Martian Lithosphere Research Articles

The case for old basaltic shergottites

The crystallization age of shergottites is currently not agreed upon. Although mineral 87Rb– 87Sr, 147Sm– 143Nd, 176Lu– 176Hf, and U–Pb isochrons all give very young ages, typically in the range of 160–180 Ma, 207Pb– 206Pb data support a much older crystallization age at 4.1 Ga, which is consistent with published whole-rock 87Rb– 87Sr data on basaltic shergottites. Different isotopic systems present different complexities, but crater-counting chronology, which shows that a substantial fraction of the Martian surface was resurfaced during the late heavy bombardment, is in favor of an old Martian lithosphere with ages in accordance with Pb–Pb and Rb–Sr isotopic data. A ∼ 4.1 Ga Pb–Pb age of shergottites also agrees with the 142Nd and 182W anomalies found in these rocks and concur with the presence of an actively convecting mantle during the first 500 Myr of the planet's history. We here present new Sm–Nd, Lu–Hf, and Pb–Pb mineral isochrons for the basaltic shergottites Shergotty and Los Angeles complementing our previous results on Zagami [Bouvier A., Blichert-Toft J., Vervoort J.D. and Albarède F. (2005). The age of SNC meteorites and the antiquity of the Martian surface, Earth Planet. Sci. Lett. 240, 221–233]. The internal 147Sm– 143Nd and 176Lu– 176Hf isochrons give young ages of, respectively, 172 ± 40 (MSWD = 2.0) and 188 ± 91 (MSWD = 3.1) for Shergotty, and 181 ± 13 (MSWD = 0.14) and 159 ± 42 (MSWD = 0.01) for Los Angeles. In contrast, the Pb isotope compositions of the leached whole-rock fragments and maskelynite separates of Shergotty and Los Angeles fall on the whole-rock isochron previously established for Zagami and other shergottite samples and collectively yield a Pb–Pb age of 4050 ± 70 Ma for the crystallization of the basaltic shergottite suite. The contrast between the ∼ 170 Ma ages of internal isochrons and the 4.1 Ga age supported by Pb–Pb and 87Rb– 87Sr on whole-rocks simply reflects that the younger age dates the perturbation of a suite of rocks of Noachian age. The internal Rb–Sr, Sm–Nd, Lu–Hf, and U–Pb errorchrons are heavily biased by the presence of disseminated phosphate minerals and inclusions, for which D/H ratios ( δD~ + 4600‰) indicate strong interaction with Martian subsurface waters. In contrast, baddeleyite, occasionally present in SNCs and also having extremely young U–Pb ages, reflect resetting under the shock conditions that prevailed either during shergottite extraction from the planet or from impacts associated with a major break-up event of planetesimals in the main asteroid belt. We finally also re-examine 39Ar– 40Ar data on SNC meteorites and suggest that they as well support old crystallization ages.

Phase equilibrium investigations of the Adirondack class basalts from the Gusev plains, Gusev crater, Mars

Abstract— Phase equilibrium experiments have been performed on a synthetic analog of the Gusev plains basalt composition from the Spirit landing site on Mars. Near‐liquidus phase relations were determined over the pressure range of 0.1 to 1.5 GPa and at temperatures from 1125 to 1390 °C in a piston cylinder apparatus and 1 atm gas mixing furnace. The composition is multiply saturated with olivine, orthopyroxene, and spinel near its liquidus at 1320 °C and 1.0 GPa, or 85 km depth on Mars, placing an upper limit constraint on the thickness of the Martian lithosphere at the time of eruption. Our experimental work suggests that the Gusev basalts are anhydrous batch melts of a primitive Martian mantle similar to the composition estimated by Dreibus and Wänke (1984). The temperature of multiple saturation indicates the persistence of high mantle potential temperatures on Mars, similar to those on the modern Earth, until at least the very latest Noachian (3.7 Ga). These high mantle temperatures would be responsible for persistent basaltic volcanism throughout the southern highlands during the first billion years of Mars's history. The source for Gusev basalts differs strongly from the source for shergottite meteorites, reinforcing the idea of the absence of global mantle convection and mixing on Mars. The existence of a relatively primitive mantle reservoir requires that at least part of the mantle underwent little modification during early planetary differentiation.

