Regional Plains Research Articles

Radar properties as clues to relative ages of ridge belts and plains on Venus

An ongoing issue regarding the crustal evolution of Venus is whether or not certain features and processes occurred in the same sequence and at the same time globally. This paper seeks to test one aspect of “directional” models for the geologic history: the inference that all ridge belts are older than regional plains. This test involves traditional geologic analysis, but also involves the use of radar properties in a unique way. We use a simplified model for the ridge shape, and an estimate of the diffuse scattering behavior of the Venus surface, to determine the “true” backscatter coefficient of the slope of the ridge tilted away from the radar, and ratio this value to the echoes from nearby plains units. In general, ridge‐forming terrain in these belts appears to have greater inferred backscatter coefficients, and thus decimeter‐scale roughness, than the regional plains. Plains units closely adjacent to many ridge belts have higher backscatter than plains at some distance, which we infer is generally due to aeolian deposition of debris characterized by centimeter‐scale microdunes or ripples where the belt topography interferes with wind patterns. The rougher apparent surface texture within the belt‐forming terrain is not consistent with the gentle folding that forms the ridges, and so is likely a result of weathering or tectonic processes operating prior to belt deformation. We thus conclude that the material incorporated in the ridge belts we have studied predates the surrounding regional plains.

Kinematic analysis of radial structures around Irnini Mons, Venus

Radial structures to the north-east of Irnini Mons are discernable as ridges at the full Magellan image resolution of 75 m/pixel using FMAP tiles. While a magma source modeled as a pressurized hole in an elastic plate usually allows for only extensional radial structures, the perturbation of regional stresses around the hole can explain the presence of radial ridges. Sixteen numerical models were run to see what effect changes in magma pressure and regional stresses had on the magnitude of the maximum principal stresses and the orientation of potential features around a hole in a plate. These models were arranged into four groups based on model input and setup: 1) uniaxial compressive stress; 2) uniaxial tensile stress; 3) biaxial stress; and 4) multiple holes. Nine of the sixteen models result in radial ridges in the orientation observed around Irnini Mons; all of these models incorporate a regional N-S compression. Flows from Irnini Mons are superimposed on an older, regional plains material deformed by a regional set of east-west trending wrinkle ridges, implying a regional north-south compression affected the area. The existence of radial ridges on the Irnini flows implies that the regional N-S compression that caused the E-W trending wrinkle ridges was still active during the formation of Irnini Mons. However, the magnitude of the regional compressive stress required for radial ridge formation could be 1–5 MPa less than the compressive strength of the material, indicating that radial ridges could form after wrinkle ridge formation has ceased.

North‐south roughness anisotropy on Venus from the Magellan Radar Altimeter: Correlation with geology

Analysis of the Doppler spectrum of Magellan Radar Altimeter echoes has revealed the spatially coherent deflection of the echo center from the nadir point (Tyler et al., 1992). We present and analyze a global map of the Doppler centroid of the radar altimeter echo, which, for the plains of Venus, characterizes the degree of anisotropy of the surface roughness in the north–south direction at scales from decimeters to kilometers. We found a global hemispherical trend of roughness anisotropy: on average, equator‐facing slopes are steeper than pole‐facing slopes. We searched globally for correlations between contrasts in roughness anisotropy and surface geology. We found that sharp contrasts in anisotropy often coincide with boundaries of geological units, although not consistently. We found a number of examples where radar‐bright (rough) lava flows have higher asymmetry than their surroundings. Diffuse radar‐dark crater‐related parabolic features and halos often have a symmetric roughness signature, while typical regional plains away from these diffuse features often have an anisotropic roughness. There is not, however, any direct and uniform correlation between the roughness anisotropy signature and geomorphologic terrain type or appearance in the radar images. We believe that asymmetric topography of deposits of windblown material, e.g., microdunes or small‐scale wind streaks in the wind shadow of obstacles, is the main contributor to the observed roughness anisotropy.

