GeologyRocks

Hi Gus,

Interesting stuff. I know absolutely nothing about Lizard Point - I'll see if time will allow some extra reading.

Jon

P.S I moved this to the general geology section as it is serious stuff and should not be alongside threads on Big Brother

Jon

I can tell you what I know about it if you like. It might go on for a bit though. I want to also know if there's anywhere on the planet where something similar is happening. Not asking much am I?

Gus

What don't you understand ( or is niggling) Gus. The rocks at Lizard speak for themselves. The whole sequence of oceanic crust is on display. Just have some faith.
I've got a book called Beneath the Skin of the Lizard by Robin Bates & Bill Scolding £3.75 ISBN 1-898166-09-9 - sub titled - Seven Coastal Walks Exploring The Geology of the Lizard. Its a bit basic in places, but really meant for beginners. Some good explanation though. I will use it as a basis if my beginners class want a field trip down there. If they do you'll be welcome.

John

John

I don't have a problem with what's there. It's why it's there that's niggling me. I've got the heavy-duty tome on the subject and I've done the research because I lecture on the subject (ok maybe it's only evening classes). It's the idea that oceanic crust in the form of an ophiolite can develop during an orogen (I know about extension), which then gets exhumed from a considerable depth, if current thinking is to be believed. It somehow doesn't quite click with me. The bits about nappes and all that doesn't bother me. And do you know of somewhere on the planet where it's occurring now, cos I don't.

There, a serious post for a change.

Gus

It's quite easy really (honest!)... all you need is about 8km of uplift! To be serious, the lizard is a bit of an enigma and it is still not 100% understood. You can get parts of ophiolite all over the place - for example, the alps and the gobi altai mountains in mongolia contain a number of sections of ocean fllor - or ophiolite - that have been obducted into the mountain ranges. The thing to remember is that typically, ophiolites are not 'typical' mid-ocean ridge oceanic crust - they are generally formed in back-arc or supra-subduction zone settings (where you get a spreading ridge overlying a subducting plate). When (as happened in the alps the ocean crust gets caught up int two plates moving together, the oceanic crust gets caught up in the orogeny and forms a nappe (containing all parts of the ophiolite from serpentinised mantle through to pillow lavas and sediments) - that can eventually be found some 100's of metres above sea level. What you are probably seeing in the lizard is a section of ocean crust from a basin between two colliding places (colliding in the variscan/hercynian orogeny I guess, although I may be wrong about this). This basin eventually got amalgamated, along with sequences of rocks from either side of the oceanic basin - into an orogenic mountain range, with the granite and magmatic parts of cornwall at the base of the system. Eventually the mountains would have been eroded (forming large areas of sediments) and exposing the lizard and granites of cornwall as seen today. Vastly simplified, and probalby wrong in a number of places, but I would say this is generally how most ophiolites form. One notable exception is the Troodos ophiolite on Cyprus (had to get a plug in somewhere) where no 'obduction' in a strict sense has taken place - instead uplift has been achieved by wetting and serpentinisation of the mantle sequence - as seen also on the lizard. This serpentinite, once hydrated, formed a low density mass, or diapir, which moved upward, taking the rest of the ophiolite with it, until forming an island, up to ~2km above sealevel - as seen today.

I've already figured all this out, although, if you read the standard textbooks, the conditions for back-arc spreading appear to be lacking when you look a bit closer. Nobody can even decide which direction subduction took place, although this must also have a bearing on it. Is it possibly due an extensional fault system forming along a conservative margin?

Gus

As far as i understand the lizard formed in a rifted basin called Gramscatho to the south of the lizard in the cambrian. The rifting (probably submarine) caused a thinning of the crust allowing the formation of oceanic crust. This oceanic crust (which contained the peridotites) was obducted by listric faulting (a normal fault which is re activated as a reverse fault) as the basin closed during the closure of the massif central ocean during the Variscan orogeny. The ocean crust was transported northwards and thrust over the devonian sediments on the 'cornish' coast along with the associated thrust nappes.

Thats as best i understand it, though there are still some gaps which need filling in in this history.

Please forgive my complete lack of technical knowledge & understanding, but I shall endevour to add something of worth to the conversation (probably totally irrelevant but since when has that stopped me?)

L.R.M. Cocks, in his presidential address delivered 22 April 1999 The Early Palaeozoic geography of Europe states that faunal evidence shows that at least some of the Lizard formed part of Armorica during the Palaeozoic, and was sutured along the South Portugese Zone during the Devonian.

Never let it be said that I am just a pretty face eh?