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Lunar and Planetary Science XXXII (2001) 1063.Pdf Lunar and Planetary Science XXXII (2001) 1063.pdf NORTHERN LOWLANDS ON MARS: EVIDENCE FOR WIDESPREAD VOLCANIC FLOODING AND TECTONIC DEFORMATION IN THE EARLY HESPERIAN. James W. Head1, Mikhail A Kreslavsky1,2 and Stephen Pratt1. 1Department of Geological Sciences, Brown University, Providence, RI 02912 USA; 2Astronomical Observatory, Kharkov National University, Kharkov, Ukraine. ([email protected]) Abstract. Detrended northern lowland topography shows data to analyze basin topography and surface roughness char- that surface deposits are underlain by an Early Hesperian re- acteristics, removing regional slopes to detect and assess sub- gional unit with a basin-wide system of sub-parallel wrinkle tle topographic variations and geologic structure [8]. We find ridges; the orientation and location of these wrinkle ridges that the northern lowlands are underlain by a regional unit completes a global circum-Tharsis ridge system ~7000 km containing a basin-wide system of sub-parallel ridges that we wide. interpret to be wrinkle ridges and arches. This unit also con- Introduction and stratigraphy: The northern lowlands of tains highly modified ghost or stealth craters, the number of Mars form the central part of a larger drainage basin that has which suggests an Early Hesperian age [18]. The unit can be comprised about three-fourths of the surface for most of its traced laterally and is contiguous with Hesperian-aged ridged history [1]; the northern lowlands may have contained a large plains in the southern hemisphere. Mapping of the orientation standing body of water in earlier Mars history [2], and could and location of the ridges completes a global circum-Tharsis have been the location of plate tectonic activity in its early ridge system forming a band approximately 7000 km wide, history [3]. Two fundamental issues remain unresolved: 1) and extending over the whole circum-Tharsis region. when and how did the distinctive northern lowlands form, and The recognition of this unit and the superposed very de- 2) what were the processes involved in their evolution? graded craters permits us to assess the stratigraphy and geome- In ascending stratigraphic order, the exposed surface units try of subsequent units and structure. We find that the present of the northern lowlands [4] include: 1) Noachian-aged rem- height of wrinkle ridges and geometry of buried craters sug- nants including a small unit dissected by channels in Acidalia gest that the Vastitas Borealis Formation has a minimum Planitia, old degraded crater rims, and Scandia Colles, a col- thickness of about a hundred meters. The polar layered terrain lection of domes interpreted to be old degraded Noachian deposits completely cover the wrinkle ridge system, and the crust; 2) the Hesperian-aged Vastitas Borealis Formation, an circumpolar deposits obscure it, supporting the interpretation unusual unit interpreted to be degraded lava flows and sedi- that the circumpolar deposit thickness is of the order of several ments, 3) Hesperian-aged channel deposits at the margins of hundred meters and that wrinkle ridge formation was not as the lowlands in Chryse Planitia, 4) various local Amazonian- active in the Amazonian as in the Hesperian. The trends of aged plains units, 5) Elysium channel deposits, 6) the north Hesperian-aged channels entering Chryse Planitia are con- polar cap, consisting of Late Amazonian ice and layered ter- trolled by the orientation and topography of wrinkle ridges rain deposits, and 7) Amazonian-aged circumpolar mantling deep into the basin. Some Amazonian-aged smooth plains material. The predominantly sedimentary nature of many of units of volcanic origin in Amazonis Planitia and South the northern plains units and modification processes since their Arcadia Planitia further bury and obscure the underlying formation have obscured the character and age of underlying wrinkle ridges [19]. Fretted terrain, particularly in the terrain, and thus its origin and evolution. We analyze these Deuteronilus Mensae region, formed subsequent to the Early units with newly processed MOLA data to asses the Hesperian-aged ridged plains [10], and remnants can be seen stratigraphy and structure of the northern lowlands. to extend beneath the Vastitas Borealis Formation. New MOLA results: The Mars Orbiter Laser Altimeter Amazonian-aged Elysium Formation channel deposits are (MOLA) experiment on board the Mars Global Surveyor mis- distinctly different in morphology and structure from the sion has provided a new global characterization of the topog- Chryse channels and very clearly overlie the wrinkle-ridged raphy and slope characteristics of Mars [1,5]. The combina- plains and Vastitas Borealis Formation in Utopia Planitia [4, tions of gravity