Nakamura 1977_volcanoes as Possible Indicators of Tectonic Stress

8/15/2019 Nakamura 1977_volcanoes as Possible Indicators of Tectonic Stress

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J o u r n a l o f V o l c a n o lo g y a n d G e o t h e r m a l R e s ea r c h, 2 1 9 7 7 ) 1 - - 1 6 1

© E l s ev i er S c ie n t i fi c P u b l is h i n g C o m p a n y , A m s t e r d a m - - P r i n t e d i n T h e N e t h e r l a n d s

V O L C A N O E S A S P O S S I B L E I N D I C A T O R S O F T E C T O N I C S T R E S S

O R I E N T A T I O N - - P R I N C I P L E A N D P R O P O S A L l

K A Z U A K I N A K A M U R A 2

La mo nt -D oh e r t y Ge olog ic a l Obse rv a t ory o f C ol um bi a Uni v e rs it y , Pa li sades , N . Y . 1096 4

U.S .A . )

R e c e i v e d J u l y 1 4 , 1 9 7 6 ; r e v is e d a n d a c c e p t e d N o v e m b e r 3 0 , 1 9 7 6 )

A B S T R A C T

N a k a m u r a , K . , 1 9 77 . V o l c a n o e s a s p o s s i b l e i n d i c a t o r s o f t e c t o n i c s t re s s o r i e n t a t i o n - -

p r i n c i p l e a n d p r o p o s a l . J. V o l c a n o l . G e o t h e r m . R e s . , 2 : 1 - - 1 6 .

A m e t h o d i s p r o p o s e d f o r d e t e r m i n i n g t h e o r i e n t a t i o n o f a v er a g e t e c t o n i c s t re s s, u s in g

s u r fa c e f e a t u r e s i n d i c a t i n g r a d i a l d i k e p a t t e r n s o f v o lc a n o e s . T h e a p p r o x i m a t e p a t t e r n o f

r a d i al d i k e s is r e v e a l ed b y t h e d i s t r i b u t i o n o f s i t e s o f f l a n k e r u p t i o n s o n t h e s l o p e o f p o l y -

g e n e t ic v o lc a n o e s . T h i s c o n c l u s i o n i s d e d u c e d f r o m t h e u n d e r s t a n d i n g t h a t f l a n k e ru p t i o n s

a r e c a u s e d b y t h e m a g m a t h a t l a t e r a ll y o f f s h o o t s f r o m t h e m a i n p o l y g e n e t i c p i p e c o n d u i t

a n d t h a t c o n d u i t s o f f l a n k v o l c a n o e s a r e m o s t p r o b a b l y f i s s u r e -s h a p e d b e c a u s e m o s t o f

t h e m a r e m o n o g e n e t i c v o l c a n o e s . R a d i a l d i k e s a r e m o r e l i k e l y t o d e v e l o p i n a d i r e c ti o n

n o r m a l t o t h e m i n i m u m h o r i z o n t a l c o m p r e s s i o n o f t h e re g i o n a l s tr e ss . T h u s , t h e d i s t r i b u t i o n

o f f l a n k c ra t e r s w il l b e e l o n g a t e in t h e d i r e c t i o n o f t h e m a x i m u m h o r i z o n t a l c o m p r e s s i o n o f

t h e r e g i o n a l s t re s s .

T h e r e g i o n a l s t re s s c a n s o m e t i m e s b e a s c r i b e d s o l e ly t o t h e e f f e c t o f t h e g r a v i ty r a th e r

t h a n t e c to n i c s tr e ss . W h e n a n u m b e r o f i n d e p e n d e n t p o l y g e n e t i c v o l c a n o e s d o t t e d w i t h

m o r e t h a n s e v e r a l fl a n k v o l c a n o e s , a r e d i s t r i b u t e d i n a b e l t o r o v e r a b r o a d a r e a , i t is p o s s i b l e

t o d i s t i n g u i s h t h e t e c t o n i c s t r e s s f r o m t h e d i r e c t g r a v i t a t i o n a l e f f e c t b y t h e r e g i o n a l u n i -

f o r m i t y i n o r i e n t a t i o n o f t h e z o n es o f fl a n k v o lc a n o e s . W h e n t h e m a x i m u m c o m p r e s s i o n o f

t e c t o n i c s t r es s is h o r i z o n t a l , t h e t r e n d s o f t h e z o n e s o f f l a n k e r u p t i o n s o n p o l y g e n e t i c vo l -

c a n o e s a r e m o r e o r l e s s l i n e a r a n d p a r a l l e l, a n d a t a h ig h a n g l e t o t h e t r e n d o f t h e m a i n v o l -

c a n i c b e l t .

I N T R O D U C T I O N

V o l c a n i c p r o c e s s e s w h i c h c a u s e e x t e n s i v e f r a c t u r i n g o f r o c k s m u s t b e r e l a t e d

t o r e g i o n a l n o n - m a g m a t i c s t r e s s a c t i n g o n t h e v o l c a n i c r e g i o n . O n e s u c h p r o c e s s

i n v o l v i n g f r a c t u r i n g i s f l a n k v o l c a n i c e r u p t i o n s w h i c h a r e r e a s o n a b l y e x p l a i n e d

a s s u r f a c e m a n i f e s t a t i o n s o f t h e f o r m a t i o n o f r a d i a l d i k e s . B e c a u s e t h e o f t e n -

I L a m o n t - D o h e r t y G e o l o g i c a l O b s e r v a t o r y C o n t r i b u t i o n N o . 24 6 0 .

2 On l e a v e f r o m E a r t h q u a k e R e s e a r c h I n s t i t u t e , U n i v e r s it y o f T o k y o , T o k y o , 1 1 3 , J a p a n .

