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1.
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. 相似文献
2.
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). 相似文献
3.
George E. McGill 《Icarus》2004,172(2):603-612
A major ongoing controversy concerns the style of crustal evolution on Venus. At one extreme is a directional model that proposes a sequence of depositional and deformational events that occur at specific times in the evolution of the crust and that are global in extent. At the other extreme is a model that argues for different ages of these events in different places on the planet. A test of the directional model is here focused on whether wrinkle ridges formed at a single time in the recorded crustal history of Venus. Where sets of wrinkle ridges intersect it commonly is possible to determine that one set is older than the other. Also, the deformation responsible for wrinkle ridges is, in places, clearly progressive with respect to stratigraphic material units. These observations are not consistent with a specific single time for the formation of wrinkle ridges within the stratigraphic sequence. Within an area including about 1/3 of the surface of Venus 15% of craters that are younger than regional plains are older than wrinkle ridges, 85% are younger than wrinkle ridges. Taking 750 myr as a reasonable mean age for the regional plains, this implies that the mean age of wrinkle ridges is ∼110 myr younger than the mean age of plains. Solomon et al. (1999, Science 286, 87) propose that the emplacement of a large volume of plains lava would lead to a major atmospheric temperature increase. Their model predicts thermal stresses in the lithosphere that, at shallow depth, would reach peak compressive stresses in about 100 myr, a number very similar to the time lag between plains emplacement and wrinkle ridge formation indicated by the crater data. The thermal compressive stresses responsible for wrinkle ridges would be maintained at a level sufficient to deform basalt for at least 100 myr and possibly for as long as 350 myr. These time intervals are not really short compared to the mean age of the plains. Finally, because wrinkle ridges are demonstrably younger than the plains they deform, they cannot be related to the processes that formed the plains and thus should not be used to define a “plains with wrinkle ridges” unit. 相似文献
4.
Improved measurements of the target elevations of 885 impact craters on Venus indicate that they are nearly random with respect to elevation. Although a slight deficit of craters at high elevations and an excess at low elevations is observed, the differences are marginally significant. Using a high-resolution digital map and database of all major volcanic, tectonic and impact features, we examine the distribution of impacts within volcanic and tectonic features, and the distribution of volcanism and tectonism with elevation. We show that the observed crater hypsometry results from resurfacing at higher elevations by volcanic and tectonic features superimposed on less active plains.The distribution of impacts in the map units has two distinct patterns: (1) the plains and shield fields (70%) have high crater densities and low proportions of tectonized or embayed craters; and (2) the remaining volcanic and tectonic features (30%) have low crater densities and high proportions of modified craters. The plains and shield fields appear to represent a much lower level of resurfacing activity. Simple area-balance calculations indicate that resurfacing at higher elevations by tectonic and volcanic features plausibly explains the observed crater hypsometry. However, the subtlety of the effects suggests that either (1) little resurfacing has occurred during the period of crater accumulation, or (2) resurfacing acts almost equally at all elevations. The apparent low activity of the plains and their abundance at lower elevations makes it unlikely that resurfacing is balanced with respect to elevation. It appears that the plains have been mostly quiescent since their emplacement, and that subsequent resurfacing occurs mostly in the highlands as a result of volcanism, corona formation, and rifting. We estimate that since the end of plains emplacement about 14% of Venus has been resurfaced by volcanism and about 6% by tectonic deformation. 相似文献
5.
We have quantitatively assessed the resurfacing sources and styles in eighteen mapped venusian quadrangles, about 30% of the venusian surface. Each quadrangle was split into 0.5° by 0.5° boxes, which were then identified as corona materials, large volcano materials (>100 km diameter), intermediate volcano materials (10-100 km), small edifice materials (<10 km), flow materials from rifts or fractures, plains without an identifiable source, impact crater materials and highly deformed materials, or data gaps. We find that coronae resurface approximately 21%, small edifices 22% and large volcanoes about 6% of the surfaces analyzed. Plains with no identifiable source account for an average of 35% of the surface assessed. Small edifices resurface on a scale of 10-100 s of km2; large edifices resurface areas of 104-105 km2. Coronae have greatly varying amounts of associated volcanism, with some coronae producing negligible flow deposits and others producing deposits of 104-106 km2. The areas identified as plains with no visible source occur on small scales (102 km2) to large scales (> 105 km2). Our results indicate that the majority of plains resurfacing by volcanism can be tied to an identifiable source, that fields of small edifices contribute more to resurfacing than we had anticipated, and that resurfacing styles do not appear to have evolved over the time period represented by the surface geology in the mapped quadrangles. All of the units that we quantified occur throughout the histories of the regions mapped. We favor plains resurfacing to have occurred over at least 100 myr, which implies terrestrially reasonable resurfacing rates. 相似文献
6.
