La classe de première S La tectonique des plaques: une approche

La classe de première S
La tectonique des plaques:
une approche historique
Laurent Jolivet
02 38 41 46 56
[email protected]
La classe de première S
La tectonique des plaques:
une approche historique
Les grandes lignes de la tectonique des plaques ont été présentées au collège. Il s’agit, en
s’appuyant sur une démarche historique, de comprendre comment ce modèle a peu à
peu été construit au cours de l’histoire des sciences et de le compléter. On se limite à
quelques étapes significatives de l’histoire de ce modèle.
L’exemple de la tectonique des plaques fournit l’occasion de comprendre la notion de
modèle scientifique et son mode d’élaboration. Il s’agit d’une construction intellectuelle
hypothétique et modifiable. Au cours du temps, la communauté scientifique l’affine et le précise
en le confrontant en permanence au réel. Il a une valeur prédictive et c’est souvent l’une de ces
prédictions qui conduit à la recherche d’un fait nouveau qui, suivant qu’il est ou non découvert,
conduit à étayer ou modifier le modèle. La solidité du modèle est peu à peu acquise par
l’accumulation d’observations en accord avec lui. Les progrès techniques accompagnent le
perfectionnement du modèle tout autant que les débats et controverses.
La naissance de l’idée
Au début du XXe les premières intuitions évoquant la mobilité horizontale
s’appuient sur quelques constatations :
- la distribution bimodale des altitudes (continents/océans),
- les tracés des côtes,
- la distribution géographique des paléoclimats et de certains fossiles.
Ces idées se heurtent au constat d’un état solide de la quasi-totalité du globe
terrestre établi, à la même époque, par les études sismiques. Elle est rejetée
par une part importante de la communauté scientifique.
Osmond Fisher (1817-1914) géologue
britannique, tenant d’un modèle de Terre en
refroidissement, note les similitudes de la
forme des continents et propose qu’ils aient
été autrefois réunis.
Antonio Snider-Pellegrini,
géographe français (1802-1885)
propose une première
reconstitution de la Pangée avant
le modèle d’Alfred Wegener. Il
avait trouvé des plantes fossiles
datant du Carbonifère identiques
de part et d’autre de l’Atlantique.
Il pensait que le déluge biblique
était la cause de la séparation des
continents.
Eduard Suess (1831-1914)
Ce schéma est très
moderne bien
qu’E. Suess ait été
un partisan de la
contraction de la
Terre
(repris du cours de A.M.C. Sengör au Collège de France en 2005)
Les géologues ne sont pas fixistes et la découverte des nappes de
charriage (1884) est une grande avancée conceptuelle
Bertrand, 1884
Peach & Horne, 1884
(Figures reprises du livre d’Olivier Merle - Masson, 1994)
La réinterprétation de la tectonique des
Alpes orientales par Pierre Termier en 1903
(repris du cours de A.M.C. Sengör au Collège de France en 2005)
Carte des aires continentales par Emile Haug (1900).
Prolongement des chaînes européennes sur le continent nord-américain selon Marcel Bertrand (1887).
Les géologues pensent «global» mais
ils manquent des concepts explicatifs
(c’est l’époque du «géosynclinal»)
http://www2.ulg.ac.be/geolsed/geol_gen/geol_gen.htm
planet-terre.ens-lyon.fr/planetterre/XML/db/p...
F.B. Taylor 1910
Brève histoire de la mob
de Wegener à Hess,
Wegener utilise la distribution bimodale des altitudes
pour argumenter la dichotomie continents-océans... Il
parle de continents qui flottent sur un substratum
visqueux. Il évoque aussi le problème des vitesses de
déformation (la poix se casse mais elle flue quand on la
laisse reposer...)
Alfred Wegener: Die Entstehung der Kontinente (1912, 1915)
Die Entstehung der Kontinente und Ozeane (1915, 1929)
www.lhce.lu/Geologie/Option/derive.html
www.lhce.lu/Geologie/Option/derive.html
www.lhce.lu/Geologie/Option/derive.html
planet-terre.ens-lyon.fr/planetterre/XML/db/p...
Certains géologues sont très tôt
«mobilistes»
La continuité des structures géologiques d'un continent à l'autre selon
Alexandre Du Toit (1927).
La formation des montagnes par la
collision de deux continents selon
Emile Argand (1924). Ses visions
sont prophétiques.
planet-terre.ens-lyon.fr/planetterre/XML/db/p...
Certains géologues sont très tôt
«mobilistes»
La continuité des structures géologiques d'un continent à l'autre selon
Alexandre Du Toit (1927).
La formation des montagnes par la
collision de deux continents selon
Emile Argand (1924). Ses visions
sont prophétiques.
planet-terre.ens-lyon.fr/planetterre/XML/db/p...
Certains géologues sont très tôt
«mobilistes»
La continuité des structures géologiques d'un continent à l'autre selon
Alexandre Du Toit (1927).
La formation des montagnes par la
collision de deux continents selon
Emile Argand (1924). Ses visions
sont prophétiques.
L’évolution de la Méditerranée selon
Emile Argand (1924) !
Les Alpes résultant de la collision de deux masses continentales
Extension et océanisation en Méditerranée (Emile Argand)
(Repris du séminaire de Jean-Paul Schaer au Collège de France en 2005)
Mais la théorie de Wegener est
très largement rejetée
Le rejet de la théorie de Wegener
Harold Jeffreys (1891-1989)
Ce n’est qu’en 1922 que les géologues commencent à s’intéresser aux thèses de Wegener. Passée la réserve
du début, les hostilités deviennent de plus en plus virulentes. Les détracteurs doutent du sérieux
scientifique de Wegener et pour justifier leur rejet ils argumentent que les mesures géodésiques de
l’éloignement du Groenland sont plus qu’incertaines, que les ajustements entre continents sont imprécis et
sans doute accidentels, que les ressemblances géologiques et paléontologiques ne sont pas si évidentes et
qu’il est bien téméraire de vouloir prouver l’existence d’un ancien continent unique en cherchant à
raccorder les moraines glaciaires…
Lake, en 1922, ouvre les hostilité contre la théorie Wegener, en mettant en doute le sérieux de sa démarche
scientifique :
« Wegener lui-même n'aide pas son lecteur à se faire un jugement impartial. Même si son attitude a pu être
originale, dans son livre, il ne cherche pas la vérité, il défend une cause, et il ferme les yeux devant chaque
fait et chaque argument qui la contredit » (in U. Marxin, Continental drift : Evolution of a concept,
Washington, Smithsonian Institution Press, 1973, p. 83.)
Et que se passe-t-il sur le fond des océans et avant les 200 derniers millions d’années ? Wegener n’en dit
rien et ces deux lacunes ont certainement joué un grand rôle dans le rejet de sa théorie.
Mais les détracteurs trouvent leurs objections les plus fortes dans le mécanisme invoqué pour rendre
compte des mouvements : l’intensité des forces supposées est bien trop faible, la résistance du sima bien
trop forte pour permettre un déplacement appréciable des continents.
Le chef de fil des négateurs absolus est Harold Jeffreys (1891-1989). Il calcule que les forces supposées ont
une amplitude 2,5.105 fois trop faible pour mouvoir et déformer les blocs continentaux et pour lui la théorie
des translations est « out of the question ».
planet-terre.ens-lyon.fr/planetterre/XML/db/p...
In Holmes, 1929
Sir Arthur Holmes (1980-1965) est certainement le plus visionnaire
des géologues mobilistes. Il a pressenti de nombreux concepts en
vogue aujourd’hui
Sir Arthur Holmes (1890-1965), the British geologist who
contributed to our understanding of Earth's age. Photo courtesy
of University of Edinburgh, Department of Geology and
Geophysics
http://www.amnh.org/education/resources/rfl/web/essaybooks/earth/p_holmes.html
Chaleur de la Terre,
convection et
mouvements en surface,
chaînes de montagnes...
Arthur Holmes
Radioactivity and Earth Movements (1931)
Chaleur de la Terre,
convection et
mouvements en surface,
chaînes de montagnes...
Holmes 1931
Jean Goguel (1952 et 1962)
20
Jean Goguel (1952 et 1962)
La solution viendra de la
communauté des
géophysiciens et de la
découvert du domaine
océanique
20
L’étude du magnétisme
des roches a fait faire un
saut en avant considérable
1853: Découverte de l’aimantation des roches: Macedonio Melloni
1901, 1906: Découverte des inversions par Bernard Bruhnes; premières datations des inversions
par Matuyama
1952: Invention d’un magnétomètre capable de mesurer de très faibles champs magnétiques par
Patrick Blackett
1959: Keith Runcorn et Ted Irving mesure la mémoire magnétique des roches, ils inventent le
paléomagnétisme.
1960: John Reynolds et John Verhoogen confirment les observations de Matuyama sur les
inversions
1960: Walter Elsasser et Ted Bullard développent l’idée de la dynamo terrestre
1960-1966: Etablissement de la première échelle des inversions du champs sur les derniers 4 Ma
par Alan Cox, Richard Doell et Brent Dalrymple (US Geological Survey), et Ian McDougall et
François Chamalun (Australian National University).
Le
!"#$%#&#$'()*+%,$)
!"#$%&#'!"#
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paléomagnétisme a fourni les premières preuves indépendantes
de la dérive des continents
Keith Runcorn
($%'')$%%*+
pôle
eulerien
88.5'N-27.7'E
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Runcorn, S.K.: Palaeomagnetic evidence for continental drift and its
geophysical cause, in S.K. Runconrn (ed) Continental Drift, New
York, Academic Press, 1962
Courbes de dérive du pôle pour l'Europe et l'Amérique
Ssm
SsDi
Actuel
Reconstruction
anté-ouverture de l'Atlantique
La première échelle des inversions du champ
magnétique a fourni la base de la théorie de
l’expansion des fonds océaniques
Allan Cox (seated), Richard Doell (L), and Brent
Dalrymple (R) at a gas mass spectrometer. This photo
was taken sometime in the early 1960s. Image courtesy
of Stanford School of Earth Sciences.
The first figure by Cox, Dalyrmple, and Doell shows
Earth’s magnetic reversals and the beginnings of the Geomagnetic Polarity Time Scale. (mod. from Science, 1963)
quake06.stanford.edu/centennial/tour/stop11.html
www.ggl.ulaval.ca/.../s1/magnetisme.terr.html
L’hypothèse de l’expansion océanique et sa vérification
Au début des années 1960, les découvertes de la topographie océanique
et des variations du flux thermique permettent d’imaginer une expansion
océanique par accrétion de matériau remontant à l’axe des dorsales,
conséquence d’une convection profonde.
La mise en évidence de bandes d’anomalies magnétiques symétriques
par rapport à l’axe des dorsales océaniques, corrélables avec les
phénomènes d’inversion des pôles magnétiques (connus depuis le début
du siècle) permet d’éprouver cette hypothèse et de calculer des vitesses
d’expansion.
! Proposed that all sea floor is
created and destroyed
! Used pattern of quakes
" Quakes occur along
plate boundaries
14
Maurice Ewing et le Vema
eanography 102, Strickland/Nuwer
Lecture 6
© 2009 University of Washington
Mary Tharp et Bruce Heezen
Marie Tharp & Bruce Heezen
Découverte
des dorsales médioocéaniques...
