Mike Searle on Ophiolite
Transcript
Note: Transcripts are not fully edited for grammar or spelling.
Oliver Strimpel
This is the 50th episode of Geology Bites. I'm Oliver Strimpel, and, to celebrate the occasion, I'm really pleased to welcome back Mike Searle, who was our guest on the inaugural episode. Our topic today is ophiolites and particularly the most extensive and best preserved ophiolite on the planet, the Semail Ophiolite in Oman. Mike Searle is Emeritus Professor of Earth Sciences at Oxford University. Until he was grounded by the pandemic, he conducted a research trip to Oman every year since the early 1980s and was part of the team that made the first geological maps of the Semail Ophiolite. Mike Searle, welcome back to Geology Bites.
Mike Searle
Thank you very much, Oliver. Very nice to be back.
Oliver Strimpel
What is an ophiolite?
Mike Searle
Well, the definition of an ophiolite is it's a slice, a thrust slice of oceanic crust and upper mantle that has been emplaced onto the continental margin. The word ophiolite comes from the Greek ophius, which means snake. And lithos, which is rock and its snake because the rocks are green. Now usually oceanic crust is out in the oceans and you very rarely see it on the continents, and where you do it is indicative of plate margins or where some sort of collision between continental crust and oceanic crust has occurred in the past. So they're extremely valuable rocks because you can't actually walk around the ocean bottom, but you can walk around the mountains where there's ophiolites preserved. And this is one of the major evidences we have for what the structure and the composition of the oceanic crust and upper mantle is like.
Oliver Strimpel
Are they pretty rare? Or do we see them all over the place?
Mike Searle
Well, they're common, but volumetrically they're pretty rare. Where you do see them, for example, along the Indus suture zone, the collision between India and Asia, they mark the relics, places where oceans have closed. In this case, the Tethyan Ocean. But they go back all the way as far as the Cambrian, and actually even further back into the Precambrian. And they mark ancient oceanic plates.
Oliver Strimpel
Can we talk a little bit about the history of their discovery?
Mike Searle
Yes, certainly. It's a very interesting story because the whole thing really starts off with the Steinmann Trinity. Gustav Steinmann noticed back in about 1890 the intimate relationship between peridotites and basalts and cherts. Peridotites are dark green rocks. Basalts are black usually, and cherts are usually bright red, so they form a lot of these multi colored mélanges you see in places like the Alps and the Zagros Mountains in Iran. And he was really the first person to say that these rocks were all intimately associated, and it became known as the Steinmann Trinity. And then the next great breakthrough was when really the plate tectonic revolution started. And that started with the great Harry Hess, who was a marine geophysicist, and, in 1955, he basically mapped the oceans using sonar. He mapped out a lot of the things like the mid-Atlantic Ridge. And dredging bits of serpentinite. So he thought that a lot of the ocean floor was made of serpentinite. And it wasn't until 1972 where a lot of people were working on ophiolite complexes like Cyprus and the Bay of Islands in Newfoundland, that this conference was convened, and they decided on an absolute definition of an ophiolite. And this is where the whole ophiolite story really began. That Penrose conference was in 1972. And all the names of the great and the good were there. So Ian Gass, who first proposed that the Cyprus Ophiolite Troodos was formed by spreading at a mid-ocean ridge. Bob Coleman, who worked on the Franciscan mélanges in California, and ophiolites all over North America. Eldridge Moores, who was part of the great cabal of geologists that discovered plate tectonics in the geological record. And, of course, John Dewey and John Bird, who wrote the absolute seminal paper “Mountain Belts and the New Global Tectonics” in 1971. And this was really the paper that set off the whole of the great mapping of ophiolites around the world because you could actually figure out where these plate boundaries were, where the ancient oceans were in time and in space. It was all a very exciting time with the discovery of plate tectonics. Vine and Matthews discovered the magnetic reversals along mirror images either side of the ocean ridges, and it was realized that the Mid-Atlantic Ridge was formed by upwelling of mantle and fractionation of basalt, pushing the older rocks aside and creating new oceans.
Oliver Strimpel
When you come across the ophiolite in Oman, what does that actually look like?
