Rock Analogue Research Articles

Experimental study of caldera formation

Scaled experiments have been carried out on caldera collapse mechanisms, using silicone as analogue magma and dry sand as analogue rock. Experiments were carried out in two and three dimensions using a range of roof aspect ratios (thickness/width 0.2 to 4.5) appropriate for caldera collapse. They reveal a general mechanism of collapse, only weakly dependent on the shape of the reservoir. For low roof aspect ratios (≤1), subsidence starts by flexure of the roof and the formation of outward dipping, reverse ring faults, which in turn trigger formation of peripheral inward dipping, normal ring faults. The subsidence always occurs asymmetrically. In cross section the reverse faults delimit a coherent piston, bounded on each side by an annular zone of inwardly tilted strata located between the reverse and normal ring fault sets. The surface depression consists of a nondeformed area (piston) surrounded by an annular extensional zone (tilted strata). For high aspect ratios (>1), multiple reverse faults break up the roof into large pieces, and subsidence occurred as a series of nested wedges (2‐D) or cones (3‐D). The extensional zone dominates the surface depression. In the case where preexisting regional faults do not play a major role, the collapse mechanics of calderas probably depends strongly on the roof aspect ratio. Calderas with low roof aspect ratios are predicted to collapse as coherent pistons along reverse faults. The annular extensional zone might be the source of the large landslides that generate intracaldera megabreccias. Collapse into magma reservoirs with high roof aspect ratios may be the origin of some funnel calderas where explosive reaming is not dominant.

Asperity interactions during creep of simulated faults at hydrothermal conditions

Grain-scale deformation processes during sliding on simulated faults, and in the presence of reactive pore fluids, are examined with in situ, see-through experiments using polycrystalline biphenyl aggregates at 60 °C as a rock analogue. Reactive pore fluids were water-alcohol-vapor mixtures and bulk shear strain rates were 5.8 × 10 −6 s −1 . Under these conditions, slip at displacement rates around 0.01 µm/s on shear fractures involves a mixture of cataclasis and frictional sliding, together with dissolution and precipitation processes. Simultaneously, dislocation flow processes accommodate distributed shearing away from slip surfaces. The evolution of fault porosity is controlled by a dynamic competition between crack growth and opening and crystal growth into cracks and pore spaces. During progressive sliding, growing crystals and other asperities on opposing fracture walls interact by fracturing, rotation, grain translation, and dissolution and precipitation. This multimechanism behavior in rock analogue materials provides insights about the mechanical behavior of faults in the presence of hydrothermal fluids at seismogenic depths. Models of the strength and slip behavior of natural faults may thus need to consider multimechanism behavior during frictional sliding.

Intergranular Pressure Solution in Nacl: Grain-To-Grain Contact Experiments under the Optical Microscope

Intergranular Pressure Solution (IPS) is an important geologic lithification, compaction and deformation mechanism in a wide variety of crustal rocks. Experimental studies of IPS in quartz aggregates have not been very successful due to the low rate of IPS, and IPS experiments performed using wet halite as a rock analogue (Spiers and Schutjens, 1990; Hickman and Evans, 1991) have left uncertainty about the detailed IPS mechanism and grain contact structure/wetting in this material. The present study reports four contact dissolution experiments performed under the optical microscope to study the mechanism and kinetics of IPS at single halite/halite and halite/glass contacts loaded under brine (room temperature). Normal constant contact forces in the range 1. 0 to 2. 6 N were applied in the presence of NaCl-saturated brine, exerting stresses of 0. 8 to 7. 4 MPa. Time-dependent mass removal and convergence were observed at all contacts. In all cases, loading of the contact (or increasing the load on the contact) led to instantaneous formation of a rough contact morphology, composed of a crystallographically-controlled pattern of islands and channels with a length scale of several micrometers. This nonequilibrium microstructure evolved with time to an optically flat contact face while contact broadening and convergence continued. The smoothing/convergence process must therefore have involved diffusion of mass out of the contact, and expulsion of brine, through a connected brine phase within the contact. Whether a fine-scale rough structure persisted in contacts which evolved to optical flatness is not known, though post-test SEM (Scanning Electron Microscopy) observations suggest that it may have. If so, its amplitude was less than 500 nm. Measurements of dissolution rates enabled comparison with a model for IPS. The analysis suggests that solute diffusion through the contact boundary was probably rate-controlling, with the contact structure and effective diffusivity varying with contact force and on-going convergence. The results agree broadly with those of previous compaction creep experiments performed using wet halite powder. Discrepancies with other workers results for single-contact dissolution experiments can be explained in terms of differences in experimental configuration and competition between driving forces.

