Paul D. Bons

Divergent double subduction: Tectonic and petrologic consequences

Divergent double subduction, involving removal of an oceanic basin from both sides and collision, has distinct fingerprints of magmatism and significant implications for the tectono-magmatic evolution of orogenic belts. The complete evolution of a divergent double subduction system involves four stages: (1) initial interaction between the two divergent subduction zones on both sides of a single oceanic plate; (2) closure of the oceanic basin when the two overriding plates meet, followed by (3) detachment of the oceanic lithosphere from the overlying crust and sedimentary sections; (4) sinking and disappearance of the oceanic slab into the mantle. Detachment of the oceanic slab leads to intense decompressional melting of mantle and triggers melting of accretionary-wedge strata and lower crust, resulting in large-scale magmatic and volcanic activity. The main consequences of a divergent double subduction system are that it produces opposed thrust systems, extensive long-lived granitoid magmatism with mantle isotopic signature, and volcanism that evolves toward bimodalism. This model can be applied to the western half of the Paleozoic Lachlan fold belt (southeastern Australia) to explain the wide-scale Silurian to early Carboniferous granitoid magmatism, its spatial and temporal relationships, and late bimodal magmatism. Closure of the oceanic basin is thought to have occurred during the Early to Middle Devonian. Detachment of the oceanic slab led to felsic to intermediate-composition magmatism. Bimodal volcanism involving local basaltic flows is a reflection of subsequent sinking of the slab.

Development of crystal morphology during unitaxial growth in a progressively widening vein: II. Numerical simulations of the evolution of antitaxial fibrous veins

The development of fibrous morphology and capability of fibres for tracking the opening trajectory were investigated using numerical simulations of a natural antitaxial fibrous vein. Starting from a non-unique best case, variation of fracture opening velocity, grain size, wall roughness, growth anisotropy and crystal growth velocity shows that these parameters differ in importance for crystal morphology and tracking capability. Fibrous veins can be simulated using crack–seal opening of the fracture. Grain boundaries track the opening trajectory if the wall roughness is high, opening increments are small and crystals touch the wall before the next crack increment starts.

Elle: the numerical simulation of metamorphic and deformation microstructures

Abstract We present a generalised framework for the numerical simulation of the evolution of rock microstructures during deformation and metamorphism. This approach is based upon a data structure that describes a polycrystalline material using a two-dimensional network of nodes and connecting boundaries that allows micro-processes to be analysed at a range of scales. The nodes may possess attributes of position, topology and chemistry; and polygonal domains defined by these nodes may possess attributes of mineralogy, rheology and lattice orientation. We represent the complex behaviour of deforming and metamorphosing rocks at the grain scale as the interaction of a set of locally-defined driving forces and micro-processes, calculated for small time steps. A central program controls the evolution of extrinsic variables such as temperature and defines the history of deformation or metamorphic processes. The central program then passes the data structure to distinct process algorithms, which interact with this data structure, both by using it to determine the local values of driving forces, and by altering the attributes to simulate the progress of the process. We outline the function of the different aspects of the modelling task, provide simple examples showing porphyroblast growth during deformation and static grain growth, and describe the data structure that we have developed which enables us to handle multi-process simulations.

New experiment to model self-organized critical transport and accumulation of melt and hydrocarbons from their source rocks

A new, simple, and easily reproducible experiment was designed to simulate the production, accumulation, and transport of melt within rock. The transport was found to be of the self-organized critical type. The emergence of self-organized criticality is explained by the availability of hydrofracture propagation as a rapid or ballistic transport mechanism. This mechanism also serves as a mechanism for stepwise accumulation. These findings are confirmed by a numerical model, which shows the emergence of self-organized critical behavior when Darcian transport cannot accommodate transport and the dormant transport mechanism of hydrofracture propagation is activated. Ballistic and self-organized critical transport may play a significant role in the transport and accumulation of geological fluids, such as melt and hydrocarbons. This conclusion has a profound impact on the modeling of many transport processes in geology (e.g., accumulation of melt, oil, and gas).

The influence of strain localisation on the rotation behaviour of rigid objects in experimental shear zones

Abstract Mica fish and tourmaline fish from natural mylonites were analysed in thin section to determine their orientation distribution. They are oriented with their long axes tilted with respect to the mylonitic foliation, and fish with a small aspect ratio exhibit a slightly larger angle than fish with a large aspect ratio. This orientation seems to be a stable orientation for the mica and tourmaline fish. Analogue experiments with two rheologically different matrix materials were performed to explain the data. One material was PDMS, a linear viscous polymer. The other was tapioca pearls, a granular material with low cohesion and Mohr–Coulomb type behaviour. In contrast to a fairly homogeneous strain distribution in PDMS, distinct small-scale shear bands developed in tapioca pearls during deformation. Experiments modelled different vorticity numbers and parallelogram-shaped rigid objects with different aspect ratios were used. Rotation rates of objects in a viscous matrix are very similar to analytical solutions for ellipses in viscous flow, but stable orientations differ from data of natural examples. In all experiments with a Mohr–Coulomb matrix elongated objects had a stable orientation due to small-scale strain localisation. We therefore suggest that small-scale strain localisation (≤mm) that might be hidden by ongoing deformation and recrystallisation processes, is an important characteristic of the rheology of mylonites.

