Accretionary Wedge #33 : Rock Garden

Rock Garden

Rock garden

This month’s accretionary wedge is being hosted by John Van Hoesen over at Geological Musings in the Taconic Mountains, who muses how geologists have incorporated geology into their homes, offices and gardens. My rock collection has been banished from the house and incorporated into an alpine rock garden at the front of the house, where a pocket-sized area on a steepish slope doesn’t lend itself to much else.

Rock Garden

The beauty of having a rock collection like this is that it brings back so many memories of places visited and geology seen.

Rock Garden

In this corner are the folds, including a flow folded rhyolite from Pembrokeshire, Wales and refolded folds from Loch Leven in Scotland. Behind are pegmatitic gabbro from Pembrokeshire, a gabbroic dreikanter from the Atacama desert from Chile, a water-lain tuff from the Lake District of England and a Welsh Old Red Sandstone conglomerate.

Rock Garden

Here are a siderite nodule from Pembrokeshire, blue john from Derbyshire, England, larvikite from Norway and rose quartz.

Rock Garden

Around the back, the acer has just come into leaf on the ‘pebble beach’. Most of the pebbles amongst the ferns and strawberry plants are actually glacial erratics collected from the local soil as we live on the edge of a glacial meltwater channel. They are mostly Triassic quartzite pebbles, but also include Carboniferous limestones and Lake District granites.

Accretionary Wedge #32- Favourite Geological Picture: West Angle Bay

Ann, at Ann’s Musings on Geology is asking for one’s favourite geological picture (only in American) for this month’s Accretionary Wedge.

I’m late for this one and most of my geology photo archive isn’t on this laptop, so I’m going to go with something that I have to hand. This image is one of my gigapans (in this case a matrix of 16 x 6 photographs stitched together). It shows the foreshore at West Angle Bay, Pembrokeshire, Wales. The view looks westwards towards Milford Haven and shows the Lower Carboniferous Limestone contorted by a series of Variscan thrust related folds. One of the thrust planes is seen in the left of the image, over-steepened by the folding. To the centre of the image are a pair of whaleback periclinal anticlines. The beds then steepen again to vertical on the right via a tight syncline.

But the beauty of a gigapan image is that one can dive in and view the detail like the slickenside lineations on the thrust plane or the writing on the buoy.

Accretionary Wedge #31: Geological Surprises – Plastic Earthquakes

Jim Lehane at The Geology P.A.G.E. has set this month’s accretionary wedge task of “What geological concept or idea did you hear about that you had no notion of before (and likely surprised you in some way)”.

I’m going to go back nearly thirty years for this one, to when I was looking for a PhD topic. In the UK in those days geology departments produced lists of potential topics which were submitted to the Natural Environment Research Council (NERC) for approval. NERC would then provide a list of approved topics and a number of grants that they were willing to support, which was fewer than the number of topics approved. As a undergraduate student, one would apply for a particular topic, not knowing if it was actually going to be funded.

I studied for a degree in geology at Imperial College specialising in geophysics in my final year. In the list of approved PhD topics for University College, Cardiff (now Cardiff University) there was a project on “Geophysical Investigation of Small Scale Geological Problems”. This was ideal for me and I applied, only to receive a reply stating that this was one of the projects that they would probably not fund, but would I be interested in applying for a project on “Deep Earthquakes” instead. To be honest, I wasn’t that keen as I was more interested in doing practical, hands on, field geophysics but in order to keep my options open I replied saying yes. In retrospect, I should have gone for Cardiff’s other funded project which involved collecting and analysing palaeomagnetic samples from New Zealand, a place I’d still love to visit. However, after being rejected by Cambridge and Durham for other potential projects I travelled down to Cardiff for interview and to my surprise was awarded the PhD grant. I now had to find out about how deep earthquakes happened, something I had not heard about, certainly surprised me, and still baffles me somewhat today.

Shallow earthquakes are relatively straight forward. Stress builds up in a block of rock containing a fracture, whose two sides are held together by friction. Eventually the stress overcomes the friction and the two sides either side of the planar fracture move past each other, releasing the energy that had been stored previously as elastic deformation in the rock mass neighbouring the fault. The system of forces acting at the source is well known and described as a “double couple”. This gives (amongst other things) the classic quadrifoliate pattern of compressional and dilatational P-waves which can be used to work out the orientation of the fault causing the earthquake.

The model is fine for shallow situations where the rocks are brittle. However, the temperature increases by about 30° C for every kilometre you go down. In areas like California where heat flow is moderately high, by the time you get to about 15 kilometres down the rocks are too soft to deform in a brittle fashion and instead flow plastically. In intraplate areas like the UK where the heat flow is less, the brittle-ductile transition is just over 20 km.

