Initiation of localized shear zones in viscoelastoplastic rocks

Journal of Geophysical Research, 1995

We present experiments and numerical simulations dealing with the growth of faults in thin brittle/ductile systems to understand deformation modes in the continentallithosphere, Experiments were uniaxial shortening of layers of dry sand and silicone putties of various viscous resistances, For large strength ratios between the brittle and ductile layers (R>5-1O), the deformation localizes into two shear bands; the fault pattern is ereated before reaching 10% shortening, and has fractal dimensions varying between 1.6 and 1.8, For small strength ratios (R<5-1O), deformation ncver localizes; the fauIt pattern is homogeneous with a trivial dimension of 2, and grows continuously during deformation, The transition between loealized and homogeneous deformation DCcurs when the mechanieal resistanee of brittle layers is 5-10 times larger than the resistance of ductile layers. This transition was a180 investigated by rncans of clcctrical analog simulations. A fuse network, which represents an elasto-brittle layer, is coupled with a eapaeitor layer which models strain-rate dependent fluids, An AC potential is applied and the fuses progressively burned out until they form a connected network, The AC-potential frequeney,j, is a tuning pararneter similar to the applied strain rate in experiments. A critical frequency is obtained marking a transition between a localization mode where the density of burned fuses decreases as the system size increases, and a delocalization mode where the density of burned fuses remains constant with increasing system size, The scaling dependency ofthe fracture process, as weil as the critical frequency, are consistent with experimental results, Available information on the rheology of the continental lithosphere shows that this mechanical transition is bracketed by the possible range of brittle-toductile strength ratios.

2003

Shear zones are the most ubiquitous features observed in planetary surfaces. They appear as a jagged network of faults at the observable brittle surface of planets and, in geological exposures of deeper rocks, they turn into smoothly braided networks of localized shear displacement leaving centimeter wide bands of ''mylonitized'', reduced grain sizes behind. The overall size of the entire shear network rarely exceeds kilometer scale at depth. Although mylonitic shear zones are only visible to the observer, when uplifted and exposed at the surface, they govern the mechanical behavior of the strongest part of the lithosphere below 10 -15 km depth. Mylonitic shear zones dissect plates, thus allowing plate tectonics to develop on the Earth. We review the basic multiscale physics underlying mylonitic, ductile shear zone nucleation, growth and longevity and show that grain size reduction is a symptomatic cause but not necessarily the main reason for localization. We also discuss a framework for analytic and numerical modeling including the effects of thermal -mechanical couplings, thermal-elasticity, the influence of water and void-volatile feedback. The physics of ductile shear zones relies on feedback processes that turn a macroscopically homogenously deforming body into a heterogeneously slipping solid medium. Positive feedback can amplify strength heterogeneities by cascading through different scales. We define basic, intrinsic length scales of strength heterogeneity such as those associated with plasticity, grain size, fluid-inclusion and thermal diffusion length scale.

Visual Geosciences, 1999

Continental breakup, or compressive lithosphere scale faulting, requires a physical mechanism for wholesale faulting of the lithosphere. We compared numerical and experimental models for the nucleation of quasi-adiabatic shear bands in polyvinylchloride (PVC) with those in an idealized viscoelastoplastic mantle with olivine rheology. In both materials fault nucleation is caused by elastic stress concentration on pre-existing imperfections, with localized yielding confined to its vicinity. Faulting occurs rapidly after the initial elastic energy in the system is charged sufficiently to cause wholesale yielding. Propagation of the fault, monitored by looking at the dissipation of plastic energy, reveals migration of a sharp, thermal-mechanical "crack"-like instability, which appears in the temperature field as a slightly diffused signal. The initial temperature rise in the crack is subtle but increases suddenly when the plate is severed. This autocatalytic behavior has also been described in ductile polymers, which can be used as mechanical analogues. We suggest that elastoplastic coupling in quasi-adiabatic shear banding is a key for fast (< 1 Ma) nucleation of shear zones. These nonlinear phenomena will be illustrated for both experimental and numerical results by 1 of 17 Springer LINK: Electronic Geosciences (1999) 4:2

