( Blog for website at http://users.indigo.net.au/don )
Fig.1. Rhegmatic fracture patterns of the mantle : schematic representation. Transform faults and spreading ridges define mantle extension and growth (not movement by convection). (a) The Atlantic ( the cradle of thought on continental drift, Plate Tectonics, and Earth expansion); passive longitudinal spreading - Americas to the left, Europe and Africa to the right; (b) Earlier forceful equatorial mantle breakthrough in the Pacific centred on the Indonesian bubble (= grey circle, which is on the front face of the sphere); older than the Atlantic and more complex. (c) Longitudinal section A-A' in (a), showing essentially vertical orientation of normal growth faults, usually described as "transform" faults ('T'); note that their vertical orientation implies their configuration as great circles, which debunks both Morgan's and McKenzie's concept of Euler poles, and by itself nails the lid on the coffin of Plate Tectonics; dashed line represents crust-mantle detachments adjusting to spin. (d) Switching roles of transform faults ('T') and spreading ridges ('R') according to gravitational loading and spreading. Grey = plane of crust-mantle detachment (dashed line in (c)); 'G' = gravity. Read this figure in the context of [1], [2], [3]
[Rhegmatic : roughly meaning global in scale, usually with systematic significance, of deep basement origin, and operative over an extensive span of geological time. The term long predates exploration of the ocean floors, but is well illustrated by transform faults and spreading ridges.]
The above figure synoptically describes mantle tectonics in the simplest possible terms. I'm posting it now for perspicaceous schoolies who might be reading this, as a thought-op. to consider in conjunction with the above-linked posts 1, 2 and 3. Hopefully those who can be bothered might have a go at lifting us out of the dark-aged, geological mess of Plate Tectonics that my generation and the previous one have landed us in, 'cos *they* won't do it. (Can't do it; allegiance is compromised).
But first, a bit of history and a tribute in passing to the work of Jan Kutina in Europe and America, and Tim O'Driscoll in Australia, who recognised the control of continental-scale lineaments on the distribution of mineralisation. They documented in no uncertain terms the empirical evidence for the commonly held view today (that wasn't one back then) that many ore deposits, particularly those of giant size, are in some way related to disturbances in the deep continental basement, and probably have a mantle control in some way.
Beginning well before the rhegmatic fracture patterns of the ocean floors (= exposed mantle in Fig.1) were becoming known, both gentlemen pursued and promoted the empirical association that was often observed to exist between crustal-scale lineaments and ore deposits. The association implied gravity-driven, global-scale continental extension that spoke for vertical tectonism and that was counter to the emerging paradigm of Plate Tectonics. It is a credit to both men that they pursued and were guided by the empirical field evidence that connected deformation in the crust to that of the mantle, and were not inveigled to follow the emerging fashion of Plate Tectonics.
The tribute is futher warranted on account of the subtext to this blog, which is the schism that exists between the fairy-floss, Hollywood, Christmas-jamboree-stocking, everyone-can-have-one, hypothesis-driven 'American Way' of doing geology - and the harder European (and mostly rest of the world) way, where 'doing it' draws battle-lines with the empirical geological facts and respects and recognises the unknown (and often unknowable) as the no-man's land in between, not the up-for-grabs freebie territory for colon-isation according to the Principle of Multiple Working Stories of the American Way (make up any old hypothesis and see if, by spreading a few numbers around, you can get it to work).
Tim's work in defining lineament patterns across Australia is well known. I never properly knew him. He left the company I worked for literally the day I joined as a newbie. He left in the morning, I arrived in the afternoon. It would be a number of years later when I met him briefly in the late seventies in Adelaide airport. Can't remember exactly when, but it was literally within a day or so of him having received from Western Mining acknowledgement and credit for his pivotal role in the use of structural lineaments in identifying Olympic Dam (Roxby Downs) as a drilling target, .. an acknowledgement that was apparently quite difficult to winkle out of them. 'Lineament' you see, was a naughty word for many. Lineaments were the stuff of the supreme magic art of arm-waving linesmanship. Combining as much geological information as possible, you could recognise them by closing one eye and squinting the subject information with the half-closed other one. They were regarded by many as very much like ley lines, and dismissed as such - more as a joke, than something to be seriously regarded.
The geological significance of such 'ley lines' lay in the control that the deformation of the basement was thought to exert on the crust, particularly in relation to the occurrence of ore deposits which in many cases appeared, somewhat equivocally depending on the degree of one's squinted belief in the principle, to be located along them. Clusters of deposits occurred where two lines (or lineaments) intersected.