In the shallow part of subduction zones, cold lithosphere is subducted and so cold, brittle material can be found at larger depths and earthquakes within the upper parts of the Wadati-Benioff are still explainable. Earthquakes in subduction zones tend to tail off around 200km down but in some start to reappear around 400 km and continue to the bottom of the upper mantle at 660km. It is these deep earthquakes that are particularly problematic as even in the heart of the subducted slab temperature will be well above the brittle-ductile transition. Brittle shear earthquakes simply can’t occur at these depths.

The depths at which the earthquakes switch off, on and off again (220, 410, and 660km) appear to correspond to changes in mineral structure due to the increased temperature and pressure with depth so this would strongly implicate mineral phase changes in deep earthquake sources. Down to 220 km the mantle is olivine + pyroxene [enstatite], from 220 – 410 km olivine + garnet [pyrope]. At 410 km olivine transforms to ringwoodite [the spinel form of olivine]. At the lower-upper mantle boundary at 660 km there is a further phase change to silicate perovskite (Mg,Fe,Al)SiO3. Since the lower mantle comprises 55% of the Earth by volume this makes silicate perovskite the Earth’s most abundant mineral.

The task of my PhD was to investigate the source mechanics of deep earthquake to see if they had the same double-couple mechanism as the shallow earthquakes or whether they had a non-double-couple source and, due to mantle phase changes, to see if there was a volumetric change component to the source.

Most earthquake source mechanism determination was in those days, and still is today, undertaken using inversion methods which produce a best fit between the observed seismograms and synthetic ones calculated by computer. Some of the inversion solutions for large deep earthquakes had shown a significant non-double-couple component to their source, suggesting that the earthquakes may not be due to planar shear.

The technique that I was to use involved forward, rather than inverse modelling. It had been devised by my PhD supervisor for discriminating between natural earthquakes and underground nuclear explosions. By making measurements of the amplitudes of certain phases (P, S and the near-surface reflections pP and sP) with error bounds, all the possible source types and orientations of those sources that are compatible with those observations are saved. By combining the compatible sources for earthquake records from several recording stations you end up (hopefully) with only a very small range of possible sources and orientations.

The image in the header shows P, pP and sP phases together with a rare S-to-P conversion from the lower-upper mantle boundary S660P

It should be remembered at this time that digital waveform data was in its infancy and my earthquake seismograms were taken from analogue records. These were kept on microfiche in the basement of the British Geological Survey in Edinburgh (next to the core store so it was always cold, even in summer). The correct microfiche had to be found for the right day, at the right time (decoding the time pips by eye – these are the white blobs next to the seismogram) at the right station, for the right component. The record was then blown up on a fiche reader and printed on a wet photocopier (so one finished up the day reeking of petroleum spirt). The earthquake data was also a minimum of two years old as it took that long to collect the records from the recording stations, copy them to microfiche and then distribute those copies. The contrast to today when I can watch earthquake seismograms from around the world on my computer desktop, pretty much in real time, could not be more stark.

Anyway, the upshot of the study was that I never found a deep earthquake that could not be explained with a non-double-couple or any significant volume change. They were all double couple, exactly like the shallow ones. Those earthquakes that had inversion mechanisms suggesting non-double-couple sources on closer examination of the actual seismograms revealed that they were actually multiple earthquakes with significant pulses of energy several seconds apart. I could analyse these sub-events and found that they were double couple but with slightly different orientations. This multiple shearing with different orientations was confusing the inversion solutions which at the time only used long period waveforms and could not distinguish the sub-events giving apparent non-double-couple solutions.

I could also use a similar method using the phase pulse lengths instead of the amplitudes to estimate the speed of the rupture and shape of the rupture surface. The up-going phases are much higher frequency than the down-going ones, showing a strong doppler shift. This means that the ruptures are travelling very fast, certainly faster than the speed of shear waves through the material. This is something different to shallow earthquakes, where rupture velocities supersonic to S-wave velocity are impossible.

So we have a geological conundrum. How is a material that should flow plastically accumulating enough stress to generate a magnitude 8.3 earthquake such as the one that occurred on June 9, 1994 636km beneath Bolivia and generates a shear mechanism indistinguishable from a shallow earthquake (other than perhaps by rupture velocity)?

There has to be some processes (probably involving mineral phase changes) that can cause some shear instability runaway condition that generates a supershear, rupturing at fast velocities generating deep earthquakes in a plastic material. What that process is uncertain, and something we may never know.

This work is over 20 years old and I’ve not been keeping up with the literature, so apologies if some/all of this is out of date. This was never written up other than in the PhD Thesis which maybe still available from the University of Wales

Accretionary Wedge 28 : Deskcrop & Rock365 #300

October’s Accretionary Wedge is being hosted by Matt Kuchta at Research at a Snail’s Pace on the topic of deskcrops. The deadline for the wedge is fortuitous as I can get it to coincide with my three hundredth deskcrop this year!

Back at last New Year’s Eve I was sitting with a group of fellow travellers around a camp fire at a Bedouin encampment on the edge of the Sahara Desert in Morocco exchanging new year’s resolutions. I rashly suggested that I would take a photograph of a rock every day in 2010 and Project Rock365 was born. How long ago that seems. It has been a long slog since but I have now made it to day / rock 300.