P r e s e n t ad d r e s s : D e p a r t m e n t o f G e o p h y s i c s , S t a n f o r d U n i v e r s it y , S t a n f o r d , C a l if . 9 4 3 0 5 ,

U . S . A .


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tation of dikes is affected by the existing regional stress, the distribution of

sites of flank eruptions may reveal the orie ntat ion of regional stresses, in ad-

dition to the stress generated by magmatic forces. In other words, flank

eruptions of polygenet ic volcanoes may be regarded as large-scale repeat ed ex-

periments of hydraulic fracturing using magma instead of water.

This paper is concerned with such a feature of radial dikes, as indicated

mainly by flank crater distribution, and proposes to make use of this distri-

bution as an indicator for the average orient ation of tecton ic stress which has

prevailed in the upper crust of the volcanic area.

MONOGENETIC AND POLYGENETIC VOLCANOES

Volcanoes may be classified as either monogenetic or polygenetic, based on

the numbe r of eruptions which formed the volcanic edifices Thorarinsson,

1960; Rittmann, 1962}.

A monogenet ic volcano erupts only on ce f rom a vent or a fissure and con-

structs a relatively small volcanic edific e such as a maar, a scoria cone, a lava

dome or a shield volcano of Icelandic type. Most fissure volcanoes are mono-

genetic. Some mo nogen etic volcanoes are co mpo un d volcanoes, such as a

scoria cone in a maar or a lava dome in a pyroclastic cone. They are the result

of a change of the externa l circu mferenc e during the course of the eruption.

A monogenetic volcano erupts only once and, therefore, its underground

conduit is newly formed every time it erupts. Common occurrence of xeno-

liths in the eruptive product s o f monogene tic volcanoes may be related to this

characteristics of their conduits.

The initial channelway for the ascent of the magma is expe cte d to be a

fissure and no t a cylinder. Therefore, the c ondu it of a monog eneti c volcano

may be fissured-shaped in its essential portion, but not pipe-shaped. At a

shallower depth, however, the conduits of monogenetic volcanoes may con-

verge to form pipes, e.g. kimberlite pipes, many of which grade downward into

dikes Dawson, 1967). Monogeneti c volcanoes com mon ly occur in groups,

either as independent swarms like Westeifel maars and scoria cones, or as a

part of a polygenet ic volcano where the y form flank volcanoes and post-

caldera cones.

Polygenetic volcanoes, on the othe r hand, e rupt r epeat edly or roughly peri-

odically from the same general vent or vents during a life-time of up to abo ut

10 s years. Comp osite vo lcanoe s and shield volca noes of the Hawaiian typ e are

the best k nown examples of polygene tic volcanoes. They form large volcanic

edifices of the orde r of 102 km 3 com pos ite volcano es) and 104 km 3 shield

volcanoes of the Hawaiian type) in volume, are more or less conical in shape

with a summit or central crater or craters. Repea ted e ruptions f rom the same

vent may form a cone ar ound the ven t as well as a pipe-shaped vertical cond uit

which is stable enough to be continua lly used as the channel way fo r the

ascending magma.


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M E C H N I S M O F F L N K E R U P T I O N S

Most flank volcanoes are monogenetic e.g. Nakamura, 1961). Not only

flank-fissure eruptions bu t flank erupt ions in general are regarded as surface

manifestations of the format ion of radial dikes around the vertical, central

pipe-conduit of a polygenetic volcano. This interpretation appears well

established for shield volcanoes of Hawaiian type e.g. Macdonald, 1972).

What is suggested here is that essentially the same interpretation may apply

to composite volcanoes.

Flank eruptions of composite volcanoes may be interpreted in the following

way: prior to the eruption, magmatic pressure in the central condui t increases

to the sum o f both the tensile strength of the surrounding rocks and the mini-

mum compressional stress of external origin. I f the magrnatic pressure is not

sufficiently relieved by an eruption from the summit, a radial vertical fracture

develops laterally from the central pipe and is simultaneously filled with the

source magma. Here the initiation of a radial dike resembles the opening of a

hydraulic fracture Haimson, 1975). Flank eruptions occur where the dike

first reaches the surface. If the available magma is of sufficient quantity and

its viscosity is low enough, then the dike arrives at the surface causing a

flank-fissure eruption.

This interpretation of the flank eruptions of composite volcanoes is sup-

ported by the following examples and the conclusion that composite vol-

canoes have stable centra l pipe conduits, while monogene tic volcanoes there-

fore most flank volcanoes) erupt from newly made fissures.

1) Flank eruptions are often accompanied or immediately preceded by

summit eruptions. Explosive eruptions are more common from summi t craters

and effusive eruptions are more com mon from flank craters Vesuvius, 1906,

Perret, 1924; Miyake-sima, 1940, Tsuya, 1941; Etna, Rittmann, 1964).

2) In the case of flank-fissure eruptions, the eruptive fissure usually grows

down-slope and the eruptive activity endures longer at t he lower portion of

the fissure, erupting lava flows Vesuvius, 1906, Perret, 1924; Hekla, 1947-48,

Thorarinsson, 1950).

3) Some flank eruptions occur simultaneously at two opposite sides on

the slope, their location being symmetrical with respect to the summi t Saku-

ra-jima, 1914, Omori, 1914--1916; Hekla, 1970, Thorarinsson and Sigvalda-

son, 1972; Villarrica, 1971, Gonz~lez, 1972; Tiatia, 1973, Doubik, personal

communication, 1974).

4) Flank volcanoes are more numerous when the magmahas a low viscosity

and the summit is higher Etna volcano and Fuji volcano).

