Small planitiae on Venus: Tensional or compressional tectonics?

The two small planitiae south of Fortuna Tessera define distinct crustal units not very unlike to small crustal plates or subplates. The mobile transformation zone between Fortuna Tessera and Allat Planitia is caused by colliding crustal plates and evidently indicates the mobilization of the planitia unit foiled by the parquet terrain. Ridges parallel to this zone and in dextral ridge groups on planitia support the idea of the main N(W)-S(E) compression. Allat Planitia has been pushed approximately from the south and southeast against Fortuna Tessera which, in contrary, has spreaded to the southeast. Within the smaller planitia there are two conjugate ridge sets and a third ridge set parallel to the parquet border. The crossing ridge sets favour the existence of a compressional NW-SE force, as do the N-S directed ridges of the middle planitia area.At least three tectonic phases within Allat Planitia can be found. The main compression was in N(W)-S(E) direction. Prominent right-handed en echelon ridge groups and long parallel ridges of the northern planitia area indicate this comrpessional environment as well as the transformation zone against Fortuna Tessera. Short dome-like ridges indicate the tension gash opening during a NW-SE compression phase. An E-W (or NWW-SEE) compression resulted in the formation of the long linear wrinkle ridge-like N-S structures on Allat Planitia. The NW-SE compression, which has caused the formation of the dextral, E-W oriented major fault, was then the youngest of the main tectonic phases involved within the area studied.     

Superposed ridges of the Hesperia planum area on Mars

Mare ridges of the Hesperia Planum area form linear, reticular and circular structures. The main factors effective in mare ridge formation have been (i) a large areal, or maybe even global, shortening and compression, (ii) major crustal tectonics, and (iii) the moderation of tectonic movements by the megaregolith discontinuity layer(s) between surface lavas and the bedrock leaving the compressional thrust to dominate over other fault movements in surface tectonics.     

Lunar ‘en echelon within en echelon’ structures

Some en echelon structures, tension gashes and compressional ridges may form similar patterns. The N-S compression activates diagonal conjugate zones of weakness with tension gashes in the vicinity of the compressional direction. In the case of E-W compression similar arrangements of en echelon compression ridges are generated.The global N-S compression existing at the time of fracturing of the lava-flooded Oceanus Procellarum basin is arguable. It is possible to interpret some different scale mare ridge arrangements as en echelon within en echelon structures. Major ridge ranges evidently have Riedel and opposite Riedel orientations and they consist of minor en echelon structures which may in places be intruded tension gashes but are evidently mostly sheared and compressed Riedel fractures.The en echelon withinen echelon structures of mare ridges manifest the significance of different scale strike-slip movements along the lithosphere zones of weakness indicated by present mare ridge zones. The orientation of these Riedel-fracture-like en echelon structures also points to the existence of an areal compression during shearings along the zones of weakness. The Oceanus Procellarum basin sinking caused by lava loadings and lunar internal cooling led to the lithosphere shortening and to compressional circumstances. The angle between proposed Riedel structures and the mare ridge zones varies within this area, possibly indicating differences in compression and shearing in distinct parts of the shortened basin lithosphere.     

Ridge belt tectonics of Ganiki Planitia on Venus

The main major ridge belts of Ganiki Planitia on Venus (Lama, Ahsonnutli and Pandrosos Dorsa) are part of the fan-shaped ridge belt complex along the 200 parallel of longitude. These ridge belts with evidence of crustal shortening support the idea of a large-scale E-W compression. The ridge belt patterns indicate a N-S shear component. These forces are explained by a triangular planitia area which compressed by surrounding terrains. The crustal shortening and ridge belt formation indicates compressional plate movement stresses in the uppermost lithosphere.Three sizes of ridge belt structure are to be found within Ganiki Planitia. (1) The ridge belt spacing of 200–400 km can be used to estimate the depth of the major uppermost homogeneous layer of Venus. There are numerous volcanic coronae, paterae and montes located along the main ridge belts or at their junctions. (2) Mid-size ridge groups or subbelts are to be found within the major ridge belts. These are formed by more local responses to tectonic stresses in the stratified uppermost crust. A wavelength of 40–70 km can be seen as a result of bending of the crustal strata and may relate to its thickness. (3) Small individual ridges are connected with most local stresses, defining places where the surface layers broke along the crests of large ridge belts or mid-scale subbelts. Radial and concentric mare ridge-like structures around coronae indicate that corona formation was effective at a sufficiently close vicinity to fault the surface.     

