Reconstructing ocean spreading when half your record is now in the mantle (or: a plug for my new paper)

A post by Chris RowanIf you’re studying the last 100 million years or so of plate tectonics, the history of sea-floor spreading recorded by the magnetic stripes that parallel and extend away from the Earth’s ocean ridges is a key source of information. Each stripe represents a section of oceanic crust that is magnetised either in the same direction as the Earth’s magnetic field generated in the outer core, or in the opposite direction. These crustal magnetisations act to slightly reinforce or slightly cancel out the core field, respectively, and are the result of continuous production of oceanic crust at the mid-ocean ridges, combined with the periodic reversals of the Earth’s magnetic field

S Pacific magnetic anomalies

Magnetic anomalies in the South Pacific: part of the World Magnetic Anomaly Map, via a Useful KML file from San Diego State University

Key to using these anomalies to reconstruct plate motions are the boundaries between positively and negatively magnetised stripes of crust. These boundaries represent oceanic crust that was produced at the spreading ridge at the time when the Earth’s magnetic field reversed. A boundary marking each reversal can be found on both plates at a spreading ridge, representing crust that must have been in contact at the ridge axis during the time of the reversal, before being separated by continued oceanic spreading between the two plates. Just find the rotation that causes these two lines of equal time, called isochrons, to overlap with each other, and you have a well-determined record of how fast, and in what direction, the plates have moved apart from each other since the time of that magnetic reversal. Nice and easy.

Conjugate magnetic anomaly fitting

If you can identify the same magnetic anomaly boundary (t1) on both sides of an active spreading ridge, you can reconstruct the spreading since that reversal by finding the rotation that puts the paired anomalies back on top of each other.

These data are not just important for describing the behaviour of individual ridge systems, but also for understanding plate tectonic processes on a global scale. The 60,000 km of oceanic spreading ridge that wind their way through the modern ocean basins, 4 kilometres beneath the ocean surface, are a major conduit of heat and mass transfer between the interior of the Earth and its surface. There is some debate over whether the rate of oceanic crust production has varied over geological time; this could potentially effect the global climate, by increasing or decreasing the flux of carbon dioxide into the oceans and atmosphere. Faster spreading rates would also mean a larger proportion of young, hot, buoyant oceanic crust flooring the ocean basins, making them shallower and raising global sea levels – conversely, a reduced global spreading rate would lower them.

It turns out that for the global spreading rate picture, reconstructing the history of what is now the East Pacific Rise is a very important part of the puzzle. The modern East Pacific Rise is the fastest spreading ridge on Earth, accommodating the rifting of the Pacific and Nazca plates at rates of up to 15 centimetres a year (compared to a global average of about 4.5 centimetres a year).

Distibrution of global oceanic spreading rates

A graph showing the distribution of spreading rates on the different oceanic ridges. The EPR, in red, has much faster spreading rates than any other ridge.

It has also had a very interesting tectonic history. The modern East Pacific Rise is mostly restricted to the Southern hemisphere, running into North America in the Gulf of California; but 80 million years ago, it was part of a much longer ridge system, almost 10,000 km long, that extended much further north. At this point, the plate on the eastern side of the ridge was known as the Farallon Plate.

80 Ma Global plate tectonic reconstruction

A global plate reconstruction at 80 million years before the present, showing a much longer Pacific-Farallon Ridge in the Western Pacific. Source: Seton et. al. (2012).

Just like today, the Pacific ocean 80 million years ago was ringed by subduction zones, and it turned out that the northern part of the Pacific-Farallon ridge was living on borrowed time; the rate at which old Farallon plate was being returned to the mantle by subduction beneath the west coast of North America was greater than the rate at which new Farallon crust was being added at the ridge. And thus the ridge itself gradually approached, and then collided with, North America, and the once-mighty Farallon Plate broke up into a number of smaller fragments. The animation below, courtesy of Tanya Atwater shows this process.

There are two significant points here. Firstly, for most of the last 100 million years or so, the spreading ridge in the Pacific ocean has been very long, or very fast spreading, or potentially both. In fact, our best estimates suggest that this single ridge system is responsible for about 40-45% of all of the oceanic crust produced since about 83 million years ago. Thus a change in spreading on this ridge at some point in the geological past could potentially have significant global effects. But secondly, accurately reconstructing the spreading history is made significantly more difficult by the almost total destruction of the Farallon plate by subduction; it is hard to find the rotation between matching isochrons when once of those isochrons is now somewhere in the mantle beneath Iowa.

So what to do? One approach, which is the basis of a paper I’ve just had published in Geophysical Journal International, is to make the best of what is left: the record of spreading still retained on the unsubducted Pacific plate. We can’t rotate matching anomaly boundaries on the two different plates back together to find the total spreading, but we can rotate adjacent anomaly boundaries on the same plate (the Pacific Plate, in this case) back together to find how much spreading occurred on the western side of the ridge between the two reversals, which should be mirrored by spreading on the (now-destroyed) eastern side of the ridge in the same time period.