Les campagnes océanographiques des
années 50 ont permis la mise en
évidence de montagnes sous la mer sur
plus de 50 000 km. Chaque océan à la
sienne.
Atlantique
- 5000 m
Pacifique
- 5000 m
Harry Hess, 1962
Au départ l’expansion des fonds océaniques est
une hypothèse formulée en particulier par
Robert S. Dietz (1961) et Harry H. Hess (1962)
BUDDINGTON VOLUME
603
HISTORY OF OCEAN BASINS
CONTINENTAL
COLUMN
Krn
OCEANIC
COLUMN
!
9/(
I I
I
-
-
:$:",+SEDIMENT
-
LAYER 3-SERPENTINE
I
LAYER 4
1
3.34
I
6 16
BUDDINGTON VOLUME
Depth Sea Floor in Abyssal Areo in Km
X
3
B
BUDDINGTON VOLUME
Figure 2.
Balance
of
oceanic
and
0,
Figure 7. Diagram to represent (1) apparent progressive overlap of ocean sediments on
a mid-ocean ridge which would actually be the effect of the n ~ a n t l cmoving laterally away
from ridge crest, and (2) the postulated fracturing where convective flow changes direction
from vertical to horizontal. Fracturing and higher temperature could account for the lower
seismic velocities o n ridge crests, and cooling and healing of the fractures with time, the
return to normal velocities on the flanks.
=r
a
5=r
0
-*
2
continental crustal columns
2
3
-.
3
3
HISTORY OF
BASINS
that basalt flows poured out on the ocean floor could be so uniform in thickness.
Crr
0
0
Rather, one would expect them to be thick near the fissures or vents from which
3
g
they were erupted and thin or absent at great distance from the vents. T h e only
2
a
likely manner in which a layer of uniform thickness could be formed would be if
g'
r
a
its bottom represented a present or past isotherm, at which temperature and
L
Figure 9. Diagram to show progressive migration of volcanic peaks, guyots, and atolls, from
pressure a reaction occurred. Two such reactions can be suggested: (1) CORE
the basalt
a ridge crest to the flanks. suggesting that the wave-cut surfaces of guyots or the bases
of atolls may become older laterally away from the crest
to eclogite inversion (Sumner, 1954; Kennedy, 1959), and (2) the hydration of
: 11. Graph portraying depth to the hl discontinuity under continents us. dep
abyssal areas in oceans, computed from balance of crustal columns
sidence compared to present topographic ridges could be explained by the above
olivine to serpentine at about 500°C (Hess, 1954). T h e comnlon
Figure 8. Possibleoccurrence
geometry of a mantle of
convection cell
reasoning.
peridotitic inclusions in oceanic basaltic volcanic
rocks
(Ross,
Foster,
and
Myers,
Turning to a reconsideration of the Mid-Atlantic Ridge it appears that layer
old ridge (Fig. 9). On this basis it would be very interesting to examine the fauna
3, with a thin and probably discontinuous cover of sediments, forms the sea
1954) and absence of eclogite inclusions lead the
writer
to northern
acceptmargin
postulate
(2).or to drill atolls near
on guyots
near the
of the old ridge
the
floor. T h e dredging of serpentinized peridotite from fault scarps at three places
southern margin to see if the truncated surfaces or bases have a Triassic or even
on the ridge (Shand, 1949) points to such a conclusion. T h e abnormally lo~v
Furthermore, the dredging of serpentinized peridotites
from
scarps
the
Permian age. At
any ratefault
the greater
width in
of the
old ridge and its belt of subseismic velocity, if this is layer 3, might be attributed to intense fracturing and
oceans (Shand, 19-19)? where the displacement on the faults may have been
dilation where the convective flow changes direction from vertical to horizontal.
Figure 7. Diagram to represent (1) apparent progressive overlap of ocean sediments on
T h e underlying material, which ordinarily would have a velocity of 8 km/sec or
sufficient
expose
layer
to this
supposition. This choice of
a mid-ocean ridgeto
which
would actually
be the 3,
effectadds
of the ncredence
~ a n t l cmoving laterally
away
12. Diagram
illustrale tliicLe~ling
of a continent
by tlcfor~nation.
1niti;lll)
;I
Fig~ilcmore,
has ato velocity
approximately
7.4 kmlsec
partly for
the same
reason but
from ridge crest, and (2) the postulated fracturing where convective flow changes direction
~ n obecause
u ~ l t a isystem
n of its
andabnormally
much larger root
aretemperature
formed, but both
spread
postulates
is
made
here
and
will
control
much
of
the
subsequent
reasoning.
T
h
e
also
high
(Fig.
7).
T
h
e
interface
between
from vertical to horizontal. Fracturing and higher temperature could account for the lower
time and isostatic adjustment
layer 3 and laterally
the 7.4with
km/sec
material below is thus the hI discontinuity. T h e inseismic velocities o n ridge crests, and cooling and healing of the fractures with time, the
seismic
velocity of layer 3 is highly variable; it ranges from 6.0 to 6.9 km/sec and
return to normal velocities on the flanks.
crease in velocity of layer 3 to about 6.7 km/sec and of the sub-bIoho material to
OCEAN
km of water and equate this to continental columns for the same pressure at 40
L’introduction du terme «sea-floor
spreading» par l’océanographe américain
Robert S. Dietz (1914-1995) en 1961
(repris du cours de C. Sengör au Collège de
France en 2005)
La découverte des anomalies
magnétiques par Raff et Mason
(1961) puis leur interprétation
indépendamment par L. Morley
et Vine & Matthews fournissent
des preuves de la réalité de ce
processus
!"#$%&%'())*$+,
-./0
© 1963 Nature Publishing Group
Lawrence W. Morley
Matthews, Vine et McKenzie
Il faut attendre la publication du «magic profile»
pour que l’ensemble des géophysiciens soit
convaincu
21
20
19
Routes du navire océanographique Eltanin en 1960 (Columbia
University) pour enregistrer les variations du champ magnétique
Profil magnétique
enregistré par l’Eltanin
la ligne rouge représente l’absence d’anomalie
Anomalie magnétique, nT
profondeur, km
Extrait d’un cours de Cornell Univ.
Pitman et Heirtzler, 1966
ESE
WNW
WNW
ESE
total intensity anomaly calculated from model
image miroir du profil pour
montrer la symétrie
profil mesuré
la symétrie du profil Eltanin 19
Extrait d’un cours de Cornell Univ.
Pitman et Heirtzler, 1966
ESE
WNW
WNW
ESE
total intensity anomaly calculated from model
image miroir du profil pour
montrer la symétrie
profil mesuré
la symétrie du profil Eltanin 19
Extrait d’un cours de Cornell Univ.
Pitman et Heirtzler, 1966
JOURNALOF GEOPHYSICAL
R•-S•-AaC•
VOL. 73, No. 6, MARCH 15, 1968
Marine Magnetic Anomalies,GeomagneticField Reversals,
and Motions of the Ocean Floor and Continents
J. R. HEIRTZLER,
2 G. O. DICKSON,E. M. I-IERRON,
W. C. PITMAN,III, ANDX. LE PICHON3
Lamont GeologicalObservatory, Columbia University
Palisades,New York 10964
INTRODUCTION
HERRON,
o
PITMAN,
•o
www-istp.gsfc.nasa.gov/earthmag/mill_6.htm
DICKSON,
Carte des anomalies magnétiques su sud de
l’Islande [Heirtzler, 1968]
o
AND LE PICHON
are the mo,deI blocks, derived for the North
Pacific profile and adjusted to fit the other
The three previouspapersof this series[Pito.ceans.The North Pacific profile is a c,omposite
man et al., 1968; Dickson et al., 196.8; Le
using sectionsat differen,tlatitudes becauseof
Pichon and Heirtzler, 19'68] have shown that
complexstructure.
a magnetic anomaly pattern, parallel to and
It is our purpose in this paper to derive a
bilaterally symmetric about the mid-oceanic
time scalefor the sequenceof magneticpolarity
ridge system, exists over extensiveregions of
even'rspredicted by the basic model and to
the North Pacific,South Pacific,SouthAtlantic,
discussthe conceptso.f continental drift. and
and Indian oceans. It has been further demono,ceanfloor spreadingin terms of the anomaly
strated that the pattern is the same in each pattern and the time scale.
of these oceanic areas and that the pattern
FollowingVine and Matthews [1963], we will
may be simulatedin each region by the same
assumethat all the linear magnetic anomalies,
sequenceof source blocks. These blocks comsu'b-parallelto the ridge axis, are due to geoprise a series of alternate strips of normally
magneticfield reversals.If that theory is b.asiand reversely magnetizedmaterial, presumably
cally in error, the conclusions
in this paper do
basalt (Figure 1).
not apply. Yet, if that theory is applied careThe symmetric and parallel nature of the
fully, many geophysicalan'dgeologicalobserv'aan'omalypattern and the general configuration
tions, previously thought to be unrelated or
of the crustal model conform to that pre'dicted
casuallyrelated,are seento have a simpleuniby Vi.r•eand Matthews [1963] and may be re- fied explanation.
•arded as strong support for the concept of
C•o•cE OF TX•E SCALE
oceanfloor spreading[Dietz, 1961; Hess,1962].
Figure 1 sh,ows
sampleanomalypro,fliesfrom
The most obvi'ousmethod for deriving a
t,hevariousregionsunder discussion.
Alsoshown
HEIRTZLER,
Gondwanaland.
2124
This paper summarizesthe results of the three previous papers in this series, which have
shown the presence of a pattern of magnetic anomalies, bilaterally symmetric about the
crest of the ridge in the Pacific, Atlantic, and Indian oceans.By assumingthat the pattern is
causedby a sequenceof normally and reversely magnetized blocks that have been produced
by sea floor spreading at the axes of the ridges, it is shown that the sequencesof blocks
correspondto the same geomagnetictime scale.An attempt is made to determine the absolute
ages of this time scale using palcomagnetic and paleontological data. The pattern of opening
of the oceansis discussedand the implications on continental drift are considered.This pattern
is in good agreement with continental drift, in particular with the history of the break up of
•
0
Mesure
de la vitesse
d’ouverture
océanique
35
Extrait d’un cours de Cornell Univ.
d’après Bott, 1967, 1982; dans Keary et al., Global tectonics, Blackwell, 2009
Les concepts de lithosphère et d’asthénosphère
Au voisinage des fosses océaniques, la distribution spatiale des foyers des
séismes en fonction de leur profondeur s’établit selon un plan incliné.
Les différences de vitesse des ondes sismiques qui se propagent le long de
ce plan, par rapport à celles qui s’en écartent, permettent de distinguer : la
lithosphère de l’asthénosphère.
L’interprétation de ces données sismiques permet ainsi de montrer que la
lithosphère s’enfonce dans le manteau au niveau des fosses dites de
subduction.
La limite inférieure de la lithosphère correspond généralement à l’isotherme
1300°C.
