Plate Tectonics at 50 (William Smith Meeting, October 2017) Session 6

I'm I'm Nydia noggin from Imperial College and I'm privileged I'm honored to be able to chair this session this afternoon and we're moving from ocean subduction zone sort of finally or you know onto the continents and we'll stay on the continents for the rest of the afternoon so the first person this afternoon is our keynote John Dewey they talking about plate tectonics and the geology of plate boundary zones so if I may just say one little anecdote John I don't know like I have lots John is MA was my PhD supervisor so there are lots of things I could say but I'll restrict myself to why I first heard John talk in 1998 in Trinity College in Dublin and he came to give a keynote talk that evening on his current theme which was extensional collapse in origins and we had got the biggest one of the biggest lecture lecture theatres in the university in the arts building it was full there were people from the two universities in Dublin people from the Geological Survey and I expect people from around the country and that was of any of you know Irish geology there is there are basically no rocks younger than the Carboniferous or at the Pennsylvanian so anything you ever studied was to do with the Caleta knives and john as was mentioned yesterday you know his fundamental contribution to all of this debate along with them Brian was M was actually on taking if they're applying this marvelous news exciting theory to what happened on the continents and one of the difficult challenges was obviously the Kineton eyes they are Pat is ocean before the Atlantic and John you know had seminal ideas that were of great interest in Ireland so to get to the point he was scheduled to talk for 50 minutes here 10 minutes questioning an hour went by John was still in fo flight an hour and 30 minutes went by and he was still there and there had been no mass exodus so anyway over to John today and you you have 25 minutes and the clock is starting now your video is one of the fiercest graduate students I'd ever and she and she clearly still is now water Pitman and hex P and several other people and I designed him and produce this map in 1973 or thereabouts we call it rather arrogantly the age of everything but you can see that the there's absolutely difference in the oceans and the continents are beautifully expressed in this diagram it shows the plate tectonics looks like it produces a rather simple result of the strong relatively strong strong this is fear with wrist strong torsional rigidity going back to about 160 million years generated the ridges abducted and lost in the trenches and you can see that the continents look rather different if they are very very de Kock geology is very very complicated as any geologist with boots on the hammer knows and gave at Jim bryden's point plate tectonics wasn't made a mess by Joel a geologist the the plate tectonics made continents a mess as we'll see so this enormously complicated pattern was facing us and geologists tended to be very conservative people who mapped an area and they got involved in a little bit and something very important happened to me in 1965 I just been appointed as a young lecturer in Cambridge in 64 and Tuesday Wilson came on sabbatical for the 6465 academic year Harry Hess was also there at the same time they used to come into the Sedgwick and chat and have coffee and so forth and I learned an enormous amount from from Tuesday and also at the same time from Teddy Bullard Teddy Bullard him I won't go into the details of this but he helped me in my career enormous ly just by being a very kind and gentle and thoughtful and and suggestive man but anyway Tuesday Wilson is coming to my room one day he came in in early 65 and John I've discovered a new class so forth my said nonsense we know about every kind of formula exists on the surface of the earth he surely is now a famous model he opened it and closed it I did this night I suddenly risers in the presence of a very clever man and he said I'm not not only that I'm producing a new global global techtronic map with trenches and ridges and transforms and subduction zones and many late later and later in the seventies Kevin Burke and I were talking to Missy if only I'd known Oilers theorem at the time I might have gone on a sphere I might have had the whole theory in 1965 but he oh he here he was producing he called them plates and he recognized recognized them their main character but the most thing the thing he did for me with mostly was not just teach me at this and Harry Hess the same about seafloor spreading and and so forth but also get out of your little local environment in the west of armed I was obsessed with the West of Ireland I loved it I was it was a thing of beauty for me and and I was mapping it and looking at it and in great detailing but religion that your chemistry everything everything to do with every kind of rock there now as I was becoming a little sort of constipated in about this little little area I think so basically to said you know I said go to North America the Appalachians obviously we closed the Atlantic the Appalachians clearly go on there's a continuation the Caledon ideas spread your wings a bit go onto the Appalachians and and what happened at the same time roughly the same time Charles Drake was was in in he came a came and went quite a lot Charles Drake from from from Lamont Columbia was in Cambridge and he he was a very important figure in that he's him for me and he said look John I can I can give you some cash and if you'd like to come to Lamont anytime you want for short long periods and I started doing that in 66 I went across for appears in the summer and I got to know Walter Pitman and Lynn Sykes and lots of people who became very important in the science were very important in the science and they gave me tutorials so I used to flip back and forth between and then I went to a long sabbatical 1967 and I used to fit back and forth between Cambridge and and America and in terms of my office site and also in terms of my geological site I started working Newfoundland and I went down as the Appalachians and it became very exciting so the two became melded and I was caught up this poor innocent young lad was caught up in a maelstrom of geophysicists both in Cambridge and and and in and I was berated by all sides in many many ways I was encouraged tremendously by the Geo visitors to its it's not doing geology in relation to the new global tectonics but the geologists in Cambridge were extremely suspicious of my dealing with geophysicist they didn't like it at all whenever I said I was going down to Mattingly rise they've taught touch it and said it's direct read please he's a traitor and and they had the other the other the other thing was that that in in Lamont have cost doc Ewing Morris Ewing was basically and non-believer hidden plate take 20s I mean I was taught plate tectonics as a fact as an undergraduate and it was it wasn't it was normal to me I think about plate tectonics and not plate tech companies for the oceans and and opening up between continents moving is that jollies would go to the edge of the ocean peer into it and it was a big blank and also didn't care it didn't matter oh yes if a continental drift was happening but it's not relevant to us we're dealing with the detail of continents ok enough enough of that general kind of introduction so it's a nice picture to Tuesday this is the man who really really changed changed my career but it's strongly but I want to mention also a number of geologists I should say at this this stage that if you had a million geologists for a million years working in on the constants they would never discover plate tectonics yet how would you take all this incredible mists and make make a plate tectonic picture Argonne tried in terms of a the collision of India with Asia realized the Himalayas were the result of that collision but the bulk of the gorge of the