Adjacent magnetic anomaly fitting

If one plate has been subducted, you don’t have pairs of magnetic anomaly boundaries that formed at the same time. But you can reconstruct rotations between anomaly boundaries on the same plate, and use that to extrapolate how much crust was produced in the same time period on the now-subducted plate.

It’s not ideal, because the shape of the ridge can and does change over time, so that adjacent anomaly boundaries are not an exact geometric match to each other. There’s also the fact that the East Pacific Rise is an asymmetric spreading ridge: based on the increasingly limited record in the South Pacific, only about 42% of all the oceanic crust produced in the past 50 million years has been added on the west (Pacific) side of the ridge, with the remaining 58% being added to the eastern (Farallon/Nazca) side. Our new paper attempts to better quantify how this spreading asymmetry varies over time where we do have a record, to set limits on what may have happened prior to 50 million years ago, when we have no record at all.

So what have we determined? The plot below shows how the average spreading rate of the Pacific-Farallon ridge has changed over time. It has never been exactly slow, but there seems to have been a transition from ‘pretty darn fast’ to ‘extremely darn fast’ around 50 million years ago.

Pacific-Farallon ridge spreading rates

Reconstructed spreading rate of the Pacific-Farallon ridge as it progressively collided with North America and shortened to form the modern East Pacific Rise. Also shown: major fragmentation events on the Farallon plate, and the broad variation in spreading asymmetry. From Rowan and Rowley (2014).

This increase coincides with the first stages of Farallon plate break-up, but although the timing is right, in some ways the response is the opposite of what you’d expect. Surface plate motions are largely thought to be driven by the pull of dense subducting slabs as they sink into the mantle; if this is the case, reducing the amount of subducting slab attached to your plate by breaking a bit off should also reduce the driving forces and slow the remaining plate down, not accelerate it. Additionally perplexing is the fact that the direction of spreading does not really change at all during this transition, either, and is in fact remarkably stable through the entire period that the ridge is colliding with North America, when you would expect there to be large changes in the balance of slab pull forces.

So what is driving this change? We’re still working on that bit (my AGU talk last December was on this very subject), but currently the speculative finger is pointing at something acting at the ridge axis itself. The potential clue? As is also shown in the plot above, there are indications that periods of higher spreading rate over the past 50 million years coincide with periods of higher spreading asymmetry. If this correlation is real, only forces acting at the ridge axis can easily explain it. As is always the case in science, you answer one question, and several more pop up to replace it.

Rowan, C.J., and Rowley, D.B. (2014). Spreading behaviour of the Pacific-Farallon ridge system since 83 Ma. Geophys. J. Int., http://dx.doi.org/10.1093/gji/ggu056.

Categories: geology, geophysics, palaeomagic, tectonics

Mammals March Madness and slight silliness from your bloggers

A post by Anne JeffersonA post by Chris RowanA post by Geo Kid

It’s time for Mammals March Madness, the tournament in which animals battle for supremacy based on their physiology and behavior, with a little bit of luck thrown in just as you would want in any competition. Note: This is a completely pretend tournament, and absolutely no animals are in any way actually fighting. There are 64 animals going through a tournament of head-to-head match ups based on their fight – and flight – capabilities. This year’s divisions include social, marine, fossil, and “who in the what now”, and there are even a few non-mammal contenders thrown in. Rather than just being passive bystanders, we humans are encouraged to make our best guesses as to who will come out on top. In making your picks, you have to consider where the battle will be fought, and whether the animals are working as individuals or in groups. During the match ups themselves, the contest organizers will tweets facts and fun for the play-by-play. So it’s a wonderful opportunity to learn about cool mammals both in the bracket making and in watching the results unfold.

GeoKid wants to know why no lions are in this year's lineup. She's too excited to stay still for a photo too.

GeoKid wants to know why no lions are in this year’s lineup. She’s too excited to stay still for a photo too.

The tournament started tonight with a wild card game between Australpithecus afarensis versus Australpithecus sediba, but it’s not too late to join in the fun. You can follow along using the #2014MMM hashtag on Twitter, or by checking in for updates on the MMM website. Organized by Harvard’s Katie Hinde, Mammals March Madness had hundreds of participants last year, and looks to have even more this year. We played along last year, but this year we’re going public with our brackets and daring our readers to join in the fun. Anne’s graduate students have taken up the challenge; will you?

Chris's bracket. How mad will he be if the orca wins after he crossed it out in favor of short-faced bear.

Chris’s bracket. How mad will he be if the orca wins after he crossed it out in favor of short-faced bear.

Chris imitates a short-faced bear. This may be the least reserved picture you'll ever see of him.

Chris imitates a short-faced bear. This may be the least reserved picture you’ll ever see of him.

GeoKid's bracket. Polar Bear wins it all.

GeoKid’s bracket. Polar Bear wins it all.

We have no polar (or short-faced) bear apparel. This will have to do.

We have no polar (or short-faced) bear apparel. This will have to do.

Anne's bracket. The fossil mammals category was tough. And fun. But she's seen a skeleton of a short-faced bear, and it was terrifying, so she takes it all the way.

Anne’s bracket. The fossil mammals category was tough. And fun. But she’s seen a skeleton of a short-faced bear, and it was terrifying, so she takes it all the way.

GeoKid looks better in the panda hat than Anne does. This time the panda is standing in for a short-faced bear.

GeoKid looks better in the panda hat than Anne does. This time the panda is standing in for a short-faced bear.

Short-faced bear skeleton, as seen as the Field Museum. I would not want to mess with this guy,

Short-faced bear skeleton, as seen as the Field Museum. I would not want to mess with this guy,

Categories: bloggery, fossils, general science

Scenic Saturday: Frozen waterfall, end of winter

A post by Anne Jefferson It’s been so cold, the geodog has been a bit shortchanged on long walks this winter. But we might finally be seeing a break in the weather and we took advantage of it for an end-of-day stroll along the Cuyahoga River in Kent. Along the path was groundwater seeping from fractured sandstone – or… a little frozen waterfall. Hopefully, soon, the weather warms enough that these icy wonders cease and all we notice on our strolls is a bit of damp rock here and there along our path.

Icy waterfall, over buff sandstone, with vines in upper portion of picture.

Photo by A. Jefferson, 6 March 2014

Categories: by Anne, geology, ice and glaciers, photos

28-ish days of #sciwrite are over, but we’ve got momentum

A post by Anne JeffersonA post by Chris Rowan
Sciwrite logo, by Chris Rowan We blew past our final check in for the 28-day, February edition of #sciwrite, but that’s because we’ve got some momentum going and didn’t want to say stop. 28. 33. It’s within the margin of error of a month, right?

In the final tally, Anne got her abstracts written, her posters made and presented, and made some figures and progress on the introduction of one paper. On the second paper, she didn’t write as much as planned, plus a new look at the data threw us for a loop. In the added bonus category, she also contributed to a meeting report in review at Eos and had her first MS student at Kent State schedule his defense. But she’s bound and determined to get both papers out ASAP, so she’s going to keep practicing daily writing, and is pretty sure she’ll get one into review by the end of this month. So #sciwrite lives on.

Chris will confess he got a little shown up in the final tally by his co-blogger, but got his grant written and is roughly 60% done with his NZ tectonics paper draft – he now has most of an introduction, methods and results sections, and lots of pretty figures. There is, however, a fair amount still to get done, including some new calculations that are needed to tie everything together in the discussion. However, he has succeeded in injecting new life into an important paper, and he is hopeful that another month of consistent effort will net him a completed draft. And that is a definite win.

Let us know how you’ve all done!

Categories: geology

Final throes of 28 days of #sciwrite

A post by Anne JeffersonA post by Chris Rowan
Sciwrite logo, by Chris Rowan Last week we intrepid #sciwriters seemed to be feeling a bit discouraged by lack of progress, but hopefully this week is looking a bit better. It could be that, come Saturday, we decided to put off writing this check-in post until we had something positive to report, or we could just be that far behind in our to-do lists. In any case, here’s our next-to-last update on February writing progress.

In the hydrology realm, Anne is pleased to report that the two extended abstracts were submitted on Monday the 17th and they look great. Two posters arising from these abstracts have also been created and submitted. The posters are for the virtual workshop on laser spectrometers for field hydrology and biogeochemistry and will be available on Scribd by mid-week. The posters will be presented on Friday live on the internet, capping off what has been an informative and interesting experience in web-enabled science sharing. Anne says:

“Even though these abstracts and posters aren’t publications per se, I’m really happy to have worked so hard on them. One is about an educational grant that I’m leading, and making the poster has caused my team to pull together information about the project that will be useful later. It has also given me ideas for a possible publication this summer. Fortunately, I wouldn’t be the lead author on it. The other poster is work I’m doing with an undergraduate, which has been tremendously rewarding and is turning out to be a neat little dataset that we are excited to keep exploring. A paper will probably come out of this sooner or later too, depending on how ambitious we want to be in additional analyses. Fun stuff!”

All of Anne’s enthusing about the abstracts and posters is not meant to completely hide the very slow progress on the actual papers she was meant to be drafting this month. Those will be coming to the forefront this week and for the #sciwrite epilogue in March and beyond. At least this week Anne managed to sort out some of her figure woes.

Chris can report a week of steady progress:

Since it is the thing with an actual deadline, most of my writing time this week was spent working on an internal grant that will support a substantial rewrite of the Geophysics course I teach, before the next time it is offered. I want to develop some exercises that give my students opportunities to analyse real datasets, and code up a set of animations and interactive visualisation tools that show more intuitively how changes in subsurface structures affect the geophysical signals you see at the surface. I’m still relatively new to grant writing, but I think its coming together nicely – I’ll at least have something to report as finished at the end of February!

As for the New Zealand paper – there was some progress there, too, although I have a couple of niggling data processing issues to sort out before I can wrap up my results section and move onto the discussion in earnest. But just like with the grant, it is coming together, just more slowly.

Chris is currently vacillating between being disappointed that he is not going to finish February with a completed paper draft, and pleased that he’s got a long-shelved project moving in the right direction again. More on that next week.

In the meantime, let us know how you’re getting on. All progress is good progress!

Categories: geology