Mike Searle
It looks like nothing on earth because they are spectacular mountains. There's whole mountain ranges made of almost entirely olivine and orthopyroxene, and it's a real lunar landscape. It's a dry desert country, so there's very little vegetation except in the wadis. And if you climb up some of the big peaks there, Jebel Akhdar, Jebel Nakhl, and you look over, you can see 20-30 ridges stretching off into the distance of this spiky black brown rock. It looks like something out of Lord of the Rings, you know. It's not real. And it's beautiful. It's absolutely beautiful, desolation personified. But they're very tricky rocks to work on because most of it is harzburgite, olivine, orthopyroxene, and it weathers to serpentinite. So it's very loose and crumbly. And you spend your life falling over, scratching your knees and crawling up these scree slopes. And it's amazing how the rocks have survived. You know, if it was a wet country like India that it would all have been eroded down to nothing.
Oliver Strimpel
OK, so naively one would not expect to see oceanic lithosphere on top of continental rocks. Especially since oceanic rocks are denser than continental rocks. So how can we go about explaining the existence of ophiolites?
Mike Searle
Well, that's the critical point is that oceanic crust and mantle is denser than continental crust, and that's why the continents sit on top of the mantle because the continents are made dominantly of quartz, plagioclase, sedimentary rocks, metamorphic rocks, whereas the oceans are made almost entirely of basaltic, gabbroic foliated rocks, and the mantle, of course, is made almost entirely of peridotite, olivines and pyroxenes. So the only way you can get a bit of denser oceanic crust on top of the continent is to basically thrust it, and these are the great thrust sheets that you see along the base of the ophiolite complexes. So, for example, in Oman, which is the most spectacular ophiolite of all, you can actually walk on the Semail thrust at the base of this 15- to 20-kilometer-thick thrust slice of ophiolite as it's emplaced from northeast to southwest across continental crust. Now, of course, ophiolites form in all sorts of different environments, so the one I've just described, which is the classic Tethyan thrust sheets of ophiolites emplaced as thin sheets across continental margins. But they also occur in places like the Franciscan in California, where they occur in accretionary prisms. And they're much more broken up here. They're quite often in mélanges, and they're associated with high pressure rocks, blueschists and eclogites and deep sea sediments. And they're much more complicated. And the third place you get them is along sutures zones, like the one along the India-Eurasia suture zone in the Himalayas that I've just described. And that's really the trace of the relict ocean where there was once a very wide Tethyan Ocean, most of which was subducted. 99% of the oceanic crust in Tethys was subducted to the north, and it was only very rare bits of ophiolite were emplaced onto the Indian continent prior to the collision that formed the Himalayas.
Oliver Strimpel
So those are the contexts, if you like. Do we think all of these have a common explanation, or do they point to different kinds of origin for the ophiolite?
Mike Searle
They point to different mechanisms of emplacement of the ophiolite, but all ophiolites originated in the oceans. By definition, they are oceanic crust and mantle, and there was a great debate whether they were formed along mid-ocean ridges like the Middle Atlantic Ridge or the East Pacific Rise. Or whether they were formed in back-arc basins and spreading centers behind island arcs like you see in the Marianas and the Bonin Islands, for example.
Oliver Strimpel
Let me just interject here to explain what island arcs and back-arc basins are. When one ocean plate subducts beneath another, water released by dehydration reactions in the down-going oceanic plate rises into the wedge of mantle material above it, which causes it to melt. The melt erupts through the overriding ocean plate, and if the eruptions are big enough, they emerge from the ocean to form a chain of volcanic islands, which is referred to as an island arc. Can you explain what back-arc basins and back-arc spreading centers are, and how they relate to subduction zones?
Mike Searle
If you look at the Western Pacific, for example, you have the oldest ocean crust known, which is Jurassic in age, subducting towards the west under the Philippine Island arc and the Izu - Bonin Island Arc, which goes all the way up to Japan, and that's where you have the trench. The trench is the actual plate boundary. And in the Marianas the trench is 12 kilometers deep. It's the deepest part of the Earth's crust. Much greater than the highest mountains, of course. Mount Everest is only 8,850 meters and the bottom of the Mariana Trench is about 12 kilometers. So above the trench you have the scraping off of the down-going plate, so it's a bunch of sediments, blueschists, all sorts of oceanic rocks in a thrust belt that sits in what we call the forearc region. And as you're going away from the subduction zone, you have the major island arc, which in the case of the Western Pacific, is the Mariana - Bonnin arc. And then behind that to the west, you have actually a back-arc spreading center. It's like a mini mid-ocean ridge that is spreading but above an active subduction zone.
Oliver Strimpel
OK. So you were saying that there was a great debate as to where the rocks that made up the ophiolite were made.
Mike Searle
Well, originally the idea was that all these ophiolites were formed at mid-ocean ridges. And a giant leap was made by Miyashiro, the Japanese geologist, in 1973, who was the first person to say that Cyprus, the Troodos Ophiolite, was formed at an island arc, which was a major departure from the mid-ocean ridge model. Miyashiro based that almost entirely on major elements, notably titanium. And that led on to a whole series of geochemistry work being done on ophiolite basalts all over the world, and the leading proponents of this were Joe Cann and Julian Pearce, who invented all the trace-element discrimination diagrams. So what they did was very clever. They were using geochemistry data, in particular the immobile trace elements, so we're talking about things like titanium, yttrium, zirconium, niobium, chromium. And these would give you an indication as to the tectonic setting. So, for example, the alkali volcanics that you see in places like Hawaii and the hotspots were geochemically very distinct from basalts you see erupted along the mid-ocean ridges. And again, both of those are very distinct from this sort of andesitic type lavas that you see along places like the island arcs of the Western Pacific. So then a lot of geochemists started looking at geochemistry of the ophiolite basalts, and, lo and behold, almost all of them come out as being not related to mid-ocean ridge basalts at all. Much more related to island arc material. In particular these rocks called boninites, which were originated from the Bonin Islands. And these are high magnesium andesites that you only find in forearc regions and island arc regions. For example, all the upper pillow lavas in Oman in the ophiolite and in Cyprus all have a boninitic chemistry, so there's no way those ophiolites could have formed at a Mid-Atlantic type ridge or any Pacific Rise type Ridge. They had to have formed at a spreading center above a subduction zone.
Oliver Strimpel
In addition to the geochemistry, is there also evidence from the physical structure of the ophiolite, structural geology if you like, as to how the ophiolite formed?
Mike Searle
Well, if you take the case of the Omani, if you like, which is by far the biggest and best exposed and best studied ophiolite anywhere in the world today. It is located in Eastern Arabia. The Semail ophiolite there is about 700 kilometers long, it's nearly 200 kilometers wide. It's an absolutely spectacular thrust slice of extremely well preserved oceanic crust and mantle. So you can actually walk around magma chambers in Oman. You can walk from the gabbros up through the sheeted dykes to the pillow lavas at the top. You can walk along the Moho for 500 kilometers along strike. Of course, it's all desert mountains. So two things about the ophiolite in Oman is one is the geochemistry of the lavas, which all suggest that they formed above a subduction zone. The second major point is the metamorphic sole. So the metamorphic sole is an absolutely critical piece of information. These are rocks composed of amphibolites, in which all pressure - temperature conditions are upside down. Highest pressure and temperature rocks are at the top. Lowest pressure - temperature rocks are at the bottom, which means that the heat to make the metamorphism can only have come out of the mantle wedge.
Oliver Strimpel
OK, so as the subducting plate dives under the overriding plate and enters the mantle, the rocks on its upper surface are exposed to the very much hotter material in the mantle wedge. So the topmost rocks reach the highest temperatures, as they're in direct contact with the mantle. While the rocks below are a bit shielded and so only reach lower peak temperatures, and we say this is inverted because in an equilibrium situation it is always the lower rocks that are hottest. Have I got that right?
Mike Searle
Yes, that's exactly right. So then you can do thermo-barometry on these amphibolites. And we find that they're formed at pressures anything up to 1213 kilobars, which is equivalent lithostatically to depths of between 40 and 50 kilometers. So that's much deeper than the thickness preserved in the ophiolite, which, remember, is 15 to 20 kilometers thick maximum. So you must have a subduction zone that takes basaltic rocks and sedimentary rocks down to depth. At the same time as the ophiolite, volcanics, and gabbros are forming in the crust. And this is the key, the geochronology of the ophiolite. So when you date an ophiolite, you're dating tiny zircon crystals of the ophiolite. And you can also date zircons from the amphibolites in the sole. And they're both exactly the same. They all overlap within error, within one million years between 96.5 and 95.5. And when this geochronology was done, it was astonishing. They came out absolutely exactly the same. The ages of the sole and the ages of the ophiolite. And that pretty well convinced people that ophiolites cannot have been formed along a mid-ocean ridge. They must have been formed along spreading centre above a subduction zone. And because they're in Oman, there's no real big arc. You know, you don't see big arc volcanics like you see in Japan or even the Bonin Islands or the Marianas. What you see is a pile of pillow lavas with boninitic geochemistry. Which is why we now call them supra-subduction zone ophiolites rather than island arc ophiolites.
Oliver Strimpel
OK, now that we know that the ophiolite rocks were made above a subducting oceanic plate, where was all this happening in relation to the Arabian continent, and can you run through the sequence of events that emplaced the ophiolite onto the Arabian continental margin?
Mike Searle
Yes, we know that the subduction initiation occurred about 96 to 95. We know that the ophiolite crust formed at the same time. So the ophiolite, the spreading centers, the pillow lavas were forming at the same time as old Triassic - Jurassic basalts were subducted beneath it to form the metamorphic sole. And then the whole thrusting episode started. And then in Oman 15 million years of shortening an ocean that was 3 or 400 kilometers wide with the ophiolite sitting on top. Fifteen million years after subduction initiation, the continent, the Arabian continent arrived at the subduction zone. And the Arabian continent is composed of all sorts of quartz and plagioclase-rich rocks that are not happy going down a subduction zone. It just happens that the rocks that we see at the deeper structural levels in Oman are absolutely spectacular eclogites. Eclogites are made of garnet, clinopyroxene, omphacite. In the case of Oman, we have beautiful blue glaucophane, which is an amphibole, the sodic, amphibole, and phengite, which is a white mica. And those rocks are occurring in the basement, so they're the protoliths of the eclogites in Oman are actually Permian sills that were emplaced into the base of the shelf carbonate sequence way back in the Permian, 250 million years ago. And when the continent reached the subduction zone, the continental crust got thinned and pulled and stretched, and it was pulled down to eclogite facies depths. So the eclogites had pressures of about 21 to 22 kilobars, which equates to about 100 kilometers depth. And then you must have had some sort of slab break off, otherwise those rocks would have continued to subduct. But because carbonates are definitely not happy in the mantle, they're way, way lower density than anything in the mantle. So they pop up. It's a bit like taking a cork down to the bottom of the swimming pool and then letting it go. So it's really just a conveyor belt, the subduction zone in Oman is a conveyor belt of material subducting from the continent down to 100 kilometers depth, breaking off the slab. The mantle says whoa, that's enough. Cannot possibly have you horrible quartz-carbonate-bearing rocks down in the mantle. And they pop straight up using buoyancy. We know they go from about 100 kilometers depth to something like 20 or 30 kilometers depth within one million years. And the ages in Oman are approximately 80 million years, so 15 million years after the ophiolite formed and the metamorphic sole formed. And then the really high pressure exhumation of the subduction zone was the final thing that happened in Oman. So the ophiolite process in Oman took 23 million years from start to finish, and then after that there was a 20-million-year gap of totally quiescent shallow water deposition where the whole ophiolite and everything was overlain by limestones.
Oliver Strimpel
OK, let me see if I understand the sequence of events here. The subduction zone we're discussing started 3 to 400 kilometers off the Arabian continental margin, somewhere out in the middle of the Tethys Ocean. As the plate subducted to the northeast, it generated back-arc basin type volcanism with the spreading centers and pillow lavas that you described. Which was how the material that was later to become the ophiolite was formed. Eventually, after 15 million years, all three to 400 kilometres of the subducting plate had been consumed and the margin of the Arabian continent, all the while being dragged along behind the subducting oceanic lithosphere, subducting to the northeast, arrived at the subduction zone trench. The pull of the dense subducting oceanic lithosphere dragged the continental margin down the trench, in effect causing the overriding plate consisting of the ophiolite rocks to be thrust on top of it. This is the actual obduction mechanism that emplaced the denser ophiolites on top of the lighter continental rock. Then, when the leading edge of the continent had been pulled down to a depth of about 100 kilometers, the oceanic slab, which perhaps having heated up and become weaker, broke off, and the subducted continental rocks then shot back up to the surface. And these are the eclogites that you mentioned. Have I got that about right?
