Brett Davis of Olinda Gold PTY LTD reported that this contribution has been sponsored by some of the posts he has seen in which faults are shown. Most of the comments here will already be common knowledge to many geologists. So, if you know all about faults and shear zones and associated terminology, please skip the post and save the rotten tomatoes for someone else. However, he have seen enough posts to the contrary, and seen enough examples at mining and exploration projects globally, to make him think a discussion is needed.
We all know what faults and shear zones are. And we all know of the various kinematic indicators used to determine movement sense, even though there are common disagreements on the interpretation. But this report is not about movement sense per se. It is more to do with descriptions.
Firstly, there is the terminology that uses the word ‘shallow’ for a low dip. Shallow means depth. What if a fault has a dip of 10° and we intersect it hundreds of metres below the surface? Is it a shallow deep fault? We should use the terms low, moderate and steep for fault dip, and reserve the term shallow for depth in the crust (or for people incapable of serious thought, like some he has met in recent times, but he digressed). He notes that low dips are sometimes referred to as gentle dips. This is acceptable (gentle is preferable to wild and unruly).
Next comes the ascription of movement sense to faults. More commonly than not, geologists will say ‘it is a sinistral fault’ or ‘it has a dextral sense of shear. This is not a problem if it is a true displacement along the movement vector, which typically manifests as a lineation. Rarely, however, do geologists say it is an apparent sense of shear, even though they have no idea of the movement vector. However, the implications are obvious. Namely, the opposite sense of movement may exist such as in his diagram below. It shows the importance of knowing the movement vector. In this case the movement sense in
plan is apparent, and the distance between the same unit on either side of the fault is called a separation, not a displacement. These are two totally different things (although separation and displacement are the same, but I digress again).
Read on .... worth the folksy read.
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