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Transcript of Capitol2-Techniques of Structural Geology and Tectonics
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8/9/2019 Capitol2-Techniques of Structural Geology and Tectonics
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CHAPTER
Techniques of Structural
eology
and
Tectonics
Investigations in struct ura l geology require familiarity
with basic techniques of observation and of reporting
and displaying three-dimensional formation. Thus we
use standardized methods for measuring the orienta-
tions of planes and lines in space. Techniques for dis-
tinguishing the relative ages of strata in a sequence of
layered-sediments are crucial to interpreting their sig-
nificance. Various graphical displays have proved most
useful for plot t ing and interpret ing orientat ional or
three-dimensional data. Such displays include geologic
maps and cross sect ions that present da ta in a geographic
framework, as well as histograms and spherical pro-
jections th at portray only orientat io nal data w ithout
regard to geographic locat ion. Geophysical data such
as seismic reflection profiles and gravity measurements
are essent ial to the interpretat ion of many large-scale
structures, and a s tr uctura l geologist mu st understand
how these dat a are obtained an d interpreted in order
to take their l imitat ions into acco unt . Mo st discussions
of s tructural geology and tectonics assume the reader
is familiar with all these techniques.
Proper interpretat ion of geologic s tructures de-
pends critically on the ability to visualize spatial rela-
t ionships am ong various features. Th is abi l ity to think
in three dimensions does not necessarily come easily,
but one can learn i t with pract ice. Learning to think
in
3-D is a m ajor goal of labo ratory courses in s truc-
tural geology.
The Orientation of Structures
Many of the structures observed in outcrops are ap-
proximately planar or l inear features. Planar features
include bedding, fractures, fault planes, dikes, uncon-
formities, and planar preferred orientations of micas.
Linear features include grooves and streaks o n a surface,
intersect ions of tw o pla nar fea tures, and l inear preferred
orientat ions of mineral grains. We can represent features
tha t are not planar, such as folded surfaces and folded
linear features, by measuring a series of tangent planes
or l ines around the s t ructure .
T hu s the at t i tude of a plane or a line-that is, i ts
orien tation in space-is fund ame ntal to the description
of structures. We specify the attitudes both of .planes
and of lines with two angles measured, respectively,
from geographic north and from a horizontal plane.
The attitude of a plane is specified by its strike and its
dip, or by the t rend of the dip l ine and the dip angle.
Th e at t i tude of a l inear feature is given by i ts t rend an d
its plunge.
T he strike is the horizontal angle, measured relat ive
to geographic north, of the horizontal l ine in a given
planar s tructure (Figure 2.1A . This horizontal line is
the strike line, and it is defined by the intersection of a
horizontal plane with the planar s tructure. I t has a
unique orientat ion for any given orientat ion of plane
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Vertical plane
normal to strike
A I P
line
Strike and dip symbol
Figur-e 2.1 Th e stl-ikc al ~ d l11
of a
planar strucrur-c. A. T h e s t r ~ k e f a pInn;lr srruiturc
I S
tlic
azirnurli
o
a
lhoriz.o~it.ll
lne
In
the and
is
defined
b y
the
i n t c r s e c t ~ o ~ ~l
a
I~orizorir~lllane
~v i th he p i , l~ i .~rtruciurc. 7'lic dlp angle
is
rncas~rrrdhet\x~ccn or~zoiital r ~d ile p l a n a r
stl-uctul-e
111 a
vcrt~calplant 11ormal o the str-lke
l in e . T h e
dip dlrcction In this dtagraiil
1s
to thc
NF..
j
-1-hc str~kchowr1 hcrc
IS
nlcasurccl on
a c j l ~ a c i r ~ l n t
ornpnss
a s NSOW
or
o n a
360 compass a s
e ~ t h c r325 or 14.5 . Tlic att l t~ii le S ~ndicatcdon
a
map h y a T-shaped symbol wltli
t l ~ c r cn l
pa~-allclo thc dip di rec tioli . The d ~ pngle is wrlttcn besidc this s y m b o l .
except a l ior i7 .onta l one . T h e dip is t l ic s lop e of a
p l a n e
d e fi n ed b y t h e d i p a n g l e a n d t h e d i p d i r e c t io n . T h e d i p
a n g l e, a l so r e f c r re d t o s ~ r n p l y s t h e d i p , is t l ~ e n g l c
herwccn a hor i zon t a l p l ane and t he p l ana r s t ruc tu re ,
~ n c a s u r c dn
a
ve r ti c a l t ha t i s pe rp end ~cu ln r o t he
s r r - ~ k e~ n c F i gu r e 2 . 1A .
I t
i s the la rgest possible anglc
b c t w c c n t h e h o r i z o ~ l t a lp l ane and t he i nc l i ned p l ane .
For a g iven s t r i ke , pa r t i cu l a r va lue o f t he d lp ang l e
iden ti f ie s two p l anes which s l ope i n oppo s i t e d i r ec t ions .
T o d i s ti n g u is h b e t w e e n t l ~ e r n ,we spec i fy t he approx i -
n i a t e d i p d i r e c ti o n . by g iv i ng t h e q u a d r a n t ( N F , SE,
S W ,
N W ) o r t h e p r i n c i p a l c o m p a s s d i r e c ti o n (N, E S, \V)
o f t h e d o w n - d i p d i r e c t io n .
T h e d i p l in e i s p e r p e n d i c u la r t o t h e s t r i k e l in e a n d
is t he d i r ec t i on o f s t e epes t de scen t on t h c p l ana r s t ruc -
t ~ ~ r c .n so me cases, par t icular ly in Bri ti sh usnge , the
a t t i t ude
of
a p l an e is speci f ied by t h r t r end an d p lunge
(de f ined be low ) o f rhc d ip l i ne. Th e p lunge o f t he d ip
11ne is t h e s am e a s t li e d ip o f t h e p l an e .
F o r t h e a t t i t ~ l d r f a l i ne a r s t ru c t u r e , t h e t r e n d i s
t he s t r ~ k e f the ver t ica l p lane in which the l inear s t ruc-
t u r e I ~ c s ; t is un ique excep t fo r an exac t l y vc rt i ca l l i nea r
s t ruc tu re (F igure 2 . 2 A ) . T h e plunge i s the angle be tw een
the hor i zon t a l p l ane and rhc l i nea r s t ruc t~ l r - c , e a su red
i n t h e v e r t i c a l p l a n e ( F ~ g u r e
. 2 A ) .
T h e d i r e c t i o n o f
p l t r ~ ~ g e ,ike t h e d ~ p lrec t ion, can be speci f ied by the
q u a d r a n t o r b y t h e c o m p a s s d l r ec t io n
of
t h e
dow n-p lung e d i r ec t i on . A l~e rna t i ve ly , t c an be speci fi ed
impl ic i tl y by us ing t h e conven t ion t ha t t he t renc i a lways
l )e l nea su red In t he d owl l -p lun ge d i r ec t i on .
W e gene ra l ly use on e o f
c \ v o
conven t ions t o r ecord
a s t r i k e o r a t r e n d . W e can spec ify t he ang l e a s a b ea r ing
( the ang l e measured be tween 0 a n d 90 eas t o r wes t o f
n o r t h o r , in s o m e ca s es , s o u t h ) o r a s a n a z i m u t h ( t h e
ang l e be tween 0 a n d 360, i nc rea s ing c lockwi se f rom
no r th ) . Meas ure lncn t s d i f fe r ing by 150 h a ve t h e s a m e
o r i cr i ta t io n . T h u s a n o r t h e a s t s t r ik e o r t r e n d c o u l d b e
re lmr t ed a s bea r ings N45E o r S4SW a n d a s a z i m u t h s
045' o r 22SG ach pa i r d i f f e r ing by 180 . Using t he same
c o n v e n t i o ~ i s ,w e c o u l d g i ve a n o r t h w e s t s t r ik e o r t r e n d
as bea r ings
N 4 S W
o r
S4SE
and a s az imuths 13.5 o r
315 . I t i s goo d p rac t i c e a lways t o w r i t e t he az imuth a s
th ree -d ig i t number , u s ing p reced ing ze ros whe re nec -
e s s a r y , t o d i s ti n g u is h i t f r o m d i p o r p l u n g e a n g le s w h i c h
a r e a l w a y s b e t w e e n 0 a n d 90 .
v a r ie t y o f c o n v e n t i o n s a r e in c o m m o n u s e f o r
wr i t i ng t he a t t i t udes o f p lanes and l ine s. W c gene ra l ly
wr i t e t he s t r i ke and d ip i n t l i e o rde r s t r i ke , d ip , d ip
d i r e c t io n , r e g a r d le s s o f w h e t h e r b e ~ i r i n g r a z i m u t h is
used . S t r i ke bea r ings a re a lways r epor t ed r c l a t i ve t o
r~ or t l i ; o d i s ti nc t i on i s made be tween az imuth s d if f e ri ng
hy 1SO'. T h u s
N35W;.55NF, , 325;5SNE,
a n d
1 4 5 ;5 5 N E
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Horizontal plane
Trend
Vertical plane containing
linear structure
Trend and plunge
symbol
q d
3
line
Figure 2.2 The trend and plunge of a linear structure. A The
trend is thc angle between north and the strike of the vertical
p l ~ n c hat contains the I~ nea r tructure. The plunge is the
arlglc between horizontal and the linear structrlre. measured
In
the vert~cal lane that
contains
the structure.
B
Ttic trend
shown here is given
as
N 2 0 W
on
a quadrant compass or
a s
either 340 or 160 on a 360 compass. I f by convention the
trend is rccorded in the dowri-I~lungeirection, thcn it can be
given only hy S20E or 160 . O n a map, linear features are
plotted
a s a n
arrow parallel to the trend and pointing in the
dowii-plunge direction. Th e plunge anglc is written beside this
.?Trow.
a ll s p ec if y t h e s a m e a t t i t u d e o f p l a n e ( F l g ur c 2 . 1 8 ) . T h e
d i p d i r ec t io n is a l w a y s i n a q u a d r a n t a d j a c e n t t o c h at
o f t he s t r i ke .
T h e m o s t c o n v e n i e n t c o n v e n t io n f o r th e a t t i t u d e s
of l i ne s is t o me asur e t he t r end
i l l
t h e d o w n - p l u n g e
d i rec t io t i , i n wh ich ca se t ha t d i r ec t i on need no t he s t a t ed
exp l ic i tl y . W e f i rs t wr i t e t he p lunge a nd t hcn t he t r end
( the r ever se o rde r i s a l so used) so t ha t t h e measure rne t i t s
40 ;S50W and 40 ;230 bo th r e fe r t o t he sam e a t t i t ude
(Figure 2 .2U).
W e p l o t t h e a t t i t u d e o f a p l a n e o n a m a p b y d r a f t i n g
a l ine parallcl LO s t r i ke wi th a shor t ba r i nd i ca t i ng t he
d o w n - d i p d i r e c t io n . If t h e t r e n d a n d p l u n ge o f t h e d i p
111lc
\
I T I C , I S I I I - C C ~ ,
c
p lo t t hc a t t i t ude wi r l i a l l a r row
p o t ~ i [ ~ t ~ gn t h e d ow n - cl il , d ~ r c c r i o n F ~ g u r e l N . T h e
a t t i t ud e of a l i ne is i nd i ca ted hy an a r row po in t i ng i n
t he d o \ v n - p I ~ ~ ~ i g ei rcc t l on (Fig urc 2 .213; the arr ow S ~ I T I -
I )ol s usec l t o i nd i ca t e t he a t t i t ude o f a l i ne and o f t he
d ip li ne o f p l ane shou ld be d i f f e ren t) .
