Enhanced intraplate seismicity along continental margins: Some causes and consequences

Abstract

In Australia, much of the seismic activity is restricted to zones up to several hundred kilometres wide inboard of continental margins at high angle to the trend in maximum horizontal compression ( S Hmax). Intriguingly, along the margin-side of one such zone, a near optimally oriented, crustal-scale structure – the Darling Fault – has no recorded historical seismicity. To explain these enigmatic features we explore how the compositional and thermal structuring of continental passive margins may lead to variations in mechanical response to imposed forcing using forward modelling. We show how lateral heat flow across such margins can produce increases in Moho temperatures by several 10s of degrees relative to the continental interior extending up to 200 km inboard of the ocean–continent transition, provided the step in thermal lithosphere thickness into the continent is greater than 30 km. For smaller lithospheric steps Moho temperatures beneath continental margins will be cooler than beneath interiors. Our mechanical models show that such thermal effects can elevate strain rates along the margin by several factors relative to continental interiors, when subject to far-field forcing. In the shallow seismogenic-realm, the strain-rate maximum is concentrated at around 100–150 km inboard of the ocean to continent lithospheric step, providing a plausible explanation for the pattern of seismicity observed in some of Australia's marginal seismic zones. This interpretation requires that strength changes associated with Moho temperature variations of several 10s of degrees are sufficient to override the reactivation potential of near optimally oriented crustal-penetrative structures, with thermal processes providing an important “switch” controlling reactivation potential of existing structures. It also provides an insight into the mechanism of formation of continental ribbons during rifting of thermally mature margins.