Pseudotachylyte Veins Research Articles

Rough faults, distributed weakening, and off‐fault deformation

We report systematic spatial variations in fault rocks along nonplanar strike‐slip faults cross‐cutting the Lake Edison Granodiorite, Sierra Nevada, California (Sierran wavy fault) and Lobbia outcrops of the Adamello Batholith in the Italian Alps (Lobbia wavy fault). In the case of the Sierran fault, pseudotachylyte formed at contractional fault bends, where it is found as thin (1–2 mm) fault‐parallel veins. Epidote and chlorite developed in the same seismic context as the pseudotachylyte and are especially abundant in extensional fault bends. We argue that the presence of fluids, as illustrated by this example, does not necessarily preclude the development of frictional melt. In the case of the Lobbia fault, pseudotachylyte thickness varies along the length of the fault, but the pseudotachylyte veins thicken and pool in extensional bends. We conduct a quantitative analysis of fault roughness, microcrack distribution, stress, and friction along the Lobbia fault. Numerical modeling results show that opening in extensional bends and localized thermal weakening in contractional bends counteract resistance encountered by fault waviness, resulting in an overall weaker fault than suggested by the corresponding static friction coefficient. The models also predict static stress redistribution around bends in the faults which is consistent with distribution of microcracks, indicating significant elastic and inelastic strain energy is dissipated into the wall rocks due to nonplanar fault geometry. Together these observations suggest that damage and energy dissipation occurs along the entire nonplanar fault during slip, rather than being confined to the region close to the dynamically propagating crack tip.

Cretaceous age, composition, and microstructure of pseudotachylyte in the Otago Schist, New Zealand

At Tucker Hill, in Central Otago, New Zealand, a series of pseudotachylyte veins are hosted in quartzofeldspathic schist. Chilled margins, microlites, flow banding, and the crystallisation of mineral phases absent from the host rock provide unequivocal evidence for melting during pseudotachylyte formation. Whole rock analyses of pseudotachylyte reveal c. 3× enrichment of K2O, Ba, and Rb, and similar depletion of Na2O, CaO, Sr, and Eu, as compared to host schist. Formation age of pseudotachylyte is 95.9±1.8 Ma as measured by total fusion 40Ar/39Ar analyses. Stepwise heating of pseudotachylyte matrix yields an excellently defined 40Ar/39Ar plateau age of 96.0±0.3 Ma. These well-defined ages are attributed to the presence of potassium feldspar, low abundance of inherited lithic material from the host rock, and few fluid inclusions containing extraneous Ar. We propose that formation of these pseudotachylyte veins was related to Cretaceous extensional uplift and exhumation of the Otago Schist.

Oldest record of brittle deformation along the Median Tectonic Line: fission-track age for pseudotachylyte in the Taki area, Mie Prefecture

Fission-track (FT) dating of pseudotachylyte (PST) associated with the Median Tectonic Line (MTL) in the Taki area, Mie Prefecture, SW Japan, provides new constraints on the timing of movement upon this fault. A PST vein with a thickness of about 5 cm yields a zircon FT age of 60.0 ± 3.5 Ma (1σ). In contrast, a weighted average of zircon FT ages obtained for protolith samples (cataclastic mylonitized Hatai Tonalite) collected 10 cm and 15 m from the PST vein boundary is 69.8 ± 1.2 Ma, which is significantly older than the age of the PST. Decomposition of feldspars in the PST suggests that the temperature exceeded 1100°C, which is sufficient to completely erase previous fission tracks in zircon within several seconds. The distribution of fission-track lengths in zircon from the PST vein also supports the interpretation that the zircon FT age was completely reset during frictional fusion of the PST vein. Considering an apatite FT age of 38.0 ± 1.5 Ma for the host rock, the age of the PST indicates that the frictional fusion was occurred during cooling of the Ryoke granite at temperatures between 250°C (closure temperature of the zircon FT system) and 100°C (closure temperature of the apatite FT system). This PST age is comparable with the oldest K-Ar age obtained for fine fractions of MTL fault gouge derived from both the Sanbagawa pelitic schist and the Izumi Group in Shikoku, indicating that the initiation of brittle fault movement associated with formation of the PST and/or fault gouges along the MTL had occurred by 60 Ma.