5) The vertical crustal deformation associated with a flank fissure) erup-

tion is fundamentally concentric around the summit but not around the erup-

tion site. The residual horizontal deformation is dilatational across the eruptive

fissure Sakura-jima, 1914, Omori, 1914--1916; Miyake-sima, 1940, Earth-

quake Research Institute, 1941; 1962, Geographical Survey Institute, 1976).


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6) A central plutonic mass and radial dike swarm are a common association

found in dissected volcanic fields {Spanish Peaks, Knopf, 1936: Summer Coon

volcano, Lipman, 1968).

7) According to field observations on the lateral surface of the dikes, in-

trusion of magma into radial dikes proceeds outward from the center and

more or less obliquely upward Oshima, 1968; Nakamura, 1972).

From the foregoing facts and considerations, it is conc lude d that the distri-

buti on of flank craters on the slopes of a polygenetic volcano reveals the

underground pattern of radial dikes.

RADIAL DIKES AND STRESS FIELDS

Radial dikes which are ideally radial and uni form in azimuth Fig.lA ),

may show either that the magmatic pressure within the central conduit was

predominant over other stresses or that they did not differ in horizontal direc-

tion. This kind of radial dike pattern seems to be more or less realized in near-

vent areas, within a radius of a few kilometers, even though the overall patte rn

may n ot be ideally radial.

On the ot her hand, as shown in Fig.lB, at a sufficient distance from the

central condu it, radial dikes sometimes tend to curve and assume a single

general trend. Radial dikes surroun ding the Spanish Peaks Knopf, 1936), for

example, are a classic example. The pattern of radial dikes in this area was

controlled, according to the interpretation by Od~ {1957), by the superposed

stress systems of localized magmatic pressure around the West Spanish Peak

and by a genetically inde pen den t regional stress. The pl utonic mass of the

West Spanish Peak is also elongated in the d irection o f the c onc entr atio n of

radial dikes. This implies tha t th e process responsible for fo rmat ion o f the

central cond uit was also affect ed by th e same regional stress tha t distorted the

radial dike patte rn.

The radial dike pattern indicated by the distribution of flank and post-cal-

dera cones and craters Fig.2) may well be the same as the case in Fig. lB in

\

F i g .1 . I d e a l i z e d h o r i z o n t a l c r o s s - se c t io n o f a c o n d u i t o f a p o l y g e n e t i c v o l c a n o c e n tr a l

p l u t o n i c m a s s ) a n d ra d ia l d i k e s N a k a m u r a , 1 9 6 9 ) . A . P e r f e c t l y ra d ia l d i k e s u n d e r u n i f o r m

s tr e ss . B . D e f o r m e d r a d i a l d i k e s u n d e r a d i f f e r e n t i a l h o r i z o n t a l s t r es s.


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t h a t r a d i al d i k e s a r e f o r m e d m o r e f r e q u e n t l y a n d e x t e n d l o n g e r in a c e r t a in

p r e f e r r e d d i r e c t i o n . W h a t e v e r t h e u l t i m a t e o r ig i n m a y b e , t h e s i g n i fi c a n t d e -

v i a t io n f r o m t h e r a d i a l it y o f t h e d i k e p a t t e r n i m p l ie s t h a t t h e r e g i o n a l st re s s,

w h i c h is i n d e p e n d e n t i n o ri g in o f t h e l o c a l i z e d p r e s su r e o f t h e m a g m a i n th e

c o n d u i t , m a y h a v e i n f lu e n c e d t h e p a t t e r n . I n F ig .3 , t h e s t r u c t u r e o f a p o l y -

g e n e t i c v o l c a n o w i t h f l a n k v o l c a n o e s u n d e r s u c h a c o n d i t i o n is s h o w n i n a

s i m p l i f i e d m a n n e r .

DISTINCTION BETWEEN TECTONIC STRESS AND GRAVITATIONAL FOR CE

B e c a u s e t h e s t re s s in t h e h o r i z o n t a l p l a n e d u e t o g r a v it y a lo n e i s e x p e c t e d

t o b e th e s a m e in m a g n i t u d e u n d e r n o r m a l c o n d i t i o n s , t h e e f f e c t o f g r a v it y

w o u l d u s u a ll y n o t b e p a r t o f t h e p r e s e n t p r o b l e m . U n d e r c e r t a in c o n d i t i o n s

w h e r e v o l c a n o e s l i e o n a t i l t e d s u r f a c e , h o w e v e r , a d i f f e r e n t i a l s t r e s s i n t h e

h o r i z o n t a l p l a n e c o u l d b e p r o d u c e d b y g r a v i t at io n a l f o r c e . T h e r e f o r e , r e g i o n a l

s tr e ss i n d e p e n d e n t o f l oc a l m a g m a p r e s s u r e c o u l d b e t h e d i r e c t e f f e c t o f g ra vi -

t a t io n a l f o r c e a s w e l l a s t e c t o n i c f o r c e . I n d e e d , F i s k e a n d J a c k s o n 1 9 7 2 )

s h o w e d t h a t t h e s tr e ss c a u s e d b y t o p o g r a p h i c r e s p o n s e t o g r a v i ta t io n a l f o r c e

c o n t r o l l e d t h e r a d ia l d ik e p a t t e r n o f H a w a i ia n v o l ca n o e s . T h e y d e m o n s t r a t e d

t h a t w h e n a s h i e ld v o l c a n o g r o w s o n t h e s l o p e o f a n a d j a c e n t s h i e ld , g r a v i ty

p r o d u c e s a t e n s i o n a l s t r e s s i n t h e o v e r l y i n g v o l c a n i c s t r u c t u r e i n t h e d i r e c t i o n

o f t h e i n c l i n a t io n o f t h e b a s a l s u rf a c e . T h i s t e n s i o n a l s t r e ss c o n t r o l s t h e t r e n d

o f r a d i a l d i k e s o r r i f t z o n e s .

