Effects of upper mantle wind on mantle plume morphology and hotspot track: Numerical modeling

A mantle thermal plume may be tilted, deflected, or even split-up by mantle lateral flows (mantle wind) during its ascent, which in turn changes the spatial distribution of various geological-magmatic responses, such as magmatic activity in the overriding plate and hotspot tracks on the surface, affecting the reliability of the constraints on absolute plate motion history. Previous research on tilted mantle plumes has focused mainly on the lower/whole mantle regions. Whether mantle plumes formed in whole/layered mantle convection suffer lateral tilt in the upper mantle, and how this affects the magmatic activity along the surface hotspot track as well as the plume-related tectonic processes, are important scientific issues in mantle thermal-plume dynamics and plate tectonics theory. This study introduces a thermal Stokes-fluid-dynamics numerical model (in ASPECT software) and pyrolite parameters constrained by mineral physics data, and quantitatively analyzes the tilted/deflected morphology of upper-mantle plumes and the concomitant surface-hotspot location-evolution characteristics under the combined effects of overriding-plate-motion driven flow (Couette) and upper mantle counter-flow (Poiseuille). We find that this composite upper-mantle wind can lead to (1) Plume head-and-upper-conduit horizontal motion in the opposite direction of the overriding plate motion and also with respect to the conduit roots, such that the magmatic spacing is increased; (2) Near-periodic split-up and ascent of a laterally-moving plume conduit, whose split-up/ascent period depends mainly on the thermo-chemical buoyancy of the plume itself; and (3) Under specific conditions of thermo-chemical buoyancy of a main "parent" plume interacting with upper mantle winds, two secondary "child" plumes hundreds of kilometers apart can sprout and ascend sequentially/sub-simultaneously through the upper mantle in a very short period of time (2-4 Myr). The resulting oscillating/jumping behavior of hotspot locations along the overriding plate motion direction can be used to explain the observations on some of Earth's igneous provinces and hotspot tracks (for example, the Kerguelen hotspot) and related-tectonics, that: (i) younger hotspot-magmatic-tectonic regions can superimpose-to and situate-amidst older ones (surface-hotspot-motion or plume-deflection distances greater than overriding-plate-motion distances, with magmatism separated closely in space but largely in time), and (ii) plume-related magmatism can be widely separated in space but closely in time or age (near-simultaneous ascent of two distant "child" plumes from the same "parent" mantle-plume conduit). Our study suggests that the complex dynamic environment within the upper mantle should be considered when constraining absolute plate motions by the moving-hotspot-reference-frame, especially when these hotspots are located near mid-ocean ridges and/or subduction zones.