Boris J. P. Kaus

Aftershocks driven by a high-pressure CO2 source at depth

In northern Italy in 1997, two earthquakes of magnitudes 5.7 and 6 (separated by nine hours) marked the beginning of a sequence that lasted more than 30 days, with thousands of aftershocks including four additional events with magnitudes between 5 and 6. This normal-faulting sequence is not well explained with models of elastic stress transfer, particularly the persistence of hanging-wall seismicity that included two events with magnitudes greater than 5. Here we show that this sequence may have been driven by a fluid pressure pulse generated from the coseismic release of a known deep source of trapped high-pressure carbon dioxide (CO2). We find a strong correlation between the high-pressure front and the aftershock hypocentres over a two-week period, using precise hypocentre locations and a simple model of nonlinear diffusion. The triggering amplitude (10–20 MPa) of the pressure pulse overwhelms the typical (0.1–0.2 MPa) range from stress changes in the usual stress triggering models. We propose that aftershocks of large earthquakes in such geologic environments may be driven by the coseismic release of trapped, high-pressure fluids propagating through damaged zones created by the mainshock. This may provide a link between earthquakes, aftershocks, crust/mantle degassing and earthquake-triggered large-scale fluid flow.

The numerical sandbox: comparison of model results for a shortening and an extension experiment

Abstract We report results of a study comparing numerical models of sandbox-type experiments. Two experimental designs were examined: (1) A brittle shortening experiment in which a thrust wedge is built in material of alternating frictional strength; and (2) an extension experiment in which a weak, basal viscous layer affects normal fault localization and propagation in overlying brittle materials. Eight different numerical codes, both commercial and academic, were tested against each other. Our results show that: (1) The overall evolution of all numerical codes is broadly similar. (2) Shortening is accommodated by in-sequence forward propagation of thrusts. The surface slope of the thrust wedge is within the stable field predicted by critical taper theory. (3) Details of thrust spacing, dip angle and number of thrusts vary between different codes for the shortening experiment. (4) Shear zones initiate at the velocity discontinuity in the extension experiment. The asymmetric evolution of the models is similar for all numerical codes. (5) Resolution affects strain localization and the number of shear zones that develop in strain-softening brittle material. (6) The variability between numerical codes is greater for the shortening than the extension experiment. Comparison to equivalent analogue experiments shows that the overall dynamic evolution of the numerical and analogue models is similar, in spite of the difficulty of achieving an exact representation of the analogue conditions with a numerical model. We find that the degree of variability between individual numerical results is about the same as between individual analogue models. Differences among and between numerical and analogue results are found in predictions of location, spacing and dip angle of shear zones. Our results show that numerical models using different solution techniques can to first order successfully reproduce structures observed in analogue sandbox experiments. The comparisons serve to highlight robust features in tectonic modelling of thrust wedges and brittle-viscous extension.

A free plate surface and weak oceanic crust produce single-sided subduction on Earth

[1] Earth’s lithosphere is characterized by the relative movement of almost rigid plates as part of global mantle convection. Subduction zones on present-day Earth are strongly asymmetric features composed of an overriding plate above a subducting plate that sinks into the mantle. While global self-consistent numerical models of mantle convection have reproduced some aspects of plate tectonics, the assumptions behind these models do not allow for realistic single-sided subduction. Here we demonstrate that the asymmetry of subduction results from two major features of terrestrial plates: (1) the presence of a free deformable upper surface and (2) the presence of weak hydrated crust atop subducting slabs. We show that assuming a free surface, rather than the conventional free-slip surface, allows the dynamical behavior at convergent plate boundaries to change from double-sided to single-sided. A weak crustal layer further improves the behavior towards steady single-sided subduction by acting as lubricating layer between the sinking and the overriding plate. This is a first order finding of the causes of single-sided subduction, which by its own produces important features like the arcuate curvature of subduction trenches. Citation: Crameri, F., P. J. Tackley, I. Meilick, T. V. Gerya, and B. J. P. Kaus (2012), A free plate surface and weak oceanic crust produce single-sided subduction on Earth, Geophys. Res. Lett., 39, L03306, doi:10.1029/2011GL050046.

