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1.
Io: Geochemistry of sulfur 总被引：1，自引：0，他引：1
John S. Lewis 《Icarus》1982,50(1):103-114
The evidence from Voyager imaging, Earth-based spectral reflectivity studies, and thermal emission measurements combine to suggest an extremely fresh, volcanically recycled sulfur-rich crust for Io, with very shallow large-scale melting. Two present styles of volcanism are possible, depending on the thickness of local deposits of sulfur: shallow liquid sulfur magma generation with quiescent flooding, and high-temperature volcanism with violent eruption of a sulfur-iron magma driven by SO2. Evolutionary considerations preclude direct derivation of Io's lithosphere from any metal-bearing chondritic source material. Metal-free C3V- or C2M-type parent material of either primary or secondary origin is the most plausible direct antecedent of the present sulfur-rich crust. Sulfates are almost certainly important constituents of the mantle, and can participate in the recycling of reduced, dense sulfide species to prevent total extraction of sulfur into the core. 相似文献
2.
J. Raitala 《Earth, Moon, and Planets》1994,64(2):155-164
Various tectonic structures to the south and southeast of Ishtar Terra indicate areal stresses. Compression from east-southeast against Ishtar Terra has resulted in ridge belt formation and surface bending at Salme Dorsa, probably along the seam between two crustal units. En echelon fault zone indicates dextral strike-slip shear(s) resulted in the westward movement of planitia crust related to Ishtar Terra. Meshkenet Tessera displays differential dextral strike-slip faulting where the southernmost bar-like blocks have had largest relative movements. Compression against Tusholi Corona has resulted in foreland surface bending similar to that of Salme Dorsa. The tectonic zone as a whole resembles a dextral transform fault extending from a concave arc in the west to another concave arc in the east. The Cytherean surface, crust or uppermost lithosphere seems to be able to transmit stresses over distances. Deeper understanding of these processes is needed to gain a new idea of the crustal deformation on terrestrial planets. 相似文献
3.
Magmatic Differentiation of Io 总被引：1，自引：0，他引：1
If Io has been volcanically active through much of its history, it must be highly differentiated. We present an initial attempt to quantify the differentiation of the silicate portion of Io. We suggest that, on average, each part of Io has undergone about 400 episodes of partial melting. We employ a widely used thermodynamic model of silicate melts to examine the effect of such repeated differentiation. Despite many caveats, including a grossly oversimplistic model of the differentiation process, uncertainties in the initial composition of the mantle, and the failure to model more than four episodes of partial melting, we are able to make some robust conclusions. Io should have a roughly 50 km thick, low density (2600–2900 kg m−3), alkali-rich, siliceous crust composed primarily of feldspars and nepheline. The crustal magmas should have relatively low melting temperatures (<1100 °C). The bulk of the mantle should be essentially pure forsterite (magnesian olivine). It is possible that the denser iron- and calcium-rich materials are segregated into a lower mantle and thus no longer involved in surface processes. These model predictions are generally consistent with the observations of Io. The enrichment of the crust in alkalis may help to explain the composition of the neutral clouds around Io. The failure to detect silicates at the surface of Io to date might be due in part to the difficulty in detecting Fe-poor minerals such as nepheline, feldspars, and forsterite via near-IR spectroscopy. Many hot spot temperatures are too high for sulfur alone but are in line with silica-rich melts. The mountains on Io could be manifestations of large buoyant plutons. The highest temperature lavas may be the result of melts from the depleted mantle making their way to the surface from great depths. 相似文献
4.
Numerical models of mantle convection that include the ‘basalt barrier’ mechanism are explored for Venus. The ‘basalt barrier’ mechanism is due to the positive buoyancy of subducted basaltic crust between the mantle depths of 660 and 750 km. The inclusion of this mechanism in models of Earth’s evolution has been shown to cause episodic mantle layering early in Earth history and we explore whether it can also operate on Venus. The models presented here include a moderately mobile lithosphere, which is not representative of the current state of Venus, but this allows us to exclude the effects of episodic lithosphere mobility and thus to isolate the effect of the basalt barrier. This is a step in a systematic approach to models with a mostly-static lithosphere. We find the basalt barrier does yield episodically layered mantle convection in some Venus models. The likelihood of episodic layering is increased by Venus high surface temperature and by its less mobile or immobile lithosphere. Surprisingly, secondary differences from Earth, including the lower gravity, density and mantle depth also promote episodic layering. The models suggest that mantle layering and overturns may still be likely to occur in Venus. The breakdown of mantle layering and consequent mantle overturns would lead to dramatic episodes of volcanism, formation of large amounts of crust, and tectonic activity on the planet’s surface, as has been inferred to have happened on Venus around 500 Ma ago from surface morphology and cratering. These results thus suggest that a transient layering of the mantle by the ‘basalt barrier’ mechanism and mantle overturns may be part of the explanation for Venus’s recent resurfacing. 相似文献
5.
