Chandan Chakraborty

Boudinage in multilayered rocks under layer-normal compression: a theoretical analysis

Abstract This paper presents a dynamic analysis of boudinage in multilayers of alternate brittle and ductile layers under layer-normal compression. Based on the mode of fracturing of individual brittle layers, boudinage is classified into three types: tensile fracture boudinage (Type 1), shear fracture boudinage (Type 2a) and extensional shear fracture boudinage (Type 2b). The layer-thickness ratio, T r (= t b / t d ), and the strength ratio, F (= T /2 ηe ), between the brittle and the ductile units are the principal physical factors determining the type of boudinage. Type 1 boudinage develops rectangular boudins and occurs when T r is low ( F is high (>0.8). In contrast, Type 2a boudinage takes place when T r is high (>8.5) or F is low ( A r ) of all the types of boudins is inversely proportional to layer-thickness ratio ( T r ). However, Type 1 and Type 2 boudins, have contrasting aspect ratios, which are generally greater and less than 1, respectively.

Proterozoic intracontinental basin: The Vindhyan example

SummaryThe features of the Vindhyan succession clearly indicate a vast intracratonic basin that remained within tens of meters of sea level throughout its lifetime. Apparently, shallow water condition was maintained over a large area for a long period of time suggesting that the sub-Vindhyan lithosphere suffered subsidence over a larger area producing a wide shallow ramp type basin. Hundreds of meters thick accumulation of peritidal strata in sequence 5 of the Vindhyan succession indicates that the subsidence rate was in perfect concert with the rate of sediment supply for a considerably long period of time during the end phase of Vindhyan basin evolution — the hallmark of cratonic basins Sloss (1988a, b). It is inferred that during the terminal period of the Vindhyan sedimentation a self-regulating system of uplift, erosion, sedimentation and subsidence controlled the accumulation of strata.

Numerical modeling of heterogeneous flow fields around rigid objects with special reference to particle paths, strain shadows and foliation drag

Abstract With the help of two-dimensional numerical models this paper investigates three aspects of heterogeneous deformation around rigid objects: (1) the nature of particle paths ; (2) the development of strain shadow zones; and (3) the drag patterns of passive markers. In simple shear, spherical objects develop typically a concentric vortex motion, showing particle paths with an eye (double-bulge)-shaped separatrix. The separatrix has no finite dimension along the central line, parallel to the shear direction. Under a combination of pure shear and simple shear, the particle paths assume a pattern with a bow-tie shaped separatrix. With increase in the ratio of pure shear to simple shear ( S r ), the separatrix around the object shrinks in size. The axial ratio of the object ( R ) is another important factor that controls the geometry of particle paths. When R R , the loci form a doublet elliptical shell structure. Objects with R >3 do not show closed particle paths, but give rise to elliptical or circular spiral particle paths. The development of strain shadow zones against equant rigid bodies depends strongly on the strain ratio S r . When S r =0 (simple shear), they develop opposite to the extensional faces of the object, forming a typical σ -type tail. The structure has a tendency to die out with an increase in the pure shear component of the bulk deformation ( S r ). The initial angle of the long axis of the object with the shear direction ( φ ) and the axial ratio of the object ( R ) determine the development of strain shadow zones near inequant rigid objects. Objects with large R and φ between 60 and 120° form pronounced zones of low finite strain, giving rise to strain shadow structures. A geometrical classification of diverse drag patterns of passive markers around rigid objects is presented along with their conditions of formation.

Flow patterns around rigid inclusions in a multiple inclusion system undergoing bulk simple shear deformation

Abstract During deformation of an inclusion-matrix system, the velocity fields around individual inclusions mutually interfere with one another. Such interacting inclusions rotate at slower rates than non-interacting, single inclusions. This paper presents a theoretical model that describes the flow pattern of matrix (viscous) material around interacting rigid inclusions of spherical shape in bulk simple shear deformation. Numerical simulations based on the velocity functions reveal that the volume concentration of inclusions is a crucial parameter controlling the flow pattern around rotating inclusions under interacting conditions. At low volume concentrations (ρv 0.1), transforms into a pattern with a bow-tie shaped separatrix. At a large volume concentration (ρv=0.4) the separatrix assumes the geometry of a super-ellipse. We also present numerical models that illustrate the influence of volume concentration on the (1) nature of strain distribution, (2) distortion patterns of passive foliations ,and (3) mantle structures around inclusions in an interacting state. Based on this theory, it is shown that the rotational retardation of the inclusions slightly enhances the bulk viscosity of the inclusion-matrix system.

