Dynamic development of fault rocks in a crustal‐scale detachment: An example from western Norway

Abstract

The Nordfjord‐Sogn Detachment (NSD) zone of western Norway juxtaposes eclogites and gneisses of lower crustal affinity with sedimentary deposits, suggesting that the zone experienced substantial displacement during Devonian extensional denudation of the Caledonides. Renewed activation occurred in Permian and Jurassic‐Cretaceous times. The detachment zone consists of a succession of fault‐related rocks reflecting various crustal positions and deformation mechanisms during activation and unroofing. In more detail, the lower 1–2 km thick section of the NSD zone consists of mylonites with various protoliths, showing a general upward decrease in metamorphic grade. The mylonitic section is truncated by the NSD fault, defining the boundary toward hanging wall nappe rocks that are topped by sedimentary basins. In an attempt to provide precise descriptions of the observed fault rocks, we classify these rocks into (1) noncohesive breccias, (2) cemented and indurated breccias of secondary cohesion, and (3) cataclasites, phyllonites, and mylonites formed with primary cohesion. Along and across‐strike sections of the fault reveal the spatial and temporal relationships of the NSD fault. The fault zone consists of a lower section of phyllonites and cataclasites, reaching a total thickness of 50–70 m. Significant volumes of pseudotachylyte were injected into these fault rocks, and were later reworked. Reactivation of the fault is suggested by overprinting carbonate‐cemented breccias that are cut by a discrete fault itself associated with breccias and gouge. By comparing the various fault rocks and their formation mechanisms, some constraints can be applied to the unroofing history. The NSD fault developed in the ductile‐brittle transition zone of the crust, where it experienced an initial shift from mainly frictional to plastic flow caused by reaction softening, before hardening caused a return to frictional flow. At the same time, repeated pulses of pseudotachylyte were generated, possibly lubricating the fault zone and contributing to strain localization. These pseudotachylyte pulses also show that the fault was affected by seismic slip events, before returning to a creep regime within a wider section than that ruptured by earthquakes. Younger cemented breccias and noncohesive breccias and gouge reflect earthquake‐rupturing and frictional flow within the seismogenic crust.