Ancient Continental Lithosphere Dislocated Beneath Ocean Basins Along the Mid‐Lithosphere Discontinuity: A Hypothesis

Abstract

The documented occurrence of ancient continental cratonic roots beneath several oceanic basins remains poorly explained by the plate tectonic paradigm. These roots are found beneath some ocean‐continent boundaries, on the trailing sides of some continents, extending for hundreds of kilometers or farther into oceanic basins. We postulate that these cratonic roots were left behind during plate motion, by differential shearing along the seismically imaged mid‐lithosphere discontinuity (MLD), and then emplaced beneath the ocean‐continent boundary. Here we use numerical models of cratons with realistic crustal rheologies drifting at observed plate velocities to support the idea that the mid‐lithosphere weak layer fostered the decoupling and offset of the African continent's buoyant cratonic root, which was left behind during Meso‐Cenozoic continental drift and emplaced beneath the Atlantic Ocean. We show that in some cratonic areas, the MLD plays a similar role as the lithosphere‐asthenosphere boundary for accommodating lateral plate tectonic displacements.

Accretion of the cratonic mantle lithosphere via massive regional relamination

“…The existing CLM relicts below different tectonic units show that some cratons were not as stable as previously understood ( 2 , 10 , 58 ), especially the lithospheric mantle ( 9 , 40 , 58 , 75 – 77 ). The relative stability of the cold crustal section of cratons is likely accompanied by the foundering-MRR cycles at mantle depths, especially during the cratonization stage, when complex CLM structures were formed.…”

Icelandia

“…These observations suggest that the high stand of the Marion Rise is supported by buoyant mantle delaminated from beneath African cratonic lithosphere during Gondwana breakup (Zhou and Dick, 2013;Dick et al, 2014;Dick and Zhou, 2015). Independent evidence for such a process is provided by seismology, which detects interfaces that are continuous beneath both the African cratonic regions and adjacent ocean basins (Wang et al, 2017). That study suggests that continental lithosphere can extend far from the coasts, out into the oceans, as we propose for Icelandia.…”

On the Role of Lower Crust and Midlithosphere Discontinuity for Cratonic Lithosphere Delamination and Recycling

“…If the lower lithosphere is buoyant, the resultant stress ( σ II ) would be not enough to cause delamination. Under this condition, a sufficiently weak MLD would lead to lateral shearing and displacement along the MLD during plate motions and leave some cratonic buoyant roots behind and beneath oceanic basins (Wang et al, ). The resultant stress ( σ II ) is also affected by tectonic stress, topographic changes, and resistance of the ambient asthenospheric mantle, which cannot be constrained in this paper.…”

“…Recently, seismic low‐velocity layers at midlithospheric depths, henceforth midlithosphere discontinuity, MLD, were detected in many cratonic areas (80–150 km deep; Karato et al, ; Selway et al, ). The nature of this layer is controversial, and many studies proposed that the MLD is a weak, critical layer for cratonic lithosphere stability (Chen et al, ; Liao et al, ; Selway et al, ; Thybo & Perchuć, ; Wang et al, ). This idea is supported by recent seismic receiver function studies in the NCC (Chen et al, ).…”

“…Delamination along lower crustal depths near convergent plate margins has been demonstrated in many numerical modeling studies (e.g., Göğüş & Pysklywec, ; Magni et al, ), yet it has been poorly tested within cratonic interiors. The influence of a weak MLD on extension and lithosphere decoupling has been tested by previous modeling works (Liao et al, ; Wang et al, ), whereas the role of the MLD on delamination has remained unaddressed. In this study, we use 2‐D numerical models to address the stability of the lithosphere and delamination, under the effect of gravitational instabilities.…”