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1.
Higher outflow channel dissection in the martian region of southern circum-Chryse appears to have extended from the Late Hesperian to the Middle Amazonian Epoch. These outflow channels were excavated within the upper 1 km of the cryolithosphere, where no liquid water is expected to have existed during these geologic epochs. In accordance with previous work, our examination of outflow channel floor morphologies suggests the upper crust excavated by the studied outflow channels consisted of a thin (a few tens of meters) layer of dry geologic materials overlying an indurated zone that extends to the bases of the investigated outflow channels (1 km in depth). We find that the floors of these outflow channels contain widespread secondary chaotic terrains (i.e., chaotic terrains produced by the destruction of channel-floor materials). These chaotic terrains occur within the full range of outflow channel dissection and tend to form clusters. Our examination of the geology of these chaotic terrains suggests that their formation did not result in the generation of floods. Nevertheless, despite their much smaller dimensions, these chaotic terrains are comprised of the same basic morphologic elements (e.g., mesas, knobs, and smooth deposits within scarp-bound depressions) as those located in the initiation zones of the outflow channels, which suggests that their formation must have involved the release of ground volatiles. We propose that these chaotic terrains developed not catastrophically but gradually and during multiple episodes of nested surface collapse. In order to explain the formation of secondary chaotic terrains within zones of outflow channel dissection, we propose that the regional Martian cryolithosphere contained widespread lenses of volatiles in liquid form. In this model, channel floor collapse and secondary chaotic terrain formation would have taken place as a consequence of instabilities arising during their exhumation by outflow channel dissection. Within relatively warm upper crustal materials in volcanic settings, or within highly saline crustal materials where cryopegs developed, lenses of volatiles in liquid form within the cryolithosphere could have formed, and/or remained stable.In addition, our numerical simulations suggest that low thermal conductivity, dry fine-grained porous geologic materials just a few tens of meters in thickness (e.g., dunes, sand sheets, some types of regolith materials), could have produced high thermal anomalies resulting in subsurface melting. The existence of a global layer of dry geologic materials overlying the cryolithosphere would suggest that widespread lenses of fluids existed (and may still exist) at shallow depths wherever these materials are fine-grained and porous. The surface ages of the investigated outflow channels and chaotic terrains span a full 500 to 700 Myr. Chaotic terrains similar in dimensions and morphology to secondary chaotic terrains are not observed conspicuously throughout the surface of Mars, suggesting that intra-cryolithospheric fluid lenses may form relatively stable systems. The existence of widespread groundwater lenses at shallow depths of burial has tremendous implications for exobiological studies and future human exploration. We find that the clear geomorphologic anomaly that the chaotic terrains and outflow channels of southern Chryse form within the Martian landscape could have been a consequence of large-scale resurfacing resulting from anomalously extensive subsurface melt in this region of the planet produced by high concentrations of salts within the regional upper crust. Crater count statistics reveal that secondary chaotic terrains and the outflow channels within which they occur have overlapping ages, suggesting that the instabilities leading to their formation rapidly dissipated, perhaps as the thickness of the cryolithosphere was reset following the disruption of the upper crustal thermal structure produced during outflow channel excavation.  相似文献   

2.
We examine here the close spatial and temporal associations among several unique features of Xanthe and Margaritifer Terrae, specifically the Valles Marineris troughs or chasmata and their interior deposits, chaotic terrain, the circum-Chryse outflow channels, and the subdued cratered material that covers Xanthe, Margaritifer, and Meridiani Terrae. Though previous hypotheses have attempted to explain the origin of individual features or subsets of these, we suggest that they may all be related. All of these features taken together present a consistent scenario that includes the processes of sub-ice volcanism and other magma/ice interactions, results of intrusive events during Late Noachian to Early Amazonian times.  相似文献   

