Coprates Chasma Research Articles

Layering stratigraphy of eastern Coprates and northern Capri Chasmata, Mars

Distinct competent layers are observed in the slopes of eastern Coprates Chasma, part of the Valles Marineris system on Mars. Our observations indicate that the stratigraphy of Coprates Chasma consists of alternating thin strong layers and thicker sequences of relatively weak layers. The strong, competent layers maintain steeper slopes and play a major role in controlling the overall shape and geomorphology of the chasmata slopes. The topmost competent layer in this area is well preserved and easy to identify in outcrops on the northern rim of Coprates Chasma less than 100 m below the southern Ophir Planum surface. The volume of the topmost emplaced layer is at least 70 km 3 and may be greater than 2100 km 3 if the unit underlies most of Ophir Planum. The broad extent of this layer allows us to measure elevation offsets within the north rim of the chasma and in a freestanding massif within Coprates Chasma where the layer is also observed. Rim outcrop morphology and elevation differences between Ophir and Aurorae Plana may be indicative of the easternmost extent of the topmost competent layer. These observations allow an insight into the depositional processes that formed the stratigraphic stack into which this portion of the Valles Marineris is carved, and they present a picture of some of the last volcanic activity in this area. Furthermore, the elevation offsets within the layer are evidence of significant subsidence of the massif and surrounding material.

A topographic test for the existence of ground ice in the walls of Coprates Chasma, Mars

Evidence for the former presence of ground ice and its possible role in the erosion of Valles Marineris trough walls is equivocal. This study aims to determine whether variations in the topographic slope angle of the walls of the trough Coprates Chasma are consistent with predicted temperature‐driven differences in the distribution, thermal condition, and strength of ground ice within its walls. Using Mars Orbiter Laser Altimeter topographic data, 247 profiles were made rim to rim across the trough and measurements made of wall slope angle, mean annual surface temperature, and trough geometry (wall height, rim elevation, and trough width). Variations in these parameters along each trough wall, and differences between the two trough walls, were established. The results indicate that the predicted variations along and differences between the walls in the three‐dimensional distribution of ground ice, and in its thermal condition and strength, are unlikely to have caused the measured variations in trough wall slope angle. These observations imply that ground ice has been absent within the trough walls since the age of formation of the spur‐and‐gully wall morphology. The results preclude the spur‐and‐gully morphology having developed by dry mass‐wasting of material above an ice‐rich cryosphere and indicate the action of liquid water to be a more likely origin. The inferred absence of ice within the walls precludes any role of ice in initiating or mobilizing landslides where they have eroded the spur‐and‐gully morphology.

Multiple-process origin of Valles Marineris basins and troughs, Mars

An improved sequence of formation for Valles Marineris depressions is proposed. It involves three key stages; (a) dike emplacement radial to Syria Planum during Late Noachian to Early Hesperian time; (b) localized subsidence of crustal rocks during post-Early Hesperian time, forming ancestral basins such as Hebes Chasma; and (c) regional normal faulting that overprints the ancestral basins and forms the structural troughs, such as Coprates Chasma, principally during Amazonian time. Attainment of high topography at Syria Planum, completed by the Early Hesperian, was accompanied and/or followed by extrusion of ridged plains lavas, emplacement of subsurface dikes, and wrinkle-ridge deformation. The high topography contributed to a substantial hydrostatic head, leading to subsurface volume reduction, vertical displacement of crustal strata to from chaotic terrain and ancestral basins, and localized outbreaks of fluid as outflow channels. The dikes contributed a pervasive structural influence on the orientations of later nearsurface processes. Sometime after infilling of ancestral basins with their interior layered deposits, normal faults related to Tharsis centered stresses were superimposed on the relict high topography, ancestral basins, and dike network, forming large grabens. Recognition of the polygenetic origin of troughs such as Melas Chasma, and the disparate origins of irregular and rectangular troughs, provides the key to unraveling the conflicting accounts of trough origin and timing.

