Interesting discussion!
You have the effect of crack interaction, but you also have the effect of
the stress variations in the surrounding rock. Take a look at these
papers.
Moir, H., Lunn R.J., Shipton Z.K., and Kirkpatrick, J.D., 2010. Simulating
brittle fault evolution from networks of pre-existing joints within
crystalline rock, Journal of Structural Geology, v.32 1742-1753.
doi:10.1016/j.jsg.2009.08.016
Moir H., Lunn R.J., Micklethwaite S. and Shipton Z. K. 2013. Distant
off-fault damage and gold mineralization: The impact of rock
heterogeneity. Tectonophysics v. 608, p. 461-467
doi:10.1016/j.tecto.2013.08.043
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Prof. Zoe Shipton
Department of Civil and Environmental Engineering
Room 5.05c, James Weir Building,
75 Montrose Street,
Glasgow, G11XJ
0141 548 3183
NERC CDT in Oil & Gas: PhD Opportunities -
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On 21/05/2014 13:30, "Oliver, Nick" <[log in to unmask]> wrote:
>Hi Ijaz, your observations are astute. There are multiple options for a
>propagating fracture depending on the confining stresses and the relative
>strength of the pre-existing veins/fractures relative to the propagation
>of the new fractures. I once had the pleasure of observing crack
>propagation in the windscreen of the car I was driving (after I had
>slowly collided with a tree branch during field work in northern
>Australia). One crack was propagating left-to-right. Another
>near-vertical crack was already there. The moving crack (moving at about
>1cm per minute as we were driving along to get the windscreen repaired)
>collided with the other one, and then apparently stopped. After 30 or 60
>minutes, it started again, heading in the same direction, but from a
>different position about 10 cm higher along the crack it had intersected.
>The final result would conventionally have been interpreted as though the
>vertical crack was a fault that had displaced the left-to-right crack,
>whereas the reality was the other way around, and the second crack had a
>'dog-leg' shape. The cohesion of the walls of the old crack was
>sufficiently high that it stopped the direct propagation of the later
>left-to-right crack.
>
>Thin sections may help, but so will very close observations in the field,
>even with the aid of a good digital camera. If you have the opportunity
>to visit a bank or other public building with veined limestone columns or
>facing stones, common in many parts of the world, you may also see the
>same thing. I have workshops with examples of this problem but there may
>be others who can respond with publications that describe this
>specifically at the scale you are interested in. One such publication is
>listed below, but it deals with the same issue at broader scales. You
>have already understood the key problem - don't trust the simple geometry
>without looking very closely at the junction.
>
>Nortje, G.S., Oliver, N.H.S., Blenkinsop, T.G., Keys, D.L., McLellan,
>J.G., Oxenburgh, S. 2011 New faults vs Fault reactivation:
>Implications for fault cohesion, fluid flow and copper mineralization,
>Mount Gordon Fault Zone, Mount Isa District, Australia. Geological
>Society Special Publication 359, 287-311
>
>Cheers
>
>Nick Oliver
>[log in to unmask]
>Principal, Holcombe Coughlin Oliver
>Trading as HCO Associates Pty Ltd ABN 52154666707
>www.holcombecoughlinoliver.com
>Structural geology, hydrothermal pathways, exploration and mining
>ph +61-(0)417764880
>
>Also Adjunct Professor of Economic Geology, James Cook University
>[log in to unmask] for university business
>
>
>
>
>
>
>
>
>
>
>-----Original Message-----
>From: Tectonics & structural geology discussion list
>[mailto:[log in to unmask]] On Behalf Of Ijaz Ahmad
>Sent: Wednesday, 21 May 2014 7:42 PM
>To: [log in to unmask]
>Subject: Dilemma of cross-cutting relationship from outcrop-scale
>fractures‏
>
>On the outcrops of limestone rocks, it is often observed that the calcite
>filled fractures cross-cut each other in such a way that there seems
>off-set along one of the fractures. Therefore, it is generally considered
>as an off-set. But I think it must be confirmed from thin-section
>(microscopic) studies whether pulverization is occurred. The reason
>behind this idea is that during fracture propagation, the propagating
>fracture when comes in contact with the pre-existing fracture, the energy
>begins to travel along the plane or interface of the pre-existing
>fracture surfaces and at certain place when it will get the weak path to
>propagate in the form of a new fracture. According to this idea, there is
>no off-set, rather it is a fracture that cross-cut the pre-existing
>fracture.
>
>Welcome for comments and suggestions.
>
>Best regards
>Ijaz Ahmad
>geologist
>
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