Effects of sterilizing doses of gamma radiation on Mars analog rocks and minerals

Rock and soil samples from the planet Mars are due to be returned to Earth within a decade. Martian samples initially will be tested for evidence of life and biological hazard under strict biological containment. Wider distribution of samples for organic and inorganic analysis may occur only if neither evidence of life nor hazard is detected, or if the samples are first sterilized. We subjected a range of Mars analog rocks and minerals to high doses of gamma radiation in order to determine the effects of gamma sterilization on the samples' isotopic, chemical, and physical properties. Gamma photons from 60Co (1.17 and 1.33 MeV) in doses as high as 3×107 rads did not induce radioactivity in the samples and produced no measurable changes in their isotopic and chemical compositions. This level of irradiation also produced no measurable changes in the crystallographic structure of any sample, the surface areas of soil analogs, or the fluid inclusion homogenization temperature of quartz. The only detectable effects of irradiation were dose‐dependent changes in the visible and near‐infrared spectral region (e.g., discoloration and darkening of quartz and halite and an increase in albedo of carbonates) and increases in the thermoluminescence of quartz and plagioclase. If samples returned from Mars require biological sterilization, gamma irradiation provides a feasible option.

Pressure solution compaction of sodium chlorate and implications for pressure solution in NaCl

Sodium chloride (NaCl) has been extensively used as a material to develop, test and improve pressure solution (PS) rock deformation models. However, unlike silicate and carbonate rocks, NaCl can deform plastically at very low stresses (∼0.5 MPa). This could mean that NaCl is less suitable for use as an analogue for rocks that do not deform plastically at conditions where PS is important. In order to test the reliability of NaCl as a rock analogue, we carried out a series of uniaxial compaction experiments on sodium chlorate (NaClO 3) at room pressure and temperature (P–T) conditions and applied effective stresses of 2.4 and 5.0 Mpa. NaClO 3 is a very soluble, elastic–brittle salt, that cannot be deformed plastically at room P–T conditions. The results were compared with experiments on NaCl at similar conditions and show that NaClO 3 behaves in a strikingly similar way to NaCl, despite its brittleness. Like NaCl, it most likely compacts by a grain boundary diffusion controlled PS mechanism. Mechanical data were fitted to a power law in the form: ε ̇ ≈ε −ασ nd −m (with volumetric strain rate ε ̇ , volumetric strain ε, effective stress σ and grain size d). A reasonable fit was obtained, with α=2 to 4, n=1.6±0.5, and m=2.8±0.5. The similarity in mechanical behaviour of the two materials (NaCl plastic, NaClO 3 brittle) suggests that plasticity does not play a key role in PS compaction deformation of NaCl. This means that its plasticity is not a drawback for its use as a PS analogue for rocks or for deriving PS creep laws for salt from compaction experiments.

