Strain localization in pseudotachylyte veins at lower crustal conditions

Viscous shearing in the dry and strong lower crust often localizes in pseudotachylyte veins (i.e. quenched molten rocks formed by the frictional heat released during seismic slip), and it has been suggested that brittle (coseismic) grain-size reduction and fluid infiltration in the fractured domains are necessary to weaken the anhydrous granulitic lower crust. However, the deformation mechanisms responsible for the associated strain weakening and viscous shear localization in pseudotachylytes are yet to be explored. This study investigates the deformation microstructures of mylonitized pseudotachylytes in anorthosites from Nus- fjord, northern Norway, where ductile shear zones invariably nucleate in pseudotachylyte veins. Thus, pseudotachy- lytes are weaker than the host rock during superposed ductile deformation. Pristine pseudotachylytes contain microlites of plagioclase, clinopyroxene, amphibole and orthopyroxene, flow structures, and chilled margins. Some pseudotachylytes have lost the pristine microstructure and have recrystallized into a fine-grained ( < 10 \u3bc m) mixture of plagioclase, amphibole, clinopyroxene, biotite, quartz \ub1 K-feldspar \ub1 orthopyroxene. Thus, the fine grain size in the mylonites ( < 20 \u3bc m) is not the product of progressive grain-size reduction with increasing strain, but is an initial characteristic of the shear zone (pseudotachylyte) precursor. The stable mineral assemblage in the mylonitic foliation consists of plagioclase, hornblende, clinopyroxene \ub1 quartz \ub1 biotite \ub1 orthoclase. Geothermobarometry and thermodynamic modelling indicate that pristine pseudotachylytes and their mylonitized equivalents formed at ca. 700 \u30aC and 0.6-0.9 GPa. Diffusion creep and grain boundary sliding were identified as the main deformation mechanisms in the mylonite on the basis of the lack of crystallographic preferred orientations, the high degree of phase mixing, and the nucleation of hornblende in dilatant sites. In contrast with common observations that fluid infiltration is required to trigger viscous deformation, thermody- namic modelling indicates that a limited amount of fluid (0.4 wt%, similar to the bulk fluid content measured in the host rock) is sufficient to stabilize the mineral assemblage in the mylonite. This suggests that cosesimic grain size reduction resulted in fluid redistribution into the fractured domains and not necessarily in fluid infiltration. Recent experiments suggest that very small amount of water (tens of ppm) are effective in facilitating mineral reactions if sufficient porosity in present. Coseismic fracturing and creep cavitation in the mylonitized pseudotachylytes en- hance the porosity of the shear zone and result in nucleation of new phases in dilatant sites. This process keeps the grain size of the polymineralic aggregate in the grain-size sensitive creep field, thereby stabilizing strain localiza- tion in the mylonitized pseudotachylytes. This study highlights that pseudotachylytes caused by brittle faulting can be precursors of viscous, weak shear zones in the dry lower crust, indicating lower crustal earthquakes as agents of rheological change from strong, brittle lower crust, to strong lower crust with embedded fine grained, weak viscous shear zones

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

Pseudotachylyte as field evidence for lower-crustal earthquakes during the intracontinental Petermann Orogeny (Musgrave Block, Central Australia)

Geophysical evidence for lower continental crustal earthquakes in almost all collisional orogens is in con\ufb02ict with the widely accepted notion that rocks, under high grade conditions, should \ufb02ow 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 Subdomain in Central Australia was deformed under dry sub-eclogitic conditions of 600\u2013700 \u25e6 C and 1.0\u20131.2 GPa during the intracontinental Petermann Orogeny (ca. 550 Ma) and contains abundant pseudotachylyte. These pseudotachylytes are commonly foliated, recrystallized, and cross-cut by other pseudotachylytes, re\ufb02ecting repeated generation during ongoing ductile deformation. This interplay is interpreted as evidence for repeated seismic brittle failure and post- to inter-seismic creep under dry lower-crustal conditions. Thermodynamic modelling of the pseudotachylyte bulk composition gives the same PT conditions of shearing as in surrounding mylonites. We conclude that pseudotachylytes in the Fregon Subdomain are a direct analogue of current seismicity in dry lower continental crust

Extrinsic elastic anisotropy in a compositionally heterogeneous Earth's mantle

Several theoretical studies indicate that a substantial fraction of the measured seismic anisotropy could be interpreted as extrinsic anisotropy associated with compositional layering in rocks, reducing the significance of strain‐induced intrinsic anisotropy. Here, we quantify the potential contribution of grain‐scale and rock‐scale compositional anisotropy to the observations by (i) combining effective medium theories with realistic estimates of mineral isotropic elastic properties, and (ii) measuring velocities of synthetic seismic waves propagating through modelled strain‐induced microstructures. It is shown that for typical mantle and oceanic crust sub‐solidus compositions, rock‐scale compositional layering does not generate any substantial extrinsic anisotropy (<1%) because of the limited contrast in isotropic elastic moduli among different rocks. Quasi‐laminated structures observed in subducting slabs using P‐ and S‐ wave scattering are often invoked as a source of extrinsic anisotropy, but our calculations show that they only generate minor seismic anisotropy (<0.1‐0.2% of Vp and Vs radial anisotropy). More generally, rock‐scale compositional layering, when present, cannot be detected with seismic anisotropy studies, but mainly with wave scattering. In contrast, when grain‐scale layering is present, significant extrinsic anisotropy could exist in vertically limited levels of the mantle such as in a MORB‐rich lower transition zone or in the uppermost lower mantle where foliated basalts and pyrolites display up to 2‐3% Vp and 3‐6% Vs radial anisotropy. Thus, seismic anisotropy observed around the 660 km discontinuity could be possibly related to grain‐scale SPO. Extrinsic anisotropy can form also in a compositionally homogeneous mantle, where velocity variations associated with major phase transitions can generate up to 1% of positive radial anisotropy