An integrated magnetic and geological study of cataclasite- dominated pseudotachylytes in the Chiapas Massif, Mexico: a snapshot of stress orientation following slip

SUMMARY The Permian age Chiapas Massif in southeast Mexico is locally host to well-exposed pseudotachylyte vein networks. The veins are black to dark grey and aphanitic in appearance, and consist mostly of microbreccia of angular fragments of plagioclase, K-feldspar, biotite and quartz, in a cryptocrystalline (microscopically irresolvable) matrix. Evidence of melting is present in the form of glass seams, dikelets, glass clasts included in cataclasite and a distinct chemistry in the pseudotachylite veins; pristine glass represents a relatively small volume of the pseudotachylite veins. At an exposure along the Tablon River valley, where the host rock is a medium to coarse-grained equigranular quartz diorite, individual veins are consistently oriented about 280°, are up to 16 mm wide, tens of cm apart, display a consistent left-lateral offset and can be traced for several metres. Individual pseudotachylyte veins rarely cross each other, and they cannot be directly linked to a regional-scale fault. Pseudotachylytes are apparently formed by a combination of crushing, comminution and frictional melting, but they are cataclasite dominated. Textures indicate that cataclasis continued after frictional melting had ceased. A 40Ar/39Ar age determination from whole rock chips of one vein shows a climbing Ar release spectrum with a date of ∼114 Ma as the most reliable age estimate for Ar retention. This result is interpreted in the context of pseudotachylyte formation, recrystallization and resetting of K-bearing minerals for the K–Ar system in the late Early Cretaceous. Ten veins were sampled for palaeomagnetic and magnetic fabric studies, with samples collected from both the veins and their host rock. Remanence data give moderate natural remanent magnetization (NRM) intensities for both the veins (e.g. NRM mean 6.6 × 10−3 A m−1; σ= 5.5) and host rock (mean 7.7 × 10−3 A m−1; σ= 10.8). Many samples of host rock yield an ill-defined east–west directed and shallow magnetization, which we interpret as a Late Permian magnetization based on previous studies of the Chiapas Massif. This magnetization resides in haematite. Veins, as well as immediately adjacent host rock, typically have well-defined, single polarity magnetizations of north–northwest declination and moderate positive inclination and these resemble the Cretaceous expected field direction. The overall mean of the veins is of Dec = 348.7° and Inc = 33.6° (k= 30.5 and α95= 12.3°; n= 6 site means). Rock magnetic parameters suggest that the remanence in the veins principally resides in low-Ti pseudo-single domain magnetite and maghemite grains, but haematite, coarse-grained magnetite, rutile, Fe-Cr-Ni oxides and ilmenite are also present in the veins. Bulk magnetic susceptibility values range between ∼0.3 and 1.1 * 10−3 SI volume units, and host rock values do not differ significantly from vein values. The orientations of the principal susceptibility axes in the host rock and the veins are indistinguishable. In both, magnetic foliations are near vertical and are essentially parallel to the nearly east–west orientation of the veins. Nonetheless, host rock fabrics are predominantly prolate, whereas vein fabrics are oblate. The mean susceptibility tensors of host rocks and pseudotachylyte are characterized by P′/T values of 1.041/–0.327 and 1.033/+0.302, respectively. This result suggests that the fabric reflected by magnetic susceptibility anisotropy in the veins was formed under pure shear stress, during cooling of the veins. The lack of evidence of fabric rotation supports models that associate partial melt with viscous break during fault slip.