In-situ annealing experiments of octachloropropane as a rock analogue: kinetics and energetics of grain growth

Two-dimensional in-situ observation of the change of size of individual grains of octachloropropane (OCP) during post-deformation annealing revealed that grain boundary migration occurred to reduce the grain boundary energy. Within the polycrystalline aggregate studied some grains show a cyclic change in grain size, others have a stable grain size, while most grains were consumed and disappeared in order that the mean grain size could increase. During grain coarsening, some grains were dissected, then coalesced, amalgamated with others and their centers migrated across the aggregate. The average grain size (D̄) can be expressed by D ̄ =k 0 exp(−Q/RT)t n , where k 0 is a constant, Q is the activation energy, R is the gas constant, T is the temperature, t is the annealing time and n is a constant. We obtained n = 0.1–0.2 and Q = 6.2 kcal/mol. The driving force ( P) is the surface energy stored at grain boundaries. The driving force can be expressed as P∝ t m , where m is the constant, and hence the grain growth rate ( D ̄ ′ ) can be expressed as D ̄ ′∝P (n−1)/m . The value of ( n−1)/ m ranges from 4 to 10.

Micro-shear zones in experimentally deformed octachloropropane

Localisation of deformation in rocks is known to be important at all scales in ductily deforming rocks. However, relatively little is known of the significance of shear localisation at small scales (<mm–cm), where ongoing deformation and recovery may obliterate any traces of such localisation. We investigated localisation of deformation in a <100 μm thick sample of the rock analogue octachloropropane in a transparent torsional deformation cell with a circular shear zone in which arbitrarily high values of simple shear can be achieved. Photomicrographs of the deforming sample were taken every 1–2 min over eight and five minute intervals, while the sample was deforming at a bulk shear-strain rate of 4.6×10 −4 s −1, after a steady-state microstructure was established. The distribution of deformation was determined using pattern matching on digitised photomicrographs. Localisation of deformation was observed in the form of anastomosing micro-shear zones on the grain and sub-grain scale (10–100 μm). These shears left no clear indicative microstructures. The micro-shear zones shifted through the material, partly along with migrating grain boundaries, causing a homogenisation of accumulated strain. The existence of such micro-shear zones can therefore not be determined easily by microstructural analysis of deformed material, even though in these experiments the micro-shear zones accommodated up to 75% of the total deformation. A second form of localisation, which occurs at the inside and outside of the shear zone and which is often observed in this type of experiment, was also noted, and can be accounted for by the friction between the sample and the confining glass plates.

About this title

The Isle of Man lies close to the surface trace of one of the most important regional Palaeozoic structures — the lapetus Suture. Evidence suggests that this boundary, between the former Avalonian microcontinent to the south and the Laurentian continent to the north, skirts the northwestern edge of the island. Over most of the British Isles the surface trace of the suture is hidden by Upper Palaeozoic rocks. However, on the Isle of Man, where Lower Palaeozoic rocks crop out at the suture, research promises to substantially augment our knowledge of the geology of the lapetus Suture Zone and of the outboard edge of the Avalonian margin. As well as the providing an overview of a key Caledonide area adjacent to the lapetus Suture, the papers in this volume describe new work on stratigraphy, sedimentology, deformation, metamorphism, geochemistry, plutonism, palaeomagnetism and geophysics. There are descriptions and an analysis of a range of deep-water sedimentary processes on an early Ordovician continental margin and a detailed analysis of the processes occurring in the developing collision zone between Avalonia and Laurentia. Review papers cover the analogous rocks of Eastern Ireland, the Scottish Southern Uplands and the English Lake District. In Sight of the Suture is the first overview of the pre-Carboniferous geology of the Isle of Man since the 1960s. It will be of prime interest to research workers in the geology of the Caledonian/Appalachian orogenic belt, to sedimentologists interested in deep marine processes and to petroleum geologists focusing on exploration in the Irish Sea.