continents was very obscure and very difficult and and there all kinds of weird and wonderful theories like the geosync line and all those sorts of things would have developed to explain it so necromancy you might do necromancy you might call it but we had lots of data we at least had lots and lots of data geologists have traditionally have liked complexity they simply like they don't like things being simplified too much because it sort of does him out of a job I suppose eventually but anyway here I was and starting to look at look at place looking at the new global tectonics here's a list of partial list of some of the geologists who've had a very big influence on on on ideas in tectonics with time and some of their ideas were hard harbingers of ideas that became important in plate tectonics and and I'm going to mention just a few of them required more many more than this and let me just go through it in the first is Hallie Wellman having wellness New Zeeland or Bridget for Brit who went across New Zealand in the 30s and became a roving mapping geologist both in universities and for the survey and he basically discovered the Alpine Fault the Alpine Fault of New Zealand that is and he recognized that the Alpine Fault which runs here had had many hundreds of kilometres of right lateral motion on it it was also a bit of a thrust as well he'll realize it because it's transpersonal a kind of structure and he realized also or suggested that the Alpine the motion on the Alpine Fault has to end somewhere has to be sucked up somewhere and he suggested it sucked up at one end by the Pew cigar full belt structure here that the trench and was sucked up by the Hickory trench or at least is its ancient equivalent in here so he really invented the the trench to trench transform hill they didn't call it that at the time and then of course is a wonderful thing thing that Harry not Robert Harry described very early on it's a place where the lower crust of the the Alpine ranges here's a gaunt garnet granular it's gone into and Ferb likes that lower crustal rocks so I'm done it already of course have been thrust over what the Americans were called dirt it's basically homeless is young whole Essene gravels the lower crust is being thrust across a lower this is an extraordinary art crop and a recognition of that for the time Tim little has done some wonderful work subsequently to illustrate this whole concept that the next person I should mention is Tom Dibley Tom dibbley was an extraordinary man who met during the 20s and 30s probably in most of California he was an extraordinarily persistent boots and hammer man and he discovered the southern branch sudden continuation of the San Andreas Fault Andy Lawson in the 1895 rated wonderful paper on the northern part of the San Andreas from San Francisco northwards he discovered the southern part and recognized that it had a truly massive offset of many hundreds of kilometers so here are two of the big ingredients of plate tectonics recognized in in before the Second World War there we have the San Andreas Andy Lawson recognized that broke in the 1906 quake north of San Francisco and here we have the southern branch nee recognized again and also he tip W also referred to this strange Big Bend in the San Andreas which of course is a transpersonal Bend and that's why the this transverse ranges there so this is a very important contribution the next one I want to mention is Bert quenelle another news a New Zealand guy who Matt mostly in the Middle East in Jordan he got the gold medal from the king of Jordan for studying Jordans geology and and Sam I'm sorry that Cornell was quite extraordinary clever Jordan ones a wonderful mapping there but also recognized that the Dead Sea transform ditch what we now call the Dead Sea transform sorry the Dead Sea des neutrons from the Dead Sea fault it's a left-lateral fault with about 112 kilometers of displacement that clearly ended at the northern end of the Red Sea and he heaved he saw here we have an interesting interesting situation this that if you plot longitudes normal to the Dead Sea transform the old intersect of the pole someone at a point somewhere near Gibraltar said this must be the pole of rotation around way around which Arabia had the Arabian Peninsula move with respect to the African plate and moreover he said this what Jason did later of course and he also said that the emotion on this tension twelve kilometers of lateral motion must be sucked up by extension in the Red Sea so there's another important component Allen Smith who sadly just recently died was it was a very fine few strange as a physicist neutrons transfer it became a Princeton the field geologist and did a wonderful PhD on the belts and Windermere series of Northwest Canada and and then of course he got very interested in the early early early phases of computing in in Cambridge of the Titans and so forth and he he was he was really the first to with Bullard Everett and Smith in 1965 to make to make a wonderful reconstruction of the Atlantic Ocean north-south and so forth to but using Euler Euler poles to make the reconstruction a finite difference method of using Euler polls now and they say there's his reconstruction probably lar Deborah can Smith reconstruction but they want above all I really want to mention and it's not that the others aren't good they're all wonderful feel John isn't and they made these great contributions the one I really want to emphasize strongly is a man called George platica is George George is not known very strongly outside the outside except for one very important paper and it's very important George in his career with the USGS Matt almost not the whole that but he met huge terrains in Alaska terrains with a with ains at the end in Alaska and he also recognized that during the 1964 earthquake at Anchorage or near Anchorage he drew this cross-section it's quite exciting a quite extraordinary for that for the time sorry what he did was to take the earthquake about 30 kilometers or so and said oh there must be two nodal planes from the misses steep thrust high angle reverse fault node and played or there's a flat nodal plane you want to make the make a choice and and he said look the aftershocks occur along along the flat one and also when you look at the steep one which most of the geophysicist I have to say at the time chose the strike length of this was about 200 kilometers on the interface of the is a big big earthquake on the interface and he said look if had in the steep nodal plane been the plane on which that moved to produce the fault there's no way that could not have broken the surface but what did digital surface of course was caused it tremendous areas of subsidence and uplift according to the dilation and compressional quadrants of the of its known and moreover he said look this plane of Earth earthquakes point straight towards the bottom of the trench this must mean that the pacific ocean floor is slipping underneath alaska now that's the proof of subduction of flat subduction if you like and that was george after 19 1965 but one person i do want to mention not a field geologist phony means but i want to mention the work of tonya Atwater who wrote the first paper probably the most important paper ever to link click tectonics with geology taunt Tania produced a wonderful piece of piece of research that related and this is simplified horribly here of course that related the magnetic stripe history of the of the Pacific Ocean to relative motion and the gross the progressive gross of the San Andreas Fault between two transforms and she showed by plotting the ages of the volcanism along this thing and the various very variation offset on the fork she was able to show that that fault had to elongate as the two transforms and migrated south and north and I'm sorry blunt again and you can see that the volcanoes progressed we shot at shutoff northwards and southwards lesson I think is they'll probably the last of the volcanoes