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.     

Depth of faulting and ancient heat flows in the Kuiper region of Mercury from lobate scarp topography

Mercurian lobate scarps are interpreted to be the surface expressions of thrust faults formed by planetary cooling and contraction, which deformed the crust down to the brittle–ductile transition (BDT) depth at the time of faulting. In this work we have used a forward modeling procedure in order to analyze the relation between scarp topography and fault geometries and depths associated with a group of prominent lobate scarps (Santa Maria Rupes and two unnamed scarps) located in the Kuiper region of Mercury for which Earth-based radar altimetry is available. Also a backthrust associated with one of the lobate scarps has been included in this study. We have obtained best fits for depths of faulting between 30 and 39 km; the results are consistent with the previous results for other lobate scarps on Mercury.The so-derived fault depths have been used to calculate surface heat flows for the time of faulting, taking into account crustal heat sources and a heterogeneous surface temperature due to the variable insolation pattern. Deduced surface heat flows are between 19 and 39 mW m^?2 for the Kuiper region, and between 22 and 43 mW m^?2 for Discovery Rupes. Both BDT depths and heat flows are consistent with the predictions of thermal history models for the range of time relevant for scarp formation.     

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.     

The effects of strain localization on the formation of Ganymede’s grooved terrain

We investigate the effects of strain localization on the formation of Ganymede’s grooved terrain by numerically modeling the extension of an ice lithosphere in which the yield strength of the ice decreases as the magnitude of the plastic strain increases. We do this to more realistically model fault strength, which is expected to vary with slip during initial fault development. We find that the inclusion of strain weakening leads to the formation of periodic structures with amplitudes of 200-500 m, consistent with the observed amplitudes of Ganymede’s large-scale grooves. The morphology of the deformation that results from extension depends both on the thermal gradient, which sets the lithospheric thickness, and on the rate at which the yield strength of the ice decreases with increasing plastic strain. Slow weakening with strain leads to low-amplitude, periodic structures, whereas moderate to rapid weakening with strain leads to large-amplitude, non-periodic structures. The combined influence of the thermal gradient and the weakening rate leads to the formation of complex surface deformation and may help explain the variety of surface morphologies observed within the grooved terrain.     

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.     

The formation of Ganymede's grooved terrain: Numerical modeling of extensional necking instabilities

Ganymede's grooved terrain likely formed during an epoch of global expansion, when unstable extension of the lithosphere resulted in the development of periodic necking instabilities. Linear, infinitesimal-strain models of extensional necking support this model of groove formation, finding that the fastest growing modes of an instability have wavelengths and growth rates consistent with Ganymede's grooves. However, several questions remain unanswered, including how nonlinearities affect instability growth at large strains, and what role instabilities play in tectonically resurfacing preexisting terrain. To address these questions we numerically model the extension of an icy lithosphere to examine the growth of periodic necking instabilities over a broad range of strain rates and temperature gradients. We explored thermal gradients up to 45 K km⁻¹ and found that, at infinitesimal strain, maximum growth rates occur at high temperature gradients (45 K km⁻¹) and moderate strain rates (10⁻¹³ s⁻¹). Dominant wavelengths range from 1.8 to 16.4 km (post extension). Our infinitesimal growth rates are qualitatively consistent with, but an order of magnitude lower than, previous linearized calculations. When strain exceeds ∼10% growth rates decrease, limiting the total amount of amplification that can result from unstable extension. This fall-off in growth occurs at lower groove amplitudes for high-temperature-gradient, thin-lithosphere simulations than for low-temperature-gradient, thick-lithosphere simulations. At large strains, this shifts the ideal conditions for producing large amplitude grooves from high temperature gradients to more moderate temperature gradients (15 K km⁻¹). We find that the formation of periodic necking instabilities can modify preexisting terrain, replacing semi-random topography up to 100 m in amplitude with periodic ridges and troughs, assisting the tectonic resurfacing process. Despite this success, the small topographic amplification produced by our model presents a formidable challenge to the necking instability mechanism for groove formation. Success of the necking instability mechanism may require rheological weakening or strain localization by faulting, effects not included in our analysis.     

Beagle Rupes - Evidence for a basal decollement of regional extent in Mercury’s lithosphere

Thanks to its location at low latitude and close to the terminator in the outbound view of Mercury obtained during MESSENGER’s first fly-by, the Beagle Rupes lobate scarp on Mercury has been particularly clearly imaged. This enables us to interpret it as a component of a linked fault system, consisting of a frontal scarp terminated by transpressive lateral ramps. The terrain bounded by these surface manifestations of faulting is the hanging-wall block of a thrust sheet and must be underlain by a basal decollement (a detachment horizon) constituting the fault zone at depth. The decollement must extend a minimum of 150 km eastwards from the frontal scarp, and at least 400 km if displacement is transferred to features interpreted as out-of-sequence thrusts and offset lateral ramps that appear to continue the linked fault system to the east. The depth of the basal decollement could be controlled by crustal stratigraphy or by rheological change within, or at the base of, the lithosphere. Previous interpretations of mercurian lobate scarps regard their thrusts as uniformly dipping and dying out at depth, lacking lateral ramps and any extensive detachment horizon. Anticipated improvements in image resolution and lighting geometry should make it possible to document what percentage of lobate scarps share the Beagle Rupes style of tectonics.     