Sequence stratigraphy of source and reservoir rocks in the Upper Permian and Jurassic of Jameson Land, East Greenland

Approximately half of the hydrocarbons discovered in the North Atlantic petroleum provinces are found in sandstones of latest Triassic – Jurassic age with the Middle Jurassic Brent Group, and its correlatives, being the economically most important reservoir unit accounting for approximately 25% of the reserves. Hydrocarbons in these reservoirs are generated mainly from the Upper Jurassic Kimmeridge Clay and its correlatives with additional contributions from Middle Jurassic coal, Lower Jurassic marine shales and Devonian lacustrine shales. Equivalents to these deeply buried rocks crop out in the well-exposed sedimentary basins of East Greenland where more detailed studies are possible and these basins are frequently used for analogue studies (Fig. 1). Investigations in East Greenland have documented four major organic-rich shale units which are potential source rocks for hydrocarbons. They include marine shales of the Upper Permian Ravnefjeld Formation (Fig. 2), the Middle Jurassic Sortehat Formation and the Upper Jurassic Hareelv Formation (Fig. 4) and lacustrine shales of the uppermost Triassic – lowermost Jurassic Kap Stewart Group (Fig. 3; Surlyk et al. 1986b; Dam &amp; Christiansen 1990; Christiansen et al. 1992, 1993; Dam et al. 1995; Krabbe 1996). Potential reservoir units include Upper Permian shallow marine platform and build-up carbonates of the Wegener Halvø Formation, lacustrine sandstones of the Rhaetian–Sinemurian Kap Stewart Group and marine sandstones of the Pliensbachian–Aalenian Neill Klinter Group, the Upper Bajocian – Callovian Pelion Formation and Upper Oxfordian – Kimmeridgian Hareelv Formation (Figs 2–4; Christiansen et al. 1992). The Jurassic sandstones of Jameson Land are well known as excellent analogues for hydrocarbon reservoirs in the northern North Sea and offshore mid-Norway. The best documented examples are the turbidite sands of the Hareelv Formation as an analogue for the Magnus oil field and the many Paleogene oil and gas fields, the shallow marine Pelion Formation as an analogue for the Brent Group in the Viking Graben and correlative Garn Group of the Norwegian Shelf, the Neill Klinter Group as an analogue for the Tilje, Ror, Ile and Not Formations and the Kap Stewart Group for the Åre Formation (Surlyk 1987, 1991; Dam &amp; Surlyk 1995; Dam et al. 1995; Surlyk &amp; Noe-Nygaard 1995; Engkilde &amp; Surlyk in press). The presence of pre-Late Jurassic source rocks in Jameson Land suggests the presence of correlative source rocks offshore mid-Norway where the Upper Jurassic source rocks are not sufficiently deeply buried to generate hydrocarbons. The Upper Permian Ravnefjeld Formation in particular provides a useful source rock analogue both there and in more distant areas such as the Barents Sea. The present paper is a summary of a research project supported by the Danish Ministry of Environment and Energy (Piasecki et al. 1994). The aim of the project is to improve our understanding of the distribution of source and reservoir rocks by the application of sequence stratigraphy to the basin analysis. We have focused on the Upper Permian and uppermost Triassic– Jurassic successions where the presence of source and reservoir rocks are well documented from previous studies. Field work during the summer of 1993 included biostratigraphic, sedimentological and sequence stratigraphic studies of selected time slices and was supplemented by drilling of 11 shallow cores (Piasecki et al. 1994). The results so far arising from this work are collected in Piasecki et al. (1997), and the present summary highlights the petroleum-related implications.