3.
Ares Vallis is one of the greatest outflow channels of Mars. Using high-resolution images of recent missions to Mars (MGS, 2001 Odyssey, and Mars Express), we investigated Ares Vallis and its valley arms, taking advantage of 3-dimensional analysis performed using the high-resolution stereo capability of the Mars Express High Resolution Stereo Camera (HRSC). In our view, Ares Vallis is characterized by catastrophic flood landscapes partially superimposed by ice-related morphologies. Catastrophic flood landforms include erosional terraces, grooved terrains, streamlined uplands, giant bars, pendant bars, and cataract-like features. Ice-related morphologies include probable kame features, thermokarstic depressions, and patterned grounds. Our investigations outline that throughout the Hesperian age, Ares Vallis and its valley arms had been sculpted by several, time-scattered, catastrophic floods, originating from Iani, Hydaspis and Aram Chaos. Geomorphological evidence suggests that catastrophic floods were ice-covered, and that climatic conditions of Mars at this time were similar to those of the present day. At the end of each catastrophic flood, ice masses grounded, forming a thick stagnant dead-ice body. Each catastrophic flood was followed by a relatively brief period of warmer-wetter climatic conditions, originated as a consequence of catastrophic flooding. During such periods thermokarstic depressions originated, liquid water formed meandering channels, and ice-contact deposits were emplaced by ice-walled streams. Finally, the climate turned into cold-dry conditions similar to the present-day ones, and ice masses sublimated.  相似文献   

4.
The Valles Marineris canyon system of Mars is closely related to large flood channels, some of which emerge full born from chaotic terrain in canyon floors. Coprates Chasma, one of the largest Valles Marineris canyons, is connected at its west end to Melas Chasma and on its east end to chaotic terrain-filled Capri and Eos Chasmata. The area from central Melas to Eos Chasmata contains a 1500 km long and about 1 km deep depression in its floor. Despite the large volumes of groundwater that likely discharged from chaotic terrain in this depression, no evidence of related fluvial activity has thus far been reported. We present an analysis of the regional topography which, together with photogeologic interpretation of available imagery, suggests that ponding due to late Hesperian discharge of water possibly produced a lake (mean depth 842 m) spanning parts of the Valles Marineris depression (VMD). Overflow of this lake at its eastern end resulted in delivery of water to downstream chaos regions and outflow channels. Our ponding hypothesis is motivated primarily by the identification of scarp and terrace features which, despite a lateral spread of about 1500 km, have similar elevations. Furthermore, these elevations correspond to the maximum ponding elevation of the region (−3560 m). Simulated ponding in the VMD yields an overflow point at its eastern extremity, in Eos Chasma. The neighborhood of this overflow point contains clear indicators of fluvial erosion in a consistent east-west orientation.  相似文献   

5.
The origin of the martian chaotic terrains is still uncertain; and a variety of geologic scenarios have been proposed. We provide topographic profiles of different chaos landscapes, notably Aureum and Hydraotes Chaos, showing that an initial shallow ground subsidence occurred at the first step of the chaos formation. We infer that the subsidence was caused by intrusion of a volcanic sill; which could have produced consequent melting as well as release of ground water from disrupted aquifer. Signs of a volcanic activity are observed on the floor of Hydraotes Chaos, a complex and deep depression located at the junction of three channels. The volcanic activity is represented by small, 0.5 to 1.5 km diameter, rounded cones with summit pits. The cone's size and morphology, as well as the presence of possible surrounding lava flows, suggest that they are primary volcanic cones similar to terrestrial cinder cones. The identification of volcanic activity on the deepest chaos, where the lower crustal thickness and the faults/fractures system contributed to the magma rising, reveals that magmatic activity, proved by the cones, and possibly help by structural activity, has been a major factor in the formation of chaotic terrains.  相似文献   

