We headed south once again, to just beyond Sleeping Warrior (where we visited on day 3). On the google satellite image Helen had spied a crater which appeared to be cut by a strike-slip fault – it looked as though there was horizontal displacement along the fault. Having fallen in love with the street-corn here, we couldn't help but stop en-route :)
Enjoying a mid morning munch on the way
Finding this crater was to be the biggest challenge. It was a challenge I thoroughly enjoyed, as it involved me climbing up a couple of craters in the area to try spot the crater we wanted to visit. Once turning of the main Nakuru-Nairobi road it should have, in theory, been very simple to find our crater. On the satellite image we have it appeared to be the only crater in the area. The reality was quite different. Scattered over the flat base of the rift valley were small craters, cinder cones and ridges (fault scarps). We asked a group of locals if they knew the way. But as this crater doesn’t have a name, and the locals weren’t really sure what we meant when saying ‘the one with the fault’ it quickly turned into something of a wild goose chase. We were taken to the largest crater in the area by a couple of the locals who insisted it was the crater we were looking for. We could tell it wasn’t as we approached - there was no fault running through it. The locals who had joined us -Gideon and Timothy- insisted we were at the only crater in the area. It wasn’t, there were lots we could see around us. Further investigation revealed they had brought us to this one as it is the only one which people come to visit. But we are geologists...and geologists tend to need to study places that few others are intereted in. We climbed part way up the flanks of this crater to get a better view of the area and see if we could spot the crater we wanted. We had to do this at a couple of places, with some bumpy off road driving between them before we finally spotted the crater we were after.
A Panoramic looking south from the crater (when we eventually found it!) I'm in it for scale, you can see that there are a lot more than the one crater/cone/ridge shown in the satellite picture. It also shows the distance between each, and hence the amount of off-road driving (such fun) to get to the crater.
Once we had finally found our crater I climbed up to the top, and wandered around the crater rim. I was convinced this was the crater we were after. I wasn’t convinced that a strike-slip fault intersects it. There was not any convincing horizontal displacement, the only place where you could maybe argue this was here:
It was only from this angle that you could see potential horizontal displacement. The edges of each side of the fault are highlighted for clarity.
Personally I think it is simply the angle I was looking at it from which gave the impression of any horizontal movement. If it truly were horizontal movement it would be clear from more than one angle.
However, this was definitely a fault and some vertical displacement could certainly be picked out. Not much, but it was there.
I don't doubt it is a fault which cuts the crater wall at the north. A fault scarp can be seen continuing into the crater.
The north-west side of the crater does seem to be faulted slightly, but by a normal or oblique fault (a fault that displays both vertical and horizontal movement.) I definitely don’t think it is a strike-slip. Why do I think this? Firstly due to the offset observed at the crater itself; the offset was minimal, there had certainly been vertical movement of around 0.5-1m and I think that the apparent horizontal displacement (from only one angle) is due to it not being a perfect normal fault, but an oblique fault – with the horizontal movement being dominant. I also think the fault is much older than the volcano which has left this crater because the fault trace can be seen for km’s to the north of the crater. The offset on this fault trace is at minimum 5m, with no evidence of any horizontal movement. What I believe has happened here is the normal fault – which already had a vertical offset of about 5m was exploited by magma at the location of the crater, forming a small volcano. Since the volcano has become dormant (we observed fumaroles on the crater rim) there has been a small amount of movement on the fault, and for some reason (possibly the shape of and/or the space created by the magma chamber of this small volcano) this later movement has been oblique locally at the crater. Although we didn’t find quite what we had come to see here, we did get to study volcanic-tectonic interactions. And once again we have proved the importance of ground-truthing observations from digital images. Today I have discussed a few different types of faults. I will end today by summarising the main different types of fault. Normal fault – a normal fault typically dips at about 60 degrees, and one block moves down relative to the other along this line. Normal faults are typical of extensional regimes (area where the crust is moving apart – ie the East African rift.
A normal fault
Strike-slip fault – A strike-slip fault usually has a much steeper, near vertical dip, and the blocks slide by each other – ie the San Andreas fault, California.
A strike-slip fault
Oblique fault– A oblique fault is a combination of either a normal or a reverse fault and a strike-slip fault. The movement along an oblique fault is both vertical and horizontal, although one tends to be dominant over the other. The Alpine fault, NZ is an example of a reverse oblique fault.
An oblique fault
Reverse and Thrust faults – A reverse fault has a similar dip to a normal fault, but movement is in the opposite direction, so one block moves up relative to the other. A thrust fault (more common) is a low-angle reverse fault. The angle of dip on a thrust fault is less than 45 degrees, at this lower angle it is much easier for one block to be thrust on top of another. The Moine Thrust in Scotland is an ancient thrust fault. Reverse and thrust faults occur in compressional regimes (where plates collide) and often result in the formation of large mountain belts such as the Himalayas.
A reverse fault