The Nusfjord exhumed earthquake source (Lofoten, Norway): deep crustal seismicity driven by bending of the lower plate during continental collision

The origin of earthquakes in the lower crust at depth of 20-40 km, where dominantly ductile deformation is expected, is highly debated. Exhumed networks of lower crustal coeval pseudotachylytes (quenched frictional melt produced during seismic slip) and mylonites (produced during the post- and interseismic viscous creep) provide a snapshot of the earthquake cycle at anomalously deep conditions in the crust. Such natural laboratories offer the opportunity to investigate the origin and the tectonic setting of lower crustal earthquakes.The Nusfjord East shear zone network (Lofoten, northern Norway) represents an exhumed lower crustal earthquake source, where mutually overprinting mylonites and pseudotachylytes record the interplay between coseismic slip and viscous creep (Menegon et al., 2017; Campbell and Menegon, 2019). The network is well exposed over an area of 4 km2 and consists of three main intersecting sets of ductile shear zones ranging in width from 1 cm to 1 m, which commonly nucleate on former pseudotachylyte veins. Mutual crosscutting relationships indicate that the three sets were active at the same time. Amphibole-plagioclase geothermobarometry yields consistent P-T estimates in all three sets (700-750 °C, 0.7-0.8 GPa). The shear zones separate relatively undeformed blocks of anorthosite that contain pristine pseudotachylyte fault veins. These pseudotachylytes link adjacent or intersecting shear zones, and are interpreted as fossil seismogenic faults representing earthquake nucleation as a transient consequence of ongoing, localised aseismic creep along the shear zones (Campbell et al., under review).The coeval activity of the three shear zone sets is consistent with a local extensional setting, with a bulk vertical shortening and a horizontal NNW-SSE extension. This extension direction is subparallel to the convergence direction between Baltica and Laurentia during the Caledonian Orogeny, and with the dominant direction of nappe thrusting in the Scandinavian Caledonides. 40Ar‐39Ar dating of localized upper amphibolite facies shear zones in the Nusfjord area with similar orientation to the Nusfjord East network yielded an age range of 433–413 Ma (Fournier et al., 2014; Steltenpohl et al., 2003), which indicates an origin during the collisional (Scandian) stage of the Caledonian Orogeny.We propose that the Nusfjord East brittle-viscous extensional shear zone network represents the rheological response of the lower crust to the bending of the lower plate during continental collision. (Micro)seismicity in the lower crust in collisional orogens is commonly localized in the lower plate and has extensional focal mechanisms. This has been tentatively correlated with slab rollback and bending of the lower plate (Singer et al., 2014). We interpret the Nusfjord East shear zone network as the geological record of this type of lower crustal seismicity

The earthquake cycle in the dry lower continental crust: insights from two deeply exhumed terranes (Musgrave Ranges, Australia and Lofoten, Norway)

This paper discusses the results of field-based geological investigations of exhumed rocks exposed in the Musgrave Ranges (Central Australia) and in Nusfjord (Lofoten, Norway) that preserve evidence for lower continental crustal earthquakes with focal depths of approximately 25–40 km. These studies have established that deformation of the dry lower continental crust is characterized by a cyclic interplay between viscous creep (mylonitization) and brittle, seismic slip associated with the formation of pseudotachylytes (a solidified melt produced during seismic slip along a fault in silicate rocks). Seismic slip triggers rheological weakening and a transition to viscous creep, which may be already active during the immediate post-seismic deformation along faults initially characterized by frictional melting and wall-rock damage. The cyclical interplay between seismic slip and viscous creep implies transient oscillations in stress and strain rate, which are preserved in the shear zone microstructure. In both localities, the spatial distribution of pseudotachylytes is consistent with a local (deep) source for the transient high stresses required to generate earthquakes in the lower crust. This deep source is the result of localized stress amplification in dry and strong materials generated at the contacts with ductile shear zones, producing multiple generations of pseudotachylyte over geological time. This implies that both the short- and the long-term rheological evolution of the dry lower crust typical of continental interiors is controlled by earthquake cycle deformation

Dynamic Evolution of Porosity in Lower-Crustal Faults During the Earthquake Cycle