Development of crystal morphology during unitaxial growth in a progressively widening vein: I. The numerical model

Abstract A two-dimensional numerical model for the simulation of the formation of microstructures in crack-seal veins is presented here. The grain aggregate in the vein is described by grain boundary nodes that link short, straight boundary segments. Growth of crystals into a crack takes place by many small incremental movements of nodes. The rate of growth depends on the chosen growth morphology of the vein-forming material. Different growth morphologies can be defined. For every user-defined number of steps, the wall rock is moved a user-defined distance and direction, which simulates the opening of a crack, after which sealing can proceed again. The shape of the crack-surface (e.g. smooth or with ridges) can be set by the user and can be made to match fracture surfaces found in real rocks. The model is capable of reproducing realistic crack-seal textures and can be used to systematically investigate the role of different parameters on the microstructures of veins. First applications of the model are presented in Part II of this paper (this volume).

The development of oblique preferred orientations in zeolite films and membranes

We propose a model that can explain the development of 101 and other oblique crystallographic preferred orientations in thin MFI-type zeolite films and membranes. The model is a refinement of the competitive growth model. In the classic competitive growth model the preferred orientation is determined by the fastest growth direction of the crystals. We show that the lateral growth component also plays a role. By the combination of outward growth and lateral growth, those crystals that have their fastest growth direction at an angle to the normal to the membrane will dominate in the early stages of growth, and an oblique orientation develops. With prolonged growth the crystals with the fastest growth direction perpendicular to the membrane will dominate and the classic competitive growth model applies again. This concept has been verified using two-dimensional growth simulations. The new model also applies to other types of thin films.

Crystallographic preferred orientation development by dissolution–precipitation creep

Abstract Crystallographic preferred orientations (CPOs) in deformed rocks are commonly interpreted as resulting from crystal plastic deformation mechanisms, where deformation is achieved by the movement of dislocations. In this paper we investigate the possibility of CPO-development by dissolution–precipitation creep or pressure solution. A numerical model is presented, which simulates the development of a grain aggregate that deforms by reaction-controlled dissolution–precipitation creep. Grains are simulated as rectangular boxes that change their shape by growth, or dissolution of their surfaces, depending on the normal stresses acting on the individual surfaces. Grains can also rotate due to an applied vorticity (for non-coaxial deformation) and if they have a non-equidimensional shape. For each strain increment, stress that is applied to the grains is the same for all grains, while individual grains deform and rotate by different amounts. A variety of CPOs develop at moderate strains, depending on the reaction rates of the different crystal-surfaces and type of deformation (uni-axial shortening, plane strain pure shear and simple shear). The modelling results confirm that dissolution–precipitation creep may play a role in CPO-development in rocks.

Experimental simulation of the formation of fibrous veins by localised dissolution-precipitation creep

Abstract Fibrous veins are generally interpreted in terms of the crack-seal mechanism. Several aspects of fibrous veins (fibrous structure, curved fibres, symmetry of antitaxial veins) are however better explained by vein formation without fracturing. Mass transfer to such veins would be by diffusional transport rather than by fluid flow through the veins. Deformation by dissolution-precipitation creep can provide the driving force for the necessary mass transfer. Veins form when mass transfer is heterogeneous and precipitation is localised. Experiments were performed which enforced a chemical potential gradient, acting as the driving force for diffusional mass transfer. These experiments resulted in fibrous growths in aggregates of soluble salts (NaCl and KCl) saturated with brine. The experimental results support the theory that fibrous veins may form without fracturing and that rather than providing evidence for major fluid pathways, fibrous veins may instead represent localised precipitation during diffusional material transfer.

Are polymers suitable rock analogs

To evaluate if a polymer is suitable for analog modeling, it is essential to know the rheological properties of the material. Polymers used in analog modeling exhibit a complex rheological behavior; only part of which has been taken into account in most modeling studies. The mechanical behavior is strongly dependent on strain rate and temperature, and is characterized by specific dependencies of the storage and loss moduli, related to the elasticity and viscosity, on the deformation rate (frequency). We have measured the storage and loss moduli at a broad range of strain rates and strains, using an oscillatory parallel-disk rheometer. Investigated materials are polydimethylsiloxane (PDMS), mixtures of PDMS and BaSO4 (filler), Rhodorsil Gomme and mixtures of Rhodorsil Gomme and plastilina, all commonly used in analog experiments. Our measurements show that the rheological properties of mixtures of plastilina and Rhodorsil Gomme depend on its deformation history. Therefore, these mixtures are problematic for analog modeling. For mixtures of PDMS and BaSO4, the significance of the elastic component increases with increasing filler content, and accordingly, these mixtures have a limited application for modeling of viscous deformation. Pure PDMS and Rhodorsil Gomme exhibit Newtonian flow behavior at strain rates commonly used in analog modeling. D 2002 Elsevier Science B.V. All rights reserved.