Stress models for Tharsis formation, Mars 总被引：1，自引：0，他引：1
A critical survey is presented of most stress models proposed for the formation of the tectonic structures in the Tharsis volcano-tectonic province on Mars and provides new constraints for further models. First papers, in the 1970s, attempted to relate the Tharsis formation to asthenospheric movements and lithosphere loading by magma bodies. These processes were then quantified in terms of stress trajectory and magnitude models in elastic lithosphere (e.g. Banerdt et al., J. Geophys. Res. 87(B12), 9723–9733, 1982). Stresses generated by dynamic lithosphere uplift were rapidly dismissed because of the poor agreement between the stress trajectories provided by the elastic models and the structural observations. The preferred stress models involved lithosphere loading, inducing isostatic compensation, and then lithosphere flexure. Some incomsistency with structural interpretation of Viking imagery has been found. In the early 1990s, an attempt to solve this problem resulted in a model involving the existence of a Tharsis-centred brittle crustal cap, deteched from the strong mantle by a weak crustal layer (Tanaka et al., J. Geophys. Res. 96(E1), 15617–15633, 1991). Such a configuration should produce loading stresses akin to those predicted by some combination of the two loading modes. This model has not been quantified yet, however it is expected to reconcile stress trajectories and most structural patterns. Nevertheless, some inconsistencies with observed structures are also expected to remain. Parallel to this approach focused on loading mechanisms, the idea that volcanism and tectonic structures could be related to mantle circulation began to be considered again through numerical convection experiments, whose results have however not been clearly correlated with surface observations. Structural clues to early Tharsis dynamic uplift are reported. These structures have little to do with those predicted by elastic stress modelling of dynamic lithosphere uplift. They denote the existence of unsteady stress trajectories responsible for surface deformations that cannot be readily predicted by elastic models. These structures illustrate that improving current stress models for Tharsis formation shall come from deeper consideration of rock failure criterion and load growth in the lithosphere (e.g. Schultz and Zuber, J. Geophys. Res. 99(E7), 14691–14702, 1994). Improvements should also arise from better understanding rheological layering in the lithosphere and its evolution with time, and from consideration of stress associated to magma emplacement in the crust, which may have produced many tectonic structures before loading stress resulting from magma freezing became significant (Mège and Masson, Planet. Space Sci. 44, 1499–1546, 1996a). 相似文献
6.
Jouko T. Raitala 《Earth, Moon, and Planets》1987,38(3):285-297
Topographic information, surface structures and construction of the Martian Tharsis bulge are used to estimate the previous stresses across the low-lying peripheral margins of the crustal blocks in terms of simple compensation models. Hot mantle activity, crustal roots, isostasy, and late-stage extensive lithosphere thickening together with volcanic building have been in combined response to the high-elevated Tharsis bulge. The initial phases of the Tharsis building have been dominated by the mantle plume doming, followed by extrusional dome raising. The volcanism has been most important bulge building factor only after thickening of the crust. During the initial mantle-generated doming and igneous activity the thin-lithosphere block tectonics has been very important. There has been a compressional peripheral zone around the bulge giving rise to dorsa formation while the high bulge crests have been in tensional state. The situation may be favorable for comparative studies with other planets. We may have something to learn from this block tectonics on the one-plate planet Mars even in respect to the Earth's plate tectonic paradigm.On leave from Dept. of Astronomy, University of Oulu, Finland. 相似文献
7.
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
8.
