s. Wn 0

3, San D

a r t i c l e i n f o

Article history:Received 26 April 2010Received in revised form 2 April 2011Accepted 5 April 2011Available online 13 April 2011

Keywords:SillBasalt

Tectonophysics 505 (2011) 6885

Contents lists available at ScienceDirect

Tectonop

j ourna l homepage: www.e lthe sills. 2011 Elsevier B.V. All rights reserved.

1. Introduction

Anisotropy of magnetic susceptibility (AMS) is dened as thevariation in direction of the principal magnetic susceptibility axeswithin a rock sample. Magnetic fabric in volcanic rocksmay be formedby a variety of geologic processes, including primary magma ow

and deformation subsequent to emplacement (Can-Tapia, 2004;Ellwood, 1978; Hrouda, 1982; Tarling and Hrouda, 1993). Studies onvolcanic and shallow intrusive rocks have shown the magnetic fabricof these rocks is commonly related to their emplacement mode, andthat magmatic ow directions can be reconstructed from theorientation of the magnetic ellipsoids (e.g., Can-Tapia et al., 1996;Dragoni et al., 1997; Ellwood, 1978; Knight and Walker, 1988; This paper is the rst one that uses AMS fabric toboth intrusive sills and basaltic lava ows in terms ofdirections in these mac igneous rocks on the Taimyrdata are powerful to record the folding mechanism andirections. Corresponding author at: State Key Laboratory of Ge

Resources, China University of Geosciences, Beijing82320343.

E-mail address: zhang.shuwei@163.com (S. Zhang).

0040-1951/$ see front matter 2011 Elsevier B.V. Adoi:10.1016/j.tecto.2011.04.004crystals in the sill samples are the main carriers of the AMS fabrics, implying the abundance of inversemagnetic fabrics. PSD and multidomain (MD) titanomagnetite (TM) grains as well as minor hematite areidentied in the basalt ows, suggesting the occurrence of more normal magnetic fabrics in the basalts than inMagnetic fabricTitanomagnetiteTaimyra b s t r a c t

Magnetic fabric research on the emplacement of the Taimyr fold-belt is still absent. Therefore, we haveperformed magnetic fabric studies on samples collected from 23 sites frommac igneous rocks (75N, 100E)in South Taimyr. This study indicates a correlation between the anisotropy of magnetic susceptibility (AMS),magma ow and fold-related compression in the rocks. AMS measurements on 183 unheated and 122 heatedsamples reveal the magma ow directions. The magma ow direction is mainly parallel to the ESEWNW andsecondarily parallel to the SSW from the NNE in the mac sills. The unheated and heated basaltic ows reveala NE-trending ow vector. In some of the sills, AMS shows a tight clustering of the maximum K1 axes close tothe bedding pole, which is not thought to dene the ow direction. Corresponding AMS measurements onigneous rocks allow us to infer the existence of magnetic fabrics of tectonic origin linked to the main foldingepisode that occurred subsequent to theMid-Triassic magmatic event in latest TriassicEarly Jurassic times. Inthe sills, the distribution of most maximum K1 axes is close to NS that corresponds to the maximumcompressive stress or folding directions; contrarily, the minimum K3 axes have an elongated distributionalong the EW direction or parallel to the fold axis before exchanging K1 and K3 axes for inverse fabrics, butclose to the NS stress direction after exchanging K1 and K3 axes for inverse fabrics. In the basaltic ows, theminimum K3 axes almost parallel to the NS folding direction. The structural interpretation of all AMS datataken from the igneous bodies is in accordance with a NNWSSE stress-related folding taking place around198 Ma in the Taimyr Peninsula (Arctic Siberia). The relationship of susceptibility axes to bedding surfacesand magnetic foliation planes is a criterion that permits differentiation of normal magnetic fabrics frominverse fabrics in the igneous samples. Single-domain (SD) and small pseudo-single-domain (PSD) magnetiteRochette et al., 1study magma ema powerful toolow and the loc

In contrast wemplacement mintrusive rocks, tsome type of p

interpret the ow direction inthe major compressive stressPeninsula. It proved that AMSd to identify the magma ow

ological Processes and Mineral100083, China. Tel.: +86 10

ll rights reserved.State Key Laboratory of Geological Processes and CICESE, Department of Geology, P.O. Box 43484al Resources, China University of Geosciences, Beijing 100083, Chinaiego, CA 92143, USAb Department of Earth Sciences, University of Bergen, Allgaten 41, 5007 Bergen, Norwayc d MinerMagnetic fabric and its signicance in theArctic Siberia

Shuwei Zhang a,b,c,, Edgardo Can-Tapia d, Harald Ja College of Computer Engineering and Software, Taiyuan University of Technology, Taiyuaills and lava ows from Taimyr fold-belt,

alderhaug b

30024, China

hysics

sev ie r.com/ locate / tecto991, 1999). Thus the AMS technique can be used toplacement mechanisms, and it has been shown to bethat allows us to infer both the direction of magmaation of its source.ith the established utility of AMS in the study ofechanisms of undeformed volcanic and shallowhe AMS of these types of rocks that have experiencedost-emplacement events (ranging from tilting to

folding) has been seldom addressed. Nevertheless, on the few studiesof this type that have been completed it has been shown that the AMScan provide clues concerning the extent of deformation with relativefacility (Gil-Imaz et al., 2002; Henry et al., 2003a; Masquelin et al.,2009). These studies also have shown that to make meaningfulinterpretations of the AMS data it is necessary to determine the rockmagneto-mineralogy, and to examine its implications including othergeological sources of information.

In this work we use the AMS of volcanic rocks from the southernTaimyr fold-belt to investigate: (1) the directions andmode of magmaemplacement in the volcanic rocks, and (2) the regional tectonics andstress regime of the area. Preliminary results of the AMS of some sillsand basalts from the region (16 from 24 diagrams in Fig. 10) werepublished previously (Zhang et al., 2008). Nevertheless, the magneticproperties of these rocks had not been studied in detail. Thus, inaddition to presenting previously unreportedAMS results and detailedmagnetic properties of these rocks, this paper is the rst one that usesAMS fabric to interpret the ow direction in both intrusive sills andbasaltic lava ows in terms of the major compressive stress directionsin these mac igneous rocks on the Taimyr Peninsula.

2. Geological setting

The Taimyr Peninsula occupies a central position in the geologicsetting of Arctic Siberia. It lies on the northerly edge of the Eurasianlandmass, neighboring the Arctic margin of Siberia, and containsigneous,metamorphic and sedimentary rocks of ProterozoicCretaceous

ages (Figs. 1 and 2). Three ENEWSW-trending structural/stratigraphiczones are traditionally recognized in this fold area: North, Central andSouth Taimyr, separated by the Main Taimyr Thrust (Fig. 1b) betweenNorth and Central Taimyr and by the Pyasina-Faddey Suture betweenthe Central and South Taimyr (Inger et al., 1999). Each of the three unitshas its own distinct composition and PrecambrianPaleozoic history,supposedly representing different blocks in the collision/accretionevents from Proterozoic to Cretaceous.

North Taimyr is referred to as the Kara Microcontinent. It com-prises mainly metasediments (Neoproterozoic to Cambrian), meta-morphosed up to amphibolite facies, with numerous intrusions, someof which yield Permo-Carboniferous ages (Pease and Vernikovsky,1998; Walderhaug et al., 2005). Ural-age orogeny is indicated byPermo-Carboniferous granite magmatism, deformation and meta-morphism (Inger et al., 1999).

Central Taimyr contains various accreted crystalline units(Neoproterozoic and older) including gneisses, island-arc and ophiolitefragments, and Permo-Carboniferous intrusive rocks (Pease andVernikovsky, 1998; Vernikovsky, 1997; Walderhaug et al., 2005).The Precambrian accreted terranes are unconformably overlain bythe RipheanMiddle Carboniferous continental margin succession(Walderhaug et al., 2005; Zhang, 2005). Central Taimyr has beenregarded by most authors as a separate microcontinent. However, theexistence of the Pyasina-Faddey Suture has been questioned becauseall of the sedimentary rocks were deposited on a stable margin or shelfenvironment, the structural style and geometry are very similar, andthe facies transitions are not only gradational but also related to the

augsina

69S. Zhang et al. / Tectonophysics 505 (2011) 6885Fig. 1. (a) Geological and locationmap of Arctic Siberia and adjacent areas (afterWalderhand Andersen (2002) and Walderhaug et al. (2005). MTT: Main Taimyr Thrust. PFS: Pya

Taimyr Lake. See also Fig. 2.et al., 2005). (b) Geological map of the eastern Taimyr Peninsula, modied from Torsvik-Faddey Suture. The study region is indicated in a rectangle on the northern margin of

70 S. Zhang et al. / Tectonophysics 505 (2011) 6885sedimentary environment shift in space and time in both the Centraland South Taimyr (Inger et al., 1999).