Seismic Slip in the Lower Crust Inferred from Granulite-related Pseudotachylyte in the Woodroffe Thrust, Central Australia

Pseudotachylytes generated in granulite facies rocks are found in the Woodroffe thrust, central Australia. Petrologic evidence and structural and textural features show that these pseudotachylytes contain typical melting-origin features including injection occurrence, rounded and embayed clasts, and microlites within the fine-grained matrix, which formed contemporaneously with the wall granulite facies rocks under conditions of ~8 kbar and ~650–700°C. These granulite-related pseudotachylyte (G-Pt) veins are overprinted in subsequent deformation stages by three other types of pseudotachylytes including mylonite-related (M-Pt) and ultramylonite-related (Um-Pt) pseudotachylytes that are then transcut by cataclasite-related pseudotachylytes (C-Pt). The overprinting occurred following uplift of the lower crust where the G-Pt veins formed through the brittle-plastic transition zone where the M-Pt and Um-Pt veins occurred within the brittle-dominated regime of the C-Pt generation zone in the upper crust. The coexistence of multiple generations of large voluminous C-Pt, M-Pt, Um-Pt, and G-Pt veins indicates that numerous large earthquakes accompanying distinct seismic slip which produced the pseudotachylytes occurred over an extended period of seismicity and various depths of the crust within the Woodroffe thrust zone. The spatial distribution and structural features of the multiple-stage pseudotachylytes suggest that coseismic slipping caused by large earthquakes which nucleated in the brittle-dominated seismogenic zone propagated downward through the brittle-plastic transition zone into the plastic-dominated granulite facies depth from the upper to lower crust.

Slow and fast deformation in the Dora Maira Massif, Italian Alps: Pseudotachylytes and inferences on exhumation history

The Dora Maira Massif (DM), Western Alps is a type example of an ultra-high pressure (UHP) metamorphic terrain. The occurrence of pseudotachylytes within the Val Gilba area of DM led to speculation regarding the role of seismic slip in the exhumation of the massif. In this study, new field, microstructural and geochemical data on the pseudotachylyte veins and their mylonitic gneiss host are presented. The formation of pseudotachylyte veins occurred late in the exhumation history at 20.1 ± 0.5 Ma and at shallow depths (<3 km), long after the early stages and rapid exhumation that is characteristic of UHP massifs. Preseismic, coseismic and postseismic deformation features suggest that seismic slip occurred at a period where the geologic strain rate was particularly low. This observation is explained by a stick-slip model along the mylonitic foliation plane in which slip was preferentially along phengite-chlorite rich layers. Frictional sliding instability is promoted by a slow slip rate, by a phenomenon similar to “brake shuttering” in automobiles. An alternative explanation could be provided by climatically driven, accelerated exhumation ca. 20 Ma, as suggested by the 20 Ma-old white micas reported in the Tertiary Piedmont Basin.

The age and depth of exhumed friction melts along the Alpine fault, New Zealand

Laser-ablation 40 Ar/ 39 Ar step-heating analyses of 20 pseudotachylyte veins from a single location along the exhumed central portion of the active Alpine fault of New Zealand yield total gas age values between 1 and 19 Ma. Evidence shows that they are genetically related and were formed during coeval episodes of seismogenic melting at shallow crustal depth, contrasting with a spread in formation ages. The total gas ages show an exponential decrease with increasing proportion of melt matrix and K content, refl ecting incomplete degassing and mixtures of radiogenic Ar sources. Calculation of intercepts for all‐melted matrix and all‐clast end-member components indicate ca. 570 ka (Quaternary) friction-melting ages of ca. 332 Ma (Lower Carboniferous) source rock. Assuming an average exhumation rate of 6‐9 mm/yr for uplift and erosion, these results imply that friction melts were generated during major slip episodes at ~3.5‐5 km crustal depth. We conclude that reliable dating of young pseudotachylyte can be accomplished by combining chronologic study with clast-matrix quantifi cation of genetically related vein assemblages.

Retrograded eclogite-facies pseudotachylytes as deep-crustal paleoseismic faults within continental basement of Lofoten, north Norway

