Asymmetric three-dimensional topography over mantle plumes

Plume-MOR decoupling and the timing of India-Eurasia collision

The debatable timing of India-Eurasia collision is based on geologic, stratigraphic, kinematic, and tectonic evidence. However, the collision event disturbed persistent processes, and the timing of disturbance in such processes could determine the onset of India-Eurasia collision precisely. We use the longevity of Southeast Indian Ridge (SEIR)-Kerguelen mantle plume (KMP) interaction cycles along the Ninetyeast ridge (NER) as a proxy to determine the commencement of India-Eurasia collision. The geochemical signature of the KMP tail along the NER is predominantly that of long-term coupling cycles, that was perturbed once by a short-term decoupling cycle. The long-term coupling cycles are mainly of enriched mid-ocean ridge basalts (E-MORBs). The short-term decoupling cycle is mostly derived from two distinct sources, MOR and plume separately, whereas the KMP is still being on-axis. The onset of India-Eurasia collision led to continental materials recycling into the mantle; hence the abrupt enrichment in incompatible elements at ca. 55 Ma, the MOR-plume on-axis decoupling, and the abrupt slowdown in the northward drift of the Indian plate was induced by the onset of India-Eurasia collision, thereafter MOR-plume recoupled.

Causes and consequences of asymmetric lateral plume flow during South Atlantic rifting

This article tackles the longstanding question of why some continental rifting was associated with major pulses of excess volcanism that formed “volcanic rifted margins,” yet other rifting, even nearby, was not. After reviewing the South Atlantic rifting exemplar, we present results from an improved type of three-dimensional calculation of mantle flow that avoids many of the artificial boundary condition-related pitfalls in prior numerical studies that did not predict asymmetric rift-related volcanism. Our experiments match the observed development of asymmetric volcanism during South Atlantic rifting, with important implications for plume-influenced rifting of continents. In particular, the scenario does not require a conventionally assumed “starting plume head” to provide the hotter-than-average mantle that induces excess rift magmatism.

Volcanic rifted margins are typically associated with a thick magmatic layer of seaward dipping reflectors and anomalous regional uplift. This is conventionally interpreted as due to melting of an arriving mantle plume head at the onset of rifting. However, seaward dipping reflectors and uplift are sometimes asymmetrically distributed with respect to the subsequent plume track. Here we investigate if these asymmetries are induced by preexisting lateral variations in the thickness of continental lithosphere and/or lithospheric stretching rates, variations that promote lateral sublithospheric flow of plume material below only one arm of the extending rift. Using three-dimensional numerical experiments, we find that South Atlantic rifting is predicted to develop a strong southward asymmetry in its distribution of seaward dipping reflectors and associated anomalous relief with respect to the Tristan Plume that “drove” this volcanic rifted margin, and that the region where plume material drains into the rift should experience long-lived uplift during rifting—both as observed. We conclude that a mantle plume is still needed to source the anomalously hot sublithospheric material that generates a volcanic rifted margin, but lateral along-rift flow from this plume, not a broad starting plume head, is what controls when and where a volcanic rifted margin will form.

Two deep-mantle sources for Paleocene doming and volcanism in the North Atlantic

The North Atlantic Igneous Province (NAIP) erupted in two major pulses that coincide with the continental breakup and the opening of the North Atlantic Ocean over a period from 62 to 54 Ma. The unknown mantle structure under the North Atlantic during the Paleocene represents a major missing link in deciphering the geodynamic causes of this event. To address this outstanding challenge, we use a back-and-forth iterative method for time-reversed global convection modeling over the Cenozoic Era which incorporates models of present-day tomography-based mantle heterogeneity. We find that the Paleocene mantle under the North Atlantic is characterized by two major low-density plumes in the lower mantle: one beneath Greenland and another beneath the Azores. These strong lower-mantle upwellings generate small-scale hot upwellings and cold downwellings in the upper mantle. The upwellings are dispersed sources of magmatism and topographic uplift that were active on the rifted margins of the North Atlantic during the formation of the NAIP. While most studies of the Paleocene evolution of the North Atlantic have focused on the proto-Icelandic plume, our Cenozoic reconstructions reveal the equally important dynamics of a hot, buoyant, mantle-wide upwelling below the Azores.

