Spatial Radon and Helium Anomalies along Major Thrust/Faults of Himachal Himalayas, India
The Himalayan mountains are highly unstable and seismically very active. The seismicity in the Himalayan belt is closely associated with the active faults and folds trending normal or oblique to the main Himalayan trend, which leads to under thrusting of the blocks. The state of Himachal Pradesh is considered to be seismically very active because as per the Seismic Zonation Map of India, most of its area falls in two seismic zones, i.e. Very High Damage Risk Zone, zone V, and High Damage Risk Zone, zone IV. The Himachal Himalayas are broadly divided into two major tectonic zones viz. the Lesser Himalayan tectogen in the south and Tethyan Himalayan tectogen in the north. The lesser Himalayan tectogen lies mainly on the southern part of Himachal Pradesh state and is bounded between Main Central Thrust (MCT) and Main Boundary Thrust (MBT). The MCT and MBT are associated with evolution of Himalayan orogeny. Besides the longitudinal lineaments several transverse lineaments occur as faults and fractures trending normally or obliquely to Himalayan trend. In an effort to signify the role of radon and helium as a productive tool to delineate some active faults and lineaments, measurements were made in the soil-gas along some of the major thrust (MBT, MCT) areas of Himachal Himalayas. Remote sensing data provides the synoptic coverage of any desired area and has been successfully used to recognize structures having tectonic significance. As a step in identification of active faulting and structural investigation, we will discuss the specific geochemical studies applied with aim of further identifying active faults and also complementing and specifying remotely sensed structures and zones. This method to investigate active tectonic structures, using soil gas composition at faults, provides relevant information about regional stress conditions, which can be obtained rapidly and at relatively low cost. Elevated emanation of radon and helium gases were detected over some of the thrust/faults/lineaments, thus indicating anomalous permeability of these zones in comparison with the adjacent areas. The collected soil gas samples were analyzed for radon and helium using RTM-2100 (SARAD) and Helium leak detector (ALCATEL) respectively. It can be concluded from present study that soil gas radon and helium patterns, combined with morphological and geological observations, can supply useful constraints for deformation tectonic environments. These findings may have important connotations for the long-term seismic hazard assessment of the tectonically active regions of the NW Himalaya. This methodology is an inexpensive method of locating unknown faults. Consequently, our work is a preliminary study in this region and the obtained results will considerably help in constructing the radon and helium map of the faults system in the NW Himalayas, India.
India-Madagascar Conjugate Margins: Spatiospectral Localization of Isostatic Coherence Estimation
The two-dimensional (2D) nature of the coherence between Bouguer gravity anomalies and bathymetry on the Western Continental margin of India (WCMI) and Eastern Continental margin of Madagascar (ECMM) and their conjugate nature is examined. We estimated the variation of effective elastic thickness (Te) of the lithosphere for the two margins through cross-spectral analysis of gravity and bathymetry data. The results show that Te values at both margins are comparable, despite WCMI having been traversed along strike by a hotspot trace. We also compared Transitional Coherence wavelength (which is equivalent to lithospheric thickness) which are also comparable, varying from 94 km to 127 km for WCMI and 95 km to 102 km for ECMM. These results indicate that these two margins formed by the symmetrical rifting mechanism and also indicate that despite the presence of a hot spot trace along WCMI, the two margins have comparable isostatic compensation mechanism with low Te values.
Surface Deformation Analysis by Means of Fractal Dimension and Lacunarity Approaches
Fractals and scaling laws such as river networks and runoff series are abundant in nature, and geometry of river networks and basins is a superb example of this. The unrelenting competition between tectonics, surface uplift and erosional processes on the earth has resulted in a variety of drainage patterns by linearizing the normal flow patterns of river networks. These patterns are fractals and their variable spatial distribution can be used to examine the vulnerability of surface deformation. At first we extract the drainage network from Shuttle Radar Topographic Mission's digital elevation data (SRTM-90m) using D8 algorithm. We convert the drainage network into a binary image where the area of interests (AOIs) i.e. drainage are represented with pixels value of 1. The fractal dimension (D) analysis using Box Counting method is used to identify the anomalous drainage patterns of vulnerable sites. We prepare a D distribution map using a moving window of 1 arc sec. by 1 arc sec. on the binary image of river network. The space occupied by AOIs reveals variable distribution of D and lower values suggest that the drainage pattern has become linearized due to the influence of tectonics and surface processes. We use lacunarity analysis using Gliding Box method to see the relative vulnerability as two AOIs can have similar D values. The AOIs with a high lacunarity of drainage pattern are more vulnerable than AOIs with lower lacunarity values. Three AOIs i.e. Vanch and Yazgulem Basin (VYB) in northwestern Pamir, Tirch Mir Fault Zone (TMFZ) in Hindukush region, and Central Badakhshan (CB) with high vulnerability and three sites i.e. Central Pamir, Shiveh Lake Region in Afghanistan and Darvaz Fault Zone with medium vulnerability were identified using fractal dimension. The lacunarity analysis was used to diferentiate between the relative vulnerability of these AOIs. Results from Pyanj river network and adjacent areas show that VYB, TMFZ, and CB have relatively high vulnerability to surface deformation. The fractal dimensions derived from two different drainage patterns may be not sufficient for distinguishing them genetically. For this reason, lacunarity analysis is applied as a useful tool for the distinction between different textural patterns with similar fractal dimension.
