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relation of the magnetic fabric to the fold curve were found, and sim-

ple techniques for the recognition of unfoldable and homogeneous

folds were elaborated.

AMS can also be used in deciphering the sense of a shear movement

(sinistral or dextral) (Rathore and Becke, 1980). This method is based

on the assumption that the minimum susceptibility direction rotates

during a simple shear movement in a manner similar to that of the

minimum strain direction.

AMS can also be used as a strain indicator of deformed igneous

rocks. During the process of deformation, the degree of AMS, in gen-

eral, increases and the principal directions reorientate into the direc-

tions parallel to the strain directions. Studies of this type were made

in dikes, volcanic bodies, and granitic rocks (e.g., Henry, 1977).

Strain analysis is one of the most laborious techniques of structural

analysis and it is confined to rocks containing convenient strain indica-

tors (oolites, concretions, reduction spots, lapilli, fossils). For this

reason, many attempts have been made to use the AMS as a strain

indicator. After revealing a close correlation between the directions

of principal susceptibilities and principal strains (for examples, see

Tarling and Hrouda, 1993), the quantitative relationship between the

AMS and strain has been investigated theoretically through mathema-

tical modeling (for review, see Hrouda, 1993), empirically through

examining natural rocks with known strain (for review, see Borradaile,

1991), and experimentally through deforming rocks and rock analogs

in the laboratory (for review, see Borradaile and Henry, 1997).

The first set of formulae describing a quantitative relationship

between the AMS and strain was obtained through empirical studies

of the AMS and strain (determined through investigation of natural

strain markers—reduction spots) in Welsh slates (Figure M86) (Kneen,

1976; Wood et al., 1976):

L ¼ðk

Þ¼ðs

F ¼ðk

Þ¼ðs

P ¼ðk

Þ¼ðs

or in terms of natural strain:

ln L ¼ aðe

 e

ln F ¼ aðe

 e

ln P ¼ aðe

 e

where k

 k

are the principal susceptibilities , s

 s

are the principal strains, and e

¼ lns

, e

¼ lns

, e

¼ lns

, are the

principal natural strains. In later empirical studies, several other

formulae were suggested, but as shown by Hrouda (1982), all of

them can be converted, within an error of a few percent, into the

above formulae.

In theoretical studies pioneered by Owens (1974), no simple formula

for the AMS-to-strain relationship has been found. However, the curves

Figure M84 Magnetic fabric in the Flamanville granite, Normandy, France. Note steep magnetic foliations conforming the magmatic

body shape. (Adapted from Cogne

and Perroud, 1988.)

556 MAGNETIC SUSCEPTIBILITY, ANISOTROPY

representing the lnP vs (e

–e

) relationship (for summary, see Hrouda,

1993), considering various AMS carriers (magnetite, hematite, pyrrho-

tite, and paramagnetic mafic silicates), are represented by monotonously

rising and only gently curved lines (Figure M87). In small strain

ranges, mainly in their initial parts, the lines do not differ very much

from straight lines. Consequently, using a reasonable simplification,

the theoretical AMS-to-strain relationship can be described in the same

way as the empirical AMS-to-strain relationship.

In experimental studies, the AMS of rock analogs deformed in

laboratory was investigated (Borradaile and Alford, 1987, 1988). The

rock analogs were relatively strongly magnetically anisotropic before

deformation, and to describe the AMS to strain relationship the follow-

ing formula respecting the predeformational AMS was used:

 P

¼ Bðe

 e

Þ;

where P

and P

are the corrected degrees of AMS before and after

deformation, respectively, and B is a constant. Analysis by Hrouda

(1991) shows that this relationship can also be converted into the rela-

tionship presented for empirical data.

It is obvious from the above analysis that the AMS-to-strain relation-

ship obtained through empirical, theoretical, and experimental studies

Figure M85 Scheme of the AMS development in progressively deformed accretionary prisms sediments. (Adapted from Pares et al.,

1999.)

MAGNETIC SUSCEPTIBILITY, ANISOTROPY 557

can, with a reasonable simplification, be represented by the formulae first

derived for the empirical relationship. Unfortunately, the above studies

have also shown that the a values are not the same for all rock types, but

vary according to the mineral carrying the AMS (Tarling and Hrouda,

1993) and also probably by the lithology of the rock investigated.

