Skip to Main Content

Abstract

Examples of weak fault zones in and bordering Precambrian Sweden are reviewed and then analysed in terms of the factors that rendered them weak. The criterion taken here for weak zones is evidence of post-glacial uplift having reactivated old shear zones that are still active now. Strain analysis of the Singö shear zone demonstrates that it was already weak while it was deep and ductile between 1.86 and 1.6 Ga. Thus the orientations of străin ellipsoids indicate pure rather than simple shear across shear strands in which the dissolution of quartz and feldspar indicate high synshear fluid pressures. The characteristics of weak strands of the Baltic–Bothnian shear zones and the Senja–Bothnia shear zones are long histories of superimposed ductile, semi-ductile and brittle structures, indicating repeated reactivations with different kinematics. The shear zones that weakened are those that reactivated in several episodes during their history. Repeated reactivation renders major shear zones in the ‘brittle’ upper crust ‘ductile’ by rounding major angularities along subvertical and subhorizontal zones. This generates continuous seams of cataclasites in which pore fluids can pressurize so that internal strains can remain ductile and almost aseismic by frictional sliding or flow. This review ends by discussing how multiple reactivations weaken major faults and how reactivation remains focused on particular zones.

You do not currently have access to this chapter.

Figures & Tables

Contents

GeoRef

References

Related

Citing Books via

Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal