John Wakabayashi on the Franciscan Complex
Transcript
Note, transcripts are not fully edited for grammar or spelling.
Oliver Strimpel
This is Geology Bites with Oliver Strimpel.
Most of the Earth's oceans lie above a sea floor consisting of basaltic oceanic crust. This crust forms oceanic plates which are continually created at mid-ocean ridges and destroyed at subduction zones. As a point in the plate makes its way over the hundreds or thousands of kilometers from a mid-ocean ridge to a subduction zone, organic and inorganic material settles on to it, forming a layer of sediments. And when the oceanic plate enters a subduction zone, some of this sediment layer is scraped off and becomes piled up at the edge of the overriding plate. This mass of sedimentary material is called an accretionary prism. But, because the subduction zone can be long-lived, once scraped off, a lot can happen to the scraped-off sediments. And it turns out that other types of material, such as bits of the underlying oceanic basalt, can also be swept up into an accretionary prism. The Franciscan complex is an accretionary prism that lies along the Pacific Coast of California, just north of San Francisco. Unlike most accretionary prisms, which are submarine, the Franciscan complex has been accreted onto the West Coast of North America and is exposed on land. This makes it much easier to observe, and indeed it has been studied intensely for many decades. While it exemplifies many features common to accretionary prisms around the world, it is also special in several other respects, not least for its extraordinary longevity.
John Wakabayashi has devoted much of his 40-year research career to the Franciscan Complex. He builds on field observations to advance our understanding of subduction and plate tectonic transitions on large scales, as well as the evolution of strike slip faults. He is a Professor in the departments of Earth and Environmental Sciences at California State University, Fresno. John Wakabayashi, welcome to Geology Bites.
John Wakabayashi
Thanks Oliver. Good to be here.
Oliver Strimpel
We're lucky to have an equipment prism on land rather than underwater. At a high level, what does it look like?
John Wakabayashi
Well, first of all, it used to be underwater when subduction was active, but it has now been exhumed above sea level since subduction ceased. So, we are now looking at rocks that were formed along the subduction fault, well beneath the sea floor, that have been exhumed to the Earth's surface today. So, what it looks like is we see a lot of rock types that many people have heard of, like sandstones and shales, and the like, along with some basalt. And these rocks, many of them have been metamorphosed. But one of the characteristics of rock units (these “accretionary prism subduction complexes”, we call them) is they do tend to be fairly disrupted by faults and folds. And commonly, when we think of rocks, we think of layers, say, and one of the more famous aspects of the Franciscan and other accretionary complexes are these assemblages, where the spatial relationships are more like blocks in a finer matrix. And we call those assemblages or horizons mélanges. So, these are very different than what many people think of when they think of rock assemblages.
Oliver Strimpel
When you're standing on a mélange, is the mixing-up origin of them evident in the geomorphology?
John Wakabayashi
Yes, actually, in most exposures, we don't see the matrix. So, it is only at the better exposures, say an artificial one, such as So, cuts, or in stream canyons or beach cliffs for natural exposures, that we see the full block and matrix. But otherwise, as you note, since the blocks tend to be on the average stronger and more resistant to erosion than the matrix, when you walk around the countryside, a common geomorphology we see when you are in such a mélange zone, are these large rocks, pinnacles, sticking out of a rolling landscape. They're usually vegetated slopes with these craggy outcrops sticking out. But whereas the Franciscan is famous for these mélanges, it would be a misconception to say that all of the Franciscan is made of such assemblages. And, in fact, the dominant architecture is a whole series of stacked faults, where slices of one rock after another are stacked on top of each other.
Oliver Strimpel
So, if you were to dig under some of these rolling hills that are covered with vegetation, that would be in between the blocks, would those be sediments?
John Wakabayashi
The matrix, which would be the low-standing areas between the blocks, are commonly made of sandstone or shale. And the block and matrix relationship is scale-independent. So, let's suppose you have blocks sticking out of a rolling hill slope that are of the scale of tens of meters, up to several hundred meters in size. Well, if you were to excavate underneath all that brush, and look at the actual rock exposed between those blocks, you would see, in fact, this matrix again — generally shale or sandstone, sometimes serpentinite — with smaller blocks sticking out, say on meter scale or several tens-of-centimeter scale. And if you get still smaller, you can get all the way down to the scale of a thin section, when we look under the microscope, and see these relationships played out at millimeter and submillimeter scales.
