Shock-wave-fans: Laboratory results and extraterrestrial evidence

MILANA, J. P. AND TIETZE K.-W.

Leipzig

Congreso; Sediment 2011; 2011

Universität Leipzig

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.