THE VOLCANIC AND TECTONIC HISTORY OF ENCELADUS

Enceladus has a protracted history of impact cratering, cryovolcanism, and extensional, compressional, and probable strike-slip faulting. It is unique in having some of the outer Solar System's least and most h eavily cratered surfaces. Enceladus' cratering record, tectonic featur es, and relief elements have been analyzed more comprehensively than d one previously. Like few other icy satellites, Enceladus seems to have experienced major lateral lithospheric motions; it may be the only ic y satellite with global features indicating probable lithospheric conv ergence and folding. Ridged plains, 500 km across, consist of a centra l labyrinthine ridge complex atop a broad dome surrounded by smooth pl ains and peripheral sinuous ridge belts. The ridged plains have few if any signs of extension, almost no craters, and an average age of just 10(7) to 10(8) years. Ridge belts have local relief ranging from 500 to 2000 m and tend to occur near the bottoms of broad regional troughs between swells. Our reanalysis of Peter Thomas' (Dermott, S. F., and P. C. Thomas, 1994, The determination of the mass and mean density of Enceladus from its observed shape, Icarus, 109, 241-257) limb profiles indicates that high peaks, probably ridge belts, also occur in unmapp ed areas. Sinuous ridges appear foldlike and are similar to terrestria l fold belts such as the Appalachians. If they are indeed folds, it ma y require that the ridged plains are mechanically (perhaps volcanicall y) layered. Regional topography suggests that folding may have occurre d along zones of convective downwelling. The cratered plains, in contr ast to the ridged plains, are heavily cratered and exhibit extensional structures but no obvious signs of compression. Cratered plains conta in a possible strike-slip fault (Isbanir Fossa), along which two pairs of fractures seem to have 15 km of right-lateral offset. The oldest c ratered plains might date from shortly after the formation of the satu rnian system or the impact disruption and reaccretion of Enceladus. An other area of cratered plains has modified craters (e.g., Ali Baba and Aladdin), which some workers have explained by anomalous heat flow an d viscous relaxation; lateral shear and shield-building volcanism also may have been important. A young rift-like structure (northern Samark and Sulci) has few craters and a concentration of cracks or grabens an d flattened, flooded, and rifted craters. Pit chains and cratered dome s suggest explosive volcanism. Smooth plains may have formed by cryovo lcanic equivalents of flood-basalt volcanism. Pure H2O would be diffic ult to extrude through an icy crust and is cosmochemically improbable as a cryovolcanic agent. Density relations rule out eutectic brine lav as on Enceladus, but NH3-H2O volcanism is possible. Current steady-sta te tidal dissipation may cause melting of ammonia hydrate at a depth o f similar to 25 km if the crust is made of ammonia hydrate or similar to 100 km if it is made of water ice. (C) 1996 Academic Press, Inc.