Syria Planum Research Articles

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.

Tectonic history of the Syria Planum province of Mars

We attribute most of the development of extensive fractures in the Tharsis region to discrete tectonic provinces within the region, rather than to Tharsis as a single entity. One of these provinces is in Syria Planum. Faults and collapse structures in the Syria Planum tectonic province on Mars are grouped into 13 sets based on relative age, areal distribution, and morphology. According to superposition and fault crosscutting relations and crater counts we designate six distinct episodes of tectonic activity in the following chronologic order: stage I is an early to late Noachian deformation forming mostly east‐west structures (fault set IA); some large volcanoes also formed. Faults were produced possibly by flexural uplift. Also, arcuate, north trending grabens (set IB) indicate that faulting is transitional to the next stage. Stage II is late Noachian to early Hesperian radial faulting centered in Syria Planum, possibly due to isostatic uplift in late Noachian (set IIA) to early Hesperian (set IIB) time. Stage III is early to late Hesperian faulting tangential to Syria Planum that was related to local centers of uplift (sets III1‐III3) on the periphery of Syria Planum. Stage IV is a late Hesperian graben formation that was circumferential to Syria Planum (set IVA), caused either by collapse associated with eruption of magma or by flexure of the lithosphere due to volcanic loading. In association with volcanism, minor faulting occurred, producing an oval pattern of faults in southwest Syria Planum (set IVBl) and a radial pattern south of the planum (set IVB2) that apparently rejuvenated buried stage II faults. Stage V is a late Hesperian to early Amazonian development of grabens and troughs of Noctis Labyrinthus and western Valles Marineris (set V) that was probably instigated by local uplift; exposure of groundwater or ground ice zones may have produced further collapse and trough enlargement. Stage VI is early Amazonian northwest trending faulting in Noctis Fossae (set VIl), perhaps due to Tharsis Monies‐centered tectonism, and north northwest normal faulting along the eastern side of the Claritas rise (set VI2) that was due to tectonic subsidence. The duration of tectonic activity in the Syria Planum province was perhaps 2–3 b.y. Photoclinometric topographic profiles across 132 grabens and fault scarps show that Syria Planum grabens have widths (average of 2.5 km, and most range from 1 to 6 km) similar to lunar grabens, but the Martian grabens have slightly higher side walls (average about 132 m) and gentler wall slopes (average of 9° and range of 2°–25°) than lunar grabens (93 m high and 18° slopes). Scarp degradation on Mars has progressed through quakes, impact shaking, and dry slope processes; the lower slopes may be due to Mars’ higher gravity. Estimates of the amount of extension for individual grabens range from 20 to 350 m; most estimates of the thickness of the faulted layer range from 0.5 to 4.5 km (average is 1.5 km). This thickness range corresponds closely to the 0.8‐to 3.6‐km range in depth for pits, troughs, and canyons in Noctis Labyrinthus and along the walls of Valles Marineris. We propose that the predominant 1‐to 1.5‐km values obtained for both the thickness of the faulted layer and the depths of the pits, troughs, and theater heads of the canyons reflect the initial depth to the water table in this region, as governed by the depth to the base of ground ice. Maximum depths for these features may indicate lowered groundwater table depths and the base of ejecta material.

Orientation, relative age, and extent of the Tharsis Plateau Ridge System

The Tharsis ridge system is roughly circumferential to the regional topographic high of northern Syria Planum and the major Tharsis volcanoes. However, many of the ridges have orientations that deviate from the regional trends. Normals to vector means of ridge orientations, calculated in 10° boxes of latitude and longitude, show that the Tharsis ridges are not purely circumferential. Intersections of vector mean orientations plotted as great circles on a stereographic net show that the ridges are not concentric to a single point but to three broad zones. These data indicate either that the Tharsis ridge system did not form in response to a single regional compressional event with a single stress center or that the majority of the ridge system did form in a single event but with locally controlled deflections in the generally radially oriented stress field. Superposition relationships and relative ages suggest that the compressional stresses that produced the ridges occurred after the emplacement of the ridged plains volcanic units (early in the Hesperian period) but did not extend beyond the time of emplacement of the Syria Planum Formation units or the basal units of the Tharsis Montes Formation (during the latter half of the Hesperian period). Comparison of topographic data with the locations of ridges demonstrates a good correlation between the topographic edge of the Tharsis rise and outer extent of ridge occurrence. Compressional deformation related to Tharsis is present out to about 5100 km from the regional topographic center located near northern Syria Planum. The innermost extent cannot be determined because the ridged plains units have been buried by more recent volcanic units, although ridge formation appears to have extended farther inward than is presently observed. Models involving a combination of isostatic stresses and stresses resulting from flexural loading appear to explain best the observed tensional features, topography, gravity, and, to a first approximation, the locations and orientations of the compressional ridges.

Tectonic history of the Tharsis Region, Mars

The faults of the Tharsis region of Mars have been mapped and delineated on the basis of age. The orientation of each group of faults was analyzed to determine if they were radial to a point. The results indicate that there are at least four discrete centers of faulting within the Tharsis area which are of significantly different age. The four centers proposed are, from oldest to youngest, (1) in the Thaumasia highlands, (2) in northern Syria Planum, and finally, (3) and (4) near Pavonis Mons. While episodes 3 and 4 occupy the same geographic position, they are separated in time by the development of the Tharsis lava plains and shields. Each episode of tectonism was separated from the next by a period of major basaltic volcanism. The tectonic features of the Tharsis area are thus viewed as being the cumulative result of the superposition of several discrete episodes of faulting, rather than the result of one episode about a single point.