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James W. Head - One of the best experts on this subject based on the ideXlab platform.
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Morphology and deformational history of Tellus Regio, Venus: Evidence for assembly and collision
Planetary and Space Science, 2018Co-Authors: Martha S. Gilmore, James W. HeadAbstract:Abstract Tessera terrain is the oldest stratigraphic unit on Venus, but its origin and evolution are inadequately understood. Here we have performed detailed mapping of Tellus Regio, the third largest Tessera plateau on Venus. Tellus Regio is shown to have distinct marginal and interior facies. The east and west margins of Tellus rise up to 2 km above the interior and include ridges and troughs ∼5–20 km across, oriented parallel to the present plains-Tessera boundary. Structures characteristic of the interior of Tellus are found within the eastern and western margins and are deformed by the margin-parallel ridges indicating their presence during the time of the formation of the current margins. These relationships suggest that the margins formed by the application of external horizontal compressional stresses at the edges of an already-existing Tessera interior. Structural and stratigraphic relationships in southwest Tellus show the assembly of three structurally distinct Tessera regions and intervening plains that are consistent with the collision of the southwest margin into the plateau interior. This requires that Tessera terrain was formed regionally and collected into the present day Tellus plateau. The latest stages of activity in Tellus include volcanism and pervasive, distributed, 1–2 km wide graben, which may have been formed due to large-scale gravitational relaxation of the plateau topography. A large intraTessera plains unit may have formed via crustal delamination. The collisional oroclinal deformation of the margins are most consistent with models that invoke mantle downwelling for the origin of Tellus Regio and other Tessera plateaus with similar structural relationships.
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Geologic Map of the Meskhent Tessera Quadrangle (V-3), Venus: Evidence for Early Formation and Preservation of Regional Topography
2008Co-Authors: James W. Head, Mikhail A. IvanovAbstract:The area of the Meskhent Tessera quadrangle (V-3, 50-75degN, 60-120degE, Fig. 1) corresponds to a transition zone from the uplands of Ishtar Terra to the west to the lowlands of Atalanta Planitia to the east. The topographic configuration, gravity signature, and presence of large Tesserae in Ishtar Terra are consistent with extensive areas of thickened crust and tectonically stabilized lithosphere representing ancient and now extinct regimes of mantle convection. The gravity and topographic characteristics of Atalanta Planitia have been cited as evidence for large-scale mantle downwelling. Thus, the region of Meskhent Tessera quadrangle represents an important sample for the study of the regional history of long-wavelength topography (highlands, midlands, and lowlands), interaction between the downwelling and areas of thickened crust/lithosphere, formation of associated tectonic features, and emplacement of volcanic plains.
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Sequential deformation of plains at the margins of Alpha Regio, Venus: Implications for Tessera formation
Meteoritics & Planetary Science, 2000Co-Authors: Martha S. Gilmore, James W. HeadAbstract: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
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Style and sequence of extensional structures in Tessera terrain, Venus
Journal of Geophysical Research: Planets, 1998Co-Authors: Martha S. Gilmore, Mikhail A. Ivanov, Geoffrey C. Collins, Lucia Marinangeli, James W. HeadAbstract:Recent studies have focused on the question of the stratigraphic sequence and thus the stages of Tessera formation, specifically, if Tessera are formed by contractional deformation followed by extensional deformation or vice versa. A major question centers on the interpretation of specific lineaments within Tesserae as graben (bounded by faults ∼60°) or, alternatively, open tension fractures (dipping ∼90°). We document and assess the origin of extensional structures in Tesserae at several locations on Venus, noting the morphology, continuity along strike, parallelism of walls, stratigraphic position and interaction with other structures, and variability due to radar viewing geometry. In each study area, our analyses demonstrate that (1) the extensional structures have variable widths, interior subparallel lineaments, and ramp terminations; (2) ridges and lineaments are continuous across the troughs, where the floors of many of these structures contain the lowered sections of preexisting structures; and (3) intraTessera plains are seen to embay ridges and an impact crater is superposed on a ridge and in both cases these features are subsequently deformed by the extensional structures. We conclude that the morphology of these extensional structures is consistent with an origin as graben, not open tension fractures, and that these graben postdate the ridges in each study area. Both the graben and the ridges of the sizes found in our survey can be formed when the brittle crust is of the order of 1 to 10 km thick. To further test the tension fracture model, we examine the conditions of a Venus that could produce tension fractures of the dimension (∼1 km width) of extensional structures found in Tessera terrain and find that thermal gradients of a minimum of 400 to 1500 K km−1 (heat flows of 800 to 3000 mW m−2) are required for a range of diabase rheologies and strain rates thought typical of Venus during Tessera formation. Such a thermal structure would favor partial melting at depths
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Sequence of tectonic deformation in the history of Venus: Evidence from global stratigraphic relationships
Geology, 1998Co-Authors: James W. Head, Alexander T. BasilevskyAbstract:Analysis of local and regional stratigraphic relationships has permitted the assessment of the nature of tectonic structures and their distribution throughout the observed history of Venus, spanning the past several hundreds of millions of years. We find that shortening characteristic of intensely deformed Tessera terrain gave way to widespread distributed fracturing and extension within the Tessera and early post-Tessera volcanic plains. This phase was followed by distributed deformation of the widespread younger volcanic plains involving compression to form broad ridge belts and closely following—and sometimes simultaneous—extension to form fracture belts. Emplacement of the most areally extensive regional volcanic plains exposed today was followed by widely distributed compression forming wrinkle ridges on the plains' surfaces. Focused extensional deformation (localized, linear rift systems) dominated the latest stages. These major temporal trends appear well established from a stratigraphic point of view and provide guidelines and constraints on models for the geologic history of Venus.
