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This contribution presents new 40Ar/39Ar laser step-heating data for muscovite associated with four significant orogenic gold-mineralized zones in central and western Newfoundland to build upon existing geochronological constraints for orogenic gold mineralization across much of the Newfoundland Appalachians. Additionally, we provide the first 40Ar/39Ar laser step-heating data for detrital muscovite from gold-mineralized sandstone of the Wigwam Formation (Botwood Group) at the Moosehead gold prospect. Most known gold zones on the isIand occur in proximity to reactivated crustal-scale faults and related structures, and are commonly localized within competent, rigid lithological units (e.g., granitoid rocks); although, some vein systems cut less competent, clastic sedimentary rock sequences. Host rocks range in age from Neoproterozoic to Devonian and may extend into the Carboniferous. Robust Pragian to Emsian (ca. 410–375 Ma) rutile, muscovite and zircon ages constrain the timing of gold mineralization in parts of the Exploits Subzone, whereas less precise age estimates for gold mineralization in the Notre Dame Subzone and Laurentian margin range from Wenlock to Emsian (ca. 433–375 Ma). Collectively, the geochronological data suggest that fluid-pressure cycling associated with gold mineralization in central and western Newfoundland occurred during progressive Siluro-Devonian tectonics associated with the waning stages of the Salinic orogenic cycle and spanning the Acadian and Neoacadian orogenic cycles. Multiple, polyphase, overlapping orogenic events allowed for repeated, gold mineralizing fluid flow events, particularly in proximity to long-lived, reactivated crustal-scale fault zones., La présente contribution fait état de nouvelles données obtenues par chauffage échelonné au laser 40Ar/39Ar de muscovite associée à quatre zones minéralisées aurifères orogéniques stratégiques dans le centre et l’ouest de Terre-Neuve permettant de mieux préciser les limites géochronologiques existantes de la minéralisation aurifère orogénique dans la majeure partie des Appalaches terre-neuviennes. Nous fournissons de plus les premières données par chauffage échelonné au laser 40Ar/39Ar de muscovite détritique provenant de grès minéralisé aurifère de la Formation de Wigwam (groupe de Botwood) dans la zone d’intérêt aurifère de Moosehead. La majorité des zones aurifères connues sur l’île se trouve à proximité de failles d’échelle crustale et de structures connexes, et elles se situent communément à l’intérieur d’unités lithologiques rigides parallèles (p. ex. des roches granitiques), bien que certains réseaux filoniens recoupent des séquences de roches sédimentaires clastiques moins parallèles. L’âge des roches hôtes varie du Néoprotérozoïque au Dévonien et peut s’étendre au Carbonifère. Des datations robustes du Praguien à l’Emsien (env. 410 à 375 Ma) de rutile, de muscovite et de zircon limitent le moment de la minéralisation de l’or dans des parties de la sous-zone Exploits, alors que des estimations moins précises de l’âge de la minéralisation aurifère dans la sous-zone Notre-Dame et de la marge laurentienne varient du Wenlock à l’Emsien (env. 433 à 375 Ma). Les données géochronologiques laissent collectivement supposer que les cycles de pression hydraulique associés à la minéralisation d’or dans le centre et l’ouest de Terre-Neuve se sont manifestés durant la progression tectonique siluro-dévonienne associée aux stades de ralentissement du cycle orogénique salinique et s’étendant aux cycles orogéniques acadien et néoacadien. Plusieurs phénomènes orogéniques polyphasés se chevauchant ont permis des écoulements de fluides minéralisateurs aurifères répétés, en particulier à proximité de zones de failles d’échelle crustale réactivées persistantes.

Geophysical evidence for lower continental crustal earthquakes in almost all collisional orogens is in conflict with the widely accepted notion that rocks, under high grade conditions, should flow rather than fracture. Pseudotachylytes are remnants of frictional melts generated during seismic slip and can therefore be used as an indicator of former seismogenic fault zones. The Fregon Domain in Central Australia, was deformed under dry subeclogitic conditions during the intracontinental Petermann Orogeny (ca. 550 Ma) and contains abundant pseudotachylyte. These pseudotachylytes are commonly foliated, recrystallized, and crosscut by other pseudotachylytes, reflecting repeated generation during ongoing ductile deformation under generally dry conditions. This interplay is interpreted as a cycle of seismic brittle failure and post- to inter seismic creep under dry lower crustal conditions. Thermodynamic modelling of the pseudotachylyte bulk composition gives conditions of shearing of 600–700 °C and 1.0–1.2 GPa, the same as in surrounding mylonites. We conclude that pseudotachylytes in the Fregon Domain are a direct analogue of current seismicity in dry lower continental crust.

The Musgrave Block in central Australia exposes numerous large-scale mylonitic shear zones developed during the intracontinental Petermann Orogeny around 560-520 Ma. The most prominent structure is the crustal-scale, over 600 km long, E-W-trending Woodroffe Thrust, which is broadly undulate but generally dips shallowly to moderately to the south and shows an approximate top-to-N sense of movement. The estimated metamorphic conditions of mylonitization indicate a regional variation in the direction of thrusting from predominantly mid-crustal (ca. 520-620 °C and 0.8-1.1 GPa) to lower-crustal (ca. 650 °C and 1.0-1.3 GPa) levels, which is also reflected in the distribution of preserved deformation microstructures. This variation in metamorphic conditions is consistent with a south-dipping thrust plane, but is only small, implying that a ≥ 60 km long N-S segment of the Woodroffe Thrust was originally shallow-dipping at an average estimated angle of ≤ 6°. The reconstructed geometry suggests that basement-cored, thick-skinned, mid-crustal thrusts can be very shallowly dipping on a scale of many tens of kilometers in the direction of movement. Such a geometry would require the rocks along the thrust to be weak, but field observations (e.g. large volumes of syn-tectonic pseudotachylyte) argue for a strong behavior, at least transiently. Localization on a low-angle near-planar structure that crosscuts lithological layers requires a weak precursor, such as a seismic rupture in the middle to lower crust. If this was a single event, the intracontinental earthquake must have been large, with the rupture extending laterally over hundreds of kilometers.

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 the discussion meeting issue ‘Understanding earthquakes using the geological record’.

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 the discussion meeting issue ‘Understanding earthquakes using the geological record’.