Even though a great majority of the empirical studies had revealed a

relatively close correlation between AMS and strain, some studies had

obtained results suggesting no correlation or even inverse correla-

tion between the AMS and strain (Borradaile and Mothersill, 1984;

Borradaile and Tarling, 1984). This implies that some rocks may have

had complex deformation histories in which various rock components

responded in different ways to overall strain and such rocks should not

be used in the strain determination via AMS. Nevertheless, it is

believed that in homogeneously deformed rocks, the AMS can serve

as a quantitative strain indicator, which is evident from the great

majority of empirical and experimental studies.

František Hrouda

Figure M86 AMS to strain relationship in Welsh slates. (Adapted from Kneen, 1976.)

Figure M87 An example of theoretical AMS to strain relationship. Pure shear strain, March model, prolate ellipsoids magnetite grains

with variable aspect ratios (a/c). (Adapted from Hrouda, 1993.)

558 MAGNETIC SUSCEPTIBILITY, ANISOTROPY

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Cross-references

Fisher Statistics

Magnetic Remanence, Anisotropy

Magnetic Susceptibility

MAGNETIC SUSCEPTIBILITY, ANISOTROPY,

EFFECTS OF HEATING

Study of the anisotropic properties of magnetic susceptibility and rema-

nent magnetizations is an active field of research in paleomagnetism.

These studies have important applications in petrofabrics, structural

geology, metamorphism, rock magnetism, volcanology, and tectonics.

Here we concentrate on the anisotropy of magnetic susceptibility

(AMS) of rocks measured at low magnetic fields, and in particular,

on the effects of laboratory heating on the AMS of rocks. Mineralogi-

cal changes resulting from heating samples in the laboratory have long

been recognized and studied (see Table M6). The potential use of tem-

perature-induced effects to investigate composite magnetic fabrics

needs further development.

Introduction

The magnetic susceptibility of rocks given by the ratio between mag-

netization (M) and applied magnetic field (H) exhibits anisotropic

properties. These properties have been used to investigate the charac-

teristics of rock fabric (Tarling and Hrouda, 1993).

¼ k

i; j

The susceptibility k

i, j

of anisotropic samples is a second-order tensor

(Nye, 1957), which is usually represented in terms of the principal sus-

ceptibility axes (maximum k

, intermediate k

, and minimum k

). The

Table M6 Laboratory data on heating-induced thermochemical

reactions

From To



Igneous

Impure titanomagnetites Magnetite >300

Magnetite Maghemite 150–250

Olivines Magnetite >300

Pyrite Magnetite 350–500

Maghemite Hematite 350–450

Magnetite Hematite >500

Pyroxenes Magnetite >600

Sediments

Siderite Magnetite >200

Lepidocrocite Maghemite 220–270

Goethite Hematite 110–120

Maghemite Hematite 350–450

Pyrite Magnetite 350–500

Magnetite Hematite >500

Hematite Magnetite >550

Note: The actual temperatures are strongly dependent on grain size and shape, redox

conditions, rate of heating, etc., and are only preliminary and indicative. Similarly, the

magnetic product resulting from heating is dependent on the specific redox conditions

at that temperature and it is likely that many of these reactions take place slowly at lower

temperatures under natural conditions—for example, goethitemay change to magnetite at

100



C–120



C under low oxidation conditions (after Tarling, 1983; Tarling and Hrouda,

1993).

560 MAGNETIC SUSCEPTIBILITY, ANISOTROPY, EFFECTS OF HEATING

AMS is generally measured at low magnetic fields, and several instru-

mental systems have been developed and commercially produced

in the last decades (e.g., Kappabridge, Saphire, Bartington, Digico,

Molspin, and PAR-SM2). AMS is also determined from measurements

in cryoge nic magnetometers.

AMS is usually analyzed in terms of the orientation of the principal

susceptibility axes and the AMS parameters. The parameters based

on the magnitudes of principal susceptibilities have been proposed

to quantify the anisotropy degree, lineation, foliation, and shape of

susceptibility ellipsoid (Tarling and Hrouda, 1993). The magnitude

and shape of susceptibility ellipsoid have been determined from var-

ious plots, which include the Flinn diagram of lineation as a function

of foliation and the plot of shape parameter as a function of anisotropy

degree (Jelinek, 1981). They permit distinction of susceptibility ellip-

soid, with prolate, oblate, and triaxial (neutral) ellipsoids.

