MLDs, LABs, and Moho’s, Oh My! Using Geodynamical Models to Guide Interpretations of Geophysical Observations

Abstract:

As we peer deeper and in more detail into cratonic lithosphere, intriguing structures arise to peak our curiosity and imagination. Seismic discontinuity imaging reveals anomalous features that appear as depths (~100-160 km) that appear to be shallower than the base of the tomographically inferred cratonic lithosphere (~150-300 km). These are now been known as mid-lithospheric discontinuities (MLD). Magnetotelluric data shows regions of low resistivity suggesting regions of hydration or presence of carbon in graphite form. But how do we interpret these observations and how do we use them to learn more about craton formation and evolution? Some explanations for these anomalies include melt accumulation, the lithosphere-asthenosphere boundary (LAB), and phase transitions. We propose that many of the structures might actually be related to the initial formation of the cratonic lithosphere. We use a combination of geodynamic models and observations of the depths and orientations of mid-lithospheric seismic discontinuities from a compilation of recent receiver function observations within various regions of cratonic lithosphere around the world and new results from the West African Craton to test whether some of the imaged structure can be attributed to the initial formation of thickened cratonic lithosphere. The formation of thick, cratonic lithosphere should introduce complex structures that could then be preserved within the long-lived regions (to then be later captured by eager geophysicists). We performed numerical simulations of the thickening of lithosphere. We considered two types of thickening - either a process akin to (1) thrust stacking or (2) viscous thickening of the lithospheric material.. In particular, we looked to delineate regions that showed regions with mid-lithospheric discontinuities occurring at variable depths and orientations. Our geodynamic models provide an explanation for the observation that some cratonic regions exhibit mid-lithospheric discontinuities at variable depths and orientations, but others may not.