6.
The geomorphology and topography of the Cerberus Plains region of Mars show three spatially and temporally distinct, young, aqueous flood channel systems. Flood geomorphology in each of these channels, as seen in Mars Orbiter Camera images, consists of streamlined forms, longitudinal lineations, and a single occurrence of transverse dunes, features similar to those in the flood-carved terrain of the Channeled Scabland in the northwestern United States. As additional geomorphic evidence of flooding, small cones (interpreted as phreatic) are found preferentially in the channels or at their distal ends. Glaciers, lava flows, and CO2-charged density flows are each inconsistent with these geomorphic features. Mars Orbiter Laser Altimeter data show two of the three channel systems (Athabasca Valles and an unnamed northern channel system) emanating from the Cerberus Fossae; we suggest that the third channel system (Marte Vallis) also originated at the fissures. The discharges for two of the three systems (Athabasca Valles and Marte Vallis) have been estimated from surface topography to have been on the order of 106 m3/s. Crater counts indicate that the channels are not only young (extreme Late Amazonian), but also were carved asynchronously. Geomorphic evidence suggests that two of the channels (Athabasca and Marte Valles) experienced more than one flood. Emanation from volcanotectonic fissures instead of chaotic terrain distinguishes these Cerberus Plains channels from the larger, older circum-Chryse channels. Groundwater must have collected in a liquid state prior to flood onset to flow at the estimated discharge rates. Lack of large-scale subsidence near the channels' origination points along the Cerberus Fossae indicates that this groundwater was at least several kilometers deep.  相似文献   

7.
The Mangala Valles system is an ∼ ∼900 km fluvially carved channel system located southwest of the Tharsis rise and is unique among the martian outflow channels in that it heads at a linear fracture within the crust as opposed to a collapsed region of chaos as is the case with the circum-Chryse channels. Mangala Valles is confined within a broad, north–south trending depression, and begins as a single valley measuring up to 350 km wide that extends northward from a Memnonia Fossae graben, across the southern highlands toward the northern lowlands. Approximately 600 km downstream, this single valley branches into multiple channels, which ultimately lose their expression at the dichotomy boundary. Previous investigations of Mangala Vallis suggested that many of the units mapped interior to the valley were depositional, related to flooding, and that a minimum of two distinct periods of flooding separated by tens to hundreds of millions of years were required to explain the observed geology. We use infrared and visible images from the THermal EMission Imaging System (THEMIS), and topographic data from the Mars Orbiting Laser Altimeter (MOLA), to investigate the nature of the units mapped within Mangala Vallis. We find that the geomorphology of the units, as well as their topographic and geographic distribution, are consistent with most of them originating from a single assemblage of volcanic flow deposits, once continuous with volcanic flows to the south of the Memnonia Fossae source graben. These flows resurfaced the broad, north–south trending depression into which Mangala Vallis formed prior to any fluvial activity. Later flooding scoured and eroded this volcanic assemblage north of the Mangala source graben, resulting in the present distribution of the units within Mangala Vallis. Additionally, our observations suggest that a single period of catastrophic flooding, rather than multiple periods separated by tens to hundreds of millions of years, is consistent with and can plausibly explain the interior geology of Mangala Vallis. Further, we present a new scenario for the source and delivery of water to the Mangala source graben that models flow of groundwater through a sub-cryosphere aquifer and up a fracture that cracks the cryosphere and taps this aquifer. The results of our model indicate that the source graben, locally enlarged to a trough near the head region of Mangala, would have required less than several days to fill up prior to any spill-over of water to the north. Through estimates of the volume of material missing from Mangala (13,000–20,000 km3), and calculation of mean discharge rates through the channel system (∼ ∼5 × 106 m3 s−1), we estimate that the total duration of fluvial activity through the Mangala Valles was 1–3 months.  相似文献   