In paleomagnetic and rock magnetic studies, several methods have

been developed to investigate the temperature dependence of magnetic

properties and remanent magnetizations (Tarling, 1983; Dunlop and

Özdemir, 1997). These studies use laboratory heating or thermal treatment

on samples under controlled conditions (e.g., thermal demagnetization,

thermomagnetic measurements, temperature-dependent susceptibility,

and paleointensity experiments). Understanding the effects of heating

on the magnetic properties and magnetic mineralogy is then an active

research field. This note is concerned with the effects of laboratory heat-

ing on the AMS of rocks. Thermal treatment has been proposed to

enhance AMS in rocks, and used to investigate on temperature-induced

fabric changes.

The low-field magnetic susceptibility changes as a result of growth/

destruction/transformation of iron oxides, sulfides and carbonates, and

hydroxides. Oxidation and reduction processes occur at different tem-

perature ranges during thermal treatment. Magnetic oxides can result

from transformation of iron sulfides (pyrite, pyrrhotite). Several meth-

ods have been implemented to study mineral alteration during labora-

tory heating (e.g., Dekkers, 1990; Van Velzen and Zijderveld, 1992),

and to promote, reduce, or avoid oxidation/reduction of magnetic min-

erals (for instance, use of inert or reducing atmospheres in heating/

cooling chambers of ovens).

Susceptibility is a bulk property, resulting from relative contribu-

tions of the diamagnetic, paramagnetic, and ferromagnetic minerals.

It depends on its mineralogy, grain size and shape, mineral distribu-

tion, crystallographic preferred orientation, layering, etc. Interpretation

of AMS in terms of rock fabric can often be complex, particularly for

composite or multicomponent fabrics. Thermal treatment has been

proposed to enhance and quantify given components of AMS fabric

(Urrutia-Fucugauchi, 1979, 1981) , where heating produces new mag-

netic minerals from the phyllosilicates and ferromagnetic minerals,

which mimic the crystallographic structure in micas and clays.

Temper ature- depende nt mi neral trans forma tions

Mineralogical changes resulting from heating samples in the labora-

tory have long been identified and studied (e.g., Abouzakhm and

Tarling, 1975; Urrutia-Fucugauchi, 1979, 1981). Susceptibility inc-

reases and decreases during heating results mainly from formation of

iron-titanium oxides (titanomagnetites and magnetite) and transforma-

tion of oxides (hematization of magnetite), respectively. Magnetic oxi-

des can be produced from a range of minerals and processes, such as

iron sulfides, carbonates and silicate minerals, hydroxides, and other

iron oxides (e.g., Tarling, 1983; Borradaile et al., 1991; Tarling and

Hrouda, 1993; Henry et al., 2003). Lithologies in which iron oxides

are not present, other minerals play an important role and constitute

special cases that require further investigation.

Pyrrhotite-bearing rocks can display changes due to transformation

of antiferromagnetic hexagonal pyrrhotite to ferrimagnetic monoclinic

pyrrhotite (Bina et al., 1991). Most often, complex series of mineralo-

gical changes occur at different tempe rature ranges. Oxidation of pyr-

rhotite at high temperature results in further susceptibility increase

(Dekkers, 1990). Magnetite formation between 500 and 725



C result

in susceptibility increase, which can be followed by decrease at higher

temperature as a result of hematization (biotite granites; Trinidade

et al ., 2001). Mineral transformations depend not only on mineral type,

but also on grain size, mineral impurities, time, atmosphere conditions

(inert, oxidizing, or reducing conditions), and water content. Tempera-

ture-induced changes in the bulk susceptibility and AMS have often

been related to the formation of magnetite. Perarnau and Tarling

(1985) proposed magnetite formation from vermiculite at 450



C and

from illite at >850



The rmal enhan cement of AMS

Early studies of AMS thermal enhancement used samples from Late

Precambrian tillites, Late Jurassic red beds, and Late Tertiary volcanics

(Urrutia-Fucugauchi, 1981). Thermal treatment resulted in enhancing

given components of the AMS fabric in the first two cases, and no

significant changes in the AMS axis pattern for the volcanic rocks.