Oliver Strimpel
Are you able to disentangle the origin of these sediments? Because, as I said in the introduction, the sea floor started out life at the mid-ocean ridge, maybe way offshore, and then it gradually made its way to a subduction zone, which may be in closer to the shore, so, you'd think that the kind of sediments would vary as it approached the shore. Can we see those different kinds of sediments?
John Wakabayashi
Yes, we can, and this record of progressive movement of an oceanic plate toward the subduction zone is recorded in the more intact sheets of fault-bounded basalt and chert (would be what we call a pelagic sediment.) These are formed of the shells of these radiolaria – the silica rich shells that rain out on the sea floor in the deep ocean far from a land mass, so there's no sands and things like that that would otherwise drown out that kind of sedimentation. So, we see those cherts, which record a long travel history, if you will, In some places, such as the Marin Headlands north of the Golden Gate. And, in some places they record 100-million-year history of these rocks traveling toward the subduction zone. And then, the largest volume of Franciscan rocks are the materials that are dumped onto the ocean plate just before subduction in the trench. And so, these are sandstones, siltstones, conglomerates that come into the trench from the continent. That's the triad of rocks that comprises Franciscan, basalt (a little bit of that), chert (a little bit of that), and then a whole lot of sandstones and shales and conglomerates, and the like.
Oliver Strimpel
I just want to pick up on what you said about some of this chert having a 100-million-year history. Does that imply that the subduction zone that created the Franciscan complex was active for that period of time?
John Wakabayashi
Yes, that's correct to an extent… with the best section of chert. But it's one of many that was subducted at very different times during Franciscan history. But, this Marin Headlands, which has the longest record of chert sedimentation -- it actually began sedimentation on that piece of ocean floor before the Franciscan subduction episode ever got started. We know from the nature of those fossils, those tiny radiolaria, that they have morphologies that are related to certain time frames. So, this is what we call biostratigraphy. People who study radiolaria have been able to tie these to geologic time through sections that have absolute age dates throughout the world. And so those particular cherts, the very basal cherts that overlie the basalt were deposited about 190 some odd million years ago, and that is a full 20 some odd million years before the Franciscan subduction episode began. So, if you kind of imagine how this works, a mid-ocean ridge is far, far, far to the west of where the Franciscan subduction zone will form. And at the time that this piece of ocean crust is forming, shortly after it forms, we have cherts that are being deposited for 20 million years or so. And then Franciscan subduction begins far, far, far to the East. And so, if you think about the time that's encompassed and what we know from looking at the record of plate tectonics around the Pacific Basin, this all formed perhaps 10,000 kilometers away from where it would eventually enter the subduction zone and get scraped off.
Oliver Strimpel
I'm curious about the processes that result in this mélange and the fact that we can actually see this basalt-chert layer, which presumably was buried by lots and lots of sediments that came from the land as the ocean plate approached the trench and got closer to the shore. So, how did that get exposed within the accretionary prism, and, related to that, how did the basalt get exposed when that was presumably underneath everything, and that may in fact be subducting?
John Wakabayashi
If we took an inventory of rocks in the subduction complex, whether it's the Franciscan or anywhere else, most of them are this “trench fill” stuff. The clastic sedimentary rocks, the sandstones, the shales. And what this reflects is where the subduction fall bites in to the downgoing plate. And the fact that most of what we see is the trench fill, means that, usually, when the subduction fall does slice into the downgoing plate, it only does so through the sediment, the clastic sediments — the trench fill. But, occasionally, that fall will slice more deeply into the downgoing plate. It's still only the very, very top of that downgoing plate. But it’s a little further down. So, if it slices a little bit further down, it slices some of the cherts; a little bit further down than that, it slices into the basalt. And so, what we see in a subduction complex, whether we see some of the real ocean plate or not (the igneous rocks such as the basalts) is dependent on how deeply that fault sliced into that downgoing plate.
Oliver Strimpel
And that fault is the plate boundary fault, right?
John Wakabayashi
And that's the plate boundary subduction zone fault, yes.
Oliver Strimpel
And do we know what determines how deeply it slices?
John Wakabayashi
Ah! That's a primary research question that to this day people are pondering. So, we are aware of the fact that this slicing happens at a variety of depths from very near the sea floor, which is what's forming the modern day sea floor accretionary prisms, to much further down the interface, well beneath the sea floor, and these are the rocks that are recorded in places like the Franciscan, right? …which are exposing a level that is well beneath the Paleo sea floor, if you will.