En observant les mouvements verticaux liés à la
vidange du Lac Bonneville, Grove Gilbert discute en
1890 la notion d’isostasie et propose de substituer
à l’isostasie locale (voir les théories de Pratt et
Airy) la notion d’isostasie régionale impliquant o.une
WOLV
rigidité de la croûté.
pensation
tatic equiusly, such
ew of the
and rede-
ve Gilbert
which he
h the geoure is that
ust, which
more local
al Society
(Gilbert,
ons of the
these may
gidity and
s, and valrigidity of
plateaus,
f isostatic
as to den-
f the crust
ds. As an
arance of
ose abanhortly beupdoming
(Fig. 3),
e of the
ake. Using
of a solid
\
\
La notion de «lithosphère», au sens
d’une enveloppe rigide dotée d’une
certaine élasticité, vient des études des
mouvements verticaux
C H A N G I N G ROLE OF LITHOSPHERE IN MODELS OF G L A C I A L ISOSTASY
\
/
\
V/
I."
I
:~
/
/
I
c
Seo level
d e f
/ ' / / . / . ~
/~/., ~\\\\\\\\\\\\~
~'," "/Regional
Lithosphere
compensati.or~\\~l
~
Depth of
compensation
~sthenosphere~--
Fig. 4. Barrell's (1919b) conception of regional isostatic equilibrium versus conventional conception of local isostatic equilibrium.
~
Fig..3. Gilbert's (1890a) contour
neville shorelines.
On the other hand, Barrell regarded the asEn 1914 Joseph Barrell introduit la notion
thenoshere as a plastic shell with the capacity to
d’asthénosphère qu’il oppose à la lithosphère
yield
to long-enduring
stress
differences
and, thus,
rigide.
L’asthénosphère est
capable
de se déformer
adjustment.
Commenting on
to effect
isostatic
plastiquement
pour
effectuer les ajustements
glacial isostasy, he isostatiques.
took a more conservative view
and remarked (Barrell, 1914a):
2 0
'~li,tts
"It is not known, however, to what degree the
map of warped Lake Bonprevious [before deglaciation] downwarp compen(D’après Wolf, 1993)
sated for the burden of the continental ice sheet
Fig.
to ic
pub
yield to long-enduring stress differences and, thus,
Commenting on
to effect isostatic adjustment.
glacial isostasy, he took a more conservative view
and remarked (Barrell, 1914a):
"It is not known, however, to what degree the
previous [before deglaciation] downwarp compenCe type d’analyse
est for
étendu
au cas des
sated
the burden
of the continental ice sheet
réajustements
post-glaciaires
and what
degree of regional stress the crust was
(Nansen,able
1921;
1934)
toDaly,
bear."
Notwithstanding this cautionary note, Barrell's
studies seem to have been generally well-received
I A L ISOSTASY
by the proponents of glacial isostasy. 99
Several years later, the Norwegian oceanograe f
pher and polar researcher Fridtjof Nansen, in a
comprehensive study of the glacial record preserved in Fennoscandia and elsewhere, developed
a comparatively detailed model of glacial-isostatic
adjustment (Nansen, 1921, pp. 290-306) and displayed its essential features in a simple illustration (Fig. 5). In another study of the properties
and behaviour of the earth's crust and its substrac equitum, he remarked (Nansen, 1928, pp. 11-12):
c equi"The earth's crust may ... be considered as a
slowly flexible sheet of solid rock floating on a
viscous substratum. If loaded in one place this
sheet will bend slowly under the load, and the
e asplastic matter underneath will be displaced to the
ty to
sides, where the sheet will be slightly lifted in a
thus,
belt round the depressed area. If unloaded in one
g on
place the sheet will rise slowly in that area; there
be illustration
an inward of
flow
in the of
substratum
view
Fig. 5. Nansen'swill
(1921)
depression
crust due underneath,
and response
a slight subsidence
the sheet in the
to ice-sheet. Note
retarded
of crust and of
substratum.
surrounding area."
e the
A major proponent of Barrell's work was the
mpenNorth American geologist Reginald Daly, who
sheet
La notionFig.de
«lithosphère», au sens
5. Nansen's (1921) illustration of depression of crust due
to ice-sheet. Note retarded response of crust and substratum.
d’une enveloppe
rigide dotée d’une
certaine élasticité, vient des études des
published extensively on the Pleistocene. His
mouvements
verticaux
views are summarized
in a monograph, which
(D’après
Wolf,
published extensively on the
Pleistocene.
His1993)
also considers most of the work completed by
other investigators up to that time. In particular,
Daly discussed two alternative hypotheses of
glacial-isostatic adjustment. To one of them he
referred as bulge hypothesis (Fig. 6) and remarked
(Daly, 1934, p. 120):
"Below it [the flexible crust] is a weak substrat u m . . . According to the bulge hypothesis, the
basining [of the crust] is accompanied by outward,
horizontal flow in the substratum and just beneath the crust . . . . Because the substratum is
1i:!:?:.:!:::i::.:!:~i:!:::i:i:!!:i.::! ! ! i:! !.Zi:?:!:.c~:!.i:~ !:!:i i:~:!:?i:!:!:!.i.!:Z:Z:!.i..!:!.!:!:i:!:!
SUBSTRATUM
B'
B'
' . ~
~:':'.:::'.:::--'-"
:'.'.-'.'j:
2 :':..-'.5...:...'"".':
................
.~):v.~.::~:::.::~?:.E::......j.:.~:::::_...........
.....:':"
. ......
t3"
E~'
3 :::.:.:.:::::.:.5::::-:::::-.-.
"~..:'":F:.'::::%'.:.'(-.
".~-'.-".~'.:.".'-~.:.:.'.'-'.'.-:.:.'..'.'.:.:-:."
.. :.:.::,-.-.....
:-...... .....
B"
B'"
Fig. 6. Daly's (1934) illustration of bulge hypothesis of isostatic
adjustment. Note movement of peripheral bulge.
yield to long-enduring stress differences and, thus,
Commenting on
to effect isostatic adjustment.
glacial isostasy, he took a more conservative view
and remarked (Barrell, 1914a):
"It is not known, however, to what degree the
previous [before deglaciation] downwarp compenCe type d’analyse
est for
étendu
au cas des
sated
the burden
of the continental ice sheet
réajustements
post-glaciaires
and what
degree of regional stress the crust was
(Nansen,able
1921;
1934)
toDaly,
bear."
Notwithstanding this cautionary note, Barrell's
studies seem to have been generally well-received
I A L ISOSTASY
by the proponents of glacial isostasy. 99
Several years later, the Norwegian oceanograe f
pher and polar researcher Fridtjof Nansen, in a
comprehensive study of the glacial record preserved in Fennoscandia and elsewhere, developed
a comparatively detailed model of glacial-isostatic
adjustment (Nansen, 1921, pp. 290-306) and displayed its essential features in a simple illustration (Fig. 5). In another study of the properties
and behaviour of the earth's crust and its substrac equitum, he remarked (Nansen, 1928, pp. 11-12):
c equi"The earth's crust may ... be considered as a
slowly flexible sheet of solid rock floating on a
viscous substratum. If loaded in one place this
sheet will bend slowly under the load, and the
e asplastic matter underneath will be displaced to the
ty to
sides, where the sheet will be slightly lifted in a
thus,
belt round the depressed area. If unloaded in one
g on
place the sheet will rise slowly in that area; there
be illustration
an inward of
flow
in the of
substratum
view
Fig. 5. Nansen'swill
(1921)
depression
crust due underneath,
and response
a slight subsidence
the sheet in the
to ice-sheet. Note
retarded
of crust and of
substratum.
surrounding area."
e the
A major proponent of Barrell's work was the
mpenNorth American geologist Reginald Daly, who
sheet
La notionFig.de
«lithosphère», au sens
5. Nansen's (1921) illustration of depression of crust due
to ice-sheet. Note retarded response of crust and substratum.
d’une enveloppe
rigide dotée d’une
certaine élasticité, vient des études des
published extensively on the Pleistocene. His
mouvements
verticaux
views are summarized
in a monograph, which
(D’après
Wolf,
published extensively on the
Pleistocene.
His1993)
also considers most of the work completed by
other investigators up to that time. In particular,
Daly discussed two alternative hypotheses of
glacial-isostatic adjustment. To one of them he
Les
travaux
ont permis
la
referred
as plus
bulgerécents
hypothesis
(Fig. 6)d’estimer
and remarked
21
viscosités
respectives
(Daly, 1934,
p. 120):de la lithosphère (~10 Pa.s)
et de "Below
l’asthénosphère
(1-2 ordres
plus
crust] de
is agrandeur
weak substrait [the flexible
faible)
ainsi que les épaisseurs de la lithosphère
t u m . . . According to the bulge hypothesis, the
à plusis de
200 km) by outward,
basining [of(de
the50crust]
accompanied
horizontal flow in the substratum and just beneath the crust . . . . Because the substratum is
1i:!:?:.:!:::i::.:!:~i:!:::i:i:!!:i.::! ! ! i:! !.Zi:?:!:.c~:!.i:~ !:!:i i:~:!:?i:!:!:!.i.!:Z:Z:!.i..!:!.!:!:i:!:!
SUBSTRATUM
B'
B'
' . ~
~:':'.:::'.:::--'-"
:'.'.-'.'j:
2 :':..-'.5...:...'"".':
................
.~):v.~.::~:::.::~?:.E::......j.:.~:::::_...........
.....:':"
. ......
t3"
E~'
3 :::.:.:.:::::.:.5::::-:::::-.-.
"~..:'":F:.'::::%'.:.'(-.
".~-'.-".~'.:.".'-~.:.:.'.'-'.'.-:.:.'..'.'.:.:-:."
.. :.:.::,-.-.....
:-...... .....
B"
B'"
Fig. 6. Daly's (1934) illustration of bulge hypothesis of isostatic
adjustment. Note movement of peripheral bulge.
mediate and deep earthquakes were attributed to body waves.
Wadati sought to apply this diagram to the estimation of the
scale of destructive earthquakes. This diagram could be compared
with Richter's magnitude-scale, published in 1935.
4. Wadati's fourth paper
Honda tried to analyse the P-wave radiation pattern of the intermediate and deep earthquakes, and published a paper on the mechanism
of deep earthquakes in 1934, in which he discussed the spatial distribution of forty-five deep earthquakes deeper than 100 km that
occurred between 1927 and 1933. He pointed out that they took
place along the Principal (Wadati's Transverse) deep earthquake
zone, crossing central Honshu NNW-SSE, and the Soya deep earth-
Une autre approche de la lithosphère
au travers de la subduction
Wadati published his fourth paper in 1935, in which the
frequency-distribution of earthquakes occurring between
1924 and 1934, and the contour lines of equal focal depths
of intermediate and deepearthquakes, were plotted. They
revealed inclined surfaces dipping away from the oceanic
trenches—the planes that came in the West to be called the
Benioff Zones following Benioff's publication in 1955.
The name was subsequently changed to Wadati-Benioff
Zones, thus acknowledging their discovery by Wadati two
decades earlier.
