Geologic history of the Mead impact basin, Venus

Transcription

1 Geologic history of the Mead impact basin, Venus Robert R. Herrick Virgil L. Sharpton Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, Texas ABSTRACT The geologic history of the Mead impact basin on Venus, a basin similar in size to Chicxulub (Mexico), may be a guide of what to expect from future exploration of Chicxulub. During the collapse phase of crater formation in the Mead basin, radar-bright impact melt material was deposited as a topographically flat surface within a large central area, burying the transient cavity rim and other underlying structures. The central area is not flat now and has been modified by viscous relaxation and thermal cooling effects. Substantial parts of the ejecta deposits have been covered by postimpact volcanic flows that are not obvious without the topographic data. Previous global surveys of Venusian impact craters, using only image data, may have underestimated the number of craters embayed by volcanism. INTRODUCTION Data collected from the Magellan mission reveal Mead (Fig. 1) to be the largest exposed impact structure on Venus (269 km diameter) and a multiring basin. Magellan collected synthetic aperture radar (SAR) data at 100 m resolution, and carried an altimeter that provided topography, centimetre-scale roughness, power reflection coefficient, and radar emissivity data at 20 km horizontal resolution (Saunders et al., 1992). SAR images appear bright if the imaged surface is sloping toward the radar, is blocky at radar wavelengths, or is inherently reflective, and for most purposes SAR data can be treated as aerial black-andwhite photographs. In most cases the reflection coefficient data and emissivity data are mathematical complements; i.e., reflective materials have a low emissivity (Pettengill et al., 1992; Ford and Pettengill, 1992). For the 80% of the basin covered by SAR data at two different look angles, stereo photogrammetric techniques were used to generate the high-frequency component of topography and improve the horizontal resolution of the topographic data for Mead to 1 5 km (technique described in Leberl et al., 1992). Stereo data were not collected for three orbits ( 60 km) just west of the basin s center. Here the resolution is km and some minor artifacts in the final topography indicate the high-low frequency transition. Although we provide evidence that Mead has been affected by some postemplacement volcanism and tectonism, Mead is far more pristine than any terrestrial basin and thus can provide important information about basin formation on a planet with Earthlike surface gravity. In particular, the Chicxulub structure in Mexico may be similar in size to Mead (Sharpton et al., 1993), suggesting that analysis of Mead can aid the interpretation of current and future geologic and geophysical data to be collected at Chicxulub. MEAD BASIN Bounded by the inner-ring scarp, the central area of Mead is 194 km in diameter and radar bright relative to the surrounding plains (Fig. 2); the strong radar returns over this region are due primarily to the high reflectivity (and low emissivity) of this surface unit rather than a roughness effect (Weitz, 1993). This high reflectivity (highest in the northeast part of the central area) suggests that the material composing the central area has either a high dielectric constant or a high porosity (Weitz, 1993). The central area averages about 700 m below the surrounding plains and 1100 m below the elevated rim. The steep inner-ring scarp averages 400 m in height and is highly irregular in outline, reminiscent of a scalloped crater rim in lunar craters. There is considerable topographic variation within the central area; the center and periphery are depressed m compared with the intermediate region between r 30 km and r 60 km. Several concentric linear features, some of which are distinct grabens, modify the intermediate region. The central depressed area exhibits primarily radial features, some of which are distinct scarps that appear to be formed by shortening. The periphery of the central area, particularly in the southwest and northeast, is slightly more radar bright in appearance and is mottled by small, very radar bright spots (Fig. 3). The area between the inner and outer scarp rings (hereafter referred to as the terrace zone) of Mead averages 40 km in width and has radar properties indistinguishable from the plains surrounding the basin. Where ejecta cover the terrace zone it resembles ejecta-covered plains outside the outer-ring scarp; plains regions within the terrace zone are similar to those outside the outer ring, and emissivity signatures are similar for the terrace zone and the surrounding plains. However, we cannot definitely