8.
Valles Marineris, located on the flank of the Tharsis Ridge uplift on Mars, exposes layering within the canyon walls interpreted to be volcanic flood lavas. By combining 1/128°×1/128°Mars Orbiter Laser Altimeter elevation data with wide-angle Mars Orbiter Camera images using Orion structural analysis software, we computed the attitude of some of this large-scale layering. Multilinear regression was used to fit planes to three-dimensional coordinates of points selected along exposed layer traces, giving the plane attitude and various fitting statistics. By measuring the same layer using different images, we found the measurements to be quite reproduceable. Errors in dip angle were typically only a few degrees or less. Analysis of the data indicates that most layers dip gently into the adjacent chasma. We interpret this orientation to be the result of the crustal subsidence, probably related to the formation of the early collapse basins, rather than the result of rotations produced by extensional faulting. Since the dip is consistent far away from the edge of the current chasmata we suggest that the scale of the depressions was on the order of hundreds of kilometers, exceeding the dimensions of the current chasmata.  相似文献   

9.
Gray crystalline hematite on Mars has been detected in three regions, Sinus Meridiani, Aram Chaos, and Valles Marineris, first by the Thermal Emission Spectrometer (TES) onboard the Mars Global Surveyor (MGS) orbiter, and then confirmed by other instruments. The hematite-rich spherules were also detected by the Mars Exploration Rover (MER) Opportunity at Meridiani Planum (Sinus Meridiani). Formation mechanisms of the hematite-rich spherules have been discussed widely since then. Here, we argue for an alternative formation mechanism, that is, the spherules originally formed at Valles Marineris due to the interaction of volcanic deposits and acidic hydrothermal fluids, and then were transported to and deposited at Meridiani Planum and Aram Chaos as alluvial/fluvial sedimentary deposits with other materials such as sulfates and rock fragments during the wash-out flows from Valles Marineris to Meridiani Planum and Aram Chaos. Diagenesis of the hematite-rich spherules may have also been a possible mechanism following sediment transport and emplacement. The hypothesis is consistent with available relevant information to date and provides an insight into the understanding of Martian surficial processes.  相似文献   

10.
Martian altitudes were measured by radar during the oppositions of 1971 and 1963 using the 64-m antenna at Goldstone (California). The resultant topographic profiles substantiate a zonal classification of the volcanic flows blanketing the south flanks of Arsia Mons, and they confirm the existence of a secondary, parasitic shield attached from the SSW to the main Arsia shield. The secondary shield is about 400 km in diameter at its base and at least 4 km high at its center. South of Valles Marineris, the Tharsis plateau is bounded by the approximate longitudes of 80° in the east and 140° in the west. In the Sinai Planum, closely adjacent to Coprates Chasma, another rise has been detected, bounded by longitudes of 55° in the east and 80° in the west. A volcanic shield of diameter 80 km, capped with a 22 km caldera has been identified near the crest of the rise. Topographic highs of about 1 km are associated with heavily faulted tracts such as Claritas Fossae. The distribution and orientation of the lunar-mare-like ridges in Sinai Planum appear to be independent of the regional gradients. Segments of the chaotic terrain at the eastern terminus of Valles Marineris are located as much as 6 km below the level of the surrounding plains.  相似文献   

11.
《Planetary and Space Science》2007,55(13):2015-2024
The Descent Imager/Spectral Radiometer (DISR) aboard the Huygens Probe took several hundred visible-light images with its three cameras on approach to the surface of Titan. Several sets of stereo image pairs were collected during the descent. The digital terrain models constructed from those images show rugged topography, in places approaching the angle of repose, adjacent to flatter darker plains. Brighter regions north of the landing site display two styles of drainage patterns: (1) bright highlands with rough topography and deeply incised branching dendritic drainage networks (up to fourth order) with dark-floored valleys that are suggestive of erosion by methane rainfall and (2) short, stubby low-order drainages that follow linear fault patterns forming canyon-like features suggestive of methane spring-sapping. The topographic data show that the bright highland terrains are extremely rugged; slopes of order of 30° appear common. These systems drain into adjacent relatively flat, dark lowland terrains. A stereo model for part of the dark plains region to the east of the landing site suggests surface scour across this plain flowing from west to east leaving ∼100-m-high bright ridges. Tectonic patterns are evident in (1) controlling the rectilinear, low-order, stubby drainages and (2) the “coastline” at the highland–lowland boundary with numerous straight and angular margins. In addition to flow from the highlands drainages, the lowland area shows evidence for more prolific flow parallel to the highland–lowland boundary leaving bright outliers resembling terrestrial sandbars. This implies major west to east floods across the plains where the probe landed with flow parallel to the highland–lowland boundary; the primary source of these flows is evidently not the dendritic channels in the bright highlands to the north.  相似文献   

