Fracture-aperture size—frequency, spatial distribution, and growth processes in strata-bounded and non-strata-bounded fractures, Cambrian Mesón Group, NW Argentina

Abstract

In Cambrian Mesón Group, NW Argentina, small faults and three opening-mode fracture sets defined by orientation and cement texture (Sets 1–3) formed sequentially in sandstone that most likely had constant mechanical properties throughout deformation. Yet the opening-mode sets display contrasting fracture-aperture-size distributions, spacing patterns, and tendency to be bed bounded. Set 1 fractures are quartz-filled or -lined opening-mode fractures with crack-seal texture, having a wide range of opening-displacement (kinematic aperture) sizes; they are irregularly spaced and non-strata-bounded fractures. Set 1 macro and microfracture-opening-displacement sizes are well described by a power law with slope −0.8. Set 2 fractures are microscopic, mostly quartz filled and have characteristic aperture sizes, are probably not bed bounded and have either a near-random or clustered spatial distribution. Set 3 fractures are quartz-lined, opening-mode fractures with extensive open pore space, having a narrow (characteristic) opening-displacement size distribution; they are regularly spaced and stratabounded. Differences between Sets 1 and 3 can be accounted for by quartz deposition resisting fracture reopening to a greater extent for Set 1 during repeated, episodic growth, where crack-seal texture is present in fracture-spanning quartz. In contrast Set 3 fractures are nearly barren with only trace-cement deposits that did not resist opening. Power-law opening-displacement size distributions may be favored in cases where fracture growth is unequally partitioned amongst variably cemented fractures, whereas a characteristic size is favored where growth is unaffected by cementation. Results imply that thermal history and diagenesis are important for fracture-size-distribution patterning.