Spectrophotometry of Comet Austin (1982g) during post-perihelion period

Lunar crustal shortening does not seem to be restricted to the lava-filled basins alone; but there are some young scarp-like terra ridges in places around mare areas where they often continue other tectonic structures. This crustal shortening has not reached the same intensity as in the case of the lobate scarp overthrusts on Mercury. Young lunar terra ridges indicate that crustal shortening with an areal extent also took place slightly around mare basins. Thus they link tensional rille tectonics with compressional mare ridge tectonics and indicate that areal heating/bending/extension — cooling/ shortening/compression may describe an important explaining factor in lunar mare- and near-mare tectonics in addition to the volcanic extrusions.     

Elastic dislocation modeling of wrinkle ridges on Mars

Wrinkle ridges are one of the most common landforms on Mars. Although it is generally agreed that they are compressional tectonic features formed by folding and thrust faulting, there is no consensus on the number of faults involved, the geometry of the faults, or the maximum fault depth. The topography of martian wrinkle ridges in Solis Planum and Lunae Planum has been studied using MOLA data. As determined in previous studies, the topography shows that most wrinkle ridges are a composite of two landforms, a broad low relief arch and a superimposed ridge. Constrained by MOLA topographic profiles, the geometry and parameters of the faults associated with wrinkle ridges have been modeled. The best fits are obtained with a blind listric thrust fault that flattens into a décollement. The listric fault geometry is approximated by a series of linear connecting segments with varying dips. The major morphologic elements of wrinkle ridges can be matched by varying the displacement on the different fault segments. Modeling of large-scale wrinkle ridges indicates that the maximum depth of faulting or depth to the décollement is about 4.5 km. This may correspond to the depth of the contact between the ridged plains volcanic sequence and the underlying megabreccia. The results suggest that wrinkle ridge thrust faults are shallow-rooted and reflect thin-skinned deformation.     

Cytherean ridge belts connected with Tessera areas: Tensional or compressional structures?

The conclusion that the different ridge belt-bounded planitia and parquet terrains studied here define Venusian crustal plate-like units is evidently valid in the context of compressional ridge belt tectonics. The long ridge belts of Kamari and Tellus Dorsa, the ridge belts in the transition zone between Ishtar Terra and planitiae and Ausra Dorsa support the idea of NW-SE, (N-S) or E-W compression components, respectively. The planitia plates have been pushed from the south or south-east against the Ishtar Terra/Fortuna Tessera highland, which has opposed the movement, giving the impression of a relative southeast-directed force. The volcanic/diapiric transition zone between these colliding crustal units or plates evidently indicates mobilization of the subsurface unit overthrust by the parquet terrain.     

Ridge belts on Venus: Morphology and origin

Ridge belts, composed of closely spaced individual ridges 5–20 km wide, form sinuous patterns 30–400 km wide and 200–2000 km long in the plains of northern Venus. They are not homogeneously distributed, but occur primarily in two regions: between 0 ° E and 90 ° E ridge belts are associated with large blocks of tessera, and have a cumulative length of about 13,200 km; and between 150 ° E and 250 ° E, the ridge belts form a fan-shaped pattern and have a total cumulative length of about 25,800 km. Most ridge belts trend within 10 ° of N-S. Five morphologic components exist within the ridge belts: (1) broad ridges, which have no sharp crest and usually occur individually in the plains: (2) discontinuous ridges, with short ridge segments less than 20 km long; (3) paired ridges, with closely spaced ridges (less than 10 km apart) that never merge; (4) parallel ridges, with widely spaced (10–50 km), less prominent ridges; and (5) anastomosing ridges, in which ridges splay at angles up to 30 °. Subtle cross-strike lineaments cut the ridge belts at angles of 30–90 ° to the ridge belt, and augen-shaped plains are often present in anastomosing ridges. We examine the relationships between the components, plains, cross-strike lineaments, and augen-shaped plains in five ridge belts. Broad arches similar to the arches associated with wrinkle ridges on the Moon, Mars and Mercury appear in all of the ridge belts examined. Through studying each of these components individually and in the context of five specific ridge belts, we conclude that these ridge belts formed by compressional forces. The ridge belts form a continuum of deformation, from the simple broad arches (Nephele Dorsa), representing small amounts of shortening, through asymmetric ridge belts in the plains (Pandrosa Dorsa) and adjacent to tessera (Kamari Dorsa), to ridge belts in troughs representing underthrusting (Ausra and Lukelong Dorsa). Underthrusting is also observed along the borders of Lakshmi Planum, associated with Freyja and Danu Montes.The interpreted compressional origins for the ridge belt components suggests that many of the other ridge belts are of compressional origin, although complex origins (involving a combination of extension, shear, and/or compression) for some ridge belts cannot be ruled out. Global high resolution data from the Magellan mission will permit global mapping of the characteristics and distribution of ridge belts and allow further tests for their origin and evolution.'Geology and Tectonics of Venus', special issue edited by Alexander T. Basilevsky (USSR Acad. of Sci., Moscow), James W. Head (Brown University, Providence). Gordon H. Pettengill (MIT. Cambridge, Massachusetts) and R. S. Saunders (J.P.L., Pasadena).     

