The tectonic and volcanic history of Dione

The tectonic and volcanic modifications of Dione are described and interpreted. It is proposed that after the formation of a brittle outer shell, but before the end of heavy meteoritic bombardment, global expansion due to radionuclide heating (and perhaps a loss of oblateness due to tidal despinning and orbital recession) produced a global system of lineaments. An NH₃ · H₂O melt was produced and “erupted” on the surface to form plains units. Cooling of the interior (or a phase change) led to horizontal compression in the surface. Compression of a thick deposit of plains material, possibly overlying a décollement surface, is proposed to explain cratered-plains ridges developed as thrust or high-angle reverse faults. Following formation of ridges and smooth plains, the surface experienced light cratering.     

Global fracture patterns of a despun planet: Application to Mercury

We have determined the global fracture patterns resulting from combinations of stresses due to tidal despinning and contraction or expansion. We find that Mercury's lineament pattern is consistent with a history of despinning and contraction. According to our model, the observed tectonic pattern implies that the despinning process reached completion before the planet ceased contracting. Our model predicts a stress due to contraction which is up to 1.8 times the maximum despinning stress on Mercury. The maximum contractional stress could be as large as 4 times the maximum despinning stress if the oldest fractures on the planet are N-S thrust faults in the equatorial region.     

East-west faults due to planetary contraction

Contraction, expansion and despinning have been common in the past evolution of Solar System bodies. These processes deform the lithosphere until it breaks along faults. Their characteristic tectonic patterns have thus been sought for on all planets and large satellites with an ancient surface. While the search for despinning tectonics has not been conclusive, there is good observational evidence on several bodies for the global faulting pattern associated with contraction or expansion, though the pattern is seldom isotropic as predicted. The cause of the non-random orientation of the faults has been attributed either to regional stresses or to the combined action of contraction/expansion with another deformation (despinning, tidal deformation, reorientation). Another cause of the mismatch may be the neglect of the lithospheric thinning at the equator or at the poles due either to latitudinal variation in solar insolation or to localized tidal dissipation. Using thin elastic shells with variable thickness, I show that the equatorial thinning of the lithosphere transforms the homogeneous and isotropic fault pattern caused by contraction/expansion into a pattern of faults striking east-west, preferably formed in the equatorial region. By contrast, lithospheric thickness variations only weakly affect the despinning faulting pattern consisting of equatorial strike-slip faults and polar normal faults. If contraction is added to despinning, the despinning pattern first shifts to thrust faults striking north-south and then to thrust faults striking east-west. If the lithosphere is thinner at the poles, the tectonic pattern caused by contraction/expansion consists of faults striking north/south. I start by predicting the main characteristics of the stress pattern with symmetry arguments. I further prove that the solutions for contraction and despinning are dual if the inverse elastic thickness is limited to harmonic degree two, making it easy to determine fault orientation for combined contraction and despinning. I give two methods for solving the equations of elasticity, one numerical and the other semi-analytical. The latter method yields explicit formulas for stresses as expansions in Legendre polynomials about the solution for constant shell thickness. Though I only discuss the cases of a lithosphere thinner at the equator or at the poles, the method is applicable for any latitudinal variation of the lithospheric thickness. On Iapetus, contraction or expansion on a lithosphere thinner at the equator explains the location and orientation of the equatorial ridge. On Mercury, the combination of contraction and despinning makes possible the existence of zonal provinces of thrust faults differing in orientation (north-south or east-west), which may be relevant to the orientation of lobate scarps.     

