Emplacement Of Regional Plains Research Articles

History of the long-wavelength topography of Venus

A comparison of the global geological and topographic maps of Venus allows analysis of the topographic configuration of the major stratigraphic units forming the planet’s surface. The main positive relief features on Venus are manifested as prominences of tectonized units, which usually pertain to the initial episodes in the observable portion of its geological history (the beginning of the Guineverian period). The main morphostructural unit in this category is tessera, the main massifs of which constitute a special class of plateau-like highlands up to several kilometers high and tens to hundreds or thousands of kilometers wide. The younger shield and regional plains are confined to the regional slopes of the highlands formed by ancient tectonized units and are concentrated in regional lowlands. This indicates that regional lowlands (hundreds to thousands of kilometers across) as well as the highlands of ancient tectonized units had been formed in the early stages of the geological history of Venus. The estimates for the absolute model age of regional plains, which fill the lowlands, indicate that the formation of this material unit marks the first third of the observable geological history. Thus, the long-wavelength topographic pattern of Venus had formed, in general, in the early stages of the geological history, during the first third of it. In the final stages, which cover the next two thirds of the observable geological history of Venus (the Atlian period), the formation of long-wavelength topographic features is confined to dome-shaped highs, which make a minor contribution (about 15%) to the overall long-wavelength topographic pattern of Venus. This is evidence of a sharp fall in the level of endogenous activity after the emplacement of regional plains, with the style of the activity having changed from intensive crustal dislocations to extensive yet isolated uplifts of the thick thermal lithosphere.

The history of volcanism on Venus

The history of volcanism on Venus

Venus astra/novae: Estimates of the absolute time duration of their activity

In order to assess the age relations between astra/novae (features with extensive radial fracture-graben systems) and their surroundings, and to determine the duration of their activity, we undertook a photogeologic analysis of Magellan images of 78 astra, 49 dark-parabola craters and 114 clear-halo craters. For seven of these 78 astra it was found that the astrum-forming faulting started before or close to the time of emplacement of regional plains and extended into the second part of post-regional-plains time. Because the mean age of the regional plains is close to the mean surface age of Venus (which is estimated to be ∼750 m.y), this means that the duration of activity of these seven astra was several hundred millions of years. This is longer than the duration of activity of ongoing mantle plumes on Earth, but shorter than the duration of activity of the plume feeding the martian volcano Olympus Mons. The basic morphologic characteristics of these seven astra, as well as their age relations with other geologic units, are similar to those of the majority of other astra; therefore, such a long duration could also be typical of some other astra. We confirm the two-phase (pre- and post-regional-plains) evolution of astrum-forming faulting suggested in previous studies. For the first phase we show evidence for purely tectonic faulting caused by the diapiric rise of a mantle plume. For the second phase we find evidence supporting the interpretation of other studies that the observed faults resulted from subsurface dike intrusions produced by magmatism associated with the plume. We also found that faulting during the second phase was not instantaneous but distributed over a long period of time.

Beta Regio, Venus: Evidence for uplift, rifting, and volcanism due to a mantle plume

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.

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

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

Novae on Venus: Geology, classification, and evolution

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

On rates and styles of late volcanism and rifting on Venus

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

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

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

ONSET TIME AND DURATION OF CORONA ACTIVITY ON VENUS: STRATIGRAPHY AND HISTORY FROM PHOTOGEOLOGIC STUDY OF STEREO IMAGES

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