History of tectonic deformation in the interior plains of the Caloris basin,mercury

Analysis of images from the Messenger MDIS narrow angle camera imply that at least part of the radial graben of the Pantheon Fossae structure, and probably the structure as a whole, predate the deformation that led to circumferential ridges on the Caloris interior plains. This follows from structural analysis and comparison with similar geological relationships on Venus and the Moon, where graben are known to both postdate and predate ridges. Observations suggest that the Pantheon Fossae radial graben (extension) formed first, pre-dating observed circumferential graben (also extension), with ridges (compression) formed in between. This scenario puts constraints on the models for the deformation of the Caloris basin and its vicinity. Our observations and analysis are consistent with Pantheon Fossae having formed in a similar manner to Venusian astra/novae, where radial dikes that propagate away from a magmatic center led to graben formation. Our results also have implications for the length of time between the emplacement of the basin volcanic fill and the onset of the compressional stresss regime that led to ridge-formation. If the Pantheon Fossae structure formed before the emplacement of ridges, as we suggest, this means that compressional stresses took some time to develop sufficiently to deform the volcanic plains. Since the Caloris interior plains had to have been already in place when Pantheon Fossae formed, and since these plains represented a significant load to the underlying lithosphere, it is striking that compression took some time to develop. These observations may provide new information about the rigidity of the basin-filling material and will help constrain models for the mechanisms and timing of events within and around the Caloris basin.     

Analysis of phyllosilicate deposits in the Nili Fossae region of Mars: Comparison of TES and OMEGA data

Diverse phyllosilicate deposits discovered previously in the Nili Fossae region with near infrared reflectance data are a window into the complex history of aqueous alteration on Mars. In this work, we used thermal infrared data from the Thermal Emission Spectrometer (TES) in combination with near infrared data from the Observatoire pour la Minéralogie, l’Eau, les Glaces, et l’Activité (OMEGA) to better constrain the mineralogy and geologic origin of these deposits. We developed a TES spectral index for identification of clay minerals, which correctly identifies the phyllosilicates in the Nili Fossae area and points to several other interesting deposits in the Syrtis Major region. However, detailed inspection of the TES spectral features of Nili Fossae phyllosilicates shows a feature at low wavenumbers (350-550 cm⁻¹) that is not an exact match to any specific Fe³⁺-, Al-, or Mg-rich phyllosilicate phase. Instead, the feature is more similar to basaltic glass and may indicate that the phyllosilicates in this region are: (1) rich in Fe²⁺ (based on similarity to trends seen in laboratory data of clay minerals), (2) poorly crystalline/extremely disordered, and/or (3) present within a matrix of actual basalt glass. This feature is similar to spectral features seen in altered rocks in the Columbia Hills region of Gusev Crater by previous authors. By calibrating measured spectral index values against mathematical spectral mixtures of typical martian dark surfaces and known abundances of alteration minerals, we are able to estimate an enrichment in abundance of alteration minerals in the altered surfaces. Many dark, Noachian deposits in the Nili Fossae area are enriched phyllosilicates by 20-30% (±10-15%) relative to dark, volcanic surfaces in the same region. The distribution and abundance of these phases indicates that alteration in the region was pervasive, but did not completely erase the original mineralogy of what was likely an Fe-rich basalt protolith. As a group, the Nili Fossae phyllosilicate deposits are fundamentally different from those found in the Mawrth Vallis region. Nili Fossae deposits have strong thermal infrared features related to admixed pyroxene, plagioclase, and occasionally olivine, whereas the Mawrth Vallis deposits contain no mafic minerals. Comparison of TES and OMEGA data also illustrates some more general differences between the datasets, including the impact of physical character of the martian surface on detectability of minerals in each spectral range.     