Martha S. Gilmore - One of the best experts on this subject based on the ideXlab platform.
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Venus Tesserae feature layered, folded, and eroded rocks
Geology, 2020Co-Authors: Paul K. Byrne, Martha S. Gilmore, Richard Ghail, A. M. Celâl Şengör, Christian Klimczak, David A. Senske, Jennifer L. Whitten, Sara Khawja, Richard E. Ernst, Sean C. SolomonAbstract:Abstract Tesserae on Venus are locally the stratigraphically oldest units preserved on the planet. These regions are characterized by pervasive tectonic deformation including normal faults, grabens, thrust faults, and folds. In multiple Tesserae, sets of (often highly) curved, parallel linear features are also present. These features strongly resemble terracing in layered volcanic or sedimentary sequences on Earth having arcuate or sinuous outcrop patterns that follow undulating topography. Should this analogy hold for Venus, then these outcrop patterns imply some erosion of the Tessera units in which these strata occur; radar-dark materials filling proximal lows might be deposits of that eroded material. This outcrop pattern is seen in geographically dispersed Tessera units, so the preservation of layering could be common for this terrain type. If so, then Tesserae record the culmination of volcanic and/or sedimentary deposition, folding, and erosion—complex geological histories that should be considered in future studies of this enigmatic terrain.
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Morphology and deformational history of Tellus Regio, Venus: Evidence for assembly and collision
Planetary and Space Science, 2018Co-Authors: Martha S. Gilmore, James W. HeadAbstract:Abstract Tessera terrain is the oldest stratigraphic unit on Venus, but its origin and evolution are inadequately understood. Here we have performed detailed mapping of Tellus Regio, the third largest Tessera plateau on Venus. Tellus Regio is shown to have distinct marginal and interior facies. The east and west margins of Tellus rise up to 2 km above the interior and include ridges and troughs ∼5–20 km across, oriented parallel to the present plains-Tessera boundary. Structures characteristic of the interior of Tellus are found within the eastern and western margins and are deformed by the margin-parallel ridges indicating their presence during the time of the formation of the current margins. These relationships suggest that the margins formed by the application of external horizontal compressional stresses at the edges of an already-existing Tessera interior. Structural and stratigraphic relationships in southwest Tellus show the assembly of three structurally distinct Tessera regions and intervening plains that are consistent with the collision of the southwest margin into the plateau interior. This requires that Tessera terrain was formed regionally and collected into the present day Tellus plateau. The latest stages of activity in Tellus include volcanism and pervasive, distributed, 1–2 km wide graben, which may have been formed due to large-scale gravitational relaxation of the plateau topography. A large intraTessera plains unit may have formed via crustal delamination. The collisional oroclinal deformation of the margins are most consistent with models that invoke mantle downwelling for the origin of Tellus Regio and other Tessera plateaus with similar structural relationships.
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Venus Surface Composition Constrained by Observation and Experiment
Space Science Reviews, 2017Co-Authors: Martha S. Gilmore, Allan H Treiman, Jörn Helbert, Suzanne E. SmrekarAbstract:New observations from the Venus Express spacecraft as well as theoretical and experimental investigation of Venus analogue materials have advanced our understanding of the petrology of Venus melts and the mineralogy of rocks on the surface. The VIRTIS instrument aboard Venus Express provided a map of the southern hemisphere of Venus at ∼1 μm allowing, for the first time, the definition of surface units in terms of their 1 μm emissivity and derived mineralogy. Tessera terrain has lower emissivity than the presumably basaltic plains, consistent with a more silica-rich or felsic mineralogy. Thermodynamic modeling and experimental production of melts with Venera and Vega starting compositions predict derivative melts that range from mafic to felsic. Large volumes of felsic melts require water and may link the formation of Tesserae to the presence of a Venus ocean. Low emissivity rocks may also be produced by atmosphere-surface weathering reactions unlike those seen presently.
