Mode I microfracturing and fluid flow in damage zones: The key to distinguishing faults from slides

We have examined the distribution of microfractures in arenites and the evolution of vein forming fluids in the matrix of carbonate breccias within the damage zones of large detached blocks in order to characterize their modes of emplacement. Previous studies of microfractures in the damage zone associated with tectonic faulting have shown a clear pattern of increasing density as the fault is approached. Previous studies of carbonate breccia within damage zones of tectonic faults typically document evidence of multiple fluid events representing repeated rupture-healing processes. However, in this study, we find no change in the microfracture density with distance from the 45 km-displaced gravity-driven slide block at Heart Mountain, Wyoming. In a previous study of the same massive slide block there was no evidence of multiple fluid infiltration events related to emplacement. We interpret these observations as indicating the absence of rupture cycling that would be expected in the development of a process zone, instead being consistent with catastrophic emplacement of gravity-driven slide blocks. We use this distinct pattern of microfracture density and fluid infiltration to demonstrate that several large (>1 km2) detached blocks in the Basin and Range, previously thought to be allochthons related to hyperextension detachment faults, are actually slide blocks whose detachment surfaces represent no crustal extension.

► Large slide blocks do not develop a process zone during emplacement.
► Microfracture density and fluid histories are useful in characterizing detachments.
► Two large-displacement extensional allochthons are found to actually be slide blocks.