The effects of inward and outward dipping craton margin geometry on upper crustal deformation: Insights from analogue modelling

Craton margins undergo intense deformation influenced by the pre-existing crustal and lithospheric architecture, rheology, and far-field kinematics. The role of rheological contrasts and weak zones at the edge of the craton has been discussed, but it is unclear whether deformation in the upper crust is influenced by the geometry of the craton margin itself (i.e., whether the margin dips towards or away from the interior of the craton). Our analogue experiments are aimed at studying the influence of craton margin geometry on structures formed during rifting and inversion, as craton margins are prone to reworking and reactivation during superimposed tectonic events.

The experiments are designed based on the geometries of the eastern and southern margins of the North Australian Craton which has experienced multiple stages of extension and shortening. The inward vs. outward dipping craton margins in these areas were interpreted from crustal-scale seismic reflection data.  In our experiments, we see that strain and deformation style varies with proximity to the craton margin. During the extensional phase of both inward and outward dipping experiments, we observe that rifts are mainly formed by boudinage and necking in the lower crust. The inward dipping model prevents the propagation of a major normal fault at the margin, resulting in a number of smaller faults. Subsequent shortening of the inward dipping model results in modest basin inversion above the craton margin, suggesting that the majority of strain is accommodated by reactivation of normal faults away from the margin. In contrast, the outward dipping model shows the propagation of a single major normal fault along the craton margins, leading to significant thinning of the lower crust. A major rift is also being formed away from the craton margin in this model. Inversion of the outward dipping craton margin model shows more intense inversion at the margin compared to the inward dipping model, with lower strain and smaller reactivation of normal faults away from the margin. We can therefore conclude that the geometry of a craton margin exerts a first-order control on the deformation of the upper crust during rifting and subsequent inversion.