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VIRTIS emissivity of Alpha Regio, Venus, with implications for Tessera composition
Icarus, 2015Co-Authors: Martha S. Gilmore, Nils Mueller, Jörn HelbertAbstract:Abstract The composition of Venus Tessera terrain is unknown. The Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) aboard Venus Express (VEx) collects data that yields the surface emissivity at ∼1 μm, which contains information convolving a number of surface properties, including composition. We examine the variation of emissivity in the vicinity of Alpha Regio, which is the largest exposure of Tessera terrain imaged by VIRTIS. We find that the emissivity of Alpha Regio Tessera is lower than adjacent plains materials and the deposits and flows of Eve corona, both of which have previously been interpreted to be basaltic. The emissivity of the bulk of Alpha is also lower than its western boundary, which is interpreted to comprise plains structurally deformed to the same degree as Tessera terrain. This suggests that the lower emissivity of Alpha is independent of structural elements, macroscale roughness, or local sedimentation processes, and is due to material properties like composition or grain size. The deviation of the emissivity of Alpha from that of the plains for which a bulk basaltic composition is well supported corresponds to a significant difference in rock type or surface mineral assemblage. The 1 μm emissivity of Alpha is consistent with rocks with low ferrous iron content. This includes felsic igneous rocks like granitoids that form under either water-rich or water-poor conditions. A water-rich origin would require both a hydrosphere and a plate recycling mechanism and thus be limited to the lifetime of surface water on Venus. Alternatively, granitoids could form via the differentiation of basaltic melts. The production of all Tessera terrain by this mechanism would require the accumulation and preservation of felsic melts from a volume of mafic magma that exceeds what is preserved in the currently observed plains. Both mechanisms of granitoid formation would require that Tessera terrain be formed prior to the emplacement of the plains, consistent with their stratigraphic position. Anorthosites also satisfy the emissivity signature and can form from copious amounts of partial melting of a mafic source. Low emissivity values are also consistent with carbonates, sulfates, phyllosilicates and their dehydration products, which may have formed via weathering of basalts under conditions of higher atmospheric P H2O . All of these hypotheses suggest the mineralogy of Alpha Tessera records an extinct era of Venus history and is a key target for future exploration.
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sequential deformation of plains at the margins of alpha regio venus implications for Tessera formation
Meteoritics & Planetary Science, 2000Co-Authors: Martha S. Gilmore, J W HeadAbstract: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.
Mason N. Dean - One of the best experts on this subject based on the ideXlab platform.
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Endoskeletal mineralization in chimaera and a comparative guide to tessellated cartilage in chondrichthyan fishes (sharks, rays and chimaera).
Journal of the Royal Society Interface, 2020Co-Authors: Ronald Seidel, Julia Chaumel, Michael J.f. Blumer, Shahrouz Amini, Mason N. DeanAbstract:An accepted uniting character of modern cartilaginous fishes (sharks, rays, chimaera) is the presence of a mineralized, skeletal crust, tiled by numerous minute plates called Tesserae. Tesserae have, however, never been demonstrated in modern chimaera and it is debated whether the skeleton mineralizes at all. We show for the first time that tessellated cartilage was not lost in chimaera, as has been previously postulated, and is in many ways similar to that of sharks and rays. Tesserae in Chimaera monstrosa are less regular in shape and size in comparison to the general scheme of polygonal Tesserae in sharks and rays, yet share several features with them. For example, Chimaera Tesserae, like those of elasmobranchs, possess both interTesseral joints (unmineralized regions, where fibrous tissue links adjacent Tesserae) and recurring patterns of local mineral density variation (e.g. Liesegang lines, hypermineralized 'spokes'), reflecting periodic accretion of mineral at Tesseral edges as Tesserae grow. Chimaera monstrosa's Tesserae, however, appear to lack the internal cell networks that characterize Tesserae in elasmobranchs, indicating fundamental differences among chondrichthyan groups in how calcification is controlled. By compiling and comparing recent ultrastructure data on Tesserae, we also provide a synthesized, up-to-date and comparative glossary on tessellated cartilage, as well as a perspective on the current state of research into the topic, offering benchmark context for future research into modern and extinct vertebrate skeletal tissues.