In
al l cases, the
va lue o f t hc d ip o r p lunge , a s appropr i a t e , i s wr i t t e l l
b e si d e t h e s y m b o l s o t h a t t h e a t t i t u d e c,ln be seen a t a
g l ance .
Geologic
Maps
C k o lo g ic m a p s a r e t h e ba si s o f all s t ~ ~ d i c sf s t ruc tu re
a n d tectonics
T h e y a re t ~ v o - d i ~ n e l ~ s i o n dcpre sen t a -
t ions of an a rea of th e I: .a rt li 's surface ot i \vhich a re
] ,l o tt ed a va r i e ty o f da t a o f geo log i c i n t ere s t . These da t a
a r e b a se d o n o b s e r v a t i o n s f r o m I n an y o u t c r o p s a n d o n
the j ud i cious i n fe rence o f r e l a t i onsh ips t ha t a rc no t d i -
r ec t l y o l~se rvah l e .T h e d a t a p l o tt e d m a y i n c lu d e t h c
d i s t r i b u c ~ o n f t h e d i f f e r e n t r o c k t y p e s, t h e l o c a t io n a n d
n a t u r e of t he con t ac t s be tween t he rock t ypcs , and t h e
l oc at io n a n d a t t i t ~ ~ d ef s t ruc tu ra l f e a tu re s .
C o n t a c t s z r e l in e s o n t h e r o p o g r a p h ~ c u r f ac e o f
r h e E a r t h w h e r e t h e b o u n d a r y s u r f a c e be t w e e n t w o d i f -
f e ren t rock t ypes i n t e r sec t s t he t opograp l i y . Con tac t s
c Jn be s t r a t i g raph i c , whe re one un i t l i e s depos i t i ona l l y
u p o n
ano the r ; t c c ton i c , whe re t he un i t s a re f au l t ed
a g a in s t o n e a n o t h e r ; o r i n t r u s i v e , w h e r e o n e u n it i n v a d e s
a n o t h e r . On a geo log i c ma p , d i f f e ren t t ypes of co t l t a c t s
and the re l iabi l i ty of the conLact loca t ion are indica ted
13 different s ty les of l ines. . l - iie sh ap e of t h e co n t a c t o n
t h e m a p d e p e n d s b o t h o n t h e g e o m e t r y o f t h e c o n t a c t
s u r f a c e ( f o r e x ; ~ m p l e , h e t h e r i t is p l a n a r o r f o l d ed a n d
w h a t i t s a t t i t u d e i s ) a n d o n t h e t o p o g r a p h y t h a t t h e
con t ac t su r face i n t e rsec t s . An expe r i enced obse rve r c an
de t e rmine t he geom e t ry o f s t ruc tu re s in a n a rea , a s we l l
a s t he qua l i t y o f i n f o rm a t ion ava i l ab l e , s imply by ca re fu l
i nspec t i on o f a good geo log i c map .
Al l geo log i c map s a r e sma l l c r t han t he a rea t hey
repse sen t . Exac t l y how much sma l l c r i s r ep re sen t ed by
the sca l e o f t he map , wh ich i s t he r a t i o o f t he d i s t ance
o n t h e m a p t o t h e eq u i v a l e n t d i s ta n c e o n t h e g r o u n d .
A s c a le o f 1 : 2 5 , 0 0 0 ( o n e t o t w e nt y -f iv e t h o u s a n d ) , f o r
e x a m p l e , i n d ic a te s t h a t 1 u n i t o f d i s t a n c e o n t h e m a p
( s u c h a s a c e n t i m e t e r o r a n i n c h ) r e p r e se n t s a h o r i z o n t a l
d i s t a n c e o f 2 5 , 0 0 0 o f t h e s a m e u n i t o n t h e g r o u n d .
Because t he sca l e i s a r a t i o , i t app l i e s t o any de s i r ed
nit o f m e a s u r e m e n t . T h e s ca le s of m o s t m a p s u s e d i n
s t r u c t u r a l a n d t c c t o n i c w o r k r a n g e b e t w e e n 1 1 0 0 0 a n d
1 100 ,000, t hough o th e r sca l es a r e a l so used . In pa r t i c -
u l a r , maps o f
l a r g e r e g l o n s s u c h a s s t ~ t e s ,
rovinces
c o u n t r l c s , a n d c o n t i n e n t s a r e published at sca les be-
t w e e n 1 500 ,000 and 1 20,000,000.
' I 'echniqves
of
St ru c t u ra l G eo l o g y
a r ~ d
c c t o n ~ c s
3
-
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IS
1 1 1 ~ c . l \ ( O f
'1
1 1 l : l ~
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t h a t i o n l n i o r i l v oc i i l r u \ . c r 1.31 -cc
distances
ca n sl io\s.n Continental
c l r ; i r l y on l y w i t h ve r r i c a i exagge r c l t i on . A ; a r e s u l t , ve r -
t ic ;l ll y c x a g g c r a t e d c r o s s s e c ti o ri s a r c s t a n d a r d i n r n a r i n c
g e o l o g y a n d s t r a t i g r a p h y .
T h e h a h i t ~ 1 3 1 e o f v e r ti c a l e x n g g c r a t i o n t o p o r t r a y
c e r t a i n
f e a t t ~ r c s , o w e v e r , g i v es a f a ls e impression of
t h e n a t u r e
c f
t h os e f c a t ~ ~ r c s .i g u r e
2.3 ,
f o r e x a ~ n p l e ,
s h o w s c r o s s s e c t io n s o f a h ~ i ll c iu c t io n o i ie a n d a n a d -
j a c e n t v o l c a n i c i s l a n d a i - c . l ' h e t r u e s c ; ~ l e e c t i o n is
s hobvn i n F i gu r e 2.3A. N o t c t h a t t h e t o l ~ o g r a p h i c a ri -
a t i o n i s a l m o s t i m l > o s s i h l c o s e e
A
1 3 r o l ~ u l y e r t i c a l l y
e x a g g e r a t e d s c c t i o n i s s l lo \ v n i n F i g u r c 2.311. T h e v e rt ic a l
p c : ar a n c e is n o t a t a l l r e p r c s c n t a t i v e o f a c t u a l s l o p e s .
. f he e f f ec t
of
ve r t i c a l ex agg e r a t i on i s j u st a s d ra l - na t ic
o n t h e d i p of p l a n a r f e a t u r e s ( c o m p a r e t h e a n g l e
ot
t h e
s u h d u c t i n g l i t h o s p li c r i c s l a l,
r ~
i g u r c 2 3A \ v it h t h a t i n
F i g u r e 2.313). P u b l i s h e d i n f o r r n a l c r o s s s e c t i o n s o f t e n
g i ve a n e v e n m o r e c o ~ i f u s i n g ic w by c o n i l ~ i n i n g e r ti -
c a l l) , c x a g g c r a t e d t o p o g r n p f ~ yw i l h n o v e r t i c a l c x a g g c r -
a t i o ~ l x l o x . t h e s u r f ac e , a s s h o w n in F i gu r c
2.3C.
V e r t i c a l e x a g g c r ~ t i o nm a k e s d i p p i n g s t r u c tu r e s J I J -
p c a r m u c h s t e e p e r t h a n t h e y a r e . T h e e ff e ct is m u c l i
s t r onge r -
o n
s h a l l o w l y dipping p l a n c s t h a n o n s t e ep l y
d i p p ~ ~ i gn e s , a n d a t h i g h v a l u e s o f t h e v e r t i c a l e x a g -
g e r a ti o n , t he d i s t i n c t i o ~ ~e t w c c n s h a l l o w a n d s t e c p t r u c
d i p s e f fe c tl \, cl y c i ~ s a p p e a r s .
A n o t h e r e f f ec t
of
v e r ti c a l e x a g g e r a t i o n i s t o c a u s e
h e d s o f th e s a m e t h i c k n r s s b u t d i ff e re n t d i p t o a p p e a r
t o d i f f e r in t h i c k n e s s . I n F i g u r e 2.3B t h e s u t ) d u c t c d l i t h -
os p t i c r i c s l a b ap pe a r s t o be t h i r i ne r w he r e i t i s dil 3p i11g
t h a n \4 .1 ie re i t i s h o r i z o n t a l , a n e ff e ct c a u s e d s i ~ l i p l y y
e ~ a g g c r ~ i ~ i o nf t h e v c rt ic a l d i m e n s i o n .
~h s v c r ti c al e x a g g e r a t io n c a u s e s d i s t o r t i o n s of t h e
g e o m e t r y o f f e a t u r e s t h a t s e ri o u sl y a l t e r t h e w a y t h e y
l o o k . B e c au s e m u c h of s t r u c t u r a l g e o l o g y i n v o l v e s v i s -
u a l iz i n g t h e t r u e s h a p e of f e a t u r e s
in
t h r e e d i m e n s i o n s ,
t h e u s e o f v c r ti c a l e x a g g e r a t i o n w i t h s t r u c t u r a l c r o s s
s e c t i o n s s h o u l d be a v o i d e d w h e n e v e r p o s s i b l e .
I n a r e a s of c o k p l e x s t r u c t u r e w h e r e a s i n g le c r o s s
s e c t i o n c a n n o t b e c o n s t r u c t e d t o r e v e a l th e t r u e a n g u l a r
crust
Volcanic
a r c
I
Trench
e x a g g e r a t i o n n e c e s sa r y t o e m p h a s i z e t h e t o p o g r a p l i y
A
a l s o e x a g g e r a t e s t h e s u r f a c e s lo p e s s o t h a t t h e i r a p -
sections.
A.
True-sc a le cross sect ion of a subdu ct ion zone in
which t l ie horizontal and vert ical scales are equal
(1
rrlm
10 krn .
No tc tha t topogra phic var ia tions are a lmost
irnpcrceptible. R.
