seismic reflection profiling as a tool to better

Mount Isa Seismic Workshop, June 2008

Seismic reflection profiling as a tool to better understand the

geodynamic and metallogenic evolution of the Mount Isa Inlier

George Gibson (plus GA, GSQ & pmd*CRC team)

Acknowledgements: Peter Southgate, Narelle Neumann, Dave Huston (GA); Dave Mason (GSQ) & Russell Korsch (pmd*CRC)

Australian Government

Geoscience Australia


Specific Scientific Objectives:

• Acquire series of E-W and N-S transects to image crustal architecture of Western and Eastern Successions and adjacent undercover areas

• Integrate seismic with potential field and geological datasets to constrain 3D and 4D models for the Mount Isa Inlier

• Determine depth, geometry & distribution of pre-1800 Ma basement

• Capture deep-tapping faults & low-angle detachments• Determine extent & geometry of basin inversion• Determine structural & crustal setting of

mineralisation– Zn-Pb-Ag– Au, Cu (IOCG)


Getting down to business: capturing basin & crustal architecture in seismic reflection images

What gets imaged:• Moho• Non-vertical faults & shear

zones (incl. detachments)• Basement topography• Unconformity-bounded

supersequences• Basin margins & sedimentary

stacking patterns (pre-, syn- & post-rift sequences)

• Basin inversion structures (thrusts, reactivated normal faults)

From McClay 2007

From McClay 2007


Seismic maps sequence not lithostratigraphic boundaries

(Bradshaw & Scott, 1999)


Seismic maps sequence not lithostratigraphic boundaries: Upper vs. Lower Gunpowder Formation

Gun Supersequence Prize SupersequenceSequence Boundary ~20my missing


Gunpowder

Mount Oxide

Seismic maps unconformity bounded sedimentary packages:

-

Base Gun Supersequence

in mapped

Gunpowder Creek Formation -

Sharp lithological change

-

Abrupt facies change


For successful interpretation of seismic data & mineralising system need to know:

• Geodynamic evolution• Temporal Framework

– Isotopic dating– Sequence Stratigraphy (correlatable

surfaces)– syn-sedimentary (Pb-Zn?) vs syn-inversion

age (Cu) for mineralisation– Tectonothermal & magmatic history


Western Succession supersequences

&

mineralisation events

Guide

Myally

Big

Quilalar

Prize

Gun

Loretta

Term

River

Lawn

WideDoom

Leic

hhar

dtC

alve

rtIs

a

Three Superbasins:•Isa Superbasin (~1670-1575 Ma) •Calvert Superbasin (~1740-1690 Ma) •Leichhardt Superbasin (~1790-1750 Ma)

•Alternatively, Gun Supersequence is part of Calvert Superbasin

•Pb-Zn & U mineralisation associated with supersequence boundaries

ZincDeposits

UraniumDeposits


Geodynamic framework: passive margin setting or failed rift?Geological elements:

• Western & eastern successions

Stacked superbasins

• Isa (1650-1595 Ma)

• Calvert (1730-1650 Ma)

• Leichhardt (1800- 1750 Ma)

Isa Superbasin

Calvert & Leichhardt

Rift Geometrypreserved

Inversionstructuresdominant


Transgression/Sag Phase

Renewed extension

Isa superbasin –sag or foreland

basin ?

Syn-rift(Fluviatile-lacustrine)felsictholeiitic bimodal

Syn-rift(Fluviatile-shallow marine)

bimodal

Basin InversionIsa D2

(1585 Ma)

Isa D1 (1640- 1600 Ma)

Calvert Superbasin

(1730-1670 Ma)

Leichhardt Superbasin

(1800-1750 Ma)

Gun Supersequence (1670-1645 Ma?)

1800

1750

1650

1700

1575

1600

Ma Isa Superbasins

& Stacking


Sybella

Granite

Leichhardt

CalvertPost-rift sediments

Syn-rift sediments

Syn-rift sediments

Syn-rift volcanics

Syn-rift volcanics

(ECV)

(Fiery Ck)

N

10 km

Myally

Subgroup 3D

Isopach

Map0km

1km

1.5km

Leichhardt River Fault

Trough

Twenty-nine Mile FZ

Leichhardt Superbasin

-

syn-rift sediments (Myally)


NW-SE oriented growth faults

Fluviatile –shallow marine sedimentary facies (west)

Deep marine (east & NE)

Calvert Superbasin: Surprise Creek Formation (1690 Ma)

3xVertical Exaggeration

Gunpowder

Mt Isa

Mt Watson

0 Km

5.5 km

σ3

σ3

Soldiers Cap Group : Eastern Succession (1685 Ma)

N


Superbasin

formation accompanied by:Superbasin

formation accompanied by:

Leichhardt Superbasin:

•Bimodal magmatism (including continental flood basalts)

•Half-graben formation

•Mid-crustal detachment faulting (Eastern Succession)

Calvert Superbasin:

•Bimodal magmatism (including oceanic tholeiites)

•Magmatic inflation & doming

•Uplift & erosion

•Extensional unroofing of mid- crustal rocks & 1670 Ma granite

Upper Plate

Lower Plate

Wonga

GraniteDetachment

Post-rift = sag

Holcombe et al., 1991

NE SW

(1780-1740 Ma)


Crustal

Thinning

Fluviatile-

Lacustrine

Fluviatile –Shallow Marine

Deep Water–

Open Marine

IntracontinentalRift

DevelopingPassive Margin;

Off-shelf


Syn-rift Depositional Environment through timeSyn-rift Depositional Environment through time

Dyke


•Younger basins better preserved on Lawn Hill Platform (Lines M1-M2)

•Leichhardt basin geometry (M3)

•Calvert & Isa basin geometries (M2)

•D1 inversion (M1 & M2)

•D2 inversion (M2 & M3)3

M1

M2

M3

Seismic as a test of previously inferred crustal geometries

Regional tilt

Calvert-Isa

Leic

hhar

dt

D1

D2


1

2

Little Ra. FZTermite Ra. FZ

RiversleighFZ

NE-trending faults best imaged in M1


30km

BMR1

M6

M5

M4BMR2

Seismic Lines 4-6:

•Rift geometry preserved in west

•Basement exposed in central zone (LK)

•Rift basins inverted in east

•Mid-crustal detachments

sw NEAtlas MountainsINVERSION FEATURES:

•Harpoon structures

•Segmented asymmetric hangingwall

anticlines

LK

seismic reflection profiling as a tool to better

Documents