12.
A debris flow mechanism is proposed to account for the formation of chaos and the large channels debouching into Chryse Planitia from the adjacent southern uplands of Mars. The debris is thought to have originated through a mechanism of collapse in the chaotic terrains which exist at the head of these channels as well as locally along the channels. This proposition is based on the detailed morphologic similarities between Martian channel source areas and the heads of both subaerial and subaqueous terrestrial debris flows. The downslope movement of the debris produced the channels through (a) modification of earlier collapse areas, (b) active bed erosion, and (c) loading-induced collapse. The large-scale channel geometry and the assemblage of related morphologic features on Mars correspond tto that observed in subaqueous debris flow chutes on the Mississippi delta front. Through various mechanisms of strain-dependent viscosity decrease the debris flow gained mobility downstream, turned into a debris avalanche, and moved onto Chryse Planitia at very high velocities. This high-velocity avalanche eroded a series of streamlined remnants near the channel mouths and deposited its load as a thin blanket over a large area of the basin creating virtually no depositional relief.  相似文献   

13.
Galaxias Chaos deviates significantly from other chaotic regions due to the lack of associated outflow channels, lack of big elevation differences between the chaos and the surrounding terrain and due to gradual trough formation.A sequence of troughs in different stages is observed, and examples of closed troughs within blocks suggest that the trough formation is governed by a local stress field rather than a regional stress field. Moreover, geomorphic evidence suggests that Galaxias Chaos is capped by Elysium lavas, which superpose an unstable subsurface layer that causes chaotic tilting of blocks and trough formation.Based on regional mapping we suggest a formation model, where Vastitas Borealis Formation embedded between Elysium lavas is the unstable subsurface material, because gradual volatile loss causes shrinkage and differential substrate movement. This process undermines the lava cap, depressions form and gradually troughs develop producing a jigsaw puzzle of blocks due to trough coalescence.Observations of chaos west of Elysium Rise indicate that this process might have been widespread along the contact between Vastitas Borealis Formation and Elysium lavas. However, the chaotic regions have probably been superposed by Elysium/Utopia flows to the NW of Elysium Rise, and partly submerged with younger lavas to the west.  相似文献   

14.
Abstract— The late Eocene Chesapeake Bay impact structure (CBIS) on the Atlantic margin of Virginia is one of the largest and best‐preserved “wet‐target” craters on Earth. It provides an accessible analog for studying impact processes in layered and wet targets on volatile‐rich planets. The CBIS formed in a layered target of water, weak clastic sediments, and hard crystalline rock. The buried structure consists of a deep, filled central crater, 38 km in width, surrounded by a shallower brim known as the annular trough. The annular trough formed partly by collapse of weak sediments, which expanded the structure to ?85 km in diameter. Such extensive collapse, in addition to excavation processes, can explain the “inverted sombrero” morphology observed at some craters in layered targets. The distribution of crater‐fill materials in the CBIS is related to the morphology. Suevitic breccia, including pre‐resurge fallback deposits, is found in the central crater. Impact‐modified sediments, formed by fluidization and collapse of water‐saturated sand and silt‐clay, occur in the annular trough. Allogenic sediment‐clast breccia, interpreted as ocean‐resurge deposits, overlies the other impactites and covers the entire crater beneath a blanket of postimpact sediments. The formation of chaotic terrains on Mars is attributed to collapse due to the release of volatiles from thick layered deposits. Some flat‐floored rimless depressions with chaotic infill in these terrains are impact craters that expanded by collapse farther than expected for similar‐sized complex craters in solid targets. Studies of crater materials in the CBIS provide insights into processes of crater expansion on Mars and their links to volatiles.  相似文献   

