K. H. Brodie

Experimental intracrystalline plastic flow in hot-pressed synthetic quartzite prepared from Brazilian quartz crystals

Samples of synthetic, ultrafine-grained quartzite were prepared by hot-pressing aggregates of crushed, clear Brazilian quartz of mean grain size 0.4 μm at 300 MPa and 1373 and 1473 K. The samples displayed rapid grain-growth to ca. 12-20 μm with <2% porosity at 1473 K, facilitated by the 0.6 wt% of water adsorbed onto the grain boundaries during sample preparation. This water could be driven off by preheating, thereby preventing grain-growth. Sufficient water was incorporated during growth into the coarsened samples to render them weak and ductile. These were deformed experimentally in the β-quartz field using an argon gas medium apparatus at 300 MPa confining pressure at temperatures mainly between T = 1273 and 1473 K. Ductile flow was described by the flow law: ∈ = 10 - 4 . 9 3 σ 2 . 9 7 f(H 2 O)exp(-242000/RT) with stress, a, in MPa, and grain size, d, in microns and strain rate ∈ in s - 1 . R is the gas constant and f(H 2 O) is water fugacity. Based on observation of grain flattening, optical strain features, shapes and densities of dislocation arrays, and flow law parameters, samples deformed dominantly by dislocation creep, with some contribution from grain-boundary sliding. Bearing in mind possible changes in the flow law parameters over the extrapolation interval and possible effects of the α-β phase transition, extrapolation to natural strain rates and temperatures predicts plastic flow at higher flow stresses at the same water fugacity at greenschist facies temperatures than previously published flow laws.

Structural geometry, lower crustal magmatic underplating and lithospheric stretching in the Ivrea-Verbano zone, northern Italy

Abstract The Ivrea-Verbano zone is believed to represent an up-ended cross-section through the lower continental crust as it existed at the end of the Hercynian orogeny in the Southern Alps. Structurally-oriented geological mapping has been carried out in the central and northern parts of the Ivrea-Verbano zone, a region some 30 km along strike. The geology of the southern half of the zone is dominated by a large basic-ultrabasic complex (the Mafic Formation), which is in contact with a strongly banded metasedimentary + metabasic sequence to the north. Particular attention was paid to: (a) the structural relationship between the rocks of the Mafic Formation and their envelope of high-grade metasedimentary and metabasic rocks; (b) the geometrical configuration throughout the Ivrea-Verbano zone of high-temperature shear zones, which accommodated post-Hercynian crustal extension; and (c) the geometry of late, low-temperature faulting, the effects of which have been removed in order to produce a restoration of the structure as it existed during the post-Hercynian extensional phase. The intrusion of large volumes of basic magma (ca 50% of the outcrop area) to form the rocks of the Mafic Formation appears to be coeval with the onset of extension ( ca 280 Ma). The main basic body has a laccolithic form, which was originally more than 10 km thick. Overfolding developed at the northern margin of the laccolith and is interpreted in terms of the gravitational collapse of the hot, immediately subsolidus or partially molten body, incorporating its envelope of hot metasediments into a large, originally recumbent fold. A geometrical association with a high-temperature, low-angle fault zone suggests that faulting was subsequently localized by the several km of uplift associated with the laccolithic intrusion. The Ivrea-Verbano zone may therefore demonstrate at least one particular geometry of lower crustal magmatic underplating, which may aid in the interpretation of present-day deep seismic profiles. It also demonstrates the geometry of a network of conjugate, high-temperature, low-angle shear zones in a well-layered lower crustal section.

Lithosphere rheology—a note of caution

Abstract It has become common practice to use laboratory-determined, low-temperature frictional sliding data, together with power-law equations for ‘steady-state’ creep of rocks at high temperatures, to construct inferred profiles of rock strength and mode of failure with depth in the lithosphere. In some cases this may involve unwarranted extrapolation of rock mechanics behaviour beyond the region of its validity. The form of transition from frictional behaviour to intracrystalline plastic flow in the Earth is much more complex than is suggested by the above model. Within the regime of plastic flow, substantial changes in flow-strength and mode of failure (whether flow becomes localized into shear zones) may accompany microstructural changes developed over large strains. When deformation is accompanied by metamorphic changes, existing flow laws for rocks are likely to be wholly inapplicable. Until a clearer understanding has emerged of the full range of expression of the rheological behaviour of rocks, existing models of lithosphere rheology should be treated with caution.

Deformation mechanisms and rheology: why marble is weaker than quartzite

When deformed together in nature, calcite rocks invariably appear weaker and more ductile than quartz rocks. This can be reconciled with experimental flow data only by taking into account grain-size-sensitive (GSS) flow in calcite rocks, which is predicted to dominate even at grain sizes on the order of 1mm at middle metamorphic grades. Using new experimental data that demonstrate the transition between intracrystalline plastic and GSS flow of quartz rocks, we predict that unnaturally small grain sizes at temperatures of 700°C or higher are required for GSS flow of quartz in nature. Thus natural flow of quartzite is expected to occur by intracrystalline plastic processes, even after recrystallization to a fine grain size.