Mechanical modeling of thrust faults in the Thaumasia region, Mars, and implications for the Noachian heat flux

Insight into the state of the early martian lithosphere is gained by modeling the topography above surface breaking thrust faults in the southern Thaumasia region. Crater counts of key surface units associated with the faulting indicate a scarp emplacement in the late Noachian–early Hesperian periods between 4.0 and 3.7 Gyr. The seismogenic layer thickness at the time of faulting is constrained to 27–35 km and 21–28 km for the two scarps investigated, implying paleo geothermal gradients of 12–18 and 15–23 K km−1, corresponding to heat flows of 24–36 and 30–46 mW m−2. The heat flow values obtained in this study are considerably lower than those derived from rift flank uplift at the close-by Coracis Fossae for a similar time period, indicating that surface heat flow is a strong function of regional setting. If viewed as representative for magmatically active and inactive regions, the thermal gradients at rifts and scarps span the range of admissible global mean values. This implies dT/dz=17–32 Kkm−1, with the true value probably being closer to the lower bound.

Evidences for a Noachian–Hesperian orogeny in Mars

A number of tectonic structures have been located at the Thaumasia Plateau, Daedalia Planum and Aonia Terra, Mars. They include isolated folds with axial traces up to 200 km long, trains of tightly folded structures tens of km long, and thrusts. Their size and geometry are similar to those on Earth, and the direction of compression seems to have varied amply with time, suggesting a complex tectonic evolution. Crater counts on the deformed terrain point to Noachian to Early Hesperian ages. On the basis of the geometry and geological relations of these structures, we propose that they form part of an old martian orogen, the Thaumasia–Aonia Orogen, which embraced not only the Thaumasia Plateau, but areas of Daedalia Planum, Aonia Terra and Nereidum Montes as well. A regional coherent layering is previous to the deformation and could represent the trace of even older stresses on the martian lithosphere.

Constraints on the Martian lithosphere from gravity and topography data

Localized spectral admittances of the large Martian volcanoes are modeled by assuming that surface and subsurface loads are elastically supported by the lithosphere. In order to model the case where the load density differs from that of the crust, a new method for calculating gravity anomalies and lithospheric deflections is developed. The modeled gravity anomalies depend upon the elastic thickness, crustal thickness, load density, and crustal density, and these parameters were exhaustively sampled in order to determine their effect on the misfit between the observed and modeled admittance function. We find that the densities of the Martian volcanoes are generally well constrained with values of 3200 ± 100 kg m−3, which is considerably greater than those reported previously. These higher densities are consistent with those of the Martian basaltic meteorites, which are believed to originate from the Tharsis and Elysium volcanic provinces. The crustal density is constrained only beneath the Elysium rise to be 3270 ± 150 kg m−3. If this value is representative of the northern lowlands, then Pratt compensation is likely responsible for the approximately 6‐km elevation difference between the northern and southern hemispheres. The elastic thicknesses of the major Martian volcanoes (when subsurface loads are ignored) are found to be the following: Elysium rise (56 ± 20 km), Olympus Mons (93 ± 40 km), Alba Patera (66 ± 20 km), and Ascraeus Mons (105 ± 40 km). We have also investigated the effects of subsurface loads, allowing the bottom load to be located either in the crust as dense intrusive material or in the mantle as less dense material. We found that all volcanoes except Pavonis are better modeled with the presence of less dense material in the upper mantle, which is indicative of either a mantle plume or a depleted mantle composition. An active plume beneath the major volcanoes is consistent with recent analyses of cratering statistics on Olympus Mons and the Elysium rise, which indicate that some lava flows are as young as 10–30 Myr, as well as with the crystallization ages of the Shergottites, of which some are as young as 180 Myr.

