Chris H. Okubo

Igneous dikes on Mars revealed by Mars Orbiter Laser Altimeter topography

The origin of Martian grabens has remained controversial for decades, given the nonuniqueness of graben morphology and conflicting interpretations of dike-induced or faultingbased formation processes. We use measurements from the Mars Orbiter Laser Altimeter to identify the characteristic topographic signature at the surface of an igneous dike beneath a Martian graben. Our results suggest that this subtle but diagnostic signature can be used to test competing interpretations of the formation of grabens and associated fissures and collapse depressions.

Mechanical stratigraphy in the western equatorial region of Mars based on thrust fault–related fold topography and implications for near-surface volatile reservoirs

Variations in lithology and pore-volatile pressure influence the distribution of layer and interface strength (mechanical stratigraphy) within the crust. In this paper, we show how mechanical stratigraphy can be inferred from the topography of thrust fault‐related folds. A thrust fault propagating upward through mechanically wellstratified crust induces the nucleation of secondary backthrust faults. Because such backthrusts are not predicted (and do not occur) in mechanically homogeneous crust, the presence of backthrusts can be used to map variations in the mechanical strength of the crust (e.g., bedding planes, volatilesaturated reservoirs). Dip directions of faults indicate the presence of strength discontinuities within thrust-related folds. We show that the slopes of fault-related fold limbs are reliable indicators of fault-dip direction. We then apply this slopeasymmetry approach to thrust-related folds on Mars. We find that thrust-related folds that have secondary backthrusts are spatially correlated with a general lithologic sequence of lava flows overlying older impact ejecta and young lobate ejecta craters on the lava-flow surface—evidence of nearsurface volatiles such as water ice. We demonstrate that secondary backthrusts within fault-related folds in the western equatorial region of Mars formed because of volatileenhanced mechanical stratification of lavaflow and ejecta lithologic sequences.

Near-tip stress rotation and the development of deformation band stepover geometries in mode II

The propagation of deformation bands into compressive and extensional stepover geometries is investigated by integrating field observations with numerical model simulations of the effective stress state due to shear along the bands. Deformation bands are tabular discontinuities, with mm- to cm-scale thicknesses, of localized volumetric strain and shear. Deformation bands are precursors to frictional slip (i.e., faulting) in porous granular rocks and soils. Systematic rotations in near-tip principal stress orientation due to shear along the overlapping deformation bands are shown to predict band propagation paths that are consistent with characteristic stepover geometries as viewed in the mode II direction, the orientation in which the observation plane is parallel with the displacement direction. Therefore, propagation paths for deformation bands can be predicted from knowledge of the effective near-tip stress state. These results establish a mechanics-based framework for investigations of fault growth and fault-controlled fluid flow in porous granular rocks and soils.

Gridding Mars Orbiter Laser Altimeter data with GMT: effects of pixel size and interpolation methods on DEM integrity

Abstract High-resolution digital elevation models (DEMs) based on Mars orbiter laser altimeter (MOLA) data provide geospatial characterizations of Martian topography. MOLA range data are essentially two-dimensional topographic profiles. Transforming these profile data into three-dimensional DEMs requires the interpolation of a continuous surface between MOLA observations. To this end, we outline a method of generating MOLA-based DEMs using the generic mapping tools (GMT) software suite. The percentage of interpolated data within these DEMs is a function of the spatial density of the MOLA observations and is shown to vary inversely with the pixel size of the DEM. We test the relative accuracy of our DEMs by comparing interpolated elevation values against coincident MOLA observations. Tests are conducted on MOLA-based DEMs containing ∼98% interpolated data at a resolution of 200xa0pixel/°. Our results yield average elevation differences and standard deviations for the interpolated surfaces that are comparable to the uncertainty of the original MOLA data. Based on these findings, we conclude that the GMT interpolation routines produce meaningful high-resolution MOLA-based DEMs.

Thrust fault vergence directions on Mars: A foundation for investigating global‐scale Tharsis‐driven tectonics

[1]xa0Insight into the mechanical and thermal structure of the Martian lithosphere and the character of Tharsis-driven tectonics is gained from detailed mapping of the global distribution of thrust fault vergence directions with respect to the center of the Tharsis tectono-volcanic province. Our results reveal sub-equal frequencies of thrust faults that verge away from and toward Tharsis. Based on the observed distribution of thrust fault vergence directions, we conclude that the base of the Tharsis load was effectively welded, by frictional and cohesive strength, to the subjacent Noachian crust throughout its periods of sustained volcanism and faulting during the late Noachian to early Hesperian, rather than being detached along a ductile layer at its base.