High‐pressure‐high‐temperature seismic velocity structure of the midcrustal and lower crustal rocks of the Ivrea‐Verbano zone and Serie dei Laghi, NW Italy

The Ivrea-Verbano zone and the adjacent Serie dei Laghi, in the inner arc of the western Alps (NW Italy), display an upended cross section through much of the thickness of the continental crust as it existed in that region in Triassic/Jurassic time. We collected a suite of oriented rock samples that represent most of the volume of the rocks of the region. P and S wave velocity measurements were made in three orthogonal directions, related to the mesoscopic fabric of the rocks, at room temperature and up to 550 MPa confining pressure. Combined high-temperature and high-pressure measurements were made up to 700°C on a subset of the samples. The lithologic units were divided into 23 different principal rock types, and we present velocity data averaged in terms of these groups. Vertical and horizontal velocity and anisotropy sections are computed from the measured velocity data based on a restored geologic section. These show that Vp and Vs both increase and anisotropy decreases systematically with depth of burial, reflecting variations in rock type and metamorphic grade with depth. Vp and Vs anisotropy arises mainly because of crystallographic preferred orientation or mineralogical banding, with the slowest direction tending to be normal to the foliation or banding. The Vp velocity sections were used to compute synthetic seismic reflection profiles for the region when it lay at the bottom of the continental crust. These correspond well with contemporary deep reflection profiles for regions of comparable tectonic evolution.

Synthetic seismic reflection profile through the Ivrea zone–Serie dei Laghi continental crustal section, northwestern Italy

A geologic cross section, restored to its original horizontal orientation in Permian-Triassic time, has been constructed for the middle and lower continental crustal rocks of the Ivrea-Verbano zone and the adjacent Serie dei Laghi of northwestern Italy. Seismic P-wave velocities of a representative suite of rock samples were measured to high-pressure and high-temperature conditions. A synthetic seismic reflection profile, ∼76 km long and 30 km thick, was computed to compare what can be deduced from the seismic profile with what is known in much more detail from geologic mapping. Imaged features correspond closely to those seen on many present-day profiles, and the broad features of the tectonic evolution would be correctly interpreted, but important recumbent fold structures would be missed, and relationships between intrusive bodies and their country rocks would be unclear.

Role of porosity and dehydration reaction on the deformation of hot-pressed serpentinite aggregates

Experimental deformation of hot-pressed powders of lizardite serpentinite was carried out to study the effect of the dehydration reaction to olivine + talc + water under controlled pore water pressure. Use of a porous aggregate ensured free movement of pore fluid into or out of the specimen in response to volume changes. The dehydration reaction further increases the porosity of the sample, causing weakening, but progressive pore collapse leads to strain hardening. At low strain rates, a transition to linear–viscous flow was inferred to be caused by the formation of transiently fine-grained olivine in the dehydration reaction. The inability of the rock to support high loads during dehydration at low strain rates means that the production of high-pressure water by dehydration and its subsequent expulsion will favour seismogenic failure in the surrounding rocks not directly involved in the dehydration reaction, rather than the serpentinite itself.

Rock mechanics constraints on mid-crustal low-viscosity flow beneath Tibet

Abstract It has been inferred from various types of geophysical data that the Tibetan middle and upper crust is detached from the underlying lower crust and mantle by a weak, mid-crustal zone involving partial melting at about 30–35 km depth. Previous modelling of the flow has used an arbitrary mid-crustal rheology to match the constraints imposed by the overall flow regime. Here we show that extrapolation of experimental rock mechanics data for solid-state flow of a quartz-dominated Tibetan middle and upper crust, plus flow of partially molten synthetic ‘granitoid’, are consistent with the geophysical constraints and provide an experimentally constrained basis for the modelling of crustal rheology involving partially molten rocks.

Rapid stress release caused by polymorphic transformation during the experimental deformation of quartz

Cylindrical samples of polycrystalline quartz were fabricated by hot pressing of crushed Brazilian quartz at 1100 and 1200°C, then deformed in extension at 1200°C at a confining pressure of 300 or 350 MPa. After initial plastic yield, samples failed with sudden and total offloading. Samples could be reloaded in compression and deformed plastically to large strains. Microscopic examination showed extensional failure had occurred with the formation of ‘veins’, oriented normal to the extension direction, containing fibers of silica with their length and c crystal axes parallel to the extension direction. From X-ray and microstructural observations, these are inferred to have formed as mixed tridymite/cristobalite reaction products, now reverted to quartz, accommodating offloading owing to the 38% extensional strain parallel to c. Such rapid transformation and offloading can be compared to processes invoked to explain deep-focus earthquakes in appropriate rock types, but also to the slower formation of oriented, localized reaction products in crustal shear zones.

Seismic Modeling of lower and mid-crustal structure as exemplified by the Massiccio Dei Laghi (Ivrea-Verbano zone and Serie Dei Laghi) crustal section, northwestern Italy

An aim of seismic reflection profiling of the present-day deep continental crust is to infer the geologic structure and hence to infer the tectonic processes that led to the formation of that structure. An important test of the validity of such interpretations lies in comparison with exposed geologic sections that once lay at deep crustal levels. The Massiccio dei Laghi (Lakes Massif) of Northern Italy provides such a section, on a scale comparable with that of contemporary deep seismic profiles, yet illustrates heterogeneity on a wide range of scales. On account of its importance as providing a substantial composite cross section through continental crustal rocks, coupled with its relative accessibility in the inner arc of the Western Alps, the region has been subjected to an extraordinary degree of structural, petrological, geochemical and petrophysical study by geoscientists during the past few decades. The rocks of the region record Paleozoic accretion, metamorphic and magmatic processes, the effects of the Hercynian orogeny,post-orogenic magmatic underplating and associated lithospheric stretching and thinning, Mesozoic extension and effects associated with the position of the region in Alpine tectonism (see review by Handy et al., 1999). The assembly of the rock units probably dates from Permo-Triassic time onwards.Their relative positions in outcrop are likely close to how they that might be taken as a model for a magmatically underplated and extended crustal section. Such a section can be compared with interpretations drawn from present-day seismic reflection profiles taken from regions of extended lithosphere that have not had the misfortune(good fortune?) to be upended and exposed during a subsequent phase of collisional orogeny.