Cataclastic solution creep of very soluble brittle salt as a rock analogue

Until about the late 1960s, macroscopically ductile deformation of quartz was seen as a microscopically cataclastic process by most geologists (cf. the origin of the name `mylonite'). Undulatory extinction, subgrains, recrystallised grains and even crystallographic preferred orientations were interpreted as due to water-assisted brittle deformation processes. Nowadays, by contrast, the occurrence of these optical microstructures is considered as conclusive and unequivocal evidence for dislocation creep. The abundance of these microstructures in naturally deformed rocks lead to the conclusion that dislocation creep is the most important ductile deformation mechanism within the Earth's crust. We studied whether a water-assisted brittle deformation mechanism could, in principle, be able to produce apparent `crystal plastic' microstructures, and how. To this end we performed a long-term deformation experiment using soluble brittle salt (NaClO 3) as an analogue for quartz. A single crystal of NaClO 3 was uniaxially stressed for 44 days at room temperature in the presence of a saturated NaClO 3 solution under atmospheric pressure. The crystal was microscopically studied during the experiment. It slowly deformed by cataclastic creep. First, irregular free face dissolution structures developed at highly stressed portions of the crystal. These locally developed into very fine channel-like structures, mostly oriented sub-parallel to crystallographic planes. Most of the channels developed into slits and finally into fractures. Fracture walls migrated by solution transfer leading to changes in shape of the crystal fragments. The resulting polygonal deformation microstructure could easily be mistaken for a dynamically recrystallised `crystal plastic' microstructure. Therefore, care should be taken to use such optical microstructures as conclusive evidence for dislocation creep. This is important, because the occurrence of these microstructures is the strongest argument for dislocation creep in crustal rocks.

Experimental evidence of contact loss during stick-slip: possible implications for seismic behaviour

We present results of an experimental study on the stick-slip phenomenon focused on detailed measurements of the relative normal displacement. This was done mainly to determine whether or not the sliding surfaces become separated during the slip phase as suggested in previous works. Experiments were performed on PMMA as it has mechanical properties comparable to those of brittle rocks and has proved to be a good analogue for rocks in rupture mechanics experiments. Results show clearly that normal displacement occurs during and after the slip phase. The characterisation of the sliding surface enables us to compare the maximum relative normal displacement during the slip phase with asperity height. In most cases, the maximum relative normal displacement was found to be higher than the average value of the peak-to-trough relief of the surface, showing that sliding surfaces were at least partially separated. The phenomenon generally consists of a monotonous slip opening phase followed by a monotonous slip closure phase. In some cases, the slip closure phase shows oscillations which have been interpreted as elastic bounces. This last phenomenon was observed irrespective of the experimental conditions with a probability ranging from 0.1 to 0.3 but no clear variations of this probability have been found as a function of normal stress and roughness. These observations may give insights for the explanations of earthquake-related problems such as the heat flow paradox, anomalous P-wave radiation or the inferred low average friction coefficients in subduction zones.

On dynamic recrystallization during solid state flow: Effects of stress and temperature

A hypothesis is advanced that dynamic recrystallization of Earth materials undergoing solid state flow may represent a balance between grain size reduction and grain growth processes occurring directly in the boundary between the dislocation and diffusion creep fields. Accordingly, the recrystallized grain size (D) and flow stress (σ) at steady state will be related by the equation delineating the field boundary, which in general is temperature dependent. Creep experiments on a metallic rock analogue, Magnox, yielded D=101.12exp[29.3/RT]σ−1.23 and demonstrated that D (µm) decreases with increasing σ (MPa) and increasing temperature (T) in a manner which is in agreement with the field boundary hypothesis. If the model applies to rocks, the widely accepted idea that dynamic recrystallization can lead to major rheological weakening in the Earth may not hold. Moreover, empirical D‐σ relations, used in paleo‐piezometry, will need to be modified to account for temperature effects.

