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).     

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).     

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.     

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).     

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.     

Major volcanic landform formation on Venus: Possible determination by compression tectonics and crustal thickening

While low level shield volcanoes have formed on Venus, major volcanic structure formation in Ishtar Terra has been restricted to caldera formation. It is possible that the combination of compression tectonics and crustal thickening inhibits the amount of magma which reaches the surface in Ishtar Terra. In certain situations, coronae on Venus may form as undeveloped volcanic structures due to restricted magma rise in thick crustal areas.     

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).     

Mineralogy of Terra Meridiani and western Arabia Terra from OMEGA/MEx and implications for their formation

Analyses of Mars Express OMEGA hyperspectral data (0.4-2.7 μm) for Terra Meridiani and western Arabia Terra show that the northern mantled cratered terrains are covered by dust that is spectrally dominated by nanophase ferric oxides. Dark aeolian dunes inside craters and dark streaks extending from the dunes into the intercrater areas in mantled cratered terrains in western Arabia Terra have similar pyroxene-rich signatures demonstrating that the dunes supply dark basaltic material to create dark streaks. The dissected cratered terrains to the south of the mantled terrains are dominated spectrally by both low-calcium and high-calcium pyroxenes with abundances of 20-30% each retrieved from nonlinear radiative transfer modeling. Spectra over the hematite-bearing plains in Meridiani Planum are characterized by very weak but unique spectral features attributed to a mixture of a dark and featureless component (possibly gray hematite) and minor olivine in some locations. Hydrated minerals (likely hydrous ferric sulfates and/or hydrous hydroxides) associated with poorly ferric crystalline phases are found in the etched terrains to the north and east of the hematite-bearing plains where erosion has exposed ∼1 km of section of layered outcrops with high thermal inertias. These materials are also found in numerous craters in the northern Terra Meridiani and may represent outliers of the etched terrain materials. A few localized spots within the etched terrain also exhibit the spectral signature of Fe-rich phyllosilicates. The ensemble of observations show that the evidence for aqueous processes detected by the Opportunity Rover in Meridiani Planum is widespread and confirms the extended presence of surface or near-surface water over this large region of Mars. The scenarios of formation of Terra Meridiani (“dirty” acidic evaporite, impact surge or weathering of volcanic ash) cannot satisfactorily explain the mineralogy derived from the OMEGA observations. The formation of the etched terrains is consistent with leaching of iron sulfides and formation of sulfates and hydrated iron oxides, either in-place or via transport and evaporation of aqueous fluids and under aqueous conditions less acidic than inferred from rocks examined by Opportunity.     

The absence of large-scale volcanic crater subsidence in Ishtar Terra on Venus

The absence of large-scale subsidence features around the three volcanic craters in Ishtar Terra on Venus implies the continued presence of underlying magma and the possibility of current volcanic activity there.     

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).     

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).     

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.     

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.     

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.     

Geology and tectonics of the Themis Regio-Lavinia Planitia-Alpha Regio-Lada Terra area,Venus: Results from Arecibo image data

