Formation and evolution of Lakshmi Planum, Venus: Assessment of models using observations from geological mapping

Detailed geological analysis of the Lakshmi Planum region of western Ishtar Terra results in the establishment of the sequence of major events during the formation and evolution of western Ishtar Terra, an important and somewhat unique area on Venus characterized by a raised volcanic plateau surrounded by distinctive folded mountain belts, such as Maxwell Montes. These mapping results and the stratigraphic and structural relationships provide a basis for addressing the complicated problem of Lakshmi Planum formation and for testing the suite of models previously proposed to explain this structure. We review and classify previous models of formation for western Ishtar Terra into “downwelling” models (generally involving convergence and underthrusting) and “upwelling” models (generally involving plume-like upwelling and divergence). The interpreted nature of units and the sequence of events derived from geological mapping are in contrast to the predictions of the divergent models. The major contradictions are as follows: (1) The very likely presence of an ancient (craton-like) tessera massif in the core of Lakshmi, which is inconsistent with the model of formation of Lakshmi due to rise and collapse of a mantle diapir; (2) The absence of rift zones in the interior of Lakshmi that are predicted by the divergent models; (3) The apparent migration of volcanic activity toward the center of Lakshmi, whereas divergent models predict the opposite trend; (4) The abrupt cessation of ridges of the mountain ranges at the edge of Lakshmi Planum and propagation of these ridges over hundreds of kilometers outside Lakshmi; the divergent models predict the opposite progression in the development of major contractional features. In contrast, convergent models of formation and evolution of Lakshmi Planum appear to be more consistent with the observations and explain this structure by collision and underthrusting/subduction of lower-lying plains with the elevated and rigid block of tessera. These models are capable of explaining formation of the major features of western Ishtar (for example, the mountain belts), the sequences of events, and principal volcanic and tectonic trends during the evolution of Lakshmi. To explain the pronounced north-south asymmetry of Lakshmi these models need to consider the likelihood that the major focal points of collision are at the north and north-west margins of the plateau. We note that pure downwelling models, however, face three important difficulties: (1) The possibly unrealistically long time span that appears to be required to produce the major features of Lakshmi; (2) The strong north-south asymmetry of the Planum; the pure downwelling models predict the formation of a more symmetrical structure; and (3) The absence of radial contractional structures (arches and ridges) in the interior of Lakshmi that would represent the predictions of the downwelling models.     

