L Scope of aeromagnetic interpretation in structural analysis

CHAPTER 6, GEOPHYSICAL RESPONSES OF I(NOWN ROCI( TYPES 66

CHAPTER 7. CHAPTER 7. MACROSCOPrc STRUCTURES 81

T. L Scope of aeromagnetic interpretation in structural analysis

On a macroscopic scale,

structural

geologists use outcrop observations and aerial pliotograpl-Ls

to

map

structures. They

^are

limited by

lack

of

access

to

outcrops and

by the

^amount

of

soil cover. Even a cursory examination of aeromagnetic maps (Plates ¹

to

3, Figure 5.4) reveals the abundance of folds and

faults

which can be resolved.

Continuity

of

information

and

deptli

of penetration make

the

aeromagnetic method

particularly

sensitive

to

changes

in structure.

In

ihis

section, the potential and

limitations

of applying aeromagnetic interpretation

to

structural analysis are examined brieflY.

All

structures discussed

in this

chapter are macroscopic structures. Magnetic interpreta'tion on

its

own cannot be used

to

separate the effects of the sedimentary, metamorphic and igneous

history of a

region

from the structural history. Note too that

where there

is no

detectable magnetic property contrast, there can be no magnetic anomaly'

0À

oé

o

I

I

A

0

6r16000

6110000

62ø000

62æ000

61!9000

614000 301000

3?5000

3070@

ô1?4ñ

61 150m

ô20706

ô2000@

614S0m

01€ffi

gil s15ø

W læ SM

Figure 7.1: Bxamples of fold anomaües on magnetic maps. The contours are

irregularly

spaced.

Dashed-[ne contours are lower

in

value than conlinuous-line contours.

A, B,

C and

E

show the

total

magnetic field, and

D

and

F

show the vertical magnetic gradient.

\

B

a

o

o

g01m

o

ô

ò

0 ô I {

Ò

0

o

0

c

I

0

o

0

0

(l ô

I

o

a

(ì

t

Poo

q)

0

_à

()

ù 5

6

0

u\

io

3@

L\

0

0

0

E ,l

Ç

t, t-

C

\

0 F

\J

ßÐ ô

o

0

q

q s

₀0

0 0

0

CHAPTER 7.

MACROSCOPrc STRUCTURES ⁸²

7.L.L ^{Magnetic modelling}

The different geometric models ^used to model magnetic anomalies include two-dimensional mod- els

(dipping

dyke, horizontal edge and prismatic polygon) and

the

single pole

model'

For an

""ptìr,-rtioo

^{of these terms refer}

to

Section 4.1.2. The single pole model and

the

dipping dyke

-ã¿"t

^{were used}

^to

model deep-seated basement features. Dipping metasedimentary horizons, which had strike extents

of

several kilometres

oï

more, were modelled using

the

dipping dyke model and, where this was not suitable, the prismatic polygon model. The polygon model allows

for the

sides

of the body to dip at

different angles and

for the top of the

^{magnetic source to} be non-horizontal. The

term

^*dyke model"

or

"dipping-dyke model" ^refers

to the

geometrical model and

not to

the intrusive dyke.

Except

for the very few

anomalies due

to

deep-seated magnetic basement, depths

to

the

top

of magnetic ^sonrces were

within

100m and commonly

within 30m.

^The shallowness of the solrces meant

that

^the most significant parameter

to

be estimated was the dip of the magnetic

horizon. Additionally

the interpreted magnetic map

(Plate

1) is similar

to

a geological map in

that

interpreted magnetic units either outcrop

oI

ale near-surface.

When modelling magnetic anomalies, a distinction must be made between "magnetic ^dips"

(i.e. dips computed

from

magnetic modelling) and

structural dips. This distinction is

due to two

important

^reasons:

1.

The assumption

that

the direction of magnetization is the ^{same as}

that

of the present fleld of the earth is

not

always

valid.

Since

it

is

not

^possible

to

separate the effects of geometry (usually

structural dip)

^and

NRM from the

^{magnetic anomaly,}

the

use

of

an incorrect value

for

the direction of magnetization

will

result

in

incorrect dip estimates.

2.

^{The effect of} nearby magnetic sources complicates interpretation and increases the ambi-

guity

inherent

in

magnetic

interpretation. By

computing the vertical magnetic gradient,

an

improvement

in

resolution can

be

achieved and

the deflnition of the

anomalies im- proved.

But in

^areas of structural complexity, anomalies may

still

be too close together to be interpreted

in

isolation.

Additionally,

^when interpreting folded sequences, the dipping-dyke model is a simplification of a dipping horizon which changes

in

dip

in

the vertical plane. Depending on the curvature of the magnetic source, the accuracy

in

estimating the dip ^varies.

7.L.2 ^Folds

Successful

interpretation

depends on the scale of the folds, the trend of the axial plane and the number of intersections made on the

flight paths.

Folds can be mapped

by

tracing a magnetic

unit

around a

fold

closure, by changes

in

the trend of the bedding

or

the strike of the units, or by comparing the dips on the limbs of the ^suspected

fold.