15.
In order to explain the development of Central Valles Marineris, a new morphostructural model is proposed. This model involves three major phases, including (i) initiation of graben patterns and pit crater chains under an early extensional phase, (ii) formation of wide grabens during major faulting, local rifting, and erosional phase, (iii) late faulting and secondary volcanic activity, possibly related to renewed updoming of East Tharsis. Based on detailed morphologic studies presented in a companion paper (Peulvast and Masson, this issue), the role of erosional processes in Central Valles Marineris landforming is discussed.  相似文献   

16.
Previous orbital mapping of crystalline gray haematite, ferric oxides, and sulfates has shown an association of this mineralogy with light-toned, layered deposits on the floor of Valles Marineris, in chaos terrains in the canyon’s outflow channels, and in Meridiani Planum. The exact nature of the relationship between ferric oxides and sulfates within Valles Marineris is uncertain. The Observatoire pour la Mineralogie, l’Eau, les Glaces et l’Activite (OMEGA) spectrometer initially identified sulfate and ferric oxides in the layered deposits of Valles Marineris. The Thermal Emission Spectrometer (TES) has also mapped coarse (gray) haematite in or at the base of these deposits. We use Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) spectra and Context Camera (CTX) and High Resolution Imaging Science Experiment (HiRISE) imagery from the Mars Reconnaissance Orbiter (MRO) to explore the mineralogy and morphology of the large layered deposit in central Capri Chasma, part of the Valles Marineris canyon system that has large, clear exposures of sulfate and haematite. We find kieserite (MgSO4·H2O) and ferric oxide (often crystalline red haematite) in the lower bedrock exposures and a polyhydrated sulfate without ferric oxides in the upper bedrock. This stratigraphy is duplicated in many other basinal chasmata, suggesting a common genesis. We propose the haematite and monohydrated sulfate formed by diagenetic alteration of a sulfate-rich sedimentary deposit, where the upper polyhydrated sulfate-rich, haematite-poor layers either were not buried sufficiently to convert to a monohydrated sulfate or were part of a later depositional phase. Based on the similarities between the Valles Marineris assemblages and the sulfate and haematite-rich deposits of Meridiani Planum, we hypothesize a common evaporite and diagenetic formation process for the Meridiani Planum sediments and the sulfate-bearing basinal Interior Layered Deposits.  相似文献   

17.
Throughout the recorded history of Mars, liquid water has distinctly shaped its landscape, including the prominent circum-Chryse and the northwestern slope valleys outflow channel systems, and the extremely flat northern plains topography at the distal reaches of these outflow channel systems. Paleotopographic reconstructions of the Tharsis magmatic complex reveal the existence of an Europe-sized Noachian drainage basin and subsequent aquifer system in eastern Tharsis. This basin is proposed to have sourced outburst floodwaters that sculpted the outflow channels, and ponded to form various hypothesized oceans, seas, and lakes episodically through time. These floodwaters decreased in volume with time due to inadequate groundwater recharge of the Tharsis aquifer system. Martian topography, as observed from the Mars Orbiter Laser Altimeter, corresponds well to these ancient flood inundations, including the approximated shorelines that have been proposed for the northern plains. Stratigraphy, geomorphology, and topography record at least one great Noachian-Early Hesperian northern plains ocean, a Late Hesperian sea inset within the margin of the high water marks of the previous ocean, and a number of widely distributed minor lakes that may represent a reduced Late Hesperian sea, or ponded waters in the deepest reaches of the northern plains related to minor Tharsis- and Elysium-induced Amazonian flooding.  相似文献   

