Planetary Tectonics

    The on–going Mars Global Surveyor (MGS) mission [Albee, 2001] has collected topographic data via the Mars Orbiter Laser Altimeter (MOLA) [Smith et al., 2001] with greater accuracy and resolution than previously available. The MOLA instrument measures spot elevations every ~300 m along the MGS ground track with 10–cm–scale accuracy [Neumann et al., 2001]. Much of my research in planetary tectonics has utilized these MOLA data.

MOLA-based image of the Tempe Terra region of Mars.

    I have developed and tested a method for gridding the discontinuous MOLA data into continuous–surface DEMs using the Generic Mapping Tools (GMT) [Wessel and Smith, 1998] software suite. The relative accuracies of these MOLA–based DEMs are tested by comparing the interpolated elevation values against coincident non–gridded MOLA observations. I find that high–resolution surface generated MOLA–based DEMs are reliable when compared to the inherent uncertainty of the non–gridded (raw) MOLA data. This work establishes the viability of using GMT to generate MOLA–based DEMs for the purpose of investigating the mechanics of faulting on Mars. Detailed information on my MOLA-DEM work is available in a paper, "Mars Orbiter Laser Altimeter data with GMT: Effects of pixel size and interpolation methods on DEM integrity", published in Computers & Geosciences (available for download through my Publications page).

Distribution of thrust faults in the western equatorial region of Mars based on analysis of MOLA-based wrinkle ridge topography.

    I have used numerical models of MOLA-based thrust-related fold topography to establish vergence directions for subjacent wrinkle ridge faults in the western equatorial region of Mars, in order to investigate the lithospheric rheology below the Tharsis tectono–volcanic province, a hemisphere–scale topographic rise that extends up to 27 km above Mars’ datum that was a focus of large–scale volcanism and faulting. Two general models have been suggested for the lithospheric rheology below Tharsis. A ‘detached cap’ model suggests that much of the base of the Tharsis volcanic pile is ductile and overlies a brittle Noachian–aged (>3.7 b.y.o [Hartmann and Neukum, 2001]) crust, with basal ductility maintained by high heat flow due to magmatic activity localized at Tharsis [e.g. Banerdt and Golombek, 1990; Tanaka et al., 1991]. Alternatively, the ‘welded base’ class of Tharsis models suggests that the entire base of the volcanic pile is brittle and effectively welded to the subjacent Noachian crust [e.g. Solomon and Head, 1982; Phillips et al., 2001]. Through my analysis of thrust fault vergence directions, I find that the base of the Tharsis volcanic pile was most plausibly brittle and in contact with a brittle Noachian crust during the time of wrinkle ridge formation, and that the existence of an intra–lithospheric ductile layer at tens of km’s depth below Tharsis is not supported by observations. Detailed information on this research is available in a paper, "Thrust fault vergence directions on Mars: A foundation for investigating global-scale Tharsis-driven tectonics", published in Geophysical Research Letters (available for download through my Publications page).

Distribution of mechanically stratified and mechanically homogeneous crust in the western equatorial region of Mars.

    I have also used forward numerical models to show that the tendency for the nucleation of secondary backthrust faults within wrinkle ridges is greatly enhanced by mechanical stratification near the upper tip of the primary thrust. Further, backthrust nucleation is not predicted within mechanically–homogeneous crust. Therefore the distribution of wrinkle ridges that contain secondary backthrust faults provides insight into the distribution mechanically well–stratified crust on Mars. Using the aforementioned map thrust fault vergence directions on and around Tharsis, I find that wrinkle ridges that contain backthrust faults occur within several discrete 100–km–scale areas. Therefore these areas are interpreted localizations of mechanically well–stratified crust. Comparison of these locations with regional geologic maps [Scott and Tanaka, 1986, Tanaka et al., 2003] reveals that the tendency for backthrust nucleation is not strongly controlled by the distribution of lithology, but instead is likely controlled by the distribution of previously interpreted [Barlow et al., 2001] near-surface volatile reservoirs. Detailed information on this work is available in a paper, "Mechanical stratigraphy in the western equatorial region of Mars based on thrust fault-related fold topography & implications for near-surface volatile reservoirs", published in Geological Society of America Bulletin (available for download through my Publications page).

    In another research project, I use observed styles of faulting to evaluate chronologic changes in lithospheric loading stresses due to the growth of Tharsis. Based on high–resolution MOLA topography, I identify a population of strike–slip faults that exhibit Middle Amazonian–aged (~0.5–1.7 b.y.o. [Hartmann and Neukum, 2001]) displacements of regional chrono–stratigraphic units along the western periphery of the Tharsis rise. These strike–slip faults are adjacent to an older population of Late Hesperian–aged (~3.0–3.6 b.y.o. [Hartmann and Neukum, 2001]) thrust faults (wrinkle ridges). Along–strike orientations of these thrust and strike–slip faults reveal the Tharsis–radial stress to be the area’s most compressive remote principal stress during both periods of faulting, and that this stress orientation and magnitude persisted throughout the Late Hesperian to the Middle Amazonian. Accordingly, the change in the predominant style of faulting from thrust to strike–slip faulting during this time requires a decrease of the Tharsis–circumferential stress to a magnitude less than lithostatic load. A publication of this research is currently in review at the Journal of Structural Geology (see my Publications page).

    This set of research findings reveals a lithospheric stress state chronology that entails isostatic compensation of the Tharsis load during the Late Hesperian followed by lithospheric flexure around the Tharsis volcanic pile during the Middle Amazonian. This implies that widespread and sustained volcanism at Tharsis during the Late Hesperian to Early Amazonian [e.g. Wilson, 2001; Lowery and Zhong, 2003] induced flexural loading of the lithosphere, which caused the cessation of wide–spread thrust faulting and wrinkle ridge formation and promoted strike–slip faulting instead. Most importantly, this analysis provides key observation–based insights into the geodynamic evolution of lithospheric stresses due to the growth of Tharsis.