Field observations and electron microprobe analyses indicate that pseudotachylytes discovered on the Lofoten island of Flakstadoy, north Norway, represent rare examples of deep-crustal paleoseismic faults. The pseudotachylyte occurrences are restricted to the margins of eclogite-facies shear zones that sharply cut pristine granulite-facies continental basement rocks. Generally, pseudotachylyte veins are sharply truncated by the eclogite shears, but some have been sheared and folded into them, documenting prekinematic to synkinematic injection. Textures preserved in the pseudotachylyte matrix document crystallization directly from the frictional melt; for example, dendritic garnets, similar in appearance, size, and composition to those from eclogite pseudotachylytes of the Bergen Arcs and Alesund (Austrheim and Boundy, 1994; Lund and Austrheim, 2003), refl ect rapid (likely in terms of tens of seconds) crystallization, and distinct fi ning of grains toward the margins of the pseudotachylyte veins indicates quenching textures. Electron microprobe analysis and backscattered-electron imaging document that the pseudotachylyte matrix is composed of microlites of garnet (Gr 25–30 , Py 15–19 , and Al 54–58 ), orthopyroxene (En 61–64 ), low-Na clinopyroxene (Jd 6 ), amphibole (ferroan pargasite), with or without K-feldspar, quartz, biotite, various Fe opaques and FeTi opaques, kyanite, dolomite, and calcite. The cogenetic eclogite-facies shear zones and pseudotachylytes were variably retrograded during Caledonian amphibolite-facies metamorphism. Omphacite is replaced by clusters or symplectites of low-Na clinopyroxene (Jd 6 ) and oligoclase/andesine (An 20–36 ); kyanite, orthopyroxene, Na-Ca clinopyroxene, amphibole, and dolomite occur as inclusions in garnet. The Flakstadoy pseudotachylytes indicate that the rocks exposed in Lofoten were rigid and resilient parts of the lower crust of an ancient continent from ca. 1.8 Ga until the Middle Ordovician. Subduction to deepercrustal levels (depths >~45 km) caused the stiff, nonreacted granulite to accommodate aseismic, steady-state fl ow in fl eclogite shear zones by concomitant, brittle, seismogenic failure and pseudotachylyte formation. Later in the Middle Ordovician, these deep-crustal rocks were exhumed to middle-crustal levels, where they were retrograded under amphibolite-facies conditions. Our results help to explain how deep-crustal earthquakes form in modern continent-continent collisional zones like the Himalayas.

Propagation of seismic slip from brittle to ductile crust: Evidence from pseudotachylyte of the Woodroffe thrust, central Australia

The Woodroffe thrust, central Australia, is a > 1.5-km-wide mylonitized shear zone marked by large volumes of mm- to cm-scale pseudotachylyte veins. The pseudotachylytes display typical melt-origin features, including rounded and embayed clasts, spherulitic and dentritic microlites, and flow structures within a fine-grained matrix. Three types of pseudotachylyte are identified on the basis of deformation texture, vein morphology, and host-rock lithology: cataclasite-related (C-Pt), mylonite-related (M-Pt), and ultramylonite-related (Um-Pt). The M-Pt and Um-Pt veins intrude into mylonite and ultramylonite and are themselves overprinted by subsequent mylonitization. These pseudotachylytes contain an internal foliation defined by flattened porphyroclasts and layering of the fine-grained vein matrix, and the foliation is generally oriented parallel to foliation in the surrounding mylonite and ultramylonite. These observations constrain the timing and environment of M-Pt and Um-Pt pseudotachylyte formation to a protracted period of deformation and mylonitization within the ductile regime of the crust. The M-Pt and Um-Pt veins, as well as the host mylonite, are overprinted by cataclasis and multiple generations of late-stage C-Pt veins that were generated in the brittle-dominated regime of the upper crust during uplift and exhumation of the shear zone. The coexistence of multiple generations of voluminous C-Pt, M-Pt, and Um-Pt veins indicates that the pseudotachylyte veins represent a large number of large earthquakes and accompanying seismic slip over an extended period of seismicity on the Woodroffe thrust. The timing and distribution of pseudotachylyte indicate that the earthquakes nucleated at the base of the brittle-dominated seismogenic zone and propagated down through the brittle–ductile transition into the ductile-dominated regime of the crust.

