FLOW AND DEFORMATION OF VISCOUS, SILICA-OVERSATURATED DISPERSIONS IN LOW-GRADE FAULTS

Cryptocrystalline silica veins occur along, shear faults and in dilata tional cracks in low-grade cataclasites derived from leucocratic grani toids. The silica minerals occur as radiate bladed quartz, and as sphe rulitic and sheaf-like clusters of neocrystallites that nucleated on c rush fragments in a matrix of opaline and chalcedony. This texture is indicative of rapid crystallization from a viscous, oversaturated flui d. The veins are clast-loaded, and typically demonstrate a banding def ined by variation in the amount and size of feldspar crush fragments. Three types of veins are distinguished that differ in relative timing of viscous fluid flow, deformation and crystallization. Type (1) veins are dilatational cracks, in which static settling and crystallization took place. An undisturbed, planar layering is found, which has no fi xed orientation with respect to the vein wall. This banding is commonl y associated with a layering defined by periodic variations in hematit e precipitates. Type (2) veins show a gradual decrease in grain size o f the crush fragments from one vein wall to the other. This banding sh ows pinch-and-swell structures, micro boudinage and folding, suggestiv e of flow. Type (3) veins occur in shear faults and demonstrate a band ing which is asymmetric with respect to the vein walls. Apart from pin ch-and-swell structures and folds, the foliation shows sigmoidal defle ction similar to that in shear bands in ductile mylonites. It is propo sed that the cryptocrystalline, fragment loaded veins are formed by so lidification of a silica sol or hydrogel that was formed by quenching of a hot, fragment-bearing solution during brittle failure. The effect of a viscous fluid in fault rock is discussed in terms of rheology an d of seismic periodicity.