Mike Searle
Yes, you have. That's a very good description.
Oliver Strimpel
So this mechanism seems to explain quite a few things. You've mentioned the geochemistry, you mentioned the inverted metamorphism, you mentioned these high-pressure adjacent continental rocks, the ultra-high-pressure rocks, you mentioned the radiometric dating. It all seems to line up. Now does that mean that we think that all the ophiolites around the world were formed above subduction zones?
Mike Searle
Well, that's a very good question, and one that's being debated all the time. Like I said, there was this huge paradigm shift in the mid 1970s. But there are places in the oceans where the ophiolites could have originated and still be parts of the ocean crust. One of them is Macquarie Island, in the South East Indian Ocean, which is really just an uplifted slice of Indian Ocean crust. So they don't have to be, but most of the ophiolites that form large intact thrust sheets like you see along the Tethyan margins, almost all of them have to have been some sort of supra-subduction ophiolite.
Oliver Strimpel
On one of your very numerous trips to Oman you explored the coast from beach to beach, scrambling along rocks and swimming around steep, rocky headlands. And there you discovered some ultra-high-pressure continental rocks adjacent to the ophiolite, which I'm assuming are the eclogite rocks that you mentioned a moment ago. Can you tell us about that discovery, and really what the significance of that was at the time?
Mike Searle
Well, I was very lucky that I was one of the first cohorts of PhD students of Ian Gass at the Open University, and we were sent out to Oman in the late ‘70s when Oman was an absolutely spectacular country. But there were no roads, no schools, no hospitals, nothing. It was a really primitive place, so we used to just pack our kit in the Land Rover, head off into the mountains for two weeks, sand tires and arrays of Jerry cans of petrol on the roof, and just go and map this pristine mountain belt. It was absolutely wonderful. And then when I finished my PhD, I was lucky enough to get post-doctoral fellowships to carry on working there. And on one of those trips, it was very clear that as you drive from south to north across the Muscat region that the deformation got more and more intense, and some of those bodies had some of the most incredible folds that you've seen. I mean, really, really spectacular, Helvetic-type nappes on mountain scales. Right down to micro scales. So on one of these exploratory trips, I just sort of went around the coast from a little village called As Sifah. And I remember snorkeling around this headland and seeing this black rock down at the end of the beach and scrambled ashore, and it was full of garnets, the size of your fist, almost. Bright red garnets, blue glaucophane, green pyroxenes, and it was clear this was an absolute pristine eclogite that was sitting at depth beneath the Semail ophiolite. And that's really when the eclogite story, the high-pressure story of the basement really came to light. And then we spent the next few years doing pressure - temperature work and dating them to get to the whole story of the obduction process and the final stages of continental subduction and exhumation related to the ophiolite emplacement. So in Oman, it's an absolutely beautiful area where you see the complete ophiolite emplacement cycle from start to finish.
Oliver Strimpel
Is there any effort to preserve these spectacular geological outcrops in Oman?
Mike Searle
Yes, quite a lot. I've spent the last 20 years trying to get a series of geoparks preserved in Oman. You know, I was looking across the border at Dubai and Abu Dhabi and the United Arab Emirates, where the development was absolutely astonishing. Dubai has gone from a fishing village with a few dhows back in the ‘60s to a New York-type skyscraper city within 40 years. And Oman was heading towards that direction. And, of course, the mountains in Oman are absolutely beautiful. They are the most spectacular mountains anywhere in the Middle East and unique in their formation. So I thought, well, rather than you know, just going to a pristine beach and finding it demolished the next year to make a 5-star hotel or a golf course or something crazy like that. I proposed about 50 sites, including four or five UNESCO World Heritage sites, including the ophiolite, the Jebel Akdar, the main huge mountains in the middle of Oman and the Musandam Peninsula, which is this spectacular drowned fjord-type country in the north. And PDO, the Shell company in Oman, put in a million rials, a couple of million pounds, to start this whole process off. And the Omani government is behind it. There are a lot of very good Omani people working now with the government to preserve these sites, and the hope is that they will become some of the best Geo Heritage sites anywhere in the world, certainly in any ophiolite complex.
Oliver Strimpel
Mike Searle, thank you very much.
Mike Searle
My pleasure. Thank you for having me, Oliver.
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