Pseudotachylyte-generating faults in Central Otago, New Zealand

In Central Otago, New Zealand, a series of subparallel, NNE–SSW trending, gently dipping (10–30° E) brittle faults are hosted in quartzofeldspathic schist. These faults contain numerous ( n>100) pseudotachylyte veins. Pseudotachylyte fault veins are traceable along strike for distances of up to 200 m, and are between <1 mm and 5 cm thick. Fault separations inferred from offset quartz veins, dilational jogs and a piercing point analysis suggest that pseudotachylyte was generated by slip increments of <1 cm to 20 cm, comparable to those believed to occur during small to moderate-sized earthquakes [Sibson, R.H., 1989. Earthquake faulting as a structural process. Journal of Structural Geology, 11 (1–2) 1–14.]. Dilational jogs, fault drag and quartz vein separations imply a top-to-the-north (normal) sense of shear. Evidence for melting is provided by chilled margins, microlites and the formation of mineral phases not present in the host rock. Melting temperatures of 900–1100 °C are inferred from petrological observations. Heat-work calculations, utilising displacements on pseudotachylyte-bearing faults, and inferred melting temperatures suggest that the faulting occurred at depths of 6–12 km, with average frictional shear resistance exceeding 100 MPa on pseudotachylyte-generating faults. These inferred depths and relatively high levels of frictional shear resistance on pseudotachylyte-generating faults are similar to those inferred by some previous studies of pseudotachylyte (e.g. [Sibson, R.H., 1975. Generation of pseudotachylyte by ancient seismic faulting. The Geophysical Journal of the Royal Astronomical Society, 43 (3) 775–794; Killick, A.M. and Roering, C., 1998. An estimate of the physical conditions of pseudotachylite formation in the West Rand Goldfield, Witwatersrand Basin, South Africa. Tectonophysics, 284 (3–4) 247–259.]).

Occurrence of mylonite zones and pseudotachylyte veins around the base of the upper crust: An example from the southern Hidaka metamorphic belt, Samani area, Hokkaido, Japan

Geological studies on exhumed pseudotachylyte-bearing mylonite zones in S-type tonalite were carried out in the southern Hidaka metamorphic belt, Hokkaido, Japan. Mylonitization is characterized by (1) development of composite planar fabrics, (2) grain size reduction, and (3) change in modal composition an increase in mica content and a decrease in quartz content from protolith to mylonite. Mylonite zones are heterogeneously concentrated in the host rocks. At microscopic scales, shear deformation is concentrated heterogeneously in finegrained layers along C-surfaces. Most of the pseudotachylyte layers are subparallel to the C-surface, and tend to overprint thick mylonite zones. The heterogeneous development of mylonite zones, which may be activated as layers of co-seismic slip, should be incorporated into numerical modeling of seismogenic zones.

Heterogeneous material distribution, an important reason for generation of strain-localized mylonite and frictional slip zones in the Hidaka metamorphic belt, Hokkaido, Japan

Lithological heterogeneity of low P/T metamorphic rocks in southern area of Hidaka metamorphic belt (HMB) was formed through historical development of HMB while these rocks had been laid in ductile lower crust. Many strain-localized mylonite zones (<100 m in thickness) are preferentially developed within S-type tonalite and pelitic gneiss, which are characterized by a large modal amount of phyllosilicates (biotite+muscovite+chlorite) and quartz, compared to other lithofacies in HMB. Mylonitic foliations are more conspicuous with close to the center of the shear zone associated with increase in amounts of phyllosilicate minerals, indicating fluidenhanced weakening mechanisms were operated in plastic shear zones. Pseudotachylyte veins are observed exclusively in these mylonite zones, which were generated during exhumation stage of HMB. We conclude the seismic slip zones in southern HMB had been initiated in the ductile lower crust by concentration of localized plastic shear zones within the phyllosilicate- and quartz-rich lithofacies, which were heterogeneously formed by old metamorphic and magmatic events. Then these zones were further weakened by fluid-enhanced plastic deformation, and finally seismic slips occurred at the bottom of seismogenic upper crust, during exhumation of HMB.

Pseudotachylytes from Corsica: fossil earthquakes from a subduction complex

AbstractSubduction zones are the most seismically active tectonic environment on Earth. Here we report fault‐rock evidence of palaeoearthquakes from a subduction complex. Pseudotachylyte veins formed by shear heating on faults at seismic strain rates contain dendrites, spherulites and acicular minerals characteristic of blueschist facies conditions. The veins have been found in both mantle peridotite and gabbro. Our discovery from Corsica opens a new avenue of research towards understanding mechanisms related to subduction zone earthquakes and the conditions at which these are released.