Lines of equal focal depth
Hypocentres of 42 deep earthquakes between 1905 and
1923, and 62 deep and 28 intermediate earthquakes
between 1924 and April 1934 were plotted, showing the
frequency distribution of their focal depths and their geographical distributions. The data before and after 1924
were shown separately, as the seismological observatories
were better equipped with Wiechert seismographs and
offered more reliable data after 1924.
The data indicated the most frequent occurrences
were at depths of 300 to 400 km and the least were at about
200 km. Eight earthquakes were deeper than 500 km, and
the deepest was at 660 km. Most of the deep earthquakes
occurred along the Transverse and Soya deep-focus earthquake zones. The former zone extended over 2000 km
from Vladivostok to the Bonin Islands, traversing central
Honshu. The latter one ran northeast along the Chishima
Islands, extending from the northern part of the Sea of
Japan through the Strait of Soya (La Pèrouse). In addition
to those zones, the Kyushu deep earthquake zone was
revealed near the Ryukyu Islands, extending to northern
Kyushu. Based on the depth of the hypocentres of these
earthquakes, the lines of equal focal depth were shown, at
which intermediate and deep earthquakes occurred frequently ( Figure 3). The lines ran along the Chishima
(Kurile) Islands, Hokkaido, northeast Honshu, and the IzuBonin Islands, and dipped away from the Chishima, Japan
and Izu-Ogasawara trenches towards the north, west, and
southwest. Other lines ran along the Ryukyu Islands and
got deeper from the Ryukyu trench toward WNW.
Benioff, 1955
Wadati, 1935
Figure 3 Distribution of earthquakes and lines of equal focal depth for
intermediate and deep earthquakes (Wadati, 1935).
H. P. Berlage, 1937: Les isobathes des tremblements de terre de l’Asie du sud-est.
(repris du cours de C. Sengör au Collège de France en 2005)
ties involved,
coincidewith the low Q zone of other important quantities to determine. The
continuity immediately suggestsa structural
the highInQfact,
zonethe
of configuration
Figure 13 does
data from this paper provide limited informathe anomalous zone are different fromFigure
those 13.
of If relation.
of Figure
JOURNALOF GEOPtlYSICAL
RESEARCI-I
VOL. 72,13
NO. 16
AUGUST 15, 1967
indeed
represent
the lithosphere,
a with
layer
tion on
this point; consequently,
the configucouldclearly
be reconciled
an of
underthrustwaves traversing more normal parts
of the
ration of the 'anomalous zone' is not well designificant
then it or
is settling
apparent
mantle, and, less certainly, becausethe
veloci- strength,
ing, or dragging,
of that
the uppermost
theories
of tectonics
based
on outward
convection
fined.
mantle layer
on the
sidecurof the arc
be- Early in the investigationit appeared
ties of P and S waves appear to be high
in the
that the anomalous zone coincided with the
zone.
rents must take
this
into Such
account,
for its
neath
thelayer
arc itself.
a process
is in general
zone
of seismic Structures
loci. When the most recent data
Deep
Earthquake
Zones,
effects on surface
geology
could
be large
and Anomalous
From this point it is a minor and reasonably
agreement
with
the hypotheses
relating
to seaspreading
under
the in
influence
mantle
varied. Such
a layer,
when
involved
exten- ofand
safe step to explain the pertinent character
of floor
in
the
Upper
Mantle,
* communication)are
of
L.the
R. Lithosphere
Sykes (personal
the seismic waves as an effect of attenuation
convection
currents,
as proposed
Hess
sion, thrusting,
or flexing,
could introduce
great byconsidered,
however,it appearsthat the seismic
OLIVER
AND
BRYAN
ISACKS
along all, or a substantialpart, of thecomplexity
propaga- into
[1962],
[1961],
and
others.
These
hy-which
the Dietz
effects
of JACK
a driving
force
zone,
was thought to be 50 to 100 km in
for rising convection
currents
betion path. When this is done, the of
anomalous
relativelypotheses
simple call
configuration
and hence
thickness,
may, in the caseof the deep shocks
Lamont
Geological
Observatory,
Columbia
University
zone is found to be a region of low attenuation
neath
the
ocean
ridges
and
sinking
currents
in
help to explain how complex surface
geology
of
the
Tongan
arc, be lessthan 20 km in thickPalisades, New York 10964
might result from a driving mechanism,say, a ness.The zone of high Q, i.e., the anomalous
TONGA
convection
current, conclusion
of greaterofsimplicity.
although
not
well
because
of unThe principal
this paperThe
is thatzone,
regional
seismic
data
fordefined
deep and
shallow
I
'•
TRENCH
concepts
of lithosphere,
asthenosphere,
and certainties
earthquakes
associatedwith
the Tonga-Kermadec
arc show
there exists
in theand
mantle
inthat
hypocentral
locations
velocity
an anomalous
whose
thickness
is ofones
the order
of 100 km
and whose
upper
surface
is
mesosphere
are, ofzone
course,
long
standing
structures,
appears,
on the
basis
of various
ray
proximately
defined
by
the
highly
active
seismic
zone
that
dips
to
the
west
beneath
the
in geology[Daly, 1940] and need not be elabo- path locations,to be somewhatthicker, perisland
arcQand extends to depths of about 700 km. Limited data for other areas suggestthat
LOW
ratedsimilar
here. 'anomalous
The new feature
of associated
this paper
is other
haps
of the
order
100important
km. The evidence
data further
LO",N
Q
zones' are
with
island
arcs.
The of
most
200configuration
km
the evidence
that conclusion
suggestsaisnew
suggest
that the Sloci
are located
nearin
thethe
upper
for the above
that
seismicbody waves,
particularly
waves,
propagating
anomalous
zones
much
to attenuation
than are
waves
of the same
type
propafor, and
perhaps
theare
extent
ofless
thesubject
mobility
of, boundary
of the
anomalous
zone,
i.e.,
near the
gating
in parts of the mantle at similar depthsupper
elsewhere.
Oneofinterpretation
of the
above
the lithosphere.
surface
the lithosphere.
This
is also
HIGH O
results and of some additional seismic data is that the deep anomalous zone is continuous
H•GHQ
The conceptof a lithosphere
that
large the region in which crustal materials would be
HIGH
Q covers
with the uppermost mantle
east
of Tonga. Such a structure is consistentwith theories of
found
if they
werearcs.
carried
into the
partsmantle
of
theTonga,
earth's
surface
yet isbased
discontinuous
convection
with
down-going
currents
in this
the
vicinity
of island
If low down
attenuation
Fig. 13. Hypothetical sectionthrough
Fiji,
snd
Rarotonga
on
data of
mantle.
Thus,
the presence
such materials
of
seismic
waves
with strength,
this be
structure
suggests
that the of
lithosphere
has in
and,
tolow
a certain
degree,
mobile
(perhaps
driven
paper. Boundaries between high Q
and
Q zones
are correlates
not
well determined
but can
ticularly of the S wave, propagating through
HIGH•
I
taken as a first approximation.
been thrust or dragged down beneath the Tonga arc and hence implies a certain mobility for
the lithosphereelsewhere.This possibility suggests,in turn, new approachesto a wide variety
of problems ranging from theTONGA
nature of the earthquake mechanismto the relation between
complex surface
mechanismswhose configuration
ß geology and
'.. driving
......
R•Rmay
0 be relatively
simple.
....
..."
"?'..".?-'"."':":
.... ' ' •½:i!iii!::"
......
:'":':'::::::
".........•:i:!:i::'ASTHENOSPHERE
The evidencecomeslargely from the seismic
The purpose of this paper •:i:i:i:'
is to demonstrate waves that are generated by deep shocksand
that zonesof the upper mantle associatedwith that travel through different zones en route to
different recording stations. The evidence is
deep earthquakesare anomalous,i.e., that cer-MESOSPHERE
largelyand
qualitative,
but
it is not
and it
Fig. 14. of
Hypothetical
section
Fiji, Tonga,
Rarotonga,
assuming
Q subtle
correlates
tain properties
these zones
differ through
from those
definitive.
new structure
a part
with of
strength.
The lithosphere
and mesosphere
zones ofThis
significant
strength,for
and
the of.
of parts
the mantle
at comparable
depths isare
the
upper mantle
is important
becauseis
of its
asthenosphere
is a zone of vanishingstrength on
appropriate
time scale.
The terminology
elsewhere.
Seismologists
commonly
use
the
term
that of Daly [1940].
'deepearthquake'
in two s•nses.
In the stricter relation to theoriesof focal mechanismsof deep
shocks,to theoriesof the spatial and temporal
INTRODUCTION
3.:::::::'
ties involved,
coincidewith the low Q zone of other important quantities to determine. The
continuity immediately suggestsa structural
the highInQfact,
zonethe
of configuration
Figure 13 does
data from this paper provide limited informathe anomalous zone are different fromFigure
those 13.
of If relation.
of Figure
JOURNALOF GEOPtlYSICAL
RESEARCI-I
VOL. 72,13
NO. 16
AUGUST 15, 1967
indeed
represent
the lithosphere,
a with
layer
tion on
this point; consequently,
the configucouldclearly
be reconciled
an of
underthrustwaves traversing more normal parts
of the
ration of the 'anomalous zone' is not well designificant
then it or
is settling
apparent
mantle, and, less certainly, becausethe
veloci- strength,
ing, or dragging,
of that
the uppermost
theories
of tectonics
based
on outward
convection
fined.
mantle layer
on the
sidecurof the arc
be- Early in the investigationit appeared
ties of P and S waves appear to be high
in the
that the anomalous zone coincided with the
zone.
rents must take
this
into Such
account,
for its
neath
thelayer
arc itself.
a process
is in general
zone
of seismic Structures
loci. When the most recent data
Deep
Earthquake
Zones,
effects on surface
geology
could
be large
and Anomalous
From this point it is a minor and reasonably
agreement
with
the hypotheses
relating
to seaspreading
under
the in
influence
mantle
varied. Such
a layer,
when
involved
exten- ofand
safe step to explain the pertinent character
of floor
in
the
Upper
Mantle,
* communication)are
of
L.the
R. Lithosphere
Sykes (personal
the seismic waves as an effect of attenuation
convection
currents,
as proposed
Hess
sion, thrusting,
or flexing,
could introduce
great byconsidered,
however,it appearsthat the seismic
OLIVER
AND
BRYAN
ISACKS
along all, or a substantialpart, of thecomplexity
propaga- into
[1962],
[1961],
and
others.