Impact craters on regional plains on Venus: Age relations with wrinkle ridges and implications for the geological evolution of Venus

A global assessment of impact craters on Venus ≥30 km in diameter shows that 146 craters are completely or partially superposed on the regional plains that dominate the surface and typically are deformed by wrinkle ridges. In this study, we found that 10 of these craters are deformed by wrinkle ridges (pre‐WR), 93 craters are superposed on wrinkle ridges (post‐WR), and 43 craters were found to be ambiguous cases. We compared our findings with the results of a similar study by McGill (2004) done for craters of all sizes but for ∼1/3 of the surface of Venus. We found that the percentage of pre‐WR craters found by McGill (2004) among the unambiguous cases (15%) is close to our percentage (10%), but we are in disagreement on the classification of a significant number of specific craters and in this analysis we describe the major reason for the disagreements. We then summarize published data on the crater densities for different plains on Venus. We assess Monte Carlo modeling of volcanic resurfacing, which competed with the ongoing formation of impact craters. On the basis of these data and analyses, we conclude that the mean age of the regional plains is close to the mean global surface age of Venus and that the potential time span of regional plains emplacement was relatively short. We interpret our results as evidence that wrinkle ridging on Venus was a distinct tectonic phase that occurred rather early in post‐regional‐plains time, probably within the first 10%. Although the exact duration remains uncertain, regional and global stratigraphic relationships strongly support the hypothesis that wrinkle ridging on Venus predominantly occurred during a specific phase of history. The geologic record of wrinkle ridging and post‐plains tectonic evolution of volcanic plains on the Moon and Mars show interesting similarities to those of Venus.

1:1.5,000,000 Geological Map of the Aglaonice Region on Venus

Please click here to download the map associated with this article. A geological map of the Aglaonice region, Venus, is presented. It is the first published attempt at mapping at the full limit of resolution of the available NASA Magellan mission data. It is primarily based on geological interpretation of a 1:1.5,000,000 scale Synthetic Aperture Radar (SAR) image and altimetry data of the same region. The map covers an area approximately 1.6 x 106 km2—roughly 1% of the venusian surface—and demonstrates the stratigraphic history revealed by mapping at this scale. We do not observe a regional plains unit, but rather five tectonically and texturally distinct volcanic plains material units. We have found that almost two-thirds of the mapped units originate from identifiable source regions, suggesting that the origin of most units on Venus can be identified if mapped at the resolution of Magellan data.

Impact crater air fall deposits on the surface of Venus: Areal distribution, estimated thickness, recognition in surface panoramas, and implications for provenance of sampled surface materials

During their formation, impact craters on Venus larger than 11 km in diameter acquired parabolic features composed of air fall deposits that are radar‐dark in Magellan synthetic aperture radar images. A model of the parabola planimetric shape and size (the latter being a function of crater diameter) was constructed from the data of Campbell et al. [1992]. Reconstructing model parabolas around all impact craters with D > 11 km in the regions of the Venera‐Vega sites revealed that parabolas cover 80–90% of the surface. These regions are representative of the majority of the Venus surface in terms of general geology and impact crater abundance. This implies that a very significant part of the surface of Venus was covered at least once with parabola deposits, and thus it is logical to look for them in the TV panoramas of the Venus surface taken by the Venera landers. These panoramas show the presence of soil and layered rocks, with the rocks (shown by the data of several instruments) consisting of porous, mechanically weak material, implying an altered or sedimentary (in a broad sense, including volcanic tuffs and impact deposits) nature. We propose the hypothesis that (1) the layered rocks may be the partly lithified and then partially eroded parabola deposits, while (2) the soil may partly be unlithified crater deposits and partly a product of the degradation of layered rocks. If this hypothesis is correct, then the interpretations related to the provenance of the materials studied geochemically by the landers may need revision. In particular, it is very possible that deep‐seated (>1–3 km) rocks (the lower units of the regional plains complex, rocks of the plains basement, and plutonic rocks) are a part (perhaps significant) of the material analyzed by the landers. The possible presence of this deeply derived material should also be taken into account in selecting landing sites for future missions to Venus.

