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When teaching structural geology, I am forever presenting students with warnings about terminology.  Seems that structural geologists have more than their share of imprecision.  What does high angle vs low angle even mean.  Some sources say 45°, others 30°.  I agree with Alan Gibbs that one must make it clear when writing what type of feature you are describing by means other than a simple name.

Mike Valentine


On 6/9/06 8:30 AM, "Musson, Roger MW" <[log in to unmask]> wrote:

Well, you just have to read what is written below to see the problem.

A reverse fault is "a high-angle thrust fault"  - i.e., it is a thrust fault that happens to be high-angle.

A thrust fault is "a shallowly dipping fault, with reverse displacement" - i.e., it is a reverse fault that happens to be low-angle.

So all reverse faults are thrust faults, and all thrust faults are reverse faults.

In general it is clear that one is more likely to describe high-angle faults as reverse and low-angle faults as thrust, but the sources quoted below (and I wouldn't put any faith in Wikipedia) don't agree as to which is the general term. Also, if only low-angle faults are thrust, then the phrase "low-angle thrust fault" (often seen) is a tautology.

Roger Musson

-----Original Message-----

From: Tectonics & structural  geology discussion list [ mailto:[log in to unmask]]On Behalf Of  Hemin Koyi

Sent: 09 June 2006 15:34

To:  [log in to unmask]

Subject: {Fraud?} Re: Transform and  transcurrent faults

Roger,

 

I am little puzzled by your interchangeable use of  "thrust" and "reverse" faults:

"Reverse fault: A fault in  which the hanging wall has moved upward in relation to the footwall; a  high-angle thrust fault." ( http://imnh.isu.edu/digitalatlas/glossary/letter.asp?letter=R)

 

 

"Fault with a dip greater than  45 degrees at which the hanging wall (upper block) appears to have moved  upward relative to the footwall (lower block)."

( MailScanner has detected a possible fraud attempt from  "www.google.com" claiming to be www.wrds.uwyo.edu/wrds/deq/whp/whpgloss.html <http://www.google.com/url?sa=X&amp;start=5&amp;oi=define&amp;q=http://www.wrds.uwyo.edu/wrds/deq/whp/whpgloss.html> )

 

In general, to distinguish it from a  "thrust", "reverse" is used for steep (> 45 degrees) dip-slip faults where  the hanging wall has moved upward relative to the footwall. Below, is where  the term "reverse fault" comes from:

"The name reverse fault also  comes from Welsh miners as they showed the opposite sense of displacement to  the normal faults. Shallowly dipping faults with reverse displacement are  called thrusts. Reverse and thrust faults imply horizontal shortening and  vertical extension, and are commonly formed in convergent plate margins.  Reverse faults in sedimentary basins may also form in the toes of deltas  because of local shortening due to sediment loading or slumping or on the  margins of laterally expanding salt diapirs." ( http://science.enotes.com/earth-science/faults-fractures)

 

And under "thrust fault", the wikipedia website states:

 

 

"Another name for high angle thrust fault is reverse  fault. The difference between a thrust and a reverse  fault is in their influence. A  reverse fault occurs primarily across lithological units whereas a thrust  usually occurs within or at a low angle to lithological units. It is  because of this that it is often difficult to recognise thrusts because their  deformation and dislocation can be difficult to detect when they occur within  the same rocks without appreciable offset of lithological  contacts."

 

 

Hemin

 

---------------------------------------------------------------------

 

Hemin A. Koyi

Professor in Tectonics and Geodynamics

 

Head of Solid-Earth Geology

 

Department of Earth Sciences

 

Uppsala University

 

Villavägen 16

 

SE-752 36 Uppsala

 

Sweden

 

 

 

Phone: +46 18 471 25 63

 

Fax: +46 18 471 25 91

 

Web: http://www.mpt.geo.uu.se/staff/koyi/index.html

 

---------------------------------------------------------------------

 

 

 

 

On Jun 9, 2006, at 13:49, Musson, Roger MW wrote:

 

On the same tack, whereas I have always taken  "reverse fault" and "thrust fault" to be interchangeable, I see now some  people objecting to the use of these terms as if they were synonymous.

 

 

As for transform faults, I would normally only use  this term in the sense given below for spreading centres. Otherwise I would  normally use "strike-slip".

 

 

Roger Musson

 

 

 

-----Original Message-----

 

From: Tectonics & structural geology  discussion list

 

[ mailto:[log in to unmask]]On  Behalf Of Malcolm McClure

 

Sent: 09 June 2006 12:23

 

To: [log in to unmask]

 

Subject: Transform and transcurrent faults

 

 

 

There used to be a clear distinction between the  application of the 

 

terms 'transcurrent fault' and 'transform fault'.  The former, also 

 

called a 'wrench' or 'strike slip' fault implied  physical offset in 

 

either dextral or sinistral sense, of terrain  across the fault. The 

 

latter term used to be confined to the apparent  offset of spreading 

 

centers and related magnetic lineations across a  linear

 

boundary, the 

 

nature of which was problematical. However the  term

 

'Transform Fault' 

 

now seems to be applied to any so-called  'conservative' plate 

 

boundary across which lateral displacement can be  established; for 

 

example, the San Andreas Fault, which used to be  considered a wrench 

 

fault.

 

Can someone please clarify modern accepted usage  of the term 

 

'transform fault'? Are there many examples of  transform faults (in 

 

the original sense) offsetting continental  crust?

 

 

Malcolm

 

 

 

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Michael J. Valentine
Associate Professor of Geology & Dept. Chair
Geology Dept.
University of Puget Sound
Tacoma, WA  98416
(253)879-3129
Email: [log in to unmask]

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