A
cross scc tion of the same sub duct ion zone
as in par t A , here shown a t a ver t i ca l exaggera t ion of 5 X
T ha t is , the vcrt ical scale is 5 t imes the hor izonta l sca le . No te
the exaggera t ion of the dip of the subducted slab. C. Schemat ic
cross sec t ion tha t con~bi t i es e r t i ca l exaggera t ion for the to-
pography with no vert ical exaggcrat ion for structures below
the surface. Thi s mixing of vert ical scales precludes the co n-
Oceanic
crust
B
Figure 2.3
R l gh t )
T h e effect of vert ical exaggera t ion on c ross
s t r ~ ~ c t ~ o nf
an
accura tc cross sec t ion.
c
T e c h n i q u e s of
St r ~~c t u r a l
eology a n d
ectonics
1 5
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I -c l , i r~o r i s h ipsf a ll t h c s t r u i t t l r c s , n l l ~ l t i ~ l l cros s s c c t ions
a t c i ~ f t r r c l i t r i e n t a t i o n s m a y b e 1 1 sc d .T \ v o c o n s t r u c t i o n s
a r e r e g u l ar l y
e n ~ pl o ye d -b l oc k d r a g r a ~ n s n d f en ce d ~ a -
g r n ~ ~ i s .l o c k d i a g r a m s s h o w a p e ra p cc t iv e d i a g r a m of
a b l o c k of c r u s t w i t h a p p r o p r i a t e g c o r n ct r i c r e p r c s e n -
r a t io n o f t h e s t r u c t u r e o n t h e t o p , ~ v h i c h s t h e m a p
v i e w , a n d a v i e w o f t h e s ~ d e s , h i c h i n c lu d e s t w o c ro s s
s e c ti o n s g en e r a ll y a t h i gh a n g l e s t o e a c h o t h e r . W e h a v e
a l re a dy u s e d a num be r o f b loc k d t: l gra rns to i l l u s t ra t e
s t r u c t u r a l f e a t u r e s ( s c e F i g u r e s 2.1A a r~c l 2.2A . Al-
t h o u g h t h e s e v i e w s o f t h e s t r u c t u r e o n t h r e e d i f f er e n t
s u r f a c e s e n a b l e ( I S t o v i s l r a l i z c t h e t h r e e - d i m e n s i o n a l
geometry, s e c t i o n s themselves a r e n o t a c c u r a t e r e p r e -
s e n t a t i o n s o f t l i c a n g u l a r r e l a t i o n s h i p s b e c a u s e
o f
t h e
n e c e ss a r y d i s t o r t i o n o f p e r s p e c t i v e . Fence d i a g r a m s r e p -
r e s e n t t h re e - d i rn c n s i on n l s t r u c t u r e s b y s h o w i n g a p e r -
s pe c t ive v i e w
of i n t c r s c c t i n g c r o s s s e c t i o n s . T h e y a r e
d i ff i cu l t t o d r a w a n d r e q u i r e a g r e a t d e a l o f i n f o r r n a t i c ~ n ;
t h u s t h e y a r e r e la t iv e ly r a r e i n t h e p u b l i s h e d s t r u c t u r a l
g e o l o g i c l i t e r a t u r e .
Stratigraphic Sequence Indicators
rnclit,lr.y o r i g ~ l ~ ) ~ l ~ r o c k s ,n ~ o n f o r m i t i e s ,
n t i
a n 1x1-
c icpuidcrl r
knowledge
o f tl ic ~ t r a r i ~ r a p l i i ce q u e n c e .
b f a n y s tr l l ct u r e s f o r m e d d u r i n g o r s h o r tl y a f t e r s e d i-
~ n c n t a t i o n a r e u s e f u l i n d e t e r m i n i n g r e l a ti v e s t r a t i -
g r a p h ic a ge . T h e
m o s t
c o m m o n o f t h e s e st r u c tu r e s a r e
h o t t o m m a r k i n g s, g r a d e d b e d d i ng , c r o s s b e d d i ng , a n d
s c our -a nd - f i l o r c t1a nnc l s t ruc tu re s .
I h t t om m a r k i n g s a r e f e a t u r e s f o r m e d m a i n l y i n
a s s o ci n t i on \ vi tl i t u r b i d i t y c u r r e n t s a n d p r e s c r \ ~ c d
o n
t h e u n d e r s i d e o f n i a n y s a n d s t o n e b e d s in i n r e r l a y e r c ~ l
s a n d s t o n e - s h a l e s e q u e n c e s . T h e y r e p r e s e n t ca s t s o f t h e
s r na l l- s ca l e s u r f a c e t o p o g r a p h y i m p o s e d o n n ~ u c l y a n
o v e r l y i n g s a n d l a y e r , o r u p o n w h i c h t li e sa11 d l a y e r w a s
d e p o s it e d . B o t t o m m a r k i n g s i n c l u d e fl ut e c a st s a n d l o ~ c i
c a s t s . F lu t e c a s t s f o r m f r o m t h e d e p o s i t i o n
of
s a n d i n
s p o o n - s h a p e d d c p r c s s io n s s c o u r e d o u t o f t h e u n d e r ly i n g
m u d h y l ii g li - ve l oc i ty c u r r e n t s . S u b s e q u e n t l i t h i f i c a t ~ o n
ot t l ic sand pl-csc t -vcs a cct5t o f t he c lepress ion o n t i le
i , o ~ t o n ~f t h e s a r i d s t o n e layer (Figur-e 2.5A, U . Flu rc
c a s t s t e n d t o h e i n , ~ r k e d I ya s y m m e t r i c in l o n g i t u d i n a l
C I -os s e c t lo l l ; t he y in d ic a t e th e d i re c t ion in wh ic h t li e
C r u c ~ a l o t l i e
interpretation
of a ~ r r a t ~ g r a p h ~ ce q u c n c c
o f d e fc , rm e d r o c k s is d e t e r m i n a t i o ~ i
f
t he re l a trvc a gc s
o f the d i f fe re n t l a ye rs- tha t i s, t he " s t ra t ig r a ph ic LIP
o r " y o u n g i n g " d i r e c t i o n in t h e s e q u e n c e . C o n s i d e r , f o r
e x a m p l e , t h e s c h e m a t i c c r o s s s r c r l o n s In F i g u r e
2.4
w h i c h i l l u s t r a t e a s r r i c s o f l a y e r e d r o c k s f o l d e d i n t w o
d i ff e re n t g c o ~ n e r r i e s .W i t h o u t k n o w l e d ge
of
t l ie s t ra t i -
g r a p h ~ c p d ~ r e c t i o n s , h e s i m p le s t i n r c r p r c t ; it ~ o nw e
c ou ld m a k e o f t he 11111 ite d xp os u re in F igu re 2.413 w o u l d
be th a t sh o w n in 1;igui-e 2 .4A, w l i ~ c h s i n c o rr e c t .
F e a t u r e s t h a t a r e us e fu l i n t h e d e t e r m i n a t i o n o f
r e l a t i v e a g e i n c l u d e s o m e p r i m a r y s t r u c t u r e s i n s e d i -
Figure
2.4
Cross sections showing "stratigr:~phic
u p "
direc-
tions. A . A fold wirh beds r ight side L IP , s indicated by tllc
a r row . Th e s r ruc ru re
is
simple, as shown schem3tically to the
right. 11 A fold with upside-down beds, as indicated hy the
arrow. The s t ruc ture n ius t bc more complex, a s shown to
right.
C ur re nt d~ re c t ~on
____
Sand
B.
Figure 2.5
Flu te
casts . A. Flute casts on the bottom of a
sarldsr onc I>cd in a turbidite. B. Cross section o f flute casts .
shown right s idc u
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cur-rer lt w a\ m ovir lg a t the t in ic the scours w ere cu t
(Fig urc 2.513).
Load ca s t s a rc sa t i ds tone ca s t s
of
depressions in
und er lyi ng r i iud t ha t for111a s t h e d e n s e r s a n d s i n k s i n t o
t li e s o f t, w a t e r -s a t u r a te d r i i ~ ~ d .h e y a p pe a r a s b ~ ~ l g e s
o n t h e b o t t o m s o f s a n d s t o n e l a y er s a n d , in s o m e c a s es ,
can ac tua l l y hecom e i so ls t ed ba ll s o f sand i n a mud
m a t r i x . f t he n iud fo rms sha rp peaks po in t i ng ~~~~~~~
i n to t h e sand , a r id t li e sand fo rn l s ro und ed r i ia sses p ro -
t r u d i n g d o w n i n t o t h e
t i l ~ ~ d ,
h e s t r u i t u r c s a r c d e s c r ib e d
as f l ame s t ruc tu rc s wh ich r e su l t f rom the down-s lope
shea r ing o f t he s ed imen t s . Th e d i r -cc t iona l it y of t he se
fea t~ l re s e rves a s a n i nd i ca to r o f t h e r e l a t ive age o f t he
sed imen t l aye r s .
G r a d e d b e d d i ~ i gn sed imen t s o r sed ir i ie i it a ry rocks
i s cha rac t c r i xd by t he g radua l change i n g ra in s i z e and
mine ra logy w i th in a s i ng le bed , usua ll y f ro rn coa r se and
c l ay -poor a t t he bo t t om to f i ne and c l ay - r i ch a t t he t op
( see F igure 2 .6 ) . Gra d ing oc curs i n a susp ens ion o f
u n -
so r t ed sed imen t because t he coa r se s t f rac t i on se t tl e s ou t
fa s t e r t han t he f ine st f r ac t i on . Thu s t he o lde s t pa r t o f
the layer is the
coarsest,
and the youngest i s the f i r iest .
I n
s o r u e c a s e s , I i o w e v e r , m c i a r t ~ o r p h i s r ~ ia n a c t ~ l n l l y
reverse the direction of grading. Because t l ie finesr fr-.ic-
t i on ha s a h ighe r su r face a rea and i s t he re fo re ~r lo rc
cher ll ica l ly reac t ive , tha t par t of t he layer- may g ro w
coa rse r c rys t a l s du r ing t he rne ta rnorp l ii sm than t he o r ig -
i na ll y coa r se r p a r t o i the Inycr .
Cro ss bedd ing cons i s t s o f t h in beds t ha t occu r a t
ang l e s o f a s much a s 20 t o 30 t o t h e p r in c i p al b e d d i n g
p l anes . These beds r e su l t f rom depos i t i on o f ma te r i a l
Bourna 1962)
i i ;p,: Divisions
Figure 2.6 Idcal graded structure of
a
t u rb~d i re ccl, showing
grading, ripples, and upper mud-rich layer (pelite).
Figurc 2 7
Sketch illustrating a cross section of channel or
scour-a~ id-fill tructure. Th e depression was eroded in the
lower unit (siitsronc) and first conglorneratc, and then s a~ ld
deposited o n top.
i n r i pp l e s o r dune s . Chnrac t e r i s t i c al l y , t he c ross beds
a r e c o n c a v e u p w a r d , a n d t h e y a r e t a n g e n t t o t he l o w e r
m a j o r b e d d in g s u r f a c e . W h e r e s u b s e q u e n t e r o s i on h a s
re rnoved t l ie t op o f t he be d , t he c rossbeds a rc t runca t ed
aga ins t t he uppe r ma j o r bedd ing su r face . In t h i s c a se ,
t hey a rc usefu l a s i nd i ca to r s o f t he s t r a t i g raph i c uo
d i rec t i on .
Ch annc l s t ruc tu res, or scou r-and-f i l l st ruc tLt res, a re
deposi t s tha t f i l l in s t rear i l or current channels cut by
e r os io n i nt o u n d e r l y i r ~ ~e d i ~ n e n t .M os t channe l depos i t s
a re cong lomera t e s , bu t f r l er -g ra ined sed in~ en t s r e a l so
foun d . Thes e s t ruc tu rc s can be i dent i fi ed by t he cha r -
ac ter i s t ic r runca t ion of layers of the under lyin g sediment
aga ins t t he s i de o f t he chan ne l (F igure
2.7 .
Ident i fying
which l aye r ha s been e rode d , and which subsequen t ly
deposited,
m a k e s i t p o s s i b l e t o d e t e r m i n e t h e s t r a t i -
g r a p h i c u p d i r e c ti o n .
Pri??zur ~fructu?-csn 1g1zeotc.sRocks
St ruc tu re s t ha t i nd i ca t e t he s t r a t i g raph i c up d i r ec t ion i n
i g ne o us r o ck s a r e m o r e a b u ~ i d a n t n ext rusive than in
in t rus ive rocks , bu t t hey a r e no t so comnlon a s in sed -
imen ta ry rocks . Te l l t a l e f ea tu re s i n ex t rus ive rocks i n -
c lude f loiv- top brecc ia , p i l low lava st ru c ture s, and f i lled
cavi t ies .