Thrust fault vergence directions on Mars: A foundation for investigating global‐scale Tharsis‐driven tectonics

Insight into the mechanical and thermal structure of the Martian lithosphere and the character of Tharsis‐driven tectonics is gained from detailed mapping of the global distribution of thrust fault vergence directions with respect to the center of the Tharsis tectono‐volcanic province. Our results reveal sub‐equal frequencies of thrust faults that verge away from and toward Tharsis. Based on the observed distribution of thrust fault vergence directions, we conclude that the base of the Tharsis load was effectively welded, by frictional and cohesive strength, to the subjacent Noachian crust throughout its periods of sustained volcanism and faulting during the late Noachian to early Hesperian, rather than being detached along a ductile layer at its base.

Amplitude of heat flow variations on Mars from possible shoreline topography

Analyses of the effective elastic thickness of the Martian lithosphere have been previously used to calculate surface heat flow on Mars at different places and times. In this work, I use elevation differences in a putative Late Hesperian shoreline, named Deuteronilus shoreline, and the relation between thermal state and buoyancy of the lithosphere, in order to estimate the amplitude of the variations of surface heat flow on Mars, probably related to the time in which this feature was formed. The results suggest that, if the Deuteronilus shoreline is a true paleo‐equipotential surface, the relative amplitude of surface heat flow variations on the shoreline regions in the Late Hesperian were less than present‐day ones in terrestrial continental areas. The results are also roughly valid for the outer contact of the Late Hesperian Vastitas Borealis Formation, if this contact is related to the limits of an ancient ocean. These results could imply that large areas of the Martian lithosphere have been tectonothermally stable since at least that time.

Thermoremanent magnetization of the Martian lithosphere

This paper investigates the thermoremanent magnetization (TRM) of the Martian lithosphere assuming that the upper part of the lithosphere acquired TRM in the early history of the planet and in the presence of the core field (the primary magnetization), whereas the lower part has been gradually magnetized by the magnetic field of the upper part as it has cooled below the Curie temperature (secondary magnetization). The secondary magnetization is relatively weak if the magnetic properties of the lower lithosphere are taken to be similar to that of the extrusive basalt near the oceanic ridge axes on Earth. However, if the Martian lower lithosphere is more magnetic, e.g., by an order of magnitude, the secondary magnetization can be significant, although still weaker than the primary magnetization of the upper part. Such a highly magnetic lower lithosphere does not seem plausible by terrestrial standards, and it remains to be confirmed whether it is geologically plausible for Mars. I also examine a lithospheric model with a strongly magnetic layer beneath the crust, 25 times more magnetic than the extrusive basalt, representing a possible ilmenite‐rich layer with a considerable amount of hematite‐ilmenite lamellae. I assume that the layer was not magnetized by the core field because it was hotter than the Curie temperature of the lamellae when the core dynamo was active. It acquired secondary magnetization later in the presence of the magnetic field of the crust. Despite adopting an extraordinarily high magnetic layer which seems implausible by terrestrial standards, the secondary magnetization of the layer has minor contributions to the observed magnetic anomalies. In all models considered the primary magnetization is ∼20–30 A/m and has the dominant contribution to the observed magnetic anomalies. It is worth noting that the radial components of the primary and secondary magnetization are parallel, whereas the tangential components are in opposite directions. Because the strong magnetic anomalies in the south likely arise from the tangential magnetization, considering the secondary magnetization requires even stronger primary magnetization.