Mechanical Evolution of Relay Zones in Normal Faulted Terranes: Insights From Three Dimensional Elastoplastic Finite Element Models
We present a 3D nonlinear finite element model to gain insights into the evolution of relay zones in normal faulted terranes. The model comprises two listric frictional sliding surfaces that act as faults and are arranged en echelon in an elastoplastic medium. We have investigated various Synthetic and Antithetic (both convergent and divergent) relay zone configurations to study the influence of (1) fault overlap/spacing ratio (-2 to 2), (2) material strength (3) coefficient of sliding friction on the faults (0.1 - 0.6) and (4) orthogonal vs. oblique extension, on the incremental evolution of stresses and strain paths in relay zones. The results suggest that a relay zone evolves in a three dimensional strain field under a combination of rotational and distortional strains. In isotropic rocks, the maximum extensional strains in the relay zone initiate oblique to regional extension and progressively rotate toward regional extension with increasing displacement on the faults. The relay zone evolves along a non-coaxial strain path and the total strain ellipsoid shape (oblate vs. prolate) is dependent on the relative orientation of the primary faults and amount of extension on them, and structural position in the relay zone. With all other parameters being identical, magnitudes of von Mises stresses at the ground surface are highest in convergent relay zones and lowest in divergent relay zones. Thus subsidiary oblique structures are more likely to develop in convergent relay zones than in synthetic or divergent relay zones. Assuming uniform fault propagation, it is possible to gain insights into relay zone evolution during fault tip propagation by comparing models with different fault overlap/spacing ratios. Model plastic strains suggest that hard linkage can develop between adjacent faults with a gap or minimal overlap; however, the occurrence of oblique, strain transferring structures increases with increasing fault overlap. The orientations of the maximum extensional strains throughout the deformation suggest that primary faults can propagate towards each other and link-up in synthetic and convergent relay zones and propagate away from each other in divergent relay zones. For layered rock volumes, mechanical stratigraphy plays a major role by influencing both the orientations and magnitudes of strains in adjoining layers. Our results give critical insights into the variations of dominant fracture/fault trends with geometry and structural position in relay zones, and show how these trends evolve in mechanically contrasting adjoining layers.
Effect of Cohesion Uncertainty of Granular Materials on the Kinematics of Scaled Models of Fold-and-Thrust Belts
Cohesionless or very low cohesion granular materials are widely used in analogue/physical models to simulate brittle rocks in the upper crust. Selection of materials with appropriate cohesion values in such models is important for the simulation of the dynamics of brittle rock deformation in nature. Uncertainties in the magnitude of cohesion (due to measurement errors, extrapolations at low normal stresses, or model setup) in laboratory experiments can possibly result in misinterpretation of the styles and mechanisms of deformation in natural fold-and thrust belts. We ran a series of 2-D numerical models to investigate systematically the effect of cohesion uncertainties on the evolution of models of fold-and-thrust belts. The analyses employ SOPALE, a geodynamic code based on the arbitrary Lagrangian-Eulerian (ALE) finite element method. Similar to analogue models, the material properties of sand and transparent silicone (PDMS) are used to simulate brittle and viscous behaviors of upper crustal rocks. The suite of scaled brittle and brittle-viscous numerical experiments have the same initial geometry but the cohesion value of the brittle layers is increased systematically from 0 to 100 Pa. The stress and strain distribution in different sets of models with different cohesion values are compared and analyzed. The kinematics and geometry of thrust wedges including the location and number of foreland- and hinterland- verging thrust faults, pop-up structures, tapers and topography are also explored and their sensitivity to cohesion value is discussed.