These
hy-which
the Dietz
effects
of JACK
a driving
force
zone,
was thought to be 50 to 100 km in
for rising convection
currents
betion path. When this is done, the of
anomalous
relativelypotheses
simple call
configuration
and hence
thickness,
may, in the caseof the deep shocks
Lamont
Geological
Observatory,
Columbia
University
zone is found to be a region of low attenuation
neath
the
ocean
ridges
and
sinking
currents
in
help to explain how complex surface
geology
of
the
Tongan
arc, be lessthan 20 km in thickPalisades, New York 10964
might result from a driving mechanism,say, a ness.The zone of high Q, i.e., the anomalous
TONGA
convection
current, conclusion
of greaterofsimplicity.
although
not
well
because
of unThe principal
this paperThe
is thatzone,
regional
seismic
data
fordefined
deep and
shallow
I
'•
TRENCH
concepts
of lithosphere,
asthenosphere,
and certainties
earthquakes
associatedwith
the Tonga-Kermadec
arc show
there exists
in theand
mantle
inthat
hypocentral
locations
velocity
an anomalous
whose
thickness
is ofones
the order
of 100 km
and whose
upper
surface
is
mesosphere
are, ofzone
course,
long
standing
structures,
appears,
on the
basis
of various
ray
proximately
defined
by
the
highly
active
seismic
zone
that
dips
to
the
west
beneath
the
in geology[Daly, 1940] and need not be elabo- path locations,to be somewhatthicker, perisland
arcQand extends to depths of about 700 km. Limited data for other areas suggestthat
LOW
ratedsimilar
here. 'anomalous
The new feature
of associated
this paper
is other
haps
of the
order
100important
km. The evidence
data further
LO",N
Q
zones' are
with
island
arcs.
The of
most
200configuration
km
the evidence
that conclusion
suggestsaisnew
suggest
that the Sloci
are located
nearin
thethe
upper
for the above
that
seismicbody waves,
particularly
waves,
propagating
anomalous
zones
much
to attenuation
than are
waves
of the same
type
propafor, and
perhaps
theare
extent
ofless
thesubject
mobility
of, boundary
of the
anomalous
zone,
i.e.,
near the
gating
in parts of the mantle at similar depthsupper
elsewhere.
Oneofinterpretation
of the
above
the lithosphere.
surface
the lithosphere.
This
is also
HIGH O
results and of some additional seismic data is that the deep anomalous zone is continuous
H•GHQ
The conceptof a lithosphere
that
large the region in which crustal materials would be
HIGH
Q covers
with the uppermost mantle
east
of Tonga. Such a structure is consistentwith theories of
found
if they
werearcs.
carried
into the
partsmantle
of
theTonga,
earth's
surface
yet isbased
discontinuous
convection
with
down-going
currents
in this
the
vicinity
of island
If low down
attenuation
Fig. 13. Hypothetical sectionthrough
Fiji,
snd
Rarotonga
on
data of
mantle.
Thus,
the presence
such materials
of
seismic
waves
with strength,
this be
structure
suggests
that the of
lithosphere
has in
and,
tolow
a certain
degree,
mobile
(perhaps
driven
paper. Boundaries between high Q
and
Q zones
are correlates
not
well determined
but can
ticularly of the S wave, propagating through
HIGH•
I
taken as a first approximation.
been thrust or dragged down beneath the Tonga arc and hence implies a certain mobility for
the lithosphereelsewhere.This possibility suggests,in turn, new approachesto a wide variety
of problems ranging from theTONGA
nature of the earthquake mechanismto the relation between
complex surface
mechanismswhose configuration
ß geology and
'.. driving
......
R•Rmay
0 be relatively
simple.
....
..."
"?'..".?-'"."':":
.... ' ' •½:i!iii!::"
......
:'":':'::::::
".........•:i:!:i::'ASTHENOSPHERE
The evidencecomeslargely from the seismic
The purpose of this paper •:i:i:i:'
is to demonstrate waves that are generated by deep shocksand
that zonesof the upper mantle associatedwith that travel through different zones en route to
different recording stations. The evidence is
deep earthquakesare anomalous,i.e., that cer-MESOSPHERE
largelyand
qualitative,
but
it is not
and it
Fig. 14. of
Hypothetical
section
Fiji, Tonga,
Rarotonga,
assuming
Q subtle
correlates
tain properties
these zones
differ through
from those
definitive.
new structure
a part
with of
strength.
The lithosphere
and mesosphere
zones ofThis
significant
strength,for
and
the of.
of parts
the mantle
at comparable
depths isare
the
upper mantle
is important
becauseis
of its
asthenosphere
is a zone of vanishingstrength on
appropriate
time scale.
The terminology
elsewhere.
Seismologists
commonly
use
the
term
that of Daly [1940].
'deepearthquake'
in two s•nses.
In the stricter relation to theoriesof focal mechanismsof deep
shocks,to theoriesof the spatial and temporal
INTRODUCTION
3.:::::::'
Zhao, 2004
Une dernière approche de la lithosphère
au travers du flux de chaleur
L. Fleitout
Un premier modèle global:
une lithosphère découpée en plaques rigides
A la fin des années soixante, la géométrie des failles transformantes océaniques
permet de proposer un modèle en plaques rigides.
Des travaux complémentaires parachèvent l’établissement de la théorie de la
tectonique des plaques en montrant que les mouvements divergents (dorsales),
décrochants (failles transformantes) et convergents (zones de subduction) sont
cohérents avec ce modèle géométrique.
Des alignements volcaniques, situés en domaine océanique ou continental, dont la
position ne correspond pas à des frontières de plaques, sont la trace du
déplacement de plaques lithosphériques au dessus d’un point chaud fixe, en
première approximation, dans le manteau.
© 1965 Nature Publishing Group
J.T. Wilson introduit la notion de
faille transformante et invente la
tectonique des plaques
Wilson, 1965
Nature Publishing Group
Wilson, 1965
cinématique à deux plaques
Quelles données ?
LA ZONE SEISMIQUJE MEDIANE INDO-ATLANTIQIUE
BY J. P. ROTHE'
Professeur a 1'Universite de Strasbourg, Directeur du Bureau
international de Setismologie
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The distribution of earthquake epicentres in the Atlantic and Indian Oceans is discussed;
numerous new epicentres are listed. It is shown that the line of epicentres following the midAtlantic Ridge is continued round the Cape of Good Hope and joins the similar line marking
the central ridge of the Indian Ocean. It seems, therefore, that these two ridges are related
structures.
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Introduction
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deviennent de plus en plus nombreuses et grac
determinations
d'epicentres
Les
a l'installation de nouvelles stations munies d'appareils modernes la position et l
forme des zones seismiques oceaniques se precisent chaque annee davantage. L
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Les limites de plaques
la sismicité est localisée le long de ceintures étroites
(ici les séismes de magnitude > 5.5)
Les limites de plaques
la sismicité est localisée le long de ceintures étroites
(ici les séismes de magnitude > 4.5)
Les vitesses relatives: les anomalies magnétiques
Les directions des mouvements relatifs:
les mécanismes au foyer des séismes
Les directions des mouvements relatifs:
les mécanismes au foyer des séismes
Les directions des mouvements relatifs:
les failles transformantes
Les failles transformantes sont parallèles au mouvement relatif
Les failles transformantes sont parallèles au mouvement relatif
Les failles transformantes sont parallèles au mouvement relatif
cinématique à deux plaques
La géométrie eulérienne
JOURNAL
OFGEOPHYSICAL
RESEARCH
VOL. 72, No. 8
APRIL 15, 1967
Mechanismof EarthquakesandNatureof Faultingon
the Mid-OceanicRidges
LYNN 1•. SYKES
Institute
[or Earth Sciences,
ESSA,Lamont GeologicalObservatory,ColumbiaUniversity
2133
OF EARTHQUAKES
Palisades, New York
ed
t
The mechanisms
of 17 earthquakeson the mid-oceanicridgesand their continentalextent
sionswereinvestigatedusingdata from the World-WideStandardized
Seismograph
Network
a
of the U.S. Coastand GeodeticSurveyand from otherlong-period
seismograph
instruments.
DEXTRAL
SINISTRAL
Mechanism
solutions
of highprecision
cannowbe obtainedfor a large
numberof earthquakes
_100
•
TRANSFORM
FAULTS
iwith magnitudesas small as 6 in many areasof the world. Lessthan 1% of the data usedin
k.
this studyare inconsistent
with a quadrantdistributionof first motionsof the phasesP and
t
PKP; in many previous investigations15 to 20% of the data were often inconsistentwith the
publishedsolutions.Ten of the earthquakesthat were studied occurredon fracture zonesthat
intersect the crest of the mid-oceanicridge. The mechanismof each of the shocksthat is
e.
t
locatedon a fracturezone is characterized
by a predominance
of strike-slipmotion on a
SINISTRAL
DEXTRAL
steeplydippingplane;the strikeof oneof the nodalplanesfor P wavesis nearlycoincident
TRANSCURRENT
FAULTS
with the strike of the fracture zone. The se.
nse of strike-slipmotion in each of the ten soluFig.
1.
Sense
of
motion
associated
with
transr
tionsis in agreement
with that predictedfor transformfaults; it is oppositeto that expected
form faults and transcurrent faults [after
Wilson,
for a simpleoffsetof the ridge crestalongthe variousfracturezones.The spatial distribue
1965a]. Double line representscrest of mid-oceanic
tion of
earthquakes
alongfracturezonesalsoseemsto rule out the hypothesis
of simpleoffset.
is ridge; single line, fracture zone. The terms
'dex•:o ø,8:84øE
½: I0ø, 8=90ø
• ---•36ø
N
79 ø S
Fig. 6. Mechanismsolution for event 2. Table
I and Figures 3 and 4 give epicentral and other
pertinent data. Symbolssame as Figure 5.
and 'sinistral'
describe
the
Two well documented
solutionsfor earthquakes
that are locatedon the mid-Atlanticridge
but that
by a predominance
transform
faultsdo not appearto be locatedon fracturezonesare characterized
sense of motion
e of the fracture zones; for the
of faulting. The mechanismsof four earthquakes on extensionsof the mid-oceanic
of normal
se they do not denote the relative configuration
the two segments of ridge on either side
of system---one
the
ridge
near northernSiberiaand three in East Africa--are alsocharacterized
by
t fracture zone.
er
e
MECHANISM OF EARTHQUAKES
tral'
Fig. 8. Mechanismsolution for event 4. Table
I and l•igures 3 and 4 contain position and other
pertinentdata. SymbolssameasFigure 5.
a predominanceof normal faulting. The inferred axes of maximum tension for these six events
are approximatelyperpendicularto the strike of the mid-oceanicridge system.The resultsare
in agreement
of sea-floorgrowthat the crestof the mid-oceanic
ridgesystem.
other pertinent data are listed in Table
1. ReL. Sykes propose le premier
test with hypotheses
1965, 1966; Heezen and Tharp, 1961, 1965a]
fault displacements
as great as 1000km IVacquiet, 1962; Menard,1965].
sev59 Nevertheless,
eral probIemsare raisedabout interpretations
of this type. The inferredmagnitudeand sense
2137
liable solutionsprobably could be obtainedfor
earthquakes
on the middes idées
de Wilson
t several additional
INTRODUCTION
e oceanicridges, the west Chile and Macquarie
sur les failles transformantes
exic ridges,and the seismicallyactive zone
The that
discovery
of a large number of fracture
- tends from the Azores to Gibraltar. However,
zoneson
with the exceptionof one earthquake
onthe
themid-oceanicridges [Menard, 1955,
JOURNAL
OFGEOPHYSICAL
RESEARCH
VOL. 73, No. 6, MARCH15, 1968
Rises,Trenches,GreatFaults,andCrustalBlocks
Time
W. JASON MORGAN
Time 2
I
Time
Department o• Geolo.gy,Princeton University, Princeton, New Jersey08540
Fig. 3. Three crustalblocksboundedby a rise, trench,and faults are shownat three
and Department
o• Geology
and Geophysics,
Woods
Hole
Oceanographic
Institution
successive
timeintervals.
