Diapiric spoke patterns

Abstract

Not all finger-like diapirs evolve from or into linear walls as previously supposed. That would imply that all contemporaneous finger diapirs lie in rows and many do not. Physical models demonstrate that diapiric linear walls or rows of fingers have to be forced by lateral forces, lateral boundaries, or internal edge effects. By contrast, the kinematics of the simplest isothermal gravity overturns responsible for finger-like diapirs are geometrically, kinematically and dynamically similar to the “spoke patterns” of vigorous thermal convection. This is because Rayleigh-Taylor (RT) diapirism and Rayleigh-Bènard (RB) thermal convection are end members of a continuous dynamic spectrum due to any combination of compositional or thermally induced density inversions. All simple RT overturns are dynamically similar, but the vigour of thermal convection varies with the heat flow. The kinematics of spoke patterns are difficult to study in rapid, multiple overturns of thermal convection in low viscosity fluids. However, spoke patterns in slow, isothermal diapiric analogues can be analysed at any stage during their single overturn. Such studies indicate that two sets of spokes conduct fluids of different densities horizontally along the top and bottom boundaries to finger-like diapiric stems or plumes where they rise or sink to spread in polygonal bulbs at the opposite boundary. Spokes converge to hubs at the bottom boundary beneath stems feeding diapiric bulbs at the top boundary and vice versa. If the movement cells are considered centred on rising plumes, the cell corners are mapped by sinking plumes. The differences between spoke patterns and previously understood diapiric patterns are illustrated by geological examples with emphasis on relevance to the search for oil and gas. Physical arguments suggest that spoke patterns are probably the most common geometries of gravity-driven overturns in layered rocks of both the crust and the mantle.