Transition between the Delamerian and Lachlan Orogens

A ramble through the Koonenberry Belt, Stawell Zone, Dimboola Arc, Thomson Orogen, and the Macquarie Arc

Bob MusgraveWith much assistance from

John Greenfield and Phil Gilmore

Let’s look at where we are…

“Tasman line”

Koonenberry Belt

TMIGeoscience Australia

Tilt-filter on TMIGeological Survey of NSW

Koonenberry Belt: Geological outline

250 km

Elements of the Mt Wright (Koonenberry) Arc

• Arc• Back-arc• Fore-arc/accretionary prism• Continental backstop (including

Neoproterozoic rift volcanics)

Mt Wright Volcanics+ Bancannia Trough

Tilt-filter + TMI, with gravity

“worms”

Potential field modelling

Bouguer gravityTotal Magnetic Intensity

Deep structure: reflection seismics and potential field modelling

TMI

1VD

Bouguer grav

Low-pass grav

TMI

1VD of TMI

Modelling 3D structure

Koonenberry 3D model

Mt Wright Arc

Mt Arrowsmith – deep bodies

• Linear structure of Koonenberry Belt looks simple enough – but note:

• Broad magnetic anomalies in Bancannia Trough – Mt Wright Volcanic Arc under Devonian basin

• Re-entrant bend at Grasmere Knee Zone – and nearby deep magnetic sources

• Bend at the Cobham Kink Zone – and “mystery” deep reversed remanence source

• And note how the gravity “ignores” both bends in the linear belt

– Suggests the bend is restricted to shallower structure

Structures from potential fields

Curnamona

Koonenberry Belt

Bancannia Trough

Grasmere Knee Zone

Cobham Kink Zone Mystery

source

Restoring the Bancannia rift20°

New Zealand CVZ and Bancannia

Parson & Wright, 1996

2.3 A/m

S. Soengkono, Journal of Volcanology and Geothermal Research 68 (1995) 193-207

Thanks to Supri Soengkono and Glass Earth (NZ) Inc

Grasmere Knee Zone: subducted seamount in accretionary margin?

0 20

km

Seamount subduction

Dominguez et al., 1998, 2000

Kodaira et al., 2000

Thomson accretionary margin: Benambran overprint on Delamerian wedge

Seamount deformation in the Thomson Orogen?

Thomson Orogen arc-arc collision?

Stawell

Bendigo

StavelyGlenelg

Hay- Booligal

Bootheragandra

Fault

Thomson

Delamerian accretionary margin - connection to the Stawell Zone

Scheibner & Basden, 1998

Delamerian and Benambran

Ponto GroupMarine- pelagic mudstone—siltstoneFelsic tuff (Mt Wright Arc equivalent)Mte-Qtz exhalativebasalt lava, tuff (tholeiite, MORB)

~511 Ma

(Evans 1993)

Grasmere (Ponto Gp) – Besshi-style VMSLate

qtz-carb-chalveins in FW

HWNon-mag

Massive sulphide ore

Brecciated & MS ore

FWMag

Orogenic Au - Warratta Inlier

Victorian Goldfields(Ramsay, 1998)

Australian Earth Sciences Convention, Perth, July 2008Warratta Inlier (Greenfield, 2008)

Mt Wright – Menidee – Dimboola Arc?

Dimboola Arc:Cambrian calc-alkaline volcs, supra- subduction zone ophiolite (oceanic arc)

Volcanic basement of Stawell Zone: Cambrian tholeiites, Besshi-style VMS, Benambran gold

Mt Wright (Koonenberry) Arc:Cambrian calc-alk volcs in rifted arc setting on continental margin.

Ponto Group: Cambrian tholeiites, Besshi-style VMS

Warrata inlier: Benambran orogenic gold

Bendigo Zone: Benambran orogenic gold