Re: NASA Worldwind, & Large scale features in Africa

on Sat, 04 Jun 2005 11:00:18 +0100, Aidan Karley
<doIlookDAFTenoughTOpost@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> sez:

` In article <d7r4a2$5sj$1@xxxxxxxxxxxxxxxxxxxxxx>, Pete wrote:

` > What causal process is implied in the term "synclinal basin"?
` > Is it just a coincidental function of mountain building surrounding
` > the area, or is the basin the driver and the surrounding ridges its
` > result?
` >
` It's very variable. "Synclinal basin" is a descriptive term with
` no implication of a theory of origin. Like describing something as
` sweet piece of pie - it could be sweet because of the nature of it's
` ingredients, it could be sweet because the cook decided to sprinkle
` sugar on it after making it, or it could be sweet because the baker's
` delivery cart parked under a tree full of aphids excreting honeydew.
` Recalling the maps I drew up a couple of weeks ago (BTW, have
` you tinkered with Cornell's Atlas? it's a useful piece of kit ; java
` only, so should work with any system.), the northern arc of hills
` stretches from the Tibesti in the East to the vicinity of the a'Hoggar
` in the west;
` http://www.ingentaconnect.com/content/geol/jgs/2000/00000157/00000005/a
` rt00001 talks about a NW-SE lineament which would form the NE margin of
` this structure, and of late Proterozoic origin. IF that is supported
` then it would suggest that this margin at least has been somewhat
` active for an extended period (covering the age of the sediments seen
` in the middle of the basin), which would suggest uplift on this margin
` and possibly passive accumulation in a subsiding basin in the centre.
` But against this is the (disputed) interpretation of the a'Hoggar as
` being the head of a mantle plume which has caused a local (1000km
` diameter) bulge, with this basin being just one facet on it's flank.

Well, between your reply and George's, I concluded that it seems the
region's circular shape is a coincidental result of its position
between upwelling regions to the west and the southeast, though that
doesn't seem to provide an explanation for the continuation of the
arc ridge across the northwest. ...I haven't looked at the Cornell
Atlas; I am finding WorlWind to be more than enough to keep me
enthralled for now, and I see that Gates has announced u$ will be
coming out with its own sat-map viewer to take on Google's shortly,
so I guess we'll soon be inundated with them. Drifting about the
Idehan Murzuq at high res with WorldWind provides some astounding
views, showing the clusters of dark circles in unlikely locations
in the midst of seeming ultimate aridity, which are the rotating
irrigation wheels; and along the south arc of the ridge surrounding
the dune field, the corrugated edge of the ridgetop, whose lower
elevations have allowed long streamers of sand to escape and smear
across hundreds of km of dark basalts to the southwest.

I was using an old NatGeo map to identify features, and it used the
spelling "Idehan Murzuch" which yielded nothing by search engine.
(WorldWind seems to depend on contributors to provide placenames, and
while many villages are named in the region, geographical feature names
are quite sparse.) The village shown in the paper in the link George
provided used the spelling "Murzuq", and that got a couple of hits, but it
occurred to me to then try "Murzuq Basin", and that was the key to lots of
info mostly from stuff relating to petroleum exploration. I found another
nice satellite pic which nicely emphasizes the circular nature of the
basin, here:
http://www.hq.nasa.gov/office/pao/History/SP-4203/phot01.htm

and this paper, which I haven't finished reading yet, looks to
go into considerable depth on the geohistory of the basin, I hope:

http://www.unt.edu.ar/fcsnat/INSUGEO/geologia_17/33.htm


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