Pull-apart origin of the Satpura Gondwana basin, central India

The Gondwana basins of peninsular India are traditionally considered as extensional-rift basins due to the overwhelming evidence of fault-controlled synsedimentary subsidence. These basins indeed originated under a bulk extensional tectonic regime, due to failure of the attenuated crust along pre-existing zones of weakness inherited from Precambrian structural fabrics. However, disposition of the basins and their structural architecture indicate that the kinematics of all the basins cannot be extensional. To maintain kinematic compatibility with other basins as well as the bulk lateral extension, some basins ought to be of strike-slip origin. The disposition, shape and structural architecture of the Satpura basin, central India suggest that the basin could be a pull-apart basin that developed above a releasing jog of a left-stepping strike-slip fault system defined by the Son-Narmada south fault and Tapti north fault in consequence to sinistral displacement along WSW-ENE. Development of a sedimentary basin under the above-mentioned kinematic condition was simulated in model experiments with sandpack. The shape, relative size, stratigraphic and structural architecture of the experimental basin tally with that of the Satpura basin. The experimental results also provide insights into the tectono-sedimentary evolution of the Satpura basin in particular and pull-apart basins in general.

Progressive development of mantle structures around elongate porphyroclasts: insights from numerical models

Abstract This paper presents a generalized theoretical approach towards two-dimensional numerical modeling of the mantle geometry of inequant porphyroclasts of varying shapes within a Newtonian matrix during progressive, general type of bulk deformation. The analysis takes into account the effects of synkinematic size reduction of the porphyroclast with concomitant mantle development in response to dynamic recrystallization. Numerical simulations reveal that the principal factors governing the geometry of mantle structures are: (1) the initial aspect ratio of the porphyroclast (a/b), (2) the rate of clast-size reduction, and (3) the ratio of the rates of pure shear and simple shear (Sr) or the kinematic vorticity (Wk) in the general type of non-coaxial deformation. In general, porphyroclasts develop δ-, φ- and finally, σ-type mantle structures, as the rate of clast-size reduction is progressively increased. The tails of equant porphyroclasts tend to be characterized by wings with increase in bulk shear during progressive deformation. In contrast, inequant objects (a/b>1) develop composite tails with multiple wings, even at low finite shear strains. However, with increase in aspect ratio δ geometry tends to dominate the overall mantle structure. Porphyroclasts with a large aspect ratio (a/b=3) form tails with overturned δ-wings, as described in Passchier, C.W., Simpson, C., 1986. “Porphyroclast system as kinematic indicators”, Journal of Structural Geology 8, 831–844. In general the type of non-coaxial deformation, with decrease in kinematic vorticity (or increase in Sr), porphyroclasts irrespective of their initial shapes, tend to form atypical δ-like tails that do not cross the central reference plane.

Rhythmic shelf storm beds : Proterozoic Kaimur Formation, India

Abstract The shallow-marine, turbidite-like storm beds of the Proterozoic Kaimur Formation exhibit intrabed rhythmic vertical variations in sedimentary structure and texture attributable to different patterns of velocity pulsations within an event of storm current deposition. The alternate repetition of parallel-laminated (S b ) and ripple cross-laminated (S c ) divisions (S bcbc… ) within individual storm beds has been interpreted to represent periodic fluctuation in flow intensity beyond the stability limits of ripple and upper flat-bed during sedimentation. Sandstone beds characterised by successively occurring thin, internal mud partings indicate pulsing discontinuous flows with alternate periods of slack water within an event of storm current deposition. Successive mud drapes occurring within the ripple cross-laminated divisions (S c ) of some of the storm beds also represent pulsing discontinuous flows. Palaeoccurrent analyses of these storm beds of the Kaimur Formation reveal a northwesterly current mode against an E—W trending shoreline. These storm beds thus may be interpreted to have deposited from geostrophic storm currents that were pulsating in nature with a periodicity of the order of tens of hours similar to that observed on present-day storm-dominated shelves.

