Dear All,
Mary’s comment reminds me of a good field observation element: closed shear fracture will cut across one another, younger fractures will cut and displace older ones.
Younger tension fractures will abut against older open tension fractures.
I like two classic books on fractures, there have been later books on the topic but in my view never really better ones:
Neville Price, “Fault and Joint Development in Brittle and Semi Brittle Rock”, Pergamon Press, 1966.
Georg Mandl, “Rock Joints, The Mechanical Genesis”, Springer, 2005.
These two classics are really focussed on the mechanics of fracturing and really mostly differ in the presentation and the style of writing, but both are geomechanically very precise and in my view unequalled by later work.
Not that later books would not be good, they just do not seem to offer new insights into the mechanics of fracturing. Most books also offer a broader view (which I like more) and are not limited to fracturing.
Regards, Dirk
> On 21 Feb 2017, at 23:16, Mary FORD <[log in to unmask]> wrote:
>
> Hello Helge
>
> The orientation of fracture sets and their relation to each other and to larger scale structure (folds, adjacent faults or just horizontal or tilted bedding) are keys to understanding their type and origin. For example, although shear fractures do not always develop as conjugate sets, if you do have conjugate facture sets then they must be shear fractures (Mode II or III). A very nice example of orientation analysis of fractures in a folded domain (Jura) coupled with detailed observations of striae and stylolites on the surfaces, is presented in the textbook of Ramsay and Huber, Volume 2. The measuring of fracture orientations and their analysis using stereonet or rose diagram is time consuming but often well worth the effort.
>
> Regards
> Mary
>
> Le 21/02/2017 à 21:53, Helge Alsleben a écrit :
>> I was wondering what attributes, features or characteristics people commonly use in the field to differentiate between opening Mode I fractures/joints and shear fractures (Mode II or Mode III)?
>>
>> I understand that Mode I fractures may develop plumose structures, whereas Mode II/III have shear displacement or may even develop slickenlines. However, for the practicality of detailed outcrop studies, where fracture surfaces are often not exposed and displacement on small bed-bound fractures may be very small, those features might not always be observable.
>>
>> Are there any other features or characteristics that one could look for to differentiate between the different mode fractures in the field with some degree of certainty?
>>
>> I'd appreciate any comments or suggestions.
>>
>> Thanks,
>> Helge
>>
>
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