The role of magma supply in fragmentation of oceanic lithosphere

Observations suggest that the oceanic lithosphere is shaped by dike intrusions and faulting in proportions that depend on the spreading rate (Carbotte et al., 2016). Yet it remains unclear how the interplay between magmatism and faulting during seafloor spreading affects mid-ocean ridge (MOR) axial morphology, fault spacing, and the pattern of abyssal hills (Buck et al., 2005, Huybers et al., 2022). Here we present two-phase flow numerical models of oceanic lithosphere extension that reconcile the nonlinear brittle behaviour of the lithosphere with mantle melting and magma transport through the lithosphere. 

Fast-spreading ridges show symmetric normal faulting and axial highs, while slow-spreading ridges show an asymmetric fault pattern and axial valleys. Previous work has focused on explaining the MOR fault pattern by tectonic or magmatic-induced deformation. In the first scenario, faults result from tectonic stretching of the thin axial lithosphere during amagmatic periods (Forsyth 1992), while in the second scenario, dike-injection may create stresses that activate extensional faults (Carbotte et al., 2016). Current state-of-the-art models (i.e., Buck et al., 2005) use a single-phase formulation for the deformation of oceanic lithosphere in which a prescribed axial dike may accommodate both magmatic and tectonic extension. In these models, the fault pattern depends on M – the fraction of plate separation rate that is accommodated by magmatic dike opening. While M-models are able to explain a number of observations, M represents a simple parameterization of complex fracture dynamics of sills, dikes, and faults. In particular, M-value models neglect fault–dike interaction and other modes of melt transport and emplacement in the lithosphere (Keller et al., 2013). 

Here we build a 2-D oceanic lithosphere extension model that incorporates a new poro- viscoelastic–viscoplastic theory with a free surface (Li et al., in review) to robustly simulate plastic representations of dikes and faults in a two-phase magma/rock system. We hypothesise that magma supply controls the pattern of dike–fault interaction in oceanic extension settings. We present simplified model problems to compare results with those from M-value models. These enable us to address the significance of M in terms of fundamental magma and lithospheric processes. We then focus on development of fault patterns, magma pathways and crustal production at fast-/slow-spreading ridges.

 

References

Buck et al., 2005, Nature, doi:10.1038/nature03358.

Carbotte et al., 2016, Geol. Soc. London, doi:10.1144/SP420.

Forsyth, 1992, Geology, doi:10.1130/0091-7613(1992)020<0027:FEALAN>2.3.CO;2.

Huybers et al., 2022, PNAS, doi:10.1073/pnas.2204761119.

Keller et al., 2013, GJI, doi:10.1093/gji/ggt306.

Li, Y., Pusok, A., Davis, T., May, D., and Katz, R., (in review). Continuum approximation of dyking with a theory for poro-viscoelastic–viscoplastic deformation, GJI.