofHelsinki, Finland.

Larson, S.Å., Tullborg, E.L., Cederbom, C., Stiberg, J.P., 1999. Sveconorwegian andCaledonian foreland basin in the Baltic Shield revealed by fission-track thermo-chronology. Terra Nova 11, 210–215.

Lewchuk, T.M., Evans, M., Elmore, R.D., 2003. Synfolding remagnetization and de-formation: results from Paleozoic sedimentary rocks in West Virginia. Geophys. J.Int. 152, 266–279.

Lowrie, W., 1990. Identification of ferromagnetic minerals in a rock by coercivity andunblocking temperature properties. Geophys. Res. Lett. 17, 159–162.

Lubnina, N., 2004. Paleomagnetic investigations of the Ordovician rocks from St.Petersburg area: age of remagnetizations and their correlation with tectonicevents. In: Mertanen, S. (Ed.), 5th Nordic Paleomagnetic Workshop: extendedabstracts, Geological Survey of Finland, Report Q29.1/2004/1, Espoo, pp. 103–107.

Mertanen, S., Pesonen, L.J., 1995. Paleomagnetic and rock magnetic investigations of theSipoo Subjotnian quartz porphyry and diabase dykes, southern Fennoscandia. Phys.Earth Planet. Inter. 88, 145–175.

Mertanen, S., Pesonen, L.J., Huhma, H., Leino, M., 1989. Paleomagnetism of the EarlyProterozoic layered intrusions, northern Finland. Bull. Geol. Soc. Finl. 347.

Mertanen, S., Pesonen, L.J., Vuollo, J., 1998. Preliminary study of the paleomagnetism ofmafic dykes in the Kola Peninsula. Laboratory for Paleomagnetism, GeologicalSurvey of Finland, Report Q 29.1/98/2, 19 pp.

Mertanen, S., Pajunen, M., Elminen, T., 2004. Multiple remagnetization events of shearzones in southern Finland. In: Mertanen, S. (Ed.), 5th Nordic Paleomagnetic Work-shop: extended abstracts, Geological Survey of Finland, Report Q29.1/2004/1, Espoo,pp. 39–44.

Mertanen, S., Airo, M.L., Elminen, T., Niemelä, R., Pajunen, M., Wasenius, P.,Wennerström, M., 2008. Paleomagnetic evidence for Mesoproterozoic–Paleozoic

reactivation of the Paleoproterozoic crust in southern Finland. In: Pajunen, M.(Ed.), Tectonic Evolution of the Svecofennian Crust in Southern Finland— A Basisfor Characterizing Bedrock Technical Properties, vol. 47. Geological Survey ofFinland, Special Paper, Finland, pp. 215–252.

Murrell, G.R., 2003. The long-term thermal evolution of central Fennoscandia, revealedby low-temperature thermochronometry. PhD thesis, Virje University, Amsterdam.

Neuvonen, K.J., Pesonen, L.J., Pietarinen, H., 1997. Remanent magnetization in theArchean basement and cutting diabase dykes in Finland, Fennoscandian shield.Geophysica 33-1, 111–146.

Pajunen, M., Airo, M.L., Elminen, T., Niemelä, R., Salmelainen, J., Vaarma, M., Wasenius, P.,Wennerström, M., 2002. Construction model of the bedrock in urban areas. Meth-odology and Directions. Geological Survey of Finland. 21.4.42/2002/5 (in Finnish).

Pajunen, M., Airo, M.L., Elminen, T., Mänttäri, I., Niemelä, R., Vaarma, M., Wasenius, P.,Wennerström, M., 2008. Tectonic evolution of the Svecofennian crust in southernFinland. In: Pajunen, M. (Ed.), Tectonic Evolution of the Svecofennian Crust inSouthern Finland— a basis for Characterizing Bedrock Technical Properties, vol. 47.Geological Survey of Finland, Special Paper, Finland, pp. 15–60.