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

Episyenites within the Tauern Window metagranitoids: unpredictable?

The core of the Tauern tectonic window (Eastern Alps) consists of dominant pre-Alpine granitoids (∼ 295 Ma) that were metamorphosed and deformed during the Alpine orogenesis (at ∼ 30 Ma). Ductile deformation at peak conditions (550-600 ̊C and 0.5-0.7 GPa) was followed by cataclastic faulting (Pennacchioni and Mancktelow, 2007). Both deformation phases occurred in a fluid-rich environment with formation of veins filled with quartz-calcite-biotite-feldspar and quartz-chlorite-epidote-adularia-calcite, respectively. Faults are typically low displacement strike-slip structures (offset < 1m) organized in en-echelon arrays at different scales with a stepping geometry consistent with the sense of fault slip (e.g. left-stepping for dextral slip). Fault stepovers include pervasive fracturing dominated by a set of antithetic faults (Pennacchioni and Mancktelow, 2013). These faults were locally exploited by episyenitic alteration which represented the "last" event of fluid-rock interaction in the Tauern meta-granitoids. Episyenites within metagranodiorites have a macroscopic porosity in the range between 25 and 35% volume (determined by microtomography), mostly derived from dissolution of multi-mm-sized quartz. Recent glacier-polished outcrops provide a unique opportunity to investigate the relationships between episyenites and overprinted faults. Detailed field mapping of a selected outcrop indicates that episyenites: (i) are spatially linked to precursor faults and statically overprinted all previous structures; (ii) occur discontinuously along faults; (iii) have a thickness (of as much as a few meters) that does not correlate with either the amount of fault slip or the density of the fracture network; (iv) developed independently of rock type (passing "undisturbed" lithologic boundaries with conspicuous variations of quartz grain size of the protolith lithology). Although the faults in the studied outcrop are extensively decorated by relatively large volumes of episyenite, occurrences of episyenite in the Tauern granitoids are generally rare. This study indicates that there is not a simple way to predict the location and the extent of episyenite alteration from the geometry and fracturing patterns of the network of precursor cataclastic faults. The dominant quartz dissolution during episyenitization was accompanied and/or followed by: (i) pervasive substitution of oligoclase and chlorite/biotite of the metagranodiorite by albite and clay-minerals, respectively, and (ii) limited precipitation of new adularia, anatase, calcite, hematite and zeolite within pores. Isotopic data from calcite filling the episyenite porosity suggest a meteoric source of the fluids (δ18 O (SMOW) ≈ -2 per mil). In contrast, fluids synkinematic with previous episodes of fluid-rock interaction during faulting and ductile shearing had a deeper origin (δ18O (SMOW) ≈ 8-9 per mil). References Pennacchioni, G., Mancktelow, N.S., 2007. J. Struct. Geol. 29, 1757-1780. Pennacchioni, G., Mancktelow, N.S., 2013. Geol. Soc. Am. Bull. 125, 1468-1483

Structural Evolution of a Crustal-Scale Seismogenic Fault in a Magmatic Arc: The Bolfin Fault Zone (Atacama Fault System)

How major crustal-scale seismogenic faults nucleate and evolve in crystalline basements represents a long-standing, but poorly understood, issue in structural geology and fault mechanics. Here, we address the spatio-temporal evolution of the Bolfin Fault Zone (BFZ), a >40-km-long exhumed seismogenic splay fault of the 1000-km-long strike-slip Atacama Fault System. The BFZ has a sinuous fault trace across the Mesozoic magmatic arc of the Coastal Cordillera (Northern Chile) and formed during the oblique subduction of the Aluk plate beneath the South American plate. Seismic faulting occurred at 5–7 km depth and ≤ 300°C in a fluid-rich environment as recorded by extensive propylitic alteration and epidote-chlorite veining. Ancient (125–118 Ma) seismicity is attested by the widespread occurrence of pseudotachylytes. Field geologic surveys indicate nucleation of the BFZ on precursory geometrical anisotropies represented by magmatic foliation of plutons (northern and central segments) and andesitic dyke swarms (southern segment) within the heterogeneous crystalline basement. Seismic faulting exploited the segments of precursory anisotropies that were optimal to favorably oriented with respect to the long-term far-stress field associated with the oblique ancient subduction. The large-scale sinuous geometry of the BFZ resulted from the hard linkage of these anisotropy-pinned segments during fault growth

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