Amounts of crustal stretching in Valles Marineris, Mars

Terrestrial grabens and continental rifts are compared with the Valles Marineris system, in order to define the mechanisms which could be responsible for its geometry and formation. Simple shear/pure shear mechanisms, symmetric/asymmetric grabens and faults, high/low dip angles, block tilting/no block tilting, shouldering or not, lithospheric layering, and amount of sedimentation on chasma floors are discussed. Amounts of stretching on several transverse topographic profiles are then calculated, assuming either block tilting or no block tilting. On each profile initial dip angles ranging from 40 to 90, and sediment thicknesses ranging from 0 to 3 km are considered, as well as symmetric and asymmetric border fault dips. The case of Ius Chasma, the southwestern trough of the Valles Marineris system, is first considered, then the entire graben system. Considering a constant 660 km profile length, stretching increases eastward in Ius Chasma from the Noctis Labyrinthus boundary (stretching factor β = 1.01–1.02, assuming initial 60 dipping faults) to the middle part of this trough (β = 1.04–1.06). Then Ius Chasma stretching decreases, but is partly taken over by stretching in Candor and Ophir chasmata. Stretching decreases from the eastern part of Melas Chasma (β = 1.05–1.06) to almost the eastern end of Coprates Chasma (β = 1.01). Then stretching increases again eastward (β = 1.02–1.04) at the longitude Coprates Chasma splits around a horst and Gangis Chasma opens northward. A striking feature is that a low peak of extension in the centre of the Valles Marineris troughs is observed (β = 1.03–1.04). The low stretching peak in the central part of Valles Marineris may originate from the existence of buried structures in the grabens, and/or along-strike variations in sediment thickness. According to the profiles and to the hypotheses, some 9–26 km of additional normal movements along faults with dip angles equivalent to the dip angles of the walls would be expected within the central Valles Marineris grabens to get constant stretching from Ius to Coprates chasmata. The amount of this lacking offset may be partly explained by a few km subsidence of Ophir Planum, and the shallow inter-chasmata grabens. Part of the remaining lacking normal movements are best explained by buried structures (possibly shallow horst and graben alternances or other structures) in the major canyons: Melas, Candor, and/or Ophir chasmata—probably mainly in Melas Chasma. Alternatively, along-strike variations of sediment thickness (about 3 km) without taking such buried structures into account, can explain the results as well. Observation of images shows up that the most realistic structural model of Valles Marineris should probably consider both variations of sediment thicknesses and existence of buried structures. Models for the origin of Valles Marineris stretching are discussed. The role of passive rifting in crust weakened by hot spot is emphasized, although extensional stresses due to the Tharsis load should have also contributed to stretching.

Topography of Valles Marineris: Implications for erosional and structural history

Compilation of a simplified geologic/geomorphic map onto digital terrain models of the Valles Marineris permitted an evaluation of elevations in the vicinity of the troughs and the calculation of depth of troughs below surrounding plateaus, thickness of deposits inside the troughs, volumes of void spaces above geologic/geomorphic units, and volumes of deposits. The central troughs north Ophir, north and central Candor, and north Melas Chasmata lie as much as 11 km below the adjacent plateaus. In Ophir and Candor Chasmata, interior layered deposits reach 8 km in elevation. If the deposits are lacustrine and if all troughs were interconnected, lake waters standing 8 km high would have spilled out of Coprates Chasma onto the surrounding plateaus having surface elevations of only 4–5 km. In this case, interior deposits above about 4 km in the central troughs would not be lacustrine. They could be volcanic. On the other hand, the troughs may not have been interconnected at the time of interior‐deposit emplacement; they may have formed isolated ancestral basins. The existence of such basins is supported by independent structural and stratigraphic evidence. The ancestral basins may have eventually merged, perhaps through renewed faulting, to form northern subsidiary troughs in Ophir and Candor Chasmata and the Coprates/north Melas/Ius graben system. The peripheral troughs are only 2–5 km deep, shallower than the central troughs. They may have formed from a combination of erosional collapse and structural activity. Chaotic terrain is seen in the peripheral troughs near a common contour level of about 4 km on the adjacent plateaus, which supports the idea of release of water under artesian pressure from confined aquifers. The layered deposits in the peripheral troughs may have formed in isolated depressions that harbored lakes and predated the formation of the deep outflow channels. If these layered deposits are of volcanic origin, they may have been emplaced beneath ice in the manner of table mountains. Areal and volumetric computations show that erosion widened the troughs by about one‐third and that deposits occupy one‐sixth of the interior space. Even though the volume eroded is larger than the volume deposited, topographic and geologic considerations imply that material eroded from trough walls was probably part of the interior layered deposits but not their sole source. Additional material may have come from subterranean piping, from reworking of local disintegration products on the floors, such as chaotic materials, or from eolian influx. But overall it is likely that the additional material is volcanic and that it forms mostly the upper, more diversely bedded layers of the interior deposits.