F low- top b recc i a deve lops a t t he uppe r su r face o f
basa l t f l ows , t he reby m ak ing i t poss ib l e t o i dent i fy t he
or ig ina l t op o f t he l aye r (F igure 2.8) . I t f o r r i l s b y t h c
break up o f a so l id i f ied l aye r du r i ng r enewed m ovem ent
o f t he unde r ly ing l ava .
In
som e case s , t he b recc i a fo rmed
o n t o p c a ll r o ll u n d e r t h e f l o w a s i t a d va n c e s. T h u s
cau t io n i s adv i sab l e in t he use o f t h i s i nd i ca to r .
Many volcanic rocks conta in vesic les-cavi t ies
fo rm ed by so l i d i f ic a t i on a ro un d gas bubb le s (F igure2 .8 ) .
Vcsic les rnay be f i l led by minera ls prec ipa ted f rom so-
lu t i on , in w h ich ca se t hey a re ca l led a rnygdu les ( t he
G r e e k w o r d
rrrnygdalon
r i leans a lmo nd ) . In rar e in-
s t a n c e s , s e d i ~ n e n t r l a t e m a g m a m a y b e d e p o s i t e d it1
the vesic les . Because the cavi t ies f i l l f r o m t h e b o t t o m
up:
pa r t i a ll y f il led amyg du le s an indicator of
Techniques
of
S t r u c t u r a l
Geology
a n d
Tecto~lics
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es~cular F r a g m e n t a l : flow top b r e c c i a
I:ig~ire 2.8 Diagrarn of top and 1)ottorn of suhaerial l a v a flow. Vesicular
l nva
ma): bc found : ~ t
rlie cop
or
a t
rlie horrorti of a flow. Flow-top
breccia
forms
on
top
13 :
solidificarlor~ nd subscclufnr
l)r-(-aku1>
of
rhc top dilrirlg flow.
the stratigi. ,~jil i ic I ~ i r ec t io n , a s d o composite a m y g -
d u l e s i the f i l l ing scquerlce i s known.
Pi l l o \v l avas fo rm dur ing xtrusion of lavas under
Lvnre r
Lava ofte l l i ssues from a cc~nt ra l en t nt t hc t o p
of a rnc,l~~icin d f l o ~ v s o w n w a r d o n t o t h e h o ri z o n ta l
I loor . I )ecausc the hot lava
I S
quenched by the wa te r , ~t
f lows l iy forming r i rbel ike f ingers that have a chi l led
cx tc r io l- cvl li ch i ~ i su l a t c s he l ava ~v i th in . he bo t to m of
the top i s convrx upward In c ross sec t ion rnnny tubes
h n vc n ti is ti nc ti ve l ~ i l l o w - s h a p e d p ~ e r ur . facc an d a
d o w n ~ \ ~ a r - d7oili t ingciisp o n t he l o w er s l ~ r f a c c F i g ~ ~ r e
2 . 3 , r i .~creby ndi i ; l t i i ig th e o r iginal
1117
di rec t ion .
S t r i i i t ~ i r e s h a t r e\s ca l t he o r i g i i ~ ~ lI P d ~ r c c t ~ o ~ ~r e
1~111clie s s c o r l m o n in
rocks t l inrl in volcanic
r o c l i o r - s e d i m e n t s . 111 ra l -c case? , however , plutonic
rocks fo rm 1 y crystal scrtl irig a t rl ic hortom of a maglnrl
c l i a ~ n b c r ,
n d
s rd i rnen ta ry s r l-uc tu rc s Inay deve lop th a t
can he
L I S C ~
o i n f er t h e s t r a t i g r a p h ~ c p direct ion. Size
g r a d i n g
of
grniiis of a single mincra l in a layer can b r
used
a s
a s t ra t i g raph ic up ind ica to r (F igure 2 .1 0 A) . T h e
cx i s t encc of g r ada t ions hecween tw o l aye rs
of
different
mine ra log y , o r phase Inye r ing , is more comm on th an
s i zc g r a d i n g ( F ~ g u r - e.1011 . I t ~ ~ r o t > a h l yoes not resul t
I r o m c rys ta l se t t l i ng , how eve r ,
s o
cann ot be used a s an
i n d i c a t o r o l t h e stratigraphic
LIP
di rec t ion . Scour -and-
fill s t r uc t~ i r e s r e p re se n t i n s om e l aye red igneous rocks .
As in sed imenta ry rocks , t hey ind ica t e t he p re sence o f
cur ren t s dur in g depos i t i on o f chc rocks a n d are a re l iable
ind ica to r o f t he s t r a t i g raph ic up d i rec t ion .
I n
SOITIC a reas , magtnas wi th a s ign i f i can t p ropor -
t i o n o f
suspended
c rys t a l s have in t ruded o r ex t ruded a s
s i ll s o r f l ows . As th e mo l t en ma te r i a l came to re s t , t he
c rys t a l s se tt l ed , fo rm ing c rys t a l accum ula t ions t ow ard
the bu t ton1 o f t h e s il l o r f l ow.
Unconformi t i e s p rov ide a va l~ l ab lemeans of determir l -
ing re la t ive age
i n a
s t ra t i g raph ic sec t ion . I l nconform-
i t i r s may he d i sc o~ l fo rmi t i e s , h i ch a re time gaps wi th in
a s e i l ~ ~ u i c ef l aye rs (F igure 2 .11A) ; angu la r
~ ~ n c o n f o r - m i r i c s ,h i ch a re e l -osiona l su r faces t ha t cu t
I ; igurr 2 9 Pillow Iavas in cross sectlon. Sm nrtviIIe cornp iex,
ac ross o lde r beds a t an ang le and a i-e ove r l a in by pa ra i l e l
Sierra Nevada, Califor~iia. be ds (1 1g~1re 1 1 B ) ; o r 11011co11for1nities. hich ar e con -
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IYigurc2 10 S i x a n d p h a se g r a di n g in I ) l u to t i ic i g t i c o ~ t s o c k s . A . S i z e g r a d i n g
i l l
ol iv inc-c l i no-
1 3 ~ 1 - o x c n c u i n i u l ~ r c ,) ~ t k d s l a n d i l t r a m a f i c c o i n p l c x . o c k e t k n i f c f o r s c a lc . li I l- io tograpll s l i o wi ~lg
r c l> c ,l tc d g r a d e d p l ln s c l a y e r i n g in g a h b r o , V o u r i ii o s o p h ~ o l i t c o m p l e x , n o r t h e r n G r c c c c . T h i b
g r a d a t i o ~ i ~ l il ~ c r a t i o n f p y r o x c n c - r i c t l ( d a r k ) a n d p l a g i o c l a s c - r i c h ( li g h t ) l a y e r s m a y n o r h c ti le
resu l t
of
gravi iy ser r l i ng .
A Disconformitv B Angular unconformity
C onconforini ty
D Folded angu lar unconformi ty
F i g u r e 2.11
T y p e s
o f u n c o n f o r m i r i c s . P a r t s A- : a rc ~ ~ n d z f o r m e d .
I cchr~iques f S t r u c t u r a l
G e o l o g y
; trid lcc toni cs
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~ 1 c - t ~~ c t ~ v c c ~ ic c i i ~ i i c i i t , ~ ~r oc ks a ~ i d ~ ~ i c l c r l y i ~ ~ ~
; I ~C C I L I S
01 rne t :~r l lc )rpl i ic ro ck s (Figure 2 . 1
I C ) .
F i g u r e 2.1 11)
s h o w s I i ow a n a n g u l a r u n c o n f o l - m i r y ri ii gl it a p p e a r w l i e n
f o l d e d i n t o a l a r g c f o l d . N o t e t h e l ac k of p a r a l l e l i s ~ ~ if
t h e b e d s h e l o \ v a n d a t l o v e t h e u n c o n f o r n i i t y .
S o m e u n c o n i o r ~ n i t i e s lisp l:ly f o ss il s o i l h o r i z o n s .
I f o n e I pre s e n t , i t p rov i tl e s a rne , ins of cs ra l ) l i shing the
re l a t ive s t r a t ig ra l> h ic a ge . Su c h a hosi7o11 a l s o ma k e s i t
c ns y to d i s t i ngu i s h hc t \vc c n a s c i l i~ i i e n t . lry n c on fo r mi t y
a i i i l a t e c t o n i c c o n t a c t s i ~ c l i as a faul t ; thes e c.111 be
d ~ t i i c ~ i l to t e ll a p a r t , c s pc c ia l ly i f t h c r o c k s h a v r h ec n
d e f o r m e d . F . ve n i n s o m e m e t a m o r l > l i o s c d r o c k s , f o ss il
s o il s a r e s o m e t i m e s p r e s er v e d a s a t h i n b a n d o f a l u -
m i n u r n -r i c h m e t a m o r p h i c r o c k s s a n d w i c h e d b e t w e e n a
~ n e t a s e c l i m e n t a r y e q u e n c e a n d a n o l d e r m e t a m o r p h i c
M a ~ i y ro g e n i c z o n es i n cl u de b e l ts
of
roc ks c h r l ra c t e r -
~zec lby dc fo rmc c l , but l i t tl e rne t a r no rpho s e d , fo s s il i f e r-
oi ls sedinlents .
I n
t he s e roc ks , i t i s pos s ib l e to de t e rmine
t li c s r r . ~ t i g r a p l i i c e q u e n c e by m e a n s o f h i o s t r a t i g r a p l i i c
; ~ ~ i ; l l ~ s i sf t l ie s rd i rne n t s t he ms e lv e s . In h igh ly de fo r ln c d
a i i d/ o i - m e t a m o r l > h o s c d a r e a s , i t m a y h e n e c e ss a r y t o
i i i l e r s t l - a t i g r a l ~ l i i cc l n t i o n s h i p s f r o m t l i e s t r a t i g r a p h y
i i i
l ess d e f o r m e d o r m e t a m o r p h o s e d a r e a s.
In s o i n c c a s e s t h e r o c k s a r c u n f o s si l if c r o u s, t h e
s t l - a t i g r a p l ~ ys u n k n o w n , a n d t h e r o ck s d o n o t p o ss es s
a ny s t r uc tu re s tha t i r i d i c a t e the re l a t ive age .
In
s u c h
s i t ua t i o ii s , I - a d i o m c t r i c g e d e t e r n i i n a t i o n s n i a y y ie ld t h e
olr ly ;igc inforl -ua t ion av ai la l ) le . Sec i imentary pr ocesses
c i o
[ l o t J cs er r a d ~ o m e t r i c l o c ks ,
so
s e d i m e ~ i t s a n n o t b e
d a t e d d i re c t ly t h i s M a y . M e t a i n o r p h i c o r i g n e o u s e v e n t s
c al l b e d a t e d , a n d t l ie a g e o f f o r m a t i o n o l s t r u c t r ~ r a l r
t e c t o n i c f e a t ~ ~ r e sa n b e e s t a h l i s h c d
i f
r a d i o ~ i l e t r i c a l l y
d a t e d i g n e o us o r m e t a m o r p h i c e v e n t s b r a c k e t t h e t ec -
t o n i c e v e n t i n t i m e .
