The Peculiar Stratigraphy of Offset Troughs within the Martian North Polar Layered Deposits: Evidence for Deformation?
We present observations of curvilinear layering exposed at particular junctions of offset troughs, which may provide the first definitive evidence of localized ductile deformation within the North Polar Layered Deposits (NPLD). We have examined small-scale variations in layer properties within such offset troughs using high-resolution Mars Orbiter Camera (MOC) and Thermal Emission Imaging System (THEMIS) images, as well as digital elevation models derived from Mars Orbiter Laser Altimeter (MOLA) topography. Three-dimensional reconstructions, layer traces, and planar approximations constructed to study layer stratigraphy reveal a characteristic "wrinkle"-shaped feature at the junctions of some offset troughs (as shown online at http://www.gps.caltech.edu/~bcm/agu2005/). This phenomenon seems restricted to offset troughs in the quadrant of the NPLD bounded by the 270E and 0E meridians (which encompasses Chasma Boreale). Layers in these offset troughs show distinct deviations from planar fits, especially at the junctions between the troughs. In contrast, layers in more common "wrinkle"-free troughs can be well-approximated with planar fits. We find that simple erosion of plane-parallel layers cannot account for the observed three-dimensional configuration of offset trough stratigraphy. Instead, we conclude that a past episode of ductile deformation is the modification mechanism that best explains the curvilinear non-planar layering exposed in these offset troughs. Future observations from orbital subsurface radar sounders such as MARSIS and particularly SHARAD will help establish whether deformation or erosion is primarily responsible for this unusual layer stratigraphy. Confirmation of localized ductile deformation would help constrain larger-scale flow models of the evolution of the North Polar Layered Deposits.