Note the motion
of the
fourcircular
markersplaced
on the ridge
1962
Woods
Massachusetts
02543
cres•atW.time
the solidHole,
segments
showthe motion
o,fthesecircles;the dottedsegments
show
JASON1:
MORGAN
the originalcoordinates
of thesemarkers.The strikeof a transform
fault is parallelto the
Morgan
montre
pour la
difference
of the velocities
of the twosides;the crestJ.
of the
ridgedrifts
with a velocity
that
•J
RISES, TRENCtIES,
The transformisfault
concept
is extended
a two
spherical
the average
of the
velocities oftothe
sides. surface.The earth's surfaceis considered to be made of a number of rigid crustal blocks. Itpremière
is assumed
each block
is
foisthat
la réalité
de la
boundedby rises (where new surfaceis formed), trenchesor young fold mountains(where
vector
average of the
of the two
géométrie
enparallel
petit
cercle
des
of the
be
to
the or
difference
surfaceis being
destroyed),
andvelocities
great faults,
andsides.
that there
isfault
no must
stretching,
folding,
dis- in
Note
thatwithin
the two
faults
extending
velocity
of the
sides.Thus,
allblock
the faults
tortion of any,
kind
a transform
given block.
On
a spherical
surface,
thetwo
motion
of one
failles
transformantes
et
propose
to theseby
two ablocks
must of
lie one
on small
into block
2 on the
left are not
parallel.
(over the mantle)
relative
to another
block
mayAll
thencommon
be described
rotation
north
of theblock.
trenchThis
(between
blocksrequires
1 and circles
concentric
the
pole
of relative
la notion
deabout
pôle
de
rotation
block relativefaults
to the
other
rotation
three
parameters,
two
to
locate
theetmo2) wouldrun eastto,westas the oneshown,and ron.
pole
GREAT FAULTS, AND CRUSTAL BLOCKS
1965 of relative rotation and one to specify the magnitude of the angular velocity. If two adde
blocs
rigides.
Ilfaults
délimite
des
faults
of the
trench (between
blocks
The
velocity
block
relative
to another
jacent blocksall
have
as south
common
boundaries
a number
of great
faults,
allofofone
these
must
lie
2latitude'
and 3) would
have
45ø strike
as shown.
An
will
along
their
boundary;
about
theapole
of relative
rotation.
Thevary
velocity
of
onecommon
block
relative
to this
plaques
rigides
• ,/// on
/ 'circlesof example
of
where
the
strike
of
transform
faults
commonboundary;
this velocity wo•uldhave a maximumat
Time I must vary along
Time their
2
Time
• //// the' other
60
60
60
changes
in this
manner
offrelative
the coast
of
the 'equator'
vanish
attrench,
aoccurs
pole
of
rotation.
Fig. 3. Threeand
crustalwould
blocksbounded
by a rise,
and faults
are shownat three
Mexico
at,
intersection
of markers
the Middle
Amersuccessive
timeintervals.
Note
thethe
motion
of the fourto
circular
placed
on the
ridge
The
motion
of Africa
relative
South
America
is a case for which enough data are availcres•at time 1: the solidsegments
showthe motiono,fthesecircles;the dottedsegments
show
-/, / /
60
ica trench,
the The
East
Pacific
rise,
and
thetoGulf
coordinates
oftest
thesemarkers.
strike
of a transform
fault
is
parallel
the
ablethe
tooriginal
critically
this
hypothesis.
The
many
offsets
on the mid-Atlantic ridge appear to
difference
of the velocities
of the twosides;the crestof the ridgedriftswith a velocitythat
k_'
is the average of of
the California.
velocities of the two sides.
be compatible
with a pole of relative rotation at 62øN (___5ø),36øW (___2ø).The velocity
We
go tochoice
a sphere.
theorem
of geompattern
predictednow
by this
of Apole
roughly
agreeswith the spreadingvelocitiesdetervector average of the velocities of the two sides. of the fault must be parallel to the differencein
etry
states
that,
a
block
on
a
sphere
can
be
moved
mined
magnetic
anomalies.
motion
ofall the
Pacific block relative to North America
Note thatfrom
the two transform
faults extending
velocityThe
of the two
sides.Thus,
the faults
./
/.
30
these two blocksby
must
on small
block 2 on theto
leftany
are not
parallel.
All common to
other
conceivable
orientation
a liesingle
isinto
also
examined.
The
strike
of faults
from the
Gulf
of California to Alaska and the angles
faultsnorth of the trench (betweenblocks1 and circlesconcentricabout the pole of relativemo-
30
ron.
b
':
ß
o
0
•0
a properly chosen
axis.both
We imply a pole of relative rotation at
2) wouldrun east
to,rotation
westearthquake
as the oneabout
shown,andmechanism
inferred
from
solutions
all faults south of the
trench
(between
blocks
velocity ofthat
one block
relative
to another
use
thisastheorem
toThespreading
prove
relative
53øN
ofthe
the
Pacific-Antarctic
ridge showsthe best agree2 and 3) (_3ø),
would have a53øW
45ø strike (___5ø).
shown.An The
will vary
along their common
boundary;
this
example of where themotion
strike of transform
faultsrigid blocks on a sphere may
of two
ment
with this
hypothesis.
The Antarctic block is found to be moving relative to the Pacific
changesin this manner occursoff the coast of
be
described
by
an
velocity
Mexico at,about
the intersection
the Middle
block
a ofpole
at Amer71øS angular
(___2ø),
118øE vector
(_5 ø)bywith a maximum spreading rate of 5.7
•0
ica trench, the Eastusing
Pacificrise,
and the
Gulf
three
parameters,
two to specifythe loca(___0.2)
of California. cm/yr. An estimateof the motion of the Antarctic block relative to Africa is made
tion
the
one for the magnitude of
now go to a sphere.
Aof
theorem
geom-and
byWeassuming
closure
ofofpole
the
Africa-America-Pacific-Antarctica-Africa
circuit and summingthe
etry statesthat,a block
on a angular
spherecanbevelocity.
moved
the
Consider
the
left
block
in
three
angular
velocity
for the casesabove.
to any other
conceivable
orientationbyvectors
a single
0
•
Figure 4 to be stationaryand the right block to
Fig. 4. On a sphere, the motion of block 2
be moving as shown.Fault lines of great disrelative to block I must be a rotation about some
component
of pole.isAllthe
boundary
trench
at between
whichIcrustal
faults
on the type
boundary
and 2
velocity perpendicularto their strike; the strike must be small circlesconcentricabout the pole A.
rotation
about a properly chosen axis. We
30
use0 this theorem to prove that the relative
motion of two rigid blocks on a sphere may
(a)
(b)
be describedby an angular
velocity vector
bywhere there is no
placement
occur
INTRODUCTION
Fig. 8. Great circlesperpendicularto the strike of offsetsof the mid-Atlantic ridgeusing
are three parameters,two to specifythe locashownin (a). With one exception,all of these lines passwithin the circle centeredat 58øN,
tion of the pole and one for the magnitude of
36øW. Great circlesperpendicularto the strike determinedby earthquakemechanismsolutions are shown in (b).
the angular velocity. Considerthe left block in
30
60
30
0
30
60
......
30
A geometricalframeworkwith which to de-
surfaceis beingdestroyed;that is, the distance
Variation de la vitesse d’ouverture
en fonction de la distance au pôle de rotation
(Westphal et al., 2003)
Variation de la vitesse d’ouverture
en fonction de la distance au pôle de rotation
•Vitesse
maximale à 90°
du pôle de
rotation
(Westphal et al., 2003)
Variation de la vitesse d’ouverture
en fonction de la distance au pôle de rotation
•Vitesse
maximale à 90°
du pôle de
rotation
•Variation en
sinus de la
distance angulaire
(Westphal et al., 2003)
D. McKenzie et R.L. Parker
testent également de leur côté
les hypothèses de Wilson avec
des blocs rigides.
© 1967 Nature Publishing Group
cinématique à trois plaques
Les points triples
Les points triples
A
B
C
B
A
C
B
A
C
B
A
C
A
V B/A
C
B
A
B
V B/A
V C/B
C
A
B
V B/A
V C/B
V A/C
C
A
B
V B/A
V C/B
V A/C
C
Triangle des vitesses
Points triples et rigidité
JONCTION TRIPLE DE TROIS RIDES (RRR)
R AB
R AB
B
A
A
P
V AC
V BA
B
P
V CB
C
C
R CA
R BC
REPRESENTATION CARTOGRAPHIQUE
•La
R BC
R CA
ESPACE DES VITESSES
rigidité impose que vAC + vBA + vCB = 0
•Or v = ω ∧ r
•Donc ωAC + ωBA + ωCB = 0
78
T. Atwater: http://www.geol.ucsb.edu/Outreach/Download/Freeware-FR.html
T. Atwater: http://www.geol.ucsb.edu/Outreach/Download/Freeware-FR.html
cinématique à n plaques
Les modèles globaux
•'OURNALOF GEOPHYSICAL
RESEARCH
X. Le Pichon propose le
premier modèle global quantifié
en vitesses et directions de
mouvement
VOL. 73. NO. 12. JuN•- 15. 1968
Sea-FloorSpreadingand ContinentalDrift
XAvI•,•
L•, PICHON 2
Lamont Geological Observatory, Columbia University
Palisades, New York 10962
A geometricalmodel of the surfaceof the earth is obtained in terms of rigid blocks in
relative motion with respectto each other. With this model a simplifiedbut completeand
consistentpicture of the global pattern of surfacemotion is given on the basisof data on
sea-floorspreading.In particular,the vectorsof differentialmovementin the 'compressire'
belts are computed.An attemptis madeto usethis modelto obtain a reconstruction
of the
historyof spreadingduringthe Cenozoicera. This history of spreadingfollowscloselyone
previouslyadvocatedto explainthe distributionof sedimentsin the oceans.
I.
INTRODUCTION
ized belts of deformation. Recent studies of the
and of the distribution
of sediments in the
o
o
o
o
o •
o
o
,..c:•
o,..Q
o
o•
of the African or South American continents.