Temperature-dependent rheology of bouncing putties used as rock analogs

Experimental investigations have been carried out at the Hans Ramberg tectonic laboratory to determine the temperature-dependent rheology of two bouncing or silicone putties commonly used for tectonic modelling, namely Rhodorsil Gomme Silbion and Dow Corning dilatant compound no. 3179, using a capillary viscometer. Rheologic data are summarized in plots of stress, τ (Pa) versus shear strain rate, γ ̇ (s −1). Rhodorsil Gomme is generally Newtonian and hence shows constant viscosity, irrespective of strain rate and temperature. Its viscosity decreases by three orders of magnitude from 10 5 Pa s to 10 2 Pa s over the investigated temperature range (23–90)°C. By contrast, Dow-Corning (DC3179) exhibits different flow regimes at different stress/temperature combinations and the flow stress shows strong strain-rate dependence within the same temperature range. Below 60°C, DC3179 deforms above its yield strength with variable stress exponent of n>1 in the strain rate range 10 −2 to 10 −4 s −1. Above γ ̇ ≈(10 −2 s −1) the power law exponent n of this material exhibits Newtonian flow ( n≈1). Previous work has shown this behaviour of DC3179, at room temperature, to be similar to that of a Bingham plastic. Above 60°C, this material deforms as a Newtonian fluid and softens with increasing strain rate. The viscosity of DC3179 spans three orders of magnitude (from 10 6 Pa s to 10 3 Pa s) for the same temperature range. Both materials show large variations of viscosity, suggesting that they are thermally unstable (i.e. they have large activation energies). We applied the steady-state creep equation: γ ̇ =A exp(−Q/RT)τ n to characterize the flow behaviour of the silicone putties; where the universal gas constant R=8.31 J K −1 mole −1, Q is an activation energy for creep, and A and n are constants. The stress exponent n and the activation energy Q for Rhodorsil Gomme remains nearly constant Q≈42±2 kJ/mole; but for DC3179 the activation energy varies with temperature and stress at the investigated temperature range (23–90°C). Whereas, the steady-state creep equation can be used to characterize the flow behaviour of Rhodorsil Gomme, it does not provide a good fit to the experimental data for DC3179. The activation energy for DC3179 varies with temperature and stress. However, for a restricted temperature range 60–90°C, its activation energy ( Q≈82±3 kJ/mole) also remains nearly constant. As was also mentioned by previous investigators, Rhodorsil Gomme is a suitable Newtonian analogue for rock flow in nature. DC3179 seems to be appropriate both as a Newtonian and non-Newtonian rock analogue; depending on temperature, strain rate and power-law exponent. The implications of the test results to salt flow in nature are assessed.

Mesozoic metamorphic and middle to late Tertiary magmatic events on Magdalena and Santa Margarita Islands, Baja California Sur, Mexico: Implications for the tectonic evolution of the Baja California continental borderland

An 40 Ar/ 39 Ar study of metamorphic and volcanic rocks from Magdalena and Santa Margarita Islands, Baja California Sur, Mexico, provides evidence for at least four events that can be related to the tectonic evolution of this portion of the southern Baja California continental borderland prior to, and during, the transition from a convergent to a transform plate boundary system. (1) Metamorphism of amphibolites and quartz-mica schist from upper plate units occurred in Late Jurassic and Early Cretaceous time (∼158–138 Ma). Analogous metamorphic rocks from upper plate rocks in the Cedros–Vizcaino–San Benito terranes of west-central Baja California have not yet been identified. However, the Santa Margarita amphibolites are similar in age to amphibolite blocks from melange of the Puerto Nuevo terrane. (2) Metabasite and metapelitic float blocks associated with serpentinized ultramafic rocks were metamorphosed and subsequently cooled in Early Cretaceous time (129–115 Ma). Their corresponding 40 Ar/ 39 Ar age spectra do not show pronounced age gradients, and they are dissimilar in age, as compared to Late Jurassic (160–170 Ma) amphibolite and late Early Cretaceous (100–115 Ma) blueschist blocks from melange in the Cedros–San Benito terranes. (3) A 30.7 Ma lamprophyre dike that crosscuts normal faults within ophiolitic rocks indicates that synsubduction extension of the upper plate on Santa Margarita Island occurred prior to mid-Oligocene time. (4) Latest Miocene–early Pliocene age (5–6 Ma) adakites were erupted into serpentinite-matrix melange on Santa Margarita Island and indicate derivation by partial melting of a young subducted slab prior to collision of ancestral East Pacific Rise ridge segments. The discovery of adakites in the continental borderland of Baja California Sur provides additional constraints for the Neogene thermal evolution of this convergent margin.

Snow and Ice as Rock Analogs in Conveying Geologic Concepts

Snow and ice share many physical properties with rocks and minerals and can be used to illustrate geologic phenomena that occur deep in the earth or over long periods of time.