South Taimyr was the northern passive margin of Siberia. In thisunit, the Upper Carboniferous to Lower Triassic shallow-marine andcontinental clastic rocks are unmetamorphosed, interleaved andfolded with the Permo-Triassic extrusive and intrusive rocks of theTaimyr igneous suite (Figs. 1 and 2) (Pease and Vernikovsky, 1998;Vernikovsky, 1997; Vernikovsky et al., 2003;Walderhaug et al., 2005).The igneous rock types include sills, dykes, basaltic ows and a varietyof pyroclastic and volcanosedimentary rocks. In the sampling area, thesills are 35 m thick that intruded sediments lled with uids, and thebasaltic ows are 35 m thick. There are no obvious angularunconformities in the Palaeozoic carbonate-dominated sedimentaryunits in South Taimyr. The volcanic units were unconformablyoverlain by Early Jurassic sedimentary units that set the upper limiton the cessation of the Late Triassic folding event (Pease andVernikovsky, 1998; Vernikovsky, 1997; Vernikovsky et al., 2003). Inthe southern folded area, weakly lithied, at-lying Hettangian-lowerPliensbachian (Lower Jurassic) strata of the Zimnaya Formation lie onan eroded surface of Triassic and Palaeozoic rocks in the Yenisey-Khatanga Trough (Walderhaug et al., 2005). These facts, as well asisotopic data fromWalderhaug et al. (2005) and palaeomagnetic datafrom both Torsvik and Andersen (2002) andWalderhaug et al. (2005),suggest that: (1) the igneous rocks were relatively at lying duringmagnetization; (2) the strong compressional deformation in theNNWSSE direction was responsible for the ENEWSW-trending

Fig. 2. Geological map (after Walderhaug et al., 2005) of the studied area marked with a rectasite numbers labeled 1, 2, etc. These samples were from the same sampling sites of papalaeomagnetic study in Torsvik and Andersen (2002). Isotopic data are from Walderhaugstructures observed today; (3) the structural deformation occurredafter the magma episode; and, (4) there was no major deformationsince the latest Triassicearliest Jurassic.

Past research on these rocks has focused mainly on theinvestigation of their geochemistry, petrology, geochronology, paleo-magnetism and lithostratigraphy. For instance, Torsvik and Andersen(2002) suggested a paleomagnetic age of a pervasive remagnetizationat around 230220 Ma before folding of sediments of Riphean toPermian ages from north of our sampling area. Also, Walderhaug et al.(2005) presented palaeomagnetic ages of 220230 Ma and 220250 Ma for the mac sills and extrusive rocks, respectively. Thesepaleomagnetic-inferred ages are consistent with the isotope ages thatthey provided of 227229 Ma for the sills, and 248 Ma for oneextrusive sample to the south of our sampling area.

From a structural point of view, the Taimyr peninsula can bedivided into three families: foldswith ENEWSWfold axes, ENEWSWtrending thin-skinned thrusts, and EW-trending strikeslip faults(Inger et al., 1999). All of these mapped structures are associated withNNWSSE compressional stress. These dominant structural charac-teristics have led to different tectonic models, differing mainly on thetime of amalgamation/collision between the North Taimyr and CentralTaimyr (Zonenshain et al., 1990), or between South-Central Taimyrand northern Siberia (South Taimyr) (Pease and Vernikovsky, 1998;Vernikovsky, 1997). The Zonenshain et al. (1990) model needs asuture between Central and South Taimyr; however, the validity of thePyasina-Faddey Suture has been questioned (Torsvik and Andersen,

ngle in Fig. 1 and location of the sampling sites, which are marked with open circles andlaeomagnetic study in Walderhaug et al. (2005), but south to the sampling area ofet al. (2005).

71S. Zhang et al. / Tectonophysics 505 (2011) 68852002; Vernikovsky, 1997). In contrast, Inger et al. (1999) argued for atwo-stage model: (1) Permo-Carboniferous thin-skinned thrusting innorthern Central-South Taimyr (UralianOrogeny), is linked to the initialLate CarboniferousEarly Permian collision between South-CentralTaimyr and North Taimyr (Kara). Subduction in such a scenarioprobably took place beneath North Taimyr. (2) Late Triassic foldingand dextral strikeslip faulting, requires that the stratigraphywere at-lying before magma event. Torsvik and Andersen (2002) proposed anewmodel in which North Taimyr (KaraMicrocontinent) collided withnorth Siberia (South-Central Taimyr) in Late Triassic. Walderhaug et al.(2005) supported a strong proof for models provided by Inger et al.(1999) and Torsvik and Andersen (2002). Herein, we follow the Ingeret al. (1999) and Torsvik and Andersen (2002) models in which theSouth-Central Taimyr collided as a single unit with north Siberia (SouthTaimyr) in latest TriassicEarly or earliest Jurassic, with South-CentralTaimyr as a distal foreland basin during the late Palaeozoic UralianOrogeny (Torsvik and Andersen, 2002; Vernikovsky, 1997) that de-veloped thin-skinned thrusts in the Southern Taimyr (Inger et al., 1999;Torsvik and Andersen, 2002).

In particular, volcanic rocks in South Taimyr are deformed duringfolding and thrusting of the host rocks as part of the Uralian forelandbasin and show progressive deformation from south to north (Ingeret al., 1999; Walderhaug et al., 2005). The lithological similarity andstyle of deformation suggest that the southern region of Taimyrrepresents the strongly deformed margin of the Siberian craton(Yakubchuk and Nikishin, 2004). The brittle deformation impliesrotation of the sills and lava ows. Therefore, the Taimyr fold-belt is aninteresting case in which to study the effect of a specic stress regimeon the resulting AMS fabric. The discussion of the implications of theAMS study, with reference to previously published results, comprisesthe main thrust of this paper. We relegate to secondary importancethe slight possibility that rigid grains changed shape during thisrelatively brittle deformation. The application of the AMS technique todeformed sills and lava ows could be useful for interpreting the owdirection and the effect of internal deformation in igneous rocks.

3. AMS and magmatic fabric

AMS is approximated as a second rank tensor, and can be des-cribed by an ellipsoid with three principal eigenvectors and eigen-values, where K1NK2NK3. K1 is usually named magnetic lineationand K3 is normal to the so called magnetic foliation plane that isdened by the K1 and K2 axes. A number of parameters have beendened to describe the AMS fabric of a rock, such as the bulk sus-ceptibility (Km), corrected anisotropy degree (Pj) and shape param-eter (T). Km is the arithmetic mean of the susceptibilities ((K1+K2+K3)/3). Pj and T have been used to describe magnitude and shape ofthe magnetic susceptibility ellipsoid (Jelinek, 1978, 1981), respec-tively. Pj is given by equation

Pj = exp2 1 2 + 2 2 + 3 2 q

and T is given by

T = 2 23 = 13 1;

where 1=ln K1, 2=ln K2, 3=ln K3 and =(1+2+3)/3.T varies from +1 for a perfect oblate ellipsoid (pancake shaped,

K1K2K3) to 1 for a perfect prolate ellipsoid (cigar shapedK1K2K3) and is around 0 for neutral ellipsoids (i.e. 1bTb0:prolate fabric; 0bTb1: oblate fabric) (Jelinek, 1981). Other parame-ters have been described in the literature (Can-Tapia, 1994)although Pj and T are among those more commonly used to reportresults.

The AMS of any rock sample is the result of a combined con-

tribution of all its constituent minerals (Borradaile and Jackson, 2004;Jackson, 1991). In rocks where magnetic susceptibility is carried byeither Fe-bearing silicate paramagnetic minerals or antiferromagneticminerals like (titano)hematite or ferromagnetic pyrrhotite, thepreferred crystallographic orientation (magnetocrystalline anisotro-py) is more important than the grain shape (Jackson, 1991; Raposoand Berqu, 2008). Otherwise, in rocks where magnetic susceptibilityis carried by ferromagnetic minerals such as titanomagnetite (TM) ormagnetite the origin of AMS is intimately related to the grain shape(shape anisotropy), where K1 is parallel to the long axis of the particle(for multi-domain (MD) and pseudo-domain (PSD) grains). Althoughmagnetic grain interaction (Can-Tapia et al., 1996) or distribution(distribution anisotropy; Hargraves et al., 1991) within a rock mightbe important in some special cases, recent studies have shown thatthe AMS might still be dominated by the shape effect despite theoccurrence of magnetic grain interaction or distribution anisotropyin the rock (Can-Tapia, 2001; Gaillot et al., 2006; Grgoire et al.,1998).

The terms normal, intermediate and inverse fabrics wereintroduced by Rochette et al. (1991, 1992) for dike swarms. Thesefabrics are dened by considering the relationship between the dikeplane and the AMS eigenvectors. Then a normal AMS fabric occurswhen the AMS foliation (K1K2 plane) is nearly parallel to the dikeplane and the magnetic foliation pole (K3) is nearly perpendicular toit. An intermediate fabric is dened by K1 and K3 axes clustering closeto the dike plane while K2 axes are nearly perpendicular to this plane.An inverse fabric displays K2 and K3 axes forming a plane parallel tothe dike plane, with K1 perpendicular to the dike wall. For a group ofAMS fabrics, however, the intermediate fabrics can be found in atwo-component AMS such as normal and inverse fabrics (Hastieet al., 2011; Rochette et al., 1999). Furthermore, the measuring resultssometimes yield a random or disturbed distribution of magnetic axes(Rochette et al., 1999).

The abnormal fabrics (intermediate and inverse) can be primaryand secondary in origin (Borradaile and Gauthier, 2003; Herrero-Bervera et al., 2001; Raposo et al., 2007; Tauxe et al., 1998). The single-domain (SD) effect may occur and it is used to explain the inverseAMS fabric in the absence of another viable explanation (Borradaileand Gauthier, 2003; Dragoni et al., 1997; Jackson, 1991; Rochetteet al., 1999; Walderhaug, 1993). However, a mineralogical cause maybe not the only explanation for abnormal fabrics. The abnormalfabrics have usually been linked also to turbulent ow and somesecondary processes such as thermal contraction, magma convection,low-temperature oxidation and/or tectonic stress that overprintedthe normal fabrics acquired during magma emplacement (Rochetteet al., 1999). Also it is possible that a more complex fabric arises fromthe variation of shear within a single unit during emplacement(Can-Tapia and Chvez-lvarez, 2004; Can-Tapia and Herrero-Bervera, 2009).