Stratigraphy of small shield volcanoes on Venus: Criteria for determining stratigraphic relationships and assessment of relative age and temporal abundance

Small volcanic edifices, shields with a diameter less than about 20 km, are common and sometimes very abundant features on the plains of Venus. Typically, they form tight or loose clusters of features known as shield fields. Small shields are interpreted to be formed due to small‐scale eruptions through numerous and distinct sources, a mode of formation apparently significantly different from the mechanism thought to be responsible for the emplacement of the vast regional plains of Venus. Did the eruption style of small shields occur repeatedly throughout the visible part of the geologic record of Venus? Or was this style more concentrated in a specific epoch or epochs of geologic history? Do the clusters of shields represent localized development of sources over a thermal anomaly such as a plume, or do they represent exposures or kipukas of a more regional unit or units? A major step toward answering these questions is an understanding of small shield stratigraphy. Multiple criteria have been developed to assess the stratigraphic relationships of individual small shields and that of shield fields with the adjacent units. In our analysis, we expanded and developed the previous criteria and added detailed criteria to describe specific patterns of deformation within shield fields, cross‐cutting, and embayment relationships between shield fields and surrounding units. We also used secondary characteristics of shield fields such as radar albedo difference, changes in shield density and size, etc. In our study, we applied these criteria and analyzed in detail stratigraphic relationships of shield fields in a random sample of features (64 fields) and in the global geotraverse along 30°N (77 fields). The total number of analyzed shield fields (141) represents about 22% of the general population of these features catalogued by Crumpler and Aubele [2000]. The majority of the fields (98, or ∼69%) predate emplacement of material of vast regional plains with wrinkle ridges. Fifteen fields (∼11%) appear to be synchronous with regional plains, and eleven fields (∼8%) postdate the plains. Nine fields (∼6%) display ambiguous relationships with regional plains and their relative age is uncertain, and eight fields (∼6%) represent unclear cases when fields are covered by crater‐related materials or by young lava flows or are not in contact with regional plains. The results of our study provide evidence for a distinct change of volcanic style from the mode of formation of globally abundant small shields to the mode of emplacement of vast regional plains in many areas on Venus. This systematic change of volcanic style appears to be inconsistent with the “nondirectional” or quasi steady state character of the geologic record of Venus. Although individual small shields were formed throughout the majority of the visible geologic history of Venus, in the syn‐ and postregional plains time the small‐shields style of volcanism was significantly reduced in abundance. The shield fields that predate regional plains do not display a strong tendency to form a single group or a few groups and can be found virtually in all places on Venus. We interpret this observation to mean that these shield fields were globally distributed before the emplacement of regional plains. This interpretation means that the shield fields embayed by regional plains represent exposures of a specific, globally widespread unit, shield plains (psh). In contrast, shield fields that postdate regional plains occur preferentially in the Beta‐Atla‐Themis region on Venus, well known for its concentration of relatively young volcanic and tectonic activity. The spatial association of relatively young fields with the large centers of young volcanism suggests a genetic link of these fields with the formation and development of the large‐scale volcanic centers. The abrupt decrease of the number of shields that postdate the formation of shield plains (psh) strongly suggests a major change of the style of volcanism following their emplacement as a globally distributed unit.

Tectonic and stratigraphic implications of the relative ages of venusian plains and wrinkle ridges

A major ongoing controversy concerns the style of crustal evolution on Venus. At one extreme is a directional model that proposes a sequence of depositional and deformational events that occur at specific times in the evolution of the crust and that are global in extent. At the other extreme is a model that argues for different ages of these events in different places on the planet. A test of the directional model is here focused on whether wrinkle ridges formed at a single time in the recorded crustal history of Venus. Where sets of wrinkle ridges intersect it commonly is possible to determine that one set is older than the other. Also, the deformation responsible for wrinkle ridges is, in places, clearly progressive with respect to stratigraphic material units. These observations are not consistent with a specific single time for the formation of wrinkle ridges within the stratigraphic sequence. Within an area including about 1 / 3 of the surface of Venus 15% of craters that are younger than regional plains are older than wrinkle ridges, 85% are younger than wrinkle ridges. Taking 750 myr as a reasonable mean age for the regional plains, this implies that the mean age of wrinkle ridges is ∼ 110 myr younger than the mean age of plains. Solomon et al. (1999, Science 286, 87) propose that the emplacement of a large volume of plains lava would lead to a major atmospheric temperature increase. Their model predicts thermal stresses in the lithosphere that, at shallow depth, would reach peak compressive stresses in about 100 myr, a number very similar to the time lag between plains emplacement and wrinkle ridge formation indicated by the crater data. The thermal compressive stresses responsible for wrinkle ridges would be maintained at a level sufficient to deform basalt for at least 100 myr and possibly for as long as 350 myr. These time intervals are not really short compared to the mean age of the plains. Finally, because wrinkle ridges are demonstrably younger than the plains they deform, they cannot be related to the processes that formed the plains and thus should not be used to define a “plains with wrinkle ridges” unit.