Dome structures in collision orogens: Mechanical investigation of the gravity/compression interplay

Domes and basins are evidence for vertical movements in both compression and extension tectonic environments. They are thus evidence for interplay between gravity and tectonic forces in structuring the continental crust. We employ analytical and numerical techniques to investigate the respective roles of gravity and compression during the growth of crustal-scale buckle anticlines and diapirs submitted to instantaneous erosion. The analytical perturbation method, which explores the onset of both types of instability, yields a “phase-diagram” discriminating eight folding-diapirism modes, fi ve of which are geologically relevant. Numerical simulations show that the phase diagram is applicable to evolved, fi nite amplitude stages. Calculated strain fi elds in both diapirs and folds show normal sense of shear at the interface if the upper layer is thick compared to the lower layer. We conclude that the present-day structural techniques applied for distinguishing diapiric domes and folds are ambiguous if detachment folding and intense erosion take place during deformation, and that domes displaying extensional structures do not necessarily refl ect extension.

Forward and reverse modeling of the three-dimensional viscous Rayleigh-Taylor instability

A combined nite-dierence/spectral method is used to model the 3D viscous Rayleigh-Taylor instability. Numerically calculated growth rate spectra are presented for an initial sinusoidal perturbation of the interface sep- arating two fluids with amplitude 10 3 H and 0.2H ,w here H is the height of the system. At small initial amplitude, growth rate spectra closely follow linear theory,whereas the calculation with higher initial amplitude shows wavelength selection towards 3D perturbations. Numerical simulations and analytical theory are used to evaluate the applicability of previous 2D numerical models, which is shown to depend on (1) the wavelength and amplitude of an initially 2D si- nusoidal perturbation and (2) the amplitude of background noise. It is also shown that reverse (backward) modeling is capableofrestoringtheinitialgeometryaslongasoverhangs are not developed. If overhangs are present, the possibility ofrestoringtheinitialconditionsislargely dependentonthe stage of overhang development.

Numerical investigation of deformation mechanics in fold‐and‐thrust belts: Influence of rheology of single and multiple décollements

[1] Thin-skinned fold-and-thrust belts related to convergence tectonics develop by scraping off a rock sequence along a weaker basal decollement often formed by water-saturated shale layers or low-viscosity salt horizons. A two-dimensional finite element model with a viscoelastoplastic rheology is used to investigate the structural evolution of fold-and-thrust belts overlying different types of decollements. In addition, the influence of multiple weak layers in the stratigraphic column is studied. Model shale decollements are frictional, with lower friction angles as the cover sequence. Model salt layers behave linear viscous, due to a lower viscosity as the cover sequence, or with a power law rheology. Single viscous decollement simulations have been compared to an analytical solution concerning faulting versus folding. Results show that fold-and-thrust belts with a single frictional basal decollement generate thrust systems ramping from the decollement to the surface. Spacing between thrust ramps depends on the thickness of the cover sequence. The structural evolution of simulations with an additional low-frictional layer depends on the strength relationship between the basal and the intersequential decollement. Tectonic underplating and antiformal stacking occur if the within-sequence decollement is weaker. In the frontal part of models, deformation is restricted to the upper part and imbrication occurs with a wavelength depending on the depth of the intermediate weak layer. “Salt” decollement with a viscosity of 10 18 Pa⋅s leads to isolated box folds (detachment folds). Multiple salt layers (10 18 Pa⋅s) result in long-wavelength folding. Our results for both frictional and viscous decollements are in bulk agreement with the Mohr-Coulomb type, critical wedge theory. Citation: Ruh, J. B., B. J. P. Kaus, and J.-P. Burg (2012), Numerical investigation of deformation mechanics in fold-and-thrust belts: Influence of rheology of single and multiple decollements, Tectonics, 31, TC3005, doi:10.1029/2011TC003047.