Journal of Geophysical Research, 2008

A new method is introduced for quantifying the scale and the intensity of strain localization from maps of natural shear zones. The method employs autocorrelation functions to determine local areal scales of geometric homogeneity. These homogenization scales are used to calculate scale-dependent localization fractions of deformed rock. The strain localization intensity is quantified from measurements of mean relative to maximum shear strain. This approach is used to analyze shear zones on different scales from an exposure (Cap de Creus, Spain) of the fossil brittle-to-viscous transition (BVT). Changes in the scaling characteristics of shear zones are interpreted to reflect a time sequence of localization during the evolution of the continental BVT. We show that shear zone scaling is related to inherited anisotropies (older schistosity, lithological layering, pegmatite bodies) and to the predominant mode of deformation (brittle, viscous). The length-towidth ratio of shear zones increases with their length up to the meter scale and decreases for larger length scales as they evolve from isolated shear fractures to interconnected mylonitic shear zones. Variations in strain localization intensity calculated along a single shear zone indicate that such shear zones weakened from their brittle tips to their mylonitic centers, thus driving their propagation and growth to larger scales. Our results imply that the BVT evolves by ''network widening,'' a process whereby strain localizes on progressively larger scales until a dense network of weak, mylonitic layers tens to hundreds of meters wide and hundreds to thousands of meters long forms subparallel to the regional shearing plane.

Journal of Geophysical Research, 1995

Continental plates represent rheologically heterogeneous media in which complex finite strain fields may develop due to interaction of plate tectonic processes with intraplate heterogeneities. Such a deformation pattern is displayed by the Borborema shear zone system in northeastern Brazil. It involves continental-scale, curvilinear, E-W trending right-lateral transcurrent shear zones that branch off from a major NE trending, right-lateral strike-slip deformation zone formed at the northern termination of the S•o Francisco craton and that finally terminate in transpressive metasedimentary belts. We suggest that this strain field results from the compressional deformation of a highly heterogeneous continental lithosphere composed of a stiff domain (craton) and rheologically weaker domains (basins). The effect of a low-viscosity domain on the deformation of a continental plate and, in particular, the perturbation induced by this domain on the development of a shear zone formed at the termination of a stiff block were investigated using numerical modeling. The low-strength domain induces an enhanced strain localization, and the geometry of the shear zone is significantly modified. It is either split, forming a branched shear zone system in which one branch maintains its original orientation while the other rotates toward the low-viscosity domain, or completely rotated toward the weak block. The perturbation of the finite strain field depends on the ability of the weak domain to accommodate deformation, which is controlled by its initial viscosity contrast relative to the surrounding lithosphere, its orientation relative to the convergence direction, and its distance from the shear zone initiated at the termination of the stiff block. The interaction between imposed boundary conditions (tectonic forces and plate geometry) and the internal structure of the plate may give rise to highly heterogeneous strain fields, as exhibited by the Borborema shear zone system, in which intraplate rheological heterogeneities induce the development of branched or sinuous shear zones. A heterogeneous continental plate subjected to a normal convergence may therefore display significant lateral variations in strain intensity, with shear zones bordering nearly undeformed blocks, and in deformation regimes and vertical strains that would result in contrasting metamorphic and uplift histories.

American Journal of Computational and Applied Mathematics, 2013

Most of the earthquake faults in NorthEast India, China, mid Atlantic-ridge, the Pacific seismic belt and Japan are found to be predominantly d ip-slip in nature. In the present paper a dip-slip fault is taken to be situated in a viscoelastic half space representing the upper lithospheric region of the Earth. A movement of the dip-slip nature across the fault occurs when the accumulated stress due to various tectonic reasons e.g. mantle convection etc, exceeds the local frict ion and cohesive forces across the fault. The movement is assumed to be creeping in nature, expressions for displacement, stress and strains are obtained by suitable mathematical methods. A detailed study of these expressions may give some ideas about the nature of stress accumulation in the system, which in turn will be helpfu l in formu lating an earthquake pred iction programme.