Thermal enhancement was proposed as a method to study composite

fabrics (Urrutia-Fucugauchi, 1979). Subsequent studies applying ther-

mal treatment to glacial depos its, slates, sandstones, and red beds sup-

ported that heating enhanced the primary AMS fabric and pointed out

the complexity of heating-induced effects on magnetic mineralogy

(e.g., Urrutia-Fucugauchi and Tarling, 1983; Perarnau and Tarling,

1985; Borradaile et al., 1991; Borradaile and Lagroix, 2000).

An example of simple thermal enhancement of AMS is illustrated in

Figure M88 . Samples studied are Cretaceous sandstones sampled near

a fault system in Venezuela (Perarnau and Tarling, 1985). The initial

fabric is poorly defined with mixed maximum, intermediate, and max-

imum axes (Figure M88a). Maximum and minimum axes are oriented

normal to bedding, and maximum, intermediate , and minimum axes

conform a girdle distribution on the bedding plane. After heating to

350



C, the AMS axes pattern changes, resulting in minimum axes

preferentially aligned normal to bedding, and maximum axes on the bed-

ding plane and oriented to the north (Figure M88b). The thermal enhanced

fabric is interpreted as a depositional sedimentary fabric, resulting from

new magnetic minerals that mimic the crystallographic structure and

shape of micas and clays. Samples from sites closer to the fault show a

fault-parallel oblateness effect (Perarnau and Tarling, 1985).

Composite AMS fabrics are in general difficult to interpret, and sev-

eral methods have been proposed in an attempt to separate the different

mineral contributions (e.g., Hrouda and Jelinek, 1990). Other methods

using measurements at high and low fields, low-temperature observa-

tions, etc. have also been used contributions (e.g., Rochette and Fillion,

1988; Pares and Van der Pluijm, 2002).

An example of magnetic fabric enhancement of a given component

of a composite fabric in glacial sediments induced by laboratory heat-

ing is summarized in Figures M89 and M90 . Samples used come from

a Late Precambrian Port Askaig Formation from southwestern Scotland

(Urrutia-Fucugauchi, 1981; Urrutia-Fucugauchi and Tarling, 1983).

T he m ea n s us ce pt ib il it y s ho we d l it tl e c ha ng e u p t o 3 00 –400



Cand

increased markedly after heating at 400 and 500



C (Figure M90). The

orientation of principal susceptibility axes before thermal treatment

shows a pattern with vertical minimum axes and maximum axes on

the bedding plane (with some scatter; Figure M89a). This pattern con-

forms that expected for a depositional sedimentary fabric in sediments

deposited under the influence of dominant currents (indicated by the

lineation). With thermal treatment up to 500



C, there are marked

changes in bulk susceptibility and AMS axes pattern (Figure M89b).

The maximum axes are grouped close to paleovertical and minimum

axes lie in the paleohorizontal plane. Thus, heating-induced changes

result in an inverse fabric. The scatter in AMS axes is reduced, which

gives a well-defined NW-SE lineation (Figure M89a,b). This new fab-

ric was interpreted in terms of compressional stresses acting parallel to

bedding on strata free to extend in directions normal to bedding and

compression. Deformation results on lineation increase eventually

exceeding foliation, with predominantly prolate-shaped ellipsoids.

MAGNETIC SUSCEPTIBILITY, ANISOTROPY, EFFECTS OF HEATING 561

The initial fabric can be totally obliterated, resulting in oblate ellipsoids

but with different orientation to bedding. Maximum axes align normal

to bedding and minimum axes parallel to compression (Graham, 1966).

The AMS after heating to 500



C then sugge sts a magnetic fabric that is

strain-controlled with compressional stresses locally NW-SE oriented,

in agreement with geologic and petrofabric studies which indicate

strong deformation of this region during the Caledonian orogeny

(Borradaile, 1979). AF and thermal demagnetization of remanence

suggests that magnetic carriers are slightly oxidized titanomagnetites.

Thermal demagnetization to 300



C–400



C produces further oxidation

with some hematization accompanied by increase of coercive force

and Curie temperature. Heating to higher temperatures results in the for-

mation of new magnetite from paramagnetic minerals, which accounts

for the marked increase of mean susceptibility (Figure M90). Formation

of new magnetic minerals occurs preferentially controlled by existing

tectonic fabrics.