Commonly, there is thickening around the

folá

hinges aod

io

associated increased magnetic ïesponse

(Whiting,

1987).

If

the limbs of the

fold

have been thinned, a

triangular

shaped anomaly may be

all that

is there

to identify

^a fold closure.

If

the fold closure lies entirely between two flight lines, or is transected by only ^one

flight

line, the closure

will

^{probably go} unnoticed.

In

the absence of other information, isoclinal

,rra

tigt

t

folds cannot be distinguished

from

dipping magnetic units unless the

fold

closure can be mapped.

A -

Ob¡.rv.d lrrtd ¡ñtr n!,t,

A.

t200

cOmpvted l,!ld r¡lañ!rlt --- lorm |ner lrom i!rtoca

gOolOqy

0TSTANCE 0(rn)

2l

-

oo!.rv.d lrald rnlanllt compulad lrald rnlen!rlt --- lorm tnô! lrom lurloco

g.ology

0ISIANCE lrm)

2f ooo

800

f È

o

200 ooo 800 600

:

I l,\) 2

a

I

Õ2

4

4 o

5

Unil A

\\

B

c

Ë

4

o 4

I o

E

Õ¿

r 2uù

600

c

1a:J t.taî9tt .¿ñ;elao lr.ld ¡nl.nr¡lt - - - lorrn lrn€¡ lrom turfoca

!0oloqy

2l

DrSlÀNCE thñ)

A

Figule 7.2:

tr{a,gnetic rnocielling of synclinal

stluctule (aItel

ltobinson e/ a/.,1985).

Figule 7.3:

Anoma,ly on

a fault:

a) Cìeology (1-dyke,

2-fault). b)

Toral lìeld. c) Apparent susceptibiìity, (af- ter Yunsheng et a|.,1985).

o o o

_l 2miês

5km

<>

;+o

Unil A

o

2

o

¿2

a

-t o

c

CHAPTER 7.

MACROSCOPrc STRUCTURES ⁸³

Intense anomalies which appeat

to

be round

or

oval on contour maps may be due

to

srnall scale,

tight

folds. Anomalies which start off ^as wide and then taper off may indicate folded units.

Aoom¿ie,

caused by folded rock

units

change around the

fold. Part

of the reason is ^because of

the

different dips and sometimes the effect of remanence

(NRM)

compounded

with

the change

in dip. It

is

not

only the limbs of folds which can be mapped:

it

^is also possible

for

the dipping

axiaf

surface

to

give rise

to a

detectable magnetic

anomaly.

Disruptions

in the continuity

of magnetic anomalies as a result of

faulting

can lead

to

the illusion

of

a

tight fold

closure and

it

may be impossible to resolve this further.

Some examples

of

interpreted

fold

anomalies are

illustrated in Figure 7.1. In diagran A of this figure,

contours

of the vertical

magnetic gradient delineate

the

Macclesfield Syncline.

Outcrop

in the

region

is very poor

^and

the

parasitic folds seen on

the

eastern

limb

were not

previouily

known

to exist.

The second example,

B, is

taken

from the ISZ.

The

fold

closures outlined may represent early isoclinal

folds.

^{The limbs are} parallel and now

form the

^eastern

limb

of a regional syncline.

A north-plunging syncline is mapped in diagrams C and D of Figure ^7.1, and a south-plunging antiform

in

diagrams E and

F.

The contours

in

diagrams C and E are of the

total

magnetic fleld,

andin

^{diagrams D and F} of the respective verticalmagnetic gradient. Resolution is improvedin the gradient maps. Similarly, in Figure 7.3, the high-frequency content of apparent susceptibility maps helps

in the

structural mapping'

The quantitative interpretation of

folds

is

usually restricted

to the

determination

of

dips on either

limb

as shown

in

Figure

7.2

and

in the

other models presented

in this

chapter. The mapping of the axial surface trace can only be schematic.

In

^cases where the axial surface ^gives

tiru io

a magnetic anomaly, the direction of plunge can be inferred

but not

always the amount' For example, the nose of a synform might give rise

to

a triangular magnetic high' surrounded by a

low. The

magnetic anomaly over

the

closure

of

an antiform

might

be

similar

except

that it

could be marked by ^a magnetic high in the form of a

"tail"

leading along the direction of plunge.

This is

illustrated in

diagrams C and E of Figure 7.1. The ^presence of such

a "tail"

^{would also} indicate

that

the plunge of the antiform is ^shallow.

7.L.3 Faults, ^{shear zones} and lineaments

The magnetic signature of ^a

fault

^zorLemay either be

"direct" or "indirect". A "direct"

^anomaly is ^a high or low caused by the production or destruction of magnetic minerals

in

the rocks within the

fault

zone, so

that

the

fault

zone

is the

source of the magnetic anomaly. Retrograde ^shear zones

within

which magnetite has been destroyed

by

circulating fluids are examples

of

"direct"

anomalies (Henkel and Guzmán, 1977).

