Global Tectonics of Pluto: The Roles of Basin Infill Loading and True Polar Wander.

Abstract:

Pluto has numerous extensional tectonic systems, many of which have radial to oblique orientations relative to Sputnik Planitia (SP). These likely result from a combination of effects related to infill of a SP impact basin by nitrogen (N₂) ice: loading stresses and True Polar Wander (TPW). Here we examine both effects using a finite element method (FEM) formulation for stress state in a spherical shell overlying an ocean. We look at stress states for various loading scenarios and compare to the tectonic record for evaluation. First we consider 2-D axisymmetric spherical shell models of basin loading. The stress state beyond the basin center is characterized by concentric orientation of the least compressional principal stress σ₃. At radial distances r = 450-700 km from basin center, the most compressional principal stress σ₁ at the surface is oriented vertically, predicting normal faults with strikes radial to the basin. In a small strip between r = 435 and 460 km, and for r > 700 km, the orientation of σ₁ changes to basin-radial in the horizontal plane, predicting strike-slip faulting. Such faults tend to have strikes oblique to the basin-radial direction, which may explain the tendency for faults west of SP to diverge from basin-radial orientations with increasing distance from SP. We calculate spherical shell stresses for 50° of southward TPW along SP’s meridian (requiring a 3-D FEM grid), combined with a 3 km-thick surface load of N₂ ice centered on SP. Stress intensity is greatest south of SP, with E-W-oriented extension and a stress state that predicts N-S striking normal faults. These results suggest that the southerly elongation of the SP basin, which has been thought to be due to an oblique basin-forming impact, may also be due to some degree to rifting caused by an environment of strong extension south of the basin, although any faulting would be covered by the N₂ plains of SP. Stress magnitudes are far lower north of the model basin, consistent with the relative lack of tectonic features north of SP. To the east and west of the basin, the stress configuration predicts strike-slip faulting. However, stress trajectories perpendicular to σ₃ curve southward, agreeing with trends of faulting seen here under a scenario where isotropic extension from ocean freezing pushes the strike-slip stress state into the normal fault state.