here a wonderful very very clever piece of work and it's kind of approach to the quantitative quantitative plate tectonics and linking geology in a general kind of way to that to those relative motions was taken up by by Walter Viet Minh of me and Alice Smith and various other people in dealing with the Alpine system a few years a few years later now what I want to do now is to actually just show you very quickly two things that didn't have pleased me very much I can't deal the whole of geology and plate tectonics the half an hour to a new missile whatever it is so what I've chose so say I know I know so what I've done is chosen two pieces of work that have pleased me very much and one at the beginning of my career starting with with the piece of work at in the Appalachian Caledonian built in which a Bluemont foot between on the in the years 67 66 67 or 68 I basically produce a tic-tac-toe in Australia graph of the Appalachians and the Kalin IDs and a series of tick tonot stratigraphic charts and here here's what part of what the part of the first map I produced up close the Atlantic of course to get it up when we saw a very strong correlation between between structures of the Carrollton ideas and the structures of the Appalachians as you would expect of course had these two being being together okay and what I started to do was to design a series of tech Tony stratigraphic charts that's simply taking this trigger to be in the and the various events that have happened the deformations and so forth and I don't want to get involved in an indica in it in the details of this what wouldn't want you to notice is that I've covered various species different colors the these these colors down here the yellow and the the oranges are basically big rift complexes places where the continental lithosphere has been pulled apart and into India I should have mentioned this stage some of which we've actually tried to reconstruct and use the Mackenzie basin model for this network works rather well it's also shown in the blue things that look like a continental shelf with a phase change at its edge that suggests the edge of a continent and the beginning of the continental rise and Oceania crops and then outside that we find in each case each of these various cross-sections shown it's orange and in color we find dark orange color we find our crocks clearly rocks and designs day sites and rhyolites and sometimes bezoar tuganda sites that clearly fit that present present observations of the of the extent our parks of the world so here we have a situation we have a continent and suddenly we realize there's an arc adjacent to it did that are build there or did it somehow come in and collide collide with it with a couple of my students some of the students in Albany later showed it very clearly that there was a collision we thought at that time Jack Verdon andreas other people thought that in fact there was a mark marginal Basin that are closed and pushed the ophea out over the over the edge so earthly lights were always tied up some in some way with the zone between the between the arc rocks and the Continental margin constant constant rocks so that we did all along the Appalachians and we started getting involved I started mapping in Newfoundland hence in some detail seeing one as Jack described earlier earlier today wonderfully abducted a few nights and we've had this clear huge sheet of a few light that was abducted - cause the Taconic orogeny and we thought we thought something may have similar may have happened to cause the Grampian orogeny in scotland and ireland along a long strike and what became clear also from the work of students in albany mostly is that there are two there are two sets of our crops there is an older set that predate the tronic orogeny that looks like an ark constant collision and then there's another set of ark rocks on top slightly more solicit perhaps on top that that probably represent the flip in polarity flip in polarity of the of the subduction and the Nuart built on top of the whole lot to produce if like an andean kind of margin so here's one way of converting an andean margin it's converting a continental margin into a first of all orogenic belts and then into an andean margin we look this is the kind of ways in which we were thinking beginning to think of the system here's his thing as i drew in the late the late 60s early 70s and that simply shows that the concept of the flip in polarity from the age of two arcs associated well very quickly now i describe the geology so I'm not allowed to move from there I may want to describe the geology of this area of the Caledon ideas in the West of Ireland very very briefly to show that you can have a rotten e below sea level in fact our Raja knee below an arc and that's what I can show you now here is a very simplified structural map for a tectonic map of Western Ireland in which very very simply we go from ductile Radian rocks which are rift related and continental margin rated rocks through a series of what we now know is it's a blue schist and Zone in here which is probably the suture between this arc and the continental margin there's then a group of accretionary prism rocks with a backstop of an ophiolite and then there is a for art basin with huge thick span of sediments in and then strangely enough we come back into the down region in Connemara with what that was pretty strange so we thought for many years that this was a terrain that somehow slid in sideways we now know that to be incorrect what it is let's not show you on the next one there's Connemara connemara the del raiding rocks of Connemara are basically a huge pile of North North verging and North facing naps with a Peruvian metamorphism and we've got kyanite staur-o-lite and and a lucite in various parts of he put a triple function but a triple point somewhere in the middle of Connemara and so here we have this this this set and in the South Mayer trough this Ordovician for arc trough between this very tiny arc and the Continental margin we find about wonderful stratigraphy you can take apart and what we find in that stratigraphy is this here is the thematic arc if these are basaltic and the sites and a few Bala nights as well as an amazing arc that is associated with the the grossly Cambrian early or division on top of that that the arc goes solicit this is very common when an art begins to approach a continental margin and you start getting getting a magnimous contaminated with sediments from the edge of the continent going down the tube and getting to be a promoter and in fact we've got some old zircons near here from from that map so the argument goes that and then synchronous with the the art going going solicit we have the for art trough with a massive thickness of volcanic lastik turbidites plus turbidites coming in from the from the Laurentian margin from the radiant continent and we now know this is simultaneously thought for a long time this is simultaneously simultaneous with the deformation of the of the dull region rocks the grampian rocks what no it's not possible simultaneous with the so so here we have a for art Basin with no detritus in the dull radium in fact the dal'rok don't detritus fund it from the dull region origin doesn't come in until up here like that so while this deformation is going on we have an art cajuste into it and what we not not now realize is that is the central section is a cartoon section across connemara across the cross Western Ireland what we clearly have here's Pierce connemara with the North facing naps there's the four art Basin in here and the arc and the ocean lights and so far there's a huge abducted slab of affiliate and arc and for art basin that has gone across the across the continental edge has metamorphosed the system in here a recently Paul Ryan and I have have quantified this by a simple numerical model and what we find is this that what if you choose appropriate appropriate lithospheric thicknesses for a big rift