Their interest to me lay in the supporting confirmation they gave to another principle, one that had virtually no precedent and that I had myself observed, and which likewise attracted the view that I too was something of a quirky nutcase - that ore deposits were commonly located in the necks of large-scale boudinage structures, and those that were, were most likely formed by the conditions of the neck. 'Ley lines', or lineaments, were simply the necklines of crustal-scale boudinage structures.
The particular connection with Tim's work lay in the Kambalda nickel deposit, some 50km south of Kalgoorlie in Western Australia, close to where I had been assigned to work. Tim had pointed out that the orebodies lay at the intersection of certain lineament structures, and attributed the 'embayments' hosting the orebodies to the effects of shearing of the host rocks, and therefore insisted that the mineralisation was structurally controlled. The consensus of the day however (promoted by the best researches of the CSIRO) was that these 'embayments' were in fact scour channels formed by lava flows during the Archaean, and that the ores were crystal settling deposits on the basal contact - a view that is still generally held, very much mistakenly (in my opinion) to this day.
As a young fella and unaware of this altercation (which difference in exploration terms was highly significant) I had already observed the spatial coincidence that existed between the Kambalda nickel deposits and two of the best-connected large-scale boudinage structures one could hope to find anywhere. The venue for such an epiphany (as these insights often are to the partially prepared) was the office where I was being interviewed for my first job. I had just completed the write-up for my degree (Glasgow), and had travelled to London for the interview.
[ ||| Flashback ] My degree work had been an exercise in structural chronology in multiply deformed quartzo-feldspathic gneisses. Riddled with amphibolite dykes and with some calc-silicate layers these rocks formed the most excellent examples of variation in boudinage structure one is likely to find anywhere.
Essential to the work though not central to it, was recognising the importance of boudinage structure in deciphering structural chronology ; simply put, the older structures tended to be preserved in the more competent boudins, the younger structures were developed in the more ductile host. Some problem lay in scale of approach, but once that was resolved then what before seemed an indecipherable 'pother-of-bother', fitted well .. ("..as tight as a finger in a bum" - as I would later learn from my colleagues in Oz was the better way to describe it.)
A peripheral offshoot of the work was recognising the importance of *large-scale* boudinage structure - important for the implications that followed for understanding the larger chronological framework of Archaean - Proterozoic sequences in Western Scotland generally: megaboudin structures tended to preserve the older terrains, whilst 'younger' terrains (but which were part of the same deformation event) were described by the less competent host rocks - obvious today to the point of hardly needing mention, but not so obvious back then when there were virtually no descriptions of large-scale boudinage in the literature.
With such paucity in the literature I turned to whatever geologial maps I could find, to try to document their further existence. The strike extent of basement rocks along the western seaboard of Scotland was insufficient to address the question, and so I turned to whatever regional maps I could find. The department at the time had few maps of other countries at suitable scales, .. some early editions of maps of Canada and South Africa at scales I don't remember, but also some of Australia at 1:250,000, which did suggest (if not confirm), their existence. Maps of those days were not as accurate or as good as those of today. [Ends Flashback ||| ]
Fig.2. the northern end of my very nice thesis area, Durness, NW Scotland, overlooking Ceannabeinne Beach.
Back to London. This was late 1970, coming up to Christmas. As a student I was entirely unaware of the literature on ore deposits. In those days economic geology was not part of the curriculum, but something you picked up on the job by doing. Had it been, then undoubtedly I would have been suitably inducted into the consensus view, and aware of what to think and what not to. I would almost certainly *not* have formed the "blindingly_obvious_view" that the ore deposits were no more than simply the result of metamorphic reconstitution according to the conditions existing in the neck. With others, I would have had the view that all deformation was a fog to see through, and that the supreme effort of intellect was to be directed towards dismissing the fog and imagining (through it) how these ore deposits could possibly come about - if what was staring me in the face (namely the boudinage-ore association) was ignored.
Returning to the interview it was clear to me from the field maps posted on the wall that here was not only "the two best-connected large-scale boudinage structures of all time", but a graphic illustration of the importance of necks in locating mineralisation. Knowing nothing of ore deposits other than that they could be somehow a product of mobility of ore 'constituents' (if we disregarded coal and sundry alluvials), but knowing much of the variation in boudinage structure, it seemed obvious to me that some sorts of deposits *should* be located in necks; necks after all were simply 'stress sinks' towards which any mobile (ore) fluids or constituents would be drawn.