An apparatus to experimentally model the dynamics of ductile shear zones

We present a ring-shear apparatus designed to deform soft, ductile rock analogues to arbitraily high shear strains in simple shear, in experiments attempting to model processes that occur in natural ductile shear zones. The sintered specimen is deformed between two unsupported cylinders by two rotating grips, under a normal and shear stress up to 5 MPa and at temperatures up to 600 K. First results of experiments with octachloropropane (OCP, C3Cl8) and camphor (C10H16O) are presented.Experiments with pure OCP samples show that a steady-state strain rate is achieved at a shear strain of around 4, when a steady-state grain shape and crystallographic preferred orientation is established. Mechanical properties compare well with those of cylindrical samples deformed in triaxial compression. Shear localisation is relatively minor. Samples of pure camphor, on the other hand, show very strong shear localisation behaviour, and mechanical properties which do not compare well with those of cylindrical samples deformed in triaxial compression.Experiments with two-phase samples (OCP containing stronger camphor inclusions) show an initial softening of the composite, followed by hardening. This is associated with the development of a foliation, defined by camphor inclusions that stretched to lenses and layers, and the subsequent folding of this foliation at shear strains above 50. A steady-state strain rate is not reached, even at strains of 100.The experiments produce results closely resembling the kinematics observed in natural shear zones, and give a first look into the complex changes in mechanical properties which occur at very high shear strains. They show how this technique, in conjunction with transparent deformation cells, can be used to study both microstructural and rheological development during long simple shear deformation histories.

Porphyroblastic manganaxinite metapelagites with incipient garnet in prehnite–pumpellyite facies, near Meyers Pass, Torlesse Terrane, New Zealand

Moderately manganiferous siliceous pelagites near Meyers Pass, Torlesse Terrane, South Canterbury, New Zealand, have been metamorphosed in the prehnite–pumpellyite facies. A conodont colour index measurement suggests T max in the range 190–300 °C. Porphyroblastic manganaxinite, manganoan pumpellyite, manganoan chlorite and trace spessartine‐rich garnet and sphalerite have formed in an extremely fine‐grained quartz–albite–berthierine–phengite–titanite groundmass. Porphyroblastic manganaxinite semischists and schists are distinctive rocks in prehnite–pumpellyite to lower‐grade greenschist and blueschist facies of New Zealand and Japan. Mn in the manganoan pumpellyites substitutes for Ca in W sites. Total Fe/(Fe+Mg) ratios in chlorite are dependent on oxidation state, being ≤0.22 in red hematitic hemipelagites, and ≥0.61 in low‐f O2 grey metapelagites. In the low‐f O2 metapelagites, manganoan berthierine with little or no chlorite is inferred in the groundmass and iron‐rich chlorite occurs as porphyroblasts and veinlets, whereas in the red rocks, Mg‐rich chlorite occurs both in groundmasses and veinlets. Variably high Si in the manganoan chlorites correlates with evidence for contaminant phases. The Mn content of chlorite contributing to garnet growth is dependent on metamorphic grade; incipient spessartine indicates a saturation value of 6–8% MnO in chlorite in low‐f O2 rocks at Meyers Pass. Lower MnO contents are recorded for otherwise analogous rocks with increasing metamorphic grade, but at a given grade coexisting chlorite and garnet are richer in Mn where f O2 is high. Manganaxinite and manganoan pumpellyite also contributed to reactions forming grossular–spessartine solid solutions. Formation of garnet in siliceous pelagites is dependent on both Mn and Ca content. The spessartine component increases with grade into the greenschist facies. Partial recrystallization of berthierine to chlorite and the growth of porphyroblastic patches of other minerals was facilitated by brittle fracture and access of fluids to an otherwise impermeable matrix; to this extent the very low‐grade metamorphism was episodic.