18.
Paul D. Komar 《Icarus》1979,37(1):156-181
Comparisons are undertaken between the hydraulics of channelized water flows on Mars, large terrestrial rivers, deep-sea turbidity currents, and the catastrophic flow of Lake Missoula floods. Expected bottom shear stresses, velocities and discharges, flow powers, and other parameters are computed for each. Sand transport rates and the times required for channel erosion are estimated for Mangala Channel. These calculations indicate that the turbidity currents and Lake Missoula floods were similar to channelized water flow on Mars in their flow characteristics and in their abilities to erode and transport sediments. Like the Lake Missoula floods, deep-sea turbidity currents are catastrophic in character, being formed by the slumping of large masses of sediment trapped in submarine canyons or deposited on the continental slope. The repeated flows originating from submarine canyons have formed deep-sea channels similar in scale and overall morphology to the Martian outflow channels. The submarine canyon can be viewed as the counterpart of the chaotic terrain or crater which serves as sources for many Martian channels. Like most Martian outflow channels, the deep-sea channels generally lack tributaries or have only minor tributaries, instead consisting of a single pronounced channel extending for several hundred kilometers from its origin at the submarine canyon to deep abyssal depths. The channels vary considerably in dimensions, but most commonly have widths in the range 2 to 15 km with reliefs of 50 to 450 meters, again similar in scale to the Martian channels. Other similarities include sections of anastomosing channels, a general lack of pronounced meandering, and a lack of an apparent “delta” where the transported sediments are deposited. The similarities of channel morphology and flow hydraulics indicate the deep-sea channels and turbidity currents can be useful in furthering our understanding of the Martian outflow channels. Physical processes in the deep-sea occur under a reduced effective gravity because of the overlying water with its buoyancy. The deep-sea channels provide another set of Earth-based channels which can be studied to determine the effects of gravity on such factors as channel meandering and anastomosing characteristics.  相似文献   

19.
Philippe Masson 《Icarus》1977,30(1):49-62
The Coprates and Phoenicis Lacus quadrangles of Mars contain the Valles Marineris, Noctis Labyrinthus, and Claritas Fossae areas, each of which shows distinctive structural patterns. Analyses of the structural trends seen within these quadrangles show four principal trend directions. The chronological relationships among these trends and their relation to the stratigraphy has been determined. It appears that the two oldest trends (essentially WSW/ENE and NNE/ SSW), on the basis of transection relations, are best defined in what have been mapped on stratigraphic criteria as older Martian terrains (troughed and furrowed and cratered terrains). Younger trends (WNW/ESE and N/S), also on the basis of transection relations, appear to be related to opening and widening of the canyon. These are present only in the younger stratigraphic units. A comparison between the structural pattern of the Valles Marineris region and that of the Eastern African Rift system at the same scale reveals regional similarities. These suggest that a common major process, lateral extensions in the crust, was involved in the formation of both features.  相似文献   

20.
The occurence within Elysium Planitia of meltwater deposits, possible pseudocraters, collapse features within troughs, and outflow channels indicates that a layer of subsurface volatiles existed at the time of volcanic activity within this area. The pseudocraters are interpreted to be indicators of near-surface volatiles, while meltwater deposits and the degree of preservation of trough walls and floors are thought to signify greater volatile depths. A latitudinal variation in the distribution of these features indicates either that the depth to the volatile layer increased from less than about 50 m at 35°N to greater than 600 m at 24°N, or that an ice wedge that existed at 35°N thinned to nonexistence at 24°N. Braided distributary channel systems within the chaotic terrain north of Elysium Planitia show that ephemeral lakes were repeatedly created and drained at this locality. The existence of volatiles contemporaneous with volcanic activity permits a search to be made for explosively generated landforms predicted to exist by previous theoretical models. Morphological evidence for strombolian, vulcanian and plinian eruptions is lacking within western Elysium Planitia; there are no identifiable cinder cones, pyroclastic flow deposits, or mantled areas indicative of large airfall deposits at an image resolution of 50–150 m/pixel. However, the pseudocraters indicate that small-scale phreatomagmatic activity may have taken place.  相似文献   

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