Lewisian Gneiss Complex controversies - setting the scene

On a holiday in 2008 to the NW Highlands Geopark I had the opportunity to view the Lewisian Gneiss Complex that represented part of the Earths lower crust and the oldest dated rock in the Lewisian Gneiss complex is just over 3 Billion years old, a vast depth of time even by the standards of geologic time.

Lewisian Gneiss

The literature I had read beforehand was aimed at the general reader and hinted at the nature of the geology, but didn't quite prepare me for the actual field exposures. Most of the rocks I had seen up until then were of igneous, volcanic and sedimentary origin, deposited on the surface or at a high level in the continental crust, at a low grade of metamorphism and relatively undeformed. Some of the rocks of the Lewisian Gneiss Complex have undergone episodes of high grade metamorphism and intense deformation, to produce a distinctive and perplexing rock. To make sense of it requires an appreciation of the heat and forces that the rock has been subjected to.

Granite sheets intruding mafic gneiss and subjected to deformation in a tectonothermal event.

There is unanimous consensus amongst geologists that pressure and temperature increases with depth in the Earth and that mechanical properties of rock progressively changes with depth in the crust, from brittle to ductile and finally molten.  Rocks buried deep in the earth, behave more like plasticine than those at the surface.  Fig 1 is an illustration of a simplistic high grade deformation of a rock body intruded by plutonic rocks and then subjected to progressive higher grades of deformation in a single episode. A ~ original relationships. B, C and D ~ increasing deformation or strain. To get to D requires a surprisingly large dimension image files in photoshop to simulate the strain to generate Gneiss banding.

Figure 1 Simulation of the intense strain to deform rock into Gneiss  

Gneiss is the name given to a metamorphic rock that has undergone intense deformation at high temperatures and pressures.  Fig 2 illustrates how previous deformation events can be overprinted by an event the causes intense deformation.

Fig 2 -  Intense deformation overprinting previous deformation events

Rocks that are several billion years old have unsurprisingly undergone several deformation events at high temperature and pressure (tectonothermal events). The banding in gneiss allows relict structures to be preserved in areas of relatively low strain and provide an insight into the geological history. Fig 3 A is a block of banded gneiss. B is a deformation event inducing structural folds with vertical axes. C is a deformation event causing a fold with a horizontal axis. D and E represent progressive deformation. F is a detail of the recumbent isoclinal folds

Fig 3 - Deformation of banded gneiss and relict structure

Recumbent folds in Lewisian Gneiss

Lewisian Gneiss terrain and cross cutting pegmatite dykes. Note figure for scale.

Illustrated cross cutting pegmatite dykes

Cross cutting relationships detail - Gneiss cut by pegmatite dyke and then both are cut by a 2nd (pink) pegmatite dyke

Highly inclined in-weathered Scourie Dyke cutting through Lewisian Gneiss

Illustrative interpretation of in-weathered Scourie Dyke cutting Lewisian Gneiss

In the late 19th Century, officers of the Great British Geological Survey started work mapping, recording, sampling and examining the petrology of the constituent rocks of the mainland Lewisian Gneiss Complex, to elucidate its history and nature. In 1907 a memoir titled "The Geological Structure of the North West Highlands of Scotland" was published, that covered the observations and findings of some of Great Britain's most experienced and respected geologists.

Gneiss (Grey) intruded by Scourie Dyke (Black) then intruded by Granite (Pink) and deformed by tectonothermal event(s)

The Geological Survey broke down the Lewisian Gneiss Complex into two divisions 1) Gneisses, the majority of which had affinities to plutonic igneous rocks with a wide range of petrological characteristics and a minority of presumed sedimentary rocks. 2) A great series of intrusive rocks that like the gneiss had a wide petrographical range. Intrusive rocks were used as markers to constrain deformation and mineral changes within the intrusive rocks and provided evidence and clues on the phenomenon of metamorphism.

Realtively low strain outcrop of Mafic gneiss (Dark) veined by Tonalite or Tondhjemite (white). Possibly a migmatite.

The effects of earth movements and metamorphism were also recognised in the structure with planes of shear, folding and thrusts, by noting the changes in gneiss banding orientation, deformation of intrusive rocks, relict structures and changes in mineral assemblages. The deformation intensity was to some extent heterogenous and also varied in style through the districts.

Weathered exposure of gneiss with a refolded fold.

Deformation and metamorphism were heterogenous in nature at all scales, it was observed that a rocks mineral assemblages could change across a hand specimen or even in a sample prepared for microscopic examination. The phenomena of changes in mineral assemblage of rock also seemed independent of deformation and gneisses with pyroxene passed imperceptibly into hornblende by the hornblende replacing pyroxene.

Highly inclined pegmatite sheet cutting moderately inclined gneiss 

Despite the great heterogeneity of rock petrology and deformation, the Lewisian Gneiss Complex was divided into 3 districts : North, Central and South, on the basis of each district having its own fairly distinct mineral assemblages and representative styles of structural deformation.