Afar triple junction triggered by plume-assisted bi-directional continental break-up

Divergent ridge-ridge-ridge (R-R-R) triple junctions are one of the most remarkable, yet largely enigmatic, features of plate tectonics. The juncture of the Arabian, Nubian, and Somalian plates is a type-example of the early development stage of a triple junction where three active rifts meet at a ‘triple point’ in Central Afar. This structure may result from the impingement of the Afar plume into a non-uniformly stressed continental lithosphere, but this process has never been reproduced by self-consistent plume-lithosphere interaction experiments. Here we use 3D thermo-mechanical numerical models to examine the initiation of plume-induced rift systems under variable far-field stress conditions. Whereas simple linear rift structures are preferred under uni-directional extension, we find that more complex patterns form in response to bi-directional extension, combining one or several R-R-R triple junctions. These triple junctions optimize the geometry of continental break-up by minimizing the amount of dissipative mechanical work required to accommodate multi-directional extension. Our models suggest that Afar-like triple junctions are an end-member mode of plume-induced bi-directional rifting that combines asymmetrical northward pull and symmetrical EW extension at similar rates.

Long-distance impact of Iceland plume on Norway's rifted margin

Results of a 3D modeling study inspired by recent seismic tomography of the Northern Atlantic mantle suggest that a complex pattern of hot mantle distribution with long horizontal flows originating from the Iceland mantle plume has been the norm in the geological past. In the Northern Atlantic the Iceland plume has a strong long-distance impact on intraplate deformation affecting both onshore and offshore parts of Norway’s rifted margin. As a result, this margin is characterized by large magnitude differential topography sustained over at least several tens of Myr. Here we use high-resolution 3D thermo-mechanical modeling to demonstrate that the long-distance plume impact can be explained by its fast lateral propagation controlled by pre-existing lithosphere structures. Numerical models show that these structures strongly affect the style of horizontal flow of plume head material. This results in long-distance propagation of hot material emplaced at the lithosphere-asthenosphere boundary causing long-wavelength anomalies in onshore topography of Norway’s rifted margin. Short-wavelength offshore topographic domes are likely caused by joint occurrence of plume-related thermal perturbations and gravitational forces related to plate thickening (ridge push). Our 3D modeling brings together plume impingement, spreading ridge dynamics, and the formation of anomalous intraplate structures offshore Norway in one scenario.

Layering of subcontinental lithospheric mantle

Recent seismic studies reveal a sharp velocity drop mostly at ∼70-100km depth within the thick mantle keel beneath cratons, termed the mid-lithosphere discontinuity (MLD). The common presence of the MLD in cratonic regions indicates structural and property layering of the subcontinental lithospheric mantle (SCLM). The nature and origin of the MLD, and many issues associated with the layering of the SCLM are essential to understand the formation and evolution of continents, and have become frontier subjects in the Earth sciences.

Sea level change. Inherited landscapes and sea level change

Enabled by recently gained understanding of deep-seated and surficial Earth processes, a convergence of views between geophysics and sedimentary geology has been quietly taking place over the past several decades. Surface topography resulting from lithospheric memory, retained at various temporal and spatial scales, has become the connective link between these two methodologically diverse geoscience disciplines. Ideas leading to the hypothesis of plate tectonics originated largely with an oceanic focus, where dynamic and mostly horizontal movements of the crust could be envisioned. But when these notions were applied to the landscapes of the supposedly rigid plate interiors, there was less success in explaining the observed anomalies in terrestrial topography. Solid-Earth geophysics has now reached a developmental stage where vertical movements can be measured and modeled at meaningful scales and the deep-seated structures can be imaged with increasing resolution. Concurrently, there have been advances in quantifying mechanical properties of the lithosphere (the solid outer skin of Earth, usually defined to include both the crust and the solid but elastic upper mantle above the asthenosphere). The lithosphere acts as the intermediary that transfers the effects of mantle dynamics to the surface. These developments have allowed us to better understand the previously puzzling topographic features of plate interiors and continental margins. On the sedimentary geology side, new quantitative modeling techniques and holistic approaches to integrating source-to-sink sedimentary systems have led to clearer understanding of basin evolution and sediment budgets that allow the reconstruction of missing sedimentary records and past geological landscapes.