Intraplate earthquakes in Australia - Intraplate Earthquakes - page 24

slip rate of 50 m/Ma. Higher relief areas of eastern Australia are associated with erosion

rates of up to 30-50 m/Ma (Weissel and Seidl, 1998 ; Heimsath et al ., 2000 , 2001; Wilkinson
et al ., 2005 ; Tomkins et al ., 2007 ) . Several authors have suggested that as much as 200 m
of relief has been added to the Eastern Highlands over the last

10 Ma (Sandiford, 2003b ;
Holdgate et al ., 2008 ; Braun et al ., 2009 ) . Such estimates are consistent with uplift rates
estimated from strain rates derived from contemporary seismicity (Braun et al ., 2009 ) ;
however, little unequivocal evidence exists to assign this uplift to active faults (cf. Holdgate
et al ., 2006 ) . The approximate equivalence of erosion rates and fault slip rates suggests that
much of the extant relief may be inherited (e.g., Bishop et al ., 1982 ; Pickett and Bishop,
1992 ; van der Beek et al ., 2001 ) .
Perhaps the most spectacular examples of neotectonism in southeast Australia are the
invertedMesozoic Otway andGippsland Basins (D5) (see Figure 2.1 ) (Holdgate et al ., 2003 ,
2007; Sandiford, 2003a ) , which formed by extension of non-cratonic (D4) crust. Within
the Gippsland Basin in particular, the Cretaceous basin deeps now form the topographic
highs at elevations of 200-300 m above sea level. Preliminary cosmogenic radionuclide
ages obtained on overlying folded alluvial sediments (Holdgate et al ., 2007 ) suggest uplift
rates of 60 m/Ma and greater on the major relief-forming faults (McPherson et al ., 2009 ;
Clark et al ., 2011a , 2012). The relatively high fault density, combined with high slip rates
and frequent contemporary seismicity (e.g., Leonard, 2008 ) , identify these basins as among
the most actively deforming parts of the continent (compare with the Flinders Ranges -
D2).
Slip rates on faults underlying folds in the passive margin basins that dominate the
Northwest Shelf region (D6) (NWSSZ, Figure 2.1 ) , are poorly constrained. While locally
having resulted in the uplift of Miocene marine deposits to elevations of over 100 m above
sea level (e.g., van de Graaff et al ., 1976 ) , neotectonic uplift is typically more modest than
in the eastern aulacogen and passive margin basins (e.g., D5).
Long-term (i.e., averaged over several million years) vertical slip rates are known from
some of the range-bounding faults of the Flinders and Mount Lofty Ranges (D2). These
typically vary between 20 and 50 m/Ma (Bourman et al ., 1999 ; Belperio et al ., 2002 ;

Sandiford, 2003b ; Celerier et al ., 2005 ; Quigley et al ., 2006 ) . Bedrock erosion rates have
been recorded at up to 122 m/Ma, but average around 40 m/Ma (Bierman and Caffee,
2002 ; Chappell, 2006; Quigley et al., 2007a, b), allowing that relief is being produced
within the ranges. Several authors suggest that up to half of the 800 m relief has been

built in the current stress regime (Sandiford, 2003b ; Quigley et al ., 2007c ; Braun et al .,
2009 ) . Seismic reflection data indicate that the structural architecture mapped at the surface,
corresponding to the Paleozoic Adelaide Fold Belt which developed over the inversion axis
of a Neoproterozoic rift basin (Jenkins and Sandiford, 1992 ; Paul et al ., 1999 ) , extends
to depths of 10-12 km beneath the ranges (Fl ottmann and Cockshell, 1996 ) . However,
much of the seismicity recorded in the region occurs below this depth (Leonard, 2008 ) ,
potentially in cratonic crust relating to the eastern margin of the Gawler Craton (cf. Figure
2.1 ) . Therefore, it is unclear how the surface-faulting record might be related, if at all, to
most of the instrumental seismicity (cf. Braun et al ., 2009 ) .