Pierre Thomas

Wrinkle ridges of Mars: structural analysis and evidence for shallow deformation controlled by ice-rich décollements

Abstract The ridged plains of Mars display large assemblages of wrinkle ridges usually interpreted as shortening features. The formation of these ridges has been explained by (1) deep thrusting, (2) buckling of lava plains or (3) thrusts rooted on shallow decollement levels. These explanations were grounded on observations of Tharsis flank ridged plains where ridges are straight and regularly spaced. This paper shows observations about irregular ridges with curved, circular or en-echelon shapes. These different shapes are due to pre-existing faults and buried craters. Structural evidence shows that such irregular ridges are created by thrusts rooted on shallow decollement levels. The shallow crust of Mars is composed of a megaregolith sometimes overlain by volcanic stratas. Ice-rich levels are supposed to exist as deduced from observations of fluidized ejecta craters. In order to understand the nature of the decollements a simultaneous study of lavas thicknesses and decollements depths have been done. The results show that decollements are not related to lava thicknesses but to icy levels. The rheology of ground ice favours this last assumption because ductile icy levels can localize decollements. Furthermore fluidized ejecta craters are preferentially localized in ridged plains indicating a correlation between groundice and wrinkle ridges. Finally the origin of the stress creating the deformation is discussed taking into account the occurrence of shallow icy layers and the possible occurrence of deeper deformation.

Chronology of compressional deformation on Mars: evidence for a single and global origin

Abstract The chronology of compressional deformation on Mars is determined using chrono-stratigraphic relations and crater counting in order to provide new evidences for the origin of this deformation. Chronological relations between compressional tectonic features with different directions establish that a single deformation phase occurred in each studied region. On other hand, the intersections between tectonic features and craters permit to compare the relative age of the deformation in different regions. The results demonstrate that there may be a long lapse of time between the accumulation of Hesperian volcanic plains and the compressional deformation in each region. The possible age deduced for the compressional deformation on both Hesperian ridged plains and Noachian primitive terrains is Late Hesperian. A single and global phase may explain the compressional deformation because it is restricted to a single epoch. A global contraction of the planet may explain many properties of compressional tectonism on Mars according to the thermal evolution of the planets. Such process does not exclude secondary sources of stress like Tharsis bulge that would control the geometry of structures at regional scales.

Quantitative analysis of the extensional tectonics of Tharsis Bulge, Mars: Geodynamic implications

About 10% of the Tharsis bulge surface is intensely faulted, with a fault density up to 1 fault/km. In these intensely faulted areas, 54 impact craters which exhibit faulted walls and floor are now elliptical in shape. These ellipticities indicate an average strain value of 10% inside these provinces. This strain value is 4 times greater than the strain deduced from normal fault morphometric analysis. This high value indicates that the strain is not only due to 60° dip normal faults, and that some additional tectonic processes exist such as boudinage. This important strain value corresponds to an elongation of about 50 km across Thaumasia and Tempe-Mareotis provinces, and 20 km across Alba Patera Regio. The amount of elongation for the entire Tharsis Region is 2 orders of magnitude higher than the strains and motions related to standard models of bulge formation. These intensely faulted areas are restricted areas where faults are both radial to the bulge and perpendicular to the topographic slope. The stretching does not influence the regional topography and there are no topographic shoulders or important level differences between faulted and nonfaulted areas, which would indicate that the crust is not thinned under the stretched provinces. Outside these intensely faulted areas the graben width is remarkably constant all over the Tharsis bulge, indicating the existence of a brittle ductile transition or an incompetent level at about 1–2 km below the surface. A new origin for the Tharsis tectonic features is proposed as a preliminary hypothesis: the intensely faulted areas correspond to the uphill parts and to denudation zones of gravity slidings which move on a 2km-deep incompetent level. The gliding of the slab on the very low dipping flanks of Tharsis needs three conditions: (1) the viscosity of the incompetent layer must be equal to the viscosity of the ice, (2) the brittle layer must have been previously faulted by the bulging itself, (3) these preexisting radial fault directions must be perpendicular to topographic slope. The ridged plain units in Lunae Palus and Coprates provinces thus correspond to a zone of blocking inside the gliding slab, due to an interruption of the decollement level.

