www.elsevier.com/locate/tecto

Tectonophysics 377 (2003) 1–5

Editorial

Geodynamic applications of palaeomagnetism—introduction

Palaeomagnetic studies provided some of the crit- procedures and difficulties, which together make the

ical evidence which led to the development of plate

tectonic theory in the 1960s and 1970s, and 30 years

on, palaeomagnetic studies are still providing funda-

mental geodynamic insights across a broad spectrum

of spatial and temporal scales. This Special Issue

focuses on recent results obtained by the international

palaeomagnetic community in our attempts to under-

stand and quantify geodynamic processes. It partially

derives from a symposium entitled ‘‘Palaeomagnetic

contributions to plate tectonics’’, convened at the

European Geophysical Society General Assembly in

Nice in April 2002.

The basic principles of palaeomagnetism can be

stated straightforwardly as follows. A rock unit can

acquire magnetization components at various stages

in its history, from the time of its formation until the

present day. Each component can record the direc-

tion of the ambient geomagnetic field at the location

of the rock unit at the time of magnetization. Under

ideal conditions, the direction and intensity of each

of these magnetic components can be determined by

laboratory study, and their relative ages and relation-

ships to geological events established. Providing that

relatively short timescale variations in the geomag-

netic field are adequately averaged out using appro-

priate sampling strategies, the inclination (dip) of

each magnetization component is related directly to

the geographic latitude of the rock at the time of

magnetization. The declination (azimuth) of the

magnetic vector indicates the subsequent rotation of

the rock unit with respect to geographic north.

Hence, information on latitudinal and rotational

movements of the rock unit can be derived. This

underlying simplicity is, of course, complicated by a

host of factors affecting the magnetization recorded

by a rock, and by a wide range of experimental

0040-1951/$ - see front matter D 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.tecto.2003.08.014

discipline both challenging and exciting.

In the first paper, Schmidt and Williams present

new data from Palaeoproterozoic iron-formation and

chert carbonate of the 1.88 Ga Gunflint Formation,

exposed in the Thunder Bay area, Ontario, Canada.

The palaeomagnetic study of such ancient rocks

requires detailed analyses of remanence structure

and magnetic mineralogy if geologically or geomag-

netically valuable data are to be recovered. Schmidt

and Williams observe both normal and reverse po-

larity components of magnetization which are carried

by a combination of hematite and magnetite. The

data fail fold and conglomerate tests of magnetic

stability indicating widespread remagnetization of the

Gunflint Formation. The similarity of remanence

directions with those observed in the Mesoprotero-

zoic Keweenawan Supergroup (1.1 Ga) allows

Schmidt and Williams to infer remagnetization dur-

ing Keweenawan magmatism. Observed asymmetry

between normal and reversed directions of magneti-

zation is attributed to remanence acquisition during a

period of apparent polar wander during Keweenawan

times, rather than asymmetry of the Mesoproterozoic

geomagnetic field (as suggested by earlier authors).

The next contribution, by Beck and Housen,

applies the palaeomagnetic technique at the scale

of the major lithospheric plates by considering the

absolute motion of North America during the Meso-

zoic. The conceptual basis for their analysis is that if

apparent polar wander is due to motion of a crustal

block relative to the Earth’s rotation axis, then the

apparent polar wander path (APWP) should define a

small-circle centered on the Euler pole of crustal

motion. Beck and Housen apply this palaeomagnetic

Euler pole (PEP) concept to set of key North

American palaeomagnetic poles selected from the

Editorial2

IAGA Palaeomagnetic Database, with the intention

of providing a chronological framework for the

analysis of Mesozoic tectonics. After critical evalu-

ation of the effects of local tectonic rotation on data

from the Colorado Plateau and consideration of the

causes of apparent differences in APWPs based on

data from southwestern and eastern North America,

the palaeomagnetic database is shown to support

three well-defined small-circle APW tracks. These

are separated by two cusps at 200 Ma (‘‘J1’’ cusp)

and 160 Ma (‘‘J2’’ cusp) that represent significant

velocity changes in the absolute motion of the North

American plate which must relate to major tectonic

events. The authors correlate the ‘‘J1’’ cusp with the

inception of North Atlantic rifting, and the ‘‘J2’’

cusp with the beginning of rapid North Atlantic

spreading and a period of rapid NW absolute motion

of North America. The variations in absolute motion

defined by the PEP analysis contrast with the smooth

motion inferred from calculations based on the fixed-

hotspot framework. Beck and Housen’s analysis is,

therefore, of direct relevance to the on-going debate

concerning potential motion of hot-spots relative to

the spin axis.