The

magnetic

low

associated

with the

Palmer Fault

(plate 5) is

an example

of

brecciation and

albitization

of the

fault

zone

(White,

1956) during which magnetite has been destroyed.

,,Indirect"

_anomalies are much more

common. The

^offset

or truncation of

one

or

lnore magnetic markers

is typical of

cross

faults. The

non-appearance

of a

magnetic

unit on

the other side

of

a

fold

and breaks

in

continuity of magnetic markers should

both

be examined for evidence of

faulting.

^{possible faults are} indicated where there are breaks

in

the magnetic pattern (terminations of highs or lows, ^change

in

gradient, linear contour patterns) and from alignnent

of

highs and

lows.

Faults should

be

suspected where blocks

of

differing magnetic character have been brought

into contact.

Different expressions of

fault

anomalies are demonstratecl in Figure 7.3, Figure 7.6, and Figure 7.9.

Changes in the character of ^a magnetic anomaly from profile to profrle can be easily identifled

CHAPTER 7.

MACROSCOPrc STRUCTURES ⁸⁴

from maps of ^stacked profiles. Small ^scale maps such as digital images are invaluable for detecting lineaments

-

^{linear features} which run for great distances and which may be caused by steeply dipping

faults

(Gay, 1972)'

Accurate mapping of faults is dependent on the trend and sense of movement along the fault plane. Where the trend is

at

a small angle

to

the

flight

^{path, such}

that it

crosses less than two

flight lir,"r,

^the

fault will

appear

to

^be subparallel

to

the

flight path. ^The

distinction between

faults

and contacts may

not

be obvious or resolvable'

euantitative

interpretation of a

fault

involves the determination of the direction, ^{sense and} amount of movement along the

fault

^plane. \Mhere there has been mainly horizontal movemetlt, the offset of anomalies and of lineaments is ^used to estimate the ^sense and amount of movement.

Vertical

movement can

be simply

calculated

only

when

the

blocks

on

opposite sides are of

the

same

material. The direction of the

downthrown

block

can

be

determined qualitatively because where there

is

substantial

throw, the

magnetic character

of the

downthrown side is

more ttsubduedt'.

But

often there ^{has been} both horizontal ^and vertical movement at different times and bloclis

of

different rock types are

juxtaposed. In my

study area,

it is difficult to

determine whether there has been vertical movement along the faults as the basement

to

the Kanmantoo Group ^is either non-magnetic

(if

shallow), or

if

magnetic is very deep. As a result, while ^cross faults are

often

obvious,

only

one

fault with

definite vertical movement

(the

Springton

Fault)

has ^been mapped.

In

any case, aeromagnetic maps respond

better to

net horizontal movement

than

to the magnetic effect of magnetic blocks overlain

vertically (Gay,1972).

Consequently, low-angle thrusts are hard

to

deduce,

let

alone atalyze quantitatively'

Information about faults may also be obtained from aeroradiometric data which are usually collected ^as

part

of modern aelomagnetic surveys. Radiometric images indicate the ^presence of faults

in

^{different ways.}

1.

Fluids circulating

in

shear zones might have resulted

in

metasomatism and the removal of potassium (thus producing a radiometric low) or sericitization (radiometric ^high).

2. The

differential weathering profiles

of

rocks

on either

side

of the fault

can

result in

^a

radiometrically anomalous soil profile.

3.

Rivers and streams favour

fault

zones and

the

sediment which

is

brought

into the

river channel may ^possess radioactive elements

in

concentrations different

to that

^{of the rocks}

on either side of the

fault.

7.2 Structural patterns in the Kanmantoo Synclinal ^Subzone

The

KNSZ covers

the

southern and western parts

of the

study area and extends

to

the south coast of Fleurieu Peninsula.

This

subzone ^covers

just

under two-thirds of the main study area.

From

the type

section along

the

south coast

of

Fleurieu Peninsula, Kanmantoo Group rocks outcrop in synclines which form an

en

échelonsystem migrating northwards (Mancktelow, ^1979).

Metasediments of

the

Tapanappa Formation and younger Kanmantoo Group

form

^the

bulk

of the outcrop as they are found

in

the core of most of the synclines.

The base

of the

Kanmantoo Group marks

the

western margin

of the KNSZ.

As discussed previously (Section 1.3),

the nature of this

boundary

is

controversial and

the

boundary ^has

LINE: 300 EASTING: 283448318292 NORTHTNG: 609992.t 400.0

200 0

00

-200 0

2.000

ÊS

!o iI

ÈE_c

(,

!

ôo

0 000

0 21

Figure 7.4:

lVlagnetic model

of the

l\'Iacclesfield Syncline-Strrrhalbyn

Anticüne.

Anomalies

us-MS1

and

us-MS2 at

14000, NG1

at

15400, TC-MS

at

22000 and 25000.

MS

t<

I

Í Í

Y Í

TC

_)<

/

f F

f

Í x

*

I

/

/ f

t-

I

t'

x

,a

US

í

a

x

TP-EB