Global stratigraphy of Venus: analysis of a random

The age relations between 36 impact craters with dark paraboloids and other geologic units and structures at these localities have been studied through photogeologic analysis of Magellan SAR images of the surface of Venus. Geologic settings in all 36 sites, about 1000 × 1000 km each, could be characterized using only 10 different terrain units and six types of structures. These units and structures form a major stratigraphic and geologic sequence (from oldest to youngest): 1) tessera terrain; 2) densely fractured terrains associated with coronae and in the form of remnants among plains; 3) fractured and ridged plains and ridge belts; 4) plains with wrinkle ridges; 5) ridges associated with coronae annulae and ridges of arachnoid annulae which are contemporary with wrinkle ridges of the ridged plains; 6) smooth and lobate plains; 7) fractures of coronae annulae, and fractures not related to coronae annulae, which disrupt ridged and smooth plains; 8) rift-associated fractures; 9) craters with associated dark paraboloids, which represent the youngest 10% of the Venus impact crater population (Campbellet al., 1992), and are on top of all volcanic and tectonic units except the youngest episodes of rift-associated fracturing and volcanism; surficial streaks and patches are approximately contemporary with dark-paraboloid craters.Mapping of such units and structures in 36 randomly distributed large regions (each 10⁶ km²) shows evidence for a distinctive regional and global stratigraphic and geologic sequence. On the basis of this sequence we have developed a model that illustrates several major themes in the history of Venus. Most of the history of Venus (that of its first 80% or so) is not preserved in the surface geomorphological record. The major deformation associated with tessera formation in the period sometime between 0.5–1.0 b.y. ago (Ivanov and Basilevsky, 1993) is the earliest event detected. In the terminal stages of tessera formation, extensive parallel linear graben swarms representing a change in the style of deformation from shortening to extension were formed on the tessera and on some volcanic plains that were emplaced just after (and perhaps also during the latter stages of the major compressional phase of tessera emplacement. Our stratigraphic analyses suggest that following tessera formation, extensive volcanic flooding resurfaced at least 85% of the planet in the form of the presently-ridged and fractured plains. Several lines of evidence favor a high flux in the post-tessera period but we have no independent evidence for the absolute duration of ridged plains emplacement. During this time, the net state of stress in the lithosphere apparently changed from extensional to compressional, first in the form of extensive ridge belt development, followed by the formation of extensive wrinkle ridges on the flow units. Subsequently, there occurred local emplacement of smooth and lobate plains units which are presently essentially undeformed. The major events in the latest 10% of the presently preserved history of Venus (less than 50 m.y. ago) are continued rifting and some associated volcanism, and the redistribution of eolian material largely derived from impact crater deposits.Detailed geologic mapping and stratigraphic synthesis are necessary to test this sequence and to address many of the outstanding problems raised by this analysis. For example, we are uncertain whether this stratigraphic sequence corresponds to geologic events which were generally synchronous in all the sites and all around the planet, or whether the sequence is simply a typical sequence of events which occurred in different places at different times. In addition, it is currently unknown whether the present state represents a normal consequence of the general thermal evolution of Venus (and is thus representative of the level of geological activity predicted for the future), or if Venus, has been characterized by a sequence of periodic global changes in the composition and thermal state of its crust and upper mantle (in which case, Venus could in the future return to levels of deformation and resurfacing typical of the period of tessera formation).     

Morphology of the Tessera Terrain on Venus: Implications for the Composition of Tessera Material

The main goal of this paper is to estimate the possible composition of the tessera material on the basis of an interpretation of the morphology of the tessera precursor terrain. The results of detailed photogeologic analysis of tessera are presented. For the study, 56 randomly chosen areas that characterize the surface of large and small tessera massifs were selected. Each area represents a portion of the F-MAP photomosaics acquired at a 75 m/px resolution. The results of this study show that the tessera precursor terrain appears everywhere as plains. In its morphology, these plains are similar to the plains outside the tessera massifs. An overview of all possible mechanisms of the formation of plains on Venus and comparison of these mechanisms with the data of the chemical measurements on the surface of Venus suggests that the Venusian plains were formed as a result of the emplacement of low-viscous basaltic lava. This rather well-known conclusion is made here for the first time in order to estimate the possible composition of the tessera material. Thus, it is likely that the composition of the tessera precursor plains is similar to the composition of the basaltic plains on Venus. The products of posttessera volcanism in the form of morphologically smooth plains commonly occur within the tessera terrains. Morphologically, these plains are similar to the regional Venusian plains, which strongly suggests a basaltic composition of such plains. There are only two volcanic flows within the whole tessera terrain on Venus whose morphology permits one to interpret them as a manifestation of nonbasaltic, more siliceous volcanism. This means that the material of the regional tessera-bearing highlands very rarely responded to the thermal influence from below by siliceous volcanism. If some hypothetical granitelike material makes up the main portion of the tessera highlands, this material remains hidden. Therefore, the hypothesis of the granitelike bulk composition of the tessera highlands has little support from observations. At the current stage of the study of Venus, a model in which tessera highlands are composed predominantly of basalt with a possible, but insignificant component of more siliceous material is thought to be correct.     

