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8/14/2019 Boudier Nicolas Ofi Types Tema 9
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84 Earth and Planetat T Science Letters, 76 (1985 /'86 ) 84 92
Elsevier Science Publishers B.V., Amst erda m - Printed in The Nether lands
[5]
H a r z b u r g i t e a n d l h e r z o li te s u b t y p e s i n o p h i o l it i c a n d o c e a n i c e n v i r o n m e n t sF . B o u d i e r a n d A . N i c o l a s
Laboratoire de Tectonophysique, 2 rue Itoussini&e, 44072 Nantes (France)
Resubmitted May 9, 1985; revised version accepted September 18, 1985
In most ophiolites the uhramafic section is harzburgitic. It is rarely composed of residual lherzolites, except for the
local occ urrence of i mpregnation lherzolites within harzburgitic massifs. Several cha racters ( environment al formations,
thickness and composition of the crust, geometry of the asthenospheric flow, serpentinization) validate the distinction
between an harzburgite and a Iherzolite ophiolitic subtype. The harzburgite subtype can be derived from any oceanic
spreading center, provided the rate is larger than 1 era/yr. The lherzolite subtype would correspond to situations
(vicinity of transform fault, very slow spreading rates) where the lithospheric front penetrates at 20-30 km into themantle below the spreading center. These lherzolitic massifs are best explained as being derived from slow-spreading
rifts. Finally the particular asthenospheric flow discovered in these massifs is discussed.
1 . I n t r o d u c t i o n
N i c o l a s a n d J a c k s o n [1 ] h a v e s h o w n t h a t i n t h e
o p h i o l i t e c o m p l e x e s k n o w n a t t h a t t im e , t he m a n -
t l e s e c t i o n w a s c o m p o s e d o f h a r z b u r g i t e s c o m p r i s -
i n g a l s o d u n i t e a n d o r t h o p y r o x e n i t e l a y e r s . T h e y
i n t e r p r e t e d t h is s u i te a s e x p o s e d s u b o c e a n i c u p p e r -
m o s t m a n t l e . I n c o n t r a s t t h e y r e g a r d e d t h e l h e r z o -l i t e m a s s i f s a s r e p r e s e n t a t i v e o f l e s s d e p l e t e d o r
u n d e p l e t e d s u b c o n t i n e n t a l u p p e r m a n t l e or d e e p e r
s u b o c e a n i c m a n t l e . H o w e v e r , m o r e r e c e n t s t u d i e s
h a v e s h o w n t h a t t h e a s s o c i a t i o n o f p l a g i o c l a s e
l h e r z o l i t e s w i t h o p h i o l i t i c s u i t e s w a s n o t s o u n c o m -
m o n .
N i c o l a s a n d D u p u y [ 2] h a v e d i s c us s e d t h e p r o b -
l e m o f t h e o r i g i n o f p l a g i o c l a s e l h e r z o l i t e s f r o m
o c e a n i c o c c u r r e n c e s a n d f r o m o p h i o l i t e s a s w e ll a s
o t h e r u l t r a m a f i c m a s s i fs , u s in g m i c r o s t r u c t u r a l a n d
g e o c h e m i c a l c r i t e r i a . T h e i r c o n c l u s i o n s w e r e t h a t
i n t e r m s o f m a n t l e m e l t i n g , s u c h f a c i e s r e p r e s e n t
e i t h e r a s l i g h t l y d ep l e t ed m an t l e ( s ee a l s o [ 3 ,4 ] ) , o r
a m o r e d e p l e t e d m a n t l e ( h a r z b u r g i t e s a n d d u n i t e s )
a f f e c t e d b y a s e c o n d a r y m a g m a t i c i m p r e g n a t i o n
i n t r o d u c i n g c l i n o p y r o x e n e a n d p l a g io c l a se .
T h e m a s s i fs b e l o n g i n g t o th e g r o u p w i t h m o d -
e r a t e l y d e p l e t e d m a n t l e r o c k s a r e m a i n l y c o m -
p o s e d o f h o m o g e n e o u s p l a g i o c l a s e l h e r z ol i te i n
w h i c h l o c a l e v i d e n c e o f p a r t i a l m e l t i n g i s s t i l l
p r e s e r v e d ; t h is i s t h e c a s e o f t h e L a n z o m a s s i f in
t h e W e s t e r n A l p s [ 5,6 ] a n d o f t h e T r i n i t y m a s s i f i n
0012 -82 1X/ 85/$ 03. 30 ~ 1985 Elsevier Science Publishe rs B.V.
C a l i f o r n i a [ 7]. I n t h e m a s s i f s w h e r e t h e p l a g i o c l a s e
l h e r z o l i t e s a r e a s c r i b e d t o a s e c o n d a r y i m p r e g -
n a t i o n , t h e s e f a c i e s a r e o n l y l o c a l l y r e p r e s e n t e d
w i t h i n t he d o m i n a n t h a r z b u r g i t e s a n d d u n i te s . F o r
i n s t a n c e i n th e B e l e p - P o u m m a s s i f o f N e w C a l e -
d o n i a [ 8] a n d i n t h e C a n y o n M o u n t a i n m a s s i f o f
O r eg o n [ 9 ,1 0 ] t h ey a r e r e s t r i c t ed t o s h ea r zo n es ; i n
t h e B a y o f I s l a n d s o f N e w f o u n d l a n d [1 1] t h e y a r el o c a t e d a t t h e b a s e o f t h e m a s s i f i n a t h r u s t z o n e
a b o v e a m e t a m o r p h i c a u re o l e ; i n P o l a r U r a l [ 12 ]
t h e l h e r z o l i t e s a r e s c a t t e r e d i n i r r e g u l a r p a t c h e s .
I n t h i s p a p e r w e w i l l n o t d i s c u s s f u r t h e r t h e
p r o b l e m o f l h e r z o l i t i c i m p r e g n a t i o n ( s e e [ 2 ] ) , a n d
w i ll c o m p a r e f r o m a g e n e ra l a n d s t r u c tu r a l p o i n t
o f v i e w t h e l h e r z o l i t e a n d h a r z b u r g i t e r e s i d u a l
t y p e s i n t h e i r a s s o c i a t i o n w i t h o p h i o l i t e s . W e w i l l
s h o w t h a t t h e lherzolite ophiolite subtyp e ( L O S )
a n d t h e harzburgite ophiolite subtyp e ( H O S ) h a v e
d i s t i n c t c h a r a c t e r s a n d w i l l p r o p o s e t o e x p l a i n
t h e m i n t e r m s o f d i f f e r e n t l i th o s p h e r i c c o o l i n g s .
