Capitol2-Techniques of Structural Geology and Tectonics

8/9/2019 Capitol2-Techniques of Structural Geology and Tectonics

1/23

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


2/23

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


3/23

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


4/23

IS

1 1 1 ~ c . l \ ( O f

'1

1 1 l : l ~


5/23

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


6/23

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


7/23

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

17


8/23

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 -


9/23

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

19


10/23

~ 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.


11/23

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


12/23

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 ,


13/23

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.

Techniques of Structural rology dnd Tectonics

3


14/23

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


15/23


16/23

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.


17/23

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


18/23

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


19/23

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.('


20/23

+-- ;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 ,


21/23

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011

grav i t y . i l i o rua l i c s ,

a n d

Tor sinj11;ir-

rr: lsi)l ih.

I y n \:,lriety o t p ~ - o c e s e sh a t i n c l r ~ d s , st al li z nt io r i,

c o i ,l i ng , s c i i i r n c n t a t i o n , n liil c h e n ~ i c n l e a c t i o n i l l t h e

F , n ~ [ h ' s i ~ a g r i c t i c ie lc l, r o c k s c a n b c c o m c r n a g n e r i z c d in

I d i r e c t io n p a r a l l e l t o t h e a n i h i e n t f iz ld a n d c a n p r e s e r v e

t h a t n ~ a g n e t i s m v e n

r

t h e r o c k s a r c r o ta t

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Capitol2-Techniques of Structural Geology and Tectonics