Global tectonics of a despun planet

Mercury, the Moon, and many large satellites of the major planets have been tidally despun from an initially faster rotation. These bodies probably possessed equatorial bulges which relaxed as they lost their spin. An analysis of the stresses induced in an elastic shell by the relaxation of an equatorial bulge indicates that differential stresses may reach a few kilobars and that the tectonic pattern developed depends mainly upon the shell thickness. In every model studied the azimuthal stress σ_?? is larger (more compressive) than the meridional stress σθθ. For a thin elastic shell (thickness less than one-twentieth of the planet's radius) the zone from the equator to 48° latitude is characterized by strike-slip faulting. Poleward of this, normal faults and graben trending east-west are expected. Thicker elastic shells acquire an equatorial belt of thrust faults with east-west throw and rough north-south trends. These tectonic styles may be modified by a small (0.05-0.1%) radial expansion or contraction. Expansion shifts the polar normal faulting province toward the equator, while contraction shifts the equatorial provinces poleward. These patterns are not substantially altered by plastic yielding of the shell, although the equatorial thrust fault province is suppressed by strike-slip faulting until strike-slip faults occur poleward of 64.8° latitude. We conclude that there are many tectonic patterns consistent with despinning and radial contraction or expansion, but they must all be consistent with σ_?? > σθθ. These results also indicate that the polar regions of a despun planet are of particular interest in deciding whether a given lineament system is due to stresses induced by the relaxation of the planet's equatorial bulge.     

Despinning plus global contraction and the orientation of lobate scarps on Mercury: Predictions for MESSENGER

Observations by the Mariner 10 spacecraft suggest that the lobate scarps on Mercury, which have been interpreted to record at most 1-2 km of radial contraction of the planet after the end of the Late Heavy Bombardment, possess a global, preferred N-S orientation but lack a strong latitudinal dependence on their surface expression. Here, we reexamine the idea that a decrease in the planetary rotation rate (despinning) coupled with global contraction of at least 3-5.5 km prior to the end of Late Heavy Bombardment resulted in global N-S oriented thrust faults. The surface expression of these faults is assumed to have been erased by the end of the Late Heavy Bombardment, and the faults were subsequently reactivated by later global contraction, producing generally N-S oriented thrust faults from an isotropic stress field. We use the estimate of >3-5.5 km contraction prior to ∼4 Ga as an additional constraint to thermomechanical simulations of the evolution of Mercury, finding that a wide range of models are consistent with this observation. The fact that a wide range of states are consistent with the contraction of Mercury prior to the end of Late Heavy Bombardment but only a restricted set of states are consistent with the at most 1-2 km of subsequent contraction bolsters the idea that there may be hidden strain on Mercury, features unseen by Mariner 10 but likely visible to the MESSENGER spacecraft.     

Tectonic patterns on a tidally distorted planet

Tidal deformation of the lithosphere of a synchronously rotating planet or satellite produces stresses that may result in a distinctive tectonic pattern. The lithosphereis treated as a thin elastic shell which maintains the equilibrium shape of a tidally distorted body. Stresses develop as the equilibrium shape changes during orbital evolution. E. M. Anderson's theory of faulting is used to translate this stress pattern into a tectonic pattern of faults on the planet's surface (The Dynamics of Faulting, Oliver & Boyd, Edinburgh, 1951). On a body such as the Moon, which has receded from the Earth, an originally large tidal bulge has collapsed. The predicted tectonic pattern includes N-S striking thrust faults over an area extending roughly 30° in latitude and longitude around the sub-Earth point and its antipode. The polar regions above roughly 70° latitude exhibit normal faults striking from the near side of the Moon toward the far side. Strike slip faults, with offsets consistent with east-west compression, occur near the limbs. Stress differences are largest at the equator on the limbs, and may have reached several hundreds bars over the last few billion years of the Moon's history. The existence of such a tectonic pattern on the Moon can only be resolved by photogeologic mapping. At present, there is little evidence of this pattern; however, the crucial evidence probably lies in the poorly mapped lunar polar regions. These tectonic patterns, which could provide geologic evidence for large tidal distortions, may also be present on the Galilean satellites of Jupiter.     

Tectonics on Iapetus: Despinning, respinning, or something completely different?