Tectonic implications of the mare-ridge pattern of the central parts of Oceanus Procellarum on the Moon

A structural analysis is presented of the mare ridge pattern in an area of about 1 000 000 km² in the central parts of Oceanus Procellarum.The penetration of magmas through the crust at the Marius Hills and Aristarchus Plateau/Harbinger Mountains volcanic complexes may have happened along pre-existing deep zone of weakness. Associated with these zones are present mare ridge ranges, some of which can be regarded as having formed radial or subradial ridge swarms to the complexes as they were strengthened by stress field changes caused by upward doming and penetrating magmas.The present moonquake epicentres within this area seem to be connected with mare ridge ranges. Focal depths from about 800 to 1000 km indicate a decreasing trend of tectonic activity. One shallow moonquake epicentre also lies within the mare ridge sets.     

Late Hesperian plains formation and degradation in a low sedimentation zone of the northern lowlands of Mars

The plains materials that form the martian northern lowlands suggest large-scale sedimentation in this part of the planet. The general view is that these sedimentary materials were transported from zones of highland erosion via outflow channels and other fluvial systems. The study region, the northern circum-polar plains south of Gemini Scopuli on Planum Boreum, comprises the only extensive zone in the martian northern lowlands that does not include sub-basin floors nor is downstream from outflow channel systems. Therefore, within this zone, the ponding of fluids and fluidized sediments associated with outflow channel discharges is less likely to have taken place relative to sub-basin areas that form the other northern circum-polar plains surrounding Planum Boreum. Our findings indicate that during the Late Hesperian sedimentary deposits produced by the erosion of an ancient cratered landscape, as well as via sedimentary volcanism, were regionally emplaced to form extensive plains materials within the study region. The distribution and magnitude of surface degradation suggest that groundwater emergence from an aquifer that extended from the Arabia Terra cratered highlands to the northern lowlands took place non-catastrophically and regionally within the study region through faulted upper crustal materials. In our model the margin of the Utopia basin adjacent to the study region may have acted as a boundary to this aquifer. Partial destruction and dehydration of these Late Hesperian plains, perhaps induced by high thermal anomalies resulting from the low thermal conductivity of these materials, led to the formation of extensive knobby fields and pedestal craters. During the Early Amazonian, the rates of regional resurfacing within the study region decreased significantly; perhaps because the knobby ridges forming the eroded impact crater rims and contractional ridges consisted of thermally conductive indurated materials, thereby inducing freezing of the tectonically controlled waterways associated with these features. This hypothesis would explain why these features were not completely destroyed. During the Late Amazonian, high-obliquity conditions may have led to the removal of large volumes of volatiles and sediments being eroded from Planum Boreum, which then may have been re-deposited as thick, circum-polar plains. Transition into low obliquity ∼5 myr ago may have led to progressive destabilization of these materials leading to collapse and pedestal crater formation. Our model does not contraindicate possible large-scale ponding of fluids in the northern lowlands, such as for example the formation of water and/or mud oceans. In fact, it provides a complementary mechanism involving large-scale groundwater discharges within the northern lowlands for the emplacement of fluids and sediments, which could have potentially contributed to the formation of these bodies. Nevertheless, our model would spatially restrict to surrounding parts of the northern plain either the distribution of the oceans or the zones within these where significant sedimentary accumulation would have taken place.     

Main fault tectonics of Meshkenet Tessera on Venus

The lengthy Meshkenet Tessera highland located between Ishtar Terra and coronae of the Nightingale group provides evidence of large-scale crustal movements. Its complex tectonic structures have various deformation geometries, thus indicating different tectonic sequences. The main parallel faults, first explained as rotational bookshelf faults, are more likely due to relative dextral direct shear movements of rectangular blocks. These faults have been active, possibly due to endogenic stresses, as indicated by mid-size ridge ranges which connect them to some of the large coronae. There are some compressional ridge belts around Meshkenet Tessera, while deformation within the tessera blocks has mostly been extensional.     