Although magnetic lineation (K1) is generally assumed to be ow-related (Herrero-Bervera et al., 2001; Knight and Walker, 1988;Rochette et al., 1992, 1999; Tarling and Hrouda, 1993; Zhang et al.,1994), it turns out that K1 is not always an unequivocal ow directionindicator. It has been proposed that K1 can be ow-perpendicular(Caon-Tapia et al., 1995; Ellwood, 1978) and ow-parallel (Caon-Tapia and Pinkerton, 2000; Knight andWalker, 1988; Zhu et al., 2003)in lava ows and dikes, and even completely unrelated to the owlineation (Geoffroy et al., 2002; Gil-Imaz et al., 2006; Henry, 1997;Hillhouse and Wells, 1991). In some cases the K3 axes can be used fordetermining ow directions (Geoffroy et al., 2002; Gil-Imaz et al.,2006). Nevertheless, close attention to variations of orientation of themagnetic fabric relative to the boundaries of the cooling unit canprovide important clues that help us tomake a better interpretation ofresults (Can-Tapia and Herrero-Bervera, 2009).

An imbrication may occur and represents the real azimuth of owinmargins of a dike or for the same site during ow of themagma. The

imbrication angle is dened as the angle between the magnetic

foliation plane and the ow plane (or modied as the deviation of theK3 axis relative to the pole of a ow plane). In the presence of animbrication angle, the plunge direction of the magnetic foliationindicates the up-ow direction (Callot et al., 2004; Hillhouse andWells, 1991), so the obliquity of the K3 axes can be used to nd theow direction.

In the structural interpretation of AMS data, it has been acceptedthat for a normal fabric the K1 approximates the maximum stretchingaxis of a nite ellipsoid and that the magnetic foliation pole (K3)parallels to the maximum shortening axis. A consistent orientation ofK1 or K3 is a sufcient proof for shearing parallel to the lineation or forcompaction to the minimum axis (Hrouda, 1982).

4. Sampling and measurements

Igneous rocks of mac composition have been collected forpaleomagnetic analysis (Walderhaug et al., 2005) from easternSouth Taimyr. A total of 332 samples were prepared from cylindricalcores drilled at 23 sites, including 11 dolerite sills and 7 basaltic ows(Fig. 2) from both eld coring and hand sampling. The sills weresampled over a wide area, but the basaltic ows were sampled on alimb of a large open fold on the Hoffman Peninsula. Sampling was

KLY-3 Kappa Bridge with CS-3 furnace apparatus of AGICO Systems inthe plaeomagnetism laboratory at University of Windsor. Isothermalremanent magnetization versus eld (IRM-H) curves were obtainedfor 26 samples in maximum elds of up to 4.2 T. Hysteresismeasurements were measured using a Molspin Vibrating SampleMagnetometer (VSM) and a Petersen Instruments Magnetic Mea-surements Variable Field Translation Balance (MMVFTB) solenoidwith a maximum eld of 0.3 T in the plaeomagnetism laboratory atUniversity of Windsor.

5. Results

5.1. Magneto-mineralogy

5.1.1. Bulk susceptibility and optical examinationThe modied Knigsberger ratio (Q), is dened as the ratio in a

rock of remanent magnetization to the induced magnetization in theEarth's eld, i.e. Q=(4/107)(Jr/KH) SI (Barrre et al., 2009;Collinson, 1983; Dunlop and zdemir, 2010; Stacey, 1967), where His the magnitude of the geomagnetic eld. The Q-ratio is relativelyhigh (1bQb3) in most sill sites, and even higher and more variable(1bQb8) in the basalt ows (Table 1), indicating the SD or small PSD

Pj

1.1.1.1.1.1.1.1.1.1.1.1.1.1.1.1.1.1.1.

; Q

72 S. Zhang et al. / Tectonophysics 505 (2011) 6885generally not done near the margins of the ows. Tectonicorientations (strike and dip) for the igneous sites were obtainedfrom measurements of the associated sedimentary rocks, which isreferred to as bedding. Measurements of the azimuth and dip ofrocks were taken for all samples with two different marking systems,i.e. cores that were drilled directly in the eld were marked along thetop direction, while handsamples which were drilled in thelaboratory were marked along strike. Both sun (whenever possible)and magnetic compasses were used to orient the samples to avoidmagnetic confusion caused by the remanent magnetization of thesampled rocks.

AMS was measured using a KLY-2 induction bridge (sensitivity:2108 SI) in the plaeomagnetism laboratory at University ofBergen, by measuring susceptibility on a given specimen in 15different positions in a weak alternating inductive magnetic eld(1 mT) (Tarling and Hrouda, 1993). Mean tensors and parameterdetermination were calculated using Jelinek (1978) statistics. Densitycontour diagrams were used to present magnetic fabric results,allowing better visualization of the distribution of principal suscep-tibility axes. The temperature dependence of magnetic susceptibilitywas measured by heating a set of 8 samples in air to 700 C using the

Table 1Site mean anisotropy susceptibility data for unheated igneous rocks and sediments.

Site Rock type n Km(103 SI) Q

1 Sill 8 7.981 1.2942 Limestone 23 0.165 0.4123 Sill 15 7.276 1.0914 Sill 5 4.795 0.8555 Sill 5 6.345 0.6236 Sill 5 11.393 0.3548 Sill 7 9.551 1.5859 Sill 10 14.048 1.52010 Sill 17 5.581 1.89811 Sill 20 3.690 1.94212 Sill 5 14.924 2.24013 Sill 14 11.276 1.24214 Bas. Flow 8 15.270 0.26615 Bas. Flow 5 16.930 3.96216 Bas. Flow 6 20.371 1.39517 Bas. Flow 5 6.056 8.22019 Bas. Flow 7 13.126 3.24820 Bas. Flow 5 11.458 2.05523 Bas. Flow 6 5.297 2.245

n=Number of samples used to calculate means; Km=Bulk susceptibility (103 SI)

Flow=Basalt ows.grains contribute to the stable remanence (Collinson, 1983; Stacey,1967).

The bulk susceptibility (Km) of the igneous rocks is shown inTable 1 and Fig. 3. The frequency distribution of magnetic suscepti-bility reveals distinct differences in the distribution of susceptibilityvalues in the sills and basaltic ows. The lava ows yield highersusceptibility than the sills. The sills show a bimodal distribution(Fig. 3a), while the lava ows show a wide-range distribution ofmagnetic susceptibility with several intervals with maximum fre-quency (Fig. 3b). The bimodal pattern has two unequal peaks withunequal frequencies at susceptibilities 4.06.0103 SI and 12.014.0103 SI, and a dividing susceptibility of 8.09.0103 SI. Incontrast, the bulk susceptibility of the basalt ows displays severalpeaks at lower and higher values than those observed in the sills. Themost prominent peaks in the distribution are at 02103 SI, 46103 SI, 1012103 SI and 1416103 SI, and a relative one isfound at 1822103 SI.

Reected-light microscopy may assist in determining the origin ofmagnetization through assessment of the degree of deuteric and low-temperature oxidation affecting the opaque phases, observed inpolished sections. Fig. 4 shows some typical mineral textures using an

T K1(D/I) K3(D/I) BP (D/I)

021 0.299 359.9/7.9 92.7/19.4 28/81024 0.366 62.7/80.2 265.9/9.1 68/82027 0.190 211.3/13.4 321.6/55.5 101/70033 +0.021 221.0/27.9 109.7/34.4 65/83035 0.172 171.3/7.5 78.3/21.6 29/80016 0.050 219.3/17.7 101.4/55.8 1/70029 +0.161 23.7/51.7 127.9/11.0 354/62012 0.065 73.9/29.2 274.8/59.1 0/70026 +0.207 123.2/66.3 235.4/9.4 6/75054 +0.388 153.3/63.9 291.7/20.1 17/72036 +0.065 150.3/51.9 23.0/25.4 2/28022 +0.232 352.9/18.4 262.2/1.9 9/63042 0.059 206.1/67.4 1.6/20.8 347/55034 +0.341 94.0/19.7 328.8/58.2 350/54071 0.162 82.0/14.3 350.6/5.8 335/57013 +0.115 174.0/37.5 342.2/51.9 332/54010 +0.287 173.4/42.7 22.9/43.3 343/60014 0.336 125.2/21.1 350.3/61.3 339/61033 0.142 115.9/84.5 9.5/1.6 32/67

=(4/107)(Jr/KH) SI; D/I=Mean declination/Inclination; BP=Bedding pole; Bas.

oil immersion objective and high-resolution digital camera. Ingeneral, the studied sills (Fig. 4ad) contain plenty of relativelylarge TMs as the dominant magnetic mineral phase with intergrownilmenite lamellae (Fig. 4b and c). The intergrowth structures imply the

ilmenite lamellae formed as the result of high temperature oxidationand exsolution, and the large grain size attests to a relatively slowinitial cooling rate (Zhang, 2005). Such lamellae structures are smallenough to have SD-like hysteresis and magnetization (Feinberg et al.,2005), which in turn changes the hysteresis parameters and domainstates. A TM grain similar to a skeleton-shaped grain (Fig. 4d) wasobserved in one site where the margin of a magma ow cooled fasterthan the interior. Some intragrain shrinkage or curvilinear contractioncracks (Fig. 4a, c and d) and granulation features (Fig. 4ad) indicatelow-temperature alteration and formation of cation-decient phases,resulting in smaller magnetic grains due to the ner subdivision ofthe magnetic grains. The low-temperature oxidation features areconsistent with the fact that the sills intruded sediments lled withuid. In contrast, most of the basaltic ows contained homogeneousskeletal TM grains (Fig. 4e) attesting to the rapid cooling rate, andoccasional unskeletal TM grains (Fig. 4f) were observed as the interiorof the ow cooled slower than its marginal parts (Zhang, 2005). Noilmenite lamellae were observed in the basaltic ows.

5.1.2. Temperature dependence of magnetic susceptibility (kT) curvesFor the sills, all kT curves show high Curie temperature (Tc) at

~580 C on heating curves (Fig. 5ad), indicating the presence ofmagnetic minerals with chemical compositions close to magnetite.Most cooling and heating curves are generally reversible (Fig. 5a andb). Tc (~580 C), reversibility and a minor increase of susceptibilityafter the heatingcooling cycle indicate the magnetic mineral isgenerally magnetically and chemically stable and no magneticmineral has been created or destroyed. Some kT curves are almost

Fig. 3. Histogram of the mean susceptibility of (a) sills (108 samples) revealing bimodaldistribution, and (b) lava ows (56 samples) revealing unimodal distribution. See textfor explanations.

