Re: NASA Worldwind, & Large scale features in Africa

on 7 May 2005 03:58:14 -0700, don findlay <don@xxxxxxxxxxxx> sez:

` 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

At last, some replies. Looks like it's taken a while to propagate
here. ...Yes, the one on the left. The worldwind image is realcolour,
and shows the ridge visible in your image plus another concentric
and larger in the same arc. Both appear visible in the arc as in your
image, but appear to be obscured by sands at the points of disappearance,
suggesting continuing circular shape.

` 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 right hand feature doesn't stand out on the WW image at all.
....I'm guessing "loopy" is not particularly a technical term.
I can see that the shape appears due to a basin-like deformation
of a formerly flat layer whose risen edge forms the arc like the
rim of a saucer. Other than impacts pushing down on the centre,
I've seen that sort of thing due to weight from glaciers, but
obviously that's not an option for this locale. Do you know, is
the ridge arc also igneous -say an uplifted basalt flow, or is it
a sedimentary layer? In the WW colour, btw, the arc is dark reddish
against a background of pale tan/yellow. In other parts of north
africa this contrast is repeated, and you can see the red streaking
away from outcrops in long stretches of apparent wind erosion
across sand. I'm guessing the red is due to iron...

The general line of structure northwest
` -southeast is the precursor of Mediterranean opening.

If you mean the general elevation parallels the mediterranean shore,
yes, I can see that, but the arc-ridge structure completely disregards
and disrupts that trend.

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.

Thanks for responding. Still no other replies propagated here have
actually addressed my inquiry. Do check out the WW viewer if
you've got the machine for it, it's quite a buzz.

--
==========================================================================
vincent@triumf[munge].ca Pete Vincent
Disclaimer: all I know I learned from reading Usenet.
.

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