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

Abstract

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. This article is part of a discussion meeting issue ‘Understanding earthquakes using the geological record’.

Understanding earthquakes using the geological record: an introduction

“…However, the effect of the lithosphere thickness on the temperature structure is insufficient on its own to explain the depth of the earthquakes, and implies (by comparison to the oceanic lithosphere) that the continental lower crust in these regions is anhydrous [9]. This inference is consistent with observations from the geological record, where evidence of earthquake slip in the mid to lower crust recorded by the presence of pseudotachylytes is commonly associated with anhydrous metamorphic rocks [8,10], which have had melt (and therefore volatiles) extracted from them. The resulting lateral variations in lithosphere hydration state, and therefore strength, can be seen to have had an important influence on the geological history of the continents [9].…”

“…For example, the recognition of transient and mostly aseismic slip in some parts of subduction zones being coupled to field-based analysis of mixed-lithology fault zones has resulted in a new understanding of the mechanisms for generating transient fault slip at velocities intermediate between earthquakes and plate motions [1][2][3][4][5]. Similarly, observations of earthquake faulting in the lower continental crust made both petrologically (preserved as pseudotachylytes) and seismologically has resulted in a new appreciation of the diversity of rheology in the continental crust, and the importance of trace amounts of water in controlling the deformation [6][7][8][9][10]. In February 2020, the Royal Society hosted a Hooke discussion meeting to further explore links between geological and geophysical methods of studying faulting, titled 'Understanding earthquakes using the geological record'.…”

“…Dunkel et al [8], Jackson et al [9] and Menegon et al [10] investigate the links between hydration state, temperature, deformation mechanism and rheology in the continental crust. At the present day, most regions where continental earthquakes occur in the mid to lower crust (e.g.…”

“…In some situations, fluid influx associated with earthquake ruptures results in metamorphic reactions to form hydrous assemblages that then deform by ductile creep [9]. In other examples, no fluid input is recorded by the anhydrous rocks but the pseudotachylytes undergo ductile deformation, thought to be by efficient diffusion creep because of their extremely small grain size [10]. Later generations of pseudotachylyte cutting the ductile fabrics implies transient (and maybe cyclical) deformation.…”

“…There are significant differences between the deviatoric stresses implied by the presence and displacement patterns of pseudotachylyte-bearing ruptures in anhydrous lower crust compared to the (much lower) stress drops in modern-day earthquakes [19,20] and estimates for the integrated strength of the seismogenic crust [15,21,22]. Possible explanations include spatial variability in stress drops along faults, and the effects of stress concentration by ductile creep in the surrounding rocks [10]. Resolving this apparent discrepancy of views, which may be related to the extremely different length-scales of observation, will be crucial for understanding how to scale the field observations of exhumed faults up to an understanding of the evolution of the bulk strength of the lower crust during faulting and metamorphism.…”