73S. Zhang et al. / Tectonophysics 505 (2011) 6885Fig. 4. Representative micrographs of oxide magnetic minerals showing typical mineral texS1-T5B, S13-T83C, S8-T57A, S12-T82D, S16-T111U and S23-T172U, respectively. Lengthtures; the sills (ad); the basaltic ows (e and f). Micrographs (af) are from samples

of scale bars=100 m.

74 S. Zhang et al. / Tectonophysics 505 (2011) 6885reversible before cooling to ~500 C, but show a distinct increase ofsusceptibility on cooling below ~500 C (Fig. 5c and d). The heatingcurve in Fig. 5c shows two different temperatures: The rst of thesetemperatures of interest is at ~410 C where it is observed a minorincrease in magnetic susceptibility that did not appear in the coolingcurve. This feature can be interpreted as resulting from: (a) anirreversible change of a small amount of maghemite inverting tohematite; (b) oxyexsolution of ulvospinel TM towards an inverse spinelmagnetite-like mineral because TM oxyexsolution in nature occursdown to temperatures as low as 300 C; or (c) the chemical alteration ofiron suldes such as pyrite or pyrrhotite although no sulde mineralswere observed by microscopy. Conversion of maghemite to hematite

Fig. 5. Representative curves showing susceptibility versus temperature and Curie temperaindicate heating and cooling curves, respectively.can also cause a susceptibility decrease in the heating cycle and muchlower susceptibility in the cooling cycle as suggested by Zhang et al.(2010). The second temperature of interest is ~585 C which is close tothe Tc of puremagnetite. Fig. 5d shows the puremagnetite as amagneticmineral with Tc at ~580 C. The distinct susceptibility increase aftercooling is caused by an increase in the concentration of magneticminerals or a change in the domain state.

For the basalts, the magnetic susceptibility generally decreasedafter a heatingcooling cycle but some curves like Fig. 5e and f showedlittle change, consistent with the data set presented by Zhang et al.(2008). On the heating cycle, susceptibility showed a slow increase inFig. 5e in contrast to the very sharp increase observed in Fig. 5f. During

tures from the sills (ad) and from the basalt ows (eh). The solid and dashed lines

heating, an inection point at 300 C occurred in Fig. 5f, and the sharpincrease of susceptibility indicates a domain state transformation. Incontrast with the uniformity of the sills, the basalt ows displayedcontrasting thermomagnetic behaviors. Besides the reversible curvesjust described, some irreversible cases were observed, as shown byFig. 5g and h. In these curves, a distinct susceptibility decrease isobserved after the heatingcooling cycle. A major Tc occurred at~480 C after a peak in magnetic susceptibility (Hopkinson effect) anda subordinate one occurred at ~565 C on the heating curve in Fig. 5g.The cooling curve showed the rst Tc at ~600 C and a second one at~510 C. The former one could indicate the production of titanohe-matite and the latter one might indicate the existence of TM. ForFig. 5h, a distinct Tc occurred at ~530 C and the heatingcooling cycleis almost reversible back down to ~460 C. On cooling below ~460 C,the magnetic susceptibility became relatively stable but much lowerthan that during heating, indicating the decrease of the TMconcentration by chemical conversion to weak magnetic minerals.

The difference in cooling rate has contributed signicantly to thefeature of thermomagnetic curves between the two rock types. The

sample had Bcr of 67.4 mT, probably indicating a mixture of magneticminerals with high and low coercive forces and minor hematite(Zhang, 2005).

Hysteresis loop measurements indicated PSD-like characteristics(Day and Fuller, 1977) for most rock samples. The sills mainly showedhysteresis parameters for remanent coercivity versus coercivity(Bcr/Bc) ranging between 1.5 and 2.5, saturation remanent magne-tization versus saturation magnetization (Mrs/Ms) varying from 0.2to 0.5 andhighBc values of 13.522.3 mT(Fig. 7ac; Table 3). Thebasaltsyielded parameters for Bcr/Bc between 1.3 and 4,Mrs/Ms in the range of0.10.3, and relatively low Bc values of 3.518.1 mT (Fig. 7a, b, d and e;Table 3), with a few exceptions when hard (antiferromagnetic)minerals are present. Some of the coercivity behavior that the sills andbasalts exhibited suggest the existence of a vortex state, the non-uniformly magnetized remanent states between single- and multi-domain (Selkin et al., 2007; Tauxe et al., 2002) that contributes toPSDhysteresis behavior as suggestedbyTauxe et al. (2002) andWillamsand Dunlop (1995) (Fig. 7b). A mixture of SD+MD grains in thesesamples cannot be ruled out since Day's limits for PSD data agree well

75S. Zhang et al. / Tectonophysics 505 (2011) 6885slower cooling rate in the sills allowed for more extensive exsolutionto fully arrange the low-Ti magnetite and high-Ti phase. Theconsistent high Tc of the low-Ti magnetite will reveal generally areversible feature during heating (Fig. 5a and b). In contrast, the morerapid cooling rate in the basaltic ows might inhibit exsolution tocause the presence of a mixture of many metastable low-Ti and high-Ti phases in TM crystals. The variation of Tc corresponds to thecompositional dependence of the Tc of TM and reects the possibleinuence of high-temperature cation vacancies on the Tc values. Thus,the process of heating led to the irreversible transformation of themagnetic minerals (Fig. 5g and h).

5.1.3. IRM, backeld curves spectra and hysteresisThe sills showed consistent isothermal magnetic properties,

achieving 90% saturation of IRM in elds below 200 mT, and allsamples showed saturation at elds below 400 mT. Backeld IRMsindicated that the remanent coercivity (Bcr) ranged from 28.8 to38.9 mT (Fig. 6a), thus implying that the majority of magnetic carrierswere small PSD or SD TMs in the sills (Cisowski, 1981). In contrast, inFig. 6b, most basaltic ows reached saturation below 100 mT andpossessed low Bcr values in the range of 9.3 to 16.7 mT. A lessernumber of samples showed Bcr values in the range of 16.7 to 34.7 mT.It corresponds to TM grain behavior that is, respectively, large PSD toMD and small PSD to SD in character (Cisowski, 1981). Some sampleshave saturation paths with a fast (steep) increase at coercivities below300 mT and a more gentle continued increase up to a much largermagnetizing eld of 1.5 T, indicating the presence of hematite. OneFig. 6. Isothermal remanent magnetization acquisition and back-eld curves of (a) thwith SD+MD properties according to the revised limits recentlyproposed by Dunlop (2002). Pot-belly hysteresis loops of most sillsamples (Fig. 7c) are not closed above 0.4 T and relatively squared,indicating SD (small PSD)-like features or high-coercivity ferromag-netic grains (Pick and Tauxe, 1994; Tauxe et al., 1996). The hysteresisloops of the basalts are pot-bellied (Fig. 7d) or waisp-waisted (Fig. 7e),indicative of mixtures of ferrimagnetic and antiferromagnetic min-erals or different grain sizes (Roberts et al., 1995; Tauxe et al., 1996), assuggested by IRM acquisition curves.

In summary, the magnetic properties of the studied rocks indicatethat the sills contained a unique magnetic phase (most likely puremagnetite grains) both in the small PSD and SD ranges. In contrast, thebasaltic ows could have two phases. The predominant phase (TM)with a similar range of grain size (i.e. predominant large PSD to MD)was accompanied in some cases withminor amounts of hematite. Thisinterpretation is consistent with the observation of the sills showingmore homogeneous magnetic properties than the basalts. In addition,the multimodal distribution of Km values for the lava ows is deemedto be related to the change in TM concentration or the Ti content in TM(Fe3xTixO4) because susceptibility decreases systematically withincreasing Ti content in the low eld range (Jackson et al., 1998). Thisvariation can be compared to the consistently high Tc (~580 C) in thesills, which most likely indicates a relatively uniform chemicalcomposition of magnetic phase within the sills. Therefore, the mostlikely explanation for the bimodal distribution of Km of the sills is thepresence of twomagnetic populations with different concentration orgrain sizes of a relatively uniform ferromagnetic phase magnetite.e sills and (b) the basalt ows. Note change of H/mT scale for backeld curves.

pe fgne

76 S. Zhang et al. / Tectonophysics 505 (2011) 6885Fig. 7. (a) Day-diagram (Day and Fuller, 1977) showing the possible magnetic domain ty(b) Modied from Tauxe et al. (2002). The saturation remanence (Mrs) to saturation ma5.2. Magnetic fabrics

5.2.1. Correlation of Km and PjThe effect of the magnetic mineralogy content on the degree of

arrangement of the magnetic minerals can be assessed by examiningthe correlation of the bulk susceptibility (Km) with the degree ofmagnetic anisotropy (Pj). As shown in Fig. 8, it is possible todistinguish both types of rocks from the KmPj diagrams, although Pjof both lava ows and sills is similar. In the case of sills, (Fig. 8a), it ispossible to identify three types of behavior based on the values of Km.Samples with higher Km (N8.0103 SI) display an almost constantKm for the whole range of observed Pj. In contrast, for the sampleswith lower Km, Pj tends to increase as Km increases. In theintermediate range, Km tends to decrease as Pj increases. These resultssuggest that an increase in magnetic susceptibility does not alwaysimprove magnetic interaction in the sills. The two main types of fastincrease in Pj with slow decrease in Km may be related to the bimodaldistribution of Km displayed in Fig. 3a. It is likely that one of the twomain populations of magnetite grains in the sills has a relatively largerdegree of interaction than the other, each population having adistinctive size distribution, and the rock AMS may be decomposedinto two contributions. This would be consistent with our interpre-tation of sills containing both small PSD and SD grains as magneticconstituents. Thus, the regression lines (a slow decrease of Km versus afast increase of Pj) indicate that the degree of anisotropy is mainlyrelated to the shape-dominated AMS. In contrast with the sills, Fig. 8bshows that the lava ows have only a small increase of Km versus a fastincrease of Pj above 5.0103 SI. This behavior suggests that anincrease in magnetic concentration might have led to a slightlystronger interaction between the grains.

