Formation of multiple fold generations on lava flow surfaces: Influence of strain rate, cooling rate, and lava composition

Abstract

If the surface crust of an advancing lava flow experiences compression, it may deform into a series of parallel folds, whose long axes are perpendicular to the flow direction. Under continued compression, a second generation of larger folds may appear, superimposed on the first. The ratio, Λ, of the second-generation fold wavelength ( L 2) to the first-generation fold wavelength ( L 1) reflects the relative importance of shortening versus cooling at the flow surface in thickening the crust. On Earth, the dominance of one or the other of these two mechanisms can be empirically correlated with composition. The characteristic values of Λ are 5.1±1.1 for basalts, 2.5±0.7 for andesites, 2.1±0.3 for dacites, and 1.8±0.4 for rhyolites. Although there is overlap among the more evolved lava compositions, basalts have consistently and significantly different Λ values, because shortening thickens the surface crust of basalt flows more rapidly than does cooling, whereas the converse is true for more evolved lavas. With appropriate scaling, these results can be applied to extraterrestrial folded flows to constrain lava compositions. Results obtained by applying this model to folded Martian and Venusian lavas suggest that they have compositions more evolved than basalt.