Novae on Venus: Geology, classification, and evolution

Novae are “radially fractured centers” 100–300 km in diameter, and 64 have been identified on Venus. Dense radial fracturing and upraised topography are common, and massive amounts of volcanism are seen in some. Novae are interpreted to be the result of updoming and fracturing of the surface due to interaction of mantle diapirs with the lithosphere and radial fracturing caused by dike emplacement. We analyzed the topography and character of deformational and morphological structures of all 64 novae, leading to a better understanding of their evolution and relation to diapir dynamics, and their age. We subdivided novae into four topographic classes: (1) upraised, (2) annular, (3) flat and negative, and (4) plateau‐like. Novae are located mostly in areas of regional rises and rift zones, with a small number in the lowlands. About one half the novae studied show association with rifts; in some of these the rift troughs surround nova rises and form plateau‐like novae. Plateau‐like novae may predate, postdate, or form simultaneously with rifts. Our analysis generally confirms and updates previous models of novae evolution, with the following geological factors being important: (1) depth of the neutral buoyancy level of the rising mantle diapir; (2) rheological characteristics of the overlying lithosphere; (3) whether or not the visco‐plastic material above the diapir undergoes spreading; and (4) the nature and influence of regional stress and rift structures. These combined factors exert control on the direction of nova evolution and appear to determine the primary characteristics of nova, such as topography, tectonics, and volcanism. We found clear evidence for (1) the important role of dike swarm emplacement in the formation of radial patterns of novae and (2) the formation of concentric tectonic features during the relaxation stage of novae. Novae have multiple evolutionary stages and are long‐lived structures. Stratigraphic analysis of all 64 local areas showed a similarity in the sequence of regional geologic units. We found that 40.3% of the novae population started to form before emplacement of regional plains with wrinkle ridges and that 11.3% completed their activity before this time; 88.7% of the population of novae was active after regional plains formation. In contrast to novae, coronae activity was greatest before formation of regional plains, which may be due to thickening of the lithosphere with time. Detailed structural analysis shows that novae evolution does not always lead to the formation of corona‐like features.

Venus: Analysis of the degree of impact crater deposit degradation and assessment of its use for dating geological units and features

We use the degree of degradation of crater‐associated radar‐dark deposits on Venus to estimate the age of the crater and neighboring units. We analyzed craters ≥30 km in diameter superposed on regional plains (subpopulation 1; 138 craters) and on later units (subpopulation 2; 30 craters) and estimated percentages of craters with dark parabolas (DP), clear dark halo (CH), faint dark halo (FH), and no dark halo (NH). We constructed theoretical models of the evolution of these percentages with time for two possible interpretations of the upper boundary of the regional plains: (1) globally synchronous and (2) nonsynchronous (diachronous). They show that in the synchronous case the dark deposit lifetimes are proportional to the corresponding percentages observed for subpopulation 1: TDP = 0.15T, TCH = 0.3T, and TFH = 0.3T, where T is the mean global surface age of Venus. In diachronous cases the percentages may or may not be proportional to the appropriate lifetimes, depending on the range of the regional plains. If the time range of plains emplacement is not larger than ±0.5T, the use of TDP = 0.15T and TCH = 0.3T is appropriate. We consider the time ∼0.5T ago as the lower time boundary for the age of the now‐observed CH craters and the time 0.1–0.15T ago as the lower boundary for the age of DP craters. We propose that the Atlian Period of the geologic history of Venus be subdivided into Upper and Lower Epochs, with the boundary between them at ∼0.5T ago, and the lower boundary of the Aurelian Period be placed at 0.1–0.15T ago. We apply this approach to assess ages of activity of three volcanic‐tectonic structures on Venus: Beta Regio‐Devana Chasma (shows evidence of activity younger than 0.5T ago), Mylitta Fluctus (also younger than 0.5T), and Atla Regio (shows evidence of activity younger than 0.1–0.15T ago).

