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
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Principal, Holcombe Coughlin Oliver
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Structural geology, hydrothermal pathways, exploration and mining
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-----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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