Northern lowlands of Mars: Evidence for widespread volcanic flooding and tectonic deformation in the Hesperian Period

Abstract

Recent Mars Orbiter Laser Altimeter (MOLA) data have provided a new picture of the Martian northern lowland basin topography and surface roughness. In order to assess detailed topographic structure important in understanding the formation and evolution of the northern lowlands, we have removed regional slopes from the topography to produce a series of maps highlighting the local topographic and geologic structure. We find that (1) the northern lowlands are underlain by a regional unit containing a basin‐wide system of subparallel wrinkle ridges; (2) this unit contains highly modified craters, the number of which suggests an Early Hesperian age; (3) this unit is laterally contiguous with Early Hesperian‐aged ridged plains in the southern uplands; (4) the orientation and location of the wrinkle ridges in the North Polar Basin complete a global circum‐Tharsis ridge system forming a band ∼7000 km wide and extending over the whole circum‐Tharsis region; and (5) several subareas of the northern lowlands show individual wrinkle‐ridge patterns (e.g., Isidis and Utopia are basin‐like). The recognition of this unit and the superposed very degraded craters permits us to assess the stratigraphy and geometry of subsequent units and structure. We find that (1) the present spacing and height of wrinkle ridges and geometry of buried craters suggest that the Vastitas Borealis Formation is a thin sedimentary unit superposed on regional ridged plains (Hr) and that its minimum average thickness is ∼100 m; (2) the polar terrain completely obscures the wrinkle‐ridge system and some circumpolar deposits partially obscure it, supporting the interpretation that some circumpolar deposit thicknesses exceed several hundred meters and that wrinkle‐ridge formation was not active in the Amazonian; (3) Late Hesperian‐aged outflow channels entering Chryse Planitia are controlled by the orientation and topography of wrinkle ridges deep into the basin, indicating that wrinkle ridges had largely formed by this time; (4) Amazonian‐aged smooth plains units, particularly in Amazonis Planitia, further bury and obscure the underlying wrinkle ridges, and (5) fretted terrain, particularly in the Deuteronilus Mensae region, formed subsequent to the Early Hesperian‐aged ridged plains, and remnants can be seen to extend beneath the Vastitas Borealis Formation. The recognition of these units and their stratigraphic relationships permits us to outline a new perspective on the history of the northern lowlands: (1) in the Early Hesperian the majority of the northern lowlands was filled with volcanic plains similar to those presently exposed in the southern uplands; outcrop patterns of buried Noachian‐aged terrain and volcanic flooding models suggest an average thickness of 800–1000 m; (2) these plains were deformed soon thereafter by Tharsis‐circumferential and basin‐related wrinkle ridges; (3) circum‐Chryse outflow channels were emplaced in the Late Hesperian, forming subdued channels whose course was largely controlled by wrinkle‐ridge orientation and height, and these channels deposited water and sedimentary material in the basin; (4) loss of the outflow channel water resulted in formation of the Vastitas Borealis Formation as a residual sedimentary deposit on top of the ridged plains; little evidence is seen for the presence of any massive near‐surface residual ice deposits remaining from the outflow channel effluent; (5) Amazonian volcanic plains were emplaced, primarily in Amazonis Planitia and Utopia Planitia, further obscuring the Hesperian ridged plains; and (6) Late Amazonian polar and circumpolar deposits formed, further obscuring the structure of the underlying Hesperian ridged plains. The widespread emplacement of the Hesperian‐aged ridged plains of apparent volcanic origin is interpreted to mean that the volcanic phase represented by this unit was global in nature and resurfaced the northern lowlands, in addition to the ∼10% of the planet previously known, for a total resurfacing of ∼30% of Mars. This remarkable event increases dramatically the amount of water and other volatiles that might have been degassed into the atmosphere during the Early Hesperian and further underlines the global significance of the contractional deformation typical of this period.