Assemblages of geologic/morphologic units in the northern hemisphere of Venus

The geologic/morphologic map of the northern mid-to-high latitudes of Venus prepared by a Soviet science team on the basis of Venera 15/16 mission radar image coverage is analyzed and used to define six discrete assemblages of geologic/morphologic units that have well-defined geographic distributions. These assemblages have distinctive and differing geological and tectonic expressions and include: Plains Assemblage - which is dominated by lowland smooth plains and lowland rolling plains interpreted to be of volcanic origin, and a high concentration of small volcanic domes; Plains-Corona Assemblage - which is dominated by lowland smooth plains and lowland rolling plains interpreted to be of volcanic origin, at least ten coronae structures concentrated in the northern half of the region, and at least five large volcanoes, generally concentrated in the southern and western half of the region; Plains-Ridge Belt Assemblage - which is dominated by lowland smooth plains and lesser amounts of lowland rolling plains, major occurrences of ridge belts in a distinctive fan-shaped pattern, and very minor and patchy occurrences of tessera; Plains-Corona-Tessera Assemblage - which is dominated by approximately equal amounts of lowland smooth plains and lowland rolling plains, at least five coronae concentrated in the northern part of the region, a small number of large volcanoes, also in the northern part of the region, and numerous small patches of tesserae scattered throughout, and the highest abundance of small volcanic domes observed in the northern hemisphere; Tessera-Ridge Belt Assemblage — which is dominated by a few large areas (Fortuna, Laima, Tellus) and several smaller areas (Dekla, Meni) of tesserae, ridge belts generally arrayed in an angular and often orthogonal pattern different from the fan-shaped pattern of the Plains-Ridge Belt Assemblage, lowland rolling plains and lesser amounts of lowland smooth plains, and an upland rise (Bell Regio); Tessera-Mountain Belt Assemblage - which is centered on the two volcanoes Colette and Sacajawea in Lakshmi Planum, and characterized by the peripheral mountain belt/tessera pairs, with the tessera on the outboard side: Danu/Clotho (S), Akna/Atropos (W), Freyja/ltzpapalotl (N), and Maxwell/Fortuna (E).The distribution and characteristics of assemblages demonstrate that vertical and horizontal tectonic forces are operating on the crust and lithosphere of Venus in different ways in specific localized areas. Alternative models are outlined for the origin of each assemblage and the relationship between assemblages, and important unresolved questions are identified. A key to the further understanding of these assemblages is the origin of ridge belts and tessera terrain.'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 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).     

Tectonics of Venus: A review

The article presents a new tectonic scheme of Venus and gives the following interpretation of the planet's main structural units: (1) plains — areas of flood volcanism over stretched crust; (2) dome-like uplifts — areas of uplifting and volcanic activity above the mantle hot-spots; (3) coronae —former dome-like uplifts, partially subsided and diffused by gravity; (4) ridge belts — fold zones; (5) tesserae — fragments of ductile compression and shortening of crust; (6) supercoronae — coronae formed in the course of further evolution and relaxation of Beta-type uplifts. Ishtar Terra is considered to be a fragment of an ancient tessera paleocontinent, on the edge of which the Lakshmi supercorona is superimposed. Aphrodite Terra is considered as a belt of mantle hot-spot structures (dome-like uplifts, coronae, supercoronae, volcanoes, rifts).Three types of planetary belts have been distinguished on Venus: uplifted 'weakened' belts with an abundance of mantle hot-spot structures; a northern fan of ridge belts; and belts of low basalt plains. The center of the planetary system of uplifted weakened belts is situated in Atla Regio.The present tectonic structure of Venus is inferred to have formed during two stages of evolution characterized by different tectonic regimes. Stage I is a regime of soft ductile plates (formation of tessera uplifts and volcanic plains). Stage II is a formation of 'weakened' uplifted planetary belts, various tectonic regimes of mantle hot-spots, and plains-forming volcanism.'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).     