We have reinvestigated the coupled thermal and crustal evolution of Mars taking new laboratory data concerning the flow behavior of iron-rich olivine into account. The low mantle viscosities associated with the relatively higher iron content of the martian mantle as well as the observed high concentrations of heat producing elements in a crust with a reduced thermal conductivity were found to promote phases of crustal recycling in many models. As crustal recycling is incompatible with an early separation of geochemical reservoirs, models were required to show no episodes of crustal recycling. Furthermore, admissible models were required to reproduce the martian crust formation history, to allow for the formation of partial melt under present day mantle conditions and to reproduce the measured concentrations of potassium and thorium on the martian surface. Taking dehydration stiffening of the mantle viscosity by the extraction of water from the mantle into account, we found that admissible models have low initial upper mantle temperatures around 1650 K, preferably a primordial crustal thickness of 30 km, and an initially wet mantle rheology. The crust formation process on Mars would then be driven by the extraction of a primordial crust after core formation, cooling the mantle to temperatures close to the peridotite solidus. According to this scenario, the second stage of global crust formation took place over a more extended period of time, waning at around 3500 Myr b.p., and was driven by heat produced by the decay of radioactive elements. Present-day volcanism would then be driven by mantle plumes originating at the core-mantle boundary under regions of locally thickened, thermally insulating crust. Water extraction from the mantle was found to be relatively efficient and close to 40% of the total inventory was lost from the mantle in most models. Assuming an initial mantle water content of 100 ppm and that 10% of the extracted water is supplied to the surface, this amount is equivalent to a 14 m thick global surface layer, suggesting that volcanic outgassing of H2O could have significantly influenced the early martian climate and increased the planet’s habitability. 相似文献
9.
Alan B. Binder 《Earth, Moon, and Planets》1982,26(2):117-133
Evaluation of all reasonable sources of stress in the lunar crust indicates that compressional thermoelastic stresses are the only ones which have been tectonically significant on the global scale during the last 3.5×109 yr of lunar history — i.e., the post-Imbrian. However, the thermoelastic stresses calculated for lunar models which have accretional heating profiles at the beginning of lunar history; i.e., a molten zone only a few hundred kilometers deep and a cool deep interior, are less than 1 kbar in the crust. Such stresses are lower than the more than 1 to 7 kbar needed to initiate thrust faulting in the outer crust according to Anderson's theory of thrust faulting. Thus such accretional models predict that no significant global thrust faulting has occurred during the post-Imbrian and that the crust should currently be seismically quiet on the global scale.In contrast, the compressional thermoelastic stresses generated in a Moon which was initially totally molten, as is the case if the Moon formed by fission, are up to 3.5 kbar in the outer few km of the crust at present. These stresses are well within the range needed to cause thrust faulting in the outer 4 km of the crust. According to this model there should be modest scale (10 km), young ( 0.5 to 1×109 yr old) thrust fault scarps in the highlands.Photoselenological investigations confirm that scarps with the expected age and geometric characteristics are found in the highlands. Thus the currently available photoselenological data support the stress model derived for an initially totally molten Moon, but not one which was molten only in the outer few hundreds of km. 相似文献
10.
Javier Ruiz Patrick J. McGovern Valle López Jean-Pierre Williams Rosa Tejero 《Icarus》2011,215(2):508-517
Lithospheric strength can be used to estimate the heat flow at the time when a given region was deformed, allowing us to constrain the thermal evolution of a planetary body. In this sense, the high (>300 km) effective elastic thickness of the lithosphere deduced from the very limited deflection caused by the north polar cap of Mars indicates a low surface heat flow for this region at the present time, a finding difficult to reconcile with thermal history models. This has started a debate on the current heat flow of Mars and the implications for the thermal evolution of the planet. Here we perform refined estimates of paleo-heat flow for 22 martian regions of different periods and geological context, derived from the effective elastic thickness of the lithosphere or from faulting depth beneath large thrust faults, by considering regional radioactive element abundances and realistic thermal conductivities for the crust and mantle lithosphere. For the calculations based on the effective elastic thickness of the lithosphere we also consider the respective contributions of crust and mantle lithosphere to the total lithospheric strength. The obtained surface heat flows are in general lower than the equivalent radioactive heat production of Mars at the corresponding times, suggesting a limited contribution from secular cooling to the heat flow during the majority of the history of Mars. This is contrary to the predictions from the majority of thermal history models, but is consistent with evidence suggesting a currently fluid core, limited secular contraction for Mars, and recent extensive volcanism. Moreover, the interior of Mars could even have been heating up during part of the thermal history of the planet. 相似文献
11.
