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| Titel |
Lethe Vallis, Mars: a "fill and spill" flood-carved channel |
| VerfasserIn |
Matthew Balme, Colman Gallagher, Sanjeev Gupta, John Murray |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250042108
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| Zusammenfassung |
| Lethe Vallis is an approximately 230 km long, 1.5 km wide, sinuous channel connecting two
shallow basins in the Elysium Planitia region of Mars. Lethe is of particular interest because
the main basin (which we term the Western Elysium Basin) from which it begins is fed by
outflow from Athabasca Vallis, a 300 km long, 30 km wide, geologically recent, catastrophic
flood-carved channel. Lethe therefore appears to be an overspill channel that formed
as the Western Elysium Basin breached a topographic divide at its south-western
margin.
The flood deposits in this region are marked by a distinctive platy-ridged-polygonised
surface texture. We have mapped the extents of this texture in and around Lethe Vallis. From
this mapping, we have constructed topographic long profiles of the Lethe Vallis thalweg and
of the north and south contacts between the platy-ridged-polygonised either side of the
channel and older underlying terrain.
The upstream half of Lethe Vallis has a slightly concave topographic profile
with a very shallow gradient of about 0.0001. The distal half of the channel has
a slope about three times greater, with a stepped profile. These steps match the
locations of cataract systems within the channel. Each cataract system is defined by a
series of amphitheatre-shaped scarps heading long inner channels within the main
valley.
In plan view the Lethe system is divided into four separate zones, defined by constrictions
where the contacts draw close to the channel margins. Each of these constrictions marks the
downstream end of a basin, and are also associated with breaks in slope of the contact long
profiles. The cataracts are each located a few kilometres upstream of the margins of the
basins, but the inner channels below the head scarps of the cataracts terminate at the basin
margins. In addition, the mapped contact for at least three of the basins (and only not so for
the smallest basin) closely follow an equipotential surface – suggesting that the contacts
are high-stand marks and indicate ponding. That, within basins, both north and
south contacts are at the same topographic height reinforces this view. The cataracts
therefore appear to represent upstream migration of knickpoints into overflowing fluvial
basins.
In addition to the cataract systems, a variety of other landforms associated with
catastrophic flooding are seen in Lethe. These include streamlined islands, anastomosing
distributary systems, fluvial hanging valleys, and paired and unpaired terraces on the channel
margins. There are also possible depositional features, dunes and/or antidunes, located just
downstream of the cataracts. These features reinforce the fluvial nature of the flow that
carved the Lethe Channel and suggest that there was an alluvial element to the
system.
Palaeo-discharge estimates based on channel cross section and thalweg slope are of the
order of 1-5Ã104 m3s-1; of the order of the flow in the Mississippi River on Earth. The
volume of fluid within the high stand contacts in the largest of the zones divided by the Lethe
system discharge gives an estimate for the minimum length of time the system was active:
about 10-50 days, but probably much longer.
In summary, the geomorphology of Lethe Vallis points to formation as a fluvial “fill and
spill” catastrophic flood system. This fits in closely with the wider observations of linked
basins within Elysium Planitia. |
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