Christopher Wibberley

Are feldspar-to-mica reactions necessarily reaction-softening processes in fault zones?

Abstract Reaction-softening by mineralogical changes from feldspars to sericite has been documented from many fault zones. During external crystalline basement deformation in the Alpine orogeny, the Ser Barbier thrust and splay faults in the Pelvoux Massif experienced ultracataclasis and sericitisation. Microstructural information and geochemical data from the fault rocks suggest that different muscovitisation reactions occurred at different times within the evolution of the fault zone, and each reaction had its own impact on fault rheology. Early cataclasis aided chemical breakdown of orthoclase feldspars to muscovite, yet quartz release accompanying this process resulted in local cementation and consequent hardening of the ultracataclasite. Continued deformation was accompanied by muscovitisation of the albite feldspar, and resulted in the formation of mica-rich fault rocks which experienced progressive silica removal by the fluid with increasing deformation. At this stage, reaction-enhanced ductility dominated. Much of the early cemented ultracataclasites escaped later deformation, and their low permeability allowed preservation of their early geochemical characteristics by preventing later fluid access. Such findings demonstrate how the complex interplay between deformation processes and geochemical reactions may result in a changing rheology during fault zone evolution.

Permeability and flow impact of faults and deformation bands in high-porosity sand reservoirs: Southeast Basin, France, analog

Outcrops of the Cretaceous high-porosity sandstone of the Southeast Basin, France, show two main types of deformation structures: a large number of small-offset, shear-enhanced cataclastic deformation bands (DBs); and a small number of large (meters to decameters)-offset ultracataclastic fault zones. Microstructural analyses of the cataclastic DBs show that fragmentation produces strands of cataclastic fragment-supported matrix, separated by weakly fractured host rock, which cluster to form the DBs. The ultracataclastic fault zones, however, are composed of a matrix-supported ultracataclasite material. Permeability data show that the DBs reduce host-rock permeability by 0.5 to 2 orders of magnitude, whereas the ultracataclasites reduce permeability by approximately 4 orders. Simple calculations considering the structural frequency, thickness, and permeability of these faults suggest that, although the DBs may have an impact on single-phase flow, it is most likely to be less than a 50% reduction in flow rate in extensional contexts, but it may be more severe in the most extreme cases of structural density in tectonic shortening contexts. The larger ultracataclastic faults, however, despite their much lower frequency, will have a more significant reduction in flow rate, probably of approximately 90 to 95%. Hence, although they are commonly at or below the limit of seismic resolution, the detection and/or prediction of such ultracataclastic faults is likely to be more important for single-phase flow problems than DBs (although important two-phase questions remain). The study also suggests that it is inappropriate to use the petrophysical properties of core-scale DB structures as analogs to larger seismic-scale faults.

Quantifying orthoclase and albite muscovitisation sequences in fault zones

Abstract A new graphical method is described for assessing the degrees of alteration of orthoclase and albite feldspars to muscovite. The method uses major element data from whole rock samples, and given a range of samples of varying degrees of alteration, pathways of the alteration sequences may be determined. The method may be used quantitatively if likely starting compositions are known and aluminium immobility is assumed. Molar proportions of K, Na and Al are plotted in the form of an Na/(Na+K) versus (Na+K)/Al graph. Modelled plots of muscovitisation of an orthoclase/albite system are presented, for comparison with actual data, so that actual compositions may be assessed in terms of feldspar muscovitisation. An example of the application of this technique to muscovitised granitic fault rocks in the external western Alps is presented, illustrating how the method is best used in conjunction with microstructural and field constraints on the fault rock evolution. This yields an evolution pathway for muscovitisation of the feldspars during fault zone deformation, and links specific alteration steps with particular microstructural changes within the fault zone. We also apply this technique to a previously published data set on basement shear zone geochemical changes from the Pyrenees where orthoclase albitisation provides an additional interest. Given that these reactions are fluid-induced, this work provides further information on the effects of fluid flow in such fault zones in terms of basement fault zone rheologies.