It is widely accepted that lunar volcanism started before the emplacement of the mare fills ( b.p.) and lasted for probably more than 3.0 Ga. While the early volcanic activity is relatively easy to understand from a thermal point of view, the late stages of volcanism are harder to explain, because a relatively small body like the Earth's Moon is expected to cool rapidly and any molten layer in the interior should solidify rather quickly. We present several thermal evolution models, in which we varied the boundary conditions at the model surface in order to evaluate the influence on the extent and lifetime of a molten layer in the lunar interior. To investigate the influence of a top insulating layer we used a fully three-dimensional spherical shell convection code for the modelling of the lunar thermal history. In all our models, a partial melt zone formed nearly immediately after the simulation started (early in lunar history), consistent with the identification of lunar cryptomare and early mare basalt volcanism on the Moon. Due to the characteristic thickening of the Moon's lithosphere the melt zone solidified from above. This suggests that the source regions of volcanic rock material proceeded to increasing depth with time. The rapid growth of a massive lithosphere kept the Moon's interior warm and prevented the melt zone from fast freezing. The lifetimes of the melt zones derived from our models are consistent with basalt ages obtained from crater chronology. We conclude that an insulating megaregolith layer is sufficient to prevent the interior from fast cooling, allowing for the thermal regime necessary for the production and eruption of young lava flows in Oceanus Procellarum. 相似文献
12.
Steven W. Squyres 《Icarus》1982,52(3):545-559
The bands of bright resurfaced terrain on Ganymede are probably broad grabens formed by global expansion and filled with deposits of ice. Grooves within the bands are thought to be extensional features formed during the same episode of expansion. The crust of Ganymede is modeled as a viscoelastic material subjected to extensional strain. With sufficiently high strain rates and stresses, deep normal faulting will occur, creating broad grabens that may then be filled. Continuing deformation at high strain rates and stresses will cause propagation of deep faults up into the flood deposits and normal faulting at the surface, while lower strain rates and stresses will cause formation of open extension fractures or, if the crustal strength is very low, grabens at the surface. The spacing between adjacent fractures may reflect the geothermal gradient at the time of deformation. Surface topography resulting from fracturing and normal faulting will decay with time as a result of viscous relaxation and mass-wasting. 相似文献
13.
The quantification of geohazards and water resources in intraplate areas requires an integrated approach connecting monitoring, reconstruction and prediction of underlying processes. Intraplate rifts such as the Northwestern European rift system and coastal areas such as the Rhine–Meuse delta system are characterized by an interplay of climatic variations and neotectonics. The Netherlands Environmental Earth System Dynamics Initiative (NEESDI) addresses the interplay of lithosphere and surface processes through an integration of upper mantle and crustal scale studies with high-resolution analyses of the sedimentary record, geomorphology and hydrodynamic regime. Recent faulting imaged by seismic reflection data and trenching appears to exert a major control on uplift and subsidence patterns in the area, effecting coastal evolution and river dynamics in the Rhine–Meuse system. 相似文献
14.
Regions of maximum shear and tension–compression stresses in the Martian interior have been revealed using two types of models: the elastic model and the model with an elastic lithosphere of varied thickness (150–500 km) positioned on a weak layer that has partially lost its elastic properties. The weakening is simulated by a ten-fold lower value of the shear modulus down to the core boundary. The numerical simulation applies Green’s functions (load number method) with the step of 1 × 1 grade along latitude and longitude down to a depth of 1000 km. The boundary condition is the expansion of the latest data on Martian topography and the gravitational field (model MRO120D) in spherical harmonics up to the degree and order of 90 in relation to the reference surface that is assumed an equilibrium spheroid. The considered two-level compensation model assumes nonequilibrium relief and density anomalies at the crust–mantle boundary to be the sources of the anomalous gravitational field. Calculations are performed for two test models of Martian internal structure with the crust mean thicknesses of 50 to 100 km and mean density of 2900 kg/m3. Considerable tangential and simultaneously compressive stresses occur under the Tharsis region. The main regions of high shear and simultaneously extentional stresses are located in the Hellas region crust and in the lithosphere of the following regions: Argyre Planitia, Mare Acidalium, Arcadia Planitia and Valles Marineris. The zone of high maximum shear and extentional stresses has been found at the base of the lithosphere under the Olympus volcano and that under the Elysium rise. 相似文献
15.
