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

Chris H. Okubo^,1 and Richard A. Schultz^,1

¹ Geomechanics–Rock Fracture Group, Department of Geological Sciences, Mackay School of Earth Sciences and Engineering, University of Nevada, Reno, Nevada 89557-0138, USA

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 well- stratified 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, volatile- saturated 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 slope- asymmetry 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 near- surface volatiles such as water ice. We demonstrate that secondary backthrusts within fault-related folds in the western equatorial region of Mars formed because of volatile- enhanced mechanical stratification of lava- flow and ejecta lithologic sequences.

Key Words: thrust fault • wrinkle ridge • topography • mechanical stratigraphy • fluid flow • Mars tectonics

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