On rates and styles of late volcanism and rifting on Venus

We investigated the possibility of significant variations in the rates and styles of volcanism and rifting during the time postdating the formation of regional plains on Venus. We analyzed the age relations of all known impact craters ≥30 km in diameter (183 craters) with the neighboring geologic units. Of these we selected 164 craters which were superposed on regional plains and determined if post‐regional‐plains (PRP) volcanics and/or PRP rift structures were present in the crater vicinity and if these craters postdated the volcanics and rift structures or predated them. In 53 cases it was possible to determine these relations. On the basis of these relationships, it was found that the general rates of volcanism and rifting during PRP time were close to constant or at least had no drastic changes. This implies a significant change in the rates of volcanism and rifting in the vicinity of the boundary between PRP time and the preceding time, which is marked by the formation of the global wrinkle‐ridge network. It was also found that (1) the role of rift‐associated volcanism during PRP time was close to constant or slightly decreasing, (2) the role of corona‐related volcanism was noticeably decreasing, and (3) the role of noncorona, hot spot volcanism was proportionally increasing. The latter changes may be due to thickening of the lithosphere during PRP time. The results imply also that at most if not all places where PRP rifting occurred, the rift‐associated fracturing, when started, continued for a significant part of PRP time. Our conclusions are valid both (1) for the case of a globally synchronous transition from emplacement of regional plains to the PRP regime and (2) for the case of such a transition occurring at different times in different areas of Venus.

Morphology of selected novae (astra) from the analysis of Magellan images at Venus

We have carried out the photogeologic analysis and mapping of five novae (astra) considered as typical examples of the population of 64 features of this type listed by Crumpler and Aubele (In: H. Sigurdsson et al. (Eds.), Encyclopedia of Volcanoes, Academic Press, New York, p. 727). The studied novae consist of two structural components: (1) older fractures which are densely spaced (0.3– 1 km ) almost parallel within small areas, and dominate the structures; (2) younger more widely spaced (⩾2– 3 km ) sinuous fractures which have a subordinate role. The older component began to form prior to the emplacement of Psh/Pwr regional plains which cover and embay it (at least partly). This suggests that the presently observed novae are remnants of a once more numerous population of structures in which dense radial fracturing was spatially continuous. It is not clear whether the two structural components of novae correspond to two separate deformation stages or the deformation was continuous but temporarily overwritten by volcanism. The observed change in the fracture spacing may be due to an increase of thickness of the deformed layer with time. All five novae show the presence of associated volcanism (flows and small shields) which postdated regional plains and the amount of which varies from structure to structure from minor to significant. Earlier nova-associated volcanism either predating or contemporaneous to regional plains cannot be excluded. Three of five studied novae are topographically positive while the other two are negative. This may be due either to topographic relaxation of some of the structures or to different formation mechanisms. The studied novae belong to the sub-population of features not associated with typical coronae and they might have followed different evolutionary paths of and/or different formation modes than the novae more closely connected to coronae. Further study of more extended samples of populations of novae and coronae is needed to understand the differences in basic characteristic of novae and possible links between novae and coronae.

Tectonic Structure, Classification, and Evolution of Arachnoids on Venus: Preliminary Results

An analysis has been done of the topography and geologic structure of arachnoids—specific radial/concentric volcannic-tectonic structures on the surface of Venus. A representative sample (53 arachnoids) from 265 structures of this type, which are listed in the catalog of volcanic structures of the surface of Venus (Crumpler and Aubele, 2000), has been studied. The overwhelming majority of arachnoids are shown to be depressions that are commonly outlined by concentric extensional structures. Following Head et al. (1992) and Aittola and Kostama (2001), the assumption is confirmed and substantiated that arachnoids are formed by gravitational relaxation of small magmatic diapirs. Several types of arachnoids are identified on the basis of an analysis of structural patterns characteristic of such structures. It is also shown that the formation of different types of arachnoids depends on the depth of the magmatic diapir under the surface, on the thickness and reologic properties of the structures superposed on the evolving magmatic diapir, and on the character of regional stress fields that arise in the process of formation of such structures. The conclusion is drawn that most of the arachnoids were formed due to the gravitational relaxation of magmatic diapirs within the brittle part of the lithosphere, and some of them appeared as a result of the gravitational relaxation of radially fractured centers—novae. It is also shown that arachnoids are long-lived and multistep structures. At least some of them began to evolve before the formation of regional plains with wrinkle ridges, and their development ended after this event.