Evidence of vertical and horizontal motions on Venus: Maxwell Montes

Based on full-resolution Magellan radar images, the detailed structural analysis of central Ishtar Terra (Venus) provides new insight to the understanding of the Venusian tectonics. Ishtar Terra, centered on 65° N latitude and 0° E longitude includes a high plateau. Lakshmi Planum, surrounded by highlands, the most important being Maxwell Montes to the East. Structural analysis has been performed with classical remote-sensing methods. Folds and faults identified on radar images were reported on structural map. Their type and distribution allowed to define the style of the crustal deformation and the context in which these structures formed. This analysis shows that Lakshmi Planum formed under a crustal stretching associated with a volcanic activity. This area then became a relatively steady platform, throughout the formation of Maxwell Montes mountain belt. Maxwell Montes is characterized by a series of NNW-SSE trending thrust faults dipping to the East, formed during a WSW-ESE horizontal shortening. In its NW quarter, the mountain belt shows a disturbed deformation controlled by pre-existing grabens and old vertical crustal fault zone. The deformation of this area is characterized by a shortening of cover above a flat detachment zone, with a progressive accommodation to the southwest. All these tectonic structures show evidence of horizontal and vertical crustal movements on Venus, with subsidence, mountain belt raise, West regional overthrusting of this mountain belt, and regional shear zone.     

Tectonic Evolution of Eastern Ishtar Terra,Venus

The complex morphology and topography of Eastern Ishtar Terra have been interpreted as due to tectonic deformation. Models proposed to account for this deformation include: crustal flow through asthenospheric flow and thermal-gravitational sliding; rifting, gravity spreading, and fold belt formation; and horizontal convergence and crustal thickening. In this study we map the detailed structural and topographic fabric of this region in order to explore and test these hypotheses. Eastern Ishtar can be divided into four major provinces: Maxwell Montes/Western Fortuna Tessera, a high plateau and mountain belt dominated by long NNW trending ridges; Central Fortuna Tessera, a low region of orthogonally oriented short WNW trending ridges and long, NNE trending troughs; Eastern Fortuna Tessera, a broad, E-W trending topographic rise characterized by ENE trending troughs and a complex pattern of intersecting ridges; and Northern Fortuna Tessera, a region of steep, NE-facing topographic scarps and ridges that trend WNW. On the basis of structural and topographic relationships, the features within these provinces are found to be inconsistent with a formation through either downslope crustal flow or rifting. We find that the mapped features are most consistent with a formation through convergence, collison, and underthrusting of thickened crustal terranes. These terranes are suggested to have been created through processes of seafloor-type spreading and crustal collision. Based on relationships between the different terranes, several accretional events are proposed in which Eastern Ishtar is produced by the collision of crustal terranes beginning at Lakshmi Planum and extending to the east. This sequence is initiated with the formation of Maxwell Montes and Western Fortuna Tessera during east-west crustal convergence, underthrusting, and stacking. The next step involves the northeast to southwest convergence of a preexisting thick block of tessera in Central Fortuna, which produces shear deformation within Western Fortuna. This northeast to southwest convergence also produces Northern Fortuna Tessera through crustal imbrication, a process recognized along the entire northern boundary of Ishtar Terra. Finally, Laima Tessera converges with Fortuna from the southeast and collides with Eastern Fortuna Tessera producing shear within Eastern Fortuna and the linear convergence zones along the edges of Laima. High resolution images returned by the Magellan spacecraft will enable us to examine the features involved in the proposed production and suturing of crustal terranes.'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).     

Geology of the Thaumasia region,Mars: plateau development,valley origins,and magmatic evolution

