CHAPTER 7. MACROSCOPrc STRUCTURES 99

TP-EB rtt r-l

CHAPTER 7. CHAPTER 7. MACROSCOPrc STRUCTURES 99

Geologically,

the ISZ

coincides

to a large

degree

with the migmatite

zone

(Figure

1'3).

The

metamorphic grade boundaries

trend

approximately

NNW

and

this is highly

signiflcant.

Almost the entire outcrop

withil

the ISZ ^has been ascribed to the Backstairs ^Passage Formation.

Gra¡ites

(e.g. Palmer Granite) and granite gneisses (e.g. Rathjen Gneiss) and migmatized rock outcrop together

with

the meta-arkoses which are

typical

of the Backstairs Passage Formation.

Calc-silicates occasionally occur, as do amphibolite and meta-dolerite dykes.

Unlike the KNSZ and KRSZ, no simple major structure has been mapped by previous workers and

the

relations of

the

structures

in the

ISZ

to

structures outside

the

Subzone are

difficult

to analyze.

While ^a

^number

^{of folds}

have been delineated

from

magnetic

interpretation, it

^has

rarely

been possible

to

classify

them

as being synclines

or anticlines. This is

because facings are

not

available and directions

of

younging are often

unknown. Additionally,

magnetic ^dips indicate

that

most folds are overturned. Fold closures are only rarely ^seen.

Structures in this area appear to be discordant

with

structures ^elsewhere in the KNSZ and the

KRSZ. This

may be due

to the overprintingof

^{F^o¿n} folds by

later

folds because temperatures were

still

high in the ISZ which is

within

the highest grade metamorphic zone. The open, upright folds comrnon

to the

KNSZ and

the I{RSZ

are less common

in the ISZ. The

contact between the ISZ and the

I{NSZ

and KRSZ appeaïs

to

be discordant. Anomalies of the

Truro

Anticlinal structures are truncated by the ISZ and the relationship between the ISZ and KRSZ is complex.

7.4.L ^Subareas in ^the ISZ

KNSZ and KRSZ

were discussed

in terms of the major

structures

in

those subzones. The relatively complex magnetic signatures and the fact

that

the structure is poorly understood has made

it

more practical

to

consider the ISZ

in

terms of subareas of differing magnetic character.

Based

on the

magnetic characteristics,

the ISZ

can be divided

into

a number

of

subareas which are shown

in

Figure 7.18 (compare

with

Plate 3).

Subarea type A

Multiple,

closely spaced, linear anomalies (dominant trend

NNW),

are caused by migmatites ^and meta-arenites of

the

Backstairs ^Passage Formation. The

structural pattern within

subareas A1

to

A.4 is largely unknown. These ^areas show evidence of previous deformation which could ^have been

isoclinal.

The rocks have been mapped as belonging

to

the Backstairs Passage Formation probably

for

want of evidence proving otherwise (see Mancktelow, 1979). The structure within these subareas

is important, particularly if

early macroscopic isoclinal folds interpreted from magnetic data are proved

to

exist.

The meta-arenites of the Rockleigli region, subarea

41,

have been mapped

in ^part

by White (1956),

Malcktelow

⁽¹⁹⁷⁹⁾

and

Lawrence

(1980).

Structures

inferred by

Mancktelow (1979) are

too

simple

to

explain the magnetic signatures. Magnetic

units, TC-MNS

and TP-MNS are

both

folded

into the Monarto Sylclile.

^Above

TC-MNS, the

magnetic

units, BP-41,

do not close around

the Monarto

Syncline. Instead,

BP-41

has been used

to

delineate a

fold

closure which

must be

pre-F^o¡n

(Figure 7.19). The Monarto

Syncline

is

therefore

not

an F1

fold

^as interpreted

by

Mancktelow

(1979).

The rnagnetic anomaly associated

with the

^pre-F*o¿n

lold

closure suggests

the fold

was

synformal. Part of an

aeromagnetic

profiie

over subarea

A1

is shown

in

Figure 7.17.

LINE: 1160 EASTING: 313085 332747 NORTHING: 6141 183 200 0

0 000

50.00

0.@

.5000

.0000

Ê_c 00

!o iT

t-

-200 0

1.000

Ë FS

(5

E co

o

ÈE_c (5

EÀ

ôo, 0 500

Figure 7.20: Magnetic model of the migmatite

belt

near

Palmer.

Anomalies:

BP-42

between 13000 and 17000, Paimer Granite: betùeen 17500 and 18000.

MIGMATITE BAND: CROSS-SECTION

\

\¡

,

I I I I I I

\ /t

\/ t

\ I

¡l1l

¡l1l

¡l¡l

I I I I I I I I

Ê_E

!

lr0,

, ^i'\ _i''\ i"\ ^,' \

ìi \i \l .'i _{ì ; I |} ^,.,'

r, _t"' il '., li

\ _,,'

t, t, t,

\/

^|

-.5000

00 200.0 400 0 600.0 800 0

Figure 7.21: Magnetic model of simulated cross section of migmatite belt

CHAPTER 7.

MACROSCOPrc STRUCTURES ¹⁰⁰

Lawrence (1980) mapped the arkoses as belonging

to

the middle member of

the

Backstairs Passage

Formation.

