Re: NASA Worldwind, & Large scale features in Africa


pete wrote:

> Anyway, spending happy hours drifting about the virtual globe, I have
> noticed a distinct feature in western Libya, along the border with
> Algeria. It appears to be a pair of concentric arcs, representing say
9
> o'clock to noon, which if completed would encircle a region slightly
> smaller than Spain. And indeed there seem to be other features which
could
> be candidates for further outcroppings of a circular structure,
around 4
> to 6 oclock. Well, obviously the notion of meteor crater comes to
mind,
> but I haven't heard of this being proposed for this region, and were
it,
> it would be far and away the largest known. I presume this feature is
> well known, and there is another explanation for it, and I wondered
> if you folks could enlighten me about it. Thanks.

Presume you're talking about the ring-like structures here:-
http://users.indigo.net.au/don/ng/libya.jpg
No, not impacts. The 'rings' are more integrated with the regional
geology than impacts would suggest. The one on the left is a loopy
basin, the one on the right is uplifted Precambrian granites with other
younger granitoid intrusions, Tertiary acid effusives and some very
young (Neogene) basalts. The general line of structure northwest
-southeast is the precursor of Mediterranean opening. The two of them
together (and the one one the margin of the image), and the general
basinal/ platform distribution of the Jurassic all along North Africa
make a mess of the idea of plate collision (Africa into Europe). Think
of them a bit like the Southern European loopy shapes on the other side
of the Alps, dolomites etc. (Collapse structures.)

Don't know about the regular interpretation though. No doubt someone
else who has read a book on rumplecloth tectonics will answer you. I'm
just going by Unesco's map.

.