Shear zone growth by repeated generation of pseudotachylytes in the lower crust

Understanding the deformation modes of the lower crust is crucial if we are to predict the rheological behaviour of the crust in space and time.  The Nusfjord locality (Lofoten, Norway) represents a natural laboratory to study the interplay between seismic and aseismic deformation in the Earth’s lower crust. The area exposes pseudotachylytes (quenched frictional melt produced during coseismic slip) within a network of ductile shear zones bounding strong low-strain domains of granulitic anorthosites. Pseudotachylytes formed within the low-strain domains, during ongoing viscous creep in the ductile shear zones, at a depth of 25-35 km. The ductile shear zones themselves contain several generations of mylonitized pseudotachylytes suggesting repeated switches from frictional to viscous deformation within shear zones. The underlying reasons and rheological consequence of mutual overprinting relationships between ductile shear zones (generally considered to be weak) and several generations of pseudotachylytes remains enigmatic.

Field investigations, photogrammetry, structural logs, and microstructural analysis reveal that (1) pseudotachylytes invariably nucleate within the low strain domains of the anorthosite host rock located between subparallel shear zones, and not along the shear zones themselves; and (2) that the rupture migrates along the material interface provided either by the shear zone/host rock boundary or by the shear zone foliation. The observed relationships suggest transient stress pulses that are supported by variations in the recrystallized grain size of quartz along individual shear zones.

We propose that repeated episodes of pseudotachylyte generation and associated host-rock fracturing  represent a mechanism of shear zone growth and thickening, because the pseudotachylyte veins are mylonitized and become part of the actively deforming shear zones, which in turn control the further development of pseudotachylytes in the adjacent rigid blocks (low-strain domains). Structural logs show that shear zone width depends on the initial spacing between subparallel shear zones: when shear zones are widely spaced (>1 m), the rigid block in between is essentially undeformed, it contains a low density of pseudotachylytes and the shear zones themselves are thin (<10 cm thick). In contrast, closely spaced shear zones are thicker (up to 1 m thick) and are separated by highly damaged rigid blocks that contain a greater density of pseudotachylytes. Thus,  pseudotachylytes overprinting ductile shear zones are not necessarily the result of frictional-viscous switches along individual structures but may rather represent seismic fractures that initiated at stress concentrations within adjacent rigid blocks, which then followed preexisting shear zones. Importantly, repeated production of pseudotachylytes will progressively transform the lower crust from dominantly rheologically stiff to weak. Such rheological weakening will have major consequences on the dynamics of lower-crustal regions.