This interpretation agrees with the Can-Tapia (2001) model andGaillot et al. (2006) results that provide variables used to assess the

(blank circle). Hysteresis loops from raw data (c) of the sills showing pot-bellied shape, and dor our samples; most samples with a mixture of MD and SD grains fall into the PSD eld.tization (Ms) ratio is called squareness (Tauxe et al., 2002). Sills (black triangle). Basaltsrelative importance of magnetically interactive and non-interactivefractions in a rock. Since a natural rock is an assemblage of interactingand non-interacting grains, magnetic interactions may either increaseor reduce the magnitude of the whole-rock anisotropy, depending onfactors such as relative orientation in space, spatial distribution,distance between neighboring grains, nite geometry, grain size or

and e of the basaltic ows showing pot-bellied andwaisp-waisted shapes, respectively.

Fig. 8. Km versus Pj plots for specimens of the (a) sills, and (b) lava ows. See the text forexplanations.

increasing anisotropy there are triaxial fabrics in the sills and slightlylinear fabrics in the basalts.

Because the magnetic fabric of deformed rocks sometimes mightcontain an overprint that is either enhanced or removed duringheating in the laboratory (e.g., Souque et al., 2002), we remeasuredthe AMS of a selected number of samples after heating them to amaximum temperature of 600 C (which was kept for 40 min). In thesills Km had an increase of 20% or more during heating, while in thebasaltic ows Km generally decreased (Tables 1 and 2). The increase ofKm in the sills and decrease in the basalts were consistent with thethermomagnetic results (Fig. 5). Transformation of magnetic miner-alogy (chemical compositions andmagnetic grain sizes) upon heatingwas responsible for the susceptibility change (Henry et al., 2003b; Hirtand Gehring, 1991; Pan et al., 1999, 2000; Zhang et al., 2008). Thenewly createdmagneticminerals might have a different orientation ofthe fabric relative to the unheated fabric (Fig. 10). Actually, improve-ment in the grouping of AMS axes after heating has been reportedpreviously for igneous, metamorphic and sedimentary rocks (Jelenskaand Kadzialko-Hofmokl, 1990; Trindade et al., 2001; Walderhaug,1993). The reasons for such changes in the susceptibility and its tensorafter heating can reside in the fact that heat treatment can acceleratethe diffusion rate of Fe2+ ions (Liu et al., 2004) and make chemical

77S. Zhang et al. / Tectonophysics 505 (2011) 6885shape, and bulk susceptibility distribution (Can-Tapia, 2001; Gaillotet al., 2006; Stephenson, 1994).

5.2.2. Shape and orientation of magnetic ellipsoidsMagnetic susceptibility ellipsoids were plotted on PjT diagrams

(Fig. 9) and the relative values are shown in Tables 1 and 2. Bothoblate and prolate shapes are observed in the studied samples. With

Fig. 9. TPj diagrams (a) sills and (b) basalts. Unheated (solid) and heated (empty). Seetext for interpretations.

Table 2Site mean anisotropy susceptibility data for heated igneous rocks and sediments.

Site Rock type n Km(103 SI) Pj T

1 Sill 6 10.096 1.010 2 Limestone 6 23.279 1.057 +3 Sill 6 9.187 1.022 +4 Sill 6 17.465 1.005 +5 Sill 6 8.689 1.011 +6 Sill 5 16.579 1.006 8 Sill 5 11.397 1.019 9 Sill 5 16.329 1.008 10 Sill 6 10.135 1.021 11 Sill 6 6.619 1.020 +12 Sill 6 11.313 1.022 +13 Sill 6 12.644 1.015 14 Bas. Flow 5 12.181 1.020 15 Bas. Flow 5 10.827 1.011 +16 Bas. Flow 6 7.957 1.004 +17 Bas. Flow 6 6.545 1.007 +19 Bas. Flow 6 8.259 1.003 +20 Bas. Flow 6 5.261 1.004 +23 Bas. Flow 6 19.162 1.015 +

n=Number of samples used to calculate means; Km=Bulk susceptibility (103 SI);N=Normal fabric; IT=Intermediate fabric; IV=Inverse fabric.alteration faster (Henry et al., 2003b; Walderhaug, 1993). Magneticannealing by heating to 600 C also affects the crystal structure andthe grain size, thus leading to a change of mineralogy (Henry et al.,2003b), a fabric disappearance, and a decrease inmagnetic interactionbetween neighboring grains. Such newly created fabrics are signi-cant to understand the process of formation (deformation) of therocks, or they are just an artifact of laboratory conditions.

In our case, it was observed that after heating, the average valuesof the T and Pj parameters changed from +0.027 to 0.025 andfrom 1.028 to 1.015, respectively, in the sill samples (Fig. 9a; Tables 1and 2), and from +0.006 to +0.316 and from 1.031 to 1.009, in thelava ow samples (Fig. 9b; Tables 1 and 2). The observed decrease ofPj after heating in both types of rocks suggests the removal of asecondary anisotropy perhaps related to both tectonics and lowtemperature oxidation from the pre-heating fabric similar to the effectdescribed by Park et al. (1988).

The directional analysis was performed by means of stereographicplots of principal susceptibility axes. To discuss the magnetic fabric,we dene a normal fabric (such as unheated sites S12, 17, 19, andheated site S2 in Fig. 10) when the site mean K3 axis is close tothe bedding pole (i.e. the site mean magnetic foliation plane nearly

K1(D/I) K3(D/I) BP (D/I) AMS

0.187 349.9/65.5 87.6/3.5 28/81 IV0.798 261.4/7.4 45.3/80.9 68/82 N0.015 111.3/51.8 266.8/35.6 101/70 IV0.273 34.6/16.9 299.2/17.2 65/83 IT0.287 306.7/64.8 93.0/21.3 29/80 IV0.045 347.6/48.6 190.4/39.1 1/70 IV0.131 18.8/9.0 113.4/26.9 354/62 IT0.147 352.4/48.4 254.6/6.9 0/70 IV0.323 94.5/76.2 231.9/10.2 6/75 IV0.059 99.5/78.5 276.9/11.4 17/72 IV0.255 28.9/43.0 250.8/38.6 2/28 IV0.332 28.7/56.3 136.6/11.6 9/63 IV0.056 114.9/17.5 24.2/2.3 347/55 IT0.452 110.3/19.3 337.1/62.9 350/54 N0.567 195.9/52.2 343.9/33.3 335/57 N0.487 101.5/18 2.7/25.1 332/54 IT0.251 262.5/31.1 1.5/14.6 343/60 IT0.045 334.8/49.5 201.5/30.3 339/61 IV0.465 232.3/10.1 129.0/52.3 32/67 IT

D/I=Mean declination/Inclination; BP=Bedding pole; Bas. Flow=Basalt ows;

Fig. 10. Stereograms of AMS directional data for unheated (white) and heated (gray) specimens calculated for representative sites. Sills (S1, 3, 5, 8, 1013), basaltic ows (S16,1720), and limestone S2.

78 S. Zhang et al. / Tectonophysics 505 (2011) 6885

fabric patterns with very oblate AMS ellipsoids that are normal witha horizontal mean K1 axis direction in theWSWdirection. Pj increasedfrom 1.023 to 1.057 after heating. The inversed feature of unheatedS2 (Fig. 10) is most likely related to compaction and deformation ofthe sediments, during which the oblate magnitude ellipsoid changedtowards a weak prolate ellipsoid (Table 1), the K3 direction wasreoriented in a direction parallel to the shortening, while the K1direction was orientated perpendicular to the shortening direction(Hrouda, 1982). However, we cannot preclude effects of iron-bearingparamagnetic minerals such as carbonate.

6. Discussion

6.1. Magnetic grain size

It is necessary to think about causes of the abnormal fabrics in theheated sills, which is inconsistent with the usual feature of a sheet-likeintrusion. Nevertheless, as will be shown, such difference between thesills and the basalts is coherent with the distinct rock magneticproperties between the two types of rock samples.

IRM and backeld curves spectra (Fig. 5) and hysteresis results(Fig. 6; Table 3) suggest the presence of ner (SD or small PSD)magnetite grains in the sills than in the basalts that have coarser(large PSD or MD) TM grains. Direct comparison made between AMS

79S. Zhang et al. / Tectonophysics 505 (2011) 6885parallels to the bedding plane) (Rochette et al., 1991, 1999; Tauxeet al., 1998). In contrast, an inverse fabric (such as unheated sites S2,10 and 11 and heated sites S1, 3, 5, 1013 and 20 in Fig. 10) occurswhen the site mean K1 axis is close to the bedding pole (i.e. the sitemean magnetic foliation plane is nearly perpendicular to the beddingplane and the symmetry is inverted). Other cases are labeledintermediate fabrics. Using this categorization, it is clear that differenttypes of magnetic fabrics have been dened in both rock types.