Saturn’s moon Iapetus is unique in that it has apparently despun while retaining a substantial equatorial bulge. Stresses arising from such a non-hydrostatic shape should in principle cause surface deformation (tectonics). As part of a search for such a tectonic signature, lineaments (linear surface features) on Iapetus were mapped on both its bright and dark hemispheres. Lineament orientations were then compared to model stress patterns predicted for spin-down from a rotation period of 16.5 h (or less) to its present synchronous period, and for a range of lithospheric thicknesses. Many lineaments are straight segments of crater rimwalls, which may be faults or joints reactivated during complex crater collapse. Most striking are several large troughs on the bright, trailing hemisphere. These troughs appear to be extensional and are distinctive on that hemisphere, because the interior floors and walls of the troughs contain dark material. Globally, no specific evidence of strike slip or thrust offsets are seen, but this could be due to the age and degraded nature of any such features. We find that observed lineament orientations do not correlate with predicted patterns due to despinning on either hemisphere (the equatorial ridge was specifically excluded from this analysis, and is considered separately). Modest evidence for preferred orientations ±40° from north could be construed as consistent with respinning, which is not necessarily far-fetched. Assuming the rigidity of unfractured ice, predicted maximum lithospheric differential stresses from despinning range from ∼1 MPa to ∼160 MPa for the elastic spheroid and thin lithosphere limits, respectively (although it is only for thicker elastic lithospheres that we expect a nonhydrostatic state to be maintained over geologic time against lithospheric failure). The tectonic signature of despinning may have been obscured over time because the surface of Iapetus is very ancient, Iapetus’ thick lithosphere may have inhibited the full tectonic expression of despinning, or both. Several prominent lineaments strike E–W, and are thus parallel to the equatorial ridge (though not physically close to it), but a tectonic or volcanic origin for the ridge is highly problematic.     

Global contraction of planetary bodies due to despinning: Application to Mercury and Iapetus

We extend previous work on the global tectonic patterns generated by despinning with a self-consistent treatment of the isotropic despinning contraction that has been ignored. We provide simple analytic approximations that quantify the effect of the isotropic despinning contraction on the global shape and tectonic pattern. The isotropic despinning contraction of Mercury is ∼93 m (T/1 day)⁻², where T is the initial rotation period. If we take into account both the isotropic contraction and the degree-2 deformations associated with despinning, the preponderance of compressional tectonic features on Mercury’s surface requires an additional isotropic contraction ?1 km (T/1 day)⁻², presumably due to cooling of the interior and growth of the solid inner core. The isotropic despinning contraction of Iapetus is ∼9 m (T/16 h)⁻², and it is not sensitive to the presence of a core or the thickness of the elastic lithosphere. The tectonic pattern expected for despinning, including the isotropic contraction, does not explain Iapetus’ ridge. Furthermore, the ridge remains unexplained with the addition of any isotropic compressional stresses, including those generating by cooling.     

Tectonic patterns on reoriented and despun planetary bodies

Several processes may produce global tectonic patterns on the surface of a planetary body. The stresses associated with distortions of biaxial figures due to despinning or reorientation were first calculated by Vening Meinesz [Vening Meinesz, F.A., 1947. Trans. Am. Geophys. Union 28 (1), 1-23]. We adopt a mathematically equivalent, but physically more meaningful treatment for distortions associated with rotation. The new approach allows us to find analytic solutions for the general case of stresses associated with distortions of biaxial or triaxial planetary figures. Distortions of biaxial figures may be driven by variations in rotation rate, rotation axis orientation, or the combination of both. Distortions of triaxial figures may be driven by the same mechanisms and/or variations in tidal axis orientation for tidally deformed satellites. While the magnitude of the resulting stresses depends on the adopted elastic and physical parameters, the expected tectonic pattern is independent of these parameters for these mechanisms. Reorientation of the rotation/tidal axis alone is expected to produce normal/thrust faulting provinces enclosing the initial rotation/tidal poles, and thrust/normal faulting provinces enclosing the final rotation/tidal poles. Reorientation of both the rotation and tidal axis results in a wide variety of tectonic patterns for different reorientation geometries. On Europa, the tidal axis reorientation which generally accompanies rotation axis reorientations may provide an alternative explanation for tectonic features that have been interpreted as evidence for non-synchronous rotation. The observed tectonic pattern on Enceladus is more easily explained by a large reorientation (∼90°) of the rotation axis, than by rotation rate variations.     