Venus: Timing and rates of geologic activity

We employ the accumulation of the impact-crater population as a geochronometer to examine the observable record of the history of Venus (<1 b.y), analyzing age relationships between craters and the different geologic units. We show that the earlier suite of units (from heavily deformed tesserae through slightly deformed regional plains) occurred during a time period an order of magnitude shorter than the subsequent period (from the end of emplacement of the wrinkle-ridge network until the present). These results imply high global rates of endogenic (volcanic) activity during the first era (comparable to that of mid-oceanic-ridge volcanism of Earth) and much lower global rates of endogenic activity (by two orders of magnitude) for the second period.

Geology of Venus: Mapping of a global geotraverse at 30°N latitude

We mapped a swath of terrain on Venus that extends circumferentially completely around the globe at 30°N latitude representing about 11% of the surface of the planet in order to address the following questions: Do presently defined stratigraphic units prove useful for broad application to regional and global mapping? Do units described in one area occur in other regions of the planet and how laterally continuous are they? If similar sequences of units are found in different parts of the planet, do they correlate temporally (e.g., time‐correlative), or do they represent similar sequences formed at different times (e.g., time‐transgressive or independently locally repetitive)? We found that similar sequences of units are seen in different parts of the global geotraverse and that key stratigraphic units could be traced across almost the entire circumference of Venus at this latitude, providing evidence that these units are not only global in extent, but also laterally contiguous. This gives confidence that the stratigraphic sequence can be extended to other parts of Venus for further testing and refinement. On the basis of this mapping, we find that several areas, including tessera massifs and Beta Regio, show a concentration of older units and appear to have been topographically high over much of the visible history of Venus. Tessera terrain is not uniformly distributed in some areas where these older units are concentrated. Ridge belts and fracture belts are not randomly distributed throughout the area, suggesting that they represent regional patterns of deformation. The altitude distribution of the stratigraphic units and structures shows that the regional plains tend to be the lowest, with older units (tessera and earlier plains) at intermediate to high elevations, consistent with regional plains emplacement following the formation of the most significant regional topography. Sixty corona and corona‐like features were found in the geotraverse, and the vast majority show evidence of being initiated in the earliest posttessera time (early Guineverian) in an extensional tectonic environment; few show evidence of postregional plains volcanism, and no positive evidence was found for the initiation of coronae in the postregional plains period of the history of Venus. Regional mapping shows that shield plains represent a unit occurring primarily prior to the emplacement of regional plains with wrinkle ridges (early part of the middle Guineverian), perhaps overlapping somewhat with its lower member. Evidence is found for changes in styles of volcanism with time, and tectonic activity changed in time in terms of pervasiveness, style, and areal distribution. On the basis of mapping of this globally continuous swath representing about 11% of the planet, we find that our analysis supports the presence of changes in the style and relative importance of different types of geological activity as a function of time, rather than the “nondirectional” history proposed by Guest and Stofan [1999].