I have saved today’s rock for day 300 and the accretionary wedge as it is one of my favourite samples which has pride of place in my home collection. It is a pegmatite sample from the Narestø Feldspar Quarry, Flosta Island, Arendal, Aust-Agder, Norway. The rock contains some really large feldspar and biotite crystals. Unfortunately, I can’t remember but else about the rock and google is not providing me with much help.

The pegmatite was collected on the Keele Geology foreign fieldcourse to Norway in 1991 (in fact my field guide tells me it was on Friday July 12). The Norway fieldcourse was a long tradition at Keele, now sadly superseded. To keep costs down, the geology department (as it was in those days) had its own tents, folding tables and chairs, cooking equipment and gas stoves. We took the ferry to Bergen and traversed Norway twice, out to Oslo and then back to Stavanger, staying at camp sites along the way. I actually did this fieldcourse twice, once in 1989 and again in 1991. We even took enough tinned food to last a fortnight to keep the cost low as Norway can be expensive, but the same logic didn’t quite work the following year when we went across the Alps and actually took tinned Italian tomatoes back into Italy!

The fieldcourse mainly covered high-grade metamorphic and igneous rocks so I was not that much use of the teaching side apart from the structural mapping at Slemmestad and some of the Caledonian nappe structures at Röldal, my role was much more that of van driver. I did however learn a huge amount from our former mineralogist, George Rowbotham.

Matt asks if we could include a scary dimension to the post. I can’t really think of anything scary except I’ve a sneaking feeling that the sample might be a bit ‘hot’.

All three hundred deskcrop images can be found on my companion posterous blog and at my flickr site.

A google map with links to the geotagged images is embedded below.

View Rock365 in a larger map

Accretionary Wedge #27 : Important Geological Experiences

Lockwood DeWitt at Outside the Interzone is hosting this month’s accretionary wedge where he asks “What is the most important geological experience you’ve had?”. The stress here is on the word important.

Picking the most important is incredibly difficult for me. I have been fortunate in my earlier career to have all sorts of important geological experiences, from climbing the summit of Mt Fuji in Japan to exploring the deepest wastes of the Atacama desert, from standing at the top of Monte Perdido in the Pyrenees to the bottom of the Black Canyon of the Gunnison in Colorado.

However, the most important for me has to be my undergraduate mapping in Lukmanierpass, Switzerland because it was important to me on so many different levels. I’ve already covered this way back in Accretionary Wedge 11, Field Camp, so I’ll try not to repeat myself too much.

The first part of the importance is because it was a turning point in my life, the point where I grew up. Up to that point I had had something of a sheltered upbringing. I had not travelled abroad except for a “De la Beche Club” (the student geology society at the Royal School of Mines) cycling geology field weekend in Northern France, and I certainly hadn’t been abroad alone. There were three of us sharing a large frame tent in Switzerland, but the car could only take two plus the tent, so I had to make my way there by train. The Swiss railway system is incredible and runs to the second. It was the first time that I saw proper mountains. The metre gauge train from Göschenen at the northern end of the Gotthard Tunnel climbing up to Andermatt is an experience in itself. Travelling alone across Europe gave me the confidence to go to so many other places since then.

(Note: I’ve converted these to black and white because the older colour photos have faded badly)

It was also the first time that I had done proper independent mapping. Prior to this our mapping training was done as buddy pairs but here I was on my own. We wouldn’t be allowed by health and safety regulations to do this today, which is a real shame because it was a wonderful experience. It was just me against the rock. I had to sort things out for myself. It took me about four weeks to work out why in one part of the area the bedding/cleavage relationship was telling me that the beds were upside-down where as I knew from the stratigraphy that they were the right-way up. It was a struggle, but I cracked it – myself.

The geology was incredible. I’ve never really seen anything like it before of since. The sediments trapped between the internal and external basement zones of the Alps exhibit one of the highest metamorphic gradients in the world with one unit going from amphibolite grade (shown above) to sub-phyllite in just a couple of kilometres. Some of the faces with white kyanite acted a mirrors in the bright sunlight. You couldn’t examine the mineral texture without sunglasses.

My last day in the field was my 20th birthday. After almost six weeks mapping, I had just one last valley to map. And, halfway up the valley I found a rock that I was not anticipating to find. This has taught me never to assume anything where mapping is involved and always check everything out. I had to work very hard to sort out that valley’s geology because I had a train booked home the following morning. I returned to camp absolutely exhausted but ultimately triumphant.

Google Earth view of my undergraduate mapping area, from the lake to the top of the ridge in the middle distance. The foreground ridge by the lake is Precambrian external zone basement gneiss and the middle distance ridge is Precambrian internal zone basement gneiss. Between them is a sliver of highly deformed and metamorphosed Mesozoic sedimentary cover rocks.