INVESTIGADORES
MILANA Juan Pablo
congresos y reuniones científicas
Título:
Shock-wave-fans: Laboratory results and extraterrestrial evidence
Autor/es:
MILANA, J. P. AND TIETZE K.-W.
Lugar:
Leipzig
Reunión:
Congreso; Sediment 2011; 2011
Institución organizadora:
Universität Leipzig
Resumen:
We introduce here the novel concept of shock-wave fan.
A fan of this type is produced by a flow that is volumetrically large enough to
expand after an opening of the conduit (valley). As a result, these fans are
devoid of lobes, as the flow could expand well until the main sediment load is
deposited. In theory, as the drag causing sediment transport is a shock wave,
the base shear stress will decay rapidly as the flow expand at an exponential
rate similar to the amount of opening that the fan could have (usually 180°, so
it is half circle). Due to this rapid decay, and the time-limited transport
capacity of theses waves (not competence), sediment transport is expected to
cease suddenly creating a depositional sediment step at fan edge.
While these kind of fans could be generated by
aerosols (as pyroclastic flows, or large meteorite impacts in extraterrestrial
setting devoid of water), that have more expansible characteristics, but we explored
their formation using watery flows. A shock-wave in a water flows is created
when successive layers of the flood catch the slower ones in the base, delayed
due to the friction at the base. A low increase of the wet perimeter as depth
rises favors shock waves. Thus an inlet high slope channel of vertical walls
was used to simulate shock waves by retaining water with a gate and releasing
it suddenly over a minor base flow. As expected the flow created a head as a
subaerial turbidite, took all the sediment it could and expanded well over the
flat floor of the flume. A well edged terraced fan, devoid of internal lobe was
created in this way. We produced a couple of several more elongated fan, adding
slope to the flume floor where the fan was created, making a flat-top terraced
fan, but in this case semi elliptically with maximum elongation parallel to the
main sloe. As we know gravity is part of the formulation of translation speed
and hence drag power of most physical waves, indicating clearly that the
transport capacity was decided by the wave dynamics.
This type of wave transport, still unexplored, but
well repeated by those that work on large meteorite impact, is perhaps the origin
of the elusive terraced fan of Mars. Those fans, show clear edges, almost
concentric, but little displaced as if the flow inertia could be displaced in
some cases as the gravity affected our flows. In our attempt of creating
successive terraced fans we failed due to the eroding effect of the water flow
over the edges of the first-underlying terraced fan. However, gas-supported
flows would not have this gravitational effect, as what keeps them near the
ground is the sediment load, and once this has been massively deposited the
flow would not affect noticeably the ground anymore.
Therefore, although the experiments show only the half
way of the Mars terraced fans, it is well possible those fans were created by
massive shock-waves created by large meteorite impacts that fostered large
sediment tranmsport. When this transport was channeled along vertically-walled
canyons, the flow could be amplified and created the each layer of terraced
fans. This possibility shows therefore there is no need of water in Mars, to
create the curious terraced fans.