We have constructed the complex geologic history of the Thaumasia region of Mars on the basis of detailed geologic mapping and relative-age dating of rock units and structure. The Thaumasia plateau dominates the region and consists of high lava plains partly surrounded by rugged highlands, mostly of Noachian and Hesperian age. Long-lived faulting centered near Syria Planum and at lesser sites produced radiating narrow grabens during the Noachian through Early Amazonian and concentric wrinkle ridges during the Late Noachian and Early Hesperian. Fault activity peaked during the Noachian and waned substantially during Late Hesperian and Amazonian time. Volcanism on the Thaumasia plateau was particularly active in comparison with other martian cratered highlands, resulting in fourteen volcanoes and numerous outcrops of smooth, ridged, and lobate plains materials. A particularly extensive set of overlapping lava-flow units was emplaced sequentially from Thaumasia Planum to Syria Planum, spanning from the Late Noachian to the Late Hesperian; lobate flows succeeded smooth flow at the beginning of the Late Hesperian. Deep crustal intrusion and a thickened, buoyant crust may have caused the uplift of the plateau during the Noachian and Early Hesperian, resulting in outward-verging fold-and-thrust plateau margins. This structural style appears similar to that of the young ranges of the Rocky Mountains in the western U.S. Within the plateau, several sites of volcanotectonic activity and valley erosion may be underlain by large and perhaps long-lived magmatic intrusions. One such site occurs at the headland of Warrego Valles. Here, at least two episodes of valley dissection from the Noachian to Early Hesperian occurred during the formation of two nearby rift systems. The site also is a locus of intersection for regional narrow grabens during the Late Noachian and Early Hesperian. However, at the site, such faults diverge or terminate, which suggests that a resistant body of rock occurs there. The overall volcanotectonic history at Thaumasia fits into a model for Tharsis as a whole in which long-lived Syria Planum-centered activity is ringed by a few significant, shorter-lived centers of activity like the Thaumasia plateau. Valley formation, like tectonism in the region, peaked during the Noachian and declined substantially during the Hesperian and Amazonian. Temporal and spatial associations of single erosional valleys and valley networks with volcanoes, rift systems, and large impact craters suggest that the majority of valleys formed by hydrothermal, deformational, and seismic-induced processes. The origin of scattered, mainly Noachian valleys is more conjectural; possible explanations include local precipitation, seismic disturbance of aquifers, or unrecognized intrusions.     

A simple method for planetary surface ruggedness estimation

The method for mapping the pixel-scale and larger ruggednesses of the Martian ridged plains within Solis Planum is described by assuming the Lommel-Seeliger's reflection law. This method allows surface tilt angles towards and away from the Sun to be estimated and topographic elevations to be displayed by the aid of monoscopic digital image data. The restrictions of this method are also discussed.     

Geology of the southern Ishtar Terra/Guinevere and Sedna Planitae region on Venus

Recent high resolution, high incidence angle Arecibo radar images of southern Ishtar Terra and flanking plains of Guinevere and Sedna on Venus reveal details of topographic features resolved by Pioneer Venus. The high incidence angles of Arecibo images favor the detection of surface roughness-related features, and complement recently obtained low incidence angle Venera 15/16 images in which changes in surface topographic slope are well portrayed. Four provinces have been defined on the basis of radar characteristics in Arecibo images and topography. Volcanism and tectonism are the dominant processes in the mapped area, which has an average age of about 0.5–1.0 billion years (Ivanov et al., 1986). These processes vary in relative significance in the mapped provinces and it is likely that geologic activity has occurred simultaneously in all four provinces. On the basis of stratigraphic evidence, however, a general sequence is proposed which represents the major activity in each area. The low predominantly volcanic plains of Guinevere and Sedna Planitiae are the relatively oldest terrain. A major region of complex tectonic deformation, the Southern Ishtar Transition Zone, postdates much of the low plains and delineates the steep-sloped flanks of Ishtar Terra. Lakshmi Planum is characterized by a distinctive volcanic style (large low edifices, calderas, flanking plains) and at least in part postdates the Southern Ishtar Transition Zone. Relatively recent plains-style volcanism occurs locally in Sedna Planitia and embays the Southern Ishtar Transition Zone. Compressional deformation appears to dominate the mountains of the Ishtar plateau, but the nature of the tectonic deformation in the Southern Ishtar Transition Zone is very complex and likely represents a combination of extension, compression and strikeslip deformation. Arecibo data reveal additional coronae in the lowlands, suggesting that corona formation is an even more widespread process than indicated by the Venera data.     