Seismotectonics of the Amenthes Rupes thrust fault population, Mars

Thrust faults within the Amenthes Rupes population in eastern Mars accommodated contractional strains of ∼0.06% with total cumulative moment (energy) release of ∼2 × 1024MJ during Late Noachian time. “Small” faults, that scale three‐dimensionally, accommodate only 12–17% of the strain and 6–9% of the moment release, indicating that inventories of Martian faults need not be complete down to the smallest lengths for meaningful estimates of strain and moment to be obtained. Frictional stability relations for Martian lithosphere suggest that most (83%) of the largest fault, Amenthes Rupes, was seismogenic during displacement accumulation, with smaller faults being progressively less seismogenic as shallower depths are penetrated with decreasing fault lengths. Given plausible displacement rates from terrestrial intraplate thrust faults of ∼1–0.01 mm yr−1, the Amenthes fault population may have taken as little as 1.5 m.y. to grow into its present configuration, corresponding to slow homogeneous strain rates of 10−17s−1, or less, that are comparable to those of terrestrial plate interiors.

Stress differences in the Martian lithosphere: Constraints on the thermal state of Mars

Depth‐dependent density perturbations are determined inside the elastic part of the Martian lithosphere (assumed to be 300 km thick) using a thick elastic shell model, such that the deformed model under the loads of surface topography and the density perturbations gives rise to the observed topography and gravity anomalies of Mars specifies by spherical harmonics of degree 3–50. Large positive density perturbations, as high as ∼150 kg/m3, exist in the crust that are associated with Hellas, Isidis, Argyre, and Utopia basins, reflecting the mantle uplift in response to the impact excavation. Also, large positive density perturbations exist in the crust beneath Olympus (∼350 kg/m3) and Tharsis monts (∼200 kg/m3), indicating that these shield volcanoes are not isostatically compensated. On the other hand, negative density perturbations are associated with Alba Patera, Elysium rise, and the highland surrounding Hellas basin. The density perturbations gradually diminish with depth, becoming less than 20 kg/m3 by 250–300 km depth. The resulting stress differences within the crust and upper mantle are dominated by Olympus (up to 100 MPa), whereas other shield volcanoes create lesser stress differences. We estimate the brittle‐ductile transition temperature in the lithosphere, adopting the creep law of dry olivine for the rheology of the Martian lithosphere. The brittle‐ductile transition temperature beneath Olympus is ∼800C throughout the lithosphere, indicating that the upper parts of Mars must have been cold to support this volcano for 1–2 Gyr.

The rocks of Mars, from far and near

Abstract—The age, structure, composition, and petrogenesis of the martian lithosphere have been constrained by spacecraft imagery and remote sensing. How well do martian meteorites conform to expectations derived from this geologic context? Both data sets indicate a thick, extensive igneous crust formed very early in the planet's history. The composition of the ancient crust is predominantly basaltic, possibly andesitic in part, with sediments derived from volcanic rocks. Later plume eruptions produced igneous centers like Tharsis, the composition of which cannot be determined because of spectral obscuration by dust. Martian meteorites (except Allan Hills 84001) are inferred to have come from volcanic flows in Tharsis or Elysium, and thus are not petrologically representative of most of the martian surface. Remote‐sensing measurements cannot verify the fractional crystallization and assimilation that have been documented in meteorites, but subsurface magmatic processes are consistent with orbital imagery indicating thick crust and large, complex magma chambers beneath Tharsis volcanoes. Meteorite ejection ages are difficult to reconcile with plausible impact histories for Mars, and oversampling of young terrains suggests either that only coherent igneous rocks can survive the ejection process or that older surfaces cannot transmit the required shock waves. The mean density and moment of inertia calculated from spacecraft data are roughly consistent with the proportions and compositions of mantle and core estimated from martian meteorites. Thermal models predicting the absence of crustal recycling, and the chronology of the planetary magnetic field agree with conclusions from radiogenic isotopes and paleomagnetism in martian meteorites. However, lack of vigorous mantle convection, as inferred from meteorite geochemistry, seems inconsistent with their derivation from the Tharsis or Elysium plumes. Geological and meteoritic data provide conflicting information on the planet's volatile inventory and degassing history, but are apparently being reconciled in favor of a periodically wet Mars. Spacecraft measurements suggesting that rocks have been chemically weathered and have interacted with recycled saline groundwater are confirmed by weathering products and stable isotope fractionations in martian meteorites.