Borradaile and Lagroix (2000) presented data for high-grade gneisses

from the Kapuskasing granulite zone. They used measurements of AMS

Figure M88 Example of thermal enhancement of AMS (for explanation see text).

Figure M89 Example of magnetic fabric enhancement induced by laboratory heating. Orientation of principal susceptibility axes before

and after thermal treatment (for explanation see text).

562 MAGNETIC SUSCEPTIBILITY, ANISOTROPY, EFFECTS OF HEATING

and anisotropy of anhysteretic remanent magnetization (AARM) to define

the magnetic fabric. AMS was mainly due to the contributions of magne-

tite and silicate minerals. Thermal treatment does not result in increase of

mean susceptibility, but definition of AMS and AARM axial orientation is

improved. Heating results in enhancement of the magnetite subfabric,

which can then be quantified. This magnetite subfabric reveled by the

thermal treatment is also shown in the heated AARM fabric, which is also

preferentially enhanced by the laboratory heating.

Other examples of successful thermal enhancement of component of

composite fabrics have been observed in granites, particularly in

S-type granites in which fabrics arise from biotite and tourmaline

minerals (Trinidade et al., 2001; Mintsa et al., 2002). The AMS of

paramagnetic phyllosilicates in granites generally correlate well with

the crystallographic fabric (e.g., Borradaile and Werner, 1994). Mintsa

et al. (2002) presented results for the Carnmenellis granite, where ther-

mal treatment above 500



C results in enhancement of the biotite fabric

overcoming the tourmaline contribution. Secondary magnetite produced

by laboratory heating within biotite crystals mimics the phyllosilicate clea-

vage, which in turn corresponds to magmatic (linear) flow structures in the

granitic body. Comparison of the AMS fabric lineations before and after

the laboratory thermal treatment shows clear differences between the

composite initial fabric produced by the biotite fabric and the (inverse)

tourmaline fabric (Mintsa et al., 2002).

Discussion

Measurement of magnetic susceptibility has long been used to monitor

changes in magnetic minerals produced by heating of rocks in the

laboratory. The effects of heating on the AMS and the enhancement

of components of AMS fabric open new exciting possibilities in petro-

fabric studies. In the simple case, heating produces new magnetic

minerals, which mimic the crystallographic structure in micas and

clays (Urrutia-Fucugauchi, 1979, 1981).

The initial studies showed, however, that heating-induced changes

in AMS are more complex than simple enhancement of the magnetic

fabric. Recent studies investigating different lithologies (e.g., granites,

gabbros, diorites, ignimbrites, volcanoclastic sediments, loess, and

limestones) confirm that heating does not always result in simple

AMS fabric enhancement (Henry et al., 2003). Henry et al. (2003) dis-

tinguish three different cases when comparing the heating-induced

fabric with the preexisting fabric. First case is simple enhancement of

preexisting fabric or masked poorly defined initial fabric. Second case

corresponds to formation of an inverse fabric. Third case is the formation

of a different fabric, which shows principal susceptibility axes related

to petrostructural elements, but differing from the initial fabric.

In the cases where thermal treatment results in increasing the bulk

susceptibility and anisotropy degree, heating-induced changes facili-

tate the measurement of the susceptibility tensor and principal suscept-

ibility axes. Susceptibility increase during heating mainly results from

the formation of iron-titanium oxides, principally titanomagnetites and

magnetite. Susceptibility decrease results from transformation of oxi-

des, like hematization of magnetite. Iron-titanium oxides could be pro-

duced from a range of minerals and processes; most often including

iron sulfides, iron carbonates, iron silicate minerals, hydroxides, and

other iron oxides (Tarling, 1983; Henry et al., 2003). Lithologies in

which iron oxides do not play a dominant role present special cases

that are still not fully investigated for magnetic properties.

Study of AMS changes induced by progressive thermal treatment

appears as a useful method to investigate the fabric of rocks. Labora-

tory stepwise heating has potential for enhancement of AMS and

revealing masked or cryptic fabrics. Further investigation of heating-

induced effects in mineralogy, grain size, and textural changes, applied

systematically to different lithologies, is clearly required. The metho-

dology permits studying composite fabrics and may give further

insight on the relationship of AMS to the different petrofabrics arising

from stratification, depositional conditions, current-induced features,

strain effects, cleavage, microfracturing, etc.

