4/11/2008

Why the Brunton Compass is the Most Important Tool

Sometimes I forget that this is a blog and not a private diary. Ishmael reminded me by asking about some of the jargon in this post. Ishmael asks:

"What's a fault thrust? Why is compass direction important in describing features? Why does my brain hurt so?"

I'm just going to focus on question #2, as I think google can answer #1 and probably #3 has something more to do with Tony's Navigational Hazzard (sic) than with geology.

ORIENTATION OF STRUCTURES
Why do we measure the orientation of geologic features?
1. It helps us group structures into a systematic framework
2. It enables us to guess at orientations of (former or current) geologic stress which caused those structures to form
3. Stress orientation is related to tectonic events and plate motion.

Let's take the example of Kodiak Island, using this very brightly coloured map my colleague Eric Thompson put together for presentation use:
modified from Byrne and Fisher, 1982.

Faults are roughly planar or curvi-planar features in 3D* and so appear as lines or arcs on a geologic map. A first order observation one can make from this map is that the major faults on Kodiak Island are just about parallel to one another. Some of these faults are cross-cut by intrusive igneous rocks (red blobs) which have been dated to about 57-63Ma (just after the massive impact event at the end of the Cretaceous). That means some of these faults have not been active since before 63Ma. A second observation one can make, using the inset location map, is that they are parallel to the active plate boundary fault (toothed line) and if you are familiar with the area you might also notice that both modern and ancient faults are parallel to the volcanic arc.

So a hypothesis one could draw from the map, is that since old faults and modern faults are parallel, that the stress orientations have been consistent for a long time. Since today's geologic stresses are caused by the subduction of the Pacific Plate beneath Kodiak, one might hypothesize that an oceanic plate has been subducting under North America for a long time.

To get more information, we did a close-up study of one of these old faults: the Uganik Thrust where it cuts through Afognak Island. The location is shown by a little star "Big Waterfall Bay" on the geologic map.
Fig. 2, Rowe, Meneghini and Moore (in review)

In the map view (A) you can see little symbols of a line with a triangle on it and a number - these are recordings of compass measurements of textures in the rocks. These measurements are used in (C) to draw a cross section which shows how the rock textures are oriented in the third dimension.

From the data presented in this figure, we can make a few more observations. First, the rocks to the SE were thrusted underneath the rocks to the NW along the Uganik Thrust. This confirms that the Uganik Thrust has the same sense of offset as the modern subduction thrust active under Kodiak. Second, the rock textures in detail are not all parallel to the thrust fault, this suggests that the story has more complications to it. The fault cross-cuts some of the textures in the rock. The igneous rocks can be seen cross-cutting the fault (B). In fact, the rock fabrics in the fault, which cross-cut the older rock fabrics, can also be used to learn about water travelling through the fault. This is of interest because water can change the nature of the rocks, or cause fluid pressure to change the stress on a fault - affecting the earthquake behaviour of a fault.

I'll skip a few of the details here - even the reviewers on this paper might find the presentation of 12 individual stereonets to be a little much data. But the basic observations presented here are enough to hypothesize this comic novel history for the fault - this is Fig. 9 of Rowe et al., submitted which I hope to be published soon.

The cartoon doesn't just draw on my own data from this study, it's also consistent with previously reported general models for the structure and assembly of Kodiak Island. The first two panels are pretty much derived from previous theories from other geologists.

Our work at the Uganik Thrust adds panels C & D to the story - showing how the fault was a conduit for fluids and then was cross-cut and effectively killed when igneous intrusions pinned it shut. Kodiakites (Kodiakers?) may not be aware that prior to the development of today's subduction zone where the Pacific Plate goes under North America, displacing fishing boats up to Mac's Sporting Goods store at a whim, there was another plate in between NA & Pac. The long time subduction of that oceanic plate caused the creation of most of the rocks of Kodiak Island during the Permian, Triassic, Jurassic and Cretaceous periods. Eventually the entire Kula-Farallon plate was consumed in the subduction zone, and the boundary between the Pacific and Kula-Farallon plates was also swallowed. This brief event caused a hiccup in the long-term stable history of the margin - causing a misorientation of rock fabrics and the introduction of the igneous intrusions. Thereafter, the Pacific Plate continued to subduct and events returned to "normal" in south-central Alaska, as they continue to this day. In Panel D, the cross sectional view across the rocks of Kodiak is pretty similar to what you see today. In this panel, the "Ghost Rocks Fm." is at Pasagshak Point - so clearly there has been a lot more uplift in the ensuing 50+ million years. This uplift is also recorded in the orientation of structures - as it has occurred through landward tilt, the old faults in the rocks are "dipping" more steeply landward whereas they were originally gently sloping.

THE TOOLS
The geologic compass is pretty much indispensable. It differs from an ordinary compass in that it has a clinometer as well as the usual magnetic needle. Also, the dial is numbered "backwards" of a what you would see on a compass rose, so that you point the compass in a direction and it reads the bearing at which you are pointing. It also has an adjustable dial so that one can set the compass to measure to true north rather than magnetic north. This is done to avoid the introduced error due to temporal and spatial variation of magnetic north.

I am an adherent to the church of Brunton; we believe the Professional Pocket Transit can do no wrong. I have two - one balanced for the northern hemisphere and one balanced for the southern hemisphere.


* Except when they are not.

2 comments:

FIngel said...

I am a geology student at Southern Oregon University and I would like to thank you for the excellent post. At the moment we are working on structural geology and this post was a great help to my studies. You are an awesome writer and I will def be bookmarking this for future reference! thanks a lot!!!

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