An interesting question is whether the crack propagation in Nick’s windscreen would have been different if he had stopped the car……. Great example…
Maarten Krabbendam
From: Riccardo Caputo [mailto:[log in to unmask]]
Sent: 21 May 2014 15:58
To: [log in to unmask]
Subject: Re: Dilemma of cross-cutting relationship from outcrop-scale fractures
Hi Ijaz, I fully agree with the conclusion of Nick Oliver ("don't trust the simple geometry without looking very closely at the junction") and I would add to consider also the dynamic effects. However, I disagree with the interpretation
of why the fracturing in the windscreen 'jumped' higher. Indeed, the halt of the first left-to-right crack was probably due to the fact that the two sides across the vertical fracture were sufficiently mechanically decoupled (i.e. without cohesion) for impeding
the propagation of the tip stress on the right side of the windscreen. In contrast, the formation of the new horizontal fracture parallel to, but not aligned with, the first one was likely caused by a new stress concentration accumulated in the meantime (30-60
minutes). The stress accumulation could be due, for example, to the progressive cooling or to a viscoelastic transfer along the 'plastic' films of the multilayer windscreen; while the concentration and the fracture initiation could have been caused by a microscopic
defeat in the glass, a different curvature of the windscreen, a local temperature gradient or even a minimal geometrical irregularity in the 'older' vertical fracture. In other terms, I would have expected that the left-to-right fracture would have propagated
perfectly in-plane across the vertical one only if the latter (at the time of the crossing) was sealed to assure the mechanical continuity of the material.
Relative to stress variability in space and time, please consider the following papers (I apologize in advance for the self-citation):
Caputo (2005): Stress variability and brittle tectonic structures. Earth-Science Reviews, 70, 1-2, 103-127, doi: 10.1016/j.earscirev.2004.11.005
Caputo (2010): Why are extension joints more abundant than faults? a conceptual model to estimate their ratio in layered carbonate rocks. J. Struct. Geol., 32 (9), 1257-1270, doi: 10.1016/j.jsg.2009.05.011
regards
Riccardo Caputo
On 21/05/2014 14:30, Oliver, Nick 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-rig
ht 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.