identify tectonic features from the surrounding plains that appear to continue into this zone. The terrace zone is generally bounded on its outer perimeter by a single scarp to the rim (average height 700 m), but on the northnortheast is a set of descending terraces, interpreted to be step-down faults, of a few hundred metres each. The eastern quadrant of the terrace zone stands m higher than the remainder of the zone; there is a corresponding decrease in the height of the outer-ring scarp. The western quadrant is generally lower than the remainder of the terrace zone. Here radar-bright ejecta are confined to high-standing areas, whereas in the other areas of the terrace zone there is no obvious correlation between ejecta location and elevation. The topographic rim of Mead is 400 m higher than the surrounding plains. Radarbright ejecta are located around most of the rim crest, but beyond the rim the radarbright ejecta are confined to high-standing areas in the surrounding plains. Northwest of the crater there is a volcanic feature 20 km in diameter, the crest of which is 900 m above the surrounding terrain. The topography combined with the radar imagery show clearly that a flow unit 75 by 50 km extends eastward from the volcano and covers part of the ejecta blanket (Fig. 4). Geologic History The data suggest that the current appearance of Mead has been affected not only by the formation of the basin, but also by postemplacement viscous relaxation and volcanism. The similarity of the terrace zone to the surrounding plains leads us to interpret the terrace zone to be a down-dropped plains region outside the transient cavity. The available data are insufficient to allow us to distinguish whether the inner-ring scarp is the boundary of the transient cavity; it may be that this boundary is underneath the central area. The volume of the hole (beneath the elevation of the surrounding plains) roughly represents the volume of material ejected beyond the outer-ring scarp. Simple calculations (e.g., Melosh, 1989, p. 90) and comparison with other large craters on Venus and the moon indicate that the ejecta blanket is far less extensive than would be expected for a pristine basin of Mead s size. Typical estimates of the volume of melt produced for a Mead-sized crater are 30% 50% of the crater volume, and about half of this will not be excavated from the crater (Grieve and Cintala, 1992; O Keefe and Ahrens, 1977). The entire central area gener- Geology; January 1996; v. 24; no. 1; p ; 4 figures. 11

2 Figure 1. A: Radar image of Mead impact basin. Outer rim averages 269 km in diameter, and radar-bright central area averages 194 km in diameter. Note 20-km-diameter volcanic feature 50 km northwest of outer rim. B: Radar image of Mead basin with topography overlaid as color. Each color change represents 150 m change in elevation. C: Profiles taken every 45. Bottom profile is radial average. Vertical line segments mark approximate location of inner- and outer-ring scarps. A B Figure 2. Sketch map showing morphologic units and significant tectonic features. Morphologic units increase in stratigraphic age from top to bottom in legend. Dotted-line box outlines area of image in Figure 1, A and B. 12 GEOLOGY, January 1996

3 Figure 3. Full-resolution image of northeast part of Mead impact structure. Outer ring is series of step-down faults in this region. Terrace zone is similar in radar character to surrounding plains and has ballistic ejecta superposed on it. Inner-ring scarp shows evidence for mass wasting. Particularly bright features appear to poke through perimeter of radar-bright central area. Image area is km. ally appears to have been emplaced as a single unit. We interpret this unit to be the cooled impact melt sheet. The reflectivity data suggest a distinctive composition or porosity for this unit. The irregular outline of the inner ring suggests that mass wasting has significantly modified this scarp. We propose that this irregular outline is a byproduct of melt material sloshing around within this scarp during the formation of Mead. The melt washed up against, and perhaps partially melted, the steep scarp face of the inner ring, causing collapse along the inner scarp face. There may be buried terraces just beneath the periphery of the central area, and the radar-bright spots are features (e.g., large clumps of ballistically emplaced ejecta) not entirely buried by the melt sheet. If the central area was emplaced as a single unit, it seems likely that its upper boundary (the surface) was flat at the time of emplacement. If so, then this unit has clearly undergone postemplacement tectonic modification. The basin-centered nature of the elevation anomalies in the central area and the lack of apparent rim deformation indicate