A classical large-scale palaeomagnetic study is

presented by Stone, Minyuk and Kolosev who aim

to resolve a long-standing debate concerning the

palaeogeography of the Kolyma–Omolon Superter-

rane and its constituent parts, that form the core of NE

Russia. New data are provided from Silurian to

Permian continental margin rocks of the Omulevka

Terrane. A careful sampling strategy and detailed

thermal demagnetization allows identification of pre-

deformational remanences in these rocks. The data are

used to constrain the detailed palaeolatitudinal motion

of the Omulevka Terrane relative to the adjacent

Omolon Terrane through the Palaeozoic. The terranes

are shown to have been in close proximity at low

latitudes in the Silurian–Devonian, and until the Late

Devonian, the data are consistent with these terranes

being closely associated with the Siberian Platform.

Between Devonian and Permian times, the palaeola-

titude of the terranes decreased as they separated from

the Siberian Platform. The Omulevka and Omolon

Terranes then moved northwards from Permian to

Jurassic times, converging again towards the Siberian

Platform. By the Late Jurassic/Early Cretaceous, the

palaeolatitudinal constraints indicate accretion of the

Kolyma–Omolon Superterrane to the eastern Siberian

continent.

Palaeomagnetic analyses of metamorphic rocks

beyond the lowest of grades are rarely seen in the

literature, but can yield valuable results so long as the

nature and origin of the remanence carrying phases are

properly documented. In this respect, Kadzialko–

Hofmokl, Kruczyk, Mazur and Siemiatkowski pro-

vide detailed rock magnetic analyses to support their

palaeomagnetic study of the Klodzo Metamorphic

Complex (KMC) of the West Sudetes, SW Poland.

This is one of the key fragments of the internal

Variscan belt of Central Europe, but its geological

evolution remains relatively poorly constrained. The

studied area consists of strongly deformed and meta-

morphosed rocks within a tectonic collage that was

finally assembled during the Carboniferous. A com-

plex suite of remanence carriers are identified (goe-

thite, pyrrhotite, magnetite, maghemite and hematite)

which often pseudomorph other minerals and are

demonstrably of secondary origin. Negative tilt tests

also imply remagnetization of the sampled units. New

magnetic overprints are shown to have been acquired

during three remagnetization episodes: (i) during the

latest Devonian in response to deformation and meta-

morphism of the KMC; (ii) at the Carboniferous/

Permian boundary, corresponding to a time of wide-

spread overprinting in the Variscan Belt; and (iii)

during Late Triassic to Early Cretaceous uplift of

the Sudetes in response to the Alpine orogeny. Suc-

cessive overprints are consistent with a palaeoposition

of the KMC close to Baltica since at least the Late

Devonian. This study serves to illustrate how a

combination of detailed magnetic mineralogical con-

straints and palaeomagnetic analyses can elucidate

regional tectonic histories even where early magnet-

izations have been obliterated by younger tectonome-

tamorphic events.

The remaining papers in the volume consider new

palaeomagnetic constraints on the tectonic evolution

of the Tethyan orogen, a system that has attracted

continuous interest since the earliest days of palae-

omagnetic research. Villalaın, Fernandez-Gonzalez,

Casas and Gil-Imaz present a novel application of

palaeomagnetic techniques to the reconstruction of

the Early Cretaceous geometry of the extensional

Cameros Basin of Spain, which was inverted in the

Tertiary to form part of the Iberian Cordillera.