complex at the edge of edge of a continent and the in the age of that rift pompous from thermal recovery and you look at the the rocks of the abducted Kofi light and and arc we find that there is no way we can get the thing much of a sea level that is that the whole of the arc and the earth feel like climb on to come to the edge or abducted onto the edge of the continent and basically they are thrust across they deform the gel region underneath and the and the heat for the borough in metamorphism that del region we believe must come from the hot arc who is a Sima sphere but beneath which were must have unit 1300 and basically we can show that the modern achmadi virtually proves and we believe that the the del Radian defamation in the rocks down here was not any below sea level but it was below an abduction rock and it only it only it only came up to the surface to provide to yield sediment to yield settlement when the arc flipped in middle or division times and this is rebounded the whole system rebounded and and we we started up lifting this and you started getting down region detritus so you can have an origin below sea level no detritus no nothing if that's there to stay there for any length that length of time you'd never know an origin was there and what is so lovely about this is that once once the flip has taken place in South connemara you have a series of accreted seamounts you know in something it looks like the inner wall of a trench when you go a long strike you go through the lawn for down Massif you go into the southern uplands and in the southern uplands I've taken I've taken the work of Phil stone and the stratigraphic work of full stone and Jim Floyd I've simply plotted here I simply plotted the here we go from the Laurentian margin so a balance ray and air share this is the later stage eventually these collide a happy a POTUS finally closes our but in towards the end of the end of the Silurian here we have a series of packages of turbidites under the debt they begin progressing in two millionaire increments it goes like this here are some oceanic black shales the heartfelt Glen feel and shales that Lapworth described there are series of turbidites into 2 millionaire packages and the growth rate of if it is an accretionary principle what else could it be what could that Stricker vivia other than and here's a lovely illustration of their relationship to inflict tectonics and geology and it comes out of really looking at the stratigraphy and the fossils incredibly carefully as Charles lapis did this is really the work of Charles Lapworth you'd have to say so hit that one now the thing I was going to say had I had I had time for for it three more minutes I wouldn't have any any more slides I shall just talk if I may now having talked about an own system thing I've been doing what since I went to California after retiring from Oxford I didn't want to retire whereas America you don't have to retire so I went there for a while and so what I did was to was to look cast around for something interesting to do and what was most interesting in California to do of course was was to look at the near Tektronix of California the advantage of doing near tectonics as a geologist is this well they're you're confined to the brittle field and be in being a structural choice I love cabbages and sort of nice ink fabrics and all that kind of stuff but if you can get used to the idea that the brittle brittle things are telling you an awful lot about about about the upper crust then what you can do is to look at look at their faults at faults and the joints very very carefully you could map them out the character is the small folds and so forth you've got the geomorphology the landscape you've got the erosion and deposition in basins you've got drainage through of your budget systems you can deal with you can derive you've got the seismicity of course and then you also you've got the most important weapon weapon and a geologist army you've got GPS velocity field derivation and in California that turned out to be the much of the key to looking at the near tectonics of the so called trans tensional corridor I tell a lie as you have here's a simple simply very much simpler method call it telephone what we've done is to look at this little area in here okay son address with a Big Bend earthquake zone or a deformation zone that runs from the mouth mouth of the Gulf of California to Palm Springs more like the system running up the east side of the Sierra Nevada this is a micro plate or micro block whatever you like to call it and then it runs up there's a zone that runs across a compression zone as he falls and thrusts in the Great Valley in here and runs across to what what now is clearly an amendment amended sino quadruple Junction in there so he did this we know from philosophy on the velocity field that this is moving relative to the artists range on the east side of the other system we're coming to the basin range of course in here and there's a Sierra Nevada in the eastern side of this block and this is the this is a trans tensional zone we've made an incredibly detailed study of the southern end of that trans tensional zone and what we find there basically is this trans tension causes havoc at a tire on stage thank our parent parent trans tension causes havoc with rocks what it does basically is it communist rocks it breaks them down it smashes them blocks are rotating some with with with vorticity some some against participate and the ultimate thing is you go from the Sierra Nevada into the core of kozo and you go from fresh beautiful cretaceous granite are right into a mush becomes a mush almost a fluid fluid mush so even the simplest kind of tectonics can cause the most appalling geological havoc I will finish there [Music] be very carefully we very carefully have a couple of minutes for questions so who would like to go first gosh you're all cows after that well I would just like to comment I find it amazing that you've spent 50 years on that same tiny little Block in the West of Ireland but hey it's beautiful I did my first mapping there but it's still quite amazing anyone else I've been as I said calamari is an obsession with a bit of mind it still isn't obsession I just say at the age of 80 I've discovered one thing in geology geology is a disease for which there is no no cure it's it's I was hoping to settle down and people would paint watercolors and write write write things and so forth but in fact I can't I can't stop doing field work and I think anybody who likes geology can't so basically near tectonics and connemara what we discovered in Connemara myself and Macario some years ago is that one can contour the South carbonate for the surface and the sub Carboniferous surface is clearly quite strongly deformed and the question is when did that come up well I take the view of Kevin post view if you have topography it's generally fairly recent you don't you don't get topography the last last very long on this earth for various reasons can either tectonic lee or it can it can be eroded down so what we found is that we have this surface all these big faults in a big force and some with throes of a three cut three three kilometres and the question is when did these went when did when did it go up and when did all this neo tectonics take place well we looked at the profile sorry sedimentary sequences off the west coast of Ireland and they turned out to be big ones with Oligocene and we were pretty certain this is tertiary uplift so I think mud there's a lot cursory geomorphology and tech toys to be done in Britain I'm convinced of that thank you very very much John we the next thought talk in this session haha yes Richard Gordon has managed to sneak in again if you hadn't noticed he's giving no he assured me it was his twin who spoke yesterday so we have plate tectonic approximation and update over to you okay so yesterday we heard quite a lot about you know the the original vision of plate