Dynamic constraints on the crustal-scale rheology of the Zagros fold belt, Iran

Thin-skinned fold-and-thrust belts are generally considered as the result of contractional deformation of a sedimentary succession over a weak decollement layer. The resulting surface expression frequently consists of anticlines and synclines spaced in a fairly regular manner. It is thus tempting to use this spacing along with other geological constraints to obtain insights into the dynamics and rheology of the crust on geological time scales. Here we use the Zagros Mountains of Iran as a case study, as it is one of the most spectacular, well-studied thin-skinned fold-and- thrust belts in the world. Both analytical and numerical models are employed to study what con- trols fold spacing and under what conditions folding dominates over thrusting. The models show that if only a single basal decollement layer is present underneath a brittle sedimentary cover, deformation is dominated by thrusting, which is inconsistent with the data of the Zagros fold belt. If we instead take into account additional decollement layers that have been documented in the fi eld, a switch in deformation mode occurs and crustal-scale folding is obtained with the correct spacing and time scales. We show that fold spacing can be used to constrain the friction angle of the crust, which is ~5° the Zagros fold belt. This implies that on geological time scales, the upper crust is signifi cantly weaker than previously thought, possibly due to the effect of fl

Stress-strength relationship in the lithosphere during continental collision

Lithospheric strength profiles generated for a shortening continental lithosphere generally predict excessively high differential stresses in the sub-Moho continental mantle; this seems inconsistent with the relative scarcity of earthquakes at this depth. This inconsistency was put forward as evidence for weak mantle rheology. However, this argument implicitly assumes that strength envelopes are valid in actively deforming regions. We test this assumption on two end-member model lithospheres having identical upper crustal rheologies, but with (1) a weak lower crust and strong mantle, and (2) a strong lower crust and weak mantle. For this purpose, we compare one-dimensional (1-D) with 2-D visco-elastoplastic numerical models of continental shortening. The 2-D models show that strongly heterogeneous deformation typically follows initially homogeneous deformation. Lithospheric-scale buckle folds and shear zones result in strain rate variations of as much as three orders of magnitude. Differential stresses in the upper crust are close to yield, as predicted by 1-D models. Stresses in deeper lithospheric regions, however, are significantly smaller than in 1-D models, especially in actively deforming regions. Systematic numerical simulations as a function of temperature and deformation rate reveal that 1-D models are reliable in hot and/or slowly deforming lithospheres only. The relative scarcity of earthquakes at mantle levels should thus be interpreted as an intrinsic consequence of strong lithospheric deformation rather than as evidence for a weak upper mantle.

3D finite amplitude folding: Implications for stress evolution during crustal and lithospheric deformation

viscosity contrast. The results show that particular combinations of horizontal wavelengths (particular modes) grow faster than others; these ‘‘dominant’’ modes are the ones expected to develop in nature. Different than in the 2D case, wheretypicallyonlyonedominantmodeexists,in3Darange of modes amplify at nearly equal rate. The linear stability analysis is strictly valid only for infinitesimal amplitudes and assumes that the fold amplitude grows exponentially with horizontal strain (or deformation time). We perform numericalsimulations(finiteelementmethod)of3Dviscoussinglelayer folding to calculate the evolution of the fold amplitude with progressive shortening. The 2D analytical solution describing finite amplitude folding [Schmalholz and Podladchikov, 2000] is modified to accurately describe 3D finite amplitude folding. The solution is extended by an additional parameter, which is the ratio of the shortening strain rates in both horizontal directions. The numerical simulations are further applied to calculate the evolution of the averaged differential stress during 3D folding.

Intermediate-depth earthquake generation and shear zone formation caused by grain size reduction and shear heating

The underlying physics of intermediate-depth earthquakes have been an enigmatic topic; several studies support either thermal runaway or dehydration reactions as viable mechanisms for their generation. Here we present fully coupled thermomechanical models that investigate the impact of grain size evolution and energy feedbacks on shear zone and pseudotachylite formation. Our results indicate that grain size reduction weakens the rock prior to thermal runaway and significantly decreases the critical stress needed for thermal runaway, making it more likely to result in intermediate-depth earthquakes at shallower depths. Furthermore, grain size is reduced in and around the shear zone, which agrees with field and laboratory observations where pseudotachylites are embedded in a simultaneously formed mylonite matrix. The decrease in critical stress to initialize localization has important implications for large-scale geodynamics, as this mechanism might induce lithosphere-scale shear zones and subduction initiation. We suggest that the combination of grain size reduction and shear heating explains both the occurrence of intermediate-depth earthquakes and the formation of large-scale shear zones.