FORMATION OF MULTIPLE FOLD GENERATIONS ON LAVA FLOW SURFACES - INFLUENCE OF STRAIN-RATE, COOLING RATE, AND LAVA COMPOSITION

If the surface crust of an advancing lava flow experiences compression , it may deform into a series of parallel folds, whose long axes are p erpendicular to the flow direction. Under continued compression, a sec ond generation of larger folds may appear, superimposed on the first. The ratio, Lambda of the second-generation fold wavelength (L-2,) to t he first-generation fold wavelength (L-1) reflects the relative import ance of shortening versus cooling at the flow surface in thickening th e crust. On Earth, the dominance of one or the other of these two mech anisms can be empirically correlated with composition. The characteris tic values of Lambda are 5.1 +/- 1.1 for basalts, 2.5 +/- 0.7 for ande sites, 2.1 +/- 0.3 for dacites, and 1.8 +/- 0.4 for rhyolites, Althoug h there is overlap among the more evolved lava compositions, basalts h ave consistently and significantly different Lambda values, because sh ortening thickens the surface crust of basalt flows more rapidly than does cooling, whereas the converse is true for more evolved lavas. Wit h appropriate scaling, these results can be applied to extraterrestria l folded flows to constrain lava compositions. Results obtained by app lying this model to folded Martian and Venusian lavas suggest that the y have compositions more evolved than basalt. (C) 1998 Elsevier Scienc e B.V.