Thus was set my view that in this case at least, the mineralisation was related to the boudinage. It would only be later on arrival in Australia when relating this view to indulgent colleaugues, that I would realise how off-the-wall they considered it (and me) to be, in (of course) the most colourful of language. Perhaps the most extreme negativity was to be expressed by a senior CSIRO researcher of nickel ore bodies a couple of years later, that it was "an insult to the profession that such an idea should be even remotely considered, when so much is known about how nickel ores occur" - which was quite an impediment for a young lad entering the industry, .. thinking he had a hand on the Holy Grail only to spend a career discovering it to be more like a poison chalice (for reasons entirely other than the geology too.) :-)) (Ah, .. Life!!)
As I came to know Tim's work in relation to lineaments and Kambalda it was clear both accounts were saying much the same thing, only where Tim was demonstrating the deep basement connection, I was providing a much more local expression of structural signature, .. one that was not only more empirically centred in the mapping, but also provided the field evidence for the key to the organising principle whereby the ores actually were constituted. This, however, did not influence the good-natured joshing of colleagues when it came to mention of 'sausages and necking', for by 1971 descriptions of boudinage in the literature considered it to be little more than a small-scale passing curiosity. Obviously it was nonsense to suggest, as I was, that this curio was the instrument whereby the Moon really could be converted to cheese. I sincerely believe much of the language and expletives of Ozspeak must have been coined in response to my 'fanciful excursions' into cheese, ..sausages and necking (when swilled of course with copious amounts of grog as one does, when faced with cheese and sausages, and the possibility of a bit of ... ).
Shortly afterwards, I became aware of Broken Hill, one of the world's largest deposits (Pb-Zn-Ag) in the same terms, and my conviction of the importance of boudinage as a control on the location of ore deposits was set. I assembled a number of other different examples illustrating the point and in 1976 sent it to Economic Geology. A rejection was duly received in August, 1977.
In doing the write-up I was conscious that the absence of large-scale boudinage from the literature could be an impediment, but really couldn't see why, if the maxim about small-scale structures reflecting the larger-scale picture had any value - and so, in order to make a point for their existence I had included a range of boudinage structures at scales that varied from centimetres to several tens of kilometres. However structural geologists and ore deposit specialists who reviewed the manuscript were sceptical, both of the boudinage structures and of the association, and declared themselves to be ".. astonished at the way in which [I] rather superficially eliminated years of work on the origin of copper zinc and nickel deposits" - the deposits in question being Mount Isa (copper /lead-zinc), Broken Hill (lead-zinc-silver), and Kambalda (nickel), amongst others. The overall tone was similar to that previously expressed (above) by the CSIRO specialist on nickel deposits. Clearly there was something more than good-natured joshing afoot, and I considered myself very lucky this to be the only response by those colleagues closest to me, .. langwidge notwithstanding.
.................
Following rejection, a colleague suggested I send the manuscript to Jan Kutina in America for consideration for publication in the Journal Global Tectonics and Metallogeny, which I did. Jan in turn recognised the similarity with ideas current in the then Soviet Union and agreed publication. That paper published in 1982 formed the basis for later, fuller descriptions of boudinage for the invidual deposits, and a final round-up resubmitted as a note to Economic geology in 1998 was passed with substantially more positive comments than before. Evidently incubation and circulation in the interim helped somewhat.
I was priviledged to meet Jan some time later when in transit to a short job in America, in about 1983 /1984. I will always remember him for his kindness, and his absolutely indefatigable energy and determination to rise above the tribulations arising first from the Russian invasion of Czechoslovakia,
and second from the paranoid "Reds under Beds" syndrome that still posessed America at the time.
No doubt Jan's preparedness to publish stemmed partly from a small book I was unaware of, but which would have common currency in Eastern European literature at the time. It would be another year before it was drawn to my attention by the office librarian who was perusing the still-in-progress newly digitised catalogue of the libary of the Geological Society of America : G.V. Tokhtuev (1967), Boudinage structures and their role in the localisation of ore mineralisation, with examples from the Ukranian Shield and other regions, Kiev, Naukova-Dumka, 215pp (in Russian). The title for my original manuscript to Economic Geology was, "Boudinage, a structural control on the location and formation of metalliferous ore deposits in orogenic belts". However, since reviewers had objected to the existence of large-scale boudinage structures, and since I had come to the view anyway that the operative process was one of making necks rather than boudins (a fine point), I changed the first word of the paper to 'necking'. Probably not for the better. (Acknowledging precedents, .. after all.)