I f t h e r e g i o n a l s t re s s t h a t c a u s e d d e v i a t i o n t o t h e r a d i a l i ty o f d i k e s is e x -

e r t e d m a i n l y b y g r a v i t a t i o n a l f o r c e , a s i t i s i n c l u s t e r e d H a w a i i a n s h i e ld s , t h e

f o l l o w i n g t w o f e a tu r e s m a y b e e x p e c t e d . F ir st , th e t r e n d o f t h e w h o l e z o n e o f

f l a n k e r u p t i o n s c o u l d b e c u r v e d f o l l o w i n g t h e u n d e r l y i n g r e l ie f . T h is is t h e c a s e

f o r H a w a i i a n c l u s t e r e d v o l c a n o e s , b u t n o t f o r e x a m p l e s s h o w n i n F i g . 2 , i n

w h i c h t h e z o n e s o f f l a n k - e r u p t i o n s it es a re m o r e o r le ss l in e ar . S e c o n d l y , w h e n

t h e t r e n d o f t h e z o n e o f f l a n k e r u p t i o n s is l in e a r, i t c o u l d b e v a r ia b l e w i t h o u t

a n y c o m m o n t r e n d i n n e ig h b o r i n g v o l c a n o e s . T h is a ls o is t h e c a s e f o r H a w a i i a n

c l u s t e r e d v o lc a n o e s . T h e r e f o r e , w h e n a v o l c a n o o r a g r o u p o f v o l c a n o e s d o e s

n o t b e a r t h e a b o v e t w o f e a t u r e s , a n i n t e rp r e t a t i o n , o t h e r t h a n t h e g r a v i ta t io n a l

e f f e c t , s h o u l d b e s o u g h t f o r t h e d i s t r i b u t i o n o f t h e f l a n k - e r u p t i o n s it e.

I f t h e s it es o f f la n k e r u p t i o n s f o r m a l in e a r z o n e p a s s in g th r o u g h t h e s u m m i t

a n d t h e t r e n d s o f t h e z o n e s a re c o m m o n o r c h a ng e g r ad u a ll y a m o n g a d j a c e n t

v o l c a n o e s , th e n t h e f u n d a m e n t a l o r ig i n o f t h e s t r es s w h i c h g o v e r n s t h e t r e n d

o f t h e z o n e s w o u l d m o s t p r o b a b l y b e a s c r ib e d t o a t e c t o n i c s tr e ss t h a t d o m i -

n a t e s t h e e n t i r e a r e a. V o l c a n o e s o f t h e s o u t h e r n p a r t o f t h e c e n t ra l C h i l e a n

A n d e s F i g .4 ) , t h o s e in J a p a n F i g .5 ) , C e n tr a l A m e r ic a , a n d t h e A l e u t i a n s m a y

b e e x a m p l e s . I n C e n t ra l A m e r i c a a n d i n t h e A l e u t ia n s , m o r e t h a n f i f t e e n c o m -

p o s i t e v o l c a n o e s , d i s t ri b u t e d i n v o l c a n ic b e l ts e x t e n d i n g s o m e 1 1 0 0 k m a n d

2 0 0 0 k m , r e s p e c ti v e l y , h a v e m o r e o r le ss l in e a r f l a n k v o l c a n o z o n e s t r e n d i n g

a p p r o x i m a t e l y i n a n o r t h - s o u t h d i r e c t i o n i n C e n t r a l A m e r i c a W i l li am s e t a l. ,

1 9 6 4 ; M c B i r n e y a n d W i ll ia m s , 1 9 6 5 ; U i , 1 9 7 2 a ; S t o i b e r a n d C a r t , 1 9 7 3 ) a n d


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F ig .3 . D i a g r a m s h o w i n g t h e r a d i a l d i k e s a n d f l a n k v o l c a n o e s o f a p o l y g e n e t i c v o l c a n o u n d e r

a d i ff e r e n t i al h o r i z o n t a l s t re s s. T h e z o n e o f f l a n k v o l c a n o e s c o r r e s p o n d s t o t h e t r e n d o f

r a di al d i k e c o n c e n t r at i o n ; h e n c e t o t h e t r e n d o f m a x i m u m c o m p r e s s i o n o f t h e h o r i z o n ta l

c o m p o n e n t o f t h e a m b i e n t s t re ss . ( A f t e r N a k a m u r a , 1 9 7 6 . )

i n a n o r t h w e s t d i r e c t io n i n t h e A l e u t i a n s ( N a k a m u r a e t a l . , 1 9 7 7 ) . T h e r e f o r e ,

t h e e a s t - n o r t h e a s t d i r e c t i o n i n C h i le , t h e g e n e r a l w e s t - n o r t h e a s t d i r e c t i o n i n

J a p a n , t h e n o r t h w e s t d i r e c t io n i n t h e A l e u t i a n a r c a n d A l a s k a P e n i n s u l a , a n d

n o r t h - s o u t h d i r e c t i o n i n C e n t r a l A m e r i c a a r e r e g a r d e d a s d i r e c t i o n s o f th e

m a x i m u m h o r i z o n t a l c o m p r e s s i o n o f t h e t e c t o n i c s t r e ss i n th e r e sp e c t i v e

r e g i o n s .