Earthquakes in the deep continental crust - insights from studies on exhumed high-pressure rocks

SUMMARY Recent studies on two exhumed anorthositic and gabbroic high-pressure complexes in the Bergen Arcs and Western Gneiss Region of Norway demonstrate that deeply subducted dry rocks in the lowermost continental crust may resist metamorphic re-equilibration for geologically significant periods of time. This persistent metastability exercised a profound control on the mechanical properties of these rocks, as evidenced by abundant pseudo-tachylyte veins, cataclasites and fractures, documenting eclogite facies seismic failure at depths exceeding 60 km in the thickened Caledonian crust. Our observations indicate that the brittle seismogenic state of some lower crustal rocks, and their propensity to remain metamorphically metastable despite prevailing conditions of T = 600‐800 ◦ C and P = 1.5 to > 2.0 GPa, is largely controlled by

Superheated friction-induced melts in zoned pseudotachylytes within the Adamello tonalites (Italian Southern Alps)

Pseudotachylytes and cataclasites are present along a strike-slip fault zone in the tonalites of the Adamello intrusion (Italian Southern Alps). Ambient conditions during faulting were 0.25–0.3 GPa and 250–300 °C. Pseudotachylyte veins thicker than 6 mm are zoned and consist of two symmetric microlitic domains towards the vein walls and a central spherulitic domain. The thickness of the microlitic domains (x) increases linearly with the half total thickness (a) of the pseudotachylyte vein (commonly <2 cm) according to the relation: x=(0.29±0.12)a. The spherulitic and microlitic domains have similar chemical composition but the microlitic domain has a lower amount of plagioclase clasts. A numerical model explains zoning as the result of the different cooling rates and clast/melt interactions at the center and periphery of thick veins. Zoning is compatible with the injection of a single pulse of superheated friction-induced melt (Tmelt≅1450 °C). Melt temperatures estimated by the clast/matrix ratio (O'Hara, 2001) are considerably lower (316–577 °C). It is suggested that the difference in the temperature estimates reflects a more complex slip history than a single seismic slip event along the fault during production of cataclasites/pseudotachylytes.

Multiple generations of pseudotachylyte in the brittle to ductile regimes, Qinling–Dabie Shan ultrahigh‐pressure metamorphic complex, central China

Abstract Pseudotachylytes are present along the Dahezhen shear zone in the Qinling–Dabie Shan collisional orogenic belt, central China. Two types of pseudotachylyte vein are documented in the shear zone: cataclasite‐related pseudotachylyte (C‐Pt) and mylonite‐related pseudotachylyte (M‐Pt). M‐Pt is associated with mylonite‐development and is overprinted by C‐Pt. All of the quartz and most of the feldspar porphyroclasts within the M‐Pt are plastically deformed, but not in the C‐Pt. Dynamically recrystallized fine‐grained quartz and feldspar bands are oriented subparallel to the mylonite and M‐Pt foliation, and partially surround the porphyroclasts. Our results suggest that the M‐Pt formed cyclically in the ductile region at estimated conditions of 400–650°C and 400–800 MPa due to propagation of seismic fracturing associated with the thrusting‐related rapid exhumation of the ultrahigh‐pressure metamorphic complex in the brittle regime down to a greater depth than the base of the seismogenic zone. The M‐Pt and mylonite formed in the Dahezhen shear zone at estimated conditions of 400–650°C and 400–800 MPa. The coexistence of C‐Pt and M‐Pt in the same shear zone suggests that repeated seismic slips occurred in both the brittle and ductile portions of the crust during the thrusting‐related rapid exhumation of the ultrahigh‐pressure metamorphic complex.