2 . D i s t i n c t i v e c h a r a c t e r s o f th e h a r z b u r g i t e ( H O S )
a n d I h e r z o l i t e ( L O S ) s u b t y p e s
T h e s p e c if ic c h a r a c t e r s o f e a c h t y p e h a v e b e e n
d e f i n e d b y s t u d i e s i n t h e f o l l o w i n g m a s s i f s f o r t h e
H O S : O m a n [ 13 ,1 4] , B a y o f I s l a n d s in N e w f o u n d -
l a n d [ 1 1 , 1 5 - 1 7 ], Z a m b a l e s i n th e P h i l i p p i n e s
[ 1 8 ,1 9 ] , T r o o d o s i n C y p r u s [ 1 9 ] , A n t a l y a an d
P o z a n t i - K e r s a n t i i n T u r k e y [2 0,2 1], M a s s i f d u S u d
8/14/2019 Boudier Nicolas Ofi Types Tema 9
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T A B L E 1
D i s t i n c t iv e c h a r a c t e r s o f t h e h a r z b u r g i t e o p h i o l i te s u b t y p e ( H O S ) a n d t h e l h e rz o l i te o p h i o l i t e s u b t y p e ( L O S )
85
H a r z b u r g i t e o p h i o l i t e s u b t y p e ( H O S ) L h e r z o l i te o p h i o l i t e s u b t y p e ( L O S )
M a s s i f s c o n s i d e r e d ( 1 ) S e m a i l ( O m a n ) , ( 2 ) B a y o f I s l a n d s ( N e w - ( 9 ) T r i n i t y ( C a l i f o r n ia ) , ( 1 0 ) P i e m o n t - L i g u r i a
f o u n d l a n d ) , ( 3 ) Z a m b a l e s ( P h i l ip p i n e s ) , ( 4 ) ( w e s t e r n I t al y ) a n d M o n t e M a g g i o r e ( C o r s i c a ),
T r o o d o s ( C y p r u s ) , ( 5 ) A n t a l y a ( T u r k e y ) , ( 6 ) ( 1 1 ) L a n z o ( w e s t e r n I ta l y ), ( 1 2 ) O t h r i s ( G r e e c e ),
P o z a n t i - K a r s a n t i ( T u r k e y ) , ( 7) M a s s i f d u S u d ( 1 3 ) Z a b a r g a d ( E g y p t )
( N e w C a l e d o n i a ), ( 8 ) C a n y o n M o u n t a i n ( O r e g o n )
m a r i n e s e d i m e n t s a n d v o l c a n i c s : 1 , 2 , 3 . 4 , 5 , 6 , 7 ,
8
s o l e o f m e t a m o r p h i c o c e a n i c c r u s t : 1 , 2 , 5 , 6
S u r r o u n d i n g f o r m a t i o n s
C r u s t a l s e c t io n :
m a x i m u m t h ic k n e ss
c u m u l a t e g a b b r o s
n a t u r e o f i n t r u s i v e sn a t u r e o f b a s a l t d i r e c t ly
a s s o c i a t e d w i th t h e
o p h i o l i t e
M a n t l e s e c t i o n :
n a t u r e o f m a n t l e r o c k s
i n t e r n a l s t r u c t u r e s
H P p l a s t i c f l o w
n e o b l a s t s g r a i n s iz e
c h r o m i t e p o d s
d i a b a s e o c c u r r e n c e
s e r p e n t i n i z a t i o n
2 - 3 k m : 3 , 4 , 8
- 7 k m : 1 , 2 , 5 , 6
t h i c k a n d u s u a l l y c o n t i n u o u s : 1 . 2 , 3 , 5 , 6
dikes: 1, 2, 3, 4, 5, 8thole i i t i c : 1, 2, 4, 5, 6
h a r z b u r g i t e s a n d d u n i t e s :
a b u n d a n t d u n i t e s d o w n t o a b o u t 1 0 k m b e l o w
t h e t o p o f t h e m a n t l e s e c t i o n : 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8
f l a t fo l i a t i on : 1 , 2 , 5 , 6 , 7
l o c a l l y v e r t i c a l w i t h v e r t i c a l l i n e a t i o n : 3 , 4 , 5 . 8
l a rge ( - 4 mm ) : 1 , 2 . 3 , 4 , 7 , 8
pre se n t : 1 , 2 , 3 , 4~ 5 , 6 , 7 .8
u n c o m m o n : 1 , 2 , 3 , 4 , 7
l izardi te : 1, 2, 3, 4, 6, 7, 8
m a r i n e s e d i m e n t s a n d v o t c a n i c s : 9 , 1 0 . 11 , 1 2, 1 3
bre c c i a s : 9~ 10
m e t a m o r p h i c c o n t i n e n t a l c r u s t: 1 1, 1 2, 1 3
s o l e o f m e t a m o r p h i c c r u s t: 9
1 k m : 1 0
2 - 3 k m : 9 , 1 2
t h i n , l oc a l i z e d t o r e s t r i c t e d a re a : 9 , 10 , 12
a b u n d a n t d i k e s a n d s i l ls : 9, 1 0 , 1 2t ho l e i i t i c : 9 , 10 , 12
a l ka l i ne : 12~ 13
p l a g i o c l a s e I h e r z o l i t e s :
d u n i t e s a b u n d a n t w i t h i n ~< 2 k m b e l o w t h e t o p
o f t h e m a n t l e s e c t i o n : 9 , 1 1
u s u a l l y s t e e p f o l ia t i o n a n d s u b h o r i z o n t a l l i n e a-
t i on : 9 , 10 , 11 , 13
s m a l l ( - 0 . 5 m m ) : 9 , 1 1
a bse n t : 9 , 10 , 11 , 12 , 13
c o m m o n i n t o p s e c t i o n : 9 , 1 0, 1 1 , 1 3
l i z a r d i t e a n d a n t i g o r i t e : 9 , 1 0 , 1 1 , 1 2
o p h i c a l c i t e s : 1 0
o f N e w C a l e d o n i a [2 2,2 3] , C a n y o n M o u n t a i n
[9,1 0, 24 ], an d fo r L O S: T r i n i t y in t h e K l a m a t h
M o u n t a i n s [ 7 , 25 - 2 7 ] , P i e m o n t - L i g u r i a i n n o r t h e r n
I t a l i a ( [ 2 8 - 3 0 ] , a n d u n p u b l i s h e d d a t a ) , C o r s i c a
[31 ,32] , Lanzo in the wes tern Alps [33] , Othr i s in
G r eec e [3 ,4 ,3 4 ], Z a b a rg a d i n t h e R ed Sea [3 5 ,3 6 ].
T r a n s i t i o n a l o p h i o l i t e su i te s a r e r e p r e s e n t e d b y t h e
X i g a z e o p h i o l i t e i n T i b e t [ 3 7 - 3 9 ] i n w h i c h t h e
c r u s t a l s e c t io n a b o v e c l i n o p y r o x e n e - r i c h h a r z b u r -
g i t e s r e s e m b l e s v e r y m u c h t h a t d e s c r i b e d f o r t h e
L O S . T h e s e f i f t e e n m a s s i f s w e r e s e l e c t e d b e c a u s e
t h e y a r e c o m p a r a t i v e l y l it tl e d i s m e m b e r e d a n d t h e y
r e p r e s e n t t h o s e f o r w h i c h w e h a v e d i r e c t i n f o r m a -
t i o n o n m o s t p o i n t s c o n s i d e r e d i n T a b l e 1 . A l l t h e
d a t a i n t h i s t a b l e h a v e n o t t h e s a m e s t a n d a r d o f
g e n e r a l i t y a n d c o n f i d e n c e d u e t o f r a g m e n t a l i n f o r -
ma t i o n o n t h e co n s i d e red mas s i f s . F i g . 1 i l l u s t r a t e s
t y p i c a l s e c t io n s in t h e H O S a n d L O S .
2.1. Environmental formations
H O S a r e u s u a l l y c o v e r e d b y m a r i n e s e d i m e n t s ,
m a i n l y u m b e r s a n d c h e r t s . L O S c o r r e s p o n d t o
m o r e c o m p l e x e n v i r o n m e n t s . T h e e a s t e r n m a r g i n
o f t h e O t h r i s o p h i o l i t e i s i n c o n t a c t w i t h u n d e r -
s a t u r a t e d b a s a l t i c l a v a s a s s o c i a t e d w i t h f r a g m e n t s
o f o p h i o l i t e s , s h a l l o w - w a t e r m a r i n e s e d i m e n t s a n d
c o n t i n e n t a l m a r g i n f o r m a t i o n s [ 3 4 ] . T h e T r i n i t y
o p h i o l i t e is c o v e r e d b y a b r e c c i a c o n t a i n i n g o p h i o -
l i t e x en o l i t h s i n a ma t r i x o f s h a l l o w mar i n e s ed i -
m e n t s [ 25 ]. T h e P i e m o n t - L i g u r i a n l h e r z o li t e lo c a l ly
f o r m e d t h e s e a f l o o r o f t h e P i e m o n t e s e b a s i n
[ 4 0 , 4 1 ] ; t h i s i s r e c o r d e d i n l i m e s t o n e / s e r p e n t i n i t e
b r e c c i a s k n o w n a s o p h i c a l c it e s [4 2] . T h e Z a b a r g a d
p e r i d o t i t e i s i n p r i m a r y c o n t a c t w i t h m e t a m o r p h i c
r o c k s a n d i s c o v e r e d b y s h a l l o w - w a t e r m a r i n e s e d i -
ments [36 ,43] .
