Fault offsets and lateral crustal movement on Europa: Evidence for a mobile ice shell

Abstract

Right-lateral structural offsets of ∼25 km have been identified on the icy Galilean satellite Europa. These occur along dark lineaments oriented orthogonally to wedge-shaped bands, which are also ∼25 km wide. Wedge-shaped bands are interpreted as dilated tension fractures, which formed as crustal blocks (or plates) 50–100 km across separated and slipped past each other along flanking strike-slip faults. This style of deformation does not appear to be characteristic of other lineament types, and with the exception of Earth appears to be unique to Europa. Together, the subparallel wedge-shaped b bands form a broad NW-SE trending belt, ∼1500 km long and less than 500 km across, near the anti-Jovian point. This belt is interpreted as a major crustal fracture (or rift) zone, with a pole of rotation (determined by the strike-slip faults) near 47°S, 144°W, and an approximate NE-SW direction of maximum tensile stress. Extension may have been areally compensated at Agenor Linea, a bright band of possible compressional origin. Global expansion, tidal distortion, and nonsynchronous rotation do not explain the inferred minimum principal (i.e., least compressive) stress directions. Alternatively, fracturing near the anti-Jovian point may be a result of (i) solid-state convection in the lower ice crust (possibly triggered by uneven heat flow from the silicate interior), (ii) rotation of the icy shell about the sub- and anti-Jovian points induced by latitudinal lithospheric thickness variations, or (iii) preferential strain accumulation from the rest of the icy shell. No significant distortion of the crustal blocks occurred during fracturing and rotation, indicating that the icy crust was probably mechanically decoupled from the silicate interior in this region over the time scale of fracturing. Decoupling on a global scale is also likely the simple geometry of lineaments argues for formation in an icy lithosphere, not a silicate one, and other evidence for fracturing caused by nonsynchronous rotation stress is not compatible with a tidally locked silicate interior (which we show is likely) unless the ice shell rotates independently. Decoupling could have been due to either warm ice or liquid water near the base of the icy crust. Mechanical bounds on lithospheric thickness (a few to ∼10 km) lead to heat flow estimates that admit both possibilities but favor decoupling by liquid water.