High-pressure metamorphism and deep-crustal seismicity: evidence from contemporaneous formation of pseudotachylytes and eclogite facies coronas

The metamorphic complex of the Western Gneiss Region (WGR), Norway, constitutes the root of the Caledonian mountain belt and experienced temperatures of 700–800 °C and pressures in excess of 20 kbar during peak metamorphism. Mafic bodies surrounded by strongly banded felsic gneisses commonly exhibit variable reequilibration to granulite and eclogite facies conditions and locally preserve igneous minerals and textures. The Kråkeneset gabbro, located on the island of Vågsøy in the mixed HP/UHP zone of the western WGR, display evidence for extensive metastability through the entire prograde and retrograde P, T histories. Eclogite constitutes less than a few percent of the total volume of the body and high-pressure assemblages typically form thin coronas around magmatic phases or occur along localized zones of brittle deformation and fluid infiltration. The gabbro displays pseudotachylyte vein networks that define subparallel brittle fault zones, <50 cm wide, transecting the gabbro body. The pseudotachylytes contain μm- to mm-scale amoeboid and dendrite-like textures of garnet and plagioclase with inclusions of the eclogite facies minerals orthopyroxene, omphacite, amphibole, and dolomite, suggesting rapid disequilibrium growth of minerals during high-pressure conditions. Textural and petrological evidence from pseudotachylytes and corona structures show that the growth of these unusual textures occurred shortly after pseudotachylyte crystallization by a process of rapid solid-state alteration of a microcrystalline pseudotachylyte matrix. The pseudotachylyte-lined fault zones are in close spatial association with numerous amphibole±carbonate-filled hydrofractures with conspicuous fracture-parallel alteration zones defined by hydrous eclogite facies assemblages. These eclogite facies hydrofractures testify to the existence of high fluid pressures and to fluid infiltration following brittle failure during high-grade metamorphic conditions. Geothermobarometric estimates (ca. T=650–700 °C, P=∼20 kbar) and petrological data imply that hydrofracturing, pseudotachylyte crystallization, and the subsequent pseudotachylyte alteration process must have occurred during high-pressure metamorphism. Our observations are suggestive of a deep-crustal earthquake scenario where a high-pressurized fluid phase plays a double role by causing both seismic failure through the embrittlement effect and facilitating eclogitization of the metastable anhydrous gabbro. Metamorphic reaction along hydrofractures and fault planes led to the development of eclogite facies foliation fabrics and illustrate the rheological change from brittle to plastic behavior associated with the gabbro to eclogite transition. The formation of weak deep-crustal shear zones following brittle failure represents an arrested initiation of the physical breakup and metamorphic reequilibration of the Kråkeneset gabbro during its residence deep in the former Caledonian collision zone.

Frictional melt pulses during a ∼1.1 Ma earthquake along the Alpine Fault, New Zealand

Our understanding of frictional melting that occurs during large earthquake slip events has been hampered by the extremely fine-grain size of frictionally fused rock, and the lack of detailed high-resolution microscopy studies that describe features at the crystal lattice scale. In such a study, we here report the complex nature of melting and crystallization in a symmetrically layered, pseudotachylyte vein from the Alpine Fault, New Zealand. Two melt pulses are recognized, attributed to successive, but rapid, injections of frictionally generated material. The initial injection, preserved at the vein margins, was proximally derived and contains a high concentration of clasts and a Si-rich glass. This was quickly followed by a second generation of a more distally derived, melt-dominated fraction, which was injected into the weak vein center. Whereas fragments of wall-rock biotite are preserved in the marginal zones, neocrystallized microlitic biotite characterizes both margins and center. The vein biotite is different in composition, microstructure and polytypism from the metamorphic biotite of the wall rock. In all melt layers, newly formed biotite shows notable signs of syn-flow crystallization, strain features and erosion at crystal–glass contacts, with breakdown of neocrystallized microlites along both crystal edges and faces. These characteristics imply cyclic pulses of heating, melting and crystallization occurred during a single, large earthquake episode, and probably reflects the stick-slip propagation properties of coseismic faulting. 40Ar/ 39Ar total gas ages from the vein center give a 1.11±0.04 Ma date for cyclic melting which, based on current exhumation rates, occurred at a crustal of depth ∼11 km.

An evaluation of Rb-Sr dating of pseudotachylyte: Structural-chemical models and the role of fluids.