T h e a b o v e c o n c l u s i o n i s f u r t h e r s u p p o r t e d w h e n t h e t r e n d s o f z o n e s o f

f l a n k - e r u p t i o n s i t e s c o i n c i d e w i t h t h e m a x i m u m c o m p r e s s i o n a l a x i s o f t e c t o n i c

s t re s s i n t h e s a m e g e n e r a l a r e a a s d e r i v e d f r o m s t u d i e s o f a c t i v e f a u l t s a n d

f o c a l m e c h a n i s m s o l u t i o n s o f v e r y s h a l l o w e a r t h q u a k e s . T h i s is a l s o t h e c a s e

f o r J a p a n ( I c h i k a w a , 1 9 7 1 ; M a t s u d a e t ah , 1 9 7 6 ) .

C O N T R A C T I O N A L O R E X T E N S I O N A L T E C T O N I C S TR E S S F I E L D

D i k e s a r e c h a r a c t e r i s t i c a l l y v e r t i c a l i n c r o s s - s e c t i o n a n d d e v e l o p m o r e o r

le ss p a r a l le l to a p l a n e d e f i n e d b y t h e m a x i m u m a n d t h e i n t e r m e d i a t e p r i n c ip a l

s t re s s ax e s . O n t h e o t h e r h a n d , a t th e e a r t h s s u r f a c e o n e o f t h e p r i n c i p a l s t re s s

a x e s s h o u l d b e v e r t i ca l , s in c e t h e a i r is i n c a p a b l e o f s u s t a i n i n g a s h e a r i n g s t r es s

F ig . 2 . D i s t r ib u t i o n o f f l a n k a n d p o s t - c a l d e r a v o l c a n o e s o n t h e s l o p e a n d i n s id e t h e c a l d e r a

o f c o m p o s i t e v o lc a n o e s . L a r g e r d o t s : s u m m i t o r c e n t r a l c r a t er s . S m a l le r d o t s : e r u p t i o n s i t e s

on t he f l a nk a nd i ns i de t he c a l de ra . B roke n l i ne s : e rup t i on f i s sure s . S t i pp l e d a re a : l a ke .

H e i g h t i n m e t e r s u n l e ss o t h e r w i s e i n d i c a te d . M t . V e n i a m i n o f , A l a s k a ( 5 6 ° 1 0 N , 1 5 9 ° 2 3 W )

( B u r k , 1 9 6 5 ) . M t . G a r e lo i , A l e u t i a n I s la n d s ( 5 1 ° 4 8 N , 1 7 8 ° 4 8 W ) ( C o a t s , 1 9 5 9 ) . C h o k a l

v o l c an o , J a p a n ( 3 9 ° 0 5 N , 1 4 0 ° 0 2 E ) ( U i, 1 9 7 2 b ) . F u j i v o l c an o , J a p a n ( 3 5 ° 2 1 N , 1 3 8 ° 4 4 E )

( T s u y a , 1 9 4 3 ) . H e k o n e v o l c a n o , J a p a n ( 3 5 ° 1 3 N , 1 3 9 ° 0 1 E ) . R u l e d a re a s e r e t h e z o n e s o f

e x p o s e d r a d ia l d ik e s w a r m ( K u n o , 1 9 5 0 ; N a k a m u r a , 1 9 6 9 ) . H e k l a v o l c a n o , I c e l a n d

( 6 4 ° N , 1 9 ° 4 1 W ) ( T h o r a r i n s s o n , 1 9 6 7 ; T h o r a r i n s s o n a n d S i g v al d as o n , 1 9 7 2 ) .


8/16

T o l g u a c a ~

L o n q u im a y ~ ~

2 8 2 2 m

L i a i m a

312~

P ic o s d e

V i ll a r ca / .1 - - - . ._ . .._ .~

anin

3 ~ 7 m

E

O s o rn o ~ ) . . . .

Zc~ O m N

2 0K M

F ig 4 F l a n k c r a t e r d i s t r i b u t i o n o f v o l c a n o e s o f t h e C h i le a n A n d e s b e t w e e n 3 7 5 ° S ~ 4 1 5 ° S

m app ed f ro m ve r t i c a l a e r i a l pho t og raph s , 1 : 40 ,000 i n sca le . Out l i ne s o f i nd i v idua l vo l -

c a n o e s a r e r e p r e s e n t e d b y s n o w - l in e s . S y m b o l s s a m e a s i n F i g .2 , e x c e p t t h a t o u t l i n e s o f

c a l d e r as a r e d r a w n i n s te a d o f s u m m i t c r a t e r s f o r P i c o s d e L l a l li c u p e, Q u e t r u p i l la n a n d L a n i n .

F o r m o s t o f t h e v o l c a n o e s , f l a n k e r u p t i o n s i te s a r e a p p a r e n t l y c o n c e n t r a t e d i n a z o n e

t r e n d i n g N 6 5 ° E t o 6 5 ° W ± 1 0 ° . T h i s su g g e s ts t h a t t h e t r e n d i s t h a t o f th e m a x i m u m

h o r i z o n t a l c o m p r e s s i o n o f t h e t e c t o n i c s t re s s.

( H a f n e r , 1 9 5 1 ) . T w o o t h e r a x e s s h o u l d b e h o r i z o n t a l . T h i s c o n d i t i o n a p p e a r s

t o b e m a i n t a i n e d i n t h e u p p e r c r u s t . F o r e x a m p l e , in J a p a n w h e r e t h e s tr e ss

f ie ld is e x p e c t e d t o s u f f e r m u c h t e c t o n i c d i s t u r b a n c e , v e r t i c a l n u l l a x e s o f th e

f o c a l m e c h a n i s m s o l u t i o n s w e r e o b t a i n e d f o r 9 8 .2 % ( 3 3 1 o u t o f 3 3 7 ) o f t h e

e a r t h q u a k e s t h a t o c c u r r e d a t o r l e ss t h a n 2 0 k m f o r t h e p e r i o d 1 9 2 6 - - 1 9 6 8

( I c h i k a w a , 1 9 7 1 ) .