Motion of the African relative to the South
3675
paper we try to carry this attempt further and
to test whether the more uniformly distributed
data on sea-floorspreadingnow available are
DRIFT
this
CONTINENTAL
--
ments of ocean floor and continents. In
AND
oceans [Ewing and Ewing, 1964] did not reveal widespreadindicationsof compressionor
distortionof large oceanicblocks.Consequently,
the displacementsinferred in the spreadingfloor hypothesis of Hess [1962] and Dietz
[1961] shouldnot result in large-scaledeformation of the moving blocks.Morgan [1968] has
investigatedthe important implicationsof these
observationson the geometry of the displace-
SPREADING
physiographyof the oceanfloor [Heezen,1962]
SEA-FLOOR
It haslong beenrecognizedthat if continents
are beingdisplacedon the surfaceof the earth,
these displacementsshould not in general involve large-scaledistortions,exceptalonglocal-
Let us assumethat largeblocksof the earth's
surface undergo displacementsand that the
only modificationsof the blocks occur along
some or all of their boundaries,that is, the
crests of the mid-oceanridges, where crustal
material may be added, and their associated
transform faults, and the active trenchesand
regionsof active folding or thrusting, where
crustalmaterial may be lost or shortened.Then
the relative displacementof any block with
respectto another is a rotation on the spherical surface of the earth. For example, if the
Atlantic Ocean is opening along the mid-Atlantic ridge, the movement should occur in
such a way as not to deform or distort the
large bodiesof horizontallystratifiedsediments
lying in its basinsand at the continentalmargins.It shouldnot involvelarge-scaledistortion
American block (one block including the con-
3'OURNAL
OF GEOPHYSICAL
RESEARCH
Isacks, Oliver et Sykes montrent
que les observations des
sismologues confortent le
modèle de la tectonique des
plaques
VOL. 75, NO. 18, SEPTEMBER
15, 1968
Seismologyand the New Global Tectonics
BRYAN ISACKS AND JACK OLIVER
Lamont GeologicalObservatory,Columbia University, Palisades,New York 10964
LYNN R. SYKES u
Earth Sciences Laboratories, ESSA
Lamont GeologicalObservatory,Columbia University,Palisades,New York 10964
A comprehensive
study of the observationsof seismologyprovideswidely basedstrong
support for the new global tectonicswhich is founded on the hypthesesof continental drift,
sea-floorspreading,transform faults, and underthrustingof the lithosphereat island arcs.
Althoughfurther developments
will be requiredto explaincertainpart of the seismological
data, at presentwithin the entire field of seismologythere appearto be no seriousobstacles
to the new tectonics.Seismicphenomenaare generallyexplainedas the result of interactions
and other processes
at or near the edgesof a few large mobileplatesof lithospherethat spread
i.'
SEISMOLOGY AND NEW GLOBAL TECTONICS
o
N
W
apart at the ocean ridgeswhere new surficial materials arise, slide past one another along the
large strike-slip faults, and convergeat the island arcs and arc-like structureswhere surficial
AND NEW GLOBAL
materials descend.Study of world seismicity showsthatSEISMOLOGY
most earthquakes
areTECTONICS
confinedto narrow continuousbelts that bound large stable areas.In the zonesof divergenceand strike-slip
motion, the activity is moderate and shallow and consistent with the transform fault hypothesis; in the zones of convergence,activity is normally at shallow depths and includes
intermediate and deep shocksthat grosslydefine the present configuration of the down-going
slabs of lithosphere.Seismicdata on focal mechanismsgive the relative direction of motion of
adjoining plates of lithosphere throughout the active belts. The focal mechanismsof about a
5857
ITHOSPHER
5861
hundred widely distributed shocks give relative motions
that
agree
with
A
$
T
H
E remarkably
N
0
S
P well
H E
Le Pichon's simplified model in which relative motions of six large, rigid blocks of
lithosphere covering the entire earth were determined
from
M
F'
S magnetic
0
S and
P topographic
H
E
R data
E
associatedwith the zonesof divergence.In the
zones
of
convergence
the
seismic
data
provide
Fig. 1. Block diagramillustratingschematically
the configurations
and rolesof the lithosphere,asthenosphere,
and mesosphere
in a versionof the new globaltectonicsin whichthe
the only geophysicalinformation on such movements.
lithosphere,
a layer of strength,plays a key role. Arrowson lithosphereindicaterelative
movements
oœadjoining
blocks.
Arrowsin asthenosphere
possible
compensating
Two principal types of mechanismsare found
for
shallow
earthquakes
inrepresent
island
arcs:
The flow
in response
to downward
movementof segments
of lithosphere.
Onearc-to-arctransformfault
extremely active zone of seismicity under theappears
inner
margin
of thefacing
ocean
is characterized
at left
betweenoppositely
zonestrench
of convergence
(islandarcs),two ridge-toridgetransformfaults alongoceanridgeat center,simplearc structureat right.
by a predominance of thrust faulting, which is interpreted as the relative motion of two
converging plates of lithosphere; a less active
zonethe
in lithosphere
the trench
andsettled,
on beneath
the outer
an islandwall
arc or aof
continental
strike-slip faults)
is disconmargin
that
is
currently
active.
At
an
tinuous;
elsewhere
it
is
continuous.
Thus,
the
the trench is characterizedby normal faulting and is thought to be a surficial manifestation of inactive
lithosphere is composedof relatively thin margin the lithospherewould be unbrokenor
the abrupt bending of the down-goingslab
of
lithosphere.
structures
along
theshowstwo
The left side of
the diagram
blocks,
some
of enormoussize,Graben-like
which in the healed.
island-arc
structures,
back
to
back,
first approximation
be considered
infinitely
outer walls of trenchesmay provide a mechanism
formay
including
and
transportingsedimentstowith the
5874
SEISMOLOGY AND NEW GLOBAL TECTONICS
ISACKS, OLIVER, AND SYKES
5871
0
•
,,/
5878
ISACKS, OLIVER,
TRENCH•AXIS
ti
th
Volcanic
Coralline
Arc
Arc
0
Water
ß•:.:::• ,
•
1.1..%1
:. ß.
5
'":'
"::.....:
h
]0
I
I :(:'":':':•:':
::""
Depth,
km
0
Dislance
-400
from
KM
+ 200
Niumate, Tonga,
-200
200
o/
km
0
Fig. 14a. Length l is a measureof the amount
of underthrustingduring the most recent period of
sea-floor spreading.
+ 200
o
in
-o
si
6oo
Iy /
/
/ ,
mechanism that could indicate extension instead of compressionparallel to the dip of the zone.
/
400
55O
/
'
6
600
km
6OO
,
- •50
•
- •00
2oo
I,• •
Fig. 11. Vertical sectionsperpendicularto the strike of an island arc showingschematically Fig. 14b. Lithosphere is deformed along its
typical orientations of double-couplefocal mechanisms.The horizontal scale is the same as lower edge as it encountersa more resistant layer
the vertical scale. The axis of compressionis represented by a converging pair of arrows; the (the mesosphere).
axis of tension is representedby a diverging pair; the null axis is perpendicular to the
section. In the circular blowups, the senseof motion is shown for both of the two possible
slip planes.The featuresshownin the main part of the figure are based on resultsfrom the
Tonga arc and the arcs of the North Pacific. The insert showsthe orientation of a focal
200
E
- 250
km
Alternatively,Isacks and Sykes [1968] show would, therefore, be expected, especiallynear
that, if it is assumedthat the slip planesform parts of the zone with complex structure.
The important feature of the interpretation
at angleswith respectto the axis of maximum
compressive
stressthat are not significantly presented here is that the deep earthquake
NEWinterpreGLOBAL
TECTONICS
mechanisms reflect 5869
stresses in the relativelyFig. 14c. Length of seismiczone is the product
differentfrom 45ø, SEISMOLOGY
then a veryAND
simple
of rate of underthrusting and time constant for
tation can be made on the basis of the model
strong slab of lithosphere and do not directly
assimilation of slab by upper mantle.
550km
300
250
200
150
I00
5O
0
50
I00 km
the shearing motions parallel to
of Figure 1. In this interpretationthe axis of accommodate
LOCAL
of the slab as is implied by the
maximumcompressive
stressis parallel to the the motion
TENSION
simple
fault-zone
model. The shearingdeformadip of the seismiczone (i.e.,
parallel
to
the
CRUST
S
S
CRUS
tions parallel to the motion of the slab are prepresumed
motion
of
the
slab
in
the
mantle),
LITHOand the axis of least compressivestressis persumablyaccommodated
by flow or creepin the
SPHERE
P HERE
$
ductile parts of the mantle.
pendicularto the zoneor parallelto the thin 0 adjoining
dimension of the slab.
Do the stressesin the slab vary with depth?
The tendencyfor the compressire
axesto be In particular, the axis of least compressive
more stable than the other two axes can be stress,the T axis, may be parallel to the dip ////////////
ASTHENOSPHERE
the zone if the material at greater depths
,ß , of
•$THENOSPHERE
interpreted
to indicatethat the difference
between the intermediate and the least principal were sinking and pulling shallowerparts of theFig. 14d. A piece (or pieces) of the lithosphere
becomes detached either by gravitational sinking
stressesis less than the differencebetween the
slab [Elsasset,1967]. In the Tonga, Aleutian,
Fig. 7a.
or by forcesin the asthenosphere.
greatestandthe intermediate
principalstresses. and Japanesearcs,the focal mechanismsindicate
that
is under
compression
parallel to its
In general,
the stress
be quite
50 km :500
250 state
200 may150
I00 vari50
0 the slab
50
IOOkm
depths greater than about 75 to 100
able owingto contortions
of the slab,as sug- dip at allLOCAL
Figure 14 shows four possible configurations of
TENSION
arcs, therefore, any extension
in underthrust plate of lithosphere in island arcs.
gestedby Figure9. Possiblylargevariability km. In these
an
be shallower than 75 to 100 kin.
in the orientations ofS the
mechanisms
Solid areas indicates lithosphere; white area,
• deep
CRUST
S S the slab must
CRUST,
ß
Fig. 9. Vertical section oriented perpendicularto the Tonga arc. Circles represent earthquakesprojected from within 0 to 150 km north of the section; trianglescorrespondto events
projectedfrom within 0 to 150 km south of the section.All shocksoccurredduring 1965while
the Lamont network .of stations in Tonga and Fiji was in operation. Locations are based on
data from these stations and from more distant stations.[No microearthquakesfrom a sample
of ?50 events originated from within the hatched region near the station at [Niumate, Tonga
(i.e., for $-P times less than 6.5 sec). A vertical exaggerationof about 13:1 was used for the
insert showingthe topography [after Raitt et al., 1955]; the horizontal and vertical scalesare
equal in the crosssection depicting earthquake locations. Lower insert shows enlargement of
southern half of section for depths between 500 and 625 km. [Note small thickness (less than
•20 km) of seismiczone for wide range of depths.
ß
ß
ß
ß
ß
ß
ß
ß
asthenosphere;hatched area, mesosphere.
L I T H 0 S P HERE
Leur article est plein d’idées
nouvelles très à la mode
aujourd’hui sur la dynamique de
la subduction
sp
an
n
it
th
o
th
ti
an
LOCAL
TENSION
(•
$
P HE
Isacks et al., 1968
"••'""
ASTHENOSPHERE
ASTHENOSPHERE
15(
km
Fig. 7b.
