3/09/2009

The "IBD Effect" and a trip to Ring Mountain

I keep finding geologically interesting spots in South Africa. There is a never ending supply, actually. However, often times when I ask around to find out what kind of work has been done in one of these areas, I get an answer to the effect of, "It's been done". In the Saldania Belt, represented locally by the slate belt that underlies the Cape Town area, a lot of people told me "It's been done". Digging into the literature and archives of theses from the local universities, I found several pieces of really nice work. Some of them emphasized the structures in the area. But to my mind, there is actually a *giant hole* in the literature when a few theses, fewer than 10 journal articles, and a sprinkling of unpublished conference abstracts are considered "DONE" for an orogenic belt or accretionary complex (the literature is ambiguous as to which this is). After finding this phenomenon at several different localities around South Africa, my friend Jodie and I christened it the "It's Been Done Effect" or IBDE, which sounds like "Ibid". Get it?

In terms of the local slate belt, my friends at the survey agree it needs some more attention so I got a few students together and we're going for it. Peeling off the coastal belt of the Malmesbury Group for scrutiny. Hoping to get started asap with a mapping day starting at the Sea Point Contact. Starting where the last thesis left off... and importing an old friend with a global slate belt habit for help!

My beloved Franciscan Complex has suffered a bit of the IBDE. It was such a hot topic a few decades ago that people tend to think that It's Been Done. In fact, there are a lot of secrets still hidden in the Franciscan and a lot of unanswered questions. Only a small but hardcore loyal following still works on it (often as a pet project without substantive funding). As such, I took some visitors there during AGU - subduction people from New Zealand - California's geologic evil twin, in some ways - whom I thought would adequately appreciate it.

Ring Mountain has low grade sandstones and shales on the lower slopes. On top there is a plate of ultramafic rock. In between the two there is a complete mess. Sheared serpentinite melange in green, purple, white, yellow and black wraps around blocks and boulders of strange metamorphic rocks.

Here's a blueschist boulder with some intense folding of the internal foliation.
On its outside surface, the reaction rim between the silicate minerals of the boulder and the ultramafic serpentinite matrix breeds an actinolite and talc-rich reaction zone (green) wrapping the glaucophane-rich blueschist (violet).

Another boulder clast contains epidote-garnet zones where fluid has entered? or departed? along fractures in the blueschist rock. Sorry met friends, I don't know which. Can I use a lifeline? I'm going with departed.

Here's what reminded me to finally post these photos: One of my fellow members of the underground Ring Mountain club mentioned fucsite on Facebook. Is this it? I'm talking to you Naomi.



My camera took a crooked picture of me with VT and AF. Perhaps it's because it was distracted by the metamorphic block it sat on:
"Wakabayashi Block", in tribute to another die-hard Friend of the Franciscan.

5 comments:

Ava said...

Hi Christie,

My name is Ava and I'm the blog moderator for the blog on a site called The Reef Tank (http://www.thereeftank.com/blog) I had a question for you about your blog but couldn't find your e-mail address and was hoping you could e-mail me so I wouldn't have to ask it here. Please e-mail me at Charismaqueen100@gmail.com

Thank you! Keep up the great work!

andrew said...

That doesn't look like Waka . . .

The glittery green stuff certainly looks like fuchsite, a chromian mica you might well expect in a high-grade serpentinite.

But on to your real point: geologic research doesn't ever stop, unless you're a prospector maybe. New generations, or the same people with a few years more insight than before, need to revisit and reexamine the rocks. It took me a long time to learn that geologic maps are all interim, provisional documents that can always be improved.

JF said...

Christie, you may be interested to learn that one of Alex's honours student has been mapping the Sea Point contact in details (1 m = 2 cm I believe) last year: "IBDE"...
But seriously, I'm sure this piece of work could be integrated in whatever you'd be doing.

And yes indeed -- there is an amazing shortage of studies fo most SA rocks. But remember, we are just a handful of academic geologists for a country that is one million sq. km and contains such things as the Bushveld or Barberton -- to name just the two that are positively unique...

(By the way, I found a very nice fault in Barberton the other day, you would have loved it !)

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