A s s u m i n g t h e r e f o r e , t h a t o n e o f t h e p r i n c ip a l a x e s i s v e r t i c a l , t h e n t h e t r e n d

o f t h e z o n e o f f l a n k e r u p t i o n , t h a t is t h e t r e n d o f t h e r a d i a l d i k e c o n c e n t r a t i o n ,

m a y b e e i t h e r t h e d i r e c t i o n o f t h e m a x i m u m c o m p r e s s i o n a l o r i n t e r m e d i a t e


9/16

4-

÷ ~

+ 4-

o g o ~ \ +

z z

o

4 - / / + o

/

\ \ I

t

Fig . 5 . Poss ib l e tr e nd of c o nc e n t ra t i on o f r a d i a l d i ke s o f J a pa n e se vo l c a noe s . B a r on a c irc l e

s h o w s t r e n d o f t h e l o n g e r a x i s o f t h e d i s t r ib u t i o n o f m o n o g e n e t i c v o l c a n o e s , p o s t - c a l d e r a

c o n e s a n d i n d e p e n d e n t c l u s t er s o f m o n o g e n e t i c v o l c a n o e s . S m a l l er c ir c le s : Q u a t e r n a r y p o l y -

g e n e t ic v o l c a n o e s I s s h ik i et a l. , 1 9 6 8 ) , w h i c h h a v e n e i t h e r m o r e t h a n a f e w f l a n k v o l c a n o e s

n o r s h o w l in e a r o r e lo n g a t e p a t t e r n s i n t h e d i s t r i b u t i o n o f m o n o g e n e t i c v o l c a n o e s . B r o k e n

l in e : t r e n c h ax i s. N a k a m u r a , 1 9 7 5 . )

p r i n c ip a l s tr e ss ax e s . T h e s e t w o c a se s w o u l d c o r r e s p o n d t o c o n t r a c t i o n a l a n d

e x t e n s i o n a l t e c t o n i c s , r e s p e c t i v e l y .

A d i s t in c t i o n b e t w e e n t h e t w o c a s e s i s p o s s i b l e f r o m t h e s e n s e o f d i s p la c e -

m e n t o f a c t i v e f a u l t s i n t h e s a m e g e n e r a l a r e a . T h e t e r m a c t i v e f a u l t s is d e f i n e d

h e r e a s t h e f a u l t s t h a t h a v e m o v e d r e p e a t e d l y i n r e c e n t g e o l o g i c t i m e , s o t h a t

e i t h e r t h e y a r e r e c o g n i z e d b y h a v i n g f a u l t s c a r p s o r t h e y h a v e d i sp l a c e d Q u a t e r -

n a r y f o r m a t i o n s . ) I n a re a s o f a c o n t r a c t i o n t e c t o n i c r e g i m e , a c t i v e f au l t s a r e

e i t h e r s t r i k e - s l i p a n d / o r t h r u s t f a u l t s a n d t h e t r e n d o f t h e f l a n k c r a t e r z o n e s

s h o u l d b e o b l i q u e a t a r o u n d 4 5 ° t o t h e s t r i k e - s l ip o r p e r p e n d i c u l a r t o t h e t h r u s t

f a u l ts . T h i s is t h e c a s e fo r m o s t o f J a p a n , a s m e n t i o n e d e a r l ie r . T h e A n d e a n

c a se is a l s o e x p e c t e d t o b e t h e s a m e , c o n s i d e r i n g t h e l o n g n o r t h - s o u t h - t r e n d i n g ,


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10

dextral At acama fau lt and associated conjugate faults in north ern Chile

(St. Amand and Allen, 1960; Okada, 1971), and a large-scale en echelon tectom~

relief that suggest northeast cont racti on of the south ern Andes (Kaizuka, 1975)~

In the extensional area, on the other hand, normal faults with a trend parallel

to the zone of flank craters will develop as, for example, in Iceland.

There is a possibility that the two cases can be distinguished, in other words,

three principal axes of tectonic stress can be determined by a certain simple

geographical relation wit hou t the help of active faults or earth quake mecha-

nism solutions. Composite volcanoes com mon ly define linear or curvilinear

volcanic belts. From the synthesis of available data for Japan (Fig.5), south

central Andes (Fig.4), the Aleutians and Central America, the angular relation

between the trends of the volcanic belt and zones of flank craters appears to

be dif ferent for differ ent tecton ic stress conditions. This relation is shown dia-

grammatically in Fig.6. In a contr action al tectonic area where the max imu m

compressional stress is horizontal, the two trends tend to form an angle, typi-

cally a high angle, with each other (Fig.6B). In an extensional tectonic area

where the maximum compressional stress is vertical, the two tend to be more

or less parallel (Fig.6A).

The above angular relation would be explained as follows. Cont empo rary

volcanic belts are mostly classified into two major categories: (1) volcanic belts

of accreting plate b oundaries, and (2) those associated with the converging

boundaries of lithospheric plates. These correspond to volcanic belts under ex-

tensional and contractional tectonics, respectively. The trends of the ex-

tensional volcanic belts tend to become perpendicular to the separating plate

motions, which exert tensional stress on the volcanic belts. Zones of flank

I B

¢

I

7~41~..~

F i g 6 T h e a n g u l a r r e l a ti o n s h i p b e t w e e n t h e t r e n d s o f a v o l c a n i c b e l t a n d z o n e s o f f l a n k

c r a te r s . A a n d B a re i d e a l i z e d s i t u a t i o n s u n d e r e x t e n s i o n a l a n d c o n t r a c t i o n a l t e c t o n i c s t r e s s

f i el d s . A p a ir o f s o l i d a r r o w s i n d i c a t e s t h e t e n s i o n a l s t r e s s A ) a n d t h e c o m p r e s s i o n a l s t r e ss