A Stratigraphic Study of Small Volcano Clusters on Venus

Recently proposed global stratigraphy and geologic history models have led to two end member volcanic evolution models for Venus: a “directional” model that proposes Venus has progressed through a series of stages, each characterized by a particular style of volcanic activity (A. T. Basilevsky and J. W. Head 1997, J. Geophys. Res. 103, 8531–8544), and a [nondirectional] model that proposes a more Earth-like type of volcanic activity with different eruption styles occurring globally on a local scale throughout the recent history of the planet (J. E. Guest and E. R. Stofan 1999, Icarus 139, 55–66). The former infers that most small volcanoes ( D<20 km) define a global unit, referred to as the “shield plains,” that predates emplacement of the widespread plains with wrinkle ridges, or “regional plains.” To test this hypothesis, cross-cutting and superposition relations were used to determine the stratigraphic positions of small volcano clusters in seven quadrangles (11% of the surface). The seven quadrangles included 179 clusters containing over 22,000 constructs. Three materials were defined for each cluster: (1) the constructs, (2) the material immediately surrounding the constructs (called the cluster material), and (3) the regional plains. Where possible, relative ages were established between the constructs, cluster material, and the regional plains. There are six possible temporal relations between the constructs and regional plains emplacement: (1) the constructs postdate regional plains emplacement, (2) some constructs postdate regional plains emplacement, and other constructs predate regional plains deformation but their ages relative to regional plains emplacement are ambiguous, (3) the constructs predate regional plains emplacement, (4) some constructs predate regional plains emplacement, and other constructs predate regional plains deformation but their ages relative to regional plains emplacement are ambiguous, (5) the constructs both predate and postdate regional plains emplacement, and (6) the ages of the constructs relative to regional plains emplacement are ambiguous. An age relative to regional plains emplacement could not be determined for almost half of the clusters studied. Of the clusters for which a relative age could be determined, almost half postdate regional plains emplacement, and an additional third contain at least some constructs that postdate regional plains emplacement. Less than a tenth of the clusters predate regional plains emplacement. These observations indicate that many small constructs postdate the emplacement of the regional plains and therefore are consistent with the nondirectional volcanic evolution model in which small constructs represent a global process occurring locally throughout the recent history of Venus.

Rifts and large volcanoes on Venus: Global assessment of their age relations with regional plains

We report on the results of synoptic global mapping of rifts and large volcanoes on Venus and detailed mapping in six areas illustrating the relationships among these features. Rifts and large volcanoes were subdivided into old, transitional (for volcanoes only), and young features. The geologic event separating old features from young ones is the formation of wrinkle ridges on the regional plains. Global mapping showed that the postplains rifts dominate in the population of rifts both in terms of number of mapped segments and in area. The postplains volcanoes are more abundant in terms of number and area occupied than those that are transitional in age, and these, in turn, dominate over the old ones. These data show that in post‐regional‐plains time rifting and formation of large volcanoes are significantly more important processes than in the immediately preceding period. The time period during which regional plains were emplaced, and old (and partly transitional) large volcanoes formed, was much shorter than the postplains time. Therefore the mean rate of formation of large volcanoes during this first period was significantly higher than in the subsequent time. Similarly, the relatively short time duration of the plains‐emplacement time indicates that the mean rate of old rifting was higher than in more recent time. This study shows prominent changes with time in the absolute rates and roles of rifting and large volcano formation. This supports the “directional” model of the geologic evolution of Venus during the last several hundred million years, as opposed to the “nondirectional” model, which predicts a more random occurrence of geologic processes throughout history.

Geologic evolution of southern Rusalka Planitia, Venus

Geologic mapping of southern Rusalka Planitia, Venus, reveals interactions of volcanism, tectonism, and topography. We recognize three regional plains units (prR1, prR2, and prR3) based on crosscutting structural relations, embayment patterns, radar brightness, and surface roughness data. Delineation of secondary (tectonic) structures allows us to constrain the relative temporal relations between the three material units. Unit prRl, a radar dark smooth unit exposed in local topographic highs, hosts NE trending extension fractures. Low‐viscosity lava flows of prR2, the most areally extensive unit, fill local topographic lows and the NE trending fractures. A shield‐sourced lava unit, prR3, overlies prR2 on the basis of embayment relations and radar brightness. NW trending wrinkle ridges deform all three plains units and record regional contraction. Locally, flood lava flows that fill NE trending fractures are structurally inverted to form short, stepped NE trending wrinkle ridges. Map patterns indicate that prR2 comprises a thin layer (&lt;50 m thick), much thinner than previous estimates of 1–3 km. Therefore previously proposed estimates of plains flood lava flow volumes and effusion rates are much too high. The local geologic history of southern Rusalka Planitia is inconsistent with global stratigraphy models. Our study supports the view of plains evolution occurring through discrete volcanic processes working at local and regional (but not global) scales.