Folds with axes parallel to the extension direction: an experimental study

Experimental studies of single- and multilayer folding have generally considered shortening of layers oriented perpendicular to the maximum extension direction X (i.e. layers parallel to YZ), or in a more limited number of cases, oblique layers still containing the intermediate Y axis. Few experimental studies have considered the case where the extension direction X lies within the layer itself, although in nature folds with axes parallel to X are quite commonly seen. These folds have often been ascribed to passive rotation of fold axes during continued shear, but it has been shown both theoretically and experimentally that active buckle folds can also develop with axes parallel to X. Single- and multilayer analogue model experiments were performed on planar layers oriented initially perpendicular to the intermediate Y axis, and with the extension direction X lying within the layer itself. All experiments were conducted in plane strain—either in pure shear or simple shear. Paraffin waxes of different melting ranges were used as analogues for rocks with a power-law rheology (stress exponent around 2–3). With a viscosity ratio of ca. 30:1, no measurable fold amplification was discernible for shortening of 36% or shear strain of 3.6. Neither domed initial perturbations with circular sections parallel to the layer nor cylindrical perturbations elongate parallel to the initial stretching direction were significantly amplified. Only at much higher viscosity ratios (ca. 600:1) did active buckle folding develop. This folding at high viscosity ratio was associated with flow of the matrix in the X direction around the layer, developing a strong linear fabric parallel to X in the matrix immediately adjacent to the layer. The development of this flow discontinuity between matrix and layer may be characteristic of active buckling of layers parallel to XZ, with fold axes parallel to X.

Modelling of mantled porphyroclasts using non-Newtonian rock analogue materials

The development of δ-type porphyroclasts was modelled experimentally in a circular transparent shear rig using crystalline rock analogue materials. Rectangular camphor objects embedded in a matrix of octachloropropane were deformed in simple shear flow. The camphor deformed into δ-objects with progressive deformation up to a shear strain of 100. The development of these δ-objects and of gradients of flow parameters in the matrix was followed from step to step during the deformation. The δ-objects in this experiment show stair-stepping of wings that did not occur in earlier experiments with Newtonian fluids. Analysis of the flow pattern indicates that this is due to an unusual flow perturbation geometry around the relatively rigid camphor objects which rotate more slowly than equivalent objects in Newtonian flow. The non-Newtonian rheology of the experimental materials may be responsible for this deviant behaviour. This implies that non-Newtonian fluids are better analogues than Newtonian fluids to study the development of structures in rocks with non-Newtonian rheology, since the geometry of the resulting structures can be different.

Remagnetisation of a rock analogue during experimental triaxial deformation

Synthetic specimens containing pseudo-single-domain magnetite were given a univectorial, anhysteretic remanent magnetisation. They were deformed in a ductile manner at room temperature under a confining pressure of 150 MPa (1.5 kbar). The strain rate was 10 −5 s −1 and the maximum strain achieved was 23% shortening. A weak (0.035 mT) magnetic field was present in the pressure vessel during deformation. Hydrostatic compaction alone produced pressure demagnetisation of low-coercivity components of remanence but did not change remanence directions. Macroscopic differential stress was needed to deform the specimens, and this also permitted syndeformational remagnetisation for values of more than 20 MPa (200 bars). This added new components of piezo-remanent magnetisation (PRM) approximately parallel to the direction of the magnetic field in the pressure vessel. They were acquired in all coercivity intervals but the largest components were added below 35 mT. The final remanence orientation and its spatial distribution of vector components were dictated by the relative orientations of the remagnetising field and the initial remanence. PRM contributions almost completely masked the changes related to strain-induced grain rotation at these low strains. The directions of the externally imposed stress-strain axes may not significantly influence the direction of new remanence components. PRM acquisition occurs when differential stress exceeds a threshold value that probably depends on the previous stress-history of the magnetite grains. In part, this may explain the origin of syntectonic remagnetisation in some naturally deformed rocks where the remanence is not reset by recrystallisation, or thermal or chemical processes.