Lunar and Planetary Science XXIX 1895.pdf What a Global Stress Map of Venus Reveals. P. W. Tracadas and M. T. Zuber, Department of Earth, Atmospheric, and Planetary Sciences, MIT, Rm. 54-521, Cambridge, MA 02139; e-mail: [email protected], [email protected]. The distribution and scales of tectonic surface fea- stresses and that the smaller belts within the larger tures provides information about the shallow internal ones are both compressive and extensive. structure of the planet and has implications for the na- Over the entire surface, numerous occurrences of ture of regional stresses and global dynamics. By us- tectonic features with multiple tectonic wavelengths are ing Magellan mapping data of Venus, we are analyzing observed- short wavelength features occur pervasively the topography and radar roughness (hi-res SAR) of over the planet [4] while features that display much the planet's surface to find characteristic scales (widths longer length scales include, for example, the width of and spacings) and orientations of tectonic deformation the Beta Regio Rift and the width and spacings of ridge with the goal of producing a global stress map of the belts in Lavinia Planitia. The length scales of these upper crust. Our approach utilizes true computational wavelength-dependent features have previously been wavelength analysis, namely area (2-D) frequency interpreted to have been controlled by the depth of the techniques rather than commonly used 1-D spatial pro- brittle-ductile transition in the crust. Unlike this evi- files, to find and measure wavelength scales of topog- dence for a ductile lower crust (or other weak subsur- raphy and radar roughness. In most areas, the analy- face rheological layer), recent experimental rheological sis is successful at recovering short wavelengths ob- data of diabase [5] suggests a stiffer Venus crust, con- jectively (1-10 km, Fig. 1.) and is being refined for de- sistent with measured lithospheric flexure associated tection of long (>100 km) wavelength scales. The with coronae and other surface loads. However, it is global coverage anticipated by this analysis will sup- not clear how to explain pervasive short wavelength plement global wrinkle ridge studies of Venus’ crustal tectonic deformation, especially in elevated terrain, in stress [1] and extend and link various regional defor- the context of deformation of a thick lithosphere. Such mation studies. deformation may be a consequence of deformation in In our analysis, the automated algorithm for detect- an early, weaker lithosphere [6], of high strain deforma- ing fracture patterns (short wavelengths in the SAR tion which results in the development of additional short data) and longer, more gradual wavelength features wavelengths [7]. (using altimetry and mosaicked SAR data) is successful Our globally mapped deformation is applied to vari- at flagging areas of interest at many scales. In the ous mechanical models to relate observed structures to SAR data, these areas are checked for false signals properties of a lithosphere being extended or com- due to tonal variations by lava flows, wind streaks, and pressed. Global models of mantle stressing of the yardangs. In both datasets, a flagged area may be crust are considered [8]. And particular emphasis is filtered out if not surrounded by similar and consistent given on our recent work on the nature of instability features; and the response due to the orbit track is al- growth in a cooling lithosphere [9]. ways removed. The confirmed areas are classified according to prevailing geologic interpretation of the References. [1.] Bilotti and Suppe, (1997) Lunar regional features (following [2]) and the deformation Planet. Sci. Conf., XXVIII, 113-114. [2.] Hansen et al. wavelength signal interpreted as compressional, exten- (1997) Venus II, 797-844. [3.] M.T. Zuber, (1990) GRL, sional, both (separate episodes of deformation in the 17, 1369-72. [4.] S.C. Solomon et al. (1992) J. Geo- same region), or ambiguous. phys. Res., 97, 13,199-13,256. [5.] S.J. Mackwell et The most complete analysis by our algorithm to al., (1995) Rock Mechanics: Proc. 35th U.S. Sympo- date centered on Lavinia Planitia. Its 100-150km wide, sium, 207-214. [6.] M.T. Zuber, Lunar Planet. Sci. radar bright ridge belts have been detected as well as Conf., XXV, 1575-1576. [7.] Montesi and Zuber, (1997) smaller belts of associated fractures. Zuber [3] sug- Chapman conf.: Geodynamics of Venus, 14. [8.] R.J. gested that the wide belts are due to compressive Phillips et al., (1997) Venus II, 1163-1204. [9.] Neu- mann and Zuber, (1997) submitted to JGR. Lunar and Planetary Science XXIX 1895.pdf Global Stress Map of Venus: P. W. Tracadas and M. T. Zuber Figure 1. Our canonical picture showing detection in a SAR FMAP of the two perpendicular and short wave- lengths (~1 km) in Guinevere Planitia (see area near 30N334). For this frame, 512x512 pixel (38x38 km) analysis areas were used for each vector location. Each vector points perpendicular to the “ridges” of parallel lineations and the vector magnitude is related to the frequency power (not to the wavelength) of the fracture pattern..