The stratigraphy of the Amenthes region, Mars: Time limits for the formation of fluvial, volcanic and tectonic landforms

We present results of our morphologic and stratigraphic investigations in the Amenthes region for which our observations suggest a complex spatial and temporal interrelation between volcanic and possibly water-related processes. We have produced a series of self-consistent geological maps and a stratigraphic correlation chart that show the spatial and temporal distribution of volcanic, fluvial and tectonic processes.The Amenthes region consists of a broad trough-like topographic depression that has served as a path for the supply of materials from Hesperia Planum to Isidis Planitia. It is most likely that Hesperia Planum and, in particular the area north of Hesperia Planum, including Tinto Vallis, Palos crater and the surrounding dissected highlands have acted as a source region for materials that were transported into the Amenthes trough and farther into the Isidis basin. The Amenthes trough, as well as the graben of Amenthes Fossae were formed after the Isidis impact in the Noachian and represent likely the oldest features in the Amenthes region. Dendritic valley networks, that bear evidence for surface runoff, have dissected the highlands adjacent to Amenthes Planum and within the Tinto Vallis and Palos crater region before ∼3.7 Ga. The ridged volcanic plains located near the Palos crater and Tinto Vallis region, within Amenthes Planum as well as within the Isidis transitional plains were formed between ∼3.5 and 3.2 Ga and represent the volcanic activity which resulted in the flooding of the Amenthes trough. The sinuous channel of Tinto Vallis was formed in the Hesperian (?3.5 Ga) and shows characteristics, which are consistent with both ground water sapping and igneous processes. The Palos crater outflow channel was formed nearly at the same time as Tinto Vallis, between ∼3.5 Ga and ∼3.2 Ga and postdates the volcanic flooding of the Amenthes trough in the Hesperian. Small valleys (∼3.4-2.8 Ga) incised into the ridged plains of Amenthes Planum appear also within the transitional plains located between the Amenthes plains and the Isidis interior plains. Our model ages show that Tinto Vallis, the Palos crater outflow channel as well as the small valleys are unlikely formed at the same time and by the same processes as the dendritic valley networks and represent an episode that clearly postdates the volcanic activity.     

Paleolakes, paleofloods, and depressions in Aurorae and Ophir Plana, Mars: Connectivity of surface and subsurface hydrological systems

The plains of Aurorae and Ophir in the equatorial region of Mars display geomorphic evidence indicative of extensive but generally short-lived paleohydrological processes. Elaver Vallis in Aurorae Planum south of Ganges Chasma is an outflow channel system >180 km long, and here inferred to have formed by cataclysmic spillover flooding from a paleolake(s) contained in the Morella crater basin. Ganges Cavus is an enormous 5-km-deep depression of probable collapse origin located in the Morella basin. The fluid responsible for the infilling of the Morella basin likely emerged at least partially through Ganges Cavus or its incipient depression, and it may have been supplied also from small-scale springs in the basin. Similar paleohydrological processes are inferred also in Ophir Planum. It is reasonable to assume that water, sometimes sediment-laden and/or mixed with gases, was the responsible fluid for these phenomena although some of the observed features could be explained by non-aqueous processes such as volcanism. Water emergence may have occurred as consequences of ground ice melting or breaching of cryosphere to release water from the underlying hydrosphere. Dike intrusion is considered to be an important cause of formation for the cavi and smaller depressions in Aurorae and Ophir Plana, explaining also melting of ground ice or breaching of cryosphere. Alternatively, the depressions and crater basins may have been filled by regional groundwater table rising during the period(s) when cryosphere was absent or considerably thin. The large quantities of water necessary for explaining the paleohydrological processes in Aurorae and Ophir Plana could have been derived through crustal migration from the crust of higher plains in western Ophir Planum where water existed in confined aquifers or was produced by melting of ground ice due to magmatic heating or climatic shift, or from a paleolake in Candor Chasma further west.     