Graphical Presentation of
Orientation Data
0 ri cn t. lt ro 1i h i st o gr a m 5 ( ~ n, re e k ,
s t o s
rne a ns
"m'lst" o r " w c h , " a n d R l~ l l nc a n? "l ine " ) a re p lo t s o f
o n e p a r t o l h e o r i e n t a t i o n d a t a , s u c h a s s t r ik e a z i m u t h ,
a ga in st t h e f re qu en cy o f or i en t at i on s t h a t a r e f o u ~ ~ d
\ vi tl ii n p a r t i c u l a r o r i e n t a t i o n i n t e r v a l s . T h e f r c q u cr i c y
n i a y h e p l o t t c d a s
a
p e r c en t of a ll o b s c r ~ a t i o n s r a s
t h e n ~ ~ r n l ) e r
f
01,serva t ior is wi th i n each in te rva l . ? - tic
p l o t s c ~ ~ n r a c t e r i s t i c a l l yoris is t of a s e r i e s o f r e c t a ng le s ,
\v Iic r- c t h c \ r l d r h o f t h e r e c t a n g l e r e p r c s e n t s t h e o r i e n -
c nr io rl i i ir c rvn l a nd i t s he igh t r e p re s e n t s t he f re que n c y .
R o s e d i a g r a m s a r c e ss c ii ti a ll y h i s t o g r a m s f o r w h i c h
t h e o r i c ~ i t n t i o n x i s i s t r a n s f o r m e d i n t o a c ir cl e t o g i v e
a tr li e i ~ ~ l g l ~ l : ~ rl o t . T h c i ~ l t e r v a l s
f
a n g l e a r e p l o t t e d
a s p i e - sh a p e d s e g m e n t s o f a c i r c l e i n t h e i r t r u e o r i e n -
t a t i o n , a n d t h e l e n g t h
of
t h e r a d i u s i s p r o p o r t i o n a l t o
t il e f rc cl uc nc y o f t h a t o r i e n t a t i o n . T h e u s e of t h e t r u e
a n g l e c o n v e y s a n i n t u i t i v e se n s e
of
t h e orientation d i s -
t r i b u t i o n . R o s e c ii a gr a rn s a r c u s e d f o r d i s p l ay i n g s u c h
f c a t u r e s a s t h e d i r e c ti o t l o f s e d i m e n t t r a n s p o r t a n d t l ie
s t r -ike o f vc r- t ic a l j o in t s ( s c c , fo r e xa mp le , F igu rc 2.12).
1 5 0 t h I l i s t o g r a m s a n d r o s e d i a g r a m s c a n p r e s e n t
( >l il y o i ic a s p e c t o f t h e a t t i t u d e o f p l a n a r o r l i n ea r f e n -
ll joints
I
O f t e n i t is d e s i r a b l e t o p r e s e n t o r i e n t a t i o n d a t a
in
s u c h
a w a y t h a t t h e d i s t r i b u t i o n o f o r i e n t a t i o n s i s
emphasized
i n d e p e n d e n t l y o f t h e g e o g r a p h i c l o c a ti o n o f t l ie d a t a .
F o r e xa ni pl e, i t m a y b e ~ ~ s e f r ~ lo k n o w w h e t h e r t h e r e
i s a p a t t e r n
of
p r e l c r r c d o r i e n t a t i o n o f b e d s , j o i n t s , o r
l i n ea r f e a t u r e s in a n a r e a , r e g a r d le s s o f h o w t h e o r i -
e r i t a t i o n s v a r y a c r o s s a m a p . T h e t y p es o f d i a g rd i n s
m o s t f r e q u e n t ly u s e d t o p r e s en t s u c h i n f o r m a t i o n a r c
h i s t o g r a m s , r o s c d i a g r a m s , a n d s p h e ri c a l p r o je c t io n s .
20 I N T R O D U C T I O N
s Sinistral faults
Figure
2.12
Rose diagra m of the faults and joints in an area.
Aziinurli intervals are 5 . T he length of the radius from center
in any given segment is proportional to the pcrcentagc of
feat~ii-cs ith
a n
orienta tion in that sector. Th e length of the
longest radius represents
40
pcrccnt of the measurements . Thc
dlngrClrnhas a twofold axis of symmetry; that is , a 180 ro-
tarloti of the diagram is identical to the original diagram.
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ippingplane
Plotting
sphere
Figure
2.13
Plotting orientation data o n a plotting sphere.
Pla n.~r nd linear features arc considered
to
p ss
tlirougli rlie
ccnter-
of
t h e sphere and to intersect its s~~rfaic.hus the
attitude
of a
planr is defined by the great c~ rc lc hat is the
intersection of the Plane with the plotting sphere.
A
horirontal
plane and a dipping plane arc shown . Th e attitude of a line
is defiricd b y two points P and P ) that are the intersections
of the line with thc plotting sphcrc.
tures, such as the s trike or bearing, respectively. In cases
where the di p of the pl anar features is always essentially
vertical or the plunge of linear features is always es-
sentially-horizontal, or some ot her constant angle, this
limitation is of no conseque nce.
f
the dip or plunge of
the feature is an inl por ta~ it ariable in defining its at-
titude, however, the best method of plotting orientation
clata is the spherical projection.
When orientation data are plotted on a spherical
projection, all p l a y s and lines are considercd to pass
through the center of the plotting sphere, and their
attitudes are then defined by their intersections with the
surface of the sphere. For planes, the intersection is a
great circle; for lines, the intersection is a point (Figure
2.13). Using the full sphere is actually redundant; a
hemisphere is all th at is needed. Applications for str uc-
tural geology and te ctonics generally use the hemisphere
below the horizontal plane (the lower hemisphere),
whereas applications in milleralogy usually employ the
upper hemisphere.
I n
both cases, the hemisphere is pro-
jected ont o a flat plane to per mit t he convenlcnr graph-
ical presentation of data.
The re are a variety of rllethods of projecting a
sphere onto a plane, although t wo projections are most
often encountered in s truc tural geology and tectonics.
Th e first may be referred t o as a stereogrnphic projection
o r as an equal- angle projection; the other is a Larnbert
projection or an equal-area projcction. They differ only
i n thc \vay the hemispherc is prvjected on to a plane
:allcd thc imagc plane .
For both types of projections, the image plane f o r
the projection 1s tangetlt to the hcrnisphcre at 7 (Figure
2.14)
and parallcl to the plane containing the edge of
rhc hemisphere. T h e equal-nrlgle projection (Figure
2 . 14A)
is constructed by using the highest point on the
spllere opposite
t h e
image plane, the zenith point
Z
as
the project ion po int . .I-he projection of a point on the
lienlisphere is defined hy constructing a line from the
zenith point through the point on the hemisphere
(1'
o r Q o the image plane (I o r Q ).T h u s ally orientation
of line has a un iq~ ie rojection o n the imagc plane. A
great circle on the hemisphere is projccted by drawing
_
gc p s n c
Figure 2.14 Projection of the lowcr half of the plotting sphere
onto the image plane.
A.
Th e principle of stereographic, or
equal-angle, projection. The zcnith point
Z
is the projection
point. Points and
Q
on the plotting hemisphere are projected
to points
P
and Q on the irnagc plane by a line passing from
Z
thro~igh and
Q,
respectively, to. the image plane.
A
great
circle is dcscribed on the image plane by projcctingeach point
of the grear circle on the plotting hemisphere to the image
plane. El. The principle of Lambert , or equal-area, projection.
I hegoint P is projected to the irnage plane by constructing a
cord of the spherr from T (th e tangent point of the image
lane)
to P and rotating that cord in a vertical plane about T
down to C in the image plane. Thc end of the rotated cord
P
is the projected point . The great circle is projected by rotating
each point of the gredt circle on rhe plotting hemisphere down
to the image plane in a sirnilar manner.
Techn ique s
oi
St ruc tu ra l Geology a nd Tectonics
21
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lin cs tr on i t hc 7criitIl p o1 11 t h ~ o ~ ~ ~ l ill po in t\ ;1101ig
11.l1
great c i rc le . 'The locus
o f
r n t c r s e c t i o ~ ~ sf rllosc 1111:
w i t h t h e i m a g e p l a n e d e l in c s t h c projection which aga rn
i s un ique fo r any g iven o r i en t a t ion o f p l ane .
-The a d v a n ta g e of th is 0 . p ~ f pr oj ec ti on is t h ~ r
ang le s be tween l i nes o n t li c I i en~ i sphe re rc no t d i s to r t ed
by r il e p ro j ec t ion . M orco vc r , c i rc l c s on the hemisphere
rcmain c i rc les or1 the proicct ion, a l though the center ol
t l ~ c i r cl c o n t h e h e m i s p h e r e d o e s n o t p r o j e c t t o t h c
ceii ter- of t l ic c i rc le on th e ima ge plane . All gr eat c i r c l e
arc i l lso arcs of circles o n the irnage pIa11c. ,Arcas tha t
arc ecl~l31on the hemisphe re , howeve r , a rc i n gene r .11
n o t e q u a l o n t h e i m a g e p l a n e .
A n e q u a l - a r e a p r o j e c t i o n i s c o n s t r ~ i c t e dby usirlg
t h e p o i n t o f t a n g e n c y 7' be tween the he rn i sphc rc and
the imag e p l ane a s a ce r it e r o f ro t a t i o r i (F igure 3 14Rj
T h e p r o j ec t io l i of a poin t on the hemisphe re i s de t e r -
mined by cons t ruc t ing a cho rd f ror ii T t o t h e p o i n t
l ' o n the hernisp here arid rota t i ng this chor d in a ver tica l
p l ane abo11t o~ vr i o C' in th e ima ge plane . T h e encl
of t hc c l~ or d in rhc image p l ane de f ines tl ie p ro j ec tion
of
t he po in t .
l'r-elections
of g rea t c i rc l e s a re cons t l -uc t cd ,
in p r ~n c ip l e , y ro t a t r ll g all poin t s on the g rea t c r rc l e
fro111 the hc rn i sphc re do w n t o t h e image p l ane in a
s imi l a r manne r . Th i s p ro j ec t ion has t he advan tage tha t
a n y t w o d ~ f f e r e n t u t e q u a l a r e a s
o
the l icni ispl icrc are
a l s o e q u a l o n t h e i m a g e p l a n e . I t is t li er ef or c ~ ~ s c do
presen t da t a w hen the s t a ti s t i ca l concen t ra t ion o f po in t s
i s impor t an t t o t he i n t e rp re t a t i on , because those con-
ce r i t l - a t i ons a re no t d i s to r t ed
h y
t h e p r o j e c t i o n . T h e
shapes o f a rcas o n t he he rn i sphe re a re no t p re se rved by
t h e p r o j c c t i o ~ i ,h o w e v e r , s o a n g ~ i l a r e lr lt io n s hi p s a r e
distorted
al r l iougl i they can st i l l be determined i f t he
a ng le s ar e i ~ ~ e a s u r e dlong a g rea t c i rc le .