Granite sheets intruding mafic gneiss and isoclinally folded 

The fieldwork and petrological analysis by officers of the Geological Survey produced a solid foundation for geologists to continue further investigations into the geological processes that had acted upon the Lewisian Gneiss Complex. The next post will review some of the controversies the geological processes that have formed the Lewisian Gneiss Complex and its history.

Geology Controversies

I find it interesting to read about geology controversies and the rationale underpinning the opposing hypotheses, that provides insight into the evolution of the 'discussions' to a general consensus. The greater the controversy, the more likelihood it has of making it to a wider audience and that is when someone with sufficient perspective can outline the respective arguments in terminology a non specialist can understand. Geology controversies tend to be long running affairs as entrenched parties present more evidence to substantiate their hypotheses and refute opposing hypotheses. When the evidence comprises actual bedrock outcrops, there is also the opportunity to walk the same sections and understand better their significance.

Some older major controversies have been well documented with books written to provide an historical overview of the controversy. Two books that I found particular interesting as they cover rocks in areas that I have visited, deal comprehensively two notable 19th Century controversies in the UK :

The Cambrian Silurian controversy  ~ Ordering geological stratigraphy which ultimately resulted in the establishment of the Ordovician.



Highlands Controversy ~ A bitter dispute on geological stratigraphy and a triumph of geological fieldwork to elucidate the tectonic structure.



It would be reasonable to suggest that geologists have a greater appetite for robust discussion than other sciences, which does add a certain frission to reading a geology paper on a controversial topic. One current long running 'heated' controversy is Archean Plate tectonics and as an example of robust discussion I have cherry picked some statements from the Introduction to a paper by Warren B. Hamilton in a memoir by the Geological Society of America.

"This essay is a study in alternatives. Most current interpretations of geodynamics and of evolution of Earth are forced to fit popular but dubious assumptions that Earth fractionated slowly and is still largely unfractionated, and that rocks of all ages must be explained with plate-tectonic processes combined with plumes rising from basal mantle. The data accord better with opposite interpretations. Earth largely fractionated very early in its history, plate tectonics did not begin operating until late Proterozoic time, and deep-mantle plumes do not operate now and did not affect the Archean Earth."

"Our planet had a hot, violent beginning, and has evolved only slowly toward its present dynamic patterns. The geologic record of the young Earth differs profoundly from that of the modern one in crustal architecture and in rock types, assemblages, and structural and magmatic histories."

"I have seen hundreds of exposures of early Paleozoic to middle Tertiary subduction mélanges around the world, but not a suggestion of one in the Archean."

"I see no plate-tectonic interactions in the Archean record. Most plate-tectonic interpretations for the Archean are based on weak compositional analogies with modern rocks of known settings, or with imagined products of hypothetical plate-related settings that have no modern analogues. Although this is a valid approach in the search for explanations, its implicit predictions are not then tested against geologic data." 

"The cartoons drawn to illustrate such schemes (e.g., Kerrich and Polat, 2006) are unrelated to anything seen on the ground." 

"Modern-mode plate tectonics, with high-pressure, low-temperature metamorphism in its sutures, began only much later, in Neoproterozoic or very early Paleozoic time (Stern, 2005, 2007; Tsujimori et al., 2006)." 

Hamilton, W.B., 2007, Earth’s first two billion years—The era of internally mobile crust, in Hatcher, R.D., Jr., Carlson, M.P., McBride, J.H., and Martínez Catalán, J.R., eds., 4-D Framework of Continental Crust: Geological Society of America Memoir 200, p. 233–296  

In the next post I will look at an area of the UK that with an exposure of Archean bedrock and the current controversies associated with it.

Mise-en-scène

With vast sums of money in play in the film and television industries, there is an incentive to examine the variables that contribute to successful and not so successful productions. It is not an exact science as there are some productions that are phenomenally financially successful whilst failing to satisfy many variables considered important and conversely there are productions that are miserable financial failures, despite satisfying many variables considered important. One of the variables is the Mise-en-scène, a term that appears to have many definitions for what is in a scene that contributes to the film/production narrative.  The average feature film has 144,000 frames, the success or failure of a feature film rarely hinges upon a single frame, whereas landscape photography is all about a single frame.

In my humble opinion mise-en-scène has some relevance to a landscape photographer in determining a narrative for an image. For viewers of landscape photography, especially critical ones, there's the objective application of the mise-en-scène to an image. Subjectively you may not like an image, but objectively you can appreciate an image by considering the mise-en-scène.

The wearing down of mountains ~ Norway 2012

The image above was one that resulted from a desire to make a landscape photograph with a narrative of active geomorphological processes. The last glaciers melted somewhere around 12,000 years ago in the UK, so a trip was made to Norway and Google maps identified a suitable area. The foreground is an outwash plain with a braided meltwater stream undergoing channel avulsion. The backdrop is a glaciated mountain environment, with some relict glacial landforms and mantled in part by flora in autumnal colour. The lighting could be better, but I'll take dappled sunlight over clear blue sky/leaden blanket cloud. The image will not be to most viewers liking, that's the nature of the world, but appraising the mise-en-scène and hopefully an objective appreciation of the image narrative becomes apparent.