8/14/2019 Boudier Nicolas Ofi Types Tema 9
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86
} A S A L T S
D Y K E •
• . . , " .
" ' . .
G A B B R OS " . " " . . "
. . - , ,
) CUMU :_ ATES - .
T E C U M UL AT E - - -D 2b --e se---- ~ - - -
T I O N Z O N E ~ # . 7 ~ / 1
- R Z O L I T E S I I ] 1 ,]
/ i / k !I f i l l
I ,, m _ b _
Fig. 1. Typical sections (a) in the harzburgite (Sem ail ophiolitein O ma n). (b) in the lherzolite (Trinity in C alifornia) ophiolite
subtypes.
M a n y H O S c o m p l e x e s re s t o n a s o le o f m e t a -
m o r p h o s e d o c e a n i c c r u s t a l f o r m a t i o n s b e l o w a n
i n t r a o c e a n i c t h r u s t [ 44 ,4 5]. W i t h t h e e x c e p t i o n o f
t h e T r i n i t y co m p l ex [ 4 6 ,4 7 ] w h i ch h as s u ch a s o l e ,
a l l t h e o t h e r L O S m a s s i f s a r e in d i r e c t c o n t a c tw i t h, o r i n t h e v i c i n i ty o f m e t a m o r p h i c c o n t i n e n t a l
c r u s t .
2 . 2. C r u s t a l s e c t i o n
A s s e e n i n T a b l e 1 a n d F i g . 1 , t h e c r u s t a l
s e c t i o n o v e r l y i n g t h e u l t r a m a f i c s e c t i o n i s t h i c k e r
i n t h e H O S c a s e . I n O m a n , w h e r e t h i s s e c t i o n i s
t h e b e s t e x p o s e d a n d l e a s t d i s m e m b e r e d , t h e t h i c k -
n e s s r a n g e s f r o m 3 t o 7 . 5 k m i n a l o n g i t u d i n a l
c r o s s - s e c t i o n [1 4]. T h e i n t e r n a l c o m p o s i t i o n a n d
o r g a n i z a t i o n a r e c o n f o r m a b l e ( F ig . l a ) w i th t h e
g e n e r a l m o d e l o f o p h i o l i t e c r u s t [ 4 8 ] a n d i n p a r -
t i c u l a r t h e g a b b r o s e q u e n c e i s w e l l d e v e l o p e d .
I n c o n t r a s t t h e L O S c r u s t a l s e c t i o n ( F ig . l b ) i s
a l t o g e t h e r t h i n n e r ( T a b l e 1 ) a n d m o r e p o o r l y
o r g a n i z e d , o r a b s e n t . I n t h e w e l l - e x p o s e d c r u s t a l
s e q u e n c e o f t h e T r i n i t y b o d y , t h e c u m u l a t e g a b -
b r o s a n d p y r o x e n i t e s a r e n o t u b i q u i to u s : t h e y a re
a p p a r e n t l y d e p o s i t e d w i t h i n s m a l l a n d d i s c o n t i n u -
o u s m a g m a c h a m b e r s . T h e r e i s a w i d e s p r e a d d e -
v e l o p m e n t o f m a g m a t i c b r e c c i a s w h i c h w e a s c r i b e
t o h y d r o u s a n a t e x i s r e l a t e d t o h y d r o t h e r m a l c i r c u -
l a t io n . F i n a l l y t h e c r u s t a l s e c t i o n is i n v a d e d b y
s i l l s an d d i k es , t h e l a t t e r b e i n g p r ev a l en t i n i t s
u p p e r p a r t . T h e c r u s t a l s e c t i o n a s s o c i a t e d w i t h t h e
P i e m o n t - L i g u r i a o p h i o l i t e s w h e n i t e x i s t s a l s o
s h o w s a l i m i t e d e x t e n t o f c u m u l a t e s . F i n a l l y , it
s e e m s t h a t t h e d i f f e r e n c e i n c r u s t a l t h i c k n e s s b e -
t w e e n t h e t w o t y p e s i s m a i n l y d u e t o t h e d i f f e r e n t
d e v e l o p m e n t o f t h e c u m u l a t e g a b b r o s e q u e n c e s .
2 .3 . N a t u r e o f t h e u lt r a m a f i c r o c k s
T h e H O S m a s s i fs ar e d o m i n a n t l y c o m p o s e d o f
h a r z b u r g i t e s w i t h a n o r t h o p y r o x e n i t e l a y e r i n g a n d
s u b s i d i a r y d u n i t e b o d i e s . T h e d u n i t e s a r e o b s e r v e d
t h r o u g h o u t t h e e x p o s e d s e q u e n c e s , b u t t h e i r a b u n -
d a n c e i n c re a s es u p w a r d t o b e c o m e d o m i n a n t i nt h e t r a n s i t i o n z o n e b e l o w t h e c r u s t a l f o r m a t i o n s
[ 4 9 ] . I n t y p i c a l s e c t i o n s o f t h e O m a n o p h i o l i t e ,
c l i n o p y r o x e n e a n d f e l d s p a r a re l o c a l ly p r e s e n t
m a i n l y in t h e u p p e r m o s t h a r z b u r g i t e s a n d d u n i t e s .
T h e o c c u r r e n c e o f t h e s e m i n e r a l s i s a s c r i b e d t o a
s e c o n d a r y m a g m a t i c i m p r e g n a ti o n .
T h e L O S m a s s if s a r e c o m p o s e d o f h o m o g e -
n e o u s f e l d s p a t h i c l h e r z o l i t e s w i t h w e b s t e r i t i c l a y e r -
i n g. A l t h o u g h t h e d is t i n c t i o n i s s o m e t i m e s u n e a s y ,
c r i t e r i a c a n b e f o u n d [ 2 , 4 7 ] t o d e m o n s t r a t e t h a t
t h e f e l d s p a r d e v e l o p s f r o m p a r t i a l m e l t i n g o f t h er o c k w h i c h c o n t a i n s i t a n d n o t f r o m a s e c o n d a r y
i m p r e g n a t i o n . T h e o n l y s i g n i fi c a n t v o l u m e s o f
d u n i t e s a r e p r e s e n t i n th e u p p e r m o s t p a r t o f t he
s e c t i o n .
D i a b a s e d i k e s a n d s il ls a r e c o m p a r a t i v e l y m o r e
a b u n d a n t i n t h e u p p e r u l t r a m a f i c s e c t i o n o f L O S
t h a n i n H O S . A c ri t ic a l d i f f e r e n c e b e t w e e n t h e t w o
s u b t y p e s i s t h a t c h r o m i t e p o d s a r e r e s t r i c t e d t o
H O S .