B ) e x e r t e d o n t h e v o l c a n i c b e l t . 1 = a n o r m a l f a u l t ; 2 = a f i s su r e o f r e g i o n a l f i s su r e e r u p t i o n ;

3 = a s t r ik e - s li p f a u l t ; 4 = a p o l y g e n e t i c v o l c a n o w i t h a s u m m i t c r a t er d o t ) a n d a l i n e a r

z o n e o f f la n k e r u p t i o n s r u l ed a r ea ) . A f t e r N a k a m u r a , 1 9 7 4 . )


11/16

11

craters also dev elo p perpendicula r to the tensional stress nd thus parallel o

the volcanic belt. Th e volcanic belts in contractional tectonic areas exten d

parallel to the conver ging plate bounda ries ma ki ng an angle with t he direction

of relative convergence. Bec aus e the conve rging direction wo ul d be nearly

parallel to the axis of the m a x i m u m compre ssiona l stress n the volcanic belt

an d becau se the z on e of flank eruptions develops in this compr ession al direc-

tion an angular relationship results be tw ee n the volcanic belt an d zones of

flank craters of individual com po sit e volcanoes.

D I S C U S S I O N

Weak zones vs tecton ic stress

When flank volcanoes on the slopes o f polygenetic volcanoes are concen-

trated in a linear zone, this has been explained trad itional ly as indicative of

the existence of wea k zones . Even if this is true, there still remains a

question of why the weak zones of particular direction served as channelways

for magm a intrusion. Considering th at t he dikes were once filled with liquid

magma, the magma pressure would have been equal to or larger tha n the

ambie nt stress normal to the dikes, regardless of their origin whethe r the y are

new fissures or old ones. Because the tensile strength of the surroun ding rocks,

which must be very small due to high temperature , was effective only at t he

time o f initial opening of the dike space, the a mbien t stress normal to th e

overall trend of the dike would be expected very close to the direction of the

min imu m compressional stress. Thus, I ascribe the linear distribution of mono-

genetic volcanoes to the presence of a regional stress.

Other features suggesting the trend of dike concentration

The c hann elway used by magma would have a range of azimuth al distri-

buti on a roun d the su mmit crater. This is due to the effec t of the mechanical

heterogeneity of the surrounding rocks and possibly the temporal change of

the regional stress. Thus, the actual azimutha l distributio n of flank craters

(i.e. radial dikes) would scatter around the average trend of the maximum

horizonta l compression o f the regional stress. Hence, more t han a few flank

craters are needed to obtain a reliable direction for this stress comp one nt. This

means tha t volcanoes of fluid magma are more suitable for this kind of stud y,

as flank eruptions are more likely to occur whe n the viscosity of the mag ma is

lower.

In this conte xt, a significant feature w ould be the bending o f radial fissures,

an exam ple of which is shown in Fig.2. A t Mt. Gareloi the fissure starts radially

southward from the summit area and bends with increasing distance from the

summit. This new direction which is assumed to be asy mpto tic to the fissure,

would be best explained as the direction of the maxi mum horizontal com-

pression o f the regional stress, as suggested by Coats (1959). This specific


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12

example becomes more convincing, since the zones of flan k craters on adjacent

volcanoes assume similar trends {Nakamura et al., 1977). A similar bend ing

phe no men on is also observed on Etn a volcano. Here, lateral eruptive centers

and eruptive radial fissures (Rittm ann, 1964) b end a nd converge asymptot i-

cally in a NNE-SSW direction. The NNE tren d m ay be the direction o f the

intermediate principal stress axis, but not of the maximum of the regional

stress, if one takes into account the long normal fault that may extend in the

same direction along the northeast coast of Sicily (W. Alvarez, personal

communication, 1976 ).

Cont our lines of the main volcanic edifices themselves may be elongate in

the same direction, provided the volume of the material erupted from the

flank craters is large. All of the volcanoes in Fig.2 possess this feature. This

feature o f polygenetic volcanoes may also be used as an auxiliary means in

ascertaining the direction of radial dike concentration.

Active normal faults some times develop almo st exclusively within th e vol-

canic structure or field, in the form of a parallel swarm. Examples of these are:

Chokai volcano (Fig.2), Buldir volcano, Aleutian Islands (Coates, 1953) and

Cerro Panalvia, southeast Gua tema la {Williams et al., 1964). These kinds o f

normal faults would be best explained either as the surface expression of the

dikes that did not reach the surface, or as the result of a local concentration of

the regional stress. These normal faults could also be used as an auxiliary

method for obtaining the trend of the radial dikes.

V o l c a n o e s u n d e r c o n t r a c t i o n a l v s e x t e n s i o n a l t e c t o n i c s

Certain features of volcanoes are different under d iffere nt tectonic con-

ditions. Using their synthesis of chemical and age data of Cenozoic volcanic

rocks of the western United States, Christiansen and Lipman (1972) and Lip-

man et al. (1972) showed that fun dame nta lly andesitic and fund amen tall y

basaltic volcanisms correlated both in time and space with the contractio nal

and extensional tectonic styles, respectively. As they suggested, this general

correlation between the chemistry of magma and tectonic styles appears to

hold t rue for oth er volcanic regions of the world. This petrographic charac-

teristic may be used as an additional key to the distinction between tectonic

styles, but n ot as a conclusive one. This is because: (1) a physical under-

standing of the correlation has not been fully established, {2) there are

problems concerning the recognition of magma types, and {3) the correlation

applies for a group of volcanoes as a whole and not for individual volcanoes.