Pesonen, L.J., Elming, S.Å., Mertanen, S., Pisarevski, S., D`Agrella-Filho, M.S., Meert, J.,Schmidt, P.W., Abrahamsen, N., Bylund, G., 2003. Paleomagnetic configuration ofcontinents during the Proterozoic. Tectonophysics 375, 289–324.

Plado, J., Pesonen, L., 2004. Primary andsecondarymagnetizations of theEstonianPaleozoicsediments. In: Mertanen, S. (Ed.), 5th Nordic Paleomagnetic Workshop: extendedabstracts, Geological Survey of Finland, Report Q29.1/2004/1, Espoo, pp. 49–53.

Plado, J., Preeden, U., Puura, V., Pesonen, L.J., Kirsimäe, K., Pani, T., Elbra, T., 2008.Paleomagnetic age of remagnetizations in Silurian dolomites, Rõstla quarry (CentralEstonia). Geol. Q. 52, 213–224.

Preeden, U., Plado, J., Mertanen, S., Puura, V., 2008. Multiply remagnetized Siluriancarbonate sequence in Estonia. Est. J. Earth Sci. 57-3, 170–180.

Puura, V., Laitakari, I., Amantov, A., 1996. Latest events affecting the Precambrianbasement. Gulf of Finland and Surrounding Areas, vol. 21. Geological Survey ofFinland, Special Paper, Espoo, pp. 115–126.

Puura, V., Vaher, R., Tuuling, I., 1999. Pre-Devonian landscape of the Baltic Oil-ShaleBasin, NW of the Russian platform. In: Smith, B.J., Whalley, W.B., Warke, P.A. (Eds.),Uplift, Erosion and Stability: Perspectives on Long-term Landscape Development,vol. 162. Geological Society of London, Special Publication, pp. 75–93.

Smethurst, M.A., Khramov, A.N., Pisarevsky, S., 1998. Palaeomagnetism of the LowerOrdovician Orthoceras Limestone, St. Petersburg, and a revised drift history forBaltica in the early Palaeozoic. Geophys. J. Int. 133, 44–56.

Tauxe, L., Mullender, T.A.T., Pick, T., 1996. Potbellies, wasp-waists and superparamag-netism in magnetic hysteresis. J. Geophys. Res. 101, 571–583.

Torsvik, T.H., Cox, L.R.M., 2005. Norway in space and time: a centennial cavalcade. Nor. J.Geol. 85, 73–86.

Torsvik, T.H., Sturt, B.A., Swensson, E., Andersen, T.B., Dewey, J.F., 1992. Palaeomagneticdating of fault rocks: evidence for Permian and Mesozoic movements and brittledeformation along the extensional Dalsfjord Fault, western Norway. Geophys. J. Int.109, 565–580.

Torsvik, T.H., Rehnström, E.F., 2003. The Tornquist Sea and Baltica–Avalonia docking.Tectonophysics 362, 67–82.

Väisänen, M., 2002. Tectonic evolution of the Paleoproterozoic Svecofennian Orogen inSouthwestern Finland. Ann. Univ. Turk. 154.

Väisänen, M., Mänttäri, I., 2002. 1.90–1.88 Ga primitive arc, mature arc and back-arcbasin in the Orijärvi area, SW Finland. Bull. Geol. Soc. Finl. 74, 185–214.

Zharkov, M.A., Chumakov, N.M., 2001. Paleogeography and sedimentation settings duringPermian–Triassic reorganizations in biosphere. Stratigr. Geol. Correl. 9, 340–363.

Zijderveld, J.D.A., 1967. A.C. demagnetization of rocks — analysis of results. In:Collinson, D.W., Creer, K.M., Runcorn, S.K. (Eds.), Methods in Paleomagnetism.Elsevier, New York, pp. 254–286.

Zwing, A., 2003. Causes and Mechanisms of Remagnetisation in Paleozoic Rocks — aMultidiciplinary Approach. PhD thesis, Ludwig–Maximilians University, München.