For those of you who don't know I am working on a project in the Pyrenees, a very interesting [mainly] Cenozoic convergent orogen. This was my first trip to this area, and I think like many geologists visiting their field site for the first time, my appreciation for the project and of the many papers already written about the geology of the area increased dramatically.
This is also the first time that I've used thermochronology to study the uplift of a convergent orogen. In the past, I have mainly focused on extensional orogens and normal faults. Normal faults are in many ways the most ideal system for thermochronology. This is a highly simplified schematic diagram showing how large rotational normal faults operate. There is no scale on this diagram, but it is meant to depict a fairly large cross section of the brittle upper crust, let's say 8 or 10 km thick.
The three images above depict three different time slices during the extension of some chunk of crust. Normal faults are the kinds of faults that accommodate extension (you can see that the final image, on bottom, has been thinned and stretched, or extended). The faults rotate as they move, and in the end, rocks that were originally very deep in the crust (some of the grey bars), are exhumed, or brought up to the surface. This exhumation can happen relatively rapidly, and results in the cooling of originally hot rocks (hot because they were deep in the crust). This cooling is what we can measure using thermochronometers.
Thrust faults are completely different, not only in terms of what environment they form in, but also with respect to what they do thermally for a rock. The figure below shows time slices of an idealized thrust system
Although thrust systems can make some impressive topography (think the Himalayas, Rockies, and many other major mountain systems), they actually do not exhume rocks. You can see this by tracing how deep any one point is on the diagram above, or on this excellent animation of a thrust fault. Since thrust faults don't exhume rocks, they don't cool rocks (well, that isn't strictly true but we will save that discussion for later), and therefore we can't necessarily use the same thermochronologic approach to study them.
Fortunately for us, the large welts in topography often formed during thrust faulting are often associated with very high rates of erosion. Erosion removes rocks from the top of the thrust sheets, thereby exhuming and cooling the deeply buried rocks we want to study. One of the reasons the Pyrenees are such a great place to work is that the products of this huge erosional event (aka the sediments in the foreland basin) are very well preserved and in many places very well exposed. In fact, many of the older sediments are themselves caught up in younger thrust sheets, which are then eroded and exposed for us to study. Many Spanish and French geologists (along with colleagues from around the world) have done an exceptional job reconstructing the history of deformation and fault activity by mapping these sediments and associated thrust sheets. And, just as a disclaimer I would like to point out that my thermochronology would be completely and entirely meaningless were it not for the labor of the structural geologists and sedimentologists who have studied this area for decades. As another disclaimer I am not a sedimentologist, and can only pass for a structural geologist by association.
On our way to the field site, we passed through places where these sediments are well exposed. One of the more impressive things about the Pyrenees are the piles and piles of conglomerate you see, some like this exposed in thousand meter high cliffs.
these conglomerates were all deposited, then deformed, during the main contractional event in the Pyrenees (early to mid Cenozoic). Some of you who have travelled to Spain may be familiar with similar looking exposures at Montserrat, these are not directly related to the Pyrenean orogen (but instead the younger Catalan Coast Range orogen), but did add to my feeling of this part of Spain as the land of conglomerates.
The thrust sheets are themselves impressive, and although I don't have the information on the unit exposed here, it is a pretty picture
But alas what I came to sample and study were the granites. Most of these granites formed (well, crystallized from a magma) during the Hercynian orogeny (late Paleozoic) where they intruded a Paleozoic sedimentary succession. The Paleozoic sediments are now all deformed and metamorphosed (that will be a later post). Many of the granites were then exhumed to at or near the surface. We know that because we can still find places where the granites are overlain unconformably by Triassic sediments. You can see that in this picture
The reddish layered rocks at the very top lie uncomformably on the granite below (the greyish stuff). The contact is itself folded (don't let anyone every tell you granites don't fold), and has now been eroded nicely for us to sample and study.
In order to do proper thermochronologic studies, you need to sample the greatest amount of structural relief you can. In extensional terrains, this is nice. Because the blocks rotate as they exhume, you can often collect samples from many kilometers of structural relief, sometimes without climbing more than a kilometer of actual altitude. Areas where the exhumation is accomplished mainly by erosion are very different, here, for example, structural relief roughly equals actual relief. So, to sample 15o0 meters of structural relief requires hiking up 1500 meters of elevation.
Not to complain, I was with an excellent field group and was able to do some exceptional (albeit long) hikes. The pyrenees are filled with very nice marked trails that take you up big granitic mountains. I was even able to bag my first 3000 meter Pyrenean Peak (3221 to be exact). But it did give me a whole new appreciation for vertical profiles. They are definitely the way to go for thermochronology (perhaps another post as to why), but they are tough things to collect.
This was also my first trip to Europe, which was incredible. But more on that later.
Anyways, a brief intro to my field season, some of my bajillion pics, and even a little geology.