Rb-Sr geochronometry has been used in several studies in an effort to date pseudotachylyte and therefore the fault hosting the pseudotachylyte and the tectonism that produced the faulting. The usual procedure has been the “thin-slab” technique, which in metamorphic rocks involves construction of an isochron using whole rock analyses of adjacent but compositionally different slices of rock, and is based on the assumption of isotopic equilibrium between the slices. In attempts to date pseudotachylyte, analyses have been obtained using a pseudotachylyte vein and the immediately adjacent host rock. Such studies commonly yield: 1) apparent ages with high errors owing to small a small range of Rb/Sr values or to “scatterchron” data; 2) host rocks not plotting on the same isochron or scatterchron defined by the pseudotachylyte; 3) apparent dates that are inconsistent with other estimates of the time of faulting; or 4) apparent ages that are not independently constrained. These studies commonly fail to record the type of pseudotachylyte vein utilized; injection veins are commonly larger and more obvious targets for collecting but are likely to yield geologically meaningless dates. A Rb-Sr geochronometric study of pseudotachylyte from the North Cascade Mountains, Washington, reveals: 1) an apparent age older than the likely age of the pseudotachylyte; 2) pseudotachylyte and host rock chemical and isotopic characteristics indicating that the pseudotachylyte was not the result of bulk melting of normal schist; and 3) metasomatism prior to pseudotachylyte formation likely affected both the protolith and what is now the host rock. In light of previous studies, the new data, and related information regarding fault-related metasomatism, three closed system models and one open system model for pseudotachylyte formation are proposed. These models make specific predictions regarding apparent whole rock-pseudotachylyte dates, microstructural features, and the relationship between the whole-rock pseudotachylyte date and the whole rock-biotite Rb-Sr dates in the same rock. The models suggests pseudotachylyte-host rock dating is unlikely to produce real ages of pseudotachylyte formation. Worse, because many pseudotachylytes apparently form during cooling and uplift, the technique will instead yield a seemingly reasonable but geologically meaningless date. Certain approaches to the problem, including a microstructural analysis coupled with a whole rock-biotite age on the same rock, may yield an actual age of pseudotachylyte formation, but the assumptions made heretofore about the behavior of the Rb-Sr system at the time of pseudotachylyte formation are questionable.

Paleozoic deformation in the Sierras de Cordoba and Sierra de Las Minas, eastern Sierras Pampeanas, Argentina

Basement orthogneisses, paragneisses, and migmatites in the Sierras de Córdoba of the eastern Sierras Pampeanas of central Argentina represent remnants of a Cambrian arc and accretionary prism that initially formed above a subduction zone along the early Cambrian margin of Gondwana. These basement rocks contain many high strain zones that record major events in the tectonic evolution of the western margin of Gondwana during the latest Proterozoic–middle Paleozoic. Initial orthogonal shortening and chevron folding of the accretionary prism rocks occurred prior to a high temperature/low pressure, relatively static metamorphism and migmatization event during 509–525 Ma, simultaneous with the Pampean orogeny. Localized deformation along a narrow zone of dextral transpression occurred late in the Pampean cycle. After peak metamorphism, the gneisses and migmatites deformed on east-dipping, greenschist-grade, mylonite-, ultramylonite-, and pseudotachylyte-bearing zones. Kinematic data from a selection of these shear zones include field data, microstructural shear sense indicators, and quartz C-axis fabric asymmetry. Almost all show west-directed, dip-slip reverse fault movement, but most do not achieve major crustal shortening. One major ultramylonite zone in western Sierras de Cordoba may represent a major tectonic boundary with the adjacent Sierra de San Luis rocks. This high strain zone is intruded by the Devonian-aged Achala batholith. Other age constraints include a pseudotachylyte vein that has been dated by 40Ar/ 39Ar method as mid-Silurian and Ordovician I-type plutonic rocks in the Sierra de Las Minas to the west that are deformed into greenschist-grade protomylonite and pseudotachylyte-bearing zones. Our data are consistent with late Ordovician to mid-Devonian orthogonal deformation throughout the Sierras de Córdoba, related to the emplacement of the Precordillera and Chilenia terranes to the west.