The geothermal gradient of Io: Consequences for lithosphere structure and volcanic eruptive activity 总被引：1，自引：0，他引：1
We solve numerically the equations describing the transfer of heat through the lithosphere of Io by a mixture of conduction and volcanic advection as proposed by O’Reilly and Davies (O’Reilly, T.C., Davies, G.F. [1981]. Geophys. Res. Lett. 8, 313-316), removing the requirement that average material properties must be used. As expected, the dominance of advective heat transfer by volcanic eruptions means that Io’s geothermal gradient well away from volcanic centres is very small, of order 1 K km−1. This result is independent of any reasonable assumptions about the radiogenic heating rate in the lithosphere. The lithosphere temperature does not increase greatly above the surface temperature until the base of the lithosphere is approached, except in limited areas around shallow magma bodies. As a consequence, solid volatile sulphur compounds mobilized by volcanic processes and re-deposited on the surface of Io commonly remain solid until they reach great depths as they are progressively buried by ongoing activity. For current estimates of the volcanic heat transfer rate, melting of SO2 does not begin until a depth of ∼20 km and sulphur remains solid to a depth of ∼26 km in a 30 km thick lithosphere. Rising magmas can incorporate fluids from these deep sulphur compound aquifers, and we quantify the major influence that this can have on the bulk density of the magma and hence the resulting possible intrusion and eruption styles. 相似文献
16.
We produced regional geologic maps of the Hi’iaka and Shamshu regions of Io’s antijovian hemisphere using Galileo mission data to assess the geologic processes that are involved in the formation of Io’s mountains and volcanic centers. Observations reveal that these regions are characterized by several types of volcanic activity and features whose orientation and texture indicate tectonic activity. Among the volcanic features are multiple hotspots and volcanic vents detected by Galileo, one at each of the major paterae: Hi’iaka, Shamshu, and Tawhaki. We mapped four primary types of geologic units: flows, paterae floors, plains, and mountains. The flows and patera floors are similar, but are subdivided based upon emplacement environments and mechanisms. The floors of Hi’iaka and Shamshu Paterae have been partially resurfaced by dark lava flows, although portions of the paterae floors appear bright and unchanged during the Galileo mission; this suggests that the floors did not undergo complete resurfacing as flooding lava lakes. However, the paterae do contain compound lava flow fields and show the greatest activity near the paterae walls, a characteristic of Pele type lava lakes. Mountain materials are tilted crustal blocks that exhibit varied degrees of degradation. Lineated mountains have characteristic en echelon grooves that likely formed as a result of gravitational sliding. Undivided mountains are partially grooved but exhibit evidence of slumping and are generally lower elevation than the lineated units. Debris lobes and aprons are representative of mottled mountain materials. We have explored the possibility that north and south Hi’iaka Mons were originally one structure. We propose that strike-slip faulting and subsequent rifting separated the mountain units and created a depression which, by further extension during the rifting event, became Hi’iaka Patera. This type of rifting and depression formation is similar to the mechanism of formation of terrestrial pull-apart basins. With comparison to other regional maps of Io and global studies of paterae and mountains, this work provides insight into the general geologic evolution of Io. 相似文献
17.