that the source of postemplacement deformation of the central area is the impact feature itself. We propose that there has been postemplacement impact-related deformation caused by basin cooling and viscous relaxation of topography. The topography and faulting in the central area are consistent with broad updoming of the central area while the centerpoint is held down. Stretching of the rising broad topographic ring produced concentric grabens, while near the centerpoint contraction resulted in radial thrust faults. Viscous relaxation of an uncompensated hole produces updoming of the floor, whereas thermal contraction from cooling of an impact basin causes basin-centered subsidence (Solomon et al., 1982; Bratt et al., 1985). Thus, qualitatively the topography of the central area could be explained by doming of the floor due to viscous relaxation counteracted in the center by thermal subsidence. To match the observations, after cooling of the melt sheet a thermal anomaly of several hundred degrees must have extended below the basin center for at least 20 km in order to produce enough subsequent thermal contraction of the rocks to counteract the updoming by relaxation; laterally the anomaly must have been concentrated within a few tens of kilometres from basin center. The amount and spatial dimensions of thermal subsidence depend on the subsurface temperature field immediately after the melt sheet cools, and this field is unconstrained (Bratt et al., 1985). Another likely reason that the basin center has domed less than intermediate distances is that the basin was not simply an uncompensated hole after emplacement, but instead there is substantial subsurface structure influencing viscous relaxation and thermal subsidence (e.g., an uplifted mantle plug below basin center or a buried crater ring of 140 km diameter). Volcanic flows with sources exterior to the crater have covered substantial parts of the ejecta blanket both exterior to the crater and within the terrace zone. A volcanic source is easily identifiable for the small flow northwest of the crater. The flow boundary is identifiable only from its onlapping relation to the high-standing ejecta, and neither the flow nor its boundary have a radar signature distinguishable from the surrounding plains. Similarly, crater ejecta exist only in high-standing areas for the regions exterior to the rim and in the western part of the inner ring zone, implying that these regions have also been covered by postimpact volcanic flows that do not have a distinctive radar signature. Coverage by widespread postimpact volcanism also explains the unusually limited extent of the ejecta blanket. Although this volcanism cannot be traced directly to a single source, we note that the potential for volcanic resurfacing is demonstrated by several large, stratigraphically young volcanoes located in Eastern Eistla Regio a few hundred kilometres to the west of Mead. The high-resolution topographic data are vital for making a compelling case that Mead has been volcanically embayed; without the topographic data, the dark areas can be attributed to patchiness or a change in radar character across the ejecta blanket. We suggest that global surveys of Venusian impact craters based on image data alone may have missed many examples of subtle embayment of impact craters. IMPLICATIONS FOR THE CHICXULUB STRUCTURE Current evidence suggests that the Chicxulub structure is roughly the same diameter as the Mead basin (Sharpton et al., 1993). However, the Chicxulub impact was into a shallow-water marine environment with perhaps 1 2 km of sediment overlying the basement crustal layer. Nevertheless, to first order, we might expect the general dimensions (e.g., rim height, depth) and appearance to be similar for the two structures, and we can make some inferences about possible drilling and seismic results. At Chicxulub it may be that one or more of the GEOLOGY, January

4 caused some researchers to conclude that the planet is geologically quiescent (e.g., Schaber et al., 1992). If more craters are embayed than previously recognized, the possibility exists that significant volcanism is occurring in a widely distributed manner. Second, Mead has undergone viscous relaxation, but the relaxation is not a simple updoming of the central area. Thus, whereas the relaxation of Mead must be indicative of the bulk mechanical properties of the Venusian lithosphere, modeling of Mead as a simple hole in the ground with no subsurface structure would clearly yield erroneous values for those properties. However, the knowledge gained about basin subsurface structure from the ongoing exploration of Chicxulub could pave the way for realistic models of Mead s relaxation, just as Mead s surface appearance is influencing