tectonics much of which still stands in terms of the plates being rigid and and the boundaries being narrow and so I want to talk about some aspects in which one or the other of those assumption isn't strictly true anymore so in about twenty years ago I wrote a paper called the plate tectonic approximation and so this is an update to that into that earlier paper I emphasize the fact that we had a lot of boundaries not only in the continents but also in the oceans that were not narrow so sort of the - once again so the two key concepts tenets of plate tectonics classical plate tectonics if that's not an oxymoron plates our region and the boundaries are narrow so let's look at the narrowness of the boundaries first so this this is kind of a sketch map of plate boundaries slightly updated from one that I did more than 20 years ago and and so you know we've heard a lot about the narrow boundaries in the oceans but early on many workers including Peter Walmart Paul tapenade is so mad Anne's work ought to recognize that when the plate boundaries go through the continents they're often wide and diffuse and not not narrow at all but for a long time many people would would contrast that what's that well unlike in the oceans where the plates are all rigid in the boundaries are Arlen arrow in the continents the boundaries are diffuse and one of the things that we were able to establish when I was at Northwestern with my colleagues and students there and workers at other universities there are a lot of places in the oceans where the boundaries are also diffusive not narrow at all ball and Harrison wrote a paper about the diffused boundary between North America and South America all the maps we've seen so far show a little narrow line that divides them but nothing on the actual earth but follows those lines they're completely fictitious the outstanding example occurs in the Indian Ocean we often talk about an Indo Australia plate I still see this term used and lots of talks but in fact what we used to think of as being the indo-australian plate really has four different plates which we can measure their relative motion one is the Indian plate up here and now there's the Capricorn plate up here the biggest is the Australia plate and then this little piece down here the McCarney plate an interesting thing about and so these are then the India and Capricorn are divided by a diffuse boundary here Capricorn and Australia are divided by a diffuse boundary here Australia and Macquarie are divided by a diffusive Hungary here and for each of these three and our best estimates of what the pole of rotation is between these plates the pole rotation between India and Capricorn is right here in the middle of the boundary it's the fact I show a gap in the boundary because this strain rates are so low that we don't get it earthquakes observable deformation but over here we have divergence over here we have convergence similarly between Capricorn and Australia we have this different diffusive boundary here pole rotation is right here in the middle of it have divergence here and convergence here and then Macquarie plate we also have the pole of rotation right here switching from divergence on one side of it and convergence on the other in each case is distributed deformation so this is very unlike the poles of rotation for the plate boundaries where the poles tend to be a fairly remote distance in general from those plate boundaries and never live right along them so here rather than just simply being a mathematical convenience for the description of the routes of motion seems to have some physical significance or geological significance since it divides the region of divergence from the regions of convergence we also think that North America South America pole of rotation is is roughly here so I want to spend a few minutes talking about some recent work I've done in with Greg Houseman trying to model the deformation that's in these diffuse plate boundaries in and the Indian Ocean and what we're doing is is we're using the thin viscous sheet models originally from England and Mackenzie which were meant to describe and model that a diffuse plate boundaries and the continents the big continental deforming zones and so I argue along with Greg that they work just as well in the and the oceans perhaps even better in some ways because the rheology the vertical stratification neurology is simpler than in in the continents and so here is the domain that we announced analyzed in our finite element is is outlined by red right here we didn't take the all of Australia just to reduce the computational burden because we're focusing on the deformation here and the Andean ocean and we've plotted up the earthquake mechanisms essential in the actual mechanisms we've just divided them into three sets depending upon which the most closely resemble normal faulting by white circles thrust faulting by black circles and then strike-slip faulting by these red circles so you could see here in the case here's our Indian plate this is roughly where our boundary is between India and Capricorn normal faulting hair thrust and strike-slip faulting over here I think we use the pole of rotation up here but I think a better estimate it just is down here similarly for the boundary between the Capricorn in Australia plate normal faulting here thrust and strike-slip up here the polar rotation is in between them and so then we apply by kinematic boundary conditions we have good estimates of the relative motions of these and then we just at least in our initial analysis only had one adjustable parameter and that was what was the exponent that we assume for the power-law fluid that we're using to represent the rheology here and so that leaves leads to a suite of models in which this is vertical strain rate this shows the velocities that we applied this diagram should be in a reference frame that's halfway in between the india emotion and the australia motion so we see that india moving this way australia moving this way these are contours non-dimensional strain rates red end is is contractional in this case so right yeah and then we have extension all kinds of deformation here N equals 1 doesn't resemble the actual zone at all it's too diffuse in equals 3 also doesn't it exempt resemble it very much n equals 10 we're getting closer n equals 30 is the best fit I'm not going to go into the details for lack of time of how we fit that but if agrees with the observations of how wide that deformation is distributed in the central Indian basin and then N equals 100 right here the deforming zone short sharpens up but it never looks like a narrow plate boundary no matter what the physics of it really wants it to be these big wide deforming zones okay and this is just this is the second invariant of strain rate tensor here so we see the strain is heavily concentrated in these two joined convergent ha zones and then these disjoints extension zones here and here compared with the earthquakes we I think we could match the the location a little bit better with a different polar rotation right here and then this compares that with the predicted focal mechanisms if there's an we introduce another disable parameter which was essentially how much extra flux we put down this trench relative to just simply interpolating we had to put some flux into it corresponding to slap pool if you like and now this way we predict a mix of strike-slip and thrust with more more strike-slip that's thrust then we move towards strike-slip and thrust between these contours but more thrust than strike-slip and then we crossover into a combination of strike-slip and extensional faulty and on the other side and so considering just two adjustable parameters i think it's this works really well okay moving on to the rest of my talk because I'm sure my time well it's going to be strictly enforced five more minutes for the next 45 slides but I'm sure I can get through it Romanian central tenets of plate tectonics