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image analysis pipeline for segmentation of a biological porosity network the lacuno canalicular system in stingray Tesserae
MethodsX, 2020Co-Authors: Merlind Schotte, Julia Chaumel, Mason N. Dean, Daniel BaumAbstract:Abstract A prerequisite for many analysis tasks in modern comparative biology is the segmentation of 3-dimensional (3D) images of the specimens being investigated (e.g. from microCT data). Depending on the specific imaging technique that was used to acquire the images and on the image resolution, different segmentation tools are required. While some standard tools exist that can often be applied for specific subtasks, building whole processing pipelines solely from standard tools is often difficult. Some tasks may even necessitate the implementation of manual interaction tools to achieve a quality that is sufficient for the subsequent analysis. In this work, we present a pipeline of segmentation tools that can be used for the semi-automatic segmentation and quantitative analysis of voids in tissue (i.e. internal structural porosity). We use this pipeline to analyze lacuno-canalicular networks in stingray Tesserae from 3D images acquired with synchrotron microCT. • The first step of this pipeline, the segmentation of the Tesserae, was performed using standard marker-based watershed segmentation. The efficient processing of the next two steps, that is, the segmentation of all lacunae spaces belonging to a specific Tessera and the separation of these spaces into individual lacunae required recently developed, novel tools. • For error correction, we developed an interactive method that allowed us to quickly split lacunae that were accidentally merged, and to merge lacunae that were wrongly split. • Finally, the Tesserae and their corresponding lacunae were subdivided into structural wedges (i.e. specific anatomical regions) using a semi-manual approach. • With this processing pipeline, analysis of a variety of interconnected structural networks (e.g. vascular or lacuno-canalicular networks) can be achieved in a comparatively high-throughput fashion. In our study system, we were able to efficiently segment more than 12,000 lacunae in high-resolution scans of nine Tesserae, allowing for a robust data set for statistical analysis.
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calcified cartilage or bone collagens in the tessellated endoskeletons of cartilaginous fish sharks and rays
Journal of Structural Biology, 2017Co-Authors: Ronald Seidel, Michael J.f. Blumer, Kady Lyons, James C Weaver, Peter Fratzl, Elisabethjudith Pechriggl, Brian K Hall, Mason N. DeanAbstract:Abstract The primary skeletal tissue in elasmobranchs –sharks, rays and relatives– is cartilage, forming both embryonic and adult endoskeletons. Only the skeletal surface calcifies, exhibiting mineralized tiles (Tesserae) sandwiched between a cartilage core and overlying fibrous perichondrium. These two tissues are based on different collagens (Coll II and I, respectively), fueling a long-standing debate as to whether Tesserae are more like calcified cartilage or bone (Coll 1-based) in their matrix composition. We demonstrate that stingray ( Urobatis halleri ) Tesserae are bipartite, having an upper Coll I-based ‘cap’ that merges into a lower Coll II-based ‘body’ zone, although Tesserae are surrounded by cartilage. We identify a ‘supraTesseral’ unmineralized cartilage layer, between Tesserae and perichondrium, distinguished from the cartilage core in containing Coll I and X (a common marker for mammalian mineralization), in addition to Coll II. Chondrocytes within Tesserae appear intact and sit in lacunae filled with Coll II-based matrix, suggesting Tesserae originate in cartilage, despite comprising a diversity of collagens. InterTesseral joints are also complex in their collagenous composition, being similar to supraTesseral cartilage closer to the perichondrium, but containing unidentified fibrils nearer the cartilage core. Our results indicate a unique potential for tessellated cartilage in skeletal biology research, since it lacks features believed diagnostic for vertebrate cartilage mineralization (e.g. hypertrophic and apoptotic chondrocytes), while offering morphologies amenable for investigating the regulation of complex mineralized ultrastructure and tissues patterned on multiple collagens.
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ultrastructural material and crystallographic description of endophytic masses a possible damage response in shark and ray tessellated calcified cartilage
Journal of Structural Biology, 2017Co-Authors: Ronald Seidel, Sidney Omelon, Michael J.f. Blumer, David Knötel, James C Weaver, Peter Fratzl, Paul Zaslansky, Daniel R Huber, Luca Bertinetti, Mason N. DeanAbstract:Abstract The cartilaginous endoskeletons of elasmobranchs (sharks and rays) are reinforced superficially by minute, mineralized tiles, called Tesserae. Unlike the bony skeletons of other vertebrates, elasmobranch skeletons have limited healing capability and their tissues’ mechanisms for avoiding damage or managing it when it does occur are largely unknown. Here we describe an aberrant type of mineralized elasmobranch skeletal tissue called endophytic masses (EPMs), which grow into the uncalcified cartilage of the skeleton, but exhibit a strikingly different morphology compared to Tesserae and other elasmobranch calcified tissues. We use materials and biological tissue characterization techniques, including computed tomography, electron and light microscopy, X-ray and Raman spectroscopy and histology to characterize the morphology, ultrastructure and chemical composition of Tesserae-associated EPMs in different elasmobranch species. EPMs appear to develop between and in intimate association with Tesserae, but lack the lines of periodic growth and varying mineral density characteristic of Tesserae. EPMs are mineral-dominated (high mineral and low organic content), comprised of birefringent bundles of large calcium phosphate crystals (likely brushite) aligned end to end in long strings. Both Tesserae and EPMs appear to develop in a type-2 collagen-based matrix, but in contrast to Tesserae, all chondrocytes embedded or in contact with EPMs are dead and mineralized. The differences outlined between EPMs and Tesserae demonstrate them to be distinct tissues. We discuss several possible reasons for EPM development, including tissue reinforcement, repair, and disruptions of mineralization processes, within the context of elasmobranch skeletal biology as well as damage responses of other vertebrate mineralized tissues.
