EGU2020-6279
https://doi.org/10.5194/egusphere-egu2020-6279
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Neotectonic constraint on models of strain localisation within Australian Stable Continental Region (SCR) crust

Daniel Clark
Daniel Clark
  • Geoscience Australia, Community Safety Branch, Canberra, Australia (dan.clark@ga.gov.au)

The mechanisms that lead to the localisation of stable continental region (SCR) seismicity, and strain more generally, remain poorly understood. Recent work has emphasised correlations between the historical record of earthquake epicentres and lateral changes in the thickness, composition and/or viscosity (thermal state) of the lithospheric mantle, as inferred from seismic velocity/attenuation constraints. Fluid flow and the distribution of heat production within the crust have also been cited as controls on the location of contemporary seismicity. The plate margin-centric hypothesis that the loading rate of crustal faults can been understood in terms of the strain rate of the underlying lithospheric mantle has been challenged in that a space-geodetic strain signal is yet to be measured in many SCRs. Alternatives involving the release of elastic energy from a pre-stressed lithosphere have been proposed.

The Australian SCR crust preserves a rich but largely unexplored record of seismogenic crustal deformation spanning a time period much greater than that provided by the historical record of seismicity. Variations in the distribution, cumulative displacement, and recurrence characteristics of neotectonic faults provide important constraint for models of strain localisation mechanisms within SCR crust, with global application. This paper presents two endmember case studies that illustrate the variation in deformation characteristics encountered within Australian SCR crust, and which demonstrate the range and nature of the constraint that might be imposed on models describing crustal deformation and seismic hazard.

The ~0.5 m high 2018 MW 5.3 Lake Muir earthquake scarp in southwest Western Australia is representative of a class of ruptures in the Precambrian SCR of Australia where the scarps are isolated from neighbouring scarps and there is little or no landscape evidence for recurrence of morphotectonic earthquakes, or of the construction of regional tectonic relief. In contrast, scarps in the Phanerozoic SCR of eastern Australia typically occur within a scarp-length of neighbouring scarps, and demonstrate extended histories of recurrence of morphotectonic events. For example, the ~75 km-long Lake George fault scarp is associated with a vertical displacement of ~250 m which accrued as the result of many morphotectonic earthquakes over the last ca. 4 Myr. The scarp links into neighbouring scarps, forming a belt-like arrangement that defines the topographic crest of the southeast Australian highlands. The limited data available indicates that recurrence is highly episodic, with periods of fault activity potentially coinciding with changes at the plate boundaries.

How to cite: Clark, D.: Neotectonic constraint on models of strain localisation within Australian Stable Continental Region (SCR) crust, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6279, https://doi.org/10.5194/egusphere-egu2020-6279, 2020

Comments on the presentation

AC: Author Comment | CC: Community Comment | Report abuse

Presentation version 1 – uploaded on 05 May 2020
  • CC1: Comment on EGU2020-6279, Tamarah King, 05 May 2020

    Dan - a few questions for you!

    - could you elaborate on the final figure? Are they results from long-term GPS strain rate studies?

    - in your graph from Lake George (very cool!) do you think the offset at 3.2Ma is multiple events in an 'active period'? Seems way too much for a single event

    - seems there is a 'missing' offset event at Lake George between ~1.5Ma and now, or am I reading the graph incorrectly?  

    • AC1: Reply to CC1, Dan Clark, 06 May 2020

      - could you elaborate on the final figure? Are they results from long-term GPS strain rate studies?

      These are velocity residuals from Australia’s continuously monitoring GPS network, calculated in 2017. It is somewhat heterogenous as to the length of record, but in the order of a decade (with some approaching 2 decades). We hope to build in data from several campaign GPS networks in higher seismicity areas in the next few years.

      - in your graph from Lake George (very cool!) do you think the offset at 3.2Ma is multiple events in an 'active period'? Seems way too much for a single event

      Certainly, the active period relates to multiple events. We studied the slip evolution of this fault by shooting a high-resolution seismic reflection profile from the scarp out into the Lake (dry at the time). The resolution did not allow us to ‘see’ individual events, but instead the interaction of folding and faulting with lake sediments during broader active periods (constrained by paleomagnetism and CRN dating). I look to our previous work on the Cadell Fault to understand what might be happening in active periods (https://doi.org/10.1144/SP432.2). There, 20 m of relief was built in a ~40 thousand-year period which involved at least six M>7 earthquakes. A manuscript on this work is in preparation, so stay tuned!

      A broader question is what caused the Lake George Fault to turn on at ca. 4 Ma and turn off again at ca. 0.78 Ma (i.e. the most recent super-cycle). Does it relate to plate margin changes, or migration of the locus of strain release around the Australian continent (i.e. a cascading destabilisation as Eric Calais would put it)?There is some evidence for a prior super cycle of activity in the Miocene on this fault. Hopefully we will be drilling to investigate this in the next couple of months (pandemic willing).