Small-scale models of multiring basins

Small-scale sand-silicone simulations of multiring impact structures have been undertaken in order to understand the effects of the rheology of the lithosphere on the variability of natural multiring structures. For low sand-silicone thickness ratio (1:3), brittle strain is accommodated by spiral strike-slip faults. For higher sand-silicone ratios (1:1 or 2:1), an inner concentric ring affected by strike-slip faults is relayed by an external ring affected by concentric normal faults. The diameter of the inner ring decreases with the increase of the sand-silicone thickness ratio. It is suggested that the flexure of the brittle layer due to the silicone flow is responsible for the brittle strain field which is enhanced by the channel flow of the lower crust. The characteristic geometry of the intersection of conjugated strike-slip faults can be observed around large multiring basins on silicate crust such as Orientale on the Moon and on icy crust, such as Valhalla on Callisto and Gilgamesh on Ganymede. The strain field around these large craters is discussed in terms of mechanical properties of the lithospheres. On the Moon, large craters without relaxation faults, such as Imbrium are located on thin crust regions. The crust was too thin to have a ductile lower layer at the time of impact. Gilgamesh on Ganymede is surrounded mainly by strike-slip faults. Asgard on Callisto has the same diameter as Gilgamesh but is surrounded by concentric normal faults. The brittle-ductile thickness ratio is thus higher on Callisto than on Ganymede.

Are there other tectonics than tidal despinning, global contraction and Caloris related events on Mercury? A review of questions and problems

Abstract Mercurys tectonic activity was confined to its early history. A synthesis of classical references indicates that its tectonic activity was principally related to (1) a small change in the shape of its lithosphere by tidal despinning, (2) a small change in radius and area by shrinkage due to secular cooling, and (3) the Caloris related events. These activities produced the ancient tectonic grid, the lobate scarps, and the Calorian ridges scarps and grabens, respectively. This low degree of activity was ultimately due to Mercurys small size. In spite of this apparent simplicity, some features are still intriguing. Detailed compilation of lineaments on the entire planet indicates that the grid is not similar to the theoretical despinning grid. Some trends are explained by despinning, but only with unusual mechanical properties of the Mercurian lithosphere, while some other trends are not explained at all by despinning. Examples of unexplained tectonic features in the same region are presented in this paper. Some circular depressions may be interpreted as the result of tectonic or volcano-tectonic subsidence (caldera?). Some exibit narrow and particularly straight grooves which cannot be explained as impact related features, and may be interpreted as open tectonic cracks. The Tolstoj area exhibits hills and grooves which cannot be interpreted as Tolstoj impact related features. Morphological and chronological studies indicate that these features would consist of the extensional tectonic features (horsts and grabens) developed on the convex top of a tectonically uplifted bulge. The tectonic development of this area occurred over a long period of time, and is probably due to a deep and long-lived internal source. These examples show the existence of large- and small-scale internal activities which affect Mercurys surface independently from global or impact related tectonics. Such activities must be taken into account in further models of Mercurian internal structure and history and must be searched in data of future missions. A new Mercurian mission with a complete coverage of image and altimetric/gravimetric data is thus necessary to understand the geology and the tectonic of Mercury.

The thermal gradient of Callisto constrained by Asgard Basin: Rheological and chemical implications

Using the width of concentric grabens formed during the relaxation of the Asgard basin on Callisto, in combination with rheological flow laws of ice and laws concerning the variations in strain rate around a relaxing crater, the surface thermal gradient of Callisto at −4 b.y. is estimated to be near 0.5°K km−1. It is assumed that the graben width depends on the depth of the brittle/ductile transition zone. Around the Asgard basin, the depth of this brittle/ductile transition zone varies from 23 km (at 200 km from the crater rim) to 4 km (at 800 km from the crater rim). It is assumed that the thermal characteristics were constant over the extended zone and that the difference in the depth of the brittle/ductile transition zone is only due to differences in the strain rate around the basin during the relaxation of the initial cavity. Calculations show that Newtonian flow law for ice gives a strain rate of external relaxation at the crater rim of about 10−14 s−1, while a power law gives a strain rate of about 10−6 s−1, assuming a surface temperature between 100 and 150°K. The order of magnitude of the calculated thermal gradient is independent of the chosen flow law. This calculated gradient is, however, one order of magnitude lower than those calculated theoretically with a chondritic abundance of radioactive elements in the silicate part of Callisto.