F. MarchisI. de Pater A.G. DaviesH.G. Roe T. FuscoD.Le Mignant P. DescampsB.A. Macintosh R. Prangé 《Icarus》2002,160(1):124-131
Io, the innermost Galilean satellite of Jupiter, is a fascinating world. Data taken by Voyager and Galileo instruments have established that it is by far the most volcanic body in the Solar System and suggest that the nature of this volcanism could radically differ from volcanism on Earth. We report on near-IR observations taken in February 2001 from the Earth-based 10-m W. M. Keck II telescope using its adaptive optics system. After application of an appropriate deconvolution technique (MISTRAL), the resolution, ∼100 km on Io's disk, compares well with the best Galileo/NIMS resolution for global imaging and allows us for the first time to investigate the very nature of individual eruptions. On 19 February, we detected two volcanoes, Amirani and Tvashtar, with temperatures differing from the Galileo observations. On 20 February, we noticed a slight brightening near the Surt volcano. Two days later it had turned into an extremely bright volcanic outburst. The hot spot temperatures (>1400 K) are consistent with a basaltic eruption and, being lower limits, do not exclude an ultramafic eruption. These outburst data have been fitted with a silicate-cooling model, which indicates that this is a highly vigorous eruption with a highly dynamic emplacement mechanism, akin to fire-fountaining. Its integrated thermal output was close to the total estimated output of Io, making this the largest ionian thermal outburst yet witnessed. 相似文献
18.
Each of the Galilean satellites, as well as most other satellites whose initial rotations have been substantially altered by tidal dissipation, has been widely assumed to rotate synchronously with its orbital mean motion. Such rotation would require a small permanent asymmetry in the mass distribution in order to overcome the small mean tidal torque. Since Io and Europa may be substantially fluid, they may not have the strenght to support the required permanent asymmetry. Thus, each may rotate at the unknown but slightly nonsynchronous rate that corresponds to zero mean tidal torque. This behaviour may be observable by Galileo spacecraft imaging. It may help explain the longitudinal variation of volcanism on Io and the cracking of Europa's crust. 相似文献
19.
Lakshmi Planum is distinctive and unique on the surface of Venus as an expansive (~2 × 106km2), relatively smooth, flat plateau containing two large shield volcanoes and abundant volcanic plains in the midst of a region of extreme relief. It rises 3–5 km above the datum and is surrounded on all sides by bands of mountains interpreted to be of compressional tectonic origin. The major units mapped on Lakshmi are volcanic edifices, smooth, ridged and grooved plains units, and structural units referred to as ridged terrain. Three styles of volcanism are observed to dominate the surface of Lakshmi. Distributed effusive volcanism is associated with extensive plains deposits and many of the small shields, domes and cones mapped within the plateau. Centralized effusive volcanism is primarily associated with the paterae, Colette and Sacajawea, and their circumferential low-shield-forming deposits. The precise origin and evolution of these unusually large and complex structures is not understood, although a catastrophic, explosive origin is unlikely. Pyroclastic volcanism may be represented by a unit referred to as the diffuse halo. The origin and evolution of Lakshmi Planum is closely related to its compressional tectonic environment; volcanism on Lakshmi has occurred synchronously with tectonism in the surrounding orogenic belts. A model for the origin and evolution of Lakshmi Planum consisting of a continuous sequence of convergence and horizontal shortening of crustal segments against a preexisting block of tessera seems best able to account for the elevation, plateau shape and irregular polygonal outline of Lakshmi, as well as the presence of ridged terrain and its resemblance to tessera. Volcanism on Lakshmi is proposed to be the result of basal melting of a thickened crustal root. According to this model, the origin and evolution of Lakshmi Planum has consisted of the following sequence of events: (1) formation of a large, elevated block of tessera surrounded by low-lying plains; (2) convergence and underthrusting of crustal segments to produce peripheral mountain ranges, thickening, and uplift of the plateau; and (3) basal melting of the thickened crust and underthrust material and surface volcanism that occurred synchronously with continued edge deformation.'Geology and Tectonics of Venus', special issue edited by Alexander T. Basilevsky (USSR Acad. of Sci., Moscow), James W. Head (Brown University, Providence). Gordon H. Pettengill (MIT. Cambridge, Massachusetts) and R. S. Saunders (J.P.L., Pasadena). 相似文献
20.
