Intraplate Earthquakes, Lithospheric Stresses and the Driving Mechanism of Plate Tectonics

Ridge subduction follows the approach of an oceanic spreading centre towards a trench and subduction of the leading oceanic plate beneath the overriding plate. There are four possible kinematic scenarios: (1) welding of the trailing and overriding plates (e.g., Aluk–Antarctic Ridge beneath Antarctica); (2) slower subduction of the trailing plate (e.g., Nazca–Antarctic Ridge beneath Chile and Pacific–Izanagi Ridge beneath Japan); (3) transform motion between the trailing and overriding plates (e.g., San Andreas Transform); or (4) divergence between the overriding and trailing plates (e.g., Pacific – North America). In case 4, the divergence may be accommodated in two ways: the overriding plate may be stretched (e.g., Basin and Range Province extension, which has brought the continental margin into collinearity (and, therefore, transform motion) with the Pacific – North America relative motion); or divergence may occur at the continental margin and be manifest as a change in rate and direction of sea-floor spreading because the pair of spreading plates changes (e.g., from Pacific–Farallon to Pacific – North America), spawning a secondary spreading centre (i.e., Gorda – Juan de Fuca – Explorer ridge system) that migrates away from the overriding plate.Mantle upwelling associated with sea-floor spreading ridges is widely regarded as a passive consequence, rather than an active cause, of plate divergence. Geological and geophysical phenomena attendant to ridge–trench interaction suggest that regardless of the kinematic relations among the three plates, a thermal anomaly formerly associated with the ridge migrates beneath the overriding plate. The persistence of this thermal anomaly demonstrates that active mantle upwelling may continue for tens of millions of years after ridge subduction. Thus, regardless of whether the mantle upwelling was active or passive at its origin, it becomes active if the spreading continues for sufficient time and, thus, must contribute to the driving mechanism of plate tectonics.

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Global Adjoint Reconstructions of Earth’s Mantle

10.5194/egusphere-egu2020-7446 ◽

2020 ◽

Author(s):

Sia Ghelichkhan ◽

Jens Oeser

Keyword(s):

High Resolution ◽

Solid Earth ◽

Mantle Convection ◽

Plate Tectonics ◽

Earth Science ◽

Driving Mechanism ◽

Earth Observations ◽

Starting Point ◽

Wide Range ◽

High Resolution Images

Mantle convection is the driving mechanism for plate tectonics and associated geological activities, including earthquakes, surface dynamic uplift and subsidence, and volcanoes. Mantle convection can be regarded as the central framework for linking the sub-disciplines of solid Earth science, e.g., geochemistry, seismology, mineral physics, geodesy and geology. In theory, it is possible to model mantle convection by integrating the principial conservation equations in time, given a past mantle-state as the starting point. Nonetheless, there remains a fundamental lack of knowledge on any past mantle-states. Without such knowledge any direct comparison of convection models and solid Earth observations is challenging and often impractical. One can, however, pose the problem differently, and obtain a past flow history by minimising &#8216;a misfit&#8217; functional between observations and models of Earth&#8217;s mantle. The recent applications of adjoint method in geodynamics, together with the ever-increasing computational power, has facilitated solutions to such minimisation problems, where a unique flow history in Earth&#8217;s mantle can be generated, subject to assumed geodynamic modelling parameters.Here, we build on previously published adjoint models and present a suite of eight high resolution (11 kms) reconstruction models going back to 50 Ma ago. These models incorporate many improvements. First, we take advantage of the recent advances in surface and body waveform tomography to obtain high resolution images of present-day structures in Earth&#8217;s mantle. Our thermodynamic modelling of mantle structures rely on the most recent datasets of mantle mineralogy and account for effects of anelasticity. Furthermore, we assume a wide range of viscosity profiles, including published models consistent with observations of geoid, mantle mineralogy, and post-glacial rebound studies. Finally, we verify these models by comparisons against a range of different geologic observations.

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