Editorial 3

Detailed palaeomagnetic analysis of Early Creta-

ceous red-bed sequences sampled along three trans-

ects across the basin reveal a stable remanence of

consistently normal polarity carried by hematite.

Conglomerate and incremental fold tests suggest that

this magnetization represents a syn-tectonic overprint

of Albian age, acquired as a burial remagnetization

during low grade metamorphism. Since remagnetiza-

tion occurred prior to contractional inversion of the

basin sequences, Villalaın et al. are able to use the

remanence data to differentiate components of tilting

associated with the extensional phase from tilts

acquired during later inversion. Their methodology

involves comparing magnetization directions to an

expected direction for the Albian derived from palae-

omagnetic poles for the Iberian Plate. Rotations

around strike parallel axes are applied to restore

the magnetization vectors to the reference direction

and fold limbs to their inferred orientations at the

time of remagnetization. This approach reveals a

consistent pre-inversion geometry along all three

transects involving large-scale syn-sedimentary fold-

ing due to fault drag along the northern basin

bounding fault and development of an associated

roll-over anticline. Hence, the analysis of well-dated

remagnetizations is shown to represent a powerful

tool in basin analysis.

The tectonic usefulness of remagnetizations when

their origin is properly understood is also highlighted

by the contribution of Villasante-Marcos, Osete,

Gervilla and Garcıa-Duenas, who provide new

palaeomagnetic data from the Ronda peridodites,

exposed in the metamorphic Internal Zones of the

Betic Cordillera of southern Spain. Rock magnetic

experiments on samples from sites displaying variable

degrees of serpentinization consistently show the

dominance of magnetite as the remanence carrier, as

may be expected since magnetite is a common prod-

uct of the serpentinization process. A highly stable

characteristic component of magnetization is isolated

upon demagnetization, together with a northwards

component with more variable unblocking temper-

atures that appears to be strongly correlated with the

degree of serpentinization. The characteristic rema-

nence post-dates folding of the internal foliation

within the peridotites, whereas the presence of both

normal and reverse polarity components is evidence

of a period of magnetization acquisition extending

across at least one geomagnetic reversal. This rema-

nence is interpreted as a thermochemical component

acquired during initial serpentinization associated

with post-metamorphic cooling from 350 to 400 jCto ambient temperatures, between 20 and 17–18

million years ago. This interpretation is supported

by agreement between observed inclinations and that

expected for stable Iberia at this time. The northwards

component is inferred to have originated during a later

serpentinization stage associated to the extensional

dismembering of the peridotite slab. Comparison of

the mean declination of the characteristic remanence

with that predicted for stable Iberia suggests that the

Ronda peridotites experienced a ca. 40j post-Early

Miocene clockwise tectonic rotation. This is consis-

tent with the pattern of clockwise rotations seen

elsewhere in the Betic Cordillera, and must be taken

into account when interpreting structural data from the

peridotites.

The debate in the Central Mediterranean has been

dominated by discussion of the motion history of the

Adria microplate, and the extent to which it may be

considered a fixed promontory of the African plate.

Early palaeomagnetic studies suggested that Adria

(peninsular Italy) experienced a Tertiary (Eocene)