A description of surface features in north Tyrrhena Terra, Mars: Evidence for extension and lava flooding

We studied north Tyrrhena Terra, an approximately 39,000 km² area, located in the transition region straddling the Amenthes and Mare Tyrrhenum Mars Chart quadrangles 14 and 22, respectively. The study area comprises ancient terrains with infilled craters, ridges and valleys. Interpretation of orbiter data of ancient terrains is inherently difficult, but valuable information can be obtained using multiple datasets and analyzing various geological features. Using data from the High Resolution Stereo Camera on board Mars Express, complemented by Mars Global Surveyor MOLA DEM and MOC Narrow Angle datasets, we observed and interpreted surface morphologies at a scale suitable for geologic investigation. Morphometric examination of a 31 km diameter large impact crater indicated that tectonism and volcanism were responsible for its morphologic modification. Small impact crater depth/diameter relationships indicated that smooth surfaces of valleys are composed of highly consolidated material. Surface cracks and lobate fronts further suggested that the rocks are volcanic. Examination of tectonic features revealed that in the study area: a dominant NW-SE fabric is related to a ridge/bench-scarp-valley repetition consistent with synthetic and antithetic normal faulting; a NNW-SSE lineament represents the surface expression of normal faulting post-dating all other tectonic features. A weak NE-SW fabric is observable as small sublinear depressions, and at the contact between units internal to one large crater. One 20 km diameter crater in the study area was interpreted to be a caldera, infilled by thick volcanic rock layers. Identification of wrinkle ridges further indicated that thick layered lava flows infilled the main depressions of the study area. The available evidence suggests that the study area underwent multiple episodes of extension and volcanism.     

Ridge belts: Are they compressional or extensional structures?

Today one of the main questions standing before geology of Venus is comprehension of tectonic style on surface of this planet. The observed ridge belts on the images of the radar surveyors of Venera-15 and -16 are related to one aspect of a solution of this problem. In general there are two views on the origin of the ridge belts on the planet. Some of the investigators consider them as a result of compression while others as extension. These discrepancies are connected with uncertainties of interpretations of the major structures of these belts, i.e., the ridges. This paper represents a review of evidence in favor of extension and compression observed within the ridge belts. Moreover it discusses the major speculative ideas and models of the ridge belts origin. The supporters of a ridge formation by extension suggest that these linear features are results of crust cracking and intruding of magma along these cracks so they propose that the ridge belts on the plains of Venus can represent analogies of the spreading zones on Earth. Other investigators assume that the ridge of belts represent structures of folding and or thrust faulting due to the tectonic compression environment so the ridge belts can be analogies of the orogenic belts and subduction zones on Earth. So the question of the ridge belts' origin remains controversial and for a solution, set one's hope on the Magellan mission.'Geology and Tectonics of Venus', special issue edited by Alexander T. Basilevsky (USSR Acad. of Sci., Moscow), James W. Head (Brown University, Providence), Gordon H. Pettengill (MIT. Cambridge, Massachusetts) and R. S. Saunders (J.P.L., Pasadena).     

Ridge systems related to Martian impact craters

Wrinkle ridge systems within and around Martian highland craters were studied in order to find their basin-induced and regional aspects. Most prominent ridge directions indicate regional tectonic patterns. Radial ridges near large craters are often slightly deflected along regional or global ridge systems. Crater floor ridges have simpler local distributions. Smaller or older craters are less resistant against the effects of global or regional stress systems. In craters concentric ridge rings locate at 0.8 crater radius with additional minor rings at 0.66, 0.44 and 0.94 crater radius. This pattern illustrates compression of lava fill over buried topography.     

Tectonic pattern of the crater Grimaldi region of the Moon

The tectonics of the Grimaldi area are described and analyzed in detail from high-resolution Lunar Orbiter photographs.Rille grabens are long and narrow fault zone structures of lunar terra. The polygonal rille graben pattern indicates the importance of lunar internal activity with an adjoining thin lithosphere in the areal tectonics at the time of rille grabening. The graben subsidence developed during tensional bending of this thin terra lithosphere. The en échelon graben offsets indicate the existence of strikeslip movements along the main fault under tensional lithosphere conditions.In some places mare ridge ranges continue in the direction of the rille graben indicating the connection of these structures to each other as part of the lunar tectonic evolution. The very thin mare lithosphere was affected more easily and over a longer period of time by lunar internal forces. The effect of older structural units is thus less conspicuous within mare areas. Proposed Riedel-shear-like structures indicate a slight shortening and compression of the mare basin lithosphere during movements along lava-covered zones of weakness.     