2 . 4 . S t r u c t u r e s a n d m i c r o s t r u c t u r e s i n m a n t l e r o c k s
T h e p a t t e r n o f h i g h - t e m p e r a t u r e ( n e a r s o l i d u s
o r a b o v e s o l i d u s ) p l a s t i c f l o w d e d u c e d f r o m t h e
i n t e r n a l s t r u c t u r e s i s a l s o d i st i n c t. N i c o l a s a n d
V i o l e t t e [5 0] h a v e s h o w n t h a t t h e a s t h e n o s p h e r i c
f l o w p a t t e r n i n H O S i s e i t h e r f l a t w i t h a r e g u l a r
f l o w d i r e c t io n , o r o r g a n i z e d f o l l o w i n g a d i a p i ri c
m o d e l ( s t e e p f o l i a t i o n s d e f i n i n g l a r g e - s c a l e f o l d s
w i t h s t e e p a x e s , a n d c o a x i a l f l o w l i n e a t io n s ) . I n
t h e L O S w h e r e t h e p l a s ti c f lo w s t r u c t u re s h a v e
b een s t u d i ed ( T r i n i t y [ 2 6 ] , L an zo [ 3 3 ] , C o r s i ca [ 3 2 ] ,
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L i g u r i a ( p e r s o n a l o b s e r v a t i o n ) ) , f l o w p l a n e s a r e
s t eep an d p a ra l l e l , an d f l o w l i n e s f l a t o r mo d -
e ra t e l y d i p p i n g (F i g . 2 ) . T h e s h ea r f l o w r eco rd s t h e
s a m e s e n s e o f s h e a r o v e r k i l o m e t e r s a c r o s s t h e
mas s i f s .M i c r o s t r u c t u r e s i n m a n t l e r o c k s a r e p o r p h y r o -
c l a s t i c ( e x c e p t i n s h e a r z o n e s w h e r e t h e y a r e
my l o n i t i c ) w i t h n eo b l a s t s s ize a ro u n d 0 .5 mm [51 ]
i n L O S a n d a r o u n d 3 . 0 - 5 . 0 m m ( [2 3] a n d u n p u b -
l i s h ed d a t a ) i n H O S. T h i s r eco rd s a h i g h e r f l o w
s t r e s s i n L O S t h an i n H O S [5 2 ] .
2.5. Serpentinization
T h o u g h d i f f ic u l t t o g e n e r a li z e b e c a u s e t h e s t u d -
i es o n t h e s e r p e n t i n i z a t i o n o f u l t r a m a f i c s e q u e n c e s
a r e n o t e x h a u s t i v e , s o m e d i f f e r e n c e s b e t w e e n H O S
a n d L O S c a n b e e m p h a s i z e d .
T h e h a r z b u r g i t e m a s s i fs a r e v a r io u s l y
s e r p e n t i n i z e d , b u t i n a p e r v a s i v e w a y o n l y b y t h e
l o w - t e m p e r a t u r e a n d p o s t - d e f o r m a t i o n a l c h r y s o -
t i l e - b r u c i t e - l i z a r d i t e a s s e m b l a g e [ 5 3 ] . T h e d i a b a s e
d i k e s w h i c h a r e c o m p a r a t i v e l y e x c e p t i o n a l i n t h e
p e r i d o t i t e s a n d t h e g a b b r o d i k e s a r e u s u a l l y u n a l -
t e r e d . T h i s s e r p e n t i n i z a t i o n c a n b e a s c r i b e d t o
c o n t a m i n a t i o n b y s e a o r g r o u n d w a t e r a t l o w
t e m p e r a t u r e [ 5 3 - 5 5 ] . I n c o n t r a s t , m o s t l h e r z o l i t e
m a s s i f s s h o w a n e a r l y e p i s o d e o f a n t i g o r i t i c
CRUST
F i g . 2 . R e c o n s t r u c t e d s t r u c t u r e o f t h e T r i n i t y o p h i o l i t e [ 2 6 ] . I n
t h e m a n t l e s e c t i o n , t h e a c t i v e f l o w p l a n e ( f u l l l i n e s ) d e d u c e d
f r o m t h e a t t i t u d e o f t h e h i g h - t e m p e r a t u r e f o l i a t io n S 1 ( b r o k e n
l i n e ) i s s t r o n g l y d i p p i n g a n d t h e f l o w l i n e h o r i z o n t a l ; b o t h a r e
p a r a l l e l t o t h e p a l e o s p r e a d i n g a x i s , d e f i n e d b y s h e e t e d d i k e s
o r i e n t a t i o n . D i a b a s e d i k e s a n d s i l l s : t h i c k b l a c k l i n es ; d u n i t e
b o d i e s : d o t s .
87
s e r p e n t i n i z a t i o n ( w i t h a c c e s s o r y m a g n e t i t e , t r e m o -
l it e a n d t a lc ) d e v e l o p e d i n a t e c t o n i c e n v i r o n m e n t
a n d p r e c e d i n g t h e l i z a r d i t e e p i s o d e . T h e d i a b a s e
d i k e s a n d t h e g a b b r o d i k e s a r e m o r e c o m m o n l y
a l t e r e d t o r o d i n g i t e . T h e s e e a r l y s e r p e n t i n i z a t i o na n d r o d i n g i t i z a t i o n r e c o r d c o n d i t i o n s o f t h e
g reen s ch i s t f ac i e s an d t h ey a r e a s c r i b ed t o t h e
h y d r o t h e r m a l a l t e r a t i o n t a k i n g p l a c e c l o s e t o t h e
s p r e a d i n g c e n t e r w h e r e t h e c o r r e s p o n d i n g o c e a n i c
c ru s t h a s b een fo rmed [5 6 ] .
I t i s k n o w n i n r o c k m e c h a n i c s t h a t b e y o n d 1
k b a r o f c o n f i n i n g p r e s s u r e t h e o p e n c r a c k s t e n d t o
c l o s e [5 7 ] . T h i s i n d i ca t e s t h a t s ea w a t e r can
p e n e t r a t e i n t o t h e m a n t l e s e q u e n c e i n a p e r v a s i v e
w a y , o n l y i f t h e o v e r l y i n g c r u s t d o e s n o t e x c e e d 3
k m i n t h ick n es s . Co n s i d e r i n g t h e r e s p ec t i v e c ru s t a l
t h i c k n e s s e s e s t i m a t e d i n L O S ( 3 k m ) a n d H O S ( 6
k m ) m a s s i f s , t h is e x p l a i n s w h y o n l y t h e L O S u p p e r
p e r i d o t i t e s a r e a f f e c t e d i n a p e r v a s i v e w a y b y t h i s
h y d r o t h e r m a l a l t e r a t i o n .
3 . D i s c u s s i o n
H O S w h i c h r e p r e s e n t t h e m a j o r i t y o f o p h i o l i t e s
h a v e b e e n f o r m e d a t o c e a n i c s p r e a d i n g c e n t e r s
[48 ,5 8] , w h e t h e r i n a n o rm a l o c ean o r i n a b ack a r c
b a s i n. L O S o b v i o u s l y c o r r e s p o n d t o s p e ci a l
s p r e a d i n g s i t u a t i o n s i n w h i c h a p a r t o f t h e m a g m aw a s n o t e x t r a c t e d a t s h a l l o w d e p t h , n a m e l y i n t h e
p l ag i o c l a s e l h e r zo l i t e s t ab i l i t y f i e l d , t h a t i s ab o v e
3 0 k m [5 9 ] , a s s h o w n b y t h e p e r s i s t en ce , i n t h e s e
r o c k s , o f f e l d s p a t h i c l e n s e s c o r r e s p o n d i n g a t o n e
t im e t o m a g m a p o c k e ts .