Signicant changes in orientations and shapes of susceptibilityellipsoids took place as a result of heat treatment (Zhang et al., 2008).In the studied sills, the shallow-inclined K1 axes generallymoved fromclose to the bedding plane towards the bedding pole, while the K3axes moved towards the bedding plane (Fig. 10), with a fewexceptions such as site S8, thus increasing the likelihood of thecreation of new inverse fabrics. Before heating, the intermediatefabrics account for 65%, and there is a strong tendency for the K1 axesto be subvertical while many of the K3 axes are subhorizontal tosteeply inclined. After heating, the inverse fabrics account for 80%,and most of the K1 axes are steeply inclined whereas many of the K3axes are subhorizontal to semihorizontal. There are fewer normalfabric sites (ca. 9% of unheated sites but zero in heated sites,respectively) in the sills. In contrast, the basaltic sites display morenormal magnetic fabrics (ca. 60% and 40% of unheated and heatedsites, respectively) and fewer inverse sites (ca. 14% in both heatedand unheated sites). Most basaltic samples underwent an interchangeof the K1 and K2 axes during heating, while the K3 axes were relativelystable (e.g. sites S17 and 19 in Fig. 10). The K3 axes are generallyoriented along NNESSW directions. Post-heating fabrics becameslightly more scattered than the pre-heating fabrics. S20 showed afabric conversion trend from a normal to an inverse fabric.

In unheated igneous samples, many fabrics have dispersion (e.g.unheated sites S1, 3, 5, 8, 13 and 20 in Fig. 10). When the dispersionoccurs, it is necessary to consider the causes of fabric dispersion beforemaking any statistical interpretation. In any case, such dispersionusually is indicative of more than one population (in a statisticalsense) being sampled. In a owing magma within stationaryboundaries (i.e. a sill in our case), the shear is zero (or small at anyrate) in the center and increases toward the outer margins, thusresulting in an imbrication fabric towards the upper or lower contactsurfaces of a tabular intrusion (Can-Tapia, 2001; Can-Tapia et al.,1996; Can-Tapia and Chvez-lvarez, 2004; Dragoni et al., 1997;Geoffroy et al., 2002; Hillhouse and Wells, 1991; Knight and Walker,1988), with the magnetic foliation oblique to the bedding. Theobliquity angle in sites S3, 5 and 1013 might reect that most sillsites are not far from the country rocks, consistent with the fact thatthe sills are only 35 m thick. Fabric dispersion in our samples isconsistent with the fact that our sampling was generally not donenear the margins, thus indicating these samples contain more thanone population. S3 and S13 show an equal combination of inverseand intermediate or normal fabrics. Inverse fabrics dominate in S10and S11. These inverse patterns cannot be ascribed to localturbulence in the magma ow, as this process typically producesrandom orientations of all susceptibility axes (Knight and Walker,1988; Rochette et al., 1999). However, they are most likely caused bythe axis inversion of magnetic grains such as SD or small PSD grains, ashysteresis results (Fig. 6; Table 3) and rock magnetic mineralogy(Figs. 35) suggest the presence of SD (or small PSD) grains in the sills.A prolate magnetic fabric with a high degree of anisotropy, carried bytwo or more populations of TM (or titanomaghemites) may also beresponsible for an inverse fabric (Callot et al., 2001). In contrast, anormal fabric is usually carried by a single population of magnetite orTM. The dispersion of S1 might be caused by the turbulent magmaow. The prolate fabrics in S1 and S5 are mostly related to magmaow kinematics where the magnetic lineation is interpreted toindicate the ow direction (Ellwood, 1978; Gil-Imaz et al., 2006;

Knight and Walker, 1988).Magnetic fabrics in heated igneous samples have some dispersionand show different orientations relative to the unheated fabrics(Fig. 10), probably as a result of mineralogy transformation (i.e.change in chemical compositions and grain sizes) and/or crystalstructure change during heating. Creation of ner magnetic grains (SDto small PSD), due to extreme subdivision of larger magnetic grains(MD-PSD) during annealing by heating, could be responsible for theexchange of AMS axes and a higher percentage of inverse fabricsoccurring in heated sills than in unheated sills (Zhang et al., 2008).Alternatively, it could indicate the preferred nucleation of magneticphases along a zone of accumulated stress. In any case, the dominationof a consistent oblate pattern in heated basalts suggests effects ofchange in the crystal structure on the basalts during heating.

Unheated limestone (S2 in Fig. 10) displays prolate fabrics that areinversed with a horizontal mean K3 in the WSW direction, which isinterpreted to be susceptibility anisotropy with a vertical stretchingdirection. In contrast, heated limestone shows classic depositional

Table 3Direct comparison between AMS type and Q value, Bc and Mrs/Ms.

Site Rock type Q Bc (mT) Mrs/Ms AMS Type

1 Sill 1.294 15.50.2 0.220.03 IT2 Limestone 0.412 68.730.5 0.300.14 IV3 Sill 1.091 16.62.0 0.290.11 IT4 Sill 0.855 15.11.5 0.230.04 IT5 Sill 0.623 18.62.8 0.360.15 IT6 Sill 0.354 18.33.6 0.340.14 IT8 Sill 1.585 18.21.6 0.260.06 IV9 Sill 1.520 15.01.5 0.300.17 IT10 Sill 1.898 17.62.0 0.220.03 IV11 Sill 1.942 15.90.7 0.380.13 IV12 Sill 2.240 16.31.2 0.220.01 N13 Sill 1.242 16.11.7 0.240.04 IT14 Bas. Flow 0.266 6.40.9 0.140.04 IT15 Bas. Flow 3.962 77.260.0 0.260.02 N16 Bas. Flow 1.395 5.940.03 0.130.05 IT17 Bas. Flow 8.220 12.95.2 0.210.05 N19 Bas. Flow 3.248 15.02.9 0.190.06 N20 Bas. Flow 2.055 8.64.9 0.180.11 N23 Bas. Flow 2.245 8.83.3 0.220.10 IV

Bc=Hysteresis coercivity; Mrs/Ms=Saturation remanence versus saturatedmagnetization; Bas. Flow=Basalt ows; N=Normal fabric; IT=Intermediate fabric;IV=Inverse fabric.and Bc and Mrs/Ms (Table 3) indicates that the sills have a higher

average coercivity (BcN15 mT) than the basalts that have an averagecoercivity (Bcb8 mT). Inverse sites (i.e. S8, 10 and 11) displayhighest value of modied Q, Mrs/Ms and Bc (Table 3) in the sills. Inorder to test the grain size of the two types of rocks, the bilogarithmicplot (saturation remanence versus bulk susceptibility) was used. Itindicates that the sills and basalts contain magnetic minerals in grainsize b12 m and 135 m, respectively (Thompson and Oldeld,1986). The small PSD or SD magnetic size of the sills is also evidenced

by the lower susceptibility values of 5.581103 SI and 3.69103 SI for sites S10 and S11, respectively (Table 1). This is consistentwith the susceptibility value of SD or small PSD grains being lowerthan superparamagnetic (SP), PSD or MD sizes (Henry et al., 2003b;Stacey and Banerjee, 1974) of the same magnetic phase.

Notably, the information obtained from the magnetic mineralogyexperiments differs from the information obtained through opticalexamination of the samples, indicating that the physical crystal sizes

malips. Cof re3 foter

80 S. Zhang et al. / Tectonophysics 505 (2011) 6885Fig. 11. Diagram of the Taimyr igneous setting showing the nal inferred and anticipatedfold axis direction and two red arrows represent the deformation direction. The lower elsills and the basaltic ows, respectively. Dashed lines represent possible stress directionsand e have the number of specimens n=108. Heated sills have n=63 for c, d and f. e andand K3 for inverse fabrics. Unheated lavas have n=56 for h and j. Heated lavas have n=58.03, 11.54 and 11.70 in hk, respectively. No bedding correction; low hemisphere. (For in

version of this article.)gma sources and the stress regime derived from AMS data. The blue line represents thee (ESEWNW-trending) and upper ellipse (NESW-trending) indicate the source of thentour data are plotted using Johannes Duyster program in equal area. Unheated sills a, bpresent restored K3 axes of unheated and heated sills, respectively, after exchanging K1r i and k. The maximum density=10.45, 12.65, 6.32, 5.98, 12.66 and 6.34 for af, 13.29,pretation of the references to color in this gure legend, the reader is referred to the web

of magnetic minerals in the sills are coarser than in the basalts (Fig. 4).Such a discrepancy, however, can be explained if one considers thatphysical grain size and chemical composition in the sills and basaltswas controlled by the cooling and quenching rate. Lamellae textures(Fig. 4b and c) in the magnetic minerals of the sills as well as the lowtemperature oxidation (Fig. 4ad) effectively reduce the magneticgrain size. Thus, the effective magnetic grain size might be muchcoarser in the basalts than in the sills (Zhang et al., 2008), andtherefore, the presence of SD or small PSD grains could explain thehigher coercivity of the sills (Liu et al., 2004; Yu et al., 2004).

6.2. Interpretation of AMS fabric

6.2.1. Structural effects on magnetic fabricsThe Mid-Triassic magma emplacement in Taimyr could be

assumed to have been horizontal (Torsvik and Andersen, 2002;Walderhaug et al., 2005). The variation in strength between the

to an ENEWSW girdle (i.e. the fold axis in Fig. 11g). These are unlikemost cases where the K1 axes are nearly parallel to the fold axis andthe K3 axes are perpendicular to the folding deformation. However,our interpretation of this inverse relationship is supported by the rockmagnetic properties, which indicates a larger proportion of SD orsmall PSD magnetite grains in the sills than in the basalts.

As described previously, it is possible to get new normal fabrics byexchanging the K1 and K3 axes for inverse fabrics (Gil-Imaz et al.,2002; Rochette et al., 1992). The restored K3 axes corresponding tounheated (Fig. 11e) and heated (Fig. 11f) sill samples are plotted. Thedensity distribution of K3 axes shows a ca. NS distribution in unheatedsills and a distinct NNESSW distribution in heated sills, so therestored K3 axes are interpreted to indicate themaximum shortening(the folding) direction. However, the density distributions ofrestored K1 axes for both unheated and heated sills do not indicatethe fold axis direction.