Geophysical Tcchniques
A l t h o u g h m a p p i n g r o c k s t h a t a r e ex p o s e d a t t h e s r ~ r t a c e
p r o v id e s g o o d i n f o r r n a t ~ o n b o u t t h e t h r e e -d i m e n si o n a l
s t ruc tu re nea r t he su r face , i t c an no t r eveal t he s t ruc tu re
of a reas cove red by a l luv ium, deep so i l s, vege ta t ion , o r
wa te r such a s l akes , se' ls , a nd oceans . N or c an sur face
m a p p i n g p r o v i d e i n f o r m a t i o n a b o u t s t r u c tu r e a t g r e at
d e p t h . I n f o r m a t i o n a b o u t t h e s h a p e s
of
m a j o r f a u l t s a t
d e p t h , t h e p re s e n ce o f m a g m a c h a m b e r s a t d e p t h , t h e
loca t ion o f t he c rus t -mant l e bou nda ry , o r t he t h i ckness
a n d n a t u r e o f t h e l i th o s p h e r e a n d t h e l o n e r m a n t l e c a n
com e on ly f ro m the in t e rp re t a t i on o f geophys i ca l mea -
sure rnen t s , and e spec i a lly f rom se i smic , g rav i ty , and
magn e t i c measuremen t s . W e rev iew bri ef ly t he app l i -
ca t ion o f t hese a spec t s o f geophys i c s t o l a rge -sca le s t ruc -
ture and tec tonics because they have becori le essent ia l ,
a n d b e c a us e a s t r u c t l ~ r a l e o l o gi s t m u s t a t l ea st b e a w a r e
22
I N T R O D U C T I O N
01 t l ir t c c l ~ ~ i ~ q ~ ~ c s
~ r i c l
l i c ~ r
I I ~ I I I . ~ ~ I O I I S
A ~ L L ~ ~ I I I ~o5 -
cr.lge
of
tlicsc
topics
ho ~v ev es , ~o i t l d rq i l l rc a r l ea st
a . ;cpar ,l te bo ok , and \vc enco urag e studcri t s to tnkc
a p p r o p r i a t e c o u r s e s in g c o p I i y s ~ c s .
Se i smic waves a re osc i l l a t ions o f e la s t i c de forma t ion
thn t p ropaga te away f ro in a source . Waves f ron i l a rge
sources
SL IC I I
:IS m a j o r e a r t h q u a k e s a n d n u c l e a r e x p l o -
bions c a n be de t ec t ed a ll a ro und t li e wor ld . Sma l l e s -
p los ions a re o f ren used a s sources t o i nves t iga t e s t r uc t ~ i r e
a t a Inore l ocal sca l e . Body waves , which can t r ave l
a n y w h e r e t l ir o u g h a s o li d b o d y , a r e o f t w o k i n d s: com-
press iona l (1 ) w a v es , f o r ~ ' l i i c l i h e p a r t ~ c l emot ion i s
pa ra l l e l t o t he d i rec t ion o f p ropaga t ion , and shea r (S)
waves , fo r which the pa r t i c l e [no t ion i s norma l t o t he
d i rec t ion o f p ro pag a t ion , ( 'The des igna t ions P a n d
S r e fe r to p r im a r y a n d s e c o n d a r y w a v e s , s o n a m e d
because o f t he norm a l sequence o f a r r iva l o f t he waves
as revealed o a s e i s m o g r a m . ) 1) waves t r ave l f a s t e r t han
S
vLlves. [ 'hey are th erefo re the f i rst waves to arr ive a t
a
d e t e c t o r f r o m a s o u r c c a n d t h u s a r e t h e e a s i e r t o
r e co g n rz c a n d m e a s u r e . F o r t h ~ s e a s o n , a n d b e c a u s e
exp los ion s gene ra t e most ly waves , t hey a re t he waves
~ x c d o ~ n i n a n t l ysed t o i nves t iga re t he s t ruc tu re o f t he
F.ar th. .The propagat ion of se ismic waves [nay be dc-
sc r ibed by the se i smic rays , which a re l i ne s eve rywhere
pe rpend icu la r t o t he se i smic wave f ron t s . Three types
o f s ei sm ic s t ~ ~ d i e sr e p ~ r t i c u l a r l y n ~ p o r t a n tn s t ruc tu re
and tec tonics: se ismic refrac t ion, se isrnic ref lec t ion, and
fi rst -mot ion studies.
S e is m ic r e f ra c ti o n s t ~ i d ~ c snvest rgate the st ructure
oi t h e E a r t h h y m e a n s o f th o s e s e ~ s m i c a y s t h a t a r e
t ra r~srn i t t ed h roug h bound a r i e s a t which se i smic ve-
loci ty change s. Th e chan ge in se ismic velocity refrac ts ,
o r bends , t he rays ( see Box 2.1) s o t h a t i n t h e E a r t h ,
wh ere se i smic ve loc ity generally i nc reases wi th dep th ,
t h e r a y p a t h s t e n d t o b e c o n c a v e u p w a r d . T h e t r a v e l
t ime of r ays f rom the source to d i f fe ren t r ece ive rs i s
p lo t t ed aga ins t t he d i s t ances of t he rece ive rs f rom the
source . Such p lo t s m ak e i t poss ib l e t o de t e rmine the ray
ve loci ty i n t he deepes t l aye r t h rough which the ray t r av-
e l s . By measur ing t r ave l t imes fo r r ays t ha t pene t ra t e
to g rea t e r and g rea t e r dep ths , t he i nves t iga to r can de -
t e rmine the ve loc ity s t ruc tu re o f t he Ea r th .
T h e v e l o c i t i e s o f
P
a n d S w a v e s d e p en d o n t h e
d e n s it y a n d t h e e la s ti c c o n s t a n ts o f t h e r o c k . T h u s k n o w -
ing how se i smic
P
and S ve loc i t i e s va ry wi th dep th
p r o v i d e s i n f o r m a t i o n a b o u t t h e d i s t r i b u t i o n of t h e d e n -
si ty and c l r l st ic propert ies in the Earth, and locat ing
where changes in se ismrc veloci ty occur reveals where
t h e r o c k t y p e c h a n g e s .
A l t h o u g h
seismic
re f rac t ion s tud ie s g ive a good
rcconna i ssancc v i ew of t he s t ru c tu re o f a l a rge a rea ,
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eismicRefraction
The tirnc I-eq~iircdor scisrnic rays t o travel directly
from a source to different detection stations distrib-
uted around the source is affcctcd by the par-ticular
paths thc seismic rays take, and these in turn at-c
determined by the structure and the seismic vclocity
o f the material along each path.
If
a scisr-nicray travels
obliqucl~~cross a boundary from a low- to high-
seismic-velocity material, it is refracted a way f rom
t h c nor-ma1 to ) he bounda1-y (Figure
2 .1 .1 .
If the ray
travels h or n h igh- to low-velocity material, it is re-
fracted toward t he normal to the boundary.
Travel-time meaw rcme nts can be interpreted to
reveal t he va riation of scisrnic \kravc velocity with
depth. The principle is illustrated in Figure 2.1.2,
which shows the location of a seismic source and an
array of detec tors. Som e of the ray paths shown stay
within the crust; others travel in part through the
upper mantle. A time--dis tance plot indicates the ar -
rival tiriles of those difrercnt rays at the detectors.
Because t11e se ism ic velocity in th e mantle is higher
than tha t in the cr-u st, niantlc rays rcach dis ta~l tdc-
tcctor-s bcfure crust al r;lys. F or th e layered stru ctur e
shown, thc diffcre ~~cen arrival times at the different
detector-s rellccts thc spe ed of the rays th rough the
deepest laycr along the ray path. Thus the slopes of
the two lines on the time-dista nce plot are the inverse
of the velocities in the cru st an d mantle, respectively.
If a laycr that has a lo wer seismic veiocity occur s
at depth between rocks that have higher seismic vc-
locitics, rays ar c bcnt towar-d the nor- md to the bou nd-
ary upon enter ing the laycr and away h-on1 he no rmal
Normal
to
boundary
sin
i
V
sinr
Se~smic y
Figurc
2.1.1
Refraction of seismic rays. Refraction is
a w a y trom the normal to thc boundary
i f
the ray travels
f r o r n
a
low ro a high-seismic-\,clocity nia ter ~al here Vt
ro z or V, to V, . Refract~on s toward the riornial to
rhe houndar)
i f
the ray travels from a higll- to a low-
se~srnic-velocitym.~terial V 2 to V.3 .
upon leaving (Figure 2.1.1). Seismic rays, thcrcforc,
can never reach their maximum depth in that layer,
and the seismic velocity of that layer-and therefore
its very existence-cannot be detected on a time-
distance plot
A Travel time
o
crustal P waves,
Figure 2.1.2 Illustration of the principle of scisrnic refraction in a two-layer structure. The diagram
shows ray paths for P waves through the structure. The travel-time plot indicates the arrival
tlmcs of the rays at the different detectors.
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b igurc 2.15 S e ~ s m ~ ceflcctioti profiles. Illdividual sclsrnlc records are thc w avy ve rt~ cal ~ nc s
p lot ted a~ dc, : sidc along
tlle
tl~staricc xis. 7 hc vcrt~ cal x l s IS the two-way travel
t1111c.
I caks
In
each record .ire sliadcd black to show up reflcctors
tha t nn
be trnccd
f r o r r i
otlc record
to
t h
next. C;ood ho r~ zo nta l eflcctors are p:lrticularly evid cr~ t clow nhout 1.7 s in the left half of thc
A IJr3mlgrated seismic profile. B. Facitzg
page
M ~gr atcd eismic profile.
t h e t e c h l l ~ q t ~ ea s se ve ra l d ~ s a d v a n t a ~ e s .h e p r e s en c e
o f l o w - v e l o c ~ t ya y e r s c a n n o t b e d e t e ct e d ( B o x 2 .1 ) . D e e p
s tr uc tu re s o r d ~ n a r ~ l ya n b e d e t ec t e d o ti ly a t d ~ s t a n c e s
f r o m t h e s o u r c e t h a t a r e g r e a t e r t h a n t h e d e p t h . T h e
prope r t i e s o f t he Ea r th a re ave raged ove r l a rge d i s t ances ,
so de t a i ls o f s t ruc tu re a re l o s t .
A nd
n o ~ i h o r i z o n t a l r
d i s c o n t ~ n u o u s a y e r s a n d c o r n p le x s t r u c t u r e a r e d i ff ic u lt
o r ~ m p o s s i b l e o r e so l ve .
In se ismic ref lec t ion studies, re f lec t ions of P waves
off i n r c r ~ ~ a loun da r i e s a rc used t o i nves t i ga te t he s truc -
tui-e
of
t he Ea r th . Se ismic s i gna l s a re r ecorded by a s
m: iuy a s sevc ra l hun dred to seve ra l t li ousar ld geoph oncs
a t a t im e . T h e r e s u l t ~ n g a t a a r e a n a l y z e d
by
c o m p u t e r
( B o x e s 2 . 2 a n d 2 . 3 ) , a n d t h e s e i s n i o g r a m s a r e p l o t t ed
sidc by side o t i t h e d ~ s t a n c e x i s ( F i g ur e 2 . 1 5 A ) . .i l lr
24
INTRODUC I ION
i n d tv i d u al p e a k s o n t h e s e i s m o g r a m , o r e v e n ts , r e c o r d
the two- way t r ave l t ime fo r e ach re fl e c ti on , wh ich i s
t he t ime requ i red fo r a wave t o t r ave l f rom a su r face
po in t t o a r e f le c to r an d back t o t he sam e su r face po in t .
Peaks in each record a re shade d b l ack so t ha t s t ro ng
s ig n a ls in a d j a c e n t s e i s m o g r a m s a t t h e s a m e t w o - w a y
t r av c l t l m e s h o w u p a s l in es t h a t i n d i c a t e a c o n t i n u o u s
reflector.