T h i s s u g g e st s d i f f e r e n t t h e r m a l r e g im e s b e t w e e n
H O S a n d L O S s p r e a d in g c e n t e rs . G e o p h y s i c a l d a t a
s u p p o r t t h e i d e a t h a t f o r m e d i u m - t o f a s t - s p r e a d -
i n g r a t e s , l o n g - l i v e d m a g m a c h a m b e r s c a n b e p r e -
s e n t [ 6 0 - 6 2 ] . T h e c o n t i n u i t y a n d t h e t h i c k n e s s o f
c u m u l a t e s i n m a n y H O S o p h i o l i t e s , l i k e i n O m a n
s u g g e s t s c o m p a r a b l e s p r e a d i n g r a t e s f o r t h e i r s i t e
o f o r i g i n . F o r o t h e r H O S l i k e T r o o d o s [ 6 3 ] o r
X i g aze [3 7 , 3 9 ] w h e re d i s co n t i n u o u s cu mu l a t e s h av e
b e e n r e p o r t e d , i t i s e n v i s a g e d t h a t t h e m a g m a
c h a m b e r s w e r e o n l y e p i s o d i c p o i n t i n g t o l o w e r
s p r e a d i n g r a t e s . H o w e v e r , i n a l l t h e H O S m a s s i f s ,
t h e r e i s e v i d e n c e o f i n t e n s e m a g m a t i c a c t i v i t y i n
t h e u p p e r m o s t m a n t l e t h a t p r o d u c e d t h e d u n i t e s ,
t h e m a g m a t i c i m p r e g n a t i o n s , a n d t h e g e n e r a t i o n
o f ch ro mi t e p o d s [4 9 ] . T h u s , i n t h e H O S cas e , t h e
a d i a b a t i c c o n d i t i o n s a c h i e v e d i n t h e r i s i n g m a n t l e
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88
b e n e a t h t h e r i d g e w e r e m a i n t a i n e d a l l t h e w a y u p
t o t h e b a s e o f t h e c r u s t . A c o o l e r r e g i m e i n t h e
L O S c a s e i s s u g g e s t ed b y ( 1 ) t h e s c a r c i ty o f
c u m u l a t e s i n t h e c r u s t a l s e q u e n c e , ( 2 ) t h e f a c t t h a t
t h e f e l d s p a th i c m e l t s p r o d u c e d d u r i n g a d i a b a t i c
a s c e n t h a v e c r y s t a l li z e d d u r i n g t h e p l a s t ic f l o w
r e l a t ed t o t h e a s cen t [ 5 ,6 ] , an d ( 3 ) t h e h i g h e r
m a n t l e v i sc o s i ty t h a n i n H O S d e d u c e d f r o m t h e
h i g h e r f l o w s t r e s s ( s e e a b o v e ) .
I n a r i s i n g a s t h e n o s p h e r e b e l o w a s p r e a d i n g
c e n t e r , a d i a b a t i c c o n d i t i o n s a r e p r e s e r v e d u n t i l t h e
l i t h o s p h e r e i s m e t . T h e s u b s e q u e n t c o o l i n g o p e r -
a t e s b y h e a t c o n d u c t i o n w i t h a t h e r m a l g r a d i e n t
s t e e p e r t h a n t h e p e r i d o t i t e s o l i d u s o n e , t h u s p r e -
v e n t i n g f u r t h e r m e l t i n g d u r i n g a s c e n t . T h e d a t a
p r e s e n t e d a b o v e a n d i n p a r t ic u l a r t h e c o n c l u s io n
t h a t m a g m a w a s o n l y l o c a ll y e x t r a c t e d s h a l l o w e r
t h a n 3 0 k m i m p l i e s f o r L O S a t h i c k e r l i t h o s p h e r e
t h a n t h e 6 k m o n e , c o m m o n l y e s t i m a t e d f o r oc e -
a n i c s p r e a d i n g c e n t e r s h e r e r e p r e s e n t e d b y t h e
H O S c a s e w h e r e h y p e r s o l i d u s c o n d i t i o n s a r e m a i n -
t a i n e d u p t o t h e M o h o [ 4 9 ] . T o e x p l a i n t h e
a b n o r m a l t h i c k n e ss o f li t h o s p h e r e in a s p r e a d i n g
e n v i r o n m e n t , w e w i l l e n v i s a g e a p a r t i c u l a r l y s l o w
s p r e a d i n g r a t e a n d t h e l o c a l i n f l u e n c e o f a t r a n s -
f o r m f a u l t .
3.1. Very-slow-spreading rate
I n m a n t l e d i a p i rs , d e p a r t u r e f r o m a d i a b a t i c
c o n d i t i o n s i s e s t i m a t e d t o b e a r o u n d 2 5 k m d e p t h
i f t h e a s c e n t v e l o c i t y is lo w e r t h a n 1 c m / y r [6 4,6 5]
d u e t o t h e p e n e t r a t i o n o f t h e c o n d u c t i v e c o o l i n g
f r o n t a t t h e s e d e p t h s . O n t h e c o n t r a r y f o r a s c e n t
r a t e s h ig h e r t h a n 1 c m / y r t h e a d i a b a t i c r e g i m e is
p r e s e r v e d t h r o u g h o u t t h e m a n t l e s e c t i o n . A t t h e
s p r e a d i n g c e n t e r i t s e l f i t s u p p e r l i m i t i s w h e r e i t
m e e t s t h e h y d r o t h e r m a l l y - c o o l e d l i th o s p h e re . F o r
a s c e n t r a te l o w e r t h a n 1 c m / y r , w h e r e t h e m e l t in g
i n th e d i a p i r c e a s e s a t d e p t h s h a l l o w e r t h a n 2 5 k m ,
o n l y a r o u n d 1 0 % m e l t i s e x t r a c t e d , r e s u l t i n g i n a
l h e r z o l i t i c r e s i d u a l m a n t l e , a h a l f - t h i n n e r c r u s t ,
a n d p o s s i b l y a m o r e a l k a l in e m a g m a t i s m [ 65 ]. F o r
a s c e n t r a t e l a rg e r t h a n 1 c m / y r , t h e m e l t i n g c e a s e s
a t t h e M o h o , 2 0 % m e l t i s e x t r a c t e d , r e s u l t i n g i n a
h a r z b u r g i t e r e s i d u e , a t h i c k e r c r u s t a n d i n a t h o l e i -
i t i c m a g m a . T h e s e t h e o r e t i c a l e s t i m a t e s a r e s u p -
p o r t e d b y R e i d a n d J a c k s o n ' s [6 6] se i s m i c s t u d i e s
w h i c h s ho w t h a t be l o w 1 - 2 c m / y r s p r e a di n g ra t e s
( w h i c h c a n b e i n a f i r s t a p p r o x i m a t i o n e q u a t e d
w i t h a s c e n t r a t e s ) t h e t o t a l t h i c k n e s s o f o c e a n i c
l a y e r s b e c o m e s t h i n n e r t h a n 6 k m .
C o n d i t i o n s f o r v e r y s lo w s p r e a d i n g r a t e s c a n b e
a c h i e v e d in o c e a n i c e n v i r o n m e n t s [ 67 ,6 8]. H o w -
ev e r , a s a l r ea d y p r o p o s ed [ 34 ,6 9] , a j u v e n i l e r i f t i n g
e n v i r o n m e n t c a n b e a g o o d c a n d i d a t e . F o r e x a m -
p l e L e P i c h o n a n d F r a n c h e t e a u [7 0] h a v e e s ti -
m a t e d t h e s p r e a d i n g r a t e in t h e R e d S e a a t 5 .0 7 .5
m m / y r f o r t h e m o s t r e c e n t 4 5 M y r a n d a t 2 .1
m m / y r i n t h e ri ft o f E t h i o p ia .