As for the lava ows, in Fig. 11h and i, the K3 axes of both unheated

basad in

81S. Zhang et al. / Tectonophysics 505 (2011) 6885studied igneous rocks and interlayered sedimentary rocks may haveresulted in changes in the fold shape during brittle deformation, whilebed thickness could have remained relatively stable (Robert andHatcher, 1990), thus allowing application of a bedding tilt correctionto reveal the arrangement of magnetic minerals in these rocks (deWall and Warr, 2004). Application of a bedding correction using thebedding strike and bedding dip at the site level rather than using thedeclination and inclination of each specimen, decreased the anglebetween the vertical and minimum K3 axes of normal fabrics, as thecorrection assumed that the present dip is due to post-emplacementtilting, and could better average and cluster the resultant distributionof susceptibility axes (Henry et al., 2003a). Thus, AMS data afterbedding correction from the studied igneous rocks could potentiallyreveal a primary magnetic fabric.

In Fig. 11ad, the bedding uncorrected K1 and K3 axes of the sillsare plotted. For unheated samples, K1 axes are concentrated in twomain zones, one subhorizontal along an approximate NNESSWdirection and the other along a SSE subvertical position (Fig. 11a). Thedistributions of K3 axes (corresponding to an ESEWNW girdle) aredened with a maximum concentration in a WNW subhorizontalposition (Fig. 11b). For the heated samples, the K1 axes (Fig. 11c)experienced the most distinct change as a group, although notably thesubhorizontal concentration remained almost unchanged. The changein the orientation of these axes was therefore mostly observed inthose having a subvertical orientation, since these axes passed from aSSE sub-vertical direction to two subgroups, one also nearly verticalbut with an N orientation and onewith intermediate plunges and NNEorientation. The position of the heated K3 axes remained almostunchanged relative to those of the unheated samples (Fig. 11d) In thiscontext, it appears that the K1 axes are nearly parallel to the NNWSSEdirection (i.e. the folding in Fig. 11g), while the K3 axes are very close

Fig. 12. Determination of the ow vector and symmetry plane for magma like the sills orlike sill and (b) vertical plane viewmodied from Callot et al. (2001); (c) an AMS ellipsoi

2006) from the top.and heated lava samples were well clustered towards the north,perpendicular to the fold axis and parallel to the distribution ofbedding poles in Fig. 11l, indicating a maximum compressive stressfrom the north as a result of the tectonic stress responsible for theTaimyr folds in the latest Triassicearliest Jurassic. In contrast, the K1axes in Fig. 11j and k were more scattered than the K3 axes, and hadsomewhat better similarity to the fold axis direction in heated lavasthan in unheated lavas, perhaps owing to the small low samplenumbers or local turbulence of the ow.

6.2.2. Magma ow directionIn order to evaluate the effect of the magma ow on the studied

rocks, it is necessary to examine the best plane of symmetry of theaxial distribution, which is perpendicular to the ow plane (Henryet al., 2003a). This symmetry plane includes the K3 axes and theplunging direction of the magnetic foliation (Fig. 12c and d). In mostcases (i.e. normal fabrics) the K1 axes are within or close to the planeof symmetry, and ow directions inferred by invoking either the K1axes or the imbrication angle are similar (Fig. 12); that is, the directionof the azimuth of the K3 axes is parallel to the ow vector in an equal-area projection and the magnetic lineation should plunge in the up-ow or toward the ow direction (Callot et al., 2004).

For the case of sills where the K1 axes are close to the subverticaland the K3 axes are within subhorizontal planes, it is clear that such K1axes cannot indicate the ow direction. In a sill with horizontalparallel walls, or when restoring the structure of dipping sills to thehorizontal, the ow vector would be contained in a vertical planeperpendicular to the strike of the magnetic foliations measuredon each sill wall (Gil-Imaz et al., 2006). This implies that in thesecases the K3 axes can indicate the ow vector (Geoffroy et al., 2002;Gil-Imaz et al., 2006; Hillhouse andWells, 1991). As stated previously,

lts, in case of horizontal ow and normal magnetic fabrics. (a) Velocity prole of magmamagma and the symmetry plane; and (d) equal-area projection (following Gil-Imaz et l.,

all of the sediments and sills were at-lying during magma em-placement and the subsequent folding deformation is brittle;therefore, it is reasonable to infer a primary sense of ow by invokingthe bedding corrected K3 axes (Fig. 13a and b), by applying the valueof bedding dip for each site to the AMS data for all individual samplesat the site level. Most K3 directions were oriented horizontally tosubhorizontally in a major ESEWNW direction and a secondary SSWdirection, and these are considered to be the ow vectors (Fig. 13aand b). The dispersion of the bedding corrected data could beinterpreted as an original feature of the AMS fabric. The observedlow-symmetry pattern of distribution of the K3 axes may also berelated to sampling limitations.

Such ow directions can be veried by individual sill sites. Inunheated sites, both the K3 axes and K1 axes in S3 are clustered intotwo groups that are perpendicular to each other, indicating thepresence of ow-perpendicular K1 axes and ow-parallel K3 axes. Inthe normal fabrics of S3, the K1 axes are clustered in the inferred SSWand ESE ow directions, as well as some of the nearly horizontal K3axes in the WNW direction in inverse fabrics. S5 shows intermediatefabrics with the K1 axes clustered in the SSE direction. In S8, thehorizontal K3 axes of the inverse fabrics have SSE directions. Twogroups of K3 axes in the subhorizontal position and a group of K1 axesin the subvertical position again indicatedWNW and SSW ow in S10and S11. In heated sites, almost all of the K3 axes have ESEWNW orSSW orientations (e.g. sites S1, 3, 5, 8 and 1013 in Fig. 10). Heatedsites S10, 11 and 13 show well-dened axes with the K1 axes sub-vertical and the K3 axes subhorizontal, and the latter may indicate theow direction.

In the case of the lava ows, it was observed that after beddingcorrection these units often demonstrated quite scattered AMSdirections at the site level. Even if this dispersion casts some suspicion

82 S. Zhang et al. / Tectonophysics 505 (2011) 6885on the reliability of these directions, only 7 sites could not decreasesuch scatter in Fig. 13c and d, and the magnetic fabric results are stillsuitable for statistical analyses regardless of their dispersion. The

Fig. 13. Determination of the ow directions in contour diagrams. K3 axes of (a)unheated sills (n=108) and (b) heated sills (n=63). K3 axes of (c) unheated basalts(n=42) and (d) heated basalts (n=40). The maximum density=11.49, 7.30, 8.80,14.45 and 10.27 for ad, respectively. Contour data are plotted using Johannes Duysterprogram in equal area using the bedding corrected (restored) AMS data. Bold lines

show the mean ow directions.basaltic samples also exhibitedmore clustered K3 axes than the K1 andK2 axes, and therefore the direction of the K3 axes can be interpretedas a relatively strong imbrication. Bedding corrected K3 axes showed arelatively tight and subvertical concentration in unheated basalts, butthe K3 axes of the heated basalts were less well clustered and notsubvertical (Fig. 13c and d). The obliquity of the K3 with respect to thehorizontal plane (i.e. the ow plane or bedding plane after beddingcorrection) may be used to determine the ow direction; that is, theow direction could be identied by the intersection of the K3-verticalplane with the horizontal plane, with the sense of obliquity furtherdening a ow direction (Rochette et al., 1999).

For unheated basalts, the elongated distribution of beddingcorrected K3 axes is around a plane that dips strongly towards theENE to SSE (Fig. 13c), indicating an average ow direction mainlyfrom WSW to ENE and secondarily from NNW to SSE. For heatedbasalts, the bedding corrected K3 axes exhibited an elongation alongthe NNESSW direction with a strong dipping trend towards the NNE(Fig. 13d), indicating amajormagma ow from the SSW to NNE. A lessstrong trend towards the SE was also present, suggesting a secondaryow direction from the NW.

In unheated samples, sites S17 and S19 had K1 axes in the SSEdirection, agreeingwith the SSE ow. The K3 axes were clustered, withK1 and K2 axes well aligned within the imbrication plane. S19 showsthe triaxial AMS ellipsoids (0.5T0.5, with three remarkablywell-dened axes). In heated site S20, the K3 axes are slightlyscattered, and the K2 axes are better clustered in the ESE direction, butthe magnetic foliation plane is perpendicular to the bedding plane,implying a ow along the SSW direction.

The major ow direction along ENEWSW inferred from unheatedbasalts and the direction along NNESSW from the heated basalts canbe interpreted to be consistent, and the variation in inferred owdirections may be due to the low number of samples and theuncertainty and errors in the experiments.

In summary, the magnetic fabric data suggest the most signicantmagma ow direction was ESE to WNW with less signicant NNE toSSW direction for the sills, and possibly WSW to ENE with lesssignicant NW to SE direction for the basaltic ows. According toHenry et al. (2003a), these magma ow directions in south Taimyrdisagree with the azimuth of the present bedding dip shown byFig. 11g and l, indicating signicant tilting after magma emplacementas conrmed by Walderhaug et al. (2005).

6.3. Compressive stress regime

It has been proposed that the compressive tectonic stress eld(NNW) has caused signicant deformation of the rocks in SouthernTaimyr in the latest TriassicEarly Jurassic time (Inger et al., 1999), aswell as the Permo-Triassic volcanic rocks of the Noril'skKharaelakhtrough, although somewhat less intensively (Yakubchuk and Nikishin,2004).

In Southern Taimyr, the observed magnitude of this compressiveevent gradually decreases towards the south, as evidenced by thestructural shape of the Taimyr Peninsula and the cross-section in Ingeret al. (1999). As a consequence, the basaltic ows sampled fromHoffman peninsula, located to the north of our sampling area,underwent stronger compaction along the azimuth of NNWSSE thanthe sills, thus leading to shallower bedding dips in the sills than in thebasaltic ows (Tables 1 and 2).