S o p l ~ is t ic a te d c o n i p ~ ~ t e r i z c digi ta l processing of
t he sc i s rn l c r ecords , i nc lud ing t he ve ry impor t an t p ro -
cesses of s t a c k ~ l i ~n d m i g r a t i o n , al l o w c o m p l ex s t r u c -
tu re s to
be resolve t i
(Figure 2 .15B)
a n d
therefore yie ld
a il i n c o n ~ p n r a b l c n la g e o f t h e s u h s u r f a c c s t ru c t u r e . T h e
s t ack ing o f sei smic r ecords is a me tho d o f enhan c ing
the s i gna l - t o -no i se r ,~ r io y add ing t oge the r r e f l ec t ions
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Locatioi~ f observations
I~riieocation of features
Sr~aceocation of features
A
Seismic section
6
igrated seismic section
C
eologic section
Figure
2.16
Diagranls illustrating the effects of migration. A. An ~rnrnigrated eismic section with
n i ~ ~ l t i ~ l entersecting curved reflections.
0
he sarne section as in part A alter migration. The
ambiguities and artifacts of the un~nig ra tcd ection are all rcmoved. C. The corresponding gcologic
srction. The depth scale is different frorn the two-way travel-time scale because seismic velocity
varies with depth.
pattern of colnpression or rarefaction first motions that
radiate out from a sudden slip event on a fault is char-
actcristic of the orientation of the fault and the sense
of slip (Box 2.4 . First-motion studies, therefore, are
used to determine the orientation of, and sense of slip
o n , faults at depth. Regional patterns of first motions
reveal large-scale tectonic [ noti ons of tlie plates. Figure
2.17
show s the radiation patterns th at ar e characturistic
of the three basic types of faulting: normal, thrust, and
strike-slip.
Ana lys i s
o
Gra vity Alzornalies
Gravity measureme nts are perhaps the second most im-
portant geophysical technique (after seis~ni cechniques)
used in str uctur al geology and tectonics.
A
gravity
anomaly (from the,Greek word anomalia, which means
irregular ity o r unevenness ) is th e difference betweer1
a nleasured value of th e acceleration of gravity, to which
certain corrections are applied, and th e reference value
for the particular location. Th e reference value is de-
termined from a n internationally accepted formula that
gives the gravitatio nal field for an elliptically sym metric
E ar th . ~ e c a u s eravity anomalies arise from differences
in the density of rocks, the goal in str ucti~ ral eology
is to relate rhese
differences
in density to structural
features. If n o density contrasts exist, then the structure
can have no effect on the gravitational field, and grav-
i t y
anomalies cannot aid in the interpretation of thar
structure.
The structure at depth i interpreted by nlatching
tlie gravity anom aly profile observed a long a linear triiv-
erse with the anomaly profile calculated from a n as-
sumed ~ no dc l f the structure. Thc model is adjusted
until the model an omal y profile s how s a satisfactory fit
to the observed a ~lo mal y rofile. Although the mode l
can never be unique, it is usually co~lst rain ed y surface
mapping and possibly by seismic data.
I n order to calculate an anomaly, we must correct
t h e m e a s ~ ~ r e dalue to the same reference used for the
stan dard field. All measurements are therefore corrected
to sea level as a common reference level. This altitude
correcri on, thc free-air correction, results in an increase
i11
most larid-based values but leaves surface observa-
Normal Thrust
fault fault
Strike-slip
fault
Figure 2.17 Equal-area projections showing the radiation pat-
tern of coinpression first motions C ) arld rarefaction first
motions R) for the three main types of faults. Thc fault on
wh ic l~ he ear thquak e occurs is assurncd to be at the cerlter
of the plotring sphere. All orientations that plot within a given
sector are the orientations of rays when thep leave the source
that have the indicated first motion. Planes separating the
sectors are nodal planes, one of which niust be the fault plane.
Material on each side of the fault moves toward the compres-
sion sccrors, defining the sense of shear on the fault.
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tackingof
Seisl raic
Records
lir
practice
da ta a1 e gatliei-ed fi.c~ni larg e liiicai.
ai-]-a> f s l i o t po in t s and gcopI lon~%\ .,ac i gco plio nc
I-ccor-CIS arly reflCctiorls fr.i~ni l iflc~.cnt iepths, an d
the stacking is doi ic by con-iputcr o proclucc cnl i r~ncct l
sz isrnograms a t each poili t in t l ie profile. Figu re 2 . 1 5A
I \ a n
cs:umplcs of a stac i\ctl seisn lic profil e, whic h
\ l i o \ i h abunclant hor.izontal rcflccto~-s ~ t shiillow
~ l~ - [ lL l l s .
I:igurc
2 . 2 . 1
i l lusrra tcs the pr inciple in\olvcd in tlie
corliilioll clcpth-point stacking
of
xcisnlic rcco l-(is.
c.splosioiis a1.c dctoriated at shot poirlt S1,
S2
S 3
i r l l i i S4,cilcctiorls rr-om
thc
snnic poinl / on 11or.i-
z o ~ ~ t a l~ ~ h s u r f a c c,oundar.v \ri l l he ~.eccivcd t gco-
p l i o ~ ~ c s;, , G2,
;:.
;lncI G 4 , rcsji)ccti\.cly.T h c s a n i c
is ti 11c foi- all ~ 1 1 1 ~ 1ol-izontal reflectors bclo\v 111c
j~oili t 1
'I'llc
coi-I-csponcl inghot points and gc-oplioncs
.C,
aritl
Gi
1.e i~quidistan t rom the point abo \ ,c
P.
I1 t lic t i -avcl ti rnes a re co rrecte d for the di l kr enc c in
l eng th o l t he ray pa ths , t l ie I - ecords can be added
toget l icr , or stac ked . Tl ie t ime-col-rec ted sigr la ls f rom
t ic r e f l c c t i o ~ ~ st P re in force one ano the r , and the
Shot
points
Geophones
s ig na ls f r o m I - a ~ ~ d o r noisc tend to cancel o u t , t he reby st S S S4
G
G, G, GI
i~ lcre asin g hc sigrla l- to-noise ra t io . The resul t i s an
cnl iancct i se ismo gram s how ing the ref lec t ions as they
\voulcl appear
i f
the sh ot point anci r rce iver
wo.c
h o t l ~
a t p.
T'igure 2.2.1 Tlic j > r ~ l i c ~ ~ I cf stacking of sc~s niic ecords.
l < x ~ ~ i o s i o i ~ src se t oli a r sho t po in~s 1 , C7
S j
and
S4.
Tlie
r.lys
rcflecrcd froln rhc same point
1
on I ~rcflccror
; ~ t
cptli a rc rccc ~ve d, ol- each of tile cxp losi o~is , t gco-
plioiics
G I , G2 G j ,
and C4 rcspccti\,cly Adjusting the
a r r ~ v a l imes of tlicse reflectlor~s or tile d~ffc rcn r engths
of
r a y
p.lth allows the fo~~r-ccords to bc added together,
I
which cllliances the si gr~a l rom the reflection a t P nnd
: I :
canccls out rando m nolsc. -Phc cffcct is a n Increasc in thc
. ,
s i g l l a - t ~ ) - ~ i o ~ ~ eatio.
t i o n s a t s e a u n c h a n g e d .
I f
t h ~ s s the only
correction
a p p l i e d , t h e ca l c ul a te d a n o m a l y i s c ~ l l e d a frcc-a i r
a n o m a l y .
T he Bougue r cor rec t ion i s a l so f requen t ly app l i ed .
I t is assumed tha t betw een sea level and t he a l t i tude of
the
me surement
is a uiiiforrn layer of continental c r u s -
t a l rock tha t r epre sen t s an excess o f n i a ss p i l ed on the
sur face . Th i s a ssum ed excess g rav i t a t i ona l a t t r ac t io n is
the re fore su t l t r ac t ed f rom l and-based measurement s . At
sea , i t i s a ssumed tha t a l l dcp ths o f wa te r r epre sen t a
de fi ci ency o f m ass , because wa te r i s l e ss dense t l i a i~ ock .
Th e a ssume d de f ic iency in g rav i t a t i ona l a t t r ac t ion is
t h e re f o r e a d d e d t o s e a - b a se d m e a s u r e m e n t s . T h e B o u -
gue r anom aly re su l t s f rom app l i ca t ion o f bo th the f ree-
a i r an d L l o ~ ~ g u e sor rec t ions .
T h e
gravi ta t ior ia l effec ts of local topography, h o w -
ever di ffe r measu r , l b ly f rom tho se o f a un i fo rm l aye r .
Th us a re f inement of t he s im ple Bougue r an om aly cal lcci
t h e c o m p l e t e B o u g u e r a n o m a l y r e q u i r es n t e r ra in cor -
rec t ion to acco unt fo r t he l oca l e ffcccs .
T h u s , th e B o u g u e r a n o m a l y c o m p a r e s t h e m a s s of
ex i s t i ng rocks a t dep th to t he mass o f s t anda rd co i i t i -
nen ta l crus t who se e levat ion is a t sea level . Boug uer
~ ln om al i e s r e ge i le ra lly s t rong ly nega t ive ove r a re as o f
high topography, indicat ing that there i s a def ic iency of
mass be low sea l evel comp ared to s t a nda r d con t inen ta l
c rus t . Th ey a re s t rong ly pos i t i ve ove r ocean bas ins , i n -
d i ca t ing tha t t he re is an excess of m ass be low the ocenn
b o t r o i n c o m p a r e d t o s t a n d a r d c o n t i n e ~ i t a l r u st .
T h e a rea unde r a g rav i ty ano ma ly p rof il e p rov ides
a unique measure of the tota l excess or def ic iency of
mass a t dep th , and the shape o f t he p rofi l e cons t ra ins
the poss ib l c d i s t r i bu t ion of t he ano mal ous n i a ss. T he
inter pre ta r ion of mas s dist r ibut ion is not un ique, hob\ '-
e v e r , b r c a u s c a gi ve n a n o m a l y p ro f il e c a n be p r o d ~ ~ c e d
by a wide range of densi ty di fferences an d distributions.
Flgrire
2.1
S A , f o r e x a m p l e , s h o w s t h r e e s y m m e t r ic b o d -
l es o f t he sam e dens i ty , each o f which p rodu ces t he sam e
symmet r i c g rav i ty anomaly . F igure 2.18U, C i l lust ra tes
I1ow the fau l t ing of di fferent densi ty dis t r ibu t ions affec ts
Tcchrlrques
of
St r u c t u r a l
Geology and Tectonics
7
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Altliough horizonla1 or v c l ~t~all o\vly lippir ig 1.cI1c.c.-
1o1.sarc conilrlon in untlcforrrieil vtiirnc~~tary; ~ \ i i i \
(sli;lllo\v part s of Figure
2 .
15 , I I I L I I I
of
tlic s1r.uctul.c
O ii~:~,r.estn str-uct~~ralrid tcctor11(. li\cstigation\ is
a grc;it deal moi.c c~ornp1i.u dcc,1>c.r. art s of Figi~ r-c
2 . 15) .For cxarnplc, I x ~ i\
< ; i l l
s ig~~i i icantrid \*~u.i;il?le
dips and discor ~t i r iuo~ ~\c tI (possiblv tr.ilncatcc1 l ~ y
fault) ;ire common. S11c.11 tr-uc,tirri. gi1.c 1-ixc to d i \ -
tort ions a ~l d i-tifacts r ~ ei s~ ni c rofiles (F1grii.~.