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μCT images of Tesserae acquired with different resolutions.
2017Co-Authors: David Knötel, Mason N. Dean, Ronald Seidel, Steffen Prohaska, Daniel BaumAbstract:Images (A) and (B) show a single Tessera surrounded by neighboring Tesserae. (A) Synchrotron μCT image with voxel size 0.678 μm. In the center of the Tessera, many cell lacunae (cl) are visible. The close-up shows an inter-Tesseral joint consisting of inter-Tesseral contact zones (icz) with direct contact between adjacent Tesserae and fibrous zones (fz) without direct contact. (B) Voxel size: 4.89 μm. This image shows the native resolution of the μCT scans used in this paper, before being downsampled for analysis to 9.78 μm (see ‘Input Data’ below). Note that the inter-Tesseral contact zones and small fibrous zones cannot be seen since the resolution is not high enough. Hence we use the following terminology throughout this paper: Inter-Tesseral connection (co) for the entire connection between Tesserae, including both contact and fibrous zones and appearing as areas of high intensity (high gray values) between Tesserae, unmineralized pores (p) for areas of low intensity between Tesserae, and Tessera center (c) for the region around the center of a Tessera.
Vicki L. Hansen - One of the best experts on this subject based on the ideXlab platform.
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Impact origin of Archean cratons
Lithosphere, 2015Co-Authors: Vicki L. HansenAbstract:Archean cratons consist of crustal granite-greenstone terrains (GGTs) coupled to roots of strong, buoyant cratonic lithospheric mantle (CLM). Although this association is unique to the Archean and formed from ca. 4.0 to 2.5 Ga, the origins of terrestrial cratons are debated. I propose that crustal plateaus, quasi-circular craton-like features (∼1400–2400 km diameter, 0.5–4 km high), on Earth’s sister planet Venus might serve as analogs for Archean cratons. Crustal plateaus, which are isostatically supported by a compositionally controlled low-density root, host a distinctive surface called ribbon-Tessera terrain. Ribbon-Tessera also occurs as arcuate-shaped inliers in the Venus lowlands, widely interpreted as remnants of rootless crustal plateaus. Within each crustal plateau, surface ribbon-Tessera terrain comprises a vast igneous province analogous to terrestrial GGTs, and the plateau root is analogous to CLM. Crustal plateaus and ribbon-Tessera terrain collectively represent Venus’ oldest preserved features and surfaces, and they formed during an ancient period of globally thin lithosphere. To explain the linked features of crustal plateaus, a bolide impact hypothesis has been proposed in which a large bolide pierces ancient thin lithosphere, leading to massive partial melting in the sublithospheric mantle. In this model, melt escapes to the surface, forming an enormous lava pond, which evolves to form ribbon-Tessera terrain; mantle melt residue forms a strong, resilient buoyant root, leading to plateau support and long-term stability of an individual crustal plateau. Building on the similarity of GGT–CLM and Venus crustal plateaus, I propose an exogenic hypothesis for Archean craton formation in which a large bolide pierces thin Archean lithosphere, causing localized high-temperature, high-fraction partial melting in the sublithospheric mantle; melt rises, forming an igneous province that evolves to form a GGT, and melt residue develops a complementary CLM. By this mechanism, Archean cratons may have formed in a spatially and temporally punctuated fashion at a time when large bolides showered Archean Earth.