anticlockwise rotation with respect to Africa. Later

studies and syntheses highlighted uncertainties due to

poor definition of the reference APWP for Africa and

the frequent observation of localised anticlockwise

rotations of thrust sheets, and suggested that the data

do not support significant rotation of Adria relative to

Africa. Here, Marton, Drobne, Cosovic and Moro

present new palaeomagnetic analyses of platform

carbonates from stable Istria, which is unequivocally

considered to be part of the Adriatic foreland, together

with additional data from the so-called ‘‘imbricated

Adria’’ margin and platform carbonates of the Dinar-

ides in Central Dalmatia. Despite the very weak

intensities of magnetization in the sampled carbo-

nates, Marton et al. identify stable remanences which

are variably interpreted as pre-deformational (pre-

sumed primary) magnetizations and post-tilting

remagnetizations. Pre-deformational remanences from

stable Istria and ‘‘imbricated Adria’’ consistently

show northwesterly directed declinations which imply

approximately 30j of anticlockwise rotation relative

to Africa during the Tertiary, although the data do not

allow the timing of this rotation to be determined

Editorial4

precisely. In contrast, Marton et al. suggest that

Maastrichtian–Eocene platform carbonates from Cen-

tral Dalmatian were remagnetized in connection with

the late Eocene–Oligocene deformation or Miocene

hydrocarbon migration, but still provide evidence for

Tertiary anticlockwise rotation. These provocative

new data are certain to renew interest in the long-

running debate over the role of large-scale rotation in

the development of the Adriatic region.

To the east of Adria lies the complex assemblage of

microcontinental and ophiolitic terranes of the eastern

Mediterranean Tethyan belt. Late Tertiary to Quater-

nary geodynamic reconstructions of this region are

relatively well-constrained. In contrast, consensus has

yet to be reached on the Mesozoic to Early Tertiary

regional palaeogeography and a range of alternative

reconstructions have been proposed that vary in the

number of ophiolite root zones and extent of ophiolite

allochthoneity. Morris presents a synthesis and anal-

ysis of the extensive high quality palaeomagnetic data

now available from Late Cretaceous rocks in the

Troodos (Cyprus) and Baer–Bassit (Syria) ophiolites

and from coeval rocks of the eastern Pontides (Tur-

key). Inclination-only fold tests are used to demon-

strate unequivocally that the remanences recorded by

the ophiolites are pre-deformational in origin. The

data place the Late Cretaceous Neotethyan spreading

axis at a palaeolatitude of ca. 22j N, comfortably

between calculated palaeolatitudinal limits of the

Eurasian and African margins. The implications of

the full set of regional constraints and associated

confidence limits are discussed using plate tectonic

cross-sections along a line extending northwards from

the Arabian continental margin. The data do not allow

a unique, purely palaeomagnetic solution for the

palaeogeography due to: (i) the relatively narrow

width of Tethys in the eastern Mediterranean, that is

near the inherent limit of palaeolatitude resolution;

and (ii) an absence of data from Late Cretaceous rocks

of the eastern Taurides. Geological considerations,

however, strongly support models in which the Troo-

dos and Baer–Bassit ophiolites were generated in a

southern Neotethyan basin, rather than those involv-

ing generation in a northerly basin and subsequent

large-scale thrust displacement to the south.

Moving further eastwards still, the last two con-

tributions present new results from the Tethyan Hima-

laya of Nepal, which represents the deformed remnant

of the passive northern margin of the Indian subcon-

tinent. This structurally complex zone experienced

two major deformational and metamorphic events

during the Eocene (the Eo-Himalayan phase) and

the Miocene (the Neo-Himalayan phase). Crouzet,

Gautam, Schill and Appel provide evidence of

multicomponent remanences within low-grade meta-

sediments of Triassic age in the western Dolpo region.

The Dolpo–Manang synclinorium represents the larg-

est fold structure in the Tethyan Himalaya, with a

scale in excess of 100 km. Detailed demagnetization

experiments succeed in isolating components of mag-

netization carried by pyrrhotite and magnetite which

were acquired at different times in the deformation

history. The component residing in magnetite passes a

fold test and comparison with expected Triassic direc-

tions supports a primary origin. In contrast, the lower

unblocking temperature pyrrhotite component fails a

fold test and is therefore of post-folding origin.

Consideration of available geochronological data for

the age of metamorphism in these rocks and inclina-

tion matching suggests that partial remagnetization

occurred at ca. 35 Ma. This implies that the main

phase of Himalayan folding occurred prior to this time

in the western Dolpo. Similar moderate clockwise

rotation angles inferred from both components sug-

gest no significant rotation of the area relative to India

prior to 35 Ma.