let's talk about the plates are rigid okay well no really real object is truly rigid so how fast you place the form strain I mean it's not a qualitative question it's a quantitative one they have to be deforming so how does this plate tectonic approximation work how do these strain rates compared with those in diffuse plate boundaries with those in narrow plate boundaries how is this straining distributed spatially what are the main processes by which they deform so we're talking about the interiors of the place not the ones and we want to be part of the diffuse plate boundaries but I examples of how we know this is that when we do plate motions around the globe and try to sum them up North America to Nubia to Antarctica back to the Pacific plate back to North America they don't quite match they missed by about five millimeters a year when we do the global circuit so how how is this five millimeters a year if it's real distributed around the world and what causes it also we have triple junctions that fail closure Bouvet fails by a few millimeters a year Galapagus fails by fourteen plus or minus five that's a 95% confidence limit it's huge it's really hard to do this with with intraplate strain I think we're probably missing something else but if that remains that when we test plate tectonics there's places where the rigidity hypothesis fails okay so the plates aren't rigid what could cause the non rigidity two possibilities of weeks or one is plate moving over a non spherical earth Dan first pointed this out in a paper in 1972 and Turcotte in Oxford also looked at it a bit and the other is horizontal thermal contraction of oceanic lithosphere we were well-acquainted that we see the subsidence of the lithosphere from the thermal contraction it's contracting in all three directions not just one so the place has to be shrinking horizontally how big is this how big are these strains so here was just the kind of a order of magnitude loose look at this narrow plate boundaries in terms of millions of years this rate we have strains of ten or a hundred or so per million years super high strain rates stable plate interiors the lower bound here comes from the earthquake moment release and as we've heard sometimes you know the actual deformation can be a factor of ten or even a hundred higher than the seismic moment released so that might bump it up to here or up to here if we picked this we can see that we're from minus 5 to plus two seven orders of magnitude difference in the strain rates between narrow plate boundaries and our rigid plate interiors and that's why the rigid plate approximation works so well non spirity when we look at that we can get strains up to about 10 to the minus 3 per million years we can compare that with say the strain rate and the debate and Plateau which is between 1 percent and 10 percent per million years or in the diffuse oceanic plate boundaries which were about peaking at an order of magnitude slow or 1% per million years thermal contraction of the oceans is age dependent and in the young youngest lithosphere we can get very high strain rates okay two minutes for the last 39 slides we can do this so okay here's a map of strain rates we've estimated for the non spherical earth the fastest straining plate is Australia because it's at mid-latitudes and it moves north fast quickly we'll move on quickly to the next one thermal contraction it's about what goes like one over the the age in terms of what the screen rate is if we plot it up it looks like a sea floor age map the highest contraction is near the ridges and then that gets lower it gets above 10 to the minus 3 for mining and per million years almost up to 10 about to 10 to the minus 2 at the ridges 1% 4 million years like the peak rates we see in the diffuse oceanic plate boundaries and then gets really low in the older oceans and then the continents were just kind of neglecting that this if we this is the strain rate from the Pacific plate where we force it to satisfy strain compatibility to different end member assumptions won't go into them if we integrate them to get a velocity field they look very similar if we fix the old plate part of the plate we get rates of up to two millimeters a year at the other end of the plate if we put it in a reference frame by the Pacific Antarctic rise which is relevant to our plate circuits then we get these kinds of motions up here a couple millimeters a year so how can we further test predictions the shrinking plate hypothesis hard to do it with geodetic data because it's hard to get enough sites and the right places on the oceans turns out the asommus of transform faults are sensitive to this it predicts that transform faults don't exactly parallel plate motion they're off by about half a degree on average it's not a big effect the difference between right lateral and left lateral is predicted to be about one degree different what's hard to measure at this well well Dan reviewed the paper on this and he said he didn't believe any of it but should be published anyway so I'm very grateful for that so here will complainer blue and red blue is right lateral red is left lateral here's the 6 plate pairs where we have both right and left lateral ones if you average them out in every case the left lateral is clockwise of the right lateral six out of six for a coin flip I'll take those odds and and this was from the dimensional analysis we weren't didn't weren't thinking about what the transform fault should be when we did this and so I don't think there's any bias okay we're down to five seconds so I should go to my last slide so some conclusions from the top many play boundaries are not narrow but defuse defuse plate boundaries occur in both continents and oceans cover 10 to 15 percent of the Earth's surface transform faults are not parallel to plate motion but it's a really good approximation plates are not rigid but it's also a really good approximation lateral thermal contraction of plates likely sums to one or two millimeters year across every plate ranges strain rates from horizontal thermal contraction and plate movement over a non spherical earth overlap but the thermal contraction is bigger and the younger and oceanic lithosphere thank you very much I'm afraid we literally have no time for questions but no Dan Dan will respond as to why e letter fate threw the paper through and didn't agree with it in the discussion not now see you have time to prepare your response all right next up we're going to East African transform margin a very interesting part of the world and we're going potential fields to you physics and I'm pleased to welcome Andrew long ok thank you everyone great pleasure to be here and I guess part of my job is to try to keep you awake for at least another half an hour which I hope to achieve so last year I learned how to draw curvy lines over gravity and magnetic data which doesn't tend to happen very often you normally end up with you nations like this so hopefully I'm going to be able to explain to you why I'm doing that and the interpretation that I have and it's an interpretation it's not a model so it hasn't been manipulated to derive or to approve or conclude a preconceived idea whether that be a geological idea or a or any other idea and hopefully you'll understand a little bit more what I'm talking about when I actually get into that so I've probably already had about five minutes already so you're going to see this thing come up several times through the presentation and we're going to work from the top down from North Forsyth the little boxes the lines indicate the areas that we're going to be looking at as we progressed down the transport margin so the first thing I'm going to do take you through a very brief description of the data that we're working with and then I'm going to talk about a chillon structure then the correlations and their conclusions so the two different types data that we're working with complimentary both satellite data one is the