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Geologic constraints on crustal plateau surface histories, Venus: The lava pond and bolide impact hypotheses
Journal of Geophysical Research, 2006Co-Authors: Vicki L. HansenAbstract:[1] Venusian crustal plateau formation is hotly debated. Crustal plateaus are large (∼1500–2500 km) quasi-circular ∼0.5- to 4-km-high plateaus that host distinctive tectonic fabrics, called ribbon Tessera. Debate centers on plateau support and fabric formation. Detailed geologic mapping of an ∼360,000 km2 region, eastern Ovda Regio, provides critical clues for plateau evolution. Ribbon-Tessera fabrics record broadly synchronous layer contraction, extension, and flooding, as an initially strong, thin layer (
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Structures in Tessera terrain, Venus: Issues and answers
Journal of Geophysical Research: Planets, 2000Co-Authors: Vicki L. Hansen, Roger J. Phillips, James J. Willis, Rebecca R. GhentAbstract:Many workers assume that Tessera terrain, marked by multiple tectonic lineaments and exposed in crustal plateaus, comprises a global onionskin on Venus. Because Tesserae are exposed mostly within crustal plateaus, which exhibit thickened crust, issues of Tessera distribution and the mechanism of crustal plateau formation (e.g., mantle downwelling or upwelling) are intimately related. A review of Magellan data indicates that Tessera terrain does not form a global onionskin on Venus, although ribbon-bearing Tesserae reflect an ancient time of a globally thin lithosphere. Individual tracts of ribbon-bearing Tessera terrain formed diachronously, punctuating time and space as individual deep mantle plumes imparted a distinctive rheological and structural signature on ancient thin crust across spatially discrete 1600-2500 km diameter regions above hot mantle plumes. Plume-related magmatic accretion led to crustal thickening at these locations, resulting in crustal plateaus. Crustal plateau surfaces record widespread early extension (ribbon structures) and local, minor perpendicular contraction of a thin, competent layer above a ductile substrate. Within individual evolving crustal plateaus the thickness of the competent layer increased with time, and broad, gentle folds formed along plateau margins and short, variably oriented folds formed in the interior; late complex graben cut folds. Local lava flows accompanied all stages of surface deformation. In contrast to these conclusions, Gilmore et al. ( 1998) summarized post-Magellan arguments in favor of downwelling models for crustal plateau formation. In light of this discrepancy, we reexamine the regions investigated by these workers and evaluate their arguments against upwelling models. We show that Gilmore et al. ( 1998) did not differentiate ribbons from graben and therefore their proposed temporal relations are invalid; they disregarded shear fracture ribbons, thus invalidating their criticism of ribbon models; they misunderstood previous radargrammetric work that constrains ribbon geometry; and they relied solely on geometrical relations to constrain timing, violating kinematic analysis methodology. Their stratigraphic constraints on ribbon-fold temporal relations are invalid because they (1) misinterpreted implications of map relations; (2) did not isolate radar artifacts due to local radar slope effects from proposed material units; (3) chose a region for analysis that clearly shows the effects of younger tectonism and volcanism; and (4) presented map relations that cannot be reproduced. Their attempts to discount upwelling models of crustal plateau formation fail because they combine fundamentally different pre-Magellan and post-Magellan upwelling models. These misconceptions about the upwelling model and processes responsible for global warming (Phillips and Hansen, 1998), lead to serious errors in Gilmore et al.'s (1998) criticism. Furthermore, we show that the data of Gilmore et al. ( 1998) are actually more consistent with upwelling than downwelling models, consistent with arguments that Tessera terrain is not global in spatial distribution.
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Tessera terrain and crustal plateaus, Venus
Geology, 1999Co-Authors: Vicki L. Hansen, Brian K. Banks, Rebecca R. GhentAbstract:Many workers assume that Tessera terrain—marked by multiple tectonic lineaments and exposed in crustal plateaus—comprises a global “onion skin” on Venus. A growing body of structural, mechanical, magmatic, gravitational-topographic, and geologic evidence indicates that Tesserae record the local interaction of individual deep-mantle plumes with an ancient, globally thin Venusian lithosphere, resulting in local regions of thickened crust.
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Ribbon Terrain Formation, Southwestern Fortuna Tessera, Venus: Implications for Lithosphere Evolution
Icarus, 1998Co-Authors: Vicki L. Hansen, James J. WillisAbstract:Abstract The term Tessera has been used to describe regions of deformed venusian crust exhibiting two or more intersecting sets of structural elements; however, Tessera includes terrains formed by a variety of spatially and temporally discrete tectonic processes. Tessera fabric characterizes highland plateau structure, and thus understanding the nature of this deformation is critical to understanding the mode of highland plateau formation. Many Tessera fabrics include ribbons, folds, and late graben. In this paper, we refine the geometry of ribbons through geologic mapping and radargrammetric analysis of type ribbon structures at southwestern Fortuna Tessera; we extend our findings to ribbon fabrics at Thetis Regio. Any model of ribbon formation must account for the following constraints on ribbon geometry. (a) Ribbon-forming lineaments exhibit sharp contrasts relative to adjacent materials. (b) Ribbon-bounding lineaments form a distinct pattern alternating between radar-dark and radar-bright, which represent trough walls oriented away from and toward the satellite, respectively. (c) Ribbons form long, narrow troughs that alternate with parallel, narrow ridges; ridges and troughs display extreme length:width aspect ratios. (d) Trough walls are near vertical. (e) Troughs are shallow with consistent shallow depth along individual troughs and in adjacent troughs. (f) In some cases (e.g., southwest Fortuna Tessera) trough walls are parallel and matched and would exhibit a close fit if the trough was closed; in these cases trough walls merge laterally forming V-shaped terminations. Trough floors are smooth and flat, lacking small-scale interior lineaments. (g) In other cases (e.g., Thetis Regio) ribbons display (a)–(d), but trough walls are defined by a series of subparallel lineaments including local interior lineaments, and trough floors ramp up to join trough walls displaying parallel rather than V-shaped terminations. Trough walls would not display a close fit if closed. We propose two member types of ribbons, tensile-fracture ribbons and shear-fracture ribbons. Tensile-fracture ribbons display features (a)–(f) and formed by the opening of tensile fractures of a thin brittle layer above a ductile substrate. They require a near fracture-free shallow crust and very shallow depth to the brittle–ductile transition (BDT) ( The presence of ribbon structures within highland plateaus favors an upwelling model for highland plateau formation, in which crustal thickening results from magmatic underplating related to a mantle upwelling or mantle plume. In order for the plume to be able to anneal mechanically the crust as required by ribbon formation, the lithosphere would likely have to be quite thin. These implications are consistent with highland plateaus as an ancient signature of mantle plumes on thin lithosphere, whereas volcanic rises, which are presently thermally supported, reflect thick lithosphere. Phoebe Regio represents a transitional lithospheric thickness.