The relationship between Martian gravity and topography

The relationship between gravity and topography of various regions of Mars is used to estimate their effective elastic thicknesses T e using direct measurements of line of sight velocity, rather than spherical harmonic coefficients. Estimates of T e vary from 70 km for Tharsis, 29 km for Elysium, to 14.5 km for the southern hemisphere, and show that the thickness of the Martian lithosphere increases with age as the radioactive isotopes of K, Th, and U decay. A simple parameterised model of the convective thermal history is used to estimate the temperature structure of the lithosphere, and shows that the base of the elastic layer has a temperature of 300±50°C, or similar to the value for terrestrial continents. In both cases the rheology is probably affected by the presence of water. The short wavelength behaviour of the gravity field allows the density of the rocks that form the topography to be estimated, and gives values of about 3.0 Mg/m 3 for Tharsis and Elysium. This value is substantially greater than that of 2.7 Mg/m 3 obtained for Earth, and is in agreement with estimates from SNC (Shergottites–Nahklites–Chassigny) meteorites of 3.3 Mg/m 3. The density of the topography of Valles Marineris is only 2.35 Mg/m 3, and suggests that ice may be present below the surface. In the heavily bombarded southern hemisphere, isostatic compensation occurs at wavelengths as short as 700 km, which requires the effective compensation depth to be no more than 10 km. The gravity field with wavelengths greater than 1500 km may be supported dynamically, by a plume rising beneath the Tharsis region. The difference in temperature between the solidus and the present areotherm is less than 250°C, so melt generation can occur in rising plumes.

Strength of Martian lithosphere beneath large volcanoes

This paper provides an estimate of the thermal state of Martian lithosphere established since the formation of the shield volcanoes. Comparison of the spherical harmonic models of Martian topography derived using harmonics truncated at degrees 10, 11, 12, and 14 shows that the volcanoes have almost no contribution from harmonic coefficients of the topography less than degree 11. Therefore intermediate‐scale surface topography and gravity anomalies of Mars, specified by harmonics of degree 11–50, are considered in this study. These harmonics are dominated by shield volcanoes, Alba Patera, and Isidis basin. The central part of Valles Marineris is also well defined by these harmonics. The strong correlation of the topography and gravity anomalies over these harmonics and the positive Bouguer anomalies of the volcanoes suggest significant excess masses beneath these surface features in addition to their topographic masses. This indicates that the Martian lithosphere has been strong enough to support both the topography and the internal excess masses. We determine the rigidity of the lithosphere beneath a given surface feature. The lithosphere is modeled by a thin elastic spherical shell that overlies an inviscid interior of higher density and is subjected to both surface and internal loads. The thickness of the shell and the magnitude of the internal load are calculated such that the deformed structure resulting from the flexure of the shell under both surface and internal loads gives rise to the observed topography and gravity anomaly. It is shown that the lithosphere beneath Elysium Mons, Alba Patera, and the central part of Valles Marineris can be modeled by elastic shells subjected to topographic loads alone, whereas the lithosphere beneath Olympus and Tharsis Montes cannot. Internal excess masses of at least 40%, and more likely 50%, of the topographic masses are required, and the lithosphere beneath must have an elastic core of ∼100–80 km to support these volcanoes. The mean thermal gradient beneath this region of Tharsis is estimated to be ∼7–10 K/km on the basis of the strength envelopes calculated using diabase rheology for the crust of 50 km thickness and olivine rheology for the underlying mantle.

