Diffuse Cretaceous-Cenozoic rifting in the Southern Ross Sea: the influence of inheritance and kinematics

Continental Rift systems often involve narrow regions, which accommodate all the stretching. In some cases, the initial extension occurs with a diffuse style and may successively produce a narrow rift. An example is the West Antarctica Rift System, bearing evidence of the concurrent formation of multiple basins normal to the rift axis. This rift system has undergone extension between the Cretaceous and the middle Neogene age (105 to 11 Ma [1, 2]), due to the sea floor spreading in the northwestern Ross Sea. It is composed of three main basins (Victoria Land Basin, Central Trough, and Eastern Basin), which cover a present-day length of 900-1000 km, encompassing the lateral contact between the cratonic domains of East Antarctica and West Antarctica Phanerozoic lithosphere. The different basins, bounded by structural highs, exhibit significant variations in the thickness and thinning of the underlying crust and lithosphere. This multiple-basin pattern suggests that, at least for some part of the rifting, the deformation occurred in a diffuse way, instead of being localized in a small portion of the rift system [3].

The factors controlling these deformation styles have been identified in the inheritance of structures and thermal/rheological heterogeneities [4], which acted concurrently with the extensional kinematics in shaping the present-day rift architectures. Therefore, an improved knowledge on how different thermo-structural initial conditions (e.g. lateral contacts, thermal transients, accumulated strain softening) influence the outcome of rifting may help identify the most likely state at the onset of rifting. To this purpose, we implement a series of numerical models, testing several starting structural conditions (rheology, temperature, prior damage) and distribution of extensional velocity (a single phase or multiple pulses, for the same total extension) that could trigger this peculiar diffuse deformation pattern.

To build a 2-D simplified geometry of the structures of the rift system, we took as a reference the seismic profiles BGR-02 and ACRUP2, normal to the rift axis, along the 77° S parallel [5].  We assumed an initial crustal thickness of about 50 km and a kinematic pattern consisting of two main distinct extension phases, covering the Cretaceous-Cenozoic interval [1, 6].

Modelling was carried out using the open source Underworld2 code [7], which relies on Lagrangian integration point finite element approach and provides a Python API to construct, run, and visualize the output of geodynamic models. The results show that the models that are more consistent with the observations require the existence of peculiar a-priori inherited features. In addition to the role of inheritance, diffuse patterns are favoured, for the same extension amount, by slow and long-lasting rifting phases, with respect to fast and short time pulses.

This work was carried out in the context of PNRA project "Onset of Antarctic Ice Sheet Vulnerability to Oceanic conditions (ANTIPODE)".

[1] Behrendt et al. (1991) https://doi.org/10.1029/91TC00868

[2] Granot & Dyment (2018) https://doi.org/10.1038/s41467-018-05270-w

[3] Huerta & Harry (2007) https://doi.org/10.1016/j.epsl.2006.12.011

[4] Perron et al. (2021) https://doi.org/10.1051/bsgf/2020038

[5] Trey et al. (1999) https://doi.org/10.1016/S0040-1951(98)00155-3

[6] Davey & De Santis (2006) https://doi.org/10.1007/3-540-32934-X_38

[7] Mansour et al. (2020) https://doi.org/10.21105/joss.01797