As a consequence of the NNWcompressive tectonic stress eld, theAMS ellipsoid patterns in the basalts reveal a distinct shortening, i.e.the K3 axes (Fig. 11h and i) point toward the direction of themaximum compressive stress along the NNWSSE. On the other hand,the fact that the distribution of the K1 axes (Fig. 11a and c) and K3 axes(Fig. 11b and d) in sill sites is parallel to the NNWSSE compressivestress and to the ENEWSW fold axis, respectively, represents an

anomalous feature. Restored K3 axes (after interchanging K1 and K3 for

83S. Zhang et al. / Tectonophysics 505 (2011) 6885inverse fabrics) that show an elongated distribution close to theNNWSSE could indicate the maximum compressive stress direction.That is to say, magnetic foliation poles (K3 axes) point toward themaximum shortening direction for normal fabrics, but in cases ofinverse fabrics, the K1 axes can indicate the maximum shorteningdirection (Gil-Imaz et al., 2002; Rochette et al., 1992).

As a result of the folding deformation, some planes comprising theprincipal susceptibility axes (K1, K2 or K3) underwent a passive rota-tion. In the basaltic ows, the mean magnetic foliation plane (K1K2)was rotated anticlockwise from an original NNWSSE (D=160.0,I=23.9) to an ENEWSW azimuth (D=84.8, I=61.6), and themean K3 axis was rotated from a subverical (D=70.0, I=66.1) to asubhorizontal position (D=353.9, I=28.4) around a horizontalrotation axis. In contrast, such rotation is not so easy to observe in thesills. For the sills, the mean plane (K2K3) moved anticlockwisefrom an original ENEWSW (D=265.1, I=25.2) to an ENEWSW(D=256.8, I=32.6) azimuth, and the mean K1 axis of the sillsmoved from a subverical position (D=175.1, I=64.8) to a sub-verical position (D=166.8, I=57.4); however, the mean K3 axismoved from a horizontal to a sub-horizontal position, and themagnetic foliation is perpendicular to the fold axis (ENEWSW).Herein, magnetic foliation planes (K1K2) could rotate towards planesthat are parallel to the fold axis or perpendicular to the foldingdirection for normal fabrics, but in cases of inverse fabrics, planes(K2K3) might undergo such rotation. The K1 axes in the sills and theK3 axes in the basaltic ows became more inclined and moreclustered, respectively, after bedding correction than before beddingcorrection, indicating the post-magma emplacement tectonic event.

It is necessary to think about whether this brittle deformationcould cause the anomalous ellipsoid pattern in the sills after heating.Since the rock is folded, the AMS ellipsoid is assumed to be rotatedand it should retain the same orientation relative to the boundariesof the original unit. This should be the case unless folding is accom-panied by some degree of recrystallization. Since all our igneoussamples had the primary mineral assemblage without having re-crystallization of new minerals, these magnetite grains (in SD andsmall PSD size) in the sills and TM grains (in PSD and MD size) in thebasaltic ows were not expected to be created to obliterate theoriginal fabric during the folding process, at least in our studied area.As stated previously, the heated sills revealed a signicant number ofinverse fabrics while the basaltic sites showed more normal fabricsthan the sills. There was stronger compaction of the lavas, yet thesehave more normal fabrics than the sills that experienced weakercompaction. In addition, the low value of Pj in both sills and lavas istypical with a primary magma ow. Following the previous points, itcould be fair to say that these rocks carry mainly primary magneticfabrics but not an overprint of tectonic effect that includes an inversetectonic effect. Nevertheless, heating in the sills promoted growth ofmagnetic minerals that accentuated inverse fabrics. The presence of alarge proportion of ne magnetite crystals (SD or small PSD) in thesills and coarse TM grains (PSD or MD) could be the cause of the lesspronounced changes in relative orientation between the heated andunheated cases. However, we cannot preclude absolutely weak effectsof secondary fabrics due to low temperature oxidation or tectonicstress on these igneous samples. The resultant fabric could be morecomplicated since it was inuenced by the early ow and late ow oreventually by the regional compression (Hastie et al., 2011). Theinverse feature of sedimentary sites like S2 coincides with the tectoniceffect that might have modied and overprinted the normal fabricsin original sedimentary origin to inverse fabrics.

6.4. Identication of magma sources

It is useful to compare these new magma fabric results from theTaimyr igneous suite with that from igneous rocks in the western part

in the Siberian Traps. The major magmatic activity of Siberian trapswas restricted to several short intervals around the Permo-Triassicboundary (Callot et al., 2004; Vernikovsky, 1997; Vernikovsky et al.,2003). Yakubchuk and Nikishin (2004) presented a revised tectonicmodel for the Noril'sk region and suggested a lateral NW-trendingdirection for the sill intrusions in the upper crustal levels. Callot et al.(2004) reported a horizontal NESW (or ENEWSW) ow pattern ofthe basaltic ows in the Siberian traps, and the lava feeding center ismost likely located in the western or northwestern rift zone thatbound the Siberian platform. Comparing our results with the previouswork, a consistent magma ow direction may indicate a correlationbetween the igneous rocks of South Taimyr and Western SiberianBasin.

One possible way to nd the magma source is to extrapolate theow directions inferred from AMS studies to get their intersection.The source of magma ows are assumed to be the ellipse from whichthe ows radiate as shown in Fig. 11m. The clustering of the K3 axesdirection suggests a lateral extension of magma ow from ESE toWNW for the sills. The addition of data from Yakubchuk and Nikishin(2004) indicates the sill was most likely fed from a magma sourcelocated to the present ESE to south of Taimyr (Fig. 11m), consistentwith the fact that the most voluminous intrusions occur on thenorthern and northwestern periphery of the Siberian craton. Thebasalt ows were most likely sourced from the present SSW to SW ofTaimyr (Fig. 11m), which is consistent with Callot et al. (2004).However, the inferred magma ow directions could be due to amagma border effect or scatter and local sampling but more probablyis an effect of the terrane topography origin.

Following the previous comparisons, we suggest that the magmaevent in our study area is probably associated with the Siberian Traps.Walderhaug et al. (2005) compared the 40Ar/39Ar ages from theTaimyr igneous suite to isotopic age data from TriassicJurassicigneous rocks of the Siberian Traps, and suggested that this Mid-Triassic magma event in Taimyr indicates a sizeable areal extent tomagmatism on the Siberian Platform at this time.

7. Conclusions

The analysis of AMS for these rocks reveals two types of petro-structural events, i.e. the magma ow and folding.

(1) The magnetic fabric is mainly due to the orientation of low-TiTM grains close to pure magnetite in the sills and low- tomedium-Ti TM with minor hematite in the basalts. Hysteresisresults and magnetic mineralogy suggest the presence of SD tosmall PSD magnetite grains in the sills and of PSD to MD TMgrains in the basalts.

(2) Using the relationship between the principal magnetic suscep-tibility axes and the bedding plane, we recognized normal orinverse fabrics in these rocks (Fig. 9; Table 3). Some samplesites show a large dispersion of their susceptibility axes,probably indicating the true absence of preferred orientation.SD or small PSD grains are tentatively considered to be themost probable reason of inverse fabrics with a clustering of K1axes near the bedding pole of the sills.

(3) After heating, the sill sites contain a signicant number of ow-perpendicular K1 maximum axes and ow-parallel K3 mini-mum axes, and the bedding corrected K3 axes were used toobtain a main ESEWNW-trendingmagma ow direction and asecondary SSW direction (Fig. 13). The subvertical K3 axes ofthe basalts are more clustered and have an imbricatearrangement related to the bedding planes. The shallow plungeof the magnetic foliation planes after bedding correctionappears to dene an imbrication that yields an NESW owdirection (Fig. 13).

(4) AMS data are powerful to record the folding mechanism.

Detailed AMS analysis conrms the general attitude of the

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Can-Tapia, E., Walker, G.P.L., Herrero-Bervera, E., 1996. The internal structure of lavaows-insights from AMS measurements: INear-vent a'a. J. Volcanol. Geotherm.Res. 70, 2136.

Cisowski, S., 1981. Interacting vs. non-interacting single domain behaviour in naturaland synthetic samples. Phys. Earth Planet. Inter. 26, 5662.

Collinson, D.W., 1983. Methods in Rock Magnetism and Paleomagnetism Techniquesand Instrumentation. Chapman and Hall, London New York.

Day, R., Fuller, M., 1977. Hysteresis properties of titanomagnetites: grain size andcompositional dependence. Phys. Earth Planet. Inter. 13, 260267.magnetic foliation and gives a pole (K3 axis) direction for themagnetic foliation. In this study, the K1 axes of the sills pointtoward the maximum shortening direction corresponding toinverse fabrics. The restored K3 axes of the sills and K3 axes ofthe basalts before bedding correction indicate a maximumcompressive NS stress (Fig. 11). The brittle deformation in thelatest TriassicEarly Jurassic caused the rotation of the crustalblock and provides an explanation for the rotation of themagnetic ellipsoids, leading to a change in the orientation ofthe susceptibility axes.

(5) The magnetic fabrics in the sills and basalts are mainly primaryin origin. The present study has implications concerning thecommonly adopted AMS-tectonic stress relationship. AMS datashould be considered with care in this study area. The K1 axescan be perpendicular or parallel to the magma ow direction.The K3 axes cannot always indicate the maximum shorteningdirection but may sometimes indicate the maximum stretchingdirection, and the K1 axes may indicate instead the maximumstress direction when inverse fabrics occur. Changes inorientation of the axes during heating in the laboratory canprovide clues about these relationships.

Acknowledgments

The work was well supported logistically by SWEDARP andEUROPROBE, and it was made possible nancially by the NorwegianResearch Council through a grant to HJW, and grants from the NorwayState Educational Loan Fund (no. 1921471) and China ScholarshipCouncil (CSC) (NSCIS no. 2007103928) to SWZ. Dr. D.T.A. Symons andDr. M.T. Cioppa are thanked for help with the manuscript. Discussionsmade by Dr. Yongjae Yu and Dr. Zhenyu Yang are acknowledged.

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