Deformation of ductile inclusions in a multiple inclusion system in pure shear

This paper analyzes the deformational behavior of mutually interacting spherical inclusions in a multiple inclusion system, considering two physical factors: viscosity ratio between inclusion and matrix (m) and the ratio of inclusion diameter to mean inter-inclusion distance (a/b). For a given value of m, the strain partitioning between a stiff inclusion and the bulk system (i.e. ratio of their natural extension rates) increases non-linearly with increasing a/b ratios and the gradient of increase becomes steeper when the inter-inclusion distance is less than about twice their diameter (i.e. a/b>about 0.5). The strain distribution within a deformed inclusion is homogeneous when the a/b ratio is less than about 0.6. For larger values of a/b, the internal deformation becomes heterogeneous, with the strain increasing or decreasing towards the core in the case of stiff (m>1) and soft (m<1) inclusions, respectively. The deformed shape of inclusions in section also shows departure from an ideal ellipse with an increase in the a/b ratio. Stiff inclusions develop shapes similar to that of a super-ellipse in contrast to soft inclusions that resemble a sub-ellipse. The heterogeneity of internal deformation is also reflected in the distortion of passive foliations initially at right angles to the bulk extension direction, which become curved with convexity outward and inward, respectively, within stiff and soft inclusions.

Controls on the failure mode of brittle inclusions hosted in a ductile matrix

Plane strain deformation experiments were performed on elliptical inclusions of cohesive sand embedded within a slab of pitch, with the aim of investigating the mode of fracturing of brittle inclusions within a ductile matrix. Under pure and simple shear, the inclusions failed in three different modes: tensile fracturing (Mode 1), shear fracturing (Mode 2a) and extensional shear fracturing (Mode 2b). Jeffrey’s (1922) theory of the flow of a viscous medium around an ellipsoidal body was applied to the experimental results to determine the principal tensile and compressive stresses within an inclusion, and analyze the failure modes using Griffith’s Criterion. The analysis reveals that the aspect ratio (R) and the orientation (u ) of the inclusion control the principal tensile and compressive stresses within it, and in turn govern the mode of brittle deformation. At a particular inclusion orientation, the tensile stress increases, whereas the compressive stress decreases monotonically with increasing aspect ratio of the inclusion. The principal stresses also vary with inclusion orientation for a given aspect ratio, but not monotonically. The analysis delimits the fields of each mode of brittle deformation of inclusions in R- u space under pure shear and simple shear. q 2001 Elsevier Science Ltd. All rights reserved.

Internal structures of sandwaves in a tide-storm interactive system: proterozoic lower quartzite formation, India

Abstract A Proterozoic sandstone sequence belonging to the Lower Quartzite Formation of Vindhyan Supergroup, India, reveals the internal structures of near-symmetrical subtidal sandwaves formed in an area of strong tidal currents, occasionally interfered by wind-induced currents of varying magnitude. Internally, the sandwaves show decimetre-scale, herring-bone cross-laminated sets with inclined and horizontal set boundaries representing accretion on the gently inclined (around 5°) lee and stoss surfaces of the sandwaves respectively. The internal structures suggest oblique upbuilding of the sandwaves with almost equal contributions from the two reversing current modes of the tidal flow. Evidently, the sandwaves were maintained by bedload transport through migration of megaripples superimposed on the sandwaves. Occasional superimposition of short-lived, wind-induced currents on the tidal flow caused appreciable suspension transport of sand-sized sediments and led to the development of successive low-angle, unidirectional, mud-draped, cross-laminated bundles interwoven with the tide-generated structures. However, the dominant sediment type introduced during the period of wind-induced currents was in the size range of mud as reflected in the presence of exceptionally thick exotic mud layers in juxtaposition with the cross-laminated bundles. During the periods of vigorous storm currents, significant volumes of sand-sized sediments were introduced in the form of density flows, deposition from which led to the burial of tidal sandwaves. Renewal of the fair-weather tidal regime caused the development of new sandwaves.