Analysis of a Thaumasia Planum rift through automatic mapping and strain characterization of normal faults

A new semi-automatic technique is presented to map and characterize tectonic features on Mars. Automatic strain estimation associated with normal faults is achieved for synthetic and real fault scarps on Mars.The application of this new technique to a small rift located in Thaumasia Planum allowed the segmentation of the rift. The defined segmentation corresponds to changes in the strikes of faults that delimitate rift areas with different architecture.The rift is formed by several pull-apart basins developed due to the reactivation of previously formed tectonic structures. The strain spatial distribution and the overall geometry are consistent with a roughly East–West left-lateral shear transfer zone between two different lithospheric blocks.     

Spectrophotometry of Comet Austin (1982g) during post-perihelion period

Lunar crustal shortening does not seem to be restricted to the lava-filled basins alone; but there are some young scarp-like terra ridges in places around mare areas where they often continue other tectonic structures. This crustal shortening has not reached the same intensity as in the case of the lobate scarp overthrusts on Mercury. Young lunar terra ridges indicate that crustal shortening with an areal extent also took place slightly around mare basins. Thus they link tensional rille tectonics with compressional mare ridge tectonics and indicate that areal heating/bending/extension — cooling/ shortening/compression may describe an important explaining factor in lunar mare- and near-mare tectonics in addition to the volcanic extrusions.     

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.     

Tectonics of Syrtis Major Planum on Mars

Mare ridges were caused by compressional tectonics and indicate the shortening of the planum surface foiled by lavas. At least two separate tectonic phases within Syrtis Major Planum can be found. The two central calderas are located on the southwestern continuation of the Nili Fossae graben zone at the junction of the N-S and NW-SE mare ridge sets. These central calderas were formed by surface collapses into relatively shallow magma chambers. Radial and concentric mare ridges around the two calderas represent a shortened surface environment within the large compressional megacaldera. Shortening was caused by sinking of the crust due to the lava load, plumbing of the magma chambers and cooling of the interiors. The main NW-SE ridge trend parallels highland faults of the major structural zone extending from Hesperia Planum to Vastitas Borealis. These NW-SE ridges indicate the large scale areal tectonic trend along the Scopulus Oenotria - Phison Rupes fault zone and support the idea of a main SW-NE compression. The N-S directed mare ridges of the northern planum area favour a change in compressional stress direction from SW-NE in the south to E-W in the northern planum, obviously due to the buried local topography. These linear mare ridges can also be interpreted as forming a large Isidis Planitia-concentric ridge circle connecting Nili Fossae to Libya Montes. Formation of the mare ridges was the youngest of the main tectonic phases involved within the area studied.     

Elastic dislocation modeling of wrinkle ridges on Mars

Wrinkle ridges are one of the most common landforms on Mars. Although it is generally agreed that they are compressional tectonic features formed by folding and thrust faulting, there is no consensus on the number of faults involved, the geometry of the faults, or the maximum fault depth. The topography of martian wrinkle ridges in Solis Planum and Lunae Planum has been studied using MOLA data. As determined in previous studies, the topography shows that most wrinkle ridges are a composite of two landforms, a broad low relief arch and a superimposed ridge. Constrained by MOLA topographic profiles, the geometry and parameters of the faults associated with wrinkle ridges have been modeled. The best fits are obtained with a blind listric thrust fault that flattens into a décollement. The listric fault geometry is approximated by a series of linear connecting segments with varying dips. The major morphologic elements of wrinkle ridges can be matched by varying the displacement on the different fault segments. Modeling of large-scale wrinkle ridges indicates that the maximum depth of faulting or depth to the décollement is about 4.5 km. This may correspond to the depth of the contact between the ridged plains volcanic sequence and the underlying megabreccia. The results suggest that wrinkle ridge thrust faults are shallow-rooted and reflect thin-skinned deformation.     

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 , with the true value probably being closer to the lower bound.     

