Some like it hot: Proterozoic reworking of an Archean ‘craton’ driven by high radiogenic heat-producing crustStacey Curtis and Anthony Reid

Australian Geoscience Council Convention

17th October 2018

Stable vs reworked Archean cratons

stable reworked

The Gawler Craton

Sparse Archean outcrop Archean extensive in subsurface

Mesoarchean magmatism

Early Paleoproterozoic volcanism and sedimentation

Kimban Orogeny(1740-1690 Ma)

Arc

hea

nP

ale

op

rote

rozo

ic

Kararan Orogeny (1590-1570 Ma)

Neoarchean magmatism and sedimentation

Late Paleoproterozoic magmatism

Late Paleoproterozoic sedimentation

Sleafordian Orogeny (2465-2410 Ma)

The Kimban Orogeny (1740-1690 Ma)

• craton-wide

• High T, low P metamorphism

• Near-isothermal decompression paths

• Early subhorizontal LS fabrics, sheath folding and boudinage during top to the north shearing

• Late tight to isoclinal folding and listric shear zones during E-W shortening

• syn-tectonic A-type felsic and mafic magmatism

Halpin et al. submitted, Morrissey et al. 2016, Cutts et al. 2013, Dutch et al. 2010, Dutch 2009, Dutch et al. 2008

‘The ‘hot plate’ tectonic model (McLaren et al. 2005)

Developed to explain orogens of Proterozoic Australia• Repeated tectonic reactivation• Large aspect ratios• Minimal crustal thickening• High T, low P metamorphism• A-type magmatism

Enrichment in high heat producing elements (U, Th and K) in the lower crust results in thermal weakening and tectonic reworking

Our Hypothesis:

Can metamorphism during the KimbanOrogeny be explained by an elevated geothermal gradient due to the blanketing of an Archean radiogenic lower crust by a sedimentary blanket?

Testing the ‘hot plate orogen’ model for the Kimban Orogeny

radiogenic Archean crust

Paleoproterozoicsedimentary rocks

INSULATION

HEAT SOURCE

Calculating radiogenic heat production

𝜌 = 𝑑𝑒𝑛𝑠𝑖𝑡𝑦𝐶 , 𝐶 = 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑖𝑛 𝑝𝑝𝑚𝐶 = 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑖𝑛 𝑤𝑡 %

𝐶 = 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑎𝑡 𝑡𝑖𝑚𝑒 𝑡𝐶 = 𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛𝜏½ = ℎ𝑎𝑙𝑓𝑙𝑖𝑓𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑟𝑎𝑑𝑖𝑜𝑎𝑐𝑡𝑖𝑣𝑒 𝑝𝑎𝑟𝑒𝑛𝑡 𝑖𝑠𝑜𝑡𝑜𝑝𝑒

Using measured U, Th and K content from whole rock geochemistry

Adjusting for radioactive decay to obtain value at 1.74 Ga (the onset of the Kimban Orogeny)

Turcotte & Schubert (2002)

Radiogenic heat production of Archean basement in the Gawler at 1.74 Ga

Mesoarchean basement Neoarchean basementPaleoproterozoicsedimentary rocks

Numerical simulation code SHEMAT = Simulator for HEat and Mass Transport (Clauser, 2003)

The maths of conductive heat transport………

change of heat with time diffusion of heat

advection of heat

productionof heat

Input Variables

Radiogenic heat production Thermal conductivity Volumetric thermalcapacity

Porosity

( )) )

measured values fromgeochemistry

0.01based on assumptions

of lithologybased on assumptions

of lithology

Model parameters

1 Paleoproterozoic sedimentary rocks

2 Neoarchean basement3 Mesoarchean basement

4 lithospheric mantle

1

2

3

4

1550°C @ 200 km

10°C @ 0 km

200

km

cell size 1 km x 1 km

170 km

10 km

15 km

0-10 km

50 km

Test sensitivity of model to two scenarios

(i) radiogenic heat production of Mesoarchean basement

(ii) thickness of sedimentary cover

Results: radiogenic heat production of Mesoarchean lower crust

Results: thickness of Paleoproterozoic sedimentary basin

Conclusion

Contacts

Stacey Curtis, Senior Geologist

Department for Energy and Mining

11 Waymouth StreetAdelaide, South Australia 5000

GPO Box 320Adelaide, South Australia 5001

T: +61 8 8429 2577

E: Stacey.curtis@sa.gov.au