The Gawler Orocline
Is it bent and why should we care?
David Giles, Peter Betts,
Greg Swain and Martin Hand
Orocline:
“…an orogenic system which
has been flexed in plan to a
horseshoe shape.”
Carey, W.S., 1955. The orocline concept in geotectonics,
Proceedings or the Royal Society of Tasmania, 89: 255288.
1
At continental convergent margins
with subduction polarity toward
the ocean:
Subduction zone retreats and
decorates the craton margin
(recesses and salients) with with
arc, back arc and extended
terranes from the over-riding plate
eg. Carpathian Arc, Western Med
2
At continental convergent margins
with subduction polarity toward
the continent:
Variable extension or
indentation of the continental
margin depending on geometry
and buoyancy (oceanic plateaux,
continental material, active
plumes?) of the subducting plate
eg. India-Asia collision
3
Archaean
core
From “the Green
Book”
MESA Bulletin 54
The Geology of
South Australia,
Volume 1, The
Precambrian
Palaeoprot.
horse-shoe
Archaean
horse-shoe
4
~1.8-1.6 Ga
Basins
P&D
Nawa
Mt Woods
Tarcoola
Willyama
York
Peninsula
Exposed
Archaean
0
200
kilometers
400
Tallacootra sz
1680±37
dextral
transpression
0
200
kilometers
400
Kalinjala sz
1682±10
dextral
transpression
Kimban
Orogeny
1.73-1.69 Ga
0
200
kilometers
400
Dates from
Swain et al,
2005
Precam Res,
139: 164-180
5
Nuyts
Volcanic Arc
1.64-1.60 Ga
0
200
kilometers
400
Coober Pedy
Mt Woods
0
200
kilometers
400
Broken Hill
Fowler
Yorke
Peninsula
Olarian
Orogeny
1.60-1.57 Ga
0
200
kilometers
400
6
GRV
BRV
Hiltaba Event
1.59-1.57 Ga
0
200
kilometers
400
If we accept that the Gawler
craton is actually an orocline…
… we should ask how and when
did it get that way?
7
When?
Before the Gawler Range
Volcanics (mostly undeformed)
After early Olarian radial
structures
Slap bang in the middle of the
Olarian Orogeny, at which time..
…arc magmatism in the c. 1.641.60 Ga Nuyts domain was shut
down,
magmatism moved inboard of the
margin and took on a distinctly
plumey feel (c. 1.60-1.57 Ga
Hiltaba Event) and,
was broadly coincident with
crustal shortening
8
How?
Roll back of south dipping
subduction into continental
recess?
Requires horse-shoe geometry of the
craton through multiple events prior to
1.60 Ga
Olarian def and met focussed on the
wrong plate
Missing (or perhaps unrecognised) suture
between Nuyts and Gawler craton proper
How?
Olarian aged indentor during
north dipping subduction?
Possible
Lithospheric delamination due to
shortening could lead to A-type
magmatism
However, if the indentor was continental
material we don’t have a record of it
9
One possibility:
Buoyant
subduction of a
plume
GRV
BRV
Transient
shortening and
voluminous
magmatism as
plume breaches
slab
0
200
kilometers
400
Why do we care?
If the orocline concept is correct then the
tectonic setting, fault architecture and
hydrothermal systems in the eastern
Gawler Craton (Olympic Dam) can be
understood with reference to modern
analogues
Major implications for those of us who
(despite counselling) can’t stop trying to
put Australia back together
10
Deposits form in an
orogen parallel
zone, controlled by
intersection of
significant orogen
parallel structures
with oblique
structures
Hmmm, where
have I seen that
before?
11
GRV
BRV
0
200
kilometers
400
Deposits form
where magmatic/
hydrothermal
system overprints
previously
unmetamorphosed
basins, potentially
containing
evaporites
An IOCG plume trail in eastern Australia?
12