Geologic evolution and cratering history of Mercury

Among the terrestrial planets, Mercury is the smallest and has the highest bulk density. Mercury exhibits a lunar-like surface, shaped by impact basins and craters. Rapid cooling and contraction as well as tidal despinning have resulted in a large inventory of tectonic scarps and faults visible on the surface. With plans for new orbiter missions to this intriguing planet taking shape, this paper presents a summary of our current knowledge on Mercury's geology and cratering history. On the basis of improved data on asteroid populations and crater scaling, we updated the time stratigraphic sequence for the planet and made new estimates for the time of formation of impact basins such as Tolstoj and Caloris, which generally are now thought to be younger than in previous estimates. In order to advance our understanding of the geology of the planet, imaging experiments on future missions must fill the gap in the global coverage left by the Mariner spacecraft, and increase the global multispectral spatial resolution to at least 100 m/pixel. Locally, the image resolution must reach approx. 10 m/pixel. Also, stereo topographic models with global and local resolutions of 200 and 20 m, respectively, are required.     

Structural control of scarps in the Rembrandt region of Mercury

Lobate scarps, thought to be the surface expression of large thrust faults, are the most spectacular contractional tectonic features visible on Mercury. Most lobate scarps follow a general and relatively simple pattern, with a roughly arcuate or linear form in plan view, and an asymmetric cross section characterized by a steeply rising scarp face and a gently declining back scarp. In this work, we study two peculiar and complex scarps in the Rembrandt region of Mercury through MESSENGER imagery. On the one hand, the formation of these scarps resulted in the deformation of features such as impact craters, fractures, extensional faults, and volcanic plains, while on the other hand, the deformed features partly influenced the formation of the scarps. Evidence for structural control on the formation of the scarps includes their orientation, segmentation, bifurcation, change in structural trend and dip orientation, and transition into high-relief ridges or wrinkle ridge morphologies in some cases. Thus, these two lobate scarps provide examples of complex geological relations among other features, expanding the recognized richness of mercurian geology. Also, the southern scarp records a complex history of contraction, suggesting that the development of some mercurian lobate scarps may be more complex than usually thought.     

Tectonic pattern of the Crüger/Rima Sirsalis -Rima de Gasparis region of the moon

Tectonic lunar units were studied in an area of about 540 000 km² in the southwestern part of the Moon's visible disk. The area is situated in the vicinity of Mare Humorum, Oceanus Procellarum, and Mare Orientale. Zones indicated by lineament sets show three preferred orientations (about N55°W, N10°W, N35°E). Probably these zones coincide with deep faults that have been active over a long period of lunar development. The distribution of Eratosthenian and partly also Imbrian lavas coincides with the zones along which re-activations have taken place. This is best seen in the areas where strong zones intersect each other. Local catastrophic forces must also be taken into account. The trends, distribution, and age relations of tectonic zones can be explained by a combination of maximum principal stress in direction about N10°W coupled possibly with old tidal forces and local (?) volcanic expansion of lunar mantle during the last stages of flooding of Oceanus Procellarum.     