North polar region of Mars: Advances in stratigraphy, structure, and erosional modification

We have remapped the geology of the north polar plateau on Mars, Planum Boreum, and the surrounding plains of Vastitas Borealis using altimetry and image data along with thematic maps resulting from observations made by the Mars Global Surveyor, Mars Odyssey, Mars Express, and Mars Reconnaissance Orbiter spacecraft. New and revised geographic and geologic terminologies assist with effectively discussing the various features of this region. We identify 7 geologic units making up Planum Boreum and at least 3 for the circumpolar plains, which collectively span the entire Amazonian Period. The Planum Boreum units resolve at least 6 distinct depositional and 5 erosional episodes. The first major stage of activity includes the Early Amazonian (∼3 to 1 Ga) deposition (and subsequent erosion) of the thick (locally exceeding 1000 m) and evenly-layered Rupes Tenuis unit (Abrt), which ultimately formed approximately half of the base of Planum Boreum. As previously suggested, this unit may be sourced by materials derived from the nearby Scandia region, and we interpret that it may correlate with the deposits that regionally underlie pedestal craters in the surrounding lowland plains. The second major episode of activity during the Middle to Late Amazonian (1 Ga) began with a section of dark, sand-rich and light-toned ice-rich irregularly-bedded sequences (Planum Boreum cavi unit, Abb_c) along with deposition of evenly-bedded light-toned ice- and moderate-toned dust-rich layers (Planum Boreum 1 unit, Abb₁). These units have transgressive and gradational stratigraphic relationships. Materials in Olympia Planum underlying the dunes of Olympia Undae are interpreted to consist mostly of the Planum Boreum cavi unit (Abb_c). Planum Boreum materials were then deeply eroded to form spiral troughs, Chasma Boreale, and marginal scarps that define the major aspects of the polar plateau's current regional topography. Locally- to regionally-extensive (though vertically minor) episodes of deposition of evenly-bedded, light- and dark-toned layered materials and subsequent erosion of these materials persisted throughout the Late Amazonian. Sand saltation, including dune migration, is likely to account for much of the erosion of Planum Boreum, particularly at its margin, alluding to the lengthy sedimentological history of the circum-polar dune fields. Such erosion has been controlled largely by topographic effects on wind patterns and the variable resistance to erosion of materials (fresh and altered) and physiographic features. Some present-day dune fields may be hundreds of kilometers removed from possible sources along the margins of Planum Boreum, and dark materials, comprised of sand sheets, extend even farther downwind. These deposits also attest to the lengthy period of erosion following emplacement of the Planum Boreum 1 unit. We find no evidence for extensive glacial flow, topographic relaxation, or basal melting of Planum Boreum materials. However, minor development of normal faults and wrinkle ridges may suggest differential compaction of materials across buried scarps. Timing relations are poorly-defined mostly because resurfacing and other uncertainties prohibit precise determinations of surface impact crater densities. The majority of the stratigraphic record may predate the recent (<20 Ma) part of the orbitally-driven climate record that can be reliably calculated. Given the strong stratigraphic but loose temporal constraints of the north polar geologic record, a comparison of north and south polar stratigraphy permits a speculative scenario in which major Amazonian depositional and erosional episodes driven by global climate activity is plausible.     

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.     

Sequential deformation of plains at the margins of Alpha Regio,Venus: Implications for tessera formation