Martian atmosphere-like nitrogen in the orthopyroxenite ALH84001

Isotopic and elemental compositions of nitrogen, neon, and argon in the Martian meteorite ALH84001 were studied. Two whole-rock samples of ALH84001,57 were analyzed with detailed stepped combustion experiments (from 200 to 1200°C with every 100°C and 200°C, respectively, with laser ablation experiments for melting the samples). Nitrogen with δ 15N = 418 ± 44‰ and 366 ± 15‰ after correction for cosmogenic nitrogen were obtained at 800°C, which are the highest δ 15N values ever found among Shergottites, Nakhlites, Chassignite meteorites. The heavy nitrogen in ALH84001 may have the same origin as that in the glass portions of two shergottites, EETA79001 and Zagami, and is likely to have been derived from the Martian atmosphere. However, when and in which phase Martian atmosphere-like nitrogen was trapped in ALH84001 is still unknown. One possibility inferred from the released temperatures is that nitrogen is sited in maskelynite. On the plot of δ 15N versus Ar/N (diagrams of δ 15N vs. 40Ar/ 14N and 36Ar/ 14N), the data points for ALH84001 in this study form a linear trend, which apparently differs from that defined by glass portions of the shergottites EETA79001 and Zagami. The present trend also differs from the trend seen by Grady et al. for ALH84001. The 40Ar/ 14N and 36Ar/ 14N ratios obtained in this work are relatively smaller than those of the shergottites on such plots, indicating the following possibilities: (1) Martian atmospheric Ar/N ratio changed throughout geologic time (e.g., ≈4 Ga) without a large change in δ 15N, that is, ALH84001 trapped ancient Martian atmospheric nitrogen, and Ar/N ratio was initially lower, or (2) different trapping sites or mechanisms between ALH84001 and the shergottite glasses made an apparent difference in the Ar/N ratio, and elementally fractionated gases were trapped in ALH84001. In the case (1), either increase in Ar concentration in the Martian atmosphere due to preferential Ar degassing from the Martian interior or preferential escape of atmospheric nitrogen must have occurred. Nitrogen with δ 15N from −13‰ to +5‰, and with δ 15N ≥ 200‰ were released at low and high temperatures, respectively. The former may have been derived from the Martian lithosphere or terrestrial atmosphere, and the latter must be of cosmogenic nitrogen. The largest amount of 40Ar was released at temperatures similar to the largest release of heavy nitrogen of ≈800°C. According to reported K abundance in ALH84001, the majority of 40Ar in our samples is considered to be of radiogenic origin.

A nitrogen and argon stable isotope study of Allan Hills 84001: implications for the evolution of the Martian atmosphere.

The abundances and isotopic compositions of N and Ar have been measured by stepped combustion of the Allan Hills 84001 (ALH 84001) Martian orthopyroxenite. Material described as shocked is N-poor ([N] approximately 0.34 ppm; delta 15N approximately +23%); although during stepped combustion, 15N-enriched N (delta 15N approximately +143%) is released in a narrow temperature interval between 700 degrees C and 800 degrees C (along with 13C-enriched C (delta 13C approximately +19%) and 40Ar). Cosmogenic species are found to be negligible at this temperature; thus, the isotopically heavy component is identified, in part, as Martian atmospheric gas trapped relatively recently in the history of ALH84001. The N and Ar data show that ALH84001 contains species from the Martian lithosphere, a component interpreted as ancient trapped atmosphere (in addition to the modern atmospheric species), and excess 40Ar from K decay. Deconvolution of radiogenic 40Ar from other Ar components, on the basis of end-member 36Ar/14N and 40Ar/36Ar ratios, has enabled calculation of a K-Ar age for ALH 84001 as 3.5-4.6 Ga, depending on assumed K abundance. If the component believed to be Martian palaeoatmosphere was introduced to ALH 84001 at the time the K-Ar age was set, then the composition of the atmosphere at this time is constrained to: delta 15N > or = +200%, 40Ar/36Ar < or = 3000 and 36Ar/14N > or = 17 x 10(-5). In terms of the petrogenetic history of the meteorite, ALH 84001 crystallised soon after differentiation of the planet, may have been shocked and thermally metamorphosed in an early period of bombardment, and then subjected to a second event. This later process did not reset the K-Ar system but perhaps was responsible for introducing (recent) atmospheric gases into ALH 84001; and it might mark the time at which ALH 84001 suffered fluid alteration resulting in the formation of the plagioclase and carbonate mineral assemblages.