Tectonic patterns on a reoriented planet: Mars

Both geologic and free-air-gravity data suggest that the positive mass anomaly associated with the Tharsis volcanoes may have reoriented Mars' lithosphere by as much as 25°. Since Mars is oblate (with flattening $\text{? ?0.005}$), rotation of the lithosphere over the equatorial bulge by 25° produces membrane stresses of several kilobars, large enough to initiate faulting. These stresses were first evaluated by F.A. Vening-Meinesz (1947, Trans. Amer. Geophys. Union28, 1–61) who treated the lithosphere as a thin elastic shell. The fracture patterns which result from these stresses are determined by the relation between stress and faulting proposed by E.M. Anderson (1951, The Dynamics of Faulting, Oliver & Boyd, Edinburgh). Plots of the magnitude and direction of stresses in a reoriented planet show that near Tharsis the dominant fault type should be north-south- trending normal faults. This normal fault province is centered about 30°N latitude and extends about 45° east and west in longitude. Similar faults should occur at the antipodes, north of Hellas Planitia. The polar regions should be occupied by roughly north-south-trending thrust faults which extend close to the equator south of Tharsis and north of Hellas. The regions between Tharsis and Hellas are subject to compression on a NE-trending axis and extension along a NW axis east of Tharsis (west of Tharsis the directions are NW compression and NE extension), thus predicting a zone of NNW and ENE strike slip faults east of Tharsis (NNE and WNW west of Tharsis). Although these patterns, except for the north-south normal faults north of Tharsis, have not yet been recognized, the discovery of such a tectonic system of the same age as Tharsis would provide strong support for the reorientation idea. Stresses due to reorientation appear to have little to do with Valles Marineris, since the stress normal to the axis of the Valles is predicted to be compressive, whereas geologic evidence suggests extension.     

The Rotation of Europa

Theoretical predictions of non-synchronous rotation and of polar wander on Europa have been tested by comparing tectonic features observed in Voyager and Galileo spacecraft images with tidal stresses. Evidence for non-synchronous rotation comes from studying changes in global scale lineaments formed over time, from the character of strike-slip faults, and from comparison of distinctively shaped cycloidal cracks with the longitudes at which such shapes should have formed, in theory. The study of cycloids constrains the rotation period (relative to the direction of Jupiter) to less than 250 000 years, while direct comparison of the orientation of Europa in Voyager and Galileo images shows the rotation is slow, with a period of >12 000 years. Comparison of strike-slip faults with their theoretical locations of formation provides evidence for substantial polar wander, supported by the distribution of various thermally produced features.     

Patterns of fracture and tidal stresses on Europa

the hypothesis that lineaments on Europa are fractures produced by tidal distortion and planetary volume change is examined by comparing the orientations of dark bands, triple bands, and cuspate ridges to fracture patterns predicted for tidal distortion due to orbital recession and orbital eccentricity. If short, reticulate dark band nnear the anti-Jove point are tension cracks which formed in response to tidal distortion, they could only have been produced by orbital eccentricity. Long, arcuate dark band and triple bands peripheral to the anti-Jove point orientations which suggest that they are strike-slip faults which formed in response to orbital recession. If cuspate ridges are compressional features, their orientations and distribution suggest that they formed in response to combined orbital recession and a decrease in planetary volume. Stresses due to orbital eccentricit could have produced tension cracks near the anti-Jove point only if tensile failure occurred either prior to the accumulation of orbital recession stresses or after they had relaxed. Surface fracturing, if a consequence of tidal deformation, places important constraints on the orbital evolution of Europa.     

Resurfacing history of Europa from pole-to-pole geological mapping

We produced geologic maps from two regional mosaics of Galileo images across the leading and trailing hemispheres of Europa in order to investigate the temporal distribution of units in the visible geologic record. Five principal terrain types were identified (plains, bands, ridges, chaos, and crater materials), which are interpreted to result from (1) tectonic fracturing and lineament building, (2) cryovolcanic reworking of surface units, with possible emplacement of sub-surface materials, and (3) impact cratering. The geologic histories of both mapped areas are essentially similar and reflect some common trends: Tectonic resurfacing dominates the early geologic record with the formation of background plains by intricate superposition of lineaments, the opening of wide bands with infilling of inter-plate gaps, and the buildup of ridges and ridge complexes along prominent fractures in the ice. It also appears that lineaments are narrower and more widely spaced with time. The lack of impact craters overprinted by lineaments indicate that the degree of tectonic resurfacing decreased rapidly after ridged plains formation. In contrast, the degree of cryovolcanic resurfacing appears to increase with time, as chaos formation dominates the later parts of the geologic record. These trends, and the transition from tectonic- to cryovolcanic-dominated resurfacing could be attributed to the gradual thickening of Europa's cryosphere during the visible geologic history, that comprises the last 2% or 30-80 Myr of Europa's history: An originally thin, brittle ice shell could be pervasively fractured or melted through by tidal and endogenic processes; the degree of fracturing and plate displacements decreased with time in a thickening shell, and lineaments became narrower and more widely spaced; formation of chaos regions could have occurred where the thickness threshold for solid-state convection was exceeded, and can be aided by preferential tidal heating of more ductile ice. In a long-term context it is not clear at this point whether this inferred thickening trend would reflect a drastic change in the thermal evolution of the satellite, or cyclic or irregular episodes of tectonic and cryovolcanic activity.     

