Geometric complexity of the Woodroffe Thrust (Musgrave Ranges, central Australia) recorded in hanging wall Al-silicate-bearing peraluminous gneisses and hosted pseudotachylytes

The Woodroffe Thrust (WT) in the Musgrave Ranges (central Australia) is a shallowly south-dipping crustal-scale mylonitic zone extending E-W for over 600 km. The WT, developed during the intracontinental Petermann Orogeny (630-520 Ma), placed hanging wall lower-crustal granulite to upper-amphibolite facies rocks of the Fregon Subdomain (FS) over footwall amphibolite-facies mid-crustal gneisses and granitoids of the Mulga Park Subdomain (MPS). The WT mylonites largely affect the MPS and to a minor extent the FS. Towards the WT, the hanging wall hosts the largest volumes of supposedly deep-seated, tectonic pseudotachylytes (pst) worldwide, also partially involved in mylonitization adjacent to the WT. The WT has been inferred to have only a very small difference in pressure (depth) over the ca. 60 km of N-S exposure along the transport direction, from 1.0 – 1.3 GPa to 0.8 – 1.1 GPa, thus representing effectively a very shallowly dipping structure^[1]. However, it was noted that these pressure estimates had to be considered with some caution due to not always ideal mineral compositions. Here we present new pressure constraints in northern outcrops from the eastern segment of the thrust suggesting a more complex geometry than previously inferred, with significant variation in depth along the structure.

Pseudotachylyte-bearing peraluminous gneisses, from two localities ca. 80 km apart (Sentinel Bore, SB, to the east and Kelly Hills, KH, to the west) in the immediate hanging wall of the WT, were investigated to establish the ambient conditions during seismic faulting. The gneisses display mm-thick alternation of quartz-feldspar and cordierite-sillimanite-rich layers, including sparse garnet, magnetite, ilmenite, and biotite. Along microfractures of the pst damage zone (i) sillimanite was fractured and remained unaltered; (ii) cordierite broke down to either an andalusite + quartz + biotite symplectite overgrown by kyanite (SB), or just kyanite (KH); and (iii) K-feldspar developed flame perthites. The pst at SB and KH also show a different mineralogy. At SB, pst assemblages include (i) andalusite (pseudomorphosed by biotite) + quartz intergrowths rimmed by plagioclase and K-feldspar; (ii) sillimanite microlites overgrowing sillimanite clasts; (iii) microlitic kyanite, and (iv) poikilitic garnet as the latest grown phase. At KH, pst assemblages include (i) cordierite + quartz intergrowths; (ii) sillimanite microlites overgrowing sillimanite; (iii) microlites of kyanite, and (iv) poikilitic garnet. Andalusite is absent at KH.

The newly identified andalusite, stable in pst, sheared pst and along microfractures in the host rock at SB indicates pressures ≤ 0.5 GPa during seismic faulting, i.e. significantly lower than in the more southern portion close to Mount Woodroffe (ca. 60 km to the SW of SB)^[2]. The absence of andalusite at KH implies a complex undulating geometry for the WT.

 

 

1: Wex et al., 2017, Geometry of a large‐scale, low‐angle, midcrustal thrust (Woodroffe Thrust, central Australia). Tectonics, 36(11), 2447-2476.

2: Hawemann et al., 2018, Pseudotachylytes as field evidence for lower-crustal earthquakes during the intracontinental Petermann Orogeny (Musgrave Block, Central Australia). Solid Earth, 9, 629-648