Deepak C. Srivastava

Test of the frictional reactivation theory for faults and validity of fault-slip analysis

The notion that slip on faults is controlled by the balance between resolved shear stress and the magnitude of frictional resistance is examined in the light of the published orientations of ancient faults and slip lineations. By compiling data from more than 1500 faults used in 228 stress analyses, we observe that natural fault planes, as well as lineations formed by fault slip, display characteristic patterns of preferred orientation when rotated into the reference frame defined by the principal stress axes. These patterns vary systematically according to the relative magnitudes of principal stresses determined by fault-slip analysis. Furthermore, these patterns are strikingly similar to those predicted by a frictional model for fault reactivation in which the potential for slip is equated to the ratio of the components of shear stress and normal stress on the fault plane. Our findings therefore make a compelling case for the concept of frictional slip tendency for the prediction of fault reactivation and lend support to the validity of fault-slip methods for paleostress analysis.

Shear zones as a new type of palaeostress indicator

This paper proposes that shear zones act as palaeostress indicators in much the same way as brittle faults. Several hundred shear zones, from both the limbs of the Variscan Langland-Mumblcs anticline (South Wales), are treated using the graphical and numerical methods of fault slip analysis. Estimated palaeostress directions derived from shear zones are characterized by greater scatter than where brittle faults are used as stress indicators. An important reason for this is the fact that on some shear zones the direction of maximum shear stress (slip lineation) is not perpendicular to the line of S/C intersection. As a consequence, the use of slip lineations produces better solutions than cases where the slip direction is inferred as a line perpendicular to the S/C intersection. The filtering of data to reduce the scatter in the results produces insignificant improvement and is considered unnecessary.

Digital method for strain estimation and retrodeformation of bilaterally symmetric fossils

A simple algorithm based on the transformation equations of homogeneous deformation allows application of computer-graphic software for estimation of strain and restoration of shape of distorted fossils in a single operation. This paper gives a computer-aided digital method for the estimation of strain in situations where a single fossil, or a set of fossils, is found embedded in rocks. Because the digital method deciphers reciprocal strain ellipse by removing the effect of tectonic deformation, it is also useful in precise taxonomic identification. Several natural examples of trilobites and brachiopods are tested by the digital method, as well as by other conventional methods of strain analysis. The digital method is easy to use, rapid, and accurate in comparison to other existing methods of strain analysis and retrodeformation.

Geochemistry and tectonic significance of the Ongarbira metavolcanic rocks, Singhbhum District, India

Abstract The volcanic-sedimentary sequence of Ongarbira occurs to the south of the Singhbhum Shear Zone and rests uncomformably on a basement topographic-low between the Chakradharpur Granite and the Singhbhum Granite. Structural analysis reveals that the Ongarbira synform and other adjoining structures are kilometre-scale first-generation folds on E-W striking axial planes. These structures are in marked contrast with the corresponding kilometre-scale first-generation folds typical of the Iron Ore Group synclinorium towards the south of the Ongarbira belt. The Ongarbira metavolcanic rocks exhibit a limited range of tholeitic basalt compositions. The basalts are grouped according to their Mg# and the details of their REE patterns. Groups I and II are relatively unevolved and very similar to LREE- and LIL-depleted ocean ridge tholeiites. The most evolved Group III with LREE enrichment is most similar to LIL-enriched ocean floor basalts, but may be contaminated with crustal materials. In general, the Ongarbira volcanics evolve upward stratigraphically. Petrogenetic major element modelling of the Ongarbira basalts suggests that the primary melt was generated by batch melting (30%) of lherzolite at pressures of at least 1.5 GPa. Sequential fractionation of Ol, O1+Cpx+Pl and Cpx+Pl followed. Comparison of incompatible and compatible element distributions indicates a very similar batch m melting-fractional crystallization sequence for Ongarbira Groups I and II basalts. Group III basalts may have been derived from a batch melt initially more enriched in incompatible elements. Trace element discrimination of the Ongarbira basalts suggests that they were generated in an extensional environment. Primordial mantle normalized element distributions indicate that the basalts are similar to continental rift basalts but with oceanic affinities. The chemical data along with the associated sediments indicate that the Ongarbira basalts were probably emplaced in a mature continental rift, although a back-arc origin cannot be completely disregarded. It is concluded that the Ongarbira volcanics were emplaced as part of the same rifting event that resulted in outpourings of the Proterozoic Dalma and Dhanjori volcanics, implying that the Ongarbira suite is Proterozoic and not part of the Archaean Iron Ore Group. The structural geometry of the Ongarbira rocks is more conformable to that of the Singhbhum Mobile Belt than to the Iron Ore Group synclinorium in the Singhbhum Craton. It is likely that the southern limit of the Singhbhum Mobile Belt is defined by the interface of its E-W structures with the NNE structures of the Iron Ore Group rocks. Thus, the Singhbhum Shear Zone does not mark the interface between the Archaean Singhbhum Craton and the Proterozoic Singhbhum Mobile Belt in the eastern Indian Shield.