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). 相似文献
7.
Pioneer Venus radar data has provided images (resolution 20- to 40-km) of approximately 50% of the total surface of Venus in a band between 45 ° N to 15 ° S. These data are used to map the broad radar characteristics of the equatorial region on the basis of radar brightness and texture. Seven radar units are defined and are used to assess the geologic character of the equatorial region. These units fall into two distinct classes, those that are radar-bright (35% of the equatorial region) which correspond to highlands and zones of intense tectonic deformation, and radar-dark units, corresponding primarily to plains (65% of the equatorial region). The correspondence between features in the 15 ° region of overlap between the Pioneer Venus and Venera 15/16 images is examined and used to extend units mapped in the northern high latitudes into the equatorial region. On the basis of the distribution of the radar units, properties of RMS slope, reflectivity, the scattering behavior of the surface, and topographic signature, seven physiographic units are mapped in the equatorial region and are identified by increasing complexity as plains (undivided), dark halo plains, upland rises, upland plateaus, interhighland tectonic zones, tectonically segmented linear highlands, and tectonic junctions. The physiographic units are distributed in a nearly continuous interconnecting zone of volcanic rises and tectonic features that extends for nearly 360 ° around the equator of the planet. The distribution of large circular structures interpreted as coronae is also examined and it is concluded that the abundances of the largest structures, diameters greater than 500 km, is less than in the northern high latitudes with a notable absence of smaller coronae. The absence of small coronae may be due to the resolution limit of the Pioneer Venus data since analyses of higher resolution Arecibo and Goldstone imagery suggests that a number of corona-like features not identified in the PV data are present.'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). 相似文献
8.
The areas of volcanic units on Venus have been measured on the 1:5000000 geological maps published by NASA/USGS. These data were used to obtain a frequency-area distribution. The cumulative frequency-area distribution of 1544 specific occurrence of units cover six orders of magnitude from the largest unit (30 × 106 km2) to the smallest (20 km2). The probability distribution function has been calculated. The medium and large volcanic units correlate well with a power-law (fractal) relation for the dependence of frequency on area with a slope of −1.83. There are fewer small units than the expected values provided by the power-law relation. Our measurements cover 21.02% of the planetary surface, 3.59% of the study area was found to be tessera terrain and is excluded from this study of volcanism. The measurements were restricted to areas where geological maps have been published. The analysis was performed on two independent areas of the planet, with a complete coverage of published maps. In both areas the largest volcanic unit covers a significant portion of the surface (58.75% and 63.64%, respectively). For the total measured volcanic units (excluding tessera), these two largest units (that could correspond to the same unit or not) cover the 61.18% and they are stratigraphically superimposed on older volcanic units which cover 3.37% of the area. The remaining area (35.45%) is occupied by younger volcanic units stratigraphically superimposed on the large volcanic unit(s). These results are based on the independent mapping of a large number of geologists with different ideas about the geodynamical evolution of Venus and different criteria for geological mapping. Despite this fact, the presence of these very large units is incompatible with the equilibrium resurfacing models, because their generation at different ages would destroy the crater randomness. Our frequency-area distribution of the mapped volcanic units supports a catastrophic resurfacing due to the emplacement of the largest unit(s) followed by a decay of volcanism. Our data for the frequency-area distribution of volcanic units provide new support for catastrophic resurfacing models. It is difficult to make our observations compatible with equilibrium, steady-state resurfacing models. 相似文献
9.
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. 相似文献
10.
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. 相似文献
11.
G. Wesley Patterson Geoffrey C. Collins Robert T. Pappalardo Baerbel K. Lucchitta 《Icarus》2010,207(2):845-867
We have compiled a global geological map of Ganymede that represents the most recent understanding of the satellite based on Galileo mission results. This contribution builds on important previous accomplishments in the study of Ganymede utilizing Voyager data and incorporates the many new discoveries that were brought about by examination of Galileo data. We discuss the material properties of geological units defined utilizing a global mosaic of the surface with a nominal resolution of 1 km/pixel assembled by the USGS with the best available Voyager and Galileo regional coverage and high resolution imagery (100-200 m/pixel) of characteristic features and terrain types obtained by the Galileo spacecraft. We also use crater density measurements obtained from our mapping efforts to examine age relationships amongst the various defined units. These efforts have resulted in a more complete understanding of the major geological processes operating on Ganymede, especially the roles of cryovolcanic and tectonic processes in the formation of might materials. They have also clarified the characteristics of the geological units that comprise the satellite’s surface, the stratigraphic relationships of those geological units and structures, and the geological history inferred from those relationships. For instance, the characteristics and stratigraphic relationships of dark lineated material and reticulate material suggest they represent an intermediate stage between dark cratered material and light material units. 相似文献
12.