The Impact of Superplasticity and the Iron Spin Transition on Mantle Dynamics
Our understanding of mantle dynamical processes depends critically upon knowledge of the physical properties of mantle minerals at high pressure and temperature. The Earth's lower mantle is believed to consist mainly of iron bearing aluminous silicate perovskite [Al-(Mg,Fe)SiO3] and ferropericlase [(Mg,Fe)O] together with a small amount of calcium silicate perovskite (CaSiO3) and Al2O3 (eg. Ringwood, 1982). In light of recent experimental measurements it is now generally accepted that the dominant mineral of the lowermost mantle is a "post-perovskite phase" (Murakami, 2004). The exothermic perovskite-post-perovskite (pv-ppv) phase transition may accommodate a higher rate of heat extraction from the core. On the other hand the electronic spin and valence states of iron-bearing minerals may also strongly influence the properties of mantle material and in turn the dynamics of Earth's interior. Most recent experimental results suggest that the electronic iron spin transition is associated with significant changes in density, compressibility, sound velocities, radiative thermal conductivity and electrical conductivity in ferropericlase (Lin et al., 2008). These studies also show that both perovskite and post- perovskite accommodate the same intermediate-spin Fe2+ state and that their opacity and absorption behavior at infrared wavelengths are very similar in high P-T experiments. Therefore corresponding changes in the radiative thermal conductivity, electrical conductivity and iron partitioning would occur due to the crystal structural transition, rather than due to the electronic spin transition, from perovskite to post-perovskite (Lin et al., 2008). Furthermore, dislocation creep which is believed to be the dominant deformation mechanism in the post-perovskite phase would be associated with lower viscosity than in the viscosity of perovskite at the same P-T conditions (Cizkova et al., 2008). These results may have significant implications for the dynamics of Earth's lower mantle and for the extent to which the convective circulation is layered, an issue that remains outstanding. In the work to be reported in this paper we have studied the impact of the iron spin transition on the style of convective mixing. Although the density increase with depth may slightly lower the mantle mean temperature, the higher thermal conductivity due to the increase of the radiative contribution below the D" layer enhances the influence of the lower viscosity by increasing the mean mantle temperature. A further issue that we address in this paper concerns the extent to which superplastic behavior associated with the endothermic phase transition at 660 km depth may influence mantle layering. During a solid-solid phase transition a polycrystalline material may deform superplastically due to the reduction in grain size that occurs when material changes phase (e.g. Sammis & Dein, 1974; Paterson, 1983; Ranalli, 1991). Our numerical models show that the inclusion of such a superplastic layer at the level of the 660 km transition enhances the degree of mantle layering and the regularity of the avalanches of material that episodically occur across this important interface that separates the upper mantle from the lower mantle. Keywords: layered mantle convection, iron spin transition, perovskite-post perovskite phase transition, superplasticity
Modeling Pure Shear Porosity Instabilities in Compacting Porous Media: Implications for Melt Transport Beneath Mid-Ocean Ridges
When a compacting porous layer, modeled as two interpenetrating viscous fluids, is subjected to an external stress, it has been shown that if the viscosity of the solid matrix decreases with porosity, that an instability occurs that results in localized regions of high and low porosity. This instability was first predicted theoretically for a pure shear external forcing (Stevenson, 1989). More recently the experiments of Holtzman et al. (2003) and the theoretical and numerical investigations of Katz et al. (2006) have demonstrated the instability for a compacting layer subjected to simple shear. In this contribution, I present a linear theory and numerical model results for a compacting layer subjected to pure shear. As in the simple shear case, bands of high and low porosity form that are parallel to the direction of maximum compression if the viscosity of the matrix is strain- rate independent. If viscosity depends on strain rate, then two sets of bands form at angles to the direction of maximum compression that increase with the degree of strain rate dependence. These bands are shown to form when the fluid is buoyant which induces oscillations and waves. Results for more complicated background shear stresses that more closely model deformations associated with large scale mantle convection between mid-ocean ridges will also be presented.
Global Geodynamic Constraints on the Structure and Dynamic State of the Continental Lower crust
Owing to the paucity of direct observations or constraints, the structure, composition and rheology of the lower crust of continents is not as well understood as the upper crust. Knowledge of lower crustal rheology is important for understanding how deep-seated lithospheric stresses generated by the convecting mantle are transmitted to the overlying brittle crust and how these stresses maintain surface topographic inequalities. Understanding the lateral variability of lower crustal thickness and density yields important clues on the thermo-chemical processes that have controlled the evolution, growth and mineralogy of the continental crust. Here we present the initial results of a global-scale study concerned with inferring the lateral changes in the lowermost crustal thickness and/or density and the implications for the stresses acting between the lithospheric mantle and the crust. Our approach involves quantifying the relationship between components of the non-isostatic topography, inferred by stripping away the isostatically compensated CRUST2.0 (Bassin et al. 2003) model, and predictions of surface dynamic topography predicted on the basis of a mantle convection model incorporating a recent joint seismic-geodynamic tomography model (Simmons et al. 2009). The specific focus will be on the modifications needed in lower crust structure that yield an optimal match between the CRUST2.0 inferences of non-isostatic topography and convection-driven dynamic undulations. The modified crustal structure will be used to explore the implications for the gravitational potential energy (GPE) of the compensated crust and hence the basal stresses acting at crust-mantle interface. Our overall objective is to constrain the dynamics of crust-mantle coupling and its contribution to surface geophysical observables.