Thus, thanks to Jan Kutina and the Russian connection was documented the link between outcrop-to meso-scale boudinage structure to the earlier-recognised work on deep mantle lineaments of both Tim and Jan (and others still earlier of Hobbs, 1904; Hills, 1947; Sonder, 1956; Hupe, 1958, Carey 1962 - amongst others, and especially Menard, 1964). Especially Menard, because Bill Menard was hands-on with the emergent theory of Plate Tectonics and had the authority to step out from the continents to the ocean floors and detail the extent of mantle lineaments. One at least has its catastrophic surficial expression in the San Francisco Fault and leaves no doubt of the control that mantle lineaments have on the continental crust. With the recognition of mantle lineaments (Fig.1) the ridicule of "squinty-eyed, arm-waving linesmanship" in relation to mineralisation suffered by Kutina and O'Driscoll prior to the advent of Plate Tectonics was debunked. If some of the examples appeared ambiguous, 'linesmanship' itself was vindicated.
[ (Conspiracy theory?)] Despite the declared scepticism from Economic Geology it seems to me in retrospect quite possible that the manuscript circulated for peer review in 1976 could have seeded the ideas of boudinage as a model for understanding the metamorphic core complexes of the Basin-and-Range of North America ( Coney, P.J., 1980, Cordilleran metamorphic core complexes, an overview, in Crittenden et al., (eds), Cordilleran Metamorphic Core Complexes, Memoir of the Geological Society of America, v.153, pp.7-34)
In the then absence of any precedent for descriptions of regional-scale boudinage it seemed to me only natural that anyone proposing such a model for the Basin-and-Range would take care to document, as I had done, the progressive increase in scale of observed boudinage structure that led to such a conclusion, particularly when this conclusion formed a central theme of the publication. However there was none. The view of those authors promoting it appeared to be based on a common 10cm-scale illustration of boudinage taken from 'the garden' so to speak, a curiosity that matched the less-than-a-page illustrations of the day. It seemed only the idea of large-scale boudinage was described, the support from the illustration seemed slim to the point of unjustified. From the descriptions it seemed that the necks were being described as boudins and vice versa, .. and in a later compendium (1987, Geological Society special publication 28, "Continental Extension Tectonics," the same authors did not mention boudinage at all as a model for Basin-and-Range core complexes. More recently however, parallel with the increasing interest in lithospheric-scale boudinage, the authors do appear to have resurrected boudinage as a model for the Basin-and-Range. Of course, *had* my original manuscript seeded the ideas of megaboudinage, it could never be referenced (since it was rejected), so we are left with the question, is this another example of the same model arising in different places at the same time - carried on the back of a 10cm analogue on one hand, and scales ranging to kilometres on the other? Who can tell? ... such paper trails often end in dead ends.
------------
"There seems to be nothing in the arts or in nature which can be compared in mechanical origin to boudinages, which makes them the more interesting and the more worthy of study." (Quirke, T. 1923. Boudinage, an unusual structural phenomenon. Bulletin of the Geological Society of America, v.34, pp 659-650)So Mr Quirke has a point. Can it really take a century for something so obvious, to find its proper status in the geological lexicon? Apparently so. Even by 1990 a textbook in structural geology (Price and Cosgrove, 'Analysis of Geological Structures', 502pps) devoting 30 pages to boudinage, would mention neither the large-scale mportance of boudinage in continental extension, nor its importance in locating mineralisation - even as its introductory sentence states:-
"Relatively little attention has been given to geological structures that form when layered sequences, or rocks with a fabric, are compressed at a high angle to, and/or extended in, the plane of the layering or fabric. The reason for this is not clear, for 'extension' structures are much more common in nature than one would infer from the paucity of literature concerning them. In this chapter two of the main structures which result from such deformation, namely boudins and pinch-and-swell structures, are discussed."
There is little use (to my mind) discussing them if no mention is made of the value for doing so. Otherwise they are simply justifying the issue they complain about. Why would one pay attention to a structure that had no contextual value?
However, thus is brought to our attention the structure that has the potential to address the two most important questions in geology in a century:- 1. By what means is the Earth getting bigger (which has its further extrapolation in the physics of mass creation) and 2. where do we look for ore deposits to sustain our civilisation. Not only are neither of these quesions (with few exceptions) being addressed through the structure that has the greatest potential to answer them , but they are regarded by the mainstream ensconced as it is in the security of consensus, as questions not worth addressing.
One has to wonder, what sort of a scientific culture are we living in (when it comes to geology)?
(Sausages, ..ley lines, .. necking, .. and Plate Tectonics. Reck'n I'll have to see my Aussie mates for some fitting lingo to deal with this one, .. bein' about civilisation an' all. )
[ See also - "Debunking Plate Tectonics", at :-
http://www.platetectonicsbiglie.blogspot.com/ ]
0 comments:
Post a Comment