I flew back to the US for the AGU spectacle and holidays with the family. On the plane, I read Kinabo et al. (2008)'s paper in Tectonics which includes some interpretations of aeromagnetic surveys over the Okavango Rift in NW Botswana (Figure 4c, below).
These images show the magnetic susceptibility of the subsurface geology. I think that it's basically a measure of how strongly the material responds to a magnetic field - so it relates to the amount of magnetic minerals in the rocks as well as the exact minerals present and their alignment. Anyway like so many geophysical data sets, it can be used for purposes that are not really related to magnetics, like looking for changes and patterns in rocks in a more general sense: mapping. In this case Kinabo et al. have interpreted the purple-red WNW-trending features as Jurassic dolerite dykes (common in this part of the world) and the NNE-trending lines as faults offsetting them. This is reasonable and consistent with everything I know about the region and I believe they are correct. However I couldn't help noticing that the clouds over Frankfurt, Germany also contain Karoo dykes offset by normal faults:
Excuse the sarcasm, what's wrong with me? It's flippin Christmas.
Just trying to express my trepidation about "mapping" by tracing "lineaments" and making guesses about what they really are. I have extreme discomfort about doing this kind of mapping myself but my local experts assure me that I still have a lot to learn. In theory, educated guesses are just fine (as in Kinabo et al. (2008)). However, if there are real geologic surprises in that data set you'd have to be pretty careful and pretty insightful to find them. Anyway if it leads to a ground-truthing campaign in the northern Kalahari, I'm all over it.
Cold: ice pack in Hudson Bay. Hot: the baby blanket I'm knitting for Mini MacStammer the Vth (not the child's real name).
We flew in twilight across the Canadian shield. My photos didn't come out so well but I could see the shapes of the 3 billion year old rocks poking up in roche moutinees through the patterned ground between snowy frozen lakes. It looked cold and very windy and isoclinally folded with a WSW-ENE trend to everything.
Getting ahead of the sun we finally broke into California in the late afternoon. The clouds opened in front of the sea and I saw what I had been waiting for - the active margin. The Crystal Springs Reservoir in San Mateo:
Funny that the wikipedia article on the reservoir mentions that it "... lies in a rift valley created by the San Andreas Fault". This is technically true, although it might be more accurate to say "IS THE SAN ANDREAS FAULT" (see USGS page for proof).
We did the customary spin around the bay, revealing an intersection of the SAF with the coastline - the mouths of both Tomales and Bodega Bays (west Marin County).
Sunset over the Salinian Plate: Point Reyes
And on to the big party!
(Slugs breakin it down, Thirsty Bear Pub, San Francisco. Looks seismic doesn't it. I frickin love San Francisco.)