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. 相似文献
13.
E. N. Guseva 《Solar System Research》2016,50(3):184-196
The spatial distribution of rift zones of Venus, their topographic configuration, morphometric parameters, and the type of volcanism associating with rifts were analyzed. This allowed the main characteristic features of rifts to be revealed and two different types of rift-forming structures, serving for classification of rift zones as rift valleys and graben belts, to be isolated. These structural types (facies) of rift zones are differently expressed in the relief: rift valleys are individual deep (several kilometers) W-shaped canyons, while graben belts are clusters of multiple V-shaped and rather shallow (hundreds of meters) depressions. Graben belts are longer and wider, as compared to rift valleys. Rift valleys are spatially associated with dome-shaped volcanic rises and large volcanos (concentrated volcanic sources), while graben belts do not exhibit such associations. Volcanic activity in the graben belts are presented by spacious lava fields with no apparent sources of volcanism. Graben belts and rift valleys were formed during the Atlian Period of geologic history of Venus, and they characterized the tectonic style of the planet at the late stages of its geologic evolution. Formation of this or that structural facies of the rift zones of Venus were probably governed by the thickness of the lithosphere, its rheological properties, and the development degree of the mantle diapirs associating with rift zones. 相似文献
14.
《Planetary and Space Science》2007,55(14):2097-2112
We briefly describe the history of landings on Venus, the acquired geochemical data and their potential petrologic interpretations. We suggest a new approach to Venus landing site selection that would avoid the potential contamination by ejecta from upwind impact craters. We also describe candidate units to be sampled in both in situ measurement and sample return missions. For the in situ measurements, the “true” tessera terrain (tt) material is considered as the highest priority goal with the second priority given to transitional tessera terrain (ttt), shield plains (psh) and lobate plains (pl) materials. For the sample return mission, the material of regional plains with wrinkle ridges (pwr) is considered as the highest priority goal with the second priority given to tessera terrain (tt) material. Combining the desire to study materials of specific geologic units with the problem of avoiding potential contamination by ejecta from upwind impact craters, we have suggested several candidate landing sites for each of the geologic units. Although spacecraft ballistics and other constraints of specific mission profiles (VEP or others) may lead to the selection of different candidate sites, we believe that the approaches outlined in this paper can be helpful approach in optimizing mission science return. 相似文献
15.
16.
Volcanoes on Venus are classified according to size with studies on the stratigraphic position of large volcanoes proposing that most of the large volcanoes postdate the regional volcanic materials. Some studies regarding intermediate volcanoes proposed that some of these volcanic features could be large volcanoes with embayed flow aprons, a situation that would alter the previous stratigraphic considerations about large volcanoes on Venus.In this work I analyze the global population of embayed intermediate-size volcanoes and compare their summits with that of other edifices classified as large volcanoes. Intermediate-size volcanoes are considered embayed when: (1) flows from another source clearly overlap the volcano slopes, and (2) display scarps related to flank-failure processes but with the associated collapse deposits being absent (i.e. interpreted as covered). As result of the survey 88 embayed intermediate-size volcanoes have been catalogued and integrated into a Geographic Information System. These embayed volcanoes have summit sizes and characteristics similar to large volcanoes and, therefore, could be interpreted as possible large volcanoes with their flow aprons embayed. Embayment materials for these volcanoes include all the units present in the history of the volcanic plains and would indicate that this type of central volcanic edifice would occur throughout the geologic history recorded in the venusian plains. 相似文献
17.
Beta Regio, Venus: Evidence for uplift, rifting, and volcanism due to a mantle plume 总被引：1，自引：0，他引：1
Geophysical data have led to the interpretation that Beta Regio, a 2000×25000 km wide topographic rise with associated rifting and volcanism, formed due to the rise of a hot mantle diapir interpreted to be caused by a mantle plume. We have tested this hypothesis through detailed geologic mapping of the V-17 quadrangle, which includes a significant part of the Beta Regio rise, and reconnaissance mapping of the remaining parts of this region. Our analysis documents signatures of an early stage of uplift in the formation of the Agrona Linea fracture belts before the emplacement of regional plains and their deformation by wrinkle ridging. We see evidence that the Theia rift-associated volcanism occurred during the first part of post-regional-plains time and cannot exclude that it continued into later time. We also see evidence that Devana Chasma rifting was active during the first and the second parts of post-regional-plains time. These data are consistent with uplift, rifting and volcanism associated with a mantle diapir. Geophysical modeling shows that diapiric upwelling may continue at the present time. Together these data suggest that the duration of mantle diapir activity was as long as several hundred million years. The regional plains north of Beta rise and the area east and west of it were little affected by the Beta-forming plume, but the broader area (at least 4000 km across), whose center-northern part includes Beta Regio, could have experienced earlier uplift as morphologically recorded in formation of tessera transitional terrain. 相似文献
18.