3.2. Trans form fau l t env ironm ent
T h e i s o t h e rm s i n t h e m a n t l e a t s p r e a d i n g c e n t e rs
c l o s e t o a t r a n s f o r m f a u l t a r e p e r t u r b e d o v e r s e v e r a l
k i l o m e t e r s a l o n g s t r i k e b y t h e e f f e c t o f t h e o l d e r
a n d c o o l e r t r u n c a t i n g l i t h o s p h e r e w a l l . T h i s
t r a n s f o r m f a u l t e f f e c t i n c r e a s e s f o r d e c r e a s i n g r a t e s
o f s p r e a d i n g [ 71 ,7 2]. F o r s p r e a d i n g r a t e s l o w e r
t h a n 3 0 m m / y r a n d a t r a n s f o rm f a u l t b r in g i n g a
n e w l y c r e a t e d a s t h e n o s p h e r e i n c o n t a c t w i t h a
l i t h o s p h e r e o l d e r t h a n 1 0 M y r , t h e c o n d u c t i v e
c o o l i n g f ro n t r e a c h t h e d e p t h o f 2 0 - 3 0 k m w h e r e
d e p a r t u r e f r o m a d i a b a t i c c o n d i t i o n s s e e m s n e ce s -
s a r y t o g e n e r a t e l h e r z o l i t e s . S u c h a n e n v i r o n m e n t
h a s b e e n p r o p o s e d f o r t h e P i e m o n t - L i g u r i a
l h e r zo l i t e o p h i o l i t e s [ 2 8 ] .
T h e e v i d e n c e a v a i l a b l e i n t h e v a r i o u s s i t u a t i o n sa n a l y z e d s o f a r is n o w p r e s e n t e d w i t h i n v i e w t o
c o m p a r e t h e s e t w o e n v i r o n m e n t s .
( 1) P l a g i o c l a s e l h e r z o l i t e s a r e r e p o r t e d f r o m
b o t h e n v i r o n m e n t s . T h e y a r e k n o w n i n a r i f t , l i k e
i n t h e Z a b a r g a d I s l a n d ( R e d S e a ) , w h e r e t h e y
o r i g i n a t e d a l o n g a s l o w - s p r e a d i n g s e g m e n t . T h e y
h a v e b e e n d r e d g e d i n t h e l a r g e t r a n s f o r m f a u l t s
[ 7 3 , 7 4 ] w h e r e s o m e o f t h e m d i s p l a y e v i d e n c e o f a
s e c o n d a r y o r i g i n b y m a g m a i m p r e g n a t i o n [ 2 , 7 4 ] ;
o t h e r s , h o w e v e r , s h o w a c o m p l e t e s i m i l a r i t y w i t h
p r i s t i n e t e x t u r e s l i k e t h o s e f r o m t h e L a n z o m a s s i f .
O n t h e o t h e r h a n d , d r e d g e s f r o m l a rg e t r a n s f o r m
f a u l ts c o l l e c t in r e l a ti v e l y g re a t a b u n d a n c e
a m p h i b o l i t e s [7 5] w h i c h h a v e n o t b e e n o b s e r v e d i n
t h e c o n s i d e r e d l h e r z o l i t e m a s s i f s .
( 2 ) 2 3 k m t h i ck o ce an i c c r u s t s ex i s t i n
s l o w - s p r e a d i n g e n v i r o n m e n t s l i k e r i f t s s u c h a s t h e
R e d S e a [7 6] a n d t h e G u l f o f C a l i f o r n i a - I m p e r i a l
V a l l e y [7 7] o r m i d - o c e a n r i d g e s s u c h a s t h e B a f f i n
B ay [ 7 8 ] o r t h e A r c t i c O cean [ 6 7 ] . S u ch t h i n c r u s t s
a r e a l s o w e l l k n o w n i n t h e v i c i n i ty o f t r a n s f o r m
f a u l t s w h e r e t h e c r u s t t e n d t o b e t h i n n e r t h a n
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L• ~ C r u s tiill:illl'l i ) ~ L ith os ph ere
~ I ; i ' I , ) } ~ . ~ - - ~ _ a sth en osp he re
" " ' , ' 1 ' , '
m T T I T m T
i , I i , I ~ j ~ , \ \ \
i i i I I \ \ \ \ \ x ,
' - - ~ / / J , ' j ~ ' J ~h ~ \\ ,\ " "I / ~ ' . ' " / l ~ , ' l l / , i i S t ,/ t l\\ 'X \,~ " "
_a_ _b_
Fig. 3. (a) The rolling mill model [84] of m antle flow beneath a medium- to fast-spreading center (HOS. corresponding to the m ostcomm on oceanic environment), opposed to (b) the funn el model beneath slow-spreading center (LOS). Dots and dashes: zone s ofpartial melting.
a w a y f r o m t h e f a u l t ( ty p i c a l l y 2 3 k m v e r s u s 6
k m ) [ 71 ]. H y d r o t h e r m a l a l t e r a t i o n d e v e l o p i n g a n t i -
g o r i t e p l u s t a l c w h i c h i s k n o w n i n L O S h a s b e e n
d e s c r i b e d i n p e r i d o t it e s f r o m t r a n s f o r m f a u lt s
( [ 7 9 , 8 0 ] a n d u n p u b l i s h e d d a t a ) , a l t h o u g h t h e m o s t
c o m m o n s e r p e n t i n i z a t i o n d e v e l o p s o r t h o c h r y s o t i l e
a n d l i z a r d i t e a t t e m p e r a t u r e a b o u t 2 0 0 ° C [ 5 5 ] .
( 3) L O S m a s s i f s h a v e v e rt i c a l f lo w p l a n e s a n d
m o d e r a t e l y p l u n g i n g f l o w li n e s (F i g . 2) . T h i s f l o w
g e o m e t r y su g g e s t s a t r a n s f o r m f a u lt e n v i r o n m e n t .
I n d e e d , t h e P o u m - B e l e p o c c u r r e n c e o f N e w C a l e-
d o n i a [ 8] w h e r e p l a g i o c l a s e a n d s p i n e l l h e r z o l i t e s
c r o p o u t w i t h i n h a r z b u r g i t e s w i t h t h is f l o w g e o m -e t r y h a s b e e n i n t e r p r e t e d a s a p a l e o t r a n s f o r m f a u lt .
H o w e v e r , t h e s e l h e r z o l i t e s a r e t h e r e s u l t o f a s e c -
o n d a r y i m p r e g n a t i o n [2 ] a n d a r e n o t L O S . T h e
s h e a r z o n e a n a l o g y o f t h e p l a s ti c f l o w p a t t e r n w i t h
t h a t e x p e c t e d f o r a t r a n s f o r m f a u l t m a y b e m o r e
a p p a r e n t t h a n r e a l. I n th e d o c u m e n t e d c a s e s o f a
t r a n s f o r m f a u l t a c t i v i t y i n o p h i o l i t e s [ 2 2 , 8 1 , 8 2 ] , t h e
u p p e r m o s t m a n t l e - l o w e r c r u s t d o m a i n s d i s p l a y
l o w - t e m p e r a t u r e / h i g h - s t r e s s ( m y l o n i ti c to s u b -
m y l o n i t i c ) t e x t u r e s. S u c h t e x t u r e s a r e e i t h e r a b s e n t
o r l o c a l i n t h e l h e r z o l i t e m a s s i f s c o n s i d e r e d h e r e
w h e r e t h e t e x t u re s a r e o n t h e c o n t r a r y t y p i c a l o f a
h i g h - t e m p e r a t u r e / l o w - s t r e s s r e g im e . M o r e o v e r ,
t h e d o m a i n s a s c r i b e d t o p a l e o - t r a n s f o r m f a u lt s in
o p h i o l i t e s a r e o n l y 3 - 4 k m w i d e w h e r e a s i n t h e
L a n z o [ 33 ] a n d T r i n i t y [ 2 6 ] m a s s i f s t h e o b s e r v e d
w i d t h s o f th e d e f o r m e d d o m a i n s a r e r e s p e c t iv e l y
1 0 - 1 5 k m a n d 5 0 k m . A 3 - 4 k m w i d e d i s t u r b a n c e
i n u p p e r m o s t m a n t l e s t r u c t u r e s w o u l d r e q u i r e a
l a r g e f a u lt ; a 2 0 - k m d i s t u r b a n c e w o u l d r e q u i r e a
t r a n s f o r m f a u l t a s l a r g e a s t h e V e m a t r a n s f o r m
[ 831 .