enhanced magnetic field model 56 format her resolution Wow but it beats 10 degrees of Perea mag holes I guess so the idea is that we process out correctly for the IGR f year and reduce it to the pole amplitude gain corrected so it tells us something useful about deep cross and magnetization something that can be directly correlated to geology with the samwell Freire gravity again just systematic and systematically processed to derive something useful which will be directly correlated to the shadow across all density variation so the this is one of the two key messages that I want to get across it's an excellent opportunity to remind people that sound well free our gravity data is incredibly easy to acquire but unfortunately it's also incredibly easy to abuse in terms of trying to prove or disprove or to illustrate an interpretation so yeah Priya gravity it directly correlates to topic bathymetric relief so it should be correctly processed to yield something slightly more useful that directly correlates to geology and geological structure and I need it's probably been said before at least in this way but you should let the data speak for itself so it's not just for the people gathered here hopefully this is going to go online and for all those people that are using Freire gravity and modeling just sand some densities to derive something to support some preconceived model this is what I'm telling you because all the work in this presentation is derive from just essentially the the bare minimum modeling input but there is no real modeling and the same case is that the mag to lots of people are trying to to use magnetics not sure East Africa in the Indian Ocean but at the end of the day you have to consider what data is available we can see that these lines are very sparsely sampled in the indian ocean and this is a problem that that there is in determining the extent of the spreading center in the western Indian Ocean where the the translations where where the transitions between continent and ocean occur and so on and also with the Version three we can also get access to an idea of the type of the source data so I'm sure it's actually not died at all mostly GTECH data this would go offshore we get lots of pink sort of no dates for areas on the shore and all this stuff I mean it has to be made up it has to be interpolated it doesn't doesn't have any scientific significance if it's just blank data so that's why I like using the the enhanced magnetic model it's only been around for a couple of years but it provides a coherent and homogeneous field from which we can process it and then make proper interpretations so this I mean I could give some history on on them on the interpretation but basically it's all happened this year and what we have here are four blocks which really stand out and they're right stepping as we're going from south to north in a non-issue on arrangement and yeah I mean you could interpret that in a number of ways but it is actually strongly suggestive of G paxing especially if you are aware that people are talking about other intraplate strike-slip structural components such as John Dewey was talking about earlier with translation the north and trans tension in the south so I'm going to take you through a very brief I don't know how many minutes i've got left but a very brief rundown starting from anser in the north where i would like to believe that the transyl margin propagated from underneath Anza it's a very deep thick Basin it's got a thick Cretaceous sequence you can't really see below that that's already been commented we don't know what structure lies underneath it and the the two key components of the transform margins point out is four which is sorry three which is the main transform margin and this coming in here is the oblique rift of passive margin if the alarm Ian Bane and hiss are coming off shore and and then we're into the inside of the transform margin and you could almost perceive that answer has been rotated through geological time to build on that the the comments I mean even yesterday with Cynthia Avenger talking about the the periphery of Crotona carries remaining zones of weakness and and even earlier for several decades people have been emphasizing the same point well if we take some of this work here from Fritz 2013 and he's mapped out areas which resemble the suture zone between the chronic West Gondwana and the incoming zania of East Gondwana of which Madagascar's down here in the right is said to be partially comprised of as well so and you fit it all back together well answer is directly over that so there may be a suture zone underneath answer at a very great depth so that's something to think about and that we know that the the faults within the answer basin have been moving all the way through the history of the the Gondwana breakup and the rift and drift and the earth Madagascar to the south and into the tertiary where there is evidence of reactivation of fault impression so if there are at least two phases of compression which affect the structure to the west of the Davy Wally Ridge where we broadly have this sort of area of transgression so gain side from the area of transgression well what was state the five units okay so this is zero transgression here the the gravity shows an oblique fold axis which is probably related to the earlier transgression and then this four Stealth's play can be clearly mapped okay going further south into the jeep Lex's own a nice regional line for mine going from just south of some go some go right out into the oceanic margin to here and I just like to point out the outer high which correlates with the magnetic I hear and the this is the main margin this associate across all of this is continental crust so believe that if you will that's what I'm gonna say that the corrine desolate so we're going from the duplex to the trans tension zone the trans tension starts off here the southern onset which is just west of the Sanders are seen out and we also have the last Jeep let's fold Zone in here on amber so the the influence of the East African Rift system I just want to throw it out there that perhaps this isn't an offshore East African British system taking mechanize mats faults we can see that they have a normal arrangement north of the Paisley seam out and then to the South they don't show any offsets and that's pretty consistent with the duplex fold rotation of blocks and then it's just cut off and the other thing is further safe don't there's absolutely no rotation they're locked into the trans tensional structure sites mr. t can't leave seismicity out this is from new Miller Bowie from last year and we can see how even some of the site's history along this fault here this antithetic fault going 3 that the north and mozambique belt ties in with with their what everything on the Marge of the western margin of the strike-slip corridor is locked all the sites mysteries locked on the onshore western branch of the East African racism and so I could go on the transgression all responses a couple of lines from TGS just have demonstrate that north of the satellites seen out we have a pull apart feature if it's the ARS or not and then so we don't have any deformation we just have the the Devi Ridge fold axial hinge running straight down and then what is happening today so the strike-slip fault is diverted to the east and why why should that be something happened during the cretaceous so this is a it's always difficult chilean large structures which strike north side so we're looking west and we haven't talked anything of we haven't mentioned anything about clean theory okay and that where where these planes are said to have tracked and Collin berry season in his work the Marian cleanliness here there is very clearly a very distinct signature to the trail of these spoons Mentos especially when you look at the difference between the the [Music] oceanic spread the crust from which Antarctica departed so those are the