Anothe r possible correlation seems to exist between the te cton ic styles and

types of volcanoes, i.e. monog enetic and polygene tic volcanoes. To cite rather

well documented facts, {1) volcanic belts associated with converging plate

boundaries, therefore supposedly under contractional tectonics, are charac-

terized by isolated andesitic po lygenetic volcanoes, especially composi te ones,

and (2) volcanic fields of alkali basalts and bimodal magmatism, which belong

to the category fund ame nta lly basaltic of Christiansen and Lipman (1972),

consist co mmo nly o f clusters of monoge netic volcanoes, occurring mainly in


13/16


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14

stress syst em superposed on th e magmatic pressure. The trends of dikes i.e.

the trend of the flank crater distribution) may concentrate in a direction of

either the max imu m compressional or the inter mediat e stress axis of the

regional stress field, or collectively, in a direction normal to the minimum

compressional axis.

4) Some features other t han flank craters may be used as an auxiliary

means to reveal the trend of the conc entr atio n of radial dikes. These features

are the bending of radial fissures, elongation of the main volcanic edifices and

the swarm of parallel normal faults within the edifices.

5) The regional stress could be considered a tectonic stress when the trends

of dike c once ntrat ion are linear in individual volcanoes and are, at the same

time, common or change gradually among adjacent volcanoes.

6) Whether the trend o f radial dike concen trati on indicates max imu m com-

pressional or i nterme diate stress axis could be dete rmine d by the angular re-

lation between the trends of volcanic belts and of the concentration of dikes,

as well as by th e sense of displacemen t of cont emp oran eou s faults. Character-

istics of the che mistry of magma and t ype of volcanoes in the volcanic belt or

field may be used as auxiliary means for the same discrimination.

ACKNOWLEDGEMENT

I am grateful to the late Professor Hisashi Kuno for his encour agem ent

during the earlier stage of this study. I also thank T. Engleder, N. Isshiki,

K. Jacob, K.. Mogi, S. Thorarinsson and T. Ui for helpf ul suggestions and

discussions at various stages of preparation. The manusc ript was critically

reviewed by T. Engleder, L.R. Sykes, and E. Wellmon. This paper was com-

pleted while I was a recipient of the Senior Postdoctoral Fellowship at

Lamon t~Doh erty Geological Observatory from November 1975 to Septemb er

1976.

Research was supp orted by the Earth Science Section, National Science

Found ation NSF Grant DES 75--03640.

REFERENCES

Burk, C.A., 1965. Geologic map o f Alaska Peninsula. Geol. Soc. Am. Mem., 99, par t 2,

sheet 1.

C h r i s t i an s e n

P~ L. and Lipman, P.W., 1972. Cenozoic volcanism and pla te-tecto nic evol ution

of t h e w e s t e r n United States, II. Late Cenozoic. Philos. Trans. R. Soc. Lond., Ser. A,

271: 217--248.

Coats, R.R. , 1953. Geology of Buldir Island, Aleu tian Islands, Alaska. U.S. Geol. Surv.

Bull., 989-A: 8--9.

Coats, R.R., 1959. Geologic reconnai ssance of Gareloi Island, Aleutia n Islands, Alaska.

U.S. Geol. Surv. Bull., 1028-J: 249--256.

Dawson, J.B., 1967. A review of t h e g e o l o g y of Kimberlite. In: P.J. Wyllie Editor ~ Ultra-

mafic a n d R e l a t e d Rocks. Wiley, New York, N.Y., pp. 241--251.

Earthquake R e s e a r c h In s t i t u t e 1 9 4 1 . R e s u l t s o f r e - t r ia n g u l a ti o n in Miyake-sima. Bull.

Earthq. Res. Inst., 19: 544--547.


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1 5

F i s k e , R .S . a n d J a c k s o n , F ~ D ., 1 9 7 2 . O r i e n t a t i o n a n d g r o w t h o f H a w a i i a n v o l c a n i c r i f t s :

t h e e f f e c t o f r e g i o n a l s t r u c t u r e a n d g r a v i t a t i o n a l s t re s s e s. P r o c . R . S o c . L o n d . , S e r . A ,

3 2 9 : 2 9 9 - - 3 2 6 .

G e o g r a p h i c a l S u r v ey I n s t i t u t e , 1 9 7 6 . C r u s t al d e f o r m a t i o n o f M i y a k e - si m a . R e p . C o o r d .

C o m r ~ P r e d i c t. V o l c an . E r u p t . , 5 : 2 9 - - 3 2 i n J a p a n e s e

G o n z ~ l e z , F . O . , 1 9 7 2 . V i l l a r r i c a v o l c a n i c e r u p t i o n , 1 9 7 1 . I n : 1 9 7 1 A n n u . R e p . C e n t e r f o r

S h o r t -l i v e d P h e n o m e n a , p p . 1 3 4 - - 1 3 8 .

H a f n e r , W . , 1 9 5 1 . S t r e s s d i s t r i b u t i o n a n d f a u l t i n g . B u ll . G e o l . S o c . A m . , 6 2 : 3 7 3 - - 3 9 8 .

H a i m s o n ,

B . C . , 1 9 7 5 .

D e e p i n - s it u s t re s s m e a s u r e m e n t s b y h y d r o f r a c t u r i n g . T e c t o n o p h y s i c s ,

2 9 : 4 1 - - 4 7 .

I c h i k aw a , M . , 1 9 7 1 . R e a n a l y s i s o f m e c h a n i s m o f e a r t h q u a k e s o c c u r r e d i n a n d n e a r J a p a n

a n d s t a t i s t ic a l s t u d i e s o n t h e n o d a l p l a n e s o l u t i o n s o b t a i n e d , 1 9 2 6 - - 1 9 6 8 , G e o p h y s . M a g .,

3 5 : 2 0 7 - - 2 7 4 .

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