Tectonic geomorphology of the northern Upper Rhine Graben, Germany

This paper focuses on the northern Upper Rhine Graben (URG), which experienced low tectonic deformation and multiple climate changes during Quaternary times. Recently, human modifications have been high. The paper presents the results of a study into the effects of fault activity on the landscape evolution of the area. The study aims to detect active faults and to determine the last phase of tectonic activity. Information on the long-term tectonic activity is gained from the geological record (drainage system, sediment distributions, fluvial terraces, fault mapping). Previous studies are reviewed and supplemented with new data on tectonic activity. The compilation of all data is presented as a series of paleogeographic maps from Late Miocene to present. It is demonstrated that differential uplift of the western margin of the northern URG had significant impact on the drainage system, the formation of fluvial terraces and the landscape of the western graben shoulder. In a second part of the paper, the imprint of tectonics on the present-day landscape is investigated at the regional scale in order to determine the location of fault scarps and tectonically influenced parts of the drainage system. This study uses an integrated analysis of topography, drainage patterns and fault network. The comparison of features suggests a structural control by numerous NNE- and NNW-oriented intra-graben faults on the flow directions of streams in the Rhine Valley. Several scarps in the Rhine Valley are identified and interpreted to result from intra-graben faulting activity, which in turn controlled fluvial dissection. The third part of the paper presents quantitative measurements of the present-day landscape shape. Calculations of geomorphic indices are used to determine the balance between erosional and tectonic processes and to identify active fault segments. The mountain-front sinuosity and valley shape indices measured along the border faults and in the footwall area are used to determine the level of activity of the faults. Stream profiles of the western and eastern catchments of the River Rhine are investigated for gradient changes at the crossing of the border faults. The combined interpretation of geomorphic indices points to active border fault segments on both sides of the graben. Based on the integration of all results it is concluded that the tectonic morphology identified for the northern URG formed in response to long-term, low level tectonic processes. Due to a significant decrease in erosional and depositional activity during the last 15,000 years, the tectonic morphology has probably been preserved until present.     

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.     

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.     

Tectonic and stratigraphic implications of the relative ages of venusian plains and wrinkle ridges

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.     

Formation and disruption of aquifers in southwestern Chryse Planitia, Mars

We present geologic evidence suggesting that after the development of Mars' cryolithosphere, the formation of aquifers in southwestern Chryse Planitia and their subsequent disruption led to extensive regional resurfacing during the Late Hesperian, and perhaps even during the Amazonian. In our model, these aquifers formed preferentially along thrust faults associated with wrinkle ridges, as well as along fault systems peripheral to impact craters. The characteristics of degraded wrinkle ridges and impact craters in southwestern Chryse Planitia indicate a profound role of subsurface volatiles and especially liquid water in the upper crust (the upper one hundred to a few thousands of meters). Like lunar wrinkle ridges, the martian ones are presumed to mark the surface extensions of thrust faults, but in our study area the wrinkle ridges are heavily modified. Wrinkle ridges and nearby plains have locally undergone collapse, and in other areas they are associated with domical intrusions we interpret as mud volcanoes and mud diapirs. In at least one instance, a sinuous valley emanates from a modified wrinkle ridge, further indicating hydrological influences on these thrust-fault-controlled features. A key must be the formation of volatile-rich crust. Primary crustal formation and differentiation incorporated juvenile volatiles into the global crust, but the crustal record here was then strongly modified by the giant Chryse impact. The decipherable rock record here begins with the Chryse impact and continues with the resulting basin's erosion and infilling, which includes outflow channel activity. We propose that in Simud Vallis surface flow dissection into the base of the cryolithosphere-produced zones where water infiltrated and migrated along SW-dipping strata deformed by the Chryse impact, thereby forming an extensive aquifer in southwestern Chryse Planitia. In this region, compressive stresses produced by the rise of Tharsis led to the formation of wrinkle ridges. Zones of high fracture density within the highly strained planes of the thrust faults underlying the wrinkle ridges formed regions of high permeability; thus, groundwater likely flowed and gathered along these tectonic structures to form zones of elevated permeability. Volatile depletion and migration within the upper crustal materials, predominantly along fault systems, led to structurally controlled episodic resurfacing in southwestern Chryse Planitia. The erosional modification of impact craters in this region is linked to these processes. This erosion is scale independent over a range of crater diameters from a few hundred meters to tens of kilometers. According to our model, pressurized water and sediment intruded and locally extruded and caused crustal subsidence and other degradational activity across this region. The modification of craters across this wide range of sizes, according to our model, implies that there was intensive mobilization of liquid water in the upper crust ranging from about one hundred to several thousand meters deep.     

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.     

Evidence for the sedimentary origin of imbrium sculpture and lunar basin radial texture

The Imbrium sculpture texture, a distinctive ridged and furrowed pattern radial to the Imbrium basin and seen in other basins, has long been debated as to its origin (internal, formed by basin-related fractures; external, related to ejecta patterns). To test for the presence of deep radial fractures on the moon, the azimuth and length of linear rim segments of twenty-four post-Imbrium-basin craters were measured. Linear segments of crater rims parallel preexisting fracture patterns in terrestrial craters floored in an indurated substrate. Craters forming in a terrain containing pervasive fractures radial to Imbrium should show evidence of this tectonic influence by forming rim crest segments (terrace scarps) preferentially along these directions. No systematic relation of these segments with Imbrium radial structure was found. This suggests that the surface radial grooves may not extend to depth. The relatively young Orientale basin shows two types of radial structures: (1) pervasive subparallel ridges and furrows formed by a spectrum of sizes of secondary crater chains emanating from the main crater, and from flowage of material during secondary cratering; (2) parallel, generally radial ridges which appear to have formed on top of outward flows of debris. These types of radial textures (both depositional and erosional) appear unrelated to major faults or fractures. Therefore, these two lines of evidence suggest that much of the Imbrium-type sculpture surrounding major lunar basins is sedimentary, rather than tectonic, in origin.