Oliver Strimpel
So, you're now talking about the depth at which the slicing off happens. That is, how far down from the level of the sea floor the subducting plate has travelled at that point. As opposed to the depth within the thickness of the subducting plate that you've been talking about so far. The thickness of the sliced-off parts of the subducting plate, such as the packages of basalt and chert seen in the Franciscan complex, is perhaps just 500 to 700 meters, whereas the subduction depth can be much greater, perhaps of the order of tens of kilometers.
John Wakabayashi
Yes. One wonders how do we know that these rocks were sliced off at different depths? We know this from looking at the metamorphic minerals that are present in these rocks. And these metamorphic minerals can be compared to laboratory experiments, which is how we say: “These minerals record this sort of pressure” which would be equivalent to this level of depth beneath the sea floor. So, we know that the Franciscan records depths of this slicing off from as shallow or perhaps even a bit shallower than ten kilometers beneath the sea floor to as deep as 70. So, what has to happen? For some reason, the subduction fault becomes stronger. That interface gets sticky and doesn't want to move. And things get weaker either above it or below it. Above it, we call that “subduction erosion” and we coupled the subducting plate to part of the roof, and we tear off some of the roof and that subducts into the mantle. But if the weaker point is in the top of the downgoing plate, then the subduction fault slices into the downgoing plate, and we have what's called accretion. This coupling of the downgoing plate to the upper plate, and that's what allows it to become part of this subduction complex. But it occurs across a variety of depths. So, what is controlling these physical changes? And the nature of the fault interface is something that is a subject of active research around the world. But we have not solved that problem yet in terms of what is determining why some rocks couple at ten, versus others, couple at 70 kilometers along the interface. And so that means that, if it's sliced off 70 kilometers down (and these would have the metamorphic assemblages that make rocks we call eclogites) and somehow, they are exhumed in their solid-state to the Earth's surface. And, as you could guess, how we get rocks up from those really excessive depths along the subduction interface has long been a subject of research.
Oliver Strimpel
Let's talk about that a bit. I've heard of various models like channel flow and corner flow. What are the models that are favored today for explaining how we can get both this mélange and you get material that went down as far as 70 kilometers back up to the surface?
John Wakabayashi
Today, the majority of the research community believes in return flow, in part because they believe in the results of these computer models that model a path that materials subduct and then makes what amounts to a U-turn and comes back up. So, it circulates if you will, hence return flow. And in doing so, it will sometimes tear pieces off the roof, and other pieces get removed from the downgoing plate, and so this process is responsible for the exhumation of these subducted rocks from various depths, as well as mixing of rocks of different histories in these mélanges. That's one model, but it's one that I don't agree with on the basis of my detailed investigations. But probably the most popular, and, in my opinion, the one that fits the data the best, is the model proposed by John Platt. We think of subduction as converging, and we think of thrust faulting, because that's what's going on the subduction fault. But, in spite of that, the balance of forces is such that, depending on the speed of subduction, and other things like that, we can actually have the accretionary prism, the subduction complex, and the material over it, in a state of extension, while subduction is still active. This was a theory that was worked out with John Platt in a famous paper that he published in 1986. And, in the years since, more and more field evidence has come up that really seems to support that model, and that includes field work that I've done. In addition, there's been a lot of debate over how these blocks of disparate history get mixed into a mélange, where we do have mélanges. And again, bear in mind that actually most of the Franciscan is not mélange, it's actually intact fault-bounded sheets. But where we do have them, we do have a mixture of rocks that clearly record different depths of burial and subduction, at different times, that are all mixed together into these mélange zones. So, one model would have it (again, that's based on the more computer model type) that these are mixed as a process of mega-movement during subduction. But when I look in the field, and I'm not the only one, what I see is evidence that the blocks were originally mixed by sedimentary processes, and so if you wonder well, where might this occur…? When we look at the trench, and we look at that trench fill, — sure, we have bedded sedimentary rocks, the kind we are all accustomed to. But, on modern-day trenches, they are filled not only with those sorts of sediments, that are bedded, but they're also full of submarine landslides. While those submarine landslides are incorporating material that was already exhumed to the sea floor. And so, when I look at those mélanges, I see evidence that they were deposited with these blocks already in place. Then they were subducted along with the other more well behaved, if you will, sandstones and shales and further deform. But the main mixing of these blocks of vastly different history had already. So, in my estimation, both the subduction and the exhumation of material was accommodated by much more localized discrete faulting rather than broadscale movement that was accommodated across zones that are kilometers thick or even tens of kilometers thick.