Structural development of Coprates Chasma and Western Ophir Planum, Valles Marineris Rift, Mars

A portion of Valles Marineris was mapped in detail in order to clarify the dominant processes responsible for the formation of Coprates Chasma. New crater counts indicate that the caprock on western Ophir Planum plateau has a Late Hesperian crater age, whereas trough floor preserved in western Coprates Chasma has an Early to Late Hesperian crater age. Caprock on western Ophir Planum correlates in relative age with the Syria Planum Formation, and the caprock may overlie Lower Hesperian ridged plains material. The conspicuous absence of wrinkle ridges on Ophir Planum may reflect burial by this later material. Trough floor material in Coprates Chasma correlates in relative age with ridged plains material on the adjacent Lunae Planum and Coprates plateaus and represents a structurally coherent block displaced downward by normal faulting. The crater counts and detailed structural relationships demonstrate the commonly accepted view that Coprates Chasma occupies a graben. The floor of this graben is only thinly mantled by intratrough deposits and is relatively intact; the trough shows no evidence for a chaotically broken and jumbled floor, as required by genesis models of collapse and subsurface drainage. Formation of Coprates Chasma by keystone collapse of locally elevated topography is not supported by available topographic data, but a general association between the trough and volcanotectonic activity in the Tharsis region is considered to be a likely explanation for the trough forming stresses. Subtle asymmetries in topography and boundary fault development across Coprates Chasma may imply a gentle, down‐to‐the‐north half‐graben geometry for the trough. Normal faults that define the northern margin of western Coprates Chasma and those that deform the adjacent Ophir Planum plateau change trend systematically with position, implying a 30° rotation of the local horizontal principal stresses in this region during trough growth. Faulting on Ophir Planum appears related to the faulting that created the Coprates Chasma trough. The geology and structure of Coprates Chasma are comparable to those of other troughs such as Melas, Ius, and perhaps Candor chasmata, suggesting that these troughs may also have formed as grabens. The revised crater age for western Ophir Planum indicates that the initiation of trough faulting and subsidence in the Valles Marineris system is early Late Hesperian and may have been synchronous across the system relative to the prevailing cratering rate.

Radar altimetry of South Tharsis, Mars

Martian altitudes were measured by radar during the oppositions of 1971 and 1963 using the 64-m antenna at Goldstone (California). The resultant topographic profiles substantiate a zonal classification of the volcanic flows blanketing the south flanks of Arsia Mons, and they confirm the existence of a secondary, parasitic shield attached from the SSW to the main Arsia shield. The secondary shield is about 400 km in diameter at its base and at least 4 km high at its center. South of Valles Marineris, the Tharsis plateau is bounded by the approximate longitudes of 80° in the east and 140° in the west. In the Sinai Planum, closely adjacent to Coprates Chasma, another rise has been detected, bounded by longitudes of 55° in the east and 80° in the west. A volcanic shield of diameter 80 km, capped with a 22 km caldera has been identified near the crest of the rise. Topographic highs of about 1 km are associated with heavily faulted tracts such as Claritas Fossae. The distribution and orientation of the lunar-mare-like ridges in Sinai Planum appear to be independent of the regional gradients. Segments of the chaotic terrain at the eastern terminus of Valles Marineris are located as much as 6 km below the level of the surrounding plains.