Abstract— The boundaries between the highly deformed tessera terrain and adjacent volcanic plains are primarily those of embayment, where the tessera are stratigraphically older than the plains. Previous studies show that <3% of these boundaries display evidence of tectonic tilting after the emplacement of the plains. One of these unusual boundaries is the western margin of Alpha Regio tessera, a zone ～ 100 km in width that separates the plains from the interior structures of Alpha. This zone is characterized by margin parallel, fine‐scale (1–5 km) fractures, graben, and ridges that truncate and postdate the broad‐scale (10–30 km) ridges and troughs of the interior of Alpha. The western margin is embayed by several volcanic plains units that are progressively tilted and deformed by graben with closer proximity to Alpha Regio. The earliest deformation of the plains consists of northeast‐trending graben ～1 km in width that are similar in morphology and spacing to graben that deform intratessera plains and plains at the eastern boundary of Alpha. Northwest‐trending graben then formed over an interval marked by the emplacement of two additional plains units; their similarity to northwest‐trending structures emanating from Eve corona and the Lada Terra rift suggests a possible genetic relationship. The tilting of the plains adjacent to western Alpha implies relative vertical movement of the margin, either uplift of tessera or downwarping of plains subsequent to the formation and relaxation of the interior of Alpha Regio. Subsidence of plains at this locale is supported by the presence of a basin to the west of Alpha surrounded by a fracture belt contiguous with western Alpha. Thus, the fractures and deformation at the western boundary of Alpha may be related to the formation of a basin to the west of Alpha with some influence from the northernmost extension of the Lada Terra rift. Such a basin is not present at a section along the eastern boundary of Alpha Regio, where the origin of tilted plains remains equivocal. We conclude that the deformation along the western margin of Alpha Regio is not directly related to the process of tessera formation but is an example of tessera modification and is consistent with the stratigraphic position of tessera as the oldest unit observed on Venus.     

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.     

Onset Time and Duration of Corona Activity on Venus: Stratigraphy and History from Photogeologic Study of Stereo Images

The details of stratigraphic units and structures making up six coronae and their regional surroundings on Venus were examined using full resolution Magellan images and stereoscopic coverage. Altimetry and stereoscopic coverage were essential in establishing the local stratigraphic relationships and the timing of corona-related topography. The degree of preservation of signatures of earlier corona-related activities and the scale of later corona-related activities vary significantly from corona to corona. We compared the geologic sequence in each corona to regional and global stratigraphic units, placing the coronae in the broader context of the geologic history of Venus. The results of this study were compared with earlier analyses bringing the total number of corona considered to about 15% of the total corona population. We found that corona started forming soon after tessera formation and largely spanned a significant part of the subsequent geologic history of Venus, over about 200–400 million years. Topographic annulae were initiated in early post-tessera time but were largely completely formed by the time of emplacement of regional plains with wrinkle ridges. Some coronae ceased activity by this time, while others continued until closer to the present, although showing evidence of waning activity. Coronae-associated volcanism dominated many coronae during this later stage. Convincing evidence of pre-regional plains corona- related volcanism was not found in the population examined here. We conclude that coronae formed in a two stage process; the first stage (tectonic phase) involved the annular warping of early extensive stratigraphic units of volcanic origin and the second (volcanic phase) involved coronae-related lava flow activity and local fracturing. For the vast majority of coronae, the first tectonic phase was largely complete prior to the emplacement of the regional plains (Pwr, plains with wrinkle ridges). The vast majority of corona-related volcanic activity (emplacement of Pl, lobate flows) occurred subsequent to the emplacement of regional plains. We found no evidence of coronae initiation in substantially later periods of the observed history of Venus. This revised version was published online in July 2006 with corrections to the Cover Date.     

Shallow and deep fresh impact craters in Hesperia Planum,Mars

The depths of 109 impact craters 2–16 km in diameter, located on the ridged plains materials of Hesperia Planum, Mars, have been measured from their shadow lengths using digital Viking Orbiter images (orbit numbers 417S–419S) and the PICS computer software. On the basis of their pristine morphology (very fresh lobate ejecta blankets, well preserved rim crests, and lack of superposed impact craters), 57 of these craters have been selected for detailed analysis of their spatial distribution and geometry. We find that south of 30°S, craters <6.0 km in diameter are markedly shallower than similar-sized craters equatorward of this latitude. No comparable relationship is observed for morphologically fresh craters >6.0 km diameter. We also find that two populations exist for older craters <6.0 km diameter. When craters that lack ejecta blankets are grouped on the basis of depth/diameter ratio, the deeper craters also typically lie equatorward of 30° S. We interpret the spatial variation in crater depth/diameter ratios as most likely due to a poleward increase in volatiles within the top 400 m of the surface at the times these craters were formed.     

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.