and topography have provided important new 12]. insights into crustal structure and thermal evolution [6]. New views of the northern lowlands: The recognition of These data have shown that: 1) the northern lowlands are ex- these units and their stratigraphic relationships permits us to tremely flat and smooth [1]; 2) they are irregular in shape and outline a new perspective on the history of the northern low- consist of two basins, the ancient, circular, heavily modified lands: 1) In the Noachian, little surface record remains but Utopia Basin of impact origin, and the irregularly-shaped large now-buried channels apparently distributed water and North Polar Basin [7]; 3) the dichotomy boundary (the distinc- sediments to the northern lowlands [6]; 2) in Early Hesperian, tive morphologic and local topographic slope boundary be- a significant part of the northern lowlands was filled with vol- tween the southern heavily cratered uplands and the northern canic plains similar to those presently exposed in Tharsis and lowlands) does not coincide exactly with crustal thickness much of the southern uplands; 3) these volcanic plains were trends [6]; 4) there is evidence for low-density linear gravity subsequently deformed by a Tharsis-circumferential, 7000 km anomalies interpreted to be ancient fluvial channels [6]; and 5) wide band of wrinkle ridges, as well as other basin-related the surface of the northern lowlands is exceptionally smooth at systems in Utopia and Isidis (the exact age of wrinkle ridge all scale lengths and is characterized by an unusual roughness formation is uncertain [4,9]); 4) slightly later in the Early at about the 3 km scale length [1,5]. Hesperian, portions of the basin margins, including the ridged As a further step in the analysis of the origin and evolution plains, were modified to form the fretted terrain [10], deposit- of the northern lowlands, we have used comprehensive MOLA ing sediments in the basin; 5) circum-Chryse outflow channels Lunar and Planetary Science XXXII (2001) 1063.pdf MARS NORTHERN LOWLANDS: J. W. Head, M. A. Kreslavsky and S. Pratt formed in the Late Hesperian and once in the basin, formed Outside the northern lowlands there is an observed early subdued channels whose course was largely controlled by planet-wide volcanic emplacement phase of Late Noachian- wrinkle ridge orientation and height, depositing material in the Early Hesperian age. Late Noachian ridged plains (Nplr) com- basin; 6) The Vastitas Borealis Formation was emplaced in the prise ~3% of the presently exposed surface of Mars and Hes- Late Hesperian; our analysis supports the idea that this unit perian-aged ridged plains (Hr) make up another 10%. Thus, if represents a veneer on the ridged plains of a minimum mean Hesperian-aged ridged plains underlie the northern lowlands, thickness of ~100 meters and probably a significant part of the then this would mean that the total percentage of the planet sediments forming this unit are from outflow channel resurfaced by Hesperian-aged volcanic plains would be the emplacement; 7) Amazonian volcanic plains were emplaced, 10% presently exposed in the southern uplands and the 20% primarily in the Elysium Basin and Amazonis Planitia, further making up the northern lowlands, for a total of 43.5 x 106 km2 obscuring the Hesperian ridged-plains [11]; 8) channel and or 30% of the surface of Mars. These new estimates would flow deposits quite different than those of Chryse were em- more than double the area covered by Hesperian ridged plains. placed in Utopia Planitia from the Elysium rise and overlie If the southern upland plains are typically about 500 meters both the Hesperian ridged plains and the Vastitas Borealis thick and those in the northern lowlands about 900 meters Formation [e.g., 4]; and 9) Late Amazonian polar and circum- thick (based on the average thickness of flooding models), polar deposits formed, obscuring the structure of the underly- then the total volume of Hr would be about 3.3 x 107 km 3. ing Hesperian ridged plains; extension of the regional slope of These estimates represent an increase in the importance of the Hesperian ridged plains beneath these deposits suggests Hesperian-aged volcanic plains from previous estimates of that the circumpolar deposits are relatively thin (less than a more than twice the area and almost twice the volume. Esti- few hundred meters), unless significant local polar cap loading mates of gas exsolution from volcanic effusion and input of and flexure has taken place [e.g., 12]. volatiles into the atmosphere as a function of time during Mars These data do not bear directly on the presence or absence history [16] shows that the peak input was during the Early of a large standing body of water in the northern lowlands in Hesperian. These new data
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