In contrast, complete magnetic overprinting in

Tethyan Himalaya lithologies is documented by Schill,

Appel, Godin, Crouzet, Gautam and Regmi through

their analyses of Carboniferous, Permian and Triassic

low-grade metacarbonates exposed along the Nar/Phu

Valley (central Nepal). The secondary remanence,

again carried by pyrrhotite, is shown to post-date

small-scale folding and is interpreted as a thermorem-

anent magnetization related to post-peak metamorphic

cooling after the Eo-Himalayan phase. Dispersion of

remanence directions along a north–south profile,

however, suggests that this remagnetization has been

affected by late-orogenic long-wavelength folding.

Small-circle modeling of this dispersion suggests ro-

tation around a pole close to the axis of the nearby

Chako antiform. By considering the distribution of

remanence directions and anisotropy of magnetic sus-

ceptibility principal axes, Schill et al. infer a mean

clockwise rotation of 16j for a remanence age of ca. 30

Ma. A compilation of secondary pyrrhotite remanen-

* Corresponding author. Tel.: +44-1752-233120; fax: +44-

1752-233117.1 Tel.: +49-89-2394-4238; fax: +49-89-2394-4205.

Editorial 5

ces along the Tethyan Himalaya reveals an increase in

clockwise rotations from west to east over a distance of

150 km. This is incompatible with large-scale oroclinal

bending models for the region which predict a decrease

in rotation angles towards the east.

In summary, three major fields of investigation

emerge from the studies presented in this Special Issue:

(i) the reconstruction of past motions and configura-

tions of major continental blocks. Palaeomagnetism

will continue to occupy the central role here since it is

the only source of quantitative constraints on palae-

olatitude and palaeo-azimuth. The development and

maintenance of the IAGA Global Palaeomagnetic

Database has provided a valuable research tool, and

improvements in the use of the database have resulted

from the now accepted need for adequate quality

filtering. There remains a need to improve the resolu-

tion of parts of the APWPs of several major continents

(e.g. for the late Palaeozoic to earlyMesozoic segments

of the Gondwana continents). A plethora of unresolved

issues also remains concerning palaeogeographic

reconstructions for Proterozoic and Palaeozoic times.

These issues will continue to challenge the palaeomag-

netic community over coming decades; (ii) investiga-

tion of the processes of tectonic rotations and their role

in the evolution of collisional orogenic belts. Rotations

are now seen to be near ubiquitous, even in non-arcuate

orogens. Here, the challenge will be to relate unequiv-

ocally observed rotations to the structures which ac-

commodate them (across a range of scales). Future

progress will depend upon proper integration of palae-

omagnetic, structural and magnetic fabric data, as

demonstrated by several of the papers herein; and (iii)

the recognition of remagnetization events and their

regional interpretation and significance. The value of

palaeomagnetic data from remagnetized rocks is now

recognized more fully and the increasing interest in this

field is reflected in the number of recent symposia

dedicated to the topic at major international meetings.

The analysis of remagnetized rocks has benefited from

more realistic approaches to structural correction and

improvements in the statistical basis of field tests.

Overall, the key to future progress in all these fields

will be fuller understanding of the mechanisms of

magnetization acquisition at the case-specific level.

Many of the papers in this volume highlight the critical

role of detailed rock magnetic analyses in elucidating

the physical and chemical processes that control the

acquisition, fidelity and stability of the palaeomagnetic

signal in a variety of lithologies and geological settings.

In this respect, the future will see continued progress in

integrating ‘‘traditional’’ palaeo- and rock magnetic

techniques with more novel magnetic (e.g. low tem-

perature analyses) and non-magnetic (e.g. analytical

electron microscopy) methodologies.

Finally, the volume includes a glossary of palae-

omagnetic and rock magnetic terms which we hope

will be useful to non-specialist readers.

Antony Morris*

School of Earth, Ocean and Environmental Sciences,

University of Plymouth, Drake Circus,

Plymouth PL4 8AA, UK

E-mail address: amorris@plymouth.ac.uk

Jenny Tait1

Institut fur Allgemeine und Angewandte Geophysik,

Ludwig-Maximilians-Universitat,

Theresienstrasse 41,

D-80333 Munchen, Germany

E-mail address: jenny@geophysik.uni-muenchen.de