Mars: a magnetic field due to thermoremanence?

Presently available magnetic field data suggest a dipole moment of Mars of less than 10 −4 times the Earth's dipole moment. Presumably, Mars does not have an active dynamo at present which could give rise to a significant magnetic field. Nevertheless, the presently available data do not rule out a minor intrinsic field which may originate from a magnetized lithosphere. The objective of this study is to examine how much magnetization the Martian lithosphere could have acquired in the presence of an internal dynamo in early Martian history. Two models are presented: the first one assumes a laterally homogeneous lithosphere with a globally uniform thickness; the second model takes a possible dichotomy of the Martian lithosphere into account, which is assumed to be two to three times thicker in the southern hemisphere than in the northern hemisphere. The gradual cooling of the lithosphere below the Curie temperature is reproduced by a multilayer model. Curie isotherm motion with time and the dipole moment of the dynamo field are calculated from a thermal evolution model. For simplicity, the magnetic properties of the lithosphere are parameterized by its iron content. In the homogeneous case, the magnetostatic boundary value problem is solved exactly; in addition, an approximate solution is presented which is applicable with the dichotomy model. Using either method, the demagnetizing field in the separate layers mainly leads to the resulting remanent dipole moment. The remanent dipole moment is found to depend approximately linearly on the Curie isotherm depth and approximately quadratically on the iron content of the lithosphere. To reproduce the upper limit of the dipole moment derived from the Phobos mission (∼10 18 Am 2), a mean depth of 150 to 200 km for the Curie isotherm and a free iron content ranging from 5 to 6 vol.% are needed. The mean surface field may be less than 10 nT for both the homogeneous and the dichotomy model. The latter model yields a marked difference between the northern and southern hemisphere, with a pronounced anomaly at the dichotomy boundary.

Wrinkle Ridges, Reverse Faulting, and the Depth Penetration of Lithospheric Strain in Lunae Planum, Mars

Wrinkle ridges in the Lunae Planum region of Mars formed in response to compressional stresses associated with the development of the Tharsis rise. In this study, we investigate the accomodation of lithospheric strain associated with wrinkle ridge development in order to constrain the depth penetration of deformation associated with the formation of Tharsis. Using observations that indicate that the shallow martian lithosphere in Lunae Planum consists of a surface plains unit underlain successively by an unconsolidated megaregolith and competent basement, we construct models of an elastic lithosphere in which a ridge nucleates along a single fault in either the surface plains unit or basement. We then calculate strain and displacement fields for a range of plausible lithosphere structures assuming uniform horizontal shortening of the lithosphere. Transmission of strain between the surface layer and basement is most effective for a thin megaregolith and small Young's modulus contrast between the surface layer, megaregolith, and basement. However, results under a broad range of conditions for both models show straining of both the surface layer and basement. If lithosphere structure information and strains estimated by kinematic studies of the wrinkle ridges are applicable, then Thatsis-related compressional deformation must have involved the lithosphere, rather than being restricted to the near-surface.

