There is little difference in the two diagrams you have drawn (see
below). If you draw the 'regional trend' by connecting the top tips of
each short line with one line, and the bottom tips with another line, then
both diagrams show what could be called 100% overlap within the regional
trend. The only difference is the angle the short lines make with the
regional trend. My understanding is that to be considered en echelon, this
angle should be something less than 90 degrees.
The second definition in my dictionary of 'echelon' is "a formation
of troops, ships, airplanes, etc. in which groups are disposed in parallel
lines, each to the right or left of the one in front, so that the whole
presents the appearance of steps.... To form in echelon"
Rob Twiss
+++++++++++++++++++++++++++
>I would like to hear some discussion on usage of the term "en echelon". Can
>anyone provide a reference that gives a strict definition of the correct
>usage of this term in geology? I assume it is a term that is not geologic
>in origin, but am not sure of what it is (perhaps military?).
>
>In most structural geology text books I have seen figures illustrating en
>echelon structures like so:
>
> /
> /
>/
>
>i.e. with a step either to the right or left of structures with identical
>strike, and perhaps some degree of overlap, but not necessarily.
>
>I have not seen a written definition in these same books, but have assumed
>that this is the correct definition.
>
>However, in the literature I have seen the term used to describe parallel
>structures with a strike that is oblique to an assumed regional trend,like so:
>
> / / /
>
>i.e. with 100% overlap
>
>I believe this second usage is incorrect, but is used by many to imply that
>a system is experiencing some degree of shear. I would describe the above
>as a parallel array, and NOT en echelon.
>
>Any comments?? Is this question of semantics worth pursuing??
>
>Cheers,
>Amy
>---------------------------------------------------------
>Amy E. Clifton
>Nordic Volcanological Institute
>Grensasvegur 50
>108 Reykjavik
>Iceland
>
>Tel: +354 525 4490
>Fax: +354 562 9767
>---------------------------------------------------------
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Robert J. Twiss email: [log in to unmask]
Geology Department telephone: (530) 752-1860
University of California at Davis FAX: (530) 752-0951
One Shields Ave.
Davis, CA 95616-8605, USA
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