Mikhail A. Ivanov - One of the best experts on this subject based on the ideXlab platform.
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Geologic Map of the Meskhent Tessera Quadrangle (V-3), Venus: Evidence for Early Formation and Preservation of Regional Topography
2008Co-Authors: James W. Head, Mikhail A. IvanovAbstract:The area of the Meskhent Tessera quadrangle (V-3, 50-75degN, 60-120degE, Fig. 1) corresponds to a transition zone from the uplands of Ishtar Terra to the west to the lowlands of Atalanta Planitia to the east. The topographic configuration, gravity signature, and presence of large Tesserae in Ishtar Terra are consistent with extensive areas of thickened crust and tectonically stabilized lithosphere representing ancient and now extinct regimes of mantle convection. The gravity and topographic characteristics of Atalanta Planitia have been cited as evidence for large-scale mantle downwelling. Thus, the region of Meskhent Tessera quadrangle represents an important sample for the study of the regional history of long-wavelength topography (highlands, midlands, and lowlands), interaction between the downwelling and areas of thickened crust/lithosphere, formation of associated tectonic features, and emplacement of volcanic plains.
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Style and sequence of extensional structures in Tessera terrain, Venus
Journal of Geophysical Research: Planets, 1998Co-Authors: Martha S. Gilmore, Mikhail A. Ivanov, Geoffrey C. Collins, Lucia Marinangeli, James W. HeadAbstract:Recent studies have focused on the question of the stratigraphic sequence and thus the stages of Tessera formation, specifically, if Tessera are formed by contractional deformation followed by extensional deformation or vice versa. A major question centers on the interpretation of specific lineaments within Tesserae as graben (bounded by faults ∼60°) or, alternatively, open tension fractures (dipping ∼90°). We document and assess the origin of extensional structures in Tesserae at several locations on Venus, noting the morphology, continuity along strike, parallelism of walls, stratigraphic position and interaction with other structures, and variability due to radar viewing geometry. In each study area, our analyses demonstrate that (1) the extensional structures have variable widths, interior subparallel lineaments, and ramp terminations; (2) ridges and lineaments are continuous across the troughs, where the floors of many of these structures contain the lowered sections of preexisting structures; and (3) intraTessera plains are seen to embay ridges and an impact crater is superposed on a ridge and in both cases these features are subsequently deformed by the extensional structures. We conclude that the morphology of these extensional structures is consistent with an origin as graben, not open tension fractures, and that these graben postdate the ridges in each study area. Both the graben and the ridges of the sizes found in our survey can be formed when the brittle crust is of the order of 1 to 10 km thick. To further test the tension fracture model, we examine the conditions of a Venus that could produce tension fractures of the dimension (∼1 km width) of extensional structures found in Tessera terrain and find that thermal gradients of a minimum of 400 to 1500 K km−1 (heat flows of 800 to 3000 mW m−2) are required for a range of diabase rheologies and strain rates thought typical of Venus during Tessera formation. Such a thermal structure would favor partial melting at depths
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Duration of Tessera deformation on Venus
Journal of Geophysical Research: Planets, 1997Co-Authors: Martha S. Gilmore, Mikhail A. Ivanov, James W. Head, Alexander T. BasilevskyAbstract:The density and distribution of impact craters superposed on the highly deformed Tessera terrain on Venus permit analysis of the amount and duration of deformation prior to the emplacement of the stratigraphically younger global volcanic plains. Eighty percent of Tesserae craters are undeformed. No existing craters exhibit evidence of contractional deformation, suggesting that the early compressional stage of Tessera deformation ended abruptly. The small number of craters fractured by late-stage Tessera extension constrains the duration of this phase to less than 20% of the average crater retention age of the Tesserae, or approximately 30–60 Ma. These results suggest a geologically rapid decline in the magnitude of surface strain rates associated with the transition from the terminal stages of Tessera compressional deformation to the eruption of the global volcanic plains.