Figure M90 Example of magnetic fabric enhancement induced by laboratory heating: mean susceptibility (for explanation see text).

MAGNETIC SUSCEPTIBILITY, ANISOTROPY, EFFECTS OF HEATING 563

Acknowledgments

Partial support for the magnetic fabric studies has been provided by

DGAPA projects IN-116201. Thanks are due to Editor E. Herrero-Bervera

for the invitation to contribute, encouragement, and useful comments.

Jaime Urrutia-Fucugauchi

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MAGNETIC SUSCEPTIBILITY, ANISOTROPY,

ROCK FABRIC

Although the anisotropy of magnetic susceptibility (AMS) of rocks

first called the attention of scientists because of its possible influence

on the direction of the remanent magnetization, it was relatively soon

realized that the AMS of a rock is in general too weak to exert a

noticeable influence in the paleomagnetic record (e.g., Uyeda et al.,

1963). On the other hand, due to the close relationship that exists

between the crystalline structure of minerals and their AMS (Nye,

1960), it was well established that the AMS should bear a very close

relationship with the mineral fabric of the rock. It is therefore not sur-

prising that the modern interest in the study of the AMS of rocks

resides mainly in its value as a petrofabric indicator rather than as a

paleomagnetic tool.

There are several sources for the AMS of rock. The most common

of these sources is the anisotropy arising from the crystalline structure

of the minerals (Nye, 1960). Almost all minerals will have an AMS

that is controlled by these sources, although their bulk susceptibility

will be generally small. A second source of AMS is found only in

some minerals with a strong permeability. These minerals will show

an anisotropy that arises from the overall shape of the grain rather than

from its crystalline structure. For this reason, this source of AMS is

called shape anisotropy. Finally, when two or more ferromagnetic

minerals are in close proximity to each other, they can interact magne-

tically and such interaction becomes the source of a magnetic aniso-

tropy. If the distance between the grains is too small (<2 grain

radius) the interactive effects can be so strong that they will overcome

the shape anisotropy of individual minerals (Cañón-Tapia, 1995). All

of these sources of anisotropy contribute in different amounts to the

total AMS measured in the laboratory. In general, the contributions are

not simple to express because they depend on the relative proportion

of minerals with a given value of bulk susceptibility and degree of ani-

sotropy (Cañón-Tapia, 2001). Traditionally, the presence of a mineral

phase with a high value of bulk susceptibility has been assumed to indi-

cate an AMS dominated by such phase, but recent evidence suggests

that the main source of anisotropy will reside in the most anisotropic

phase even if such phase has a lower value of bulk susceptibility

(Borradaile and Gauthier, 2003). Therefore, some attention has to be

given to the mineral composition of the rock type under examination

to assess the probable sources of its AMS, and hence to make a mean-

ingful interpretation.

The variables responsible for the acquisition of a rock fabric also

depend on the type of rock that is considered. Although a mineral

fabric is acquired as a response to deformation forces, the total amount

of deformation differs greatly from rock to rock, as do the actual

564 MAGNETIC SUSCEPTIBILITY, ANISOTROPY, ROCK FABRIC

mechanism of deformation experienced by the rock. Consequently, the

AMS of a rock specimen needs to be examined to the light of the

lithology under consideration to reach a meaningful interpretation.

Notably, this lithological dependence of the AMS has been disre-

garded through most of the history of the AMS method probably

because advances in the identification of the forces responsible for fab-

ric acquisition of different lithologies have been uneven in time.

Undoubtedly, the study of fabric structures in sedimentary and

deformed rocks has a longer history. Therefore it is not surprising that

the earlier works establishing a link between sedimentary features

(bedding) and AMS were made on these lithologies (Ising, 1942;

Granar, 1957). Among the first attempts to explain theoretically the

magnetic fabric acquisition of sedimentary rocks is the work by Rees

(1961), followed by experiments in which different sedimentation

conditions were explored (Rees, 1966, 1968; Hamilton et al., 1968).

