Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p41f..01m&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P41F-01
Physics
[5475] Planetary Sciences: Solid Surface Planets / Tectonics, [6295] Planetary Sciences: Solar System Objects / Venus, [8005] Structural Geology / Folds And Folding
Scientific paper
Several tectonic models exist to explain the formation of the heavily deformed tessera plateaus on Venus. These models differ in the style, sequence, and magnitude of compression and extension acting to create the plateaus. Here we seek to use the geometry of tessera folds, measured from Magellan radargrammetry-derived topography, to constrain the processes by which they formed. We calibrate our approach by comparing the shapes of folds formed in analog models, seismic sections, and SRTM profiles of terrestrial fold belts. We have quantified a spectrum of fold shapes for the terrestrial examples ranging from symmetrical buckle folds to asymmetrical thrust-related folds by measuring the slope and curvature of topographic profiles. We compare positive and negative slopes across profiles to assess both the magnitude and consistency of fold asymmetry. In analog models, thrust-related folds are found to have greater slope asymmetry than buckle folds over a range of shortening. We further compare the relative widths of anticlines to synclines, defined by changes in the sign of the curvature. Relatively smaller syncline widths are expected to be indicative of over-thrusting. In analog models, the ratio of anticlinal to synclinal width is also dependent on the spacing of thrust faults as well as the magnitude of slip. In models with a viscous detachment layer the initial spacing of thrusts is directly proportional to the thickness of both the detachment and the brittle layer. For a fixed brittle layer thickness, a model with a thicker detachment layer will develop more widely spaced faults and associated anticlines separated by relatively wider synclines. As slip on the thrust increases, however, this acts to narrow the synclines relative to the anticlines. In terrestrial fold belts imaged both seismically and by radargrammetery (SRTM), sedimentation and erosion tend to mask the effects described above, altering both slope asymmetry and apparent syncline and anticline widths. We have performed similar analyses on folds from a portion of Ovda Regio, Venus. Folds from this tessera terrain have nearly symmetrical slopes and anticline/syncline widths. Comparison to the earth analog examples suggests that these folds may have been generated in one of two ways: 1) growth of low-strain buckle folds with symmetrical slopes and fold widths, or 2) growth of low-strain folds associated with widely spaced thrusts that have been modified by erosion and sedimentation thus masking thrust-related slope asymmetry. Significant regolith-forming processes are not expected, therefore typical thrust faults may require a reconsideration of erosional processes on Venus. While we prefer the first hypothesis, both interpretations suggest that Ovda's folds reflect low strain, implying that the significant strain predicted for plateau formation is not accommodated by these structures. High strain may be accommodated by internal deformation instead, which would be consistent with high heat flow and/or weak rheologies.
Gilmore Martha S.
Herrick Robert R.
Mulhern J.
Resor Phillip G.
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