conclusions you can read one quick question no one we can catch up our missing time thank you you can hold questions to alert to the session at the end so I'd like to us I'll put a question John this might need your chair meaning skills to control John but it's a general question I mean when plate tectonics came out talk about the oceans it hasn't required very much modification since those early times and it offered a great promise of doing something with understanding continental geology and the early iconic papers by John and his colleagues had a go at that I'm be interested to know your perspective on what it didn't do so one of the interesting problems but you and John Casey referred to for example was a few lights and are we really any nearer now understanding how you get a bit of ocean crust from several communities below sea level on to a continental margin or is that one of the problems which has remained you know it's intractable now as it was then I think that the ophiolite problem is he never use the word solve that's an absurd term to use but never us I think we've got much further with understanding ophiolite so you use it using that the model with Jack described Jack described today I mean there's no question that geo chemically the bulk of OFI lights have Bala knights in them now there are scraps Schenker used to call over your eggs the little bits and pieces of ultra metal pieces of gabbro call Mophie lights if you want to with that I would I would discount those things I wonder I think I think I think I think that the the abduction of a few lights is easiest if a for arc hits a continental March and then there's a ready-made simple mechanism for doing stuffing that stuffing a Rifton rifted marchin underneath an arc you feel like a partial abduction a collision an arc and the abduction the foreign arc on top that's what it looks like in West New France a huge sheet several hundred kilometers wide all the way from middle middle middle of Newfoundland if you like right out onto the platform you see a game write out a crime across the rifted complex and that's something you could map you can actually see it and and and that a few light is very clearly part of an arc that is there interlacing relationships between transforms and arc arc rocks Oh Felicia crocks they all they're all bounds as Jack showed on on his diagrams by the by the uniform orientation of the sheeted dike complex that's always across the whole system it's it's it's it averages northwest-southeast that look like it looks like okay here in slab I'm a couple of hundred miles wide would abducts onto the Continental margin and it's and it clearly is part of an arc so I'd say that most arc most cofee like complexes are in that situation there may be others the two-tire complex in Chile for example is on a ridge is is on a ridge trunk that Ridge trench trench triple Junction there may be ways of doing it of special little bits here and there but but the big abducted ophiolite scattered around the world mostly Jurassic Cretaceous and Ordovician all those those three age I think that see you Glenn you've got to have a continental margin availa than about ophiolite so bobwhite would like to ask a question to richard gordon you you showed us that there were earthquakes in the south of India in that broad defamation zone and he had predictions for the directions of slip from your model but you didn't show us the slip vectors from the earthquakes do they match your model predictions oh you mean where I didn't show what the actual mechanism well you said that you just said they're normal or thrust or strike-slip but therefore you must know what the slip vector is and does the slip vector match your model of distributed deformation well I mean in a very general way the slip vectors do but we don't expect a distributed deformation we don't expect the slip vectors to line up with with it you know it's it's because you've got a combination of the displacements and the rotations of the blocks but I mean in a general sense and the equatorial Indian Ocean and that zone between the India and the Capricorn plate the normal faulting is north-south extension which is what your you would expect and in the central Indian basin and the mechanisms where we have thrusts or north-south contraction and the P axes and and the strike-slip faults in that same region line up that way but as you get closer to the trench they become more northwest-southeast there's a bit of a rotation but I don't think it's in inconsistent with it you know we also yeah I mean but you're also transitioning over to what's really arguably but the boundary between India and Australia since the convergent zones merge together but the slope is also the deformation is also affected by the presence of the of the trench but I mean it's still all system with adding up with what we think is the relative motion between the three plates because the as far as I could see from your models nineteen sorry Jim Chalmers gave us Copenhagen as far as I could see from your models ninety Ridge crosses the middle of one of your postulated plates can you suggest what is ninety what is ninety East Ridge in your model oh thank you so much well I mean so of 90 years Ridge is a volcanic construction all feature it's not a deformational feature and it looks you know in the early days when we were looking at this we thought that some of the deformation may be localized along the 90s Ridge along the transform fault next to it but it doesn't seem to me at this point in time to be very important if a little bit of localization is there because the deformation cuts right across 90s bridge I mean it's just some something else that's being deformed and strained as part of the deformation I didn't McKenzie I didn't understand what the boundary conditions were on the models that you shared with different powers of pass and pass as to whether you fixed the the velocities on the boundaries or whether you were fixing the forces oh we were definitely fixing the velocities on the boundaries we did not try to do the forces which is a much harder problem and we know reasonably well what those angular velocities are so we I mean I I've rushed through it but we had an outline around say India the Indian plate portion and we forced that boundary the edge of the plate to move rigidly and then across wherever we went across the diffuse boundary then we just tapered say from India to Capricorn and then it was a little bit of rigid Capricorn at the edge and then Capricorn to Australia we tapered that as well and we had a big taper across the Java Sumatra Trench as well which you know for the initial models and then along that boundary we made it more complicated because we did experiment then with adding an additional flux into the trench superimposed on interpolating between India and Australia because what struck me was that that actually they the the defamation that you were finding we looked like the defamation you'd expect right from trying to drive the whole thing northwards by pulling right in and the Sumatra trench and I rather wondered whether you'd actually looked at the force balance is on on that plate to see whether that was in fact where it broke we're break because look it looked to me as if it was and if it were breaking the way that you saw yes so I mean we did do a retrospective looking at what the stresses were you know in the model that were a consequence of the track just you know just placement bound that velocity boundary conditions we put on it not sure I would trust them completely it doesn't look like we necessarily got them right up in the northern part of the Indian plate exactly but so we most trust the region where we have the deformation focus not so much the far field

Channel: The Geological Society

Views: 798

Rating: 5 out of 5

Keywords: Plate Tectonics, Geology, Science and Technology, Plate boundary zones

Id: EFNL1FW02vs

Length: 72min 20sec (4340 seconds)

Published: Wed Nov 29 2017