^{Trends marked}

by

Mancktelow (1979)

follow

magnetic trends

of

BP-41 and stop

just

short

of the fold

closure.

North of the fold

closure,

similar

air-photo trends do

not

continue:

this

is consistent

with

the interpretation of an early macroscopic fold refolded by F^oin.

White (1956) called subarea A2 the "zoîe ^of veins". Subarea A2 contains the multiple, linear,

NNW

trending, magnetic and radiometric rocks west of Palmer. Magnetic anomaly amplitudes and radiometric count increase as rocks pass

into

the migmatite zone. While

linear

anomalies predominate on ^a large scale, individual trends can only be followed

for

short distances. Despite

the ptygmatic folding

associated

with the migmatite

zone,

the

overall

trend of the

magnetic

units is

dominantly

NNW. The

regional structure

is a

synform, interpreted

by

Fleming and

\Mhite (1984)

to

be

l'3.

New, closely spaced

gravity

data are being collected

in the Mt.

Lofty Ranges (Boyd, pers.

comm.). A gravity

high has been located above subarea

A2

(Middleton, 1e73).

Modelling is

difficult

and magnetic prolìles show a ^series of peaks

in

the

total

magnetic fleld and

its vertical

gradient (Figure

7.20). The

^closeness

of the

anomalous sources increases the ambiguity of the

interpretation.

Even on the vertical magnetic gradient profile, the true negative peaks are

not seen. An

experimental model showing short-wavelength folds

is illustrated

in Figure 7.21. Such a model also gives rise

to

a series of linear magnetic anomalies similar to

that

shown

in

Figure 7.20. The structure

within

such a belt is

difficult

and impossible to resolve from magnetic modelling alone.

The migmatite

^band east

of the Monarto

Granite

is

a,n extension

of

subarea

42, with

at least two anomalous magnetic units establishing the continuity (there are gaps

in

detailed aero- magnetic coverage as shown

in

Figure

I.2).

Closures from contour maps indicate the presence of isoclinal folds folded

by

F^o¿n.The limbs of the fold are parallel and trend NN\M. The closures can be seen

in

Figure 7.1-8, Plates 1

to

3 and Plate 5, and

in

Figure

5.4. Attempts to

model the isoclinal folds were

not

successful due to interference from neighbouring magnetic ^anomalies (Figure 7.22). The isoclinal folds are on the east limb of a regional synform. They may be F"o,¡u

or

-t]ro¿,, folds.

Subarea

A3 is

characterized

by linear

magnetic anomalies

trending NS.

Subarea

A3

lies between

the

southern extension

of the Monarto

Syncline and

the Wellington "Granite".

^Fold

closures

in

subarea A3 are probably of the same generation as the isoclinal folds

in

subarea 42.

A

strong magnetic gradient separates subarea A3 from the KNSZ and may indicate a structural boundary. Anomalies

in the

KNSZ adjacent

to the

contact are of

low

amplitudes (less than a few hundreds of nanoTeslas) compared

to

the high amplitude anomaües

in

subarea

43.

The magnetic character of ^subareas A2 and A4 are similar, though they are separated by ^sub- area C1. The pronounced

NNW

trend characteristic of subarea A2 is again seen

in

subarea 44.

Granitic

rocks a,lso outcrop

in this

area, and geological maps (Thomson, 1969b) indicate the presence of granitized sediments (presumably meta-arenites or migmatites). The basic structure of subarea A4 may be anticlinal in keeping

with

the major syncline, the Karinya Syncline, to the east. Faults

to

the west and northwest obscure the relation between subarea A4 and the KRSZ'

The

southwestern contact between

the Karinya

Syncline and

the

subarea

A4 of the

ISZ

is

a problem area. Carrickalinga Head Formation rocks on the west limb of the Karinya Syncline are flanked on both the east and west by rocks of the younger Backstairs Passage Formation. In the east the younger rocks are folded

into

a syncline. But for Backstairs Passage Formation rocks to outcrop on the west as well requires

further

explanation. Abbas (1975) has mapped overturned dips

in

the ISZ west of the KRSZ. The feasible alternatives include the following: the boundary between subarea A4 and the KRSZ is ^a

fault,

the combination of several deformation ^{events has}

LINE: 1250 EASTING:33'1831 341983 NORTHING: 6118502 300.0

200.0

100 0

00

-100 0

1 000

Fc Eo

iI

t-E c (,

=_o ôo

-1.000 00 160 0 320.0 480 0 640.0 0.000

1.000

Figure ⁷ .22: Magrtetic model ^of the "isocünal" ^{folds of subarea} A3 of the ISZ. Anomalies: BP-42 between 2300 and 3500.

LINE: 1690 EASTING: 312s29 340504 NORTHING: 6157150 200 0

lt

00

F

_c

!o iE

0 000

ÈE

It

-200 0

-1.0ûD

850

Figure 7.23: Magnetic model

of the

Springton

Fault.

The

fault is

indicated

by the bold

line.

Anomalies:

BP-NI(S at

4800, possible subsurface granite between 7000 and 12000, BP-Á.4 bc' tween ¹²⁰⁰⁰ and 16000.

Tc

ôC¡