      - seems there is a 'missing' offset event at Lake George between ~1.5Ma and now, or am I reading the graph incorrectly?  

      In the lower panel of part 3 you can see the upper grey sedimentary unit. This is a heavy clay relating to complete closure of all outlets of the basin. As this unit is largely structureless, we see very little to guide us in the seismic reflection data. However, we note a very large spike in sedimentation rate at ~1 Myr in borehole data, and contend that this relates to a further punctuated uplift event (i.e. the red diamond).

  • CC2: Comment on EGU2020-6279, Christoph Grützner, 08 May 2020

    Hi Dan,

    A few comments on the question at the end of your poster:
    (i) Very slow tectonic strain accumulation
    --> Difficult to prove unless we become able to measure stress directly and with sufficiently precision. Or someone comes up with the new post-GPS super tool.

    (ii) Depletion of a fossil stress pool

    I think this is unlikely, because as you say some faults seem to switch on and off every few millions of years. Wouldn't the fossil stress then be gone at some point? Shouldn't we rather see a decline in EQ activity if this was true?

    (iii) Local concentrators enhancing plate margin processes

    I guess one would have to look in detail in all available evidence from the surrounding plate boundaries in terms of changes in velocity and direction. With our proxies such as volcanic activity, perhaps erosion, palmag it might be difficult to achieve the necessary resolution (small change in subduction angle/direction might be just enoughto push your faults over their limits).

    (iv) Other?

    Have you thought about mantle processes that could lead to stress variations in the crust? Perhaps very slow long wave-length uplift & subsidence cycles or changes basal drag? No idea how to check this except with modelling...

    I really like your and Tamarah's work on Australia. It's fascinating and puzzling!
    Cheers
    Christoph

    • AC2: Reply to CC2, Dan Clark, 11 May 2020

      (iii) “guess one would have to look in detail in all available evidence from the surrounding plate boundaries in terms of changes in velocity and direction”.

       

      Yes indeed! There is a pattern remaining to be found. In Quigley et al (2010, ) we note that:

      “There is some indication that the temporal clustering behaviour emerging from single fault studies may be symptomatic of a larger picture of the more or less continuous tectonic activity from late Miocene to recent being punctuated by ‘pulses’ of activity in specific deforming regions. For example, major deformation episodes are constrained to the interval 6–4 Ma in SW Victoria (Paine et al. 2004) and 2–1 Ma in the Otway Ranges (Sandiford 2003a). An episode of deformation ceased at 1.0 Ma in the offshore Gippsland Basin although it continued onshore until c. 250 ka (Holdgate et al. 2003).”

       

      The Late Miocene saw increased coupling between the Australian and pacific plates, and the initiation of uplift along the NZ and PNG alps. Over much of Australia this is taken as the onset of the current stress regime. I can’t remember his reasoning off the top of my head, but Mike Sandiford contends that stress is still building from this event. Beau Whitney’s more recent work in the NW suggests that there is an evolving active intraplate region (the West Australia Shear Zone) extending from the northern plate boundary that involved a kick of deformation from 3 Ma. It’s not yet clear what effect this had (or is having) over the broader Australian plate.

       

      (iv) “Have you thought about mantle processes that could lead to stress variations in the crust? Perhaps very slow long wave-length uplift & subsidence cycles or changes basal drag? No idea how to check this except with modelling..."

       

      Unfortunately, mine is not the brain of a modeller – I focus on operating a shovel in the trenches! 😉

       

      One of the amazing products of the aridity and relative lack of tectonics in Australia is that it is a wonderful canvas on which to paint changes in surface processed relating to dynamic topographic change as we move to the north over a heterogeneous mantle. The Quigley et al (2010) paper mentioned above summarises earlier work by Mike Sandiford and his group that documents the modes of deformation evident on the Australian continent. In particularly there is a lithospheric scale buckling (200 km wavelength) similar to that seen on Peninsular India, and a thousand kilometre scale tilting. The former *may* be seismogenic in the Flinders Ranges (circumstantial), and the latter uplift might drive erosion, which in turn *may* promote seismicity. Hard to demonstrate! Though for the erosion hypothesis, I am part of a grant application that Stephane Mazzotti has recently put in to investigate this. There is also an uplift (and volcanism) relating to cavitation behind the southern trailing edge of the continent – so fast are we travelling ()! 😉


      Thankyou for your interest!

      • CC3: Reply to AC2, Christoph Grützner, 11 May 2020

        Thanks a lot for these interesting details and all the best for further studies!

        Christoph