Detailed mapping in a 14,000,000 km2 area of northwestern Guinevere Planitia and northern Beta Regio bounded by 264°-312°E, 24°-60°N has revealed thousands of long extensional lineaments (graben, fissures and related fractures). These can be grouped into radiating, circumferential and linear systems. Thirty four radiating systems have been identified, of which 16 have radii greater than 300 km and eight have radii greater than 1000 km. Twenty six linear (straight) systems with a length greater than 300 km have been distinguished of which six have a length greater than 1000 km. Linear systems are generally associated with rifts, although some may represent distal portions of radiating systems. In addition, 19 circumferential systems, some associated with coronae, have been identified. The distribution of each system is compared with the host geology in order to place the graben-fissure systems in a regional stratigraphic framework. The majority of systems are: (1) younger than tesserae, ridge belts and densely fractured plains, (2) coeval with, and in many cases, define fracture belts, (3) partially flooded by wrinkle-ridged plains units, and (4) older than smooth and lobate plains units and young rifts. The inventory of radiating graben-fissure systems that we catalogue represents a database of tectono-magmatic centers that complements the centers defined using other criteria, e.g., large volcanoes, coronae, and shield fields. We have attempted to identify those systems that are underlain by dike swarms in order to evaluate their relationship to mantle plumes. At least 11 of the radiating systems extend well beyond any central topographic uplift and are therefore interpreted to be underlain by dike swarms. 相似文献
19.
Kenneth L. Tanaka Corey M. Fortezzo Rosalyn K. Hayward J.Alexis P. Rodriguez James A. Skinner 《Planetary and Space Science》2011,59(11-12):1128-1142
We present a preliminary photogeologic map of the Scandia region of Mars with the objective of reconstructing its resurfacing history. The Scandia region includes the lower section of the regional lowland slope of Vastitas Borealis extending about 500–1800 km away from Alba Mons into the Scandia sub-basin below ?4800 m elevation. Twenty mapped geologic units express the diverse stratigraphy of the region. We particularly focus on the materials making up the Vastitas Borealis plains and its Scandia sub-region, where erosional processes have obscured stratigraphic relations and made the reconstruction of the resurfacing history particularly challenging. Geologic mapping implicates the deposition, erosion, and deformation/degradation of geologic units predominantly during Late Hesperian and Early Amazonian time (～3.6–3.3 Ga). During this time, Alba Mons was active, outflow channels were debouching sediments into the northern plains, and basal ice layers of the north polar plateau were accumulating. We identify zones of regional tectonic contraction and extension as well as gradation and mantling. Depressions and scarps within these zones indicate collapse and gradation of Scandia outcrops and surfaces at scales of meters to hundreds of meters. We find that Scandia Tholi display concentric ridges, rugged peaks, irregular depressions, and moats that suggest uplift and tilting of layered plains material by diapirs and extrusion, erosion, and deflation of viscous, sedimentary slurries as previously suggested. These appear to be long-lived features that both pre-date and post-date impact craters. Mesa-forming features may have similar origins and occur along the southern margin of the Scandia region, including near the Phoenix Mars Lander site. Distinctive lobate materials associated with local impact craters suggest impact-induced mobilization of surface materials. We suggest that the formation of the Scandia region features potentially resulted from crustal heating related to Alba Mons volcanism, which acted upon a sequence of lavas, outflow channel sediments, and polar ice deposits centered within the Scandia region. These volatile-enriched sediments may have been in a state of partial volatile melt, resulting in the mobilization of deeply buried ancient materials and their ascent and emergence as sediment and mud breccia diapirs to form tholi features. Similar subsurface instabilities proximal to Alba Mons may have led to surface disruption, as suggested by local and regional scarps, mesas, moats, and knob fields. 相似文献
20.
Solar System Research - We have analyzed the spatial and age relationships of volcano-tectonic units that have developed on the surface of Venus: groove belts, coronae, and rift zones. Most of the... 相似文献