Brittle tectonics and pore-fluid conditions in the evolution of the Great Boundary Fault around Chittaurgarh, Northwestern India

Abstract This paper aims at reconstructing paleostress history and deciphering the pore-fluid pressure conditions during the reactivation of a regional-scale fault. Paleostress analyses of the mesoscopic structures suggest that three successive events of reactivation on the Great Boundary Fault occurred in thrust-type, strike-slip type and thrust-type tectonic-settings, respectively. Whereas the pore-fluid pressure was supralithostatic during the first and third events of reactivation, it was sublithostatic during the second event. Each event of reactivation induced a fluid pressure gradient, which resulted in the focused and enhanced flow of syntectonic fluids within the high permeability locales, primarily fractures and faults. Fluid inclusion study on strike-slip veins reveals that the syntectonic fluids were highly dense, Na–Ca–Cl brines of formational water origin. Stratigraphic evidence in favour of a 2-km-thick column of overburden above Kaimur sandstone beds implies that the strike-slip faulting occurred at 160–202 °C temperature and 53 MPa pressure. Variation in homogenization temperature reflects fluctuation in pore-fluid pressure during entrapment of syntectonic fluids and points to seismic pumping as a possible mechanism of fluid flow during faulting. High paleogeothermal gradient, obtained by fluid inclusion data, is ascribed to the high heat flow due to crustal stretching during the Proterozoic rifting, the basal and intermittent volcanism in the basin, and occurrence of Berach granite as the basement.

The kink-band triangle: a triangular plot for paleostress analysis from kink-bands

A kink-band can be graphically represented as a point on an equilateral triangle whose vertices define the angles between external foliation and kink plane, between internal foliation and kink plane, and between internal- and external-foliations. Four typical deformation paths that correspond to the four modes of kink-band growth can be discerned on this triangle. The three linear relationships between each of the kink-band angles and the inclination of the σ1-axis with respect to the unrotated layering can be transformed into a straight line on the triangular plot. Application of this plot in paleostress analysis is demonstrated by several examples. The method, however, yields best results when a large number of data on the kink-band angles are plotted and contoured on the triangular graph.

Late brittle tectonics in a Precambrian ductile belt: evidence from brittle structures in the Singhbhum Shear Zone, Eastern India

Abstract The Singhbhum Shear Zone, a 200 km long tectonic lineament, passes close to the boundary between the Archaean ‘Singhbhum craton’ and the Proterozoic ‘Singhbhum mobile belt’. Evidence from meso-structures reveals development of this shear zone in a ductile tectonic regime (1.6–2.2 Ga.) by thrusting of the northern block over the southern block along the N-dipping shear surface. A hitherto unrecognized, late brittle tectonic regime (≤1.6 Ga.) comprising two discrete phases is established from dynamic analysis of the fractures and veins in the shear zone. Episodic extension parallel and normal to the shear zone was a major consequence of this middle Proterozoic (≤1.6 Ga.) brittle tectonic episode, and was associated with the intrusion of a swarm of basic dykes.

A New Approach for Paleostress Analysis from Kink Bands: Application of Fault‐Slip Methods

Optimal and nonoptimal methods of fault‐slip analysis are tested on the simple type of kink bands by treating these structures as if they were striated faults. Results from different graphical and numerical techniques are mutually consistent and all imply development of these kink bands in a thrust regime as a consequence of NNW‐SSE‐oriented maximum compression ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Shock pressure estimates in target basalts of a pristine crater: A case study in the Lonar crater, India

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