New radar images obtained from the Arecibo Observatory (resolution 1.5–4.0 km) for portions of the southern hemisphere of Venus show that: the upland of Phoebe Regio contains the southern extension of Devana Chasma, a rift zone extending 4200 km south from Theia Mons and interpreted as a zone of extension; Alpha Regio, the only large region of tessera within the imaged area, is similar to tessera mapped elsewhere on the planet and covers a smaller percentage of the surface than that observed in the northern high latitudes; the upland made of Ushas, Innini and Hathor Montes consists of three distinct volcanic constructs; Themis Regio is mapped as an ovoid chain of radar-bright arcuate single and double ring structures, edifices and bright lineaments. This area is interpreted as a region of mantle upwelling and on the basis of apparent split and separated features, a zone of localized faulting and extension. Linear zones of deformation in Lavinia Planitia are characterized by lineament belts that are often locally elevated, are similar to ridge belts mapped in the northern high latitudes and are interpreted to be characterized mainly by compression; radar-bright lava complexes within Lavinia Planitia are unique to this part of the planet and are interpreted to represent areas of eruption of high volumes of extremely fluid lava; the upland of Lada Terra is bound to the north by a linear deformation zone interpreted as extensional, is characterized by large ovoids and coronae, is interpreted to be associated with an area of mantle upwelling, and is in contrast to the northern high latitude highland of Ishtar Terra. Regions of plains in the southern hemisphere cover about 78%; of the mapped area and are interpreted to be volcanic in origin. Located within the area imaged (10–78 S) are 52 craters interpreted to be of impact origin ranging from 8 to 157 km in diameter. On the basis of an overall crater density of 0.94 craters/10⁶ km², it is determined that the age of this part of the Venus surface is similar to the 0.3 to 1.0 billion year age calculated for the equatorial region and northern high latitudes. The geologic characteristics of the portion of the Venus southern hemisphere imaged by Arecibo are generally similar to those mapped elsewhere on the planet. This part of the planet is characterized by widespread volcanic plains, large volcanic edifices, and zones of linear belt deformation. The southern hemisphere of Venus differs from northern high latitudes in that tessera makes up only a small percentage of the surface area and the ovoid chain in Themis Regio is unique to this part of the planet. On the basis of the analysis presented here, the southern hemisphere of Venus is interpreted to be characterized by regions of mantle upwelling on a variety of scales (ovoids, region made up of Ushas, Innini and Hathor Montes), upwelling and extension (Themis Regio) and localized compression (lineament belts in Lavinia Planitia).     

Tectonics of the southern escarpment of Ishtar Terra on venus from observations of morphology and gravity

Maxima of calculated topographical line-of-sight (LOS) gravity attractions caused by Ishtar Terra are shifted to the north with respect to the measured LOS free air gravity maxima south of the highland. This implies a tendency to isostatic compensation of central Ishtar and mass surpluses at the continental border and the southern forelands.The following scenario is compatible with the interpretation of the gravity anomalies and morphological features. Relative motions of the lowland Sedna Planitia against continental Ishtar Terra have caused buckling and flat subduction of the lowland lithospheric material. (Deep subduction can be ruled out by thermal reasons). The free air gravity high is modelled by surplus masses of the buckling and of the high density subducting plate. Evidence for this is given by several compressional features like Ut and Vesta Rupes at the southern continental border and ridges at the SW-flanks of Maxwell Montes. It is further supported by several possible volcanic-tectonic depressions located in the southern part of Ishtar. This local interpretation does not necessarily imply the existence of global plate tectonics on Venus like on Earth, but at least limited horizontal movements of the Venusian lithosphere seem to be likely. This result shows that plate recycling must be considered for heat transfer through the lithosphere beside conduction and hot spot volcanism.Contribution No. 273, Institut für Geophysik der Universität Kiel, F.R.G.     

Crustal tectonic zone on Venus

Various tectonic structures to the south and southeast of Ishtar Terra indicate areal stresses. Compression from east-southeast against Ishtar Terra has resulted in ridge belt formation and surface bending at Salme Dorsa, probably along the seam between two crustal units. En echelon fault zone indicates dextral strike-slip shear(s) resulted in the westward movement of planitia crust related to Ishtar Terra. Meshkenet Tessera displays differential dextral strike-slip faulting where the southernmost bar-like blocks have had largest relative movements. Compression against Tusholi Corona has resulted in foreland surface bending similar to that of Salme Dorsa. The tectonic zone as a whole resembles a dextral transform fault extending from a concave arc in the west to another concave arc in the east. The Cytherean surface, crust or uppermost lithosphere seems to be able to transmit stresses over distances. Deeper understanding of these processes is needed to gain a new idea of the crustal deformation on terrestrial planets.     

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.     

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.