Volcanism on Io: New insights from global geologic mapping 总被引：2，自引：0，他引：2
David A. Williams Laszlo P. Keszthelyi Jessica A. Yff Paul M. Schenk Tammy L. Becker 《Icarus》2011,214(1):91-112
We produced the first complete, 1:15 M-scale global geologic map of Jupiter’s moon Io, based on a set of monochrome and color Galileo-Voyager image mosaics produced at a spatial resolution of 1 km/pixel. The surface of Io was mapped into 19 units based on albedo, color and surface morphology, and is subdivided as follows: plains (65.8% of surface), lava flow fields (28.5%), mountains (3.2%), and patera floors (2.5%). Diffuse deposits (DD) that mantle the other units cover ∼18% of Io’s surface, and are distributed as follows: red (8.6% of surface), white (6.9%), yellow (2.1%), black (0.6%), and green (∼0.01%). Analyses of the geographical and areal distribution of these units yield a number of results, summarized below. (1) The distribution of plains units of different colors is generally geographically constrained: Red-brown plains occur >±30° latitude, and are thought to result from enhanced alteration of other units induced by radiation coming in from the poles. White plains (possibly dominated by SO2 + contaminants) occur mostly in the equatorial antijovian region (±30°, 90-230°W), possibly indicative of a regional cold trap. Outliers of white, yellow, and red-brown plains in other regions may result from long-term accumulation of white, yellow, and red diffuse deposits, respectively. (2) Bright (possibly sulfur-rich) flow fields make up 30% more lava flow fields than dark (presumably silicate) flows (56.5% vs. 43.5%), and only 18% of bright flow fields occur within 10 km of dark flow fields. These results suggest that secondary sulfurous volcanism (where a bright-dark association is expected) could be responsible for only a fraction of Io’s recent bright flows, and that primary sulfur-rich effusions could be an important component of Io’s recent volcanism. An unusual concentration of bright flows at ∼45-75°N, ∼60-120°W could be indicative of more extensive primary sulfurous volcanism in the recent past. However, it remains unclear whether most bright flows are bright because they are sulfur flows, or because they are cold silicate flows covered in sulfur-rich particles from plume fallout. (3) We mapped 425 paterae (volcano-tectonic depressions), up from 417 previously identified by Radebaugh et al. (Radebaugh, J., Keszthelyi, L.P., McEwen, A.S., Turtle, E.P., Jaeger, W., Milazzo, M. [2001]. J. Geophys. Res. 106, 33005-33020). Although these features cover only 2.5% of Io’s surface, they correspond to 64% of all detected hot spots; 45% of all hot spots are associated with the freshest dark patera floors, reflecting the importance of active silicate volcanism to Io’s heat flow. (4) Mountains cover only ∼3% of the surface, although the transition from mountains to plains is gradational with the available imagery. 49% of all mountains are lineated and presumably layered, showing evidence of linear structures supportive of a tectonic origin. In contrast, only 6% of visible mountains are mottled (showing hummocks indicative of mass wasting) and 4% are tholi (domes or shields), consistent with a volcanic origin. (5) Initial analyses of the geographic distributions of map units show no significant longitudinal variation in the quantity of Io’s mountains or paterae, in contrast to earlier studies. This is because we use the area of mountain and patera materials as opposed to the number of structures, and our result suggests that the previously proposed anti-correlation of mountains and paterae (Schenk, P., Hargitai, H., Wilson, R., McEwen, A., Thomas, P. [2001]. J. Geophys. Res. 106, 33201-33222; Kirchoff, M.R., McKinnon, W.B., Schenk, P.M. [2011]. Earth Planet. Sci. Lett. 301, 22-30) is more complex than previously thought. There is also a slight decrease in surface area of lava flows toward the poles of Io, perhaps indicative of variations in volcanic activity. (6) The freshest bright and dark flows make up about 29% of all of Io’s flow fields, suggesting active emplacement is occurring in less than a third of Io’s visible lava fields. (7) About 47% of Io’s diffuse deposits (by area) are red, presumably deriving their color from condensed sulfur gas, and ∼38% are white, presumably dominated by condensed SO2. The much greater areal extent of gas-derived diffuse deposits (red + white, 85%) compared to presumably pyroclast-bearing diffuse deposits (dark (silicate tephra) + yellow (sulfur-rich tephra), 15%) indicates that there is effective separation between the transport of tephra and gas in many Ionian explosive eruptions. Future improvements in the geologic mapping of Io can be obtained via (a) investigating the relationships between different color/material units that are geographically and temporally associated, (b) better analysis of the temporal variations in the map units, and (c) additional high-resolution images (spatial resolutions ∼200 m/pixel or better). These improvements would be greatly facilitated by new data, which could be obtained by future missions. 相似文献