( 4 ) F i n a l l y , t h e a s s o c i a t i o n o f t h e L O S w i t h
c o n t i n e n t a l f o r m a t i o n s a n d s h a l l o w - w a t e r s e d i -
m e n t s a r g u e s f o r a r if t e n v i r o n m e n t r a t h e r t h a n a n
o c e a n i c t r a n s f o r m f a u l t .
C o n s e q u e n t l y w e f a v o r , f o r t h e l h e r z o l i t e m a s -
s i fs c o n s i d e r e d h e r e , t h e e a r l i e r i n t e r p r e t a t i o n p r o -
p o s e d b y M e n z i e s [ 3 4] a s c r i b i n g a t y p i c a l L O S , t h e
O t h r i s l h e r z o l i t ic o p h i o l i t e , t o a n i n c i p i e n t o c e a n
f o r m i n g i n a r i f t o p e n i n g a t s p r e a d i n g r a t e s i n t h e
r a n g e o f o r b e l o w 1 0 m m / y r . I t s h o u l d b e c le a r ,
h o w e v e r , t h a t t h e L O S c a n a l s o o c c u r i n o c e a n i c
l i t h o s p h e r e s e g m e n t s g e n e r a t e d a l o n g t r a n s f o r m
f a u l t s . T h e r e c o n s t r u c t e d s t r u c t u r e o f t h e T r i n i t yc o m p l e x [ 2 6 ] , w h i c h i s s o f a r t h e m o s t c o m p l e t e
l h e r z o l i t i c o p h i o l i t e o f t h i s t y p e , c o n s t i t u t e s a n
e x a m p l e o f t h e g e n e r a l s t r u c t u r e a s c r i b e d h e r e t o a
r i f t e n v i r o n m e n t ( F i g s . 2 a n d 3 ). T h e r e s u l t s o f a
r e c e n t s t r u c t u r a l s t u d y o f t h e Z a b a r g a d I s l a n d
w h i c h c o r r e s p o n d s t o a n e a r l y s t a g e o f r i f t i n g i n
t h e R e d S e a [3 6 ] s u p p o r t t h i s m o d e l . T h e h i g h -
t e m p e r a t u r e f o l i a t io n s i n p l a g i o c l a s e l h e rz o l i te s a r e
v e r t i c a l a n d s t r i k e p a r a l l e l t o t h e R e d S e a d i r e c t i o n
a n d t h e f l o w l i n e a t io n s p l u n g e a t 5 0 ° t o t h e n o r t h -
w e s t .
4 . C o n c l u s i o n s
T h e l h e r z o l i t e s u b t y p e i n o p h i o l i t e s c o r r e s p o n d s
t o a p a r t i c u l a r s i t u a t i o n a t a s p r e a d i n g c e n t e r
w h e r e t h e r i s i n g m a n t l e m e t , a t d e p t h s h a l l o w e r
t h a n 3 0 k m , c o n d i t i o n s s u c h t h a t h e a t l o s s b y
c o n d u c t i o n t o t h e s u r f a c e b e c a m e m o r e e f f i c i e n t
t h a n t h e h e a t s u p p l y b y a d v e c t i o n . A s a c o n s e -
q u e n c e , ( 1 ) m e l t s w e r e n o t f u r t h e r e x p e l l e d a n d
c r y s t a l l i z e d y i e l d i n g a f e l d s p a t h i c l h e r z o l i t e i n s t e a d
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90
o f t h e c o m m o n l y o b s e r v e d h a r z b u r g i te s , ( 2 ) t h e
o v e r l y in g n e w c r u s t w a s p r o d u c e d t h i n n e r t h a n
n o r m a l ( 3 - 6 k m ) w i t h o n l y li m i t e d f o r m a t i o n o f
c u m u l a t e s i n r e l a t i v e l y s m a l l m a g m a c h a m b e r s ;
h y d r o t h e r m a l c i r c u l a t i o n r e a c h e d t h e m a n t l e f o r -
m a t i o n s ( s e r p e n t i n iz a t i o n i n a n ti g o r i te a n d r o d i n -
g i t i z a t i o n ) a n d i n t e n s e h y d r o u s a n a t e x i s o c c u r r e d
i n t h e d e e p e r c r u st , (3 ) c o m p a r a t i v e l y h i g h v i sc o s -
i t y a n d a p a r t i c u l a r d i a p i r i c s t r u c t u r e i n t h e m a n t l e
a r e d e d u c e d f r o m h i g h - s t r e s s p l a s t ic f l o w s t r u c -
t u r e s in m a n t l e r o c k s ; t h e o c c u r r e n c e o f si ll s w a r m s
i n t h e c r u s t a l s o s u g g es t s a p a r t i cu l a r s t r e s s f i e l d
[ 8 5 ] . T h e t h e r m a l r e g i m e i n t h e m a n t l e r e s p o n s i b l e
f o r t h e L O S g e n e r a t i o n c a n b e a s c r i b e d t o t h e
c o o l i n g e f f e c t o f t r a n s f o r m f a u l t s o r t o s p r e a d i n g
r a t e s s m a l l e r th a n 1 0 1 5 m m / y r . I n t h e i n v e s t i-
g a t e d m a s s i f s t h e s e c o n d p o s s i b i l i t y i s f a v o r e d a n dt h e s i t e m o s t a p p r o p r i a t e f o r t h e g e n e r a t i o n o f
L O S s e e m s t o b e a j u v e n i l e o c e a n l i k e th e R e d S e a ,
a s e x a m p l i f ie d b y t h e c a se o f th e Z a b a r g a d I s l a nd .
4 .1 . M a n t l e s tr u c t u r e in a r i f t / j u v e n i l e o c e a n a n d
s e i s m i c ' a n i s o t r o p y
A p u z z l i n g f e a t u r e o f t h e m a n t l e f l o w p a t t e r n
r e p r e s e n t e d i n F i g . 2 i s t h a t t h e f l o w l in e a t i o n s
b o r n b y t h e v e r t i c a l f o l i a t i o n p l a n e s o f t h e
a s t h e n o s p h e r e d i a p i r a r e h o r i z o n t a l o r m o d e r a t e l y
p l u n g i n g , a n d n o t v e r t i c a l a s s h o u l d b e e x p e c t e d
f o r a d i a p i r i c i n t r u s i o n . T h i s w o u l d i n d i c a t e t h a t
b e l o w a r i f t , t h e f l o w i n t h e r i s i n g m an t l e co u l d
h a v e a s t r o n g h o r i z o n t a l c o m p o n e n t , p a r a l l e l t o
t h e a x i s o f t h e r i f t . l n d e e d i n t h e Z a b a r g a d I s l a n d
o f t h e R e d S e a , th e f l o w l in e a t i o n w h i c h t r e n d s
p a r a l l e l t o t h e r i f t d i r e c t i o n i s m o d e r a t e l y p l u n g i n g
[ 3 6 ] . I n t h e w es t e r n A l p s , t h e av e r ag e s t r i k e o f t h e
f o l i a t i o n s r e c o r d e d i n t h e l h e r z o l i t e s o f C o r s i c a
[ 3 2 ] an d L an zo [ 3 3 ] ( t h e p l a s t i c f l o w s t r u c t u r e s i n
t h e L i g u r i a n l h e r z o l i t e s b e i n g m o r e o b s c u r e d u e t o
s e c o n d a r y a l p i n e d e f o r m a t i o n ) i s n o r t h - s o u t h t h a ti s p a r a l l e l t o t h e d i r e c t i o n o f t h e P i e m o n t e s e o c e a n
d e d u c e d f r o m p a l i n p s p a t i c r e c o n s t ru c t i o n s ,
w h e r e a s t h e f l o w l i n e a t i o n i n t h e s e m a s s i f s i s o n l y
m o d e r a t e l y p l u ng i n g . T h e s a m e p a t t e r n i s r e c o r d e d
i n t h e l h e rz o l i te m a s s i f s o f O j e n a n d B e n i B o u s e r a
o n e a c h s i d e o f t h e A l b o r a n S e a , w h e r e t h e f o l ia -
t i o n s w i t h f l a t l i n e a t i o n s a r e r e s p e c t i v e l y n o r t h e a s t
t r e n d i n g [ 8 6 ] , a n d n o r t h w e s t t r e n d i n g [ 8 7 ] a n d
w h e r e t h e p r e s u m e d r if t w a s o r i e n te d n o r t h - s o u t h
[8 8]. F i n a l l y , se i s m i c a n i s o t r o p y m e a s u r e m e n t s b e -
l o w t h e R h i n e g r a b e n a l s o s h o w t h a t t h e h i g h
v e l o c i t y is r o u g h l y p a r a l l e l t o t h e r i ft e l o n g a t i o n
[ 8 9 ] . I n t e r e s t i n g l y a r ecen t s t u d y [ 9 0 ] p r o p o s es an
a s t h e n o s p h e r i c f l o w m o d e l o f s i m i l a r g e o m e t r y i n
t h e c a s e o f r e d u c e d v e r t i c a l fl o w v e l o c i t y b e n e a t h a
s l o w - s p r e a d i n g r i d g e .