Tectonics of Tharsis dorsa on Mars

The tectonics of the Tharsis and adjoining areas is considered to be associated with the convection in the Martian mantle. Convection and mantle plume have been responsible for the primary uplift and volcanism of the Tharsis area. The radial compressional forces generated by the tendency for downslope movement of surface strata, vertical volcanic intrusions and traction of mantle spreading beneath Tharsis were transmitted through the lithosphere to form peripheral mare ridge zones. The locations of mare ridges were thus mainly controlled by the Tharsis-radial compression. The load-induced stresses then contributed on further ridge formation over an extended period of time by the isostatic readjustment which was reponsible for long-term stresses in the adjoining areas. Extrusions, changes in internal temperature and possible phase changes may also have caused changes in mantle volume giving rise to additional compressional forces and crustal deformations.On leave from Dept. of Astronomy, University of Oulu, Oulu, Finland     

Radar altimetry of South Tharsis,Mars

Martian altitudes were measured by radar during the oppositions of 1971 and 1963 using the 64-m antenna at Goldstone (California). The resultant topographic profiles substantiate a zonal classification of the volcanic flows blanketing the south flanks of Arsia Mons, and they confirm the existence of a secondary, parasitic shield attached from the SSW to the main Arsia shield. The secondary shield is about 400 km in diameter at its base and at least 4 km high at its center. South of Valles Marineris, the Tharsis plateau is bounded by the approximate longitudes of 80° in the east and 140° in the west. In the Sinai Planum, closely adjacent to Coprates Chasma, another rise has been detected, bounded by longitudes of 55° in the east and 80° in the west. A volcanic shield of diameter 80 km, capped with a 22 km caldera has been identified near the crest of the rise. Topographic highs of about 1 km are associated with heavily faulted tracts such as Claritas Fossae. The distribution and orientation of the lunar-mare-like ridges in Sinai Planum appear to be independent of the regional gradients. Segments of the chaotic terrain at the eastern terminus of Valles Marineris are located as much as 6 km below the level of the surrounding plains.     

Lakshmi Planum,Venus: Characteristics and models of origin

Lakshmi Planum is distinctive and unique on the surface of Venus as an expansive (~2 × 10⁶km²), relatively smooth, flat plateau containing two large shield volcanoes and abundant volcanic plains in the midst of a region of extreme relief. It rises 3–5 km above the datum and is surrounded on all sides by bands of mountains interpreted to be of compressional tectonic origin. The major units mapped on Lakshmi are volcanic edifices, smooth, ridged and grooved plains units, and structural units referred to as ridged terrain. Three styles of volcanism are observed to dominate the surface of Lakshmi. Distributed effusive volcanism is associated with extensive plains deposits and many of the small shields, domes and cones mapped within the plateau. Centralized effusive volcanism is primarily associated with the paterae, Colette and Sacajawea, and their circumferential low-shield-forming deposits. The precise origin and evolution of these unusually large and complex structures is not understood, although a catastrophic, explosive origin is unlikely. Pyroclastic volcanism may be represented by a unit referred to as the diffuse halo. The origin and evolution of Lakshmi Planum is closely related to its compressional tectonic environment; volcanism on Lakshmi has occurred synchronously with tectonism in the surrounding orogenic belts. A model for the origin and evolution of Lakshmi Planum consisting of a continuous sequence of convergence and horizontal shortening of crustal segments against a preexisting block of tessera seems best able to account for the elevation, plateau shape and irregular polygonal outline of Lakshmi, as well as the presence of ridged terrain and its resemblance to tessera. Volcanism on Lakshmi is proposed to be the result of basal melting of a thickened crustal root. According to this model, the origin and evolution of Lakshmi Planum has consisted of the following sequence of events: (1) formation of a large, elevated block of tessera surrounded by low-lying plains; (2) convergence and underthrusting of crustal segments to produce peripheral mountain ranges, thickening, and uplift of the plateau; and (3) basal melting of the thickened crust and underthrust material and surface volcanism that occurred synchronously with continued edge deformation.'Geology and Tectonics of Venus', special issue edited by Alexander T. Basilevsky (USSR Acad. of Sci., Moscow), James W. Head (Brown University, Providence). Gordon H. Pettengill (MIT. Cambridge, Massachusetts) and R. S. Saunders (J.P.L., Pasadena).