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Western Australia – the Halls Creek Orogen

The Halls Creek Orogen of Western Australia is a NE-SW trending belt of spatially extensive High-Temperature, Low-Pressure, (HTLP) metamorphism situated on the Eastern margin of the Kimberley Craton.  The orogen represents the ca. 1860-1800Ma collisional interface between the Kimberley Craton to the West and the North Australian Craton to the East (Bodorkos et al., 2002).  The orogen is sub-divided into three distinct metamorphic domains: the Western, Central and Eastern zones.  The Western and Central zones are interpreted as sections of the Kimberley Craton margin; the Eastern zone the passive margin of the North Australian Craton.  The tectono-metamorphic evolution of the Halls Creek Orogen involved protracted high geothermal gradients in the upper crust (>30Ma), of large volumes of mantle-affined mafic and ultramafic intrusions and low radiogenic heat production (Bodorkos et al., 2002). 

To investigate existing conceptual models of the tectono-metamorphic evolution of the Halls Creek Orogen, 33 petrological-thermomechanical numerical experiments were run to evaluate plate behaviour, magma generation, structure development, and tectonic evolution through time. Initial model setups are designed based on pre-existing conceptual models for the orogen: an intra-oceanic arc and active margin setting (subduction zone).  Of the 33 models simulated, those with features that are consistent with observed geological and geophysical observations, and interpreted tectonic evolution, of the Halls Creek Orogen are presented here.

Model I: Continental subduction/collision Model; Mantle source magmatism

Model I presents a 600km-wide, oceanic crust flanked by continental lithosphere to either side, which grades to a thickness of 200km.  The left margin of the oceanic crust is passive; the right is an inclined mantle lithosphere shear zone.  The continental lithosphere to either side has a 30km-thick crustal profile.  The 7km-thick oceanic crust is composed of 2km of hydrothermally-altered basalt underlain by 5km of gabbro.  An initial horizontal bulk shortening rate of 2cm/a is imposed upon the model for 16My.

During the onset of the model, the oceanic lithosphere is subducted under the continental lithosphere to the right.  A magmatic arc develops in the upper crust of the overriding continental plate c. 10Ma after the initiation of subduction, accompanied by localized mantle lithosphere metasomatism at the base of the overriding.  After ~12Ma, local horizontal bulk extension caused by roll-back of subducting plate leads to back arc basin development inboard of the subduction zone, despite continued far-field horizontal bulk shortening.  This is associated with a localized zone of strong mantle lithosphere metasomatism at the base of the overriding plate, accompanied by melting of the subducted continental crust.  Continued lithospheric extension leads to decompression melting of the asthenosphere after ~15Ma which stabilizes the extensional process.

At ~18Ma (~6Ma after back arc basin initiation) strong coupling between the subducting and overriding plate overrule extensional tectonics, and the resulting horizontal bulk shortening leads to inversion and closure of the back-arc basin.  After the initiation of bulk shortening, a plume of wet peridotite delaminates from the down-going slab and rises upwards - generating further magmatism in the overlying crust.

Figure 1a & b.

Figures 1a and b illustrate the magmatic evolution of the model with respect to magma addition (km3/km) and composition.  Two main melt generation phases are observed at ~16 Ma and ~19 Ma; the first associated with the initiation of extensional tectonics, the second associated with the flux of wet molten peridotite in mantle.  By the end of model at 21.3Ma, total melt production in the region is comprised of 60% mantle input, 28% oceanic crust component and 10% lower crustal derived.

Model II: Continental subduction/collision scenario; Crustal source magmatism

Model II presents a 200km-wide, oceanic crust flanked by continental lithosphere to either side, which grades to a thickness of 200km.  The width of the oceanic crust is 400km narrower than in Model 1.  The left margin of the oceanic crust is passive; the right is an inclined mantle lithosphere shear zone.  The continental lithosphere to either side has a 30km-thick crustal profile.  The 7km-thick oceanic crust is composed of 2km of hydrothermally-altered basalt underlain by 5km of gabbro.  An initial horizontal bulk shortening rate of 4 cm/a is imposed upon the model for 16 My. 

Subduction of the oceanic crust initiates immediately following the onset of horizontal bulk shortening.  Closure of the ocean basin occurs after ~5Ma, followed by subduction of the incoming passive margin and left-hand continental crust.  During collisional orogenesis, strong coupling between the subducting slab and overriding plate result in the horizontal bulk shortening of the overriding continental crust, precluding arc and/or back-arc basin development.  Burial of the sedimentary rocks of the passive margin and continental crust leads to slab dehydration, partial melting and the formation of a buoyant plume above the subducting slab.  

This plume rises vertically, penetrating the lithosphere of overriding plate in the form of a diapir which includes melt bearing sediments, upper continental crust, and oceanic crust.  In turn, this process induces horizontal bulk extension of the overriding plate, and emplacement of mafic hypabyssals and volcanics at shallow crustal levels.  This is associated with the development of a spatially extensive (~150km-wide) zone of strong mantle lithosphere metasomatism at the base of the overriding plate.  During this time, orogenesis and crustal thickening occurs immediately inboard of the subduction zone.  Delamination of the eclogitised oceanic lithosphere occurs at ~17Ma, followed by rebounding of the remaining subducted continental lithosphere.

Figures 2a and b illustrate the model’s magmatic evolution with respect to addition (km3/km) and composition.  The main melt generation phase is observed after 16Ma, associated with initiation of extensional tectonics and uprising of continental crust-derived magma plume.  At the conclusion of the model at 20 Ma, 52% of the total melt generation in the region is derived from melting of the lower continental crust, 18% from upper crust, 13% from oceanic crust component, 13% from sediments and 5% from the mantle.


Figure 2a & b.