interpretations of the data for Chicxulub. ACKNOWLEDGMENTS Supported by a National Aeronautics and Space Administration (NASA) grant to the Lunar and Planetary Institute and a NASA grant to Sharpton under the Venus Data Analysis Program. Software supplied by Vexcel Corporation was used to do stereo analysis of topography. Jeff Plaut at the Jet Propulsion Laboratory supplied matching stereo images of Mead. We thank T. Watters for his review. Lunar and Planetary Institute Contribution 871. Figure 4. Volcano and associated flows northwest of Mead basin. Although distinct flow boundary does not exist, combined topography and image data show clearly that flows from 20-km-diameter high-standing volcanic feature have covered parts of ejecta blanket immediately east of volcano. Thin dashed lines are topography at 300 m contour interval. Image is km. inner rings are buried by melt or breccia units, and at intermediate distances melt rocks may overlie ballistically emplaced ejecta. However, there may be places drilled that are just within the outer topographic ring, and there ejecta deposits will directly overlie apparently undeformed basement material. In addition, postemplacement viscous relaxation and cooling may make the top of the melt sheet vary in depth across the structure. Finally, the sedimentary nature of the uppermost target material at Chicxulub may cause these layers to behave as a lesscoherent rock mass than the igneous surface rocks on Venus. Thus, Chicxulub s outer rings may not be defined by a single fault scarp, but rather as a series of smaller stepdown faults, as occurs for part of the outer rim at Mead. DISCUSSION Two observations presented here have particularly far reaching implications for studies of the geologic and tectonic history of Venus. First, Mead is clearly embayed by postimpact volcanism, but an unambiguous interpretation would have been difficult without the topographic data. The material that has covered parts of Mead s ejecta does not have obvious flow margins in the Magellan data and has radar properties indistinguishable from the surrounding plains. Potentially hundreds of craters (and their ejecta deposits) classified by previous workers as unembayed (e.g., Schaber et al., 1992; Phillips et al., 1992) have been surrounded and partially covered by subtle embayment. The random and apparently unembayed nature of the majority of the craters has REFERENCES CITED Bratt, S. R., Solomon, S. C., and Head, J. W., 1985, The evolution of impact basins: Cooling, subsidence, and thermal stress: Journal of Geophysical Research, v. 90, p. 12,415 12,433. Ford, P. G., and Pettengill, G. H., 1992, Venus topography and kilometer-scale slopes: Journal of Geophysical Research, v. 97, p. 13,103 13,114. Grieve, R. A. F., and Cintala, M. J., 1992, An analysis of differential impact melt-crater scaling and implications for the terrestrial impact record: Meteoritics, v. 27, p Leberl, F. W., Thomas, J. K., and Maurice, K. E., 1992, Initial results from the Magellan stereo experiment: Journal of Geophysical Research, v. 97, p. 13,675 13,689. Melosh, H. J., 1989, Impact cratering: A geologic process: New York, Oxford University Press, p. 90. O Keefe, J. D., and Ahrens, T. J., 1977, Impact-induced energy partitioning, melting, and vaporization on terrestrial planets: Lunar and Planetary Science Conference, 8th, Proceedings, p Pettengill, G. H., Ford, P. G., and Wilt, R. J., 1992, Venus surface radiothermal emission as observed by Magellan: Journal of Geophysical Research, v. 97, p. 13,091 13,102. Phillips, R. J., Raubertas, R. F., Arvidson, R. E., Sarkar, I. C., Herrick, R. R., Izenberg, N., and Grimm, R. E., 1992, Impact crater distribution and the resurfacing history of Venus: Journal of Geophysical Research, v. 97, p. 15,923 15,948. Saunders, R. S., and 26 others, 1992, Magellan mission summary: Journal of Geophysical Research, v. 97, p. 13,067 13,090. Schaber, G. G., and nine others, 1992, Geology and distribution of impact craters on Venus: What are they telling us?: Journal of Geophysical Research, v. 97, p. 13,257 13,301. Sharpton, V. L., and nine others, 1993, Chicxulub multiring impact basin: Size and other characteristics derived from gravity analysis: Science, v. 261, p Solomon, S. C., Comer, R. P., and Head, J. W., 1982, The evolution of impact basins: Viscous relaxation of topographic relief: Journal of Geophysical Research, v. 87, p Weitz, C. M., 1993, Chapter 7. Impact craters, in Guide to Magellan image interpretation: Pasadena, California, Jet Propulsion Laboratory Publication 93-24, p Manuscript received April 28, 1995 Revised manuscript received October 2, 1995 Manuscript accepted October 10, Printed in U.S.A. GEOLOGY, January 1996

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