Oliver Strimpel
Is there any way to probe that model by examining present-day active trenches?
John Wakabayashi
The problem with this is that when we look at what we call mélanges onshore, and those such as the Franciscan, we're examining the record of the subduction interface that's well beneath our level of current observation. In other words, we're well below the sea floor, which we can only access with submersible vehicles, generally remote-controlled ones. And then our research ocean drilling only makes little pinpricks on the ocean floor and doesn't get us really deep along that interface. So, we can't really directly observe this, and, for that matter, all that we see in the Franciscan affords us just precisely that opportunity to examine what the subduction interface looks like and ask ourselves what it's telling us about the accommodation of faulting along the world's greatest faults in the regions that we can't directly observe this behavior. And this is the region, by the way, that generates the most damaging earthquakes on Earth, because it's from a depth of pretty much the sea floor down to a depth of about 40 kilometers beneath the sea floor -- that is the main zone that breaks in these huge earthquakes, sometimes exceeding magnitude 9.
Oliver Strimpel
But the Franciscan complex is in a region where subduction has finished, right, so we're not seeing subduction related earthquakes in the San Francisco area?
John Wakabayashi
No, not today. That's actually a very important thing to distinguish between these long dormant structures and the modern-day structures. As in most geology, we have ways of sorting these out. For example, we have sedimentary and volcanic rocks that postdate subduction, that lie across these Franciscan structures, so that we can be confident where we see those relationships, that the Franciscan structures really are of pre-transform age.
Oliver Strimpel
So, they are distinguished from transform faults, which are what are active now along the San Andreas Fault, and other faults that are going up as far as the far Northwest of the coast there.
John Wakabayashi
Yeah. South of what's called the Mendocino Triple Junction, which is in northernmost California, along the coast. South of that, the current plate boundary is a right lateral plate boundary. So, the Pacific side is moving northwest with respect to the interior of North America. And so, this movement is accommodated primarily by movement on these right lateral strike slip faults, of which the San Andreas Fault itself is the biggest one.
Oliver Strimpel
I've said in my introduction that the Franciscan complex is unusual in the respect that it's exposed, that it's on land and not under the water. How did it accrete to the land?
John Wakabayashi
The original accretion was submarine, so most of the exhumation of these rocks took place while subduction was still active. But that exhumation wasn't so much above the bottom of the sea, but toward the bottom of the sea. So, most of the rocks we see today were not too far beneath the sea floor at the time subduction ceased and may have become really close to it. It was after subduction ceased, however… You could think of the subduction process of pulling down the Earth's surface a bit, and to visualize that, remember that subduction is driven mainly by gravity. The sinking of the slab, rather than two things coming together, it's more of the oceanic plate is being pulled down by its own density into the mantle. As it does that, there's some drag on the upper plate as well, and the very, very lowest places on the surface of the Earth are these oceanic trenches. That's the deepest water we have in any of the modern-day oceans, are where the subduction zones are. We have an expression that subduction zones suck. But when subduction stops, boop! Things are going to bob up a little bit. That's what we believe happened after Franciscan subduction ceased. Subduction is still active offshore of Oregon and Washington, and that active accretionary prism is still submarine. But the expectation is, if subduction were to cease there, the sea floor would bob up and eventually break water.
Oliver Strimpel
Do we see examples of exposed accretionary prisms on or near now-extinct subduction zones elsewhere in the world?
John Wakabayashi
There are a number of subduction complexes exposed on land around the world. The Franciscan is probably the largest in terms of spatial extent, and the duration of a single subduction episode, but there are a number of such units, whether it be in Japan or New Zealand, parts of Europe, elsewhere in Asia. And I just returned from looking at a record of much older subduction well back 490 million some odd years ago in Newfoundland.
Oliver Strimpel
What are you working on at the moment, and, with respect to the Franciscan complex, are there some particular puzzles that still remain, other than the one you mentioned earlier about what controls where the plate boundary fault slices through the downgoing plate?
