ABOVE:   Two oblique asteroid impacts on the moon, which have created butterfly radial rays (left: Crater Proclus, right: Craters Messier (right) and Messier A (left)). The impact that created the Messier Craters was such a shallow impact that a double crater was formed.

ABOVE:   Impact craters of a similar age: Top arrow – Zhamanshin crater in Kazakhstan, large middle arrow marks possible position of undiscovered source crater for the Australasian tektites, bottom arrow marks the Mount Darwin crater, Tasmania, Australia. Butterfly Radial Rays are suggested by microtektite sampling in deep ocean sediments. A highly oblique impact is also suggested by the asymmetrical distribution of tektites, with extremely aerodynamic australite forms being most distal.

Interestingly the 13.5km diameter Zhamanshin crater in Kazakhstan, dated at 0.90+/-0.10Ma, and the ?1km Mount Darwin crater, Tasmania, Australia, dated at 0.73+/-0.04Ma, form a remarkably straight line in relation to the orientation of the Australasian strewn field (see Figure 2.2). Both these craters are too small and incorrectly positioned to be the source crater for Australasian tektites. It is certainly worth considering a multiple impact event with an age of c.0.803Ma, with the Indochinese (undiscovered) crater being the largest.


ABOVE:   A space shuttle image and NASA image of the Tonle Sap Lake in Cambodia. The bottom of the pictures is the south.

The tektite distrubution and pattern of morphologies clearly indicates a very oblique impact, under 10 degrees. The Tonle Sap Lake offers a very favourable possible impact structure. When compared with the Messier and Messier A craters on the moon one would expect an overall tektite distribution pattern similar to that observed.

It is unfortunate that the asteroid impact would have occurred along a very similar trend to the main tectonic features in this region. The Tonle Sap Lake may turn out to be a tectonic feature. Clearly more work is needed on this possible impact structure.

The author currently favours an impact in the Gulf of Tonkin (also spelt Tonking and Tongking) area. The crater would probably be around 40 km in diameter (Glass, 2003). This shallow shelfal sea may well have been land at the time of impact, due to lower sea levels during ice ages. This is based more on a hunch than any detailed research or evidence. It seems to me that the radial rays point more to this area. Furthermore, the tektite distribution suggests that the southwest Guangdong region in China is very proximal to the impact, with similar tektites being recovered from Vietnam and Thailand (but overall giving the impression they are slightly more distal in Thailand). I am wanting to place a crater somewhere to the southeast of the Red River Delta in Vietnam, in the Gulf of Tonkin. Is there a 'rift valley' feature here that is actually an impact crater? There has certainly been oil and gas exploration in this area so I would imagine the area has been shot through with seismic - is there anything on this confidential data? With the Red River delta in this region the sediment supply would very rapidy bury and disguise any possible impact structure.

Finally it is worthy of a mention that a gravity anomaly has been recorded offshore of southeastern Vietnam. This is another possible candidate, although the author does not feel that the tektite distribution fits with this location, in fact Tonle Sap seems more favourable than this location.


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