November 1, 2013: A bookshelf-faulting model for the mechanical origin of the tiger-stripe fracture system in the South Polar Terrain of Saturn’s moon Enceladus

The semi-square-shaped South Polar Terrain (SPT) of Saturn’s small moon Enceladus (500 km diameter) hosts five regularly spaced (~35 km) and similar-length (~130-150 km) linear structural zones informally named as the tiger-stripe fractures. Although the tiger-stripe fractures are geologically active and serve as conduits of erupting water vapor, their mechanical origin remains uncertain. Existing hypotheses invoke both tidal stress and internal heating and predict the tiger-stripe fractures to be extensional cracks, propagating spreading ridges, faults with alternating opening, closing, right-slip and left-slip kinematics, and transpressional shear zones. The main issue leading to such diverse views on the mechanical origin and the kinematic development of the tiger-stripe fractures is a lack of systematic determination of the geometric and kinematic relationships among diverse geologic structures in the South Polar Terrain. To address this question, a first detailed tectonic map of the South Polar Terrain is constructed based on a detailed analysis of the most recently released CASSINI images of the SPT. The structural analysis taken in this study differs from the early work in that it does not rely on the use of fracture offsets as a mean of determining fault kinematics, as termination and overstepping of younger fractures at and across older fractures can lead to erroneous interpretations on the timing and kinematics of fracture formation. Instead, the new approach used in this study emphasizes the use of geometrically linked and kinematically related secondary and termination structures to establish the kinematics of the tiger-stripe faults. Based on this new approach and guides from terrestrial examples, we show that (1) the tiger-stripe fractures are left-slip faults and (2) the SPT is bounded by an extensional zone along its leading edge, a thrust-fold belt along its trailing edge, a right-slip shear zone along its eastern edge, and a left-slip shear zone along its western edge. A westward increase in the width of the leading-edge extensional zone requires that the SPT deformation be partitioned by translation towards the trailing-edge and clockwise rotation of the SPT. The translation of the SPT is accommodated by contraction along the trailing edge and strike-slip shear along the eastern and western edges. Meanwhile, clockwise rotation of the SPT is accommodated first by initiation of the tiger-stripe fractures as conjugate Riedel faults that was followed by bookshelf faulting. Based on the topography data we attribute the lateral translation of the SPT to gravitational sliding along an inclined brittle-ductile transition zone dipping towards the trailing-edge direction. Clockwise rotation of the SPT relative to its surrounding region could either be related to internal deformation induced by gravitational sliding or by non-synchronous rotation of the SPT above a local ocean.

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