Magnitude of global contraction on Mars from analysis of surface faults: Implications for martian thermal history

Faults provide a record of a planet’s crustal stress state and interior dynamics, including volumetric changes related to long-term cooling. Previous work has suggested that Mars experienced a pulse of large-scale global contraction during Hesperian time. Here we evaluate the evidence for martian global contraction using a recent compilation of thrust faults. Fault-related strains were calculated for wrinkle ridges and lobate scarps to provide lower and upper bounds, respectively, on the magnitude of global contraction from contractional structures observed on the surface of Mars. During the hypothesized pulse of global contraction, contractional strain of −0.007% to −0.13% is indicated by the structures, corresponding to decreases in planetary radius of 112 m to 2.24 km, respectively. By contrast, consideration of all recognized thrust faults regardless of age produces a globally averaged contractional strain of −0.011% to −0.22%, corresponding to a radius decrease of 188 m to 3.77 km since the Early Noachian. The amount of global contraction predicted by thermal models is larger than what is recorded by the faults at the surface, paralleling similar studies for Mercury and the Moon, which suggests that observations of fault populations at the surface may provide tighter bounds on planetary thermal evolution than models alone.     

Young thrust-fault scarps in the highlands: evidence for an initially totally molten moon

Thermoelastic stress calculations show that if only the outer few hundred kilometers of the Moon was initially molten and if it had a cool interior, i.e., the magma ocean model of the Moon, the highlands should not have any young, compressional tectonic features. In contrast, if the Moon was initially totally molten, the highlands should have 10-km- scale, ?0.5- to 1 × 10⁹-year-old thrust faults. Observations using the Apollo panoramic imagery show that young thrust faults do exist in the highlands. Extrapolation of the data suggests that some 2000 thrust-fault scarps, whose average length is 9 km, are in the highlands. The fault scarps generally occur in series or complexes of four or five scarps. The average length of these complexes is 50 km; the largest observed complex is 120 km long. Extrapolation of the data suggests that there are about 400 such complexes. The ages of the scarps range from 60±30 to 680±250 my, with a possible bias of up to plus a factor of 2 or minus a factor of 4. These scarps are by far the youngest endogenic features on the Moon. The selenographical, size, age, morphological, and azimuth frequency distributions of the scarps can be explained by the effects of the kilobar-level thermoelastic stresses, the 100-bar-level tidal and rotational stresses, and influence by preexisting structures. These results show that the Moon has recently entered an epoch of late stage, global tectonism and favor the concept that the Moon was initially totally molten.     

A comparison of the cratered terrain escarpments on Miranda with the thrust faults on Mercury

Large escarpments on the Uranian satellite Miranda are similar in structure to thrust faults on Mercury result of crustal contraction. The tectonic lineations may be considered evidence of a crustal contraction on Miranda result of differentiation of the satellite.     

Patterns of fracture and tidal stresses due to nonsynchronous rotation: Implications for fracturing on Europa

This study considers the global patterns of fracture that would result from nonsynchronous rotation of a tidally distorted planetary body. The incremental horizontal stresses in a thin elastic or viscous shell due to a small displacement of the axis of maximum tidal elongation are derived, and the resulting stress distributions are applied to interpret the observed pattern of fracture lineaments on Europa. The observed pattern of lineaments can be explained by nonsynchronous rotation if these features formed by tension fracturing and dike emplacement. Tension fracturing can occur for a small displacement of the tidal axis, so that the resulting lineaments may be consistent with other evidence suggesting a young age for the surface.