Figure 7 showsvertical sectionsthroughan islandarc indicatinghypotheticalstructuresand
other features.Both sectionsshow down-goingslab of lithosphere,seismiczone near surface
of slaband in adjacentcrust,tensionalfeaturesbeneathoceandeepwhereslabbendsabruptly
and surfaceis free. (In both sections,S indicatesseismicactivity.) (a) A gap in mantle portion
of lithosphere beneath island arc and circulation in mantle associatedwith crustal material of
the slab and with adjoining mantle [Holmes, 1965]. (b) The overridinglithospherein contact
with the down-goingslab and bent upward as a result of overthrusting.The relation of the
bending to the volcanoesfollows Gunn [ 1947]. No vertical exaggeration.
d
d
th
an
T
e
m
s
m
c
n
th
d
p
e
d
an
th
c
w
p
tr
a
is
ep
tw
q
am
[H
ea
Z
pi
is
m
co
flysch accumulations thickening toward the
trench, and blueschistm61anges.As the oceanic
Jou•N^L
os G•O?H¾SICaL
RESE^I•CI•
plate descendsbeneath
the continental
rise to
depthsgreaterthan 100 km, submarinevolcanics
are erupted behind the volcanicfront (Figure
untain belt is schematically illustrated in
gure 10. These sequencesare based on an
alysis [Bird and Dewey, 1970] of northstern parts of the Appalachiansfor Ordovin times and on the general structure of the
sozoic Cordilleran system along the western
CONTINENTAL
SHELF
•
CONTINENTAL
RISE
.......... •--'"'---...•
-
10A). As the heat flux generatedby the rise of
volcanic
front
•
pillow
lavas
turbld•tes-] sealevelI
argllle
scaghose
volcan,cs
/
i ..... :•L
•
If
'•/.
/t •x •
• I • i / •/•
VOL. 75, NO. 14, MAY 10, 1970
argillites
••nat•
Mountain
Belts and the New
Jo•
•
Global Tectonics
F. D•w•v
m
•/
•x/
x•/•
/•
•.
_•
/
/,
fl:•_,
• .•
flysch
IIIIIIIil111111111
•xl x -- I•/ /•/x.
•
•
,-
•/....
i,,.1,•..
Department o• Geology, University of Cambridge
Cambridge, England
• flysch s•a
I'
• ""/
.,
\-.
-.•.-
.....
• • / C- / ' • _xx.-
-•.-/-',%%/•-.... x•. •.x
x•/•/
......
/ '
x%./,<.•-
. ....
JOHN M.
-- '/ •J'I andg•nod•orites
l i'
I I'1•'-•,•!•1•I I I I I I I
• Jmetamorph•c
front
Department of Geological Sciences
State University of New York at Albany
Albany, New York 12203
thermal
doming/granites
w
I
grawty
slide /
wlldflysch
- ••j_'•__•__•'
•.7•.-/
•--•..•"•--••
BIRD
I
sealevel
2642
DEWEY AND BIRD
Analysis
of
the
sedimentary,
volcanic,
structural,
and metamorphicchronologyin mountain
-- x
x •'
basalticand andesitic
volcanoes
belts, and considerationof the implicationsof the new global tectonics (plate tectonics),
spread,ng
• K•O•.
..... e.s
ing.r'•nlt•s
//
I undertucking
offlysch
rafts
I•
molasse
"•_•.=•'
'..•.--.----•2
• ½•
.
J
. ,
orogcn on conbncntal
dcformabon of sediments at
stronglyindicate
of plate evolution.It is proposed
marginthat mountainbelts are a consequence
•
flysch
wedge
/footofinner
wall
oftrench
lubtes
/ • •-•.
I develop/by the
z deformationand metamorphism
a,nd
cherts continental
that mountainxbelts
of the shelf
sedimentaryand
volcanic assemblagesof Atlantic-type continental margins. These assemblagesresult from
the events associated
with the rupture of continents and the expansionof oceans by
• bluesch•st
•
metamorp
ß- '/ ' x','x- , J' a••c
Ik"--T•'gh urn•__n.a tholeiibk:
'!
lithosphereplate generation at oceanic ridges. The earliest assemblagesthus developed
•
andes•tesand rhyohtes
are volcanic rocks and coarseclastic sedimentsdepositedin fault-boundedtroughs on a
distendingand segmentingcontinentalcrust,subsequentlysplit apart and carried away from
-'•...
• '- ' .•
•.•>-• •_
•_
•, ,', •..
andto•,te,
the ridge on essentiallyaseismiccontinentalmargins.As the continentalmarginsmove away
thrust
wedges
of
crust
,
oceanic crust shelf delta
from the ridge,Bnonvolcanic continental
and rise assemblagesof oceanic
orthoquartzite-carbonate, and lutite (shelf), and lutite, slump deposits, and turbidites
(rise)
This
'
crustaccumulate.
.hthosphere
plate
kind of continental margin is transformedinto an orogenicbelt in one of two ways. If a
trench develops near, or at, the continenal margin to consumelithosphere from the oceanic
Fig. 10. Schematicsequenceof sectionsillustrating a model for the evolution of a cordillerana mountain belt (cordilleran type) grows by dominantly thermal mechanismsrelated
type mountainbelt developedby the underthrusting
of a continentby an side,
oceanicplate.
to the rise of calc-alkaline and basaltic magmas. Cordilleran-type
mountain belts are characw•de zone of shallow earthquakes
terized by paired metamorphic belts (blueschiston the oceanic side and high temperature
on the continental side) and divergent thrusting and synorogenicsediment transport from
La nouvelle théorie est très vite the high-temperaturevolcanic axis. If the continental margin collideswith an island arc, or
with another continent, a collision-type mountain belt develops by dominantly mechanical
•l•r i i .•J•.'
•
I I I
•,
\
ea level
.
h•u
ß • cent oran•te plutons
mise à profit par les géologues. processes.Where a continent/island are collision occurs, the resulting mountains will be
small (e.g., the Tertiary fold belt of northern New Guinea), and a new trench will develop
Ils tiennent enfin le cadre on
the oceanic side of the arc. Where a continent/continent collision occurs,the mountains
conceptuel qui leur manquait et will be large (e.g., the Himalayas), and the single trench zone of plate consumptionis replaced by a wide zone of deformation. Collision-type mountain belts do not have paired
qui est venu des océans
metamorphic belts; they are characterizedby a single dominant direction of thrusting and
synorogenic Fig.
sediment
transport, sequence
away from
the siteillustrating
of the trench
over theofunderthrust
13. Schematic
of sections
the collision
two continents.
plate. Stratigraphic sequencesof mountain belts (geosynclinal sequences) match those as-
Le modèle NUVEL-1 (DeMets et al., 1990)
Le modèle NUVEL-1 (DeMets et al., 1990)
NB: tous les mouvements sont par rapport à l’Eurasie fixe
Le renforcement du modèle
par son efficacité prédictive
Le modèle prévoit que la croûte océanique est d’autant plus vieille qu’on s’éloigne
de la dorsale. Les âges des sédiments en contact avec le plancher océanique
(programme de forage sous-marins J.O.I.D.E.S.) confirment cette prédiction et les
vitesses prévues par le modèle de la tectonique des plaques.
Le modèle prévoit des vitesses de déplacements des plaques (d’après le
paléomagnétisme et les alignements de volcans intraplaques). Avec l’utilisation
des techniques de positionnement par satellites (G.P.S.), à la fin du XXe siècle,
les mouvements des plaques deviennent directement observables et leurs
vitesses sont confirmées.
carte des
anomalies
magnétiques
sites de forage
du programme
DSDP
Dès le début les idées sur
l’expansion des fonds
océaniques sont testées par des
forages. C’est le programme
DSDP
Extrait d’un cours de Cornell Univ.
anomalie magnétique
Forages
profonds
dans
l’Atlantique
(DSDP)
âge paléontologique
âge du fond de l’océan donné par les
fossiles trouvés dans les sédiments
déposés directement sur la croûte
océanique
en fonction
de l’âge des anomalies
Extrait d’un cours
de Cornell Univ.
Age (Ma) des anomalies en supposant une vitesse constante
L’âge du fonds des océans
Müller et al. 1997
demi-vitesses d’ouverture
Volcanisme de dorsale
Pillow-lavas
(laves en coussins)
Les campagnes de plongée
montrent la réalité du
volcanisme des dorsales, puis
des déformations dans les fosses
de subduction
Le système GPS
Satellite
Antenne
Récepteur
Beaucoup plus récemment les mesures des déformations et des
déplacements de la croûte sont grâce aux méthodes géodésiques (GPS...)
Sites de mesures géodésiques
 Réseaux GPS régionaux (une cinquantaine, près de 3000 sites)
 Réseau GPS global (>150)
 Réseau VLBI global [Ma & Ryan, 1998] (70 sites)
 Réseau DORIS [Crétaux et al., 1998]
(in Kreemer, 2002)
On observe une coincidence presque parfaite entre les vitesses prédites par la cinématique des
plaques «classique» et les vitesses mesurées directement par géodésie.
Vitesses mesurées vs. prédites
SLR+VLBI
(Robbins et al., 1993)
DORIS
(Soudarin, 1994)
Vitesses mesurées par géodésie comparées aux vitesses
prédites par le modèle « géologique » NUVEL-1
Dans les zones de limites de
plaques on peut mesurer la
déformation des continents
Dans les zones de limites de
plaques on peut mesurer la
déformation des continents
Frontières de plaques
C. Kreemer
Frontières de plaques
C. Kreemer
Les limites de plaques sont des zones de largeur finie,
au sein desquelles se produisent des déformations.
Prise en compte de la déformation interne des
frontières de plaques (Kreemer et al., 2002)
Le plus bel exemple est sans
conteste la déformation de la
lithosphère asiatique sous la
poussée de l’Inde
Tapponnier et Molnar, 1977
cinématique absolue
Les points chauds, passage de la
cinématique à la dynamique
Le modèle NUVEL-1 (DeMets et al., 1990)
NB: tous les mouvements sont par rapport à l’Eurasie fixe
Le modèle NUVEL-1 (DeMets et al., 1990)
cinématique relative
NB: tous les mouvements sont par rapport à l’Eurasie fixe
Le modèle NUVEL-1 (DeMets et al., 1990)
cinématique relative
Quid des mouvements /manteau profond ?
NB: tous les mouvements sont par rapport à l’Eurasie fixe
Wilson, 1963
Les premières idées sur les relations
entre convection, tectonique des
plaques et ce qu’on appelle
aujourd’hui les points chauds ont été
proposées très tôt
Les points chauds et la cinématique absolue
Morgan, 1972
Les points chauds et la cinématique absolue
• Les points
chauds semblent
~fixes les uns par
rapport aux
autres
chaîne de volcans
sous marins des Empereurs
Hawaï
• On détermine le
mouvement de la
plaque Pacifique
uniquement
Les points chauds et la cinématique absolue
80 Ma
• Les points
chauds semblent
~fixes les uns par
rapport aux
autres
chaîne de volcans
sous marins des Empereurs
43 Ma
Hawaï
Volcans actifs
• On détermine le
mouvement de la
plaque Pacifique
uniquement
Les points chauds et la cinématique absolue
80 Ma
• Les points
chauds semblent
~fixes les uns par
rapport aux
autres
chaîne de volcans
sous marins des Empereurs
43 Ma
Hawaï
l’âge augmente régulièrement
Volcans actifs
• On détermine le
mouvement de la
plaque Pacifique
uniquement
Les points chauds et la cinématique absolue
Michael Gurnis, Caltech
Michael Gurnis, Caltech