Geologic units on Venus: evidence for their global correlation

Detailed geologic mapping of approximately 30% of the surface of Venus has revealed a stratigraphic sequence that appears to be repeated in widely separated areas on the planet. This sequence shows a transition from oldest highly deformed terrain units, the tessera, to a series of widespread volcanic plains units, each with recognizable morphologic characteristics and interpreted eruption style, and finally to individual eruptive centers associated with edifices and rift zones. This sequence of events is accompanied by characteristic and repetitive patterns of tectonic deformation. In order to test the validity and broader applicability of this sequence, we compare it with local, regional, and global mapping published in 17 analyses by other authors. This comparison shows that the sequence of units, and thus the relative time sequence of the corresponding geologic events, is generally the same in virtually all of the widely distributed areas that have been studied, comprising about one-half the surface of Venus. We then address several alternative explanations for the observed sequence: (1) that the similar units and sequences correspond to events that occurred in different areas at different times (repetitive in different places but not time correlative); (2) that the sequences occurred generally synchronously in different places (repetitive and time correlative); or (3) that the situation is intermediate between the two cases. In order to distinguish among these possibilities, we analyze evidence for the lateral global continuity of units, the chronology of emplacement on the basis of the density of superposed craters, and the consistency of stratigraphic relationships based on these data. We find that this analysis supports the hypothesis that the individual sequences are repetitive in different parts of the planet, and generally time correlative between these locations. As a further test of this stratigraphy, we outline a global model for the geological evolution of Venus and assess the duration of emplacement of units, the timing and style of tectonic phases, and the volumes and implied fluxes of volcanic deposits. We find that the morphologically recognizable part of the history of Venus comprises only the last 10–20% of its total history. Emplacement of tessera-forming material and its deformation into tessera terrain are the major geologic events of the initial Fortunian time. Several stages of areally extensive volcanism occurred subsequently, burying vast areas of tessera and forming regional plains during Sigrunian, Lavinian, and Rusalkian times. The average global rate of volcanism was a few km 3 y −1, and the emplacement of plains was accompanied by alternating episodes of contraction and extension. The last global-scale distributed tectonic episode, the formation of an extensive network of wrinkle-ridges, happened near the end of the Rusalkian Period, and marked the transition to the present stage of the history of Venus, the Atlian Period. Atlian times are characterized by a predominance of regional rifting focused at several broad rises, and localized rift-associated volcanism in the form of shield volcanic constructions and lobate volcanic plains-forming units, and is the longest time duration among the stratigraphic units considered, although the resulting tectonic and volcanic features and deposits cover only 10–20% of the surface of Venus. Comparison of this sequence of events with those revealed by continued geologic mapping of Venus will permit further testing and assessment so that a consistent and comprehensive documentation of the geological record can be presented and used to test models of the geodynamic evolution of Venus.

Evidence for rapid regional plains emplacement on Venus from the population of volcanically embayed impact craters

We have reexamined the population of craters on Venus believed to be embayed by lava flows and discuss the reliability of criteria for determining the volcanic embayment state of a crater, presenting nonvolcanic processes which may produce similar crater morphologies. We conclude that 29 craters have been embayed by lava flows from the exterior, and an additional 27 craters are ambiguous cases (3–6% of the total crater population). All of the embayed craters are embayed by the stratigraphically youngest lava flows except for five to 11 craters embayed by stratigraphically older regional plains deposits which are typically deformed by wrinkle ridges. Through geologic mapping of an area covering over 8% of the surface, we examined the spatial relationships between wrinkle‐ridged plains and stratigraphically older plains units, and estimate that a large portion of the wrinkle‐ridged plains are thinner than 500 m, which is thin enough for the rims of the largest half of preexisting craters to be preserved. There are not enough craters embayed by the wrinkle‐ridged plains to support a model in which the plains were erupted directly onto an ancient surface. Given the number of large craters embayed by thin deposits of wrinkle‐ridged plains, we develop a simple statistical model to overestimate the number of craters on the preplains surface. This model suggests that the average age of the surface beneath the plains plus the average length of time over which the plains were emplaced was of the order of tens of millions of years. The short exposure age for the preplains surface indicates that some event or events must have erased the ancient population of craters from the surface before the emplacement of the wrinkle‐ridged plains.