Tectonic history of the Syria Planum province of Mars

Abstract

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.