2009

The relative importance of distributed versus localized strain remains a matter of intense debate regarding orogenic plateau development, particularly in Tibet where through-going crustal-scale faults are required to explain large magnitudes of displacement and strain-partitioning. We present results from an exhumed lower crustal shear zone that point to a rheological dichotomy during Paleoproterozoic transpressive strain in the western Canadian Shield. This dichotomy provides insight into the topography across eastern Tibet, i.e. the Longmen Shan region and Sichuan basin, which has been interpreted to be the result of strain-localization induced by differences in crustal strength at depth. The Grease River shear zone (GRsz) is a 5-7 km-wide, >400 km-long shear zone that cuts the Athabasca granulite terrane, one of Earth’s largest exposures of continental lower crust (>20,000 km2). The GRsz is dominated by penetrative NE-striking, steeply NW-dipping foliations with gently SW-plunging stretching lineations and dextral SW-over-NE kinematics. Neoarchean top-to-the-SE flow of weak lower crust (sub-horizontal fabric at ca. 2.60-2.55 Ga) in the Athabasca granulite terrane exposed SE of the GRsz was followed by >650 m.y. of near-isobaric cooling and strengthening of continental lithosphere. In contrast, melt-weakened flow and shallow SW-dipping fabric development in lower crust exposed NW of the GRsz occurred at ca. 1.93-1.90 Ga during culmination of the Taltson orogeny. Melt-weakened flow NW of the GRsz was coincident with ca. 1.92-1.90 Ga dextral transpressive strain along the GRsz. Southeast of the GRsz, deformation was restricted to discrete 10s of m-scale moderately- to steeply-dipping shear zones in rocks that had previously dehydrated and isobarically-cooled in the Neoarchean. This pattern of pervasive ductile flow contrasted with localized strain illustrates the dramatic effects of rheological heterogeneity across a transcurrent shear zone. This is analogous to the inferred low viscosity flow of crust around the rigid basins that bound the Tibetan Plateau (i.e., the Sichuan and Tarim basins).

Journal of Geophysical Research, 2012

We present numerical models of earthquake cycles on a strike-slip fault that incorporate laboratory-derived power law rheologies with Arrhenius temperature dependence, viscous dissipation, conductive heat transfer, and far-field loading due to relative plate motion. We use these models to explore the evolution of stress, strain, and thermal regime on "geologic" timescales ($10 6-10 7 years), as well as on timescales of the order of the earthquake recurrence ($10 2 years). Strain localization in the viscoelastic medium results from thermomechanical coupling and power law dependence of strain rate on stress. For conditions corresponding to the San Andreas fault (SAF), the predicted width of the shear zone in the lower crust is $3-5 km; this shear zone accommodates more than 50% of the far-field plate motion. Coupled thermomechanical models predict a single-layer lithosphere in case of "dry" composition of the lower crust and upper mantle, and a "jelly sandwich" lithosphere in case of "wet" composition. Deviatoric stress in the lithosphere in our models is relatively insensitive to the water content, the far-field loading rate, and the fault strength and is of the order of 10 2 MPa. Thermomechanical coupling gives rise to an inverse correlation between the fault slip rate and the ductile strength of the lithosphere. We show that our models are broadly consistent with geodetic and heat flow constrains from the SAF in Northern California. Models suggest that the regionally elevated heat flow around the SAF may be at least in part due to viscous dissipation in the ductile part of the lithosphere.

Earth and Planetary Science Letters, 2012

Even though it is a well-established fact that the Earth is currently in a plate-tectonics mode, the question on how to ''break'' lithospheric plates and initiate subduction remains a matter of debate.