These works showed that the influence of the magnetic moment

induced in a ferromagnetic particle that is about to be deposited will

only become important for very small particles, because the magnetic

moment induced in the particle is proportional to the cube of the par-

ticle dimension whereas the influence of either gravitational or hydro-

dynamic forces is proportional to its fourth power. Also, the formation

of ripples and cross-lamination in sediments deposited under relatively

strong currents was shown to affect the measured AMS because each

sample might contain different sets of cross-laminations each with a

different orientation. Nevertheless, it was concluded that in general

the AMS measurements of sediments reflects the gravitational and

hydrodynamic forces acting during deposition, and therefore this

method yields results equivalent to those concerning the preferred

orientation of particles as obtained from optical observation.

Departures from the AMS obtained under experimental conditions

(or so-called “primary fabrics”) were considered to be “anomalous”

by Hamilton and Rees (1970). The causes of the anomalous fabrics

were associated with deformation (whether penecontemporaneous with

sediment deposition as for example by gravitational slump or by tec-

tonic-related deformation), mineral alteration, or with the activity of

plants and burrowing animals. In all of these cases, the magnetic fabric

was predicted to yield a larger site-scatter of directions of the principal

susceptibilities of individual specimens than it would be found if the

sediments in the site had formed by normal depositional processes.

Consequently, the scatter of AMS axes became to be synonymous of

fabric alteration processes.

In contrast to sediments, prior to the 1970s the interest in the fabric

of igneous rocks was almost null (Smith, 2002). Therefore, it is not

surprising that most assumptions concerning the interpretation of

AMS in sediments were adopted indiscriminately when the first stu-

dies of the AMS of igneous rocks were made. In particular, the idea

of a mineral fabric as the result of the preferred orientation of minerals

already present during viscous flow of melted rock became easily

adopted as equivalent to the processes of fabric acquisition studied

in sediments (Khan, 1962). As time passed, however, it was realized

that the AMS of igneous rocks presents much more variability than

the AMS of sediments, leading to a series of confusions and contradic-

tions concerning the utility of AMS in this type of rocks (for a recent

historical recount focusing on this type of rocks see Cañón-Tapia,

2004). Although some discrepancies can exists concerning the exact

mechanism responsible for the fabric acquisition of a particular rock,

most workers will nowadays acknowledge that the mechanisms of

deformation of igneous rocks are varied and complex, and have a

different character than the (deformation) mechanisms responsible for

the acquisition of a mineral fabric in sedimentary rocks. Consequently,

different assumptions should be made when interpreting AMS mea-

surements in either rock type.

Another consequence of the advances made in the understanding of

emplacement mechanisms of igneous rocks and magmatic deforma-

tion, is that the AMS of this type of rocks is not unique and it does

not remain constant in the same unit, as previously assumed. These

characteristics of the AMS are common to all mineral fabrics that are

acquired during magmatic deformation, provided the density of sus-

pended minerals is not very large. In these cases, the movement of

the particles of almost any shape can be approximated by the equations

of movement of ellipsoidal particles derived by Jeffery (1922). The

movement of these particles will be periodic, and therefore a cyclic

behavior of the mineral fabric (passing from well-defined mineral orien-

tations to random distributions of mineral grains) will be found as a

function of total shear. Consequently, systematic variations of AMS

are predicted as the result of the velocity gradients commonly found

in igneous rocks, and therefore, a nonunique AMS signal is to be

expected. Nevertheless, as the variations of the fabric are directly related

to the magmatic deformation, it is possible to devise sampling strategies

(e.g., Cañón-Tapia and Chávez-Álvarez, 2004) that provide enough evi-

dence allowing us to extract useful information concerning the late

aspects of the emplacement of these type of rocks (including but not

limited to) the local flow direction.

Although the same basic equations could be used in principle to

model the mineral fabric of a sedimentary rock, it should be noted that

in these cases, other factors like the rolling-over effect of elongated

grains over the deposition bed or the mechanical interaction of a much

larger population of grains, will result in conditions that differ from

those found in igneous rocks. Consequently, the AMS of both rock

types might reflect completely different processes.

In summary, although the AMS of all rock types bears a close relation

to their mineral fabric, the basic assumptions required for the correct

interpretation of this type of results are lithology-dependent. Neverthe-

less, the AMS method remains a very useful petrofabric tool, provided

that the interpretation of results is made with a clear understanding of

the physical conditions relevant for each particular case.

Edgardo Cañón-Tapia

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MAGNETIC SUSCEPTIBILITY, ANISOTROPY, ROCK FABRIC 565