It's a Demon of a Fault

Many people have requested that I do a post on the Demon Fault. I've struggled to put something together because structural geology is not one of my strong points and secondly because there was so little published information about it, except for some specific papers in the 1970’s. Thankfully, a few months ago Babaahmdi & Rosenbaum (2013) published a detailed paper summarising what was known in the 1970s, presenting how the fault appears, how it seems to have developed and how it may fit into the development of eastern Australia. It is worth noting that Gideon Rosenbaum from The University of Queensland has been the major researcher on New England structural geology for the last 5 years. If it was not for him and his student’s research we would be struggling to understand some of the basic features of the older rocks of the region including the Demon Fault.

Large faults are usually have quite distinctive landscape features. The Demon Fault is mainly a transverse type fault (movement on the fault horizontally rather than vertically) which displays very obvious topographical features.  Transverse faults often form valleys, where the rock of the fault has been broken down into what is called gouge or rock-flour. Gouge is very weak material. It is easily eroded and rivers often preferentially follow the route of the fault carving out the gouge into deep valleys.  The presence of deformational features in the surrounding rock can give an indication of how deep the fault was when it was active.

The Demon Fault is a prominent feature because it is evident from a series of valleys from near the Queensland Border to Dorrigo. At the Queensland end it is partly obscured by the Cenozoic Main Range Volcanics and in the Dorrigo area the end is obscured by the Cenozoic aged Ebor Volcanics. Geological maps of the area show a nice linear feature with obvious truncation of pre-existing geological units. Aerial photos also show the fault up nicely with streams preferentially flowing along the trace of the fault and contrasting with the rugged forested mountains surrounding it. I’ve never taken a photo of any part of the Demon Fault but a nice photo taken from an aeroplane can be found here: http://www.panoramio.com/photo/35890361

The Timbarra River has followed the Demon Fault creating linear valley
http://www.panoramio.com/photo/35890361 (used with permission)

Korsch et al (1978) observed that the Demon Fault had displaced several geological units including intrusions of the Bungulla Monzogranite (now known as the Rocky River Monzogranite), Dumbudgery Granodiorite and Newton Boyd Granodiorite as well as the Drake Volcanics. The fault was interpreted as a dextral strike-slip fault (a fault where the eastern side had moved south relative to the western side). Korsch et al (1978) calculated that the fault had displaced these units 17km which is substantial in Eastern Australia. Dating of the displaced granite intrusions provides a possible maximum date of within Triassic period (249-232 million years). The nature of deformation features adjacent to the faulting indicates that the fault was shallow and/or was created in a brittle environment. Badaahmadi & Rosenbaum (2013) speculate that the timing of the faulting may actually be similar to that of faulting and extension in the earth’s crust that formed the Ipswich and Clarence-Moreton Basins (more about this in future posts).

There are many factors in understanding the Demon Fault. It is interesting to note that other authors have come up with different lengths of displacement including 30km in the northern part of the fault and 23Km in the central part. Badaahmadi & Rosenbaum (2013) have calculated that the northern part of the fault displaced 35km, in the centre by 25km and south by 19km. Some components of reverse faulting (where one side of the fault slid down and away from the other side) were observed. Additionally, it was noted that the Demon Fault did not appear to follow one big long line but instead had numerous splays (deviations, splitting, etc) especially in the south. Badaahmadi & Rosenbaum (2013) suspect that there may be two causes to the different lengths:

1. Splays may have created movement of the fault which had a vertical component as well as horizontal.
2. There may have been some fault reactivation of the northern part of the fault as recently as the Cenozoic era.

Both of these possibilities really need a discussion in their own right, rather than cursory mention. So, I’ll get back to these in a future post.  I also want to cover the significance of the Demon Fault in formation of the Texas and Coffs Harbour Oroclines which are incredibly large features that I’ve briefly touched on in an earlier post about the South Solitary Island.

References/bibliography:
*Babaahmadi, A. & Rosenbaum, G. 2013. Kinematics of the Demon Fault: Implications for Mesozoic strike-slip faulting in eastern Australia. Australian Journal of Earth Sciences. V.60
*Korsch, R.J., Archer, R. & McConachy, G.W. 1978. The Demon Fault. Journal and Proceedings, Royal Society of New South Wales. V111.