The shallow structure of the Martian lithosphere in the vicinity of the ridged plains

Wrinkle ridges in the smooth plains of the Coprates and Lunae Planum regions of Mars form linear or concentric patterns with a regular spacing of 25–50 km. We test the hypothesis that the periodic development of deformation was a consequence of unstable horizontal compression, that occurred prior to probable ridge‐related faulting, of a strength‐stratified lithosphere which consists of a mechanically strong surface plains unit that successively overlies a weak megaregolith and a strong lithospheric basement. Results show that a range of models with both rigid and deformable megaregolith‐basement interface conditions yield solutions which can explain the ridge spacing within the constraint provided by the estimated thickness of the smooth plains materials. In models that incorporate viscous and plastic rheologies, uniform and exponentially varying vertical strength distributions, and the presence or absence of interfacial slip at the base of the surface plains unit, the ridge spacing is primarily controlled by the power law exponent of the lithosphere, the megaregolith/plains unit thickness ratio, and the plains unit/megaregolith strength contrast. Deformable basement models with a viscous rheology (1 ≤ n ≲ 3) can explain the ridge spacing if the megaregolith was thicker than and approximately 1–3 orders of magnitude weaker than the plains unit. Similar models with a perfectly plastic (n → ∞) rheology require plains unit thicknesses greater than 5 km, which exceeds most observational estimates. The dominant wavelength in the plastic models is not sensitive to internal strength contrasts and provides no constraint on the relative competence of the plains and megaregolith. Ridge spacing in the combined viscous and plastic models can be consistent with either a dry, water‐rich, or ice‐rich megaregolith at the time of ridge formation. If the ridge spacing was controlled primarily by the mechanical properties of lithospheric basement with little or no influence from the plains unit, then the minimum megaregolith thickness in the vicinity of the ridges must have been of the order of 0.4 km. The fact that ridges are most commonly found in the smooth plains but ridge spacing can be explained by models without a plains unit suggests that the unit may have facilitated the nucleation of reverse faults that are interpreted to characterize ridge structure but conceivably could have folded passively in response to unstable compression of the basement.

Martian Tharsis bulge: insight to mantle-related processes

When examining the tectonics of the one-plate planet Mars it is possible to test and even to develop further some ideas with respect to Earth's plate-tectonic paradigm. The definition of terrestrial plate movements does not unambiguously identify the actual mantle processes involved. One way to leam more about the endogenous development of planets is to undertake comparative studies. The main tectonic structures of Mars are interpreted here to give information concerning mantle processes which may relate to those processes driving Earth's plate tectonics, but which in the case of Mars, have been insufficient to break Martian lithosphere into plates, but which have been sufficient to cause rift-block tectonics and to build the Tharsis bulge.

Elysium Region, Mars: Tests of lithospheric loading models for the formation of tectonic features

The second largest volcanic province on Mars lies in the Elysium region. Like the larger Tharsis province, Elysium is marked by a topographic rise and a broad free air gravity anomaly and also exhibits a complex assortment of tectonic and volcanic features. We test the hypothesis that the tectonic features in the Elysium region are the product of stresses produced by loading of the Martian lithosphere. We consider loading at three different scales: local loading by individual volcanoes, regional loading of the lithosphere from above or below, and quasi‐global loading by Tharsis. A comparison of flexural stresses with lithospheric strength and with the inferred maximum depth of faulting confirms that concentric graben around Elysium Mons can be explained as resulting from local flexure of an elastic lithosphere about 50 km thick in response to the volcano load. Volcanic loading on a regional scale, however, leads to predicted stresses inconsistent with all observed tectonic features, suggesting that loading by widespread emplacement of thick plains deposits was not an important factor in the tectonic evolution of the Elysium region. A number of linear extensional features oriented generally NW‐SE may have been the result of flexural uplift of the lithosphere on the scale of the Elysium rise. The global stress field associated with the support of the Tharsis rise appears to have influenced the development of many of the tectonic features in the Elysium region, including Cerberus Rupes and the systems of ridges in eastern and western Elysium. The comparisons of stress models for Elysium with the preserved tectonic features support a succession of stress fields operating at different times in the region. While the order in which those stress fields operated cannot be determined from present geological observations, thermal and mechanical arguments favor the hypothesis that any flexural uplift of the lithosphere by a mantle thermal anomaly preceded or occurred contemporaneously with emplacement of the largest volcanic loads.