Hi Helge,
Differentiating Mode I from shear fractures in the field can indeed be
quite challenging in many situations. Keep in mind that plumose
structure is very delicate. A fracture could form in pure Mode I, and
then later be reactivated in shear, which would likely destroy the
plumose structure. There are also several other types of other surface
structures that can accompany the fracturing process. The Structural
Geology text by Twiss & Moores has lovely descriptions of these features
in Chapter 2, and later in Chapter 9, they discuss these features in
more detail, including a short section on differentiating extension
fractures from shear fractures. With this said, and as you point out,
these fracture surface features are only useful if you can see the
fracture surfaces.
Another useful feature to look for are the so-called wing cracks,
horsetails, or tail cracks. These form near the tips of mode II
fractures, and can be rather useful (if present) as a sense of slip
indicator on fractures with very small amounts of slip or no clear
offset features. Because these wing cracks develop in response to near
crack perturbed stresses that are much larger than the remote driving
stresses, they can form along fractures with very small amounts of slip
(a few mm or so). Of course, this is only useful, if the fracture tip is
exposed/visible, but these were quite useful for me back when I was
working in the Valley of Fire State Park in Nevada, USA.
There have been many papers written on wing cracks (including some of
mine), but my two favorites are:
Cruikshank, K.M., Zhao, G., Johnson, A.M., 1991. Analysis of minor
fractures associated with joints and faulted joints.
J. Struct. Geol. 13 (8), 865–886.
Willemse, E.J.M., Pollard, D.D., 1998. On the orientation and patterns
of wing cracks and solution surfaces at the tips of a sliding flaw or
fault. J. Geophys. Res. 103 (B2), 2427–2438
I hope this helps. Best of luck with your work.
Cheers,
-Scott
On 2/21/2017 3:53 PM, Helge Alsleben wrote:
> 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
|