R e f e r e n c e s
1 A. Nicolas and E.D. Jackson, R6partition en deux provinces
des p6ridotites des cha]nes alpines longeant la m6diterran6e:
implications geotectoniques, Bull. Suisse Mineral. P6trogr.
52, 479-495, 1972.
2 A. Nicolas and C. Dupuy, Origin of ophiolitic and oceanic
lherzolites, Tectonop hysi cs 110, 177 187, 1984.
3 M. Menzies, Rare earth geochemistry of fused ophiolitic
and alpine lherzolites. Othris, Lanzo and Troodos, Geo-
chim. Cosm ochim . Acta 40, 645 656, 1976.
4 M. Menzies and C. Allen, Plagioclase iherzolite-residual
mantle relationships within two eastern Mediterranean
ophiolites, Contrib. Mineral. Petrol. 45, 197-213, 1974.
5 F. Boudier and A. Nicolas, Fusion partielle gabbroique
dans la lherzolite de Lanzo (Alpes Pi6montaises), Schweiz.
Mineral. Petrogr. Mitt. 52, 39-56, 1972.
6 F. Boudier and A. Nicolas, Structural controls on partial
melting in the Lanzo peridotite. Oreg. Dep. Geol. Min. Ind.
Bull. 96, 33-78, 1977.
7 J.E. Quick, Petrology and pet rogenesis of the Trinity peri-
dotite, an upper mantle diapir in the Eastern Klamath
Mountains, northern California, J. Geophys. Res. 86,
837-863, 1981.
8 D. Secher, Les lherzolites ophiol itiques de Nouvelle-Caledonie et leurs gisements de chromite, 228 pp., These,
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9 G.R . Hi mmel berg and R.A. Loney, Petrology of ultrama fic
and gabbroic rocks of the Canyon Mountain ophiolite,
Oregon, Am. J. Sci. 280A, 232 268. 1980.
10 M. Misseri and F. Boudier, Structures in the Canyon
Mountain ophiolite indicate an Island arc intrusion,
Tectonophysics, in press, 1985.
11 J. Girardeau, Structure des ophiolites de Terre Neuve et
mod61e de crofite oc6anique , 154 pp.. Th6sc, Univers it+ de
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12 A.A. Savelyev and G. N. Savelyeva, Ophioli tes of the Voykar
massif (Polar Ural), Memorial Univ. Newfoundl and, Dep.
Geol. Rep. 8, 127-140, 1979.13 F. Boudier and R.G. Coleman, Cross section through the
peridotite in the Samail Ophiolite, southeastern Oman
Mount ains , J. Geophy s. Res. 86, 2573-2592, 1981.
14 J.J. Pallister and C.A. Hopson, Samail ophiolite plutonic
suite: field relations, phase variation, cryptic variation and
layering, and a model is a spreading ridge magma chamber,
J. Geophys. Res. 86, 2593 2644, 1981.
15 J. Girardeau and A. Nicolas, The structures of two ophiolite
massifs, Bay of Islands, Newfoundland: a model for the
oceanic crust and upper mantle, Tectonophysics 77, 1-34,
1981.
16 W.R. Church and R.K. Stevens, Early Paleozoic complexes
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o f t h e N e w f o u n d l a n d A p p a l a c h i a n s a s m a n t l e -o c e a n i c c r u s t
s e q u e n c e s , J . G e o p h y s . R e s . 7 6, 1 4 6 0 - 1 4 6 6 , 1 9 7 1.
1 7 C . J . S u e n , F .A . F r e y a n d J . M a l p a s , B a y o f Is l a n d s o p h i o l i te
s u i te , N e w f o u n d l a n d : p e t r o l o g i c a n d g e o c h e m i c a l c h a r a c t e r -
i s ti c s w i t h e m p h a s i s o n r a r e e a r t h e l e m e n t s g e o c h e m i s t r y ,
E a r t h P l a n e t . S c i . L e t t . 4 5 , 3 3 7 - 3 4 8 , 1 9 7 9 .1 8 J. W . H a w k i n s a n d C . A . E v a n s , G e o l o g y o f th e Z a m b a l e s
R a n g e , L u z o n , P h i l i p p i n e I s l a n d s : o p h i o l i t e d e r i v e d f r o m a n
I s l an d A r c - B a c k A r c b a s i n p a ir , A m . G e o p h y s . U n i o n ,
G e o p h y s . M o n o g r . S e r . 2 7~ 9 5 - 1 2 3 , 1 9 8 2 .
1 9 J . F . V i o l e tt e , S t r u c t u r e d e s o p h i o l i t e s d e s P h i l i p p i n e s
( Z a m b a l e s e t P a l a w a n ) e t d e C h y p r e , 1 5 2 p p ., T h 6 s e , U n i -
v e r s i te d e N a n t e s , 1 9 8 0 .
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L e b l a n c , P . P a r i s a n d M . R a b i n o v i t c h , S t r u c t u r e s i n t h e
N e w C a l e d o n i a p e r i d o t i t e s - g a b b r o s : i m p l i c a t i o n s f o r o c e -
a n i c m a n t l e a n d c r u s t , T e c t o n o p h y s i c s 6 9 , 8 5 - 1 1 2 , 1 9 8 0 .
2 3 D . C a s s a r d , S t r u c t u r e e t o r i g i n e d e s g i s e m e n t s d e c h r o m i t e
d u m a s s i f d u S u d ( o p h i o l it e s d e N o u v e l l e C a l 6 d o n i e ), 2 3 9
p p . , T h 6 s e U n i v e r s i t6 d e N a n t e s , 1 98 0.
2 4 H . G . A v e L a l l e m a n t, S t r u ct u r e o f t h e C a n y o n M o u n t a i n
( O r e g o n ) o p h i o l i t e c o m p l e x a n d i ts i m p l i c a t i o n f o r s e a f lo o r
s p r e a d i n g , G e o l . S o c . A m . S p e c . P a p . 1 7 3 , 1 - 4 9 , 1 9 7 6 .
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