Microfracturing associated with reactivated fault zones and shear zones: what can it tell us about deformation history?

Abstract

Abstract Deformation in fault zones is commonly characterized by grain-scale microfracturing, with microcrack density typically increasing toward the middle of the zone. The cracks can form under a wide variety of conditions and need to be used with great caution in making tectonic interpretations, particularly in areas with a complex history of fault reactivation. Microcracks may be intragranular (contained within single grains) or intergranular (with a length of several grain diameters). Intragranular cracks formed under dominantly plastic deformation conditions are crystallographically controlled and may not be directly related to regional stresses. Intragranular cracks formed during initial fracturing under cataclastic conditions develop only in grains that are optimally oriented to the deforming stresses. Intergranular cracks form during progressive cataclasis as intragranular cracks grow to join one another: they may develop as transgranular cracks that cut across several grains or as grain-boundary cracks. Once formed, microcracks may be preserved in a variety of ways (e.g. sintering, healing, cementation) depending on post-deformation conditions, and may be distinguished from one another on the basis of microstructural characteristics. Distinguishing between successive generations of microcracks in areas of fault reactivation is particularly important in determining the deformation history and obtaining deformation conditions. For example, Proterozoic quartzites collected from the central Utah Sevier belt have undergone multiple episodes of contractional deformation followed by Basin-and-Range extension. The use of polarized and dark-field optical microscopy and scanning electron microscopy allows microcracks related to the separate episodes of deformation to be distinguished on the basis of morphology, mode of preservation and consistent cross-cutting relationships. Variations in microcrack density and volume of cataclasized rock for the different generations of microcracks are used to establish the patterns of overprinting during fault reactivation. Anastomosing zones of intense deformation formed during successive episodes of faulting may not coincide with one another, as grain-size reduction and cementing during each episode hardens the zones, causing deformation to shift to adjoining weaker rock. However, the fault zone as a whole is a sufficiently large inhomogeneity that it is reactivated during successive faulting events.