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Tessera terrain on Venus: A survey of the global distribution, characteristics, and relation to surrounding units from Magellan data
Journal of Geophysical Research: Planets, 1996Co-Authors: Mikhail A. Ivanov, James W. HeadAbstract:The Tessera terrain on Venus, comprised of areas of high radar backscatter, complex deformation patterns relative to other units, and topography standing higher than surrounding plains, covers ∼35.33 × 106 km2, about 8% of the surface of Venus, and is nonrandomly distributed, being preferentially located at equatorial and higher northern latitudes with a distinct paucity below about 30°S. Individual Tessera occurrences range in area from the lower limits of our measurements (about 200 km2) up to the largest Tessera, Ovda, with an area of about 8.6 × 106 km2, or about 2% of the surface area of Venus. The size-frequency distribution of Tessera patches is strongly unimodal and skewed toward smaller sizes, reflecting the great abundance of small Tessera fragments. Modes of occurrence include (1) large clusters (e.g., Aphrodite Terra and Ishtar Terra); (2) arc-like segments which may extend for thousands of kilometers and are either concave inward toward the major Tessera cluster development or away from it; (3) areas where Tesserae are rare or absent which occur both as low-lying plains (e.g., Guinevere Planitia), and as elevated regions (e.g., Atla Regio). Tessera terrain has a bimodal elevation-frequency distribution, with the main peak at about 0–1 km and an additional peak at about 3 km above mean planetary radius. In terms of number of occurrences, however, Tesserae do not display a correlation with elevation at the global scale, since small Tessera patches commonly occupy low-lying regions. Although Tessera exhibit a range of gravity signatures, many occurrences are interpreted to represent relatively shallow (crustal) levels of compensation. Tessera boundaries include Type I (sinuous/embayed, dominated by adjacent lava plains embaying Tessera massifs; 73% of total Tesserae boundaries) and Type II (linear/tectonic). Only a small percentage of the length of all boundary types show no lava embayment and could be interpreted as tectonically active for long periods subsequent to initial Tessera formation. Occurrence of broad slopes of post-Tessera embayed plains away from Tessera boundaries suggests that regional tilting occurred subsequent to final Tessera deformation in some places. Several lines of evidence suggest the possibility that a widespread Tessera-like basement, comprising at least 55% of the surface of Venus, is buried under a cover of lava plains a few hundred meters to as much as 2–4 km thick. A wide variety of deformational structures and patterns is observed within the Tessera including those representing extension, compression, shear, and transpression; in some cases the apparently complex patterns can be resolved into single-event kinematic interpretations involving noncoaxial deformation (e.g., Itzpapalotl), while in other cases, polyphase deformation is more likely (e.g., central Ovda and Thetis). Where relations can be determined stratigraphically, earliest deformation within the Tessera is primarily related to crustal shortening and compression (Phase I), followed by pervasive extensional deformation commonly oriented normal to the strike of Phase I features, generally along the same principal stress direction (Phase II). Evidence also exists for the contemporaneous formation of these distinctive deformation patterns. Lava plains within and adjacent to the Tessera embay both of these fabrics but sometimes overlap in time with Phase II extensional deformation and with regional tilting. Tessera terrain as a geologic unit occupies the lowest portion of the stratigraphic column in all areas that we have observed, an observation consistent with many other mapping studies. We see no evidence for transitional stages between Tessera and volcanic rises and/or lowlands, that might represent a long-term sequence of upwelling or downwelling followed by crustal deformation and Tessera formation. No impact craters deformed by Phase I deformation have yet been observed on Tessera, suggesting that Phase I Tessera deformation sufficiently intense to eradicate earlier impact craters ceased relatively abruptly somewhat before ∼300–500 m.y. ago; however, the starting time, and thus duration of Tessera formation, is unknown. On the basis of the very small number of on-Tessera craters deformed by Phase II extensional deformation, this period probably did not last more than several tens of millions of years after the cessation of Phase I. Little observable deformation of the Tessera terrain appears to have taken place in the last several hundred million years, during which time the vast volcanic plains were emplaced, although tilting of early plains along some Tessera margins is observed. Building on the global synthesis presented here, future analyses of individual Tessera occurrences will provide the detailed descriptions, kinematic interpretations, and strain histories necessary to assess and distinguish between the several catastrophic and uniformitarian models for Tessera formation.
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Density and morphology of impact craters on Tessera Terrain, Venus
Geophysical Research Letters, 1993Co-Authors: Mikhail A. Ivanov, Alexander T. BasilevskyAbstract:Densities of impact craters on Tessera, which is complex ridged terrain of tectonic origin, and on the remainder of the planet, which is mostly volcanic plains, were studied using Magellan images for about 96% of the surface of Venus. The density of large (D>16 km) impact craters on Tessera is higher by a factor of about 1.4 than on the remainder of the planet. This means that the Tessera crater retention age is larger than the age of the plains. This is in agreement with the well known fact that Tessera is embayed by the surrounding volcanic plains. The density of small (D