.
IS),
which m ust be coi-rcctcd by mig ratiorl.
LVc shall describ c thc PI- in~ .ipl rf niig~-:~tion
y
~rsing n example
for
which thc. seismic velocity of
the m aterial is coris ta~l t, nti the so urc e and clctector-
arc at th e sam e point
p
(Figuw 2 . 3 . 1 A .
A
pal.ticulai.
rcflcction that apparently plots at I1bclo\v tlic dctcc-
tor could corn? h-oin a boundary that is tangcnt to
in
point on a circular a rc of constant t\vo-way travel
tiinc having I-adius
pP
around
p
Cor cxarnplc, from
I . O n t\Vo adj;tcent rcflcc tion scibmogi arns (Figure
2 . 3 .
I
R ,
a rcflcclion z~ppai.cntlyplots
a
beneath
and at Pz bcnc;~th
12
[ t ~vo uid llcrcfo~-c ppear
that the rc[lcctor had the dip of thc linc PIP2 . 111 fact,
howcvcr, the
reflector
musf bc thc corum on tangcnt
to lht' t\vo constan t-trav cl-tiri~c u-cs of I - ~ C ~ ~ L I SIPI
about pl and p P abo~11 2. Tlir ~rcflcctions mus t
thcrcForc colnc from points I [ ancl 1 ;. T ~ L ~ sn un-
Apparent position
of reflecting polnt
cor I-c~ctccii-or~lcho\ vs cl.l-onc%oi \ oc;itions
arid
clips
for. dil~pi iig .cflcctor.\,n d the. I-clicctionpoints PI a ~ ~ d
I J 2
rnirst Ilc 111igr-ntcdtlu~ig hcir r.e\pccti\.c cori\tnnt-
ti-;t\.cl-tirncL I I . ~ . ~o th e cor rect location s at ar1c1
1 3. TI?' 11igllc1.ile tru c dip, t h e gr~3;atcr lie d is to r~ io r~ .
i'c,~-tii.al.ctlcctor a plot on un~iiigr ;itetl cisinic profilcs
:I\
:in alignmelit of rcflcctio~isla~.irig 45 ip.
1s th e scisrnic sollr-ce and the clctcctor ar c not at
thc s;unle po ir~ t, lic a rc of consta~l t wo-way
travel
t ~ m c ccorncs
a n
cllipsc, a nd it is fui-thcr- distor ted i f
the vclocity is [rot constan t. These a - c co~nplicatioris
that ninst he accourlted for
in
any analysis or a real
scisniic. i.ccor-cl, alth oug h the pr-inciple rcmain s the
sanlc.
Another problem occurs
i f a
reflector is discon-
tinuous. The end of the reflector acts as a defracrion
point wliich tnkcs cnergy from a ny anglc o f inciderrcc
and I-adia tcs it i r ~ ll directions as though the point
\\,ci-c.
a
ncw sour-c:c (point n Figul-c 2.3.2 . 'l'hc
sig~wtls1.ccordcd by the nearby
detector s for
cs-
; ~ r i ~ l ~ l c ,t
171
p2, ancl p3-t1icxn plot along a pal-abolic
ar c on ;in unco srcctcd scisrnic prolilc (the dotte d line
in Figul-c 2.3.2; scc also Figures 2 . 1 5 A , 2.16A , bc-
caus e th e t\vo-way tl.avc1 times for the signal incrcasc
as t l ~ c istance or th e detector From tllc encl of the
r.cflcctor illel-cases. The lo cation for
l
possiblc dif-
Figure 2 3 1 Tile migration of seismic signals corrects the sc~srnic ecords to give the true location
and dip of rrflectorr. In t h ~ s xample, seismic velocity is considered constant, and the shot point
and receiver are hotti located at the same polnt. A . A reflection rece~vcd at appears on the
seismrc record a t a two-way travel time that plots at
P .
In fact, the signal could come from any
rcflcctor, such as
P ,
that is tangent to the seinicircular arc of radius p P around p . That arc is
the locus of constanr rwo-way travel tlme. B. Reflections deiccted at p , and p2 plot vertically
below each point at 1 ; and P z , respectively, giving the reflector- tlic apparcnt dip and location
of
the linc l',Pz. Thc t ~ u r o ~ ~ ~ t i o nnd d ~ pf thc reflector, ho~vrver,must be given by the lint: P l r P z ' ,
which 15 thc corlltnoll tangent to the constant two-way travel-time arcs about pl and p z , respec-
tively. Notc that
PI
is the act~~nlocation of rhr reflector below p 2
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I~.:rcti:)li oinis that could gc nc r a ~lic si;:n:tl r-cc.ol.clcd
at a given rc:cci\ier, however, rnltst lie along ;in 2u.c. of
coilstant t\r,o-\vay l.a\.c*l i n ~ e llout that rcci~ivci. t11c
tl;tslied arcs
i l l
Figure
2 . 3 . 2 .
I'hc true locatioi~
f
t11c
c iffr;li,tion poiiit is th e co mm on intcrscct ion uf t he
arc\ const~~uctei i.01- several dctectol-s. Thus
rnigl-sting
cach signal along its arc to the colilrnon po ii~ t ticn-
tifics the t rue locatior~
f
the diffrac:tion point 11.
111practice, the process of migration consists of
taking cach individual event on a reflection seismo-
gram , migrating it
along
its arc of constant two-way
travel tinic, an d adding t hat event t o arly othcl- seis-
mogram intersected by that arc at the point of inter-
section. The r es~ dti ng eisnlic profile is then
a
series
of'seisniogra~ns,each of which corisists of the original
~-i.('
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+-- ;r;lvlty
i310f
C?
Thrust fa;: t
.
I
A
Figure 2.18 Illustrntiou of tlic . lri lhig~~ity
and the non~~ niqucn css nherent i n tllc 1 I
a n d normal tlisplaccrncnt of a dense base-
Thrust
Incnr ovcria~nby Icss densc str ata . Th e
nsyrnrnctry
of
thc: a t i o m a l y rcflccrc
r l ~ a r
f
thc 11ndc1-ly~ng
~ I . L I C I I I ~ C , ,
t i le d~stinc-
tion among thc thr cc d~lfcrcrirsrructurcs
\ ,~ ln iosr eg l~~ il , lc , . T l ~ cffcct 011 grav-
i t y
. ~ ~ i o ~ i i . l l ~ c sf \ .c~t icnl ,hrust, and nor-
m a l dirplacerncrit of a
cicnsc
laycr within
Normal
less
dcllsc
layers.
Th c rhrcc stru tures pro
e
Gravity effectof a
fau l ted dense
dtccc m a rkadly diffcrcnr gravity .~~ ior na lie s. basement
Thrust
Normal
C ravity effect of a faulted
dense
layer
thc gravi ty ano ma ly profi le . If a low-dc nsi ty laycr ov-
e r li e s t li ick h ighe r -dens i ty l ayc r and t l ~ c t ruc tu re is
f . 1 ~ 1 l t e d F ~ g u r c
.1813),
t he g rav l ty an o~ na ly rof il c is
nsvmnle t r l c , bu t t h e d iHeren tgco rne t r i e s of f au l t i ng havc
only
:I
rlilllor cffcct
o n
t h e ; l n o m a l y s h a p e .
f
tlie dcnscr-
n i n t c r ~ ~ l lS in a relatively t h i n l a y c r ( F i g ~ ~ r e.18C:), t hc
g r av i ty a r l om a ly ~ r o f i l es ag ain a s ym m et ric , t l i o ~ ~ ~ l lhe
s h a p c i s d i f f c r c n t f r o m t h a t i n F i g u r e 2.15l3 nd the
cffcct of di fferent faul t geometry is s ignif icant . Thus
a l t l i o ~ ~ g l ih e a n o m a l y s h a p e i m p o s e s c o n s t r a i n ts o n t h e
possible s t rucr urc , to be re l iable , gra vi ty 111odels should
he based o n a d d ~ t i o n a l t ru c t u r a l a n d g e op l iy s lc a l 111
f o r r n a t ~ o n .
Geomagne t i c S t ud i c s
A mag ne t i c fi eld is a vec to r quan t i t y t ha t has bo th tnag-
n i tud e and d i rec t ion . F or t he E a r th s magne t i c f ie lc l, t he
n i a g n i t u d e c a n b e specified
b y
t hc magni tudes o f t he
h o r i z o i ~ t a l n d v e r t ic a l
components
of t he f i e ld . The
orienta t ion is speci f ied b y t he dec l ina t ion and inc l ina -
t i on , \ vh l ch a r e e ssen t i a l ly t he t r end an d p lunge o f t he
f ie ld l i ne , t hough the inc l ina t ion a l so i~ i c lud es he po-
l a r i ty , which de f ines whc the r t he mag ne t i c vec to r po in t s
u p or do wn . S tud ie s o f t he Ea r th s m agne t i c f ic ld i nc lude
t h e s t ~ ~ d yf m agne t i c ano mal i e s a nd o f pn leornag ti e tr sm.
Magn e t i c ano lna l i e s a rc mcas urcme nt s o f thc va r i-
a t i on of th e Earth s mag net ic f icld re la t ive to sorne lo-
s , l l iy t l e f i l l ed re fc re~~cc . here
IS
no in t e rna t iona l
s r ; l n d n ~ - dcfcrencc f ie ld fro m which a nom al ies are meas-
111-cil,hc ia use th e Earth s niagnet ic f ield
S
not cons t an t
and c l i : l nges s~g~ l i f i can t lyvcn on a human t lme sca l c .
Reg iona l ma ps o f magne t i c anom al i e s a rc made hy
sing both ae r i a l and sur face measurements. T h e p r i n-
c ipal u se o f c o n t in e n t al m a g n et ic a ~ ~ o m a l yaps is to
i n f e r t h e p r e s en c e o f rock types and s t ruc tu re s t ha t a re
cove red by o t h e r r o c k s, s e d i m e n t s , o r w a t e r. I n s o m e
cases , t he p re scnce o f pa r t i cu l a r rock types a t dep th can
be in fe r rcd on the bas i s o f cha rac te r i s t ic pa t t e rns o n a
magnetic anomaly map . For example , rhc ex t ens ion o f
rocks o f t he Can ad ia n sh i e ld benea th th rus t f au l t s o f
the Canad i l n Rocky M oun ta ins c an be in fe r red f rom
t l ie ex t ens ion of t he sh i e ld m agne t i c pa t t e rn bcnea th the
t h r u s t f r o n t .
M ar in e ~n agn c t i c u rveys have re su l t ed in t he we ll -
kn ow n m aps o f t he sym met r i c pa t t e rns o f r ii agnet ic
a n o m a l i e s w h ~ c h a v c b e e n s o f u n d a m e n t a l t o t h e d e-
\ , c lop men t o f p l a t e t ec ton ic t heory . Wh en cor re l a ted
~v i t l i l ic ~ na gn e t i c eve rsa l t im e sca l e , the se maps can
he in t e rp re t ed to g ive a ma p o f t he age of ocean bas ins.
Magne t i c anorna l i c s a l so can be used in a mannc r
s i n ~ i l u r o g r a v it y a n o n l a l i e s t o i n f e r s tr u c t u r c a t d e p t h ,
-
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