Earthquake-induced fracturing of the dry and strong lower crust can transiently increase permeability for fluids to flow and trigger metamorphic and rheological transformations. However, little is known about the porosity that facilitates these transformations. We analyzed microstructures that have recorded the mechanisms generating porosity in the lower crust from a pristine pseudotachylyte (solidified earthquake-derived frictional melt) and a mylonitized pseudotachylyte from Lofoten, Norway to understand the evolution of fluid pathways from the coseismic to the post- and interseismic stages of the earthquake cycle. Porosity is dispersed and poorly interconnected within the pseudotachylyte vein (0.14 vol%), with a noticeably increased amount along garnet grain boundaries (0.25–0.41 vol%). This porosity formed due to a net negative volume change at the grain boundary when garnet overgrows the pseudotachylyte matrix. Efficient healing of the damage zone by fluid-assisted growth of feldspar neoblasts resulted in the preservation of only a few but relatively large interconnected pores along coseismic fractures (0.03 vol% porosity). In contrast, porosity in the mylonitized pseudotachylyte is dramatically reduced (0.02 vol% overall), because of the efficient precipitation of phases (amphibole, biotite and feldspars) into transient pores during grain-size sensitive creep. Porosity reduction on the order of >85% may be a contributing factor in shear zone hardening, potentially leading to the development of new pseudotachylytes overprinting the mylonites. Our results show that earthquake-induced rheological weakening of the lower crust is intermittent and occurs when a fluid can infiltrate a transiently permeable shear zone, thereby facilitating diffusive mass transfer and creep

Development of crystallographic preferred orientation and microstructure during plastic deformation of natural coarse-grained quartz veins

The microstructure and crystallographic preferred orientation (CPO) of quartz were quantified in 17 samples of natural monomineralic tabular veins. The veins opened and were deformed, up to shear strain \u3b3 > 15, in a small temperature window (about 25\ub0C) above 500\ub0C, as established by Ti-in-quartz thermometry. The veins filled a set of fractures within the Adamello tonalite (southern Alps, Italy) and localized homogeneous simple shear during postmagmatic cooling. The local (square millimeter scale) and bulk (square centimeter) CPO were investigated by computer-integrated polarization microscopy (CIP) and X-ray texture goniometry. Weakly deformed veins (WDV: \u3b3 < 1) consist of millimeter- to centimeter-sized crystals with a strong CPO showing a c-axis girdle slightly inclined, mostly with the shear sense, to the foliation (XY) plane and a strong maximum close to the lineation (X). Moderately deformed veins (MDV: 2 < \u3b3 < 3) consist of elongated nonrecrystallized ribbon grains and most have a CPO showing a strong Y maximum of c axes some with weak extension into a YZ girdle. Strongly deformed veins (SDV: \u3b3 = 4 to 15) are pervasively to completely recrystallized to fine (34\u201340 \u3bcm grain size) aggregates with a strong CPO similar to that of MDV. The slip systems during plastic deformation were dominantly prism \u3008a\u3009 with subordinate rhomb and basal \u3008a\u3009 slip. Recrystallization occurred rather abruptly for 3 < \u3b3 < 4. In contrast to dislocation creep experiments in quartz (and other minerals), a steady-state recrystallized fabric is achieved at early stages of deformation (\u3b3 48 4) as there is no evidence, with increasing strain, of strengthening of the CPO, of rotation of the fabric skeleton, or of change in grain size. WDV represent weakly deformed relicts of veins with an initial CPO believed to have developed during crystal growth but unsuitably oriented for prism \u3008a\u3009 slip during subsequent shear. MDV and SDV appear to derive from veins different from WDV, where the vein crystals grew with orientation favorable for prism \u3008a\u3009 slip. The relationship between the initial growth CPO and the kinematic framework suggests that veins opened at a temperature close to that at which there is a switch between the activity of prism \u3008c\u3009 and prism \u3008a\u3009 slip, with the temperature of growth causing growth of crystals well oriented for slip. The initial CPO of veins, from which quartz mylonites are commonly derived, plays a critical role in the fabric evolution. The strong growth- and strain-induced CPOs of these sheared veins inhibited significant reworking during lower temperature stages of pluton cooling when basal \u3008a\u3009 slip would have been dominant