Archive for August, 2005|Monthly archive page

On Ontario

I expected to be updating this thing regularly over the summer, and for various reasons that hasn’t been happening. Nonetheless, here’s an update on some current projects:

Superior Province tomography: The Superior Province is an Archean craton — that is, one of the oldest chunks of stable continent in the world — which makes up most of Ontario, along with a decent slice of Quebec and (buried under sediment) about half of Manitoba. The deep roots of these cratons (below the crust) are the subject of much interest, though the Superior has only really been looked at in piecemeal fashion.

Until now, that is. The FEDNOR array has put a coarsely-spaced set of seismic stations out on the Superior; thanks to two St├ęphanes, we’ve been able to include data from temporary deployments in the region as well. The result is a 3-D image of seismic velocity beneath most of Ontario:

On_tomo_250resulting from recording earthquake traveltimes at stations from FEDNOR/POLARIS, the Canadian National Seismograph Network, the Abitibi and TW~ST experiments, and (thanks to some digital archeology and the IRIS Data Management Centre) the older APT89 experiment. The overall coverage is pretty good (blacked-out areas are not well covered), and in these early results, there appears to be a major difference between the eastern (mostly red, i.e. seismically slow) and western (mostly blue, fast) regions of the Superior Province. A few more FEDNOR stations have just gone in, and will hopefully help close up that resolution hole in the middle; still, even what we have now is a pretty good-sized data set, representing (among other things) a lot of trvel-time picking work by Soo-Kyung Miong, the student who’s been working on this.

Soo also did SKS splitting analysis (a technique which detects directional fabric) at the same stations for her honours thesis; the splits at the western stations are stronger and more consistent in the east. A full presentation of this will go into a paper soonish (once again, it’ll be nice to have those new stations), but in the meantime, in collaboration with Ian and Dave, we’ve been looking at…

Mantle Fabric in Eastern Ontario: There’s nothing like a journal special issue (with deadline) to make one write a paper in a hurry. This one combines the aforementioned SKS splitting in eastern Ontario with magnetotelluric measurements of geoelectric strike (another measure of fabric, this time in electrical rather than seismic properties) and some receiver function studies of individual stations (which I’ve mentioned before) in order to get an idea of how the fabric varies with depth. There’s no simple punch line, but here’s a map:

E_on_fabricshowing electrical strike directions (black) along with seismic fast-axis directions (red; lighter red arrows are Soo’s new results). They’re not the same, but they don’t seem to be uncorrelated, either. I still don’t entirely understand the guts of the magnetotelluric method, but watching Ian work on this has been instructive — hanging around smart people is never a bad thing.

The latter paper’s already submitted — I may put up a preprint at some point, if my co-authors don’t mind — and hopefully the other project will be a paper before the end of the year. Then it’ll be time for the next project — and I haven’t even mentioned the project my graduate student, Jinling Zhang, is working on, which is looking extremely promising.

Now I need to decide how much of this I can cram into AGU posters…