Intraplate earthquakes in Australia - Intraplate Earthquakes - page 31

2.6.1 Mechanical and thermal influences
It has been proposed that intraplate regions with higher seismic potential have pre-existing
zones of weakness (Sykes, 1978 ; Talwani and Rajendran, 1991 ; Stuart et al ., 1997 ; Kenner
and Segall, 2000 ; Dentith and Featherstone, 2003 ) , intersecting faults (Talwani, 1988 ,
1999), elevated heat flow (Liu and Zoback, 1997 ; Celerier et al ., 2005 ; Hillis et al .,
2008 , Holford et al ., 2011 ) , crustal anomalies (Campbell, 1978 ; Kenner and Segall, 2000 ;
Gangopadhyay and Talwani, 2003 ; Pandey et al ., 2008 ; Assump¸ ao and Sacek, 2013 ) or
can be identified by crustal boundaries inferred from potential field data (Langenheim
and Hildenbrand, 1997 ; Lamontagne et al ., 2003 ; van Lanen and Mooney, 2007 ; Dentith
et al ., 2009 ) . It is likely that the variety of models reflects the range of mechanisms
that are operating, and although all of the above mechanisms have demonstrated local
applicability in the Australian context, counter-examples are abundant. This is particularly
the case where models relying on thermal mechanisms for strain localisation have been
proposed.
Sandiford and Egholm ( 2008 ) argue that enhanced seismicity along some parts of the
Australian continental margin is a consequence of thermal weakening due to steady-state
heat flow across the lithospheric thickness steps between oceanic and continental crust (e.g.,
SESZ, Figure 2.1 ) and extended continental and non-extended cratonic crust (e.g., SWSZ,
Figure 2.1 ) (Fishwick et al ., 2008 ; Kennett et al ., 2013 ) . While the hypothesis is intuitively
appealing, in Australia and along the eastern seaboard of North America (Wheeler and
Frankel, 2000 ) , several lines of evidence suggest that the contribution of this mechanism to
the continental strain budget over geological timescales is minor.
For example, east of the large lithospheric thickness step on the western boundary of
the SWSZ (i.e., across the Darling Fault), fault scarps are randomly distributed (Clark,
2010 ) and the topography predicted if instrumental seismic moment release rates are
extrapolated to million-year timescales (Braun et al ., 2009 ) is absent. Given extremely
low rates of bedrock erosion (Bierman and Caffee, 2002 ; Belton et al ., 2004 ; Chappell,
2006), this finding implies that the locus of seismicity in the SWSZ is transitory, rather
than responding to steady-state heat flow at the margin. Furthermore, very little seismicity,
paleo- or instrumental, can be correlated with the dramatic transition from cratonic to non-
cratonic lithospheric thickness in eastern Australia (i.e., the Tasman Line, Figures 2 . 1 and
2.3), and significant instrumental seismicity proximal to the eastern seaboard (the SESZ)
is not associated with a large lithospheric thickness step (Fishwick et al ., 2008 ) .
More generally, Holford et al . ( 2011 ) propose that the thermal properties of the crust and
upper mantle exert a regional-scale (100-1000 km) modulating control on which parts of the
Australian lithosphere undergo (seismogenic) failure and which parts experience relatively
less deformation (cf. Celerier et al ., 2005 ; Sandiford and Quigley, 2009 ) . Specifically,
these authors invoke relatively high heat flow to explain localisation of seismic moment
release and deformation in the Flinders Ranges Seismic Zone (FRSZ, Figure 2.1 ) compared
to the flanking Murray Basin and Nullarbor Plain. The correlation is imperfect; the heat
flow anomaly does not extend to the Gippsland and Otway Basins (Densley et al ., 2000 ;