John Wakabayashi
There's a number of unresolved issues I'm interested in pursuing more and more evidence to constrain this mélange mixing, because, whereas I think that I have good evidence that the primary mixing is by sedimentary process, this is far from a consensus view. And, in fact, it represents kind of an endmember view, whereas the majority of the community still favors the large-scale return flow. And, finally, the process of subduction wipes out most of the ocean crust -- most of that ocean crust subducts where we never see it. When we think about plate motions, our first cut at past motions of plates are from preserved ocean floor and looking at their ages from the magnetic anomalies and the like, and that's the record. As we go back in time, it gets thinner and thinner and thinner because there is no oceanic crust preserved that's older than about what the middle of the Jurassic. The vast majority of ocean crust subducts, but little bits of pieces become incorporated into subduction complexes such as the Franciscan. So, by using those little pieces, we have the potential of reconstructing the history of subduction of all of these lost plates. So, I'm engaged in some collaborative work, that we're trying to unravel some of that history.
Oliver Strimpel
That’s fascinating. Just to pick up on that last point, what is it about these entrained blocks of subducted oceanic plate that we see in the accretionary complexes? What is it about them that you can use to try and document the history that goes back before the middle of the Jurassic?
John Wakabayashi
First of all, look at just the age alone, we could say, “Ah, the radiolaria in this chert, the ones that are right above the basalt, that this basalt (say, in the Marin Headlands) formed at a mid-ocean ridge.” And, how do you know it's a mid-ocean ridge? “Because we've looked at the geochemistry of those basalts, and they give us a geochemical signature that resembles the geochemical signature of basalts erupted at a mid-ocean Ridge.” OK, so we're fairly confident that that's the real thing. Then we say, “OK, this piece of that subducted plate formed 190 some odd million years ago. It reaches the trench at about 95, based on the age of the trench fill -- the clastic sedimentary rocks, the sandstones that overlie the cherts.” So, at this time in Franciscan history, we accreted a piece of ocean floor that had been formed at this time, and so you can go around and systematically do that. Another tool we have would be paleomagnetism. Paleomagnetism can work out the latitudinal shift that this piece of ocean floor made — you know, how far South of where it accreted did it come from or how far North, and so we can then start to get a handle on not only the timing, but the trajectories of plate motions from these little scraps. And, there really are more of them than people realize. But there's only been a few that have been studied to date, so, we plan to go after some of the neglected ones, as well as revisiting some of those that have been studied.
Oliver Strimpel
So, we've already talked to other couple of ways in which the Franciscan complex is special, its longevity, the fact that it's on land, and now you just mentioned that actually it's quite enormous in physical dimension. Are there other reasons that made you spend the good 40 years of your research career on the Franciscan complex?
John Wakabayashi
Part of what drew me to this field of study was that I was very outdoor-oriented, and I've been hiking from a very early age. But one memorable trip – two, actually, before I became a geologist would really echo later… Sometime in the '60s, I hiked up a little mountain in the coast range; it's called Pacheco Peak, and I picked up this rock 'cause it was attractive because they had a quartz vein that had nice quartz crystals in it. And I didn't know that the sandstone around it was Franciscan sandstone and that that Franciscan sandstone had high pressure metamorphic minerals like jadeite in it, and that there was a brilliant young professor at UCLA at that time who was about to change our whole research direction on subduction's own geology. Right as plate tectonics emerged several years later, this would be Gary Ernst, who would publish a paper that connected high-pressure, low-temperature metamorphism to the process of subduction. And one of his key areas was the Pacheco Pass area, where he did a lot of field work. And so, that was the first sort of touch with it. The second was when I was in high school, and I was sort of a rockhound, and I was interested in looking for this mineral called lawsonite, another high-pressure, low- temperature mineral. And this little book I had said there was this locality on Tiburon Peninsula (this is north of San Francisco) that was the type locality for this mineral lawsonite. I went to the wrong place, but I wandered around and eventually stopped at this rock, which was actually a garnet amphibolite. And I didn't know that I was looking at the rocks that formed at the very initiation of subduction. And little did I know that 14 years later I would publish the electron microprobe data from that same rock in a study I did on the metamorphism of such rocks and connecting the details of that metamorphic history to subduction initiation process.
Oliver Strimpel
John Wakabayashi, thank you very much.
John Wakabayashi
Thanks for having me, Oliver. It's been my pleasure.
Oliver Strimpel
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