Volume 104, Issue 3
September 1995, pages 1-537
pp 1- September 1995
pp 327-347 September 1995
The supracrustal enclave within the Peninsular Gneiss in the Honakere arm of the Chitradurga-Karighatta belt comprises tremolite-chlorite schists within which occur two bands of quartzite coalescing east of Jakkanahalli(12°39′N; 76°41′E), with an amphibolite band in the core. Very tight to isoclinal mesoscopic folds on compositional bands cut across in the hinge zones by an axial planar schistosity, and the nearly orthogonal relation between compositional bands and this schistosity at the termination of the tremolite-chlorite schist band near Javanahalli, points to the presence of a hinge of a large-scale, isoclinal early fold (F1). That the map pattern, with an NNE-plunging upright antiform and a complementary synform of macroscopic scale, traces folds 'er generation (F2),is proved by the varying attitude of both compositional bands (S0) and axial pranar schistosity (S1), which are effectively parallel in a major part of the area. A crenulation cleavage (S2) has developed parallel to the axial planes of theF2 folds at places. TheF2 folds range usually from open to rarely isoclinal style, with theF1 andF2 axes nearly parallel. Evidence of type 3 fold interference is also provided by the map pattern of a quartzite band in the Borikoppalu area to the north, coupled with younging directions from current bedding andS0-S1 inter-relation.
Although statistically theF1 andF2 linear structures have the same orientation, detailed studies of outcrops and hand specimens indicate that the two may make as high an angle as 90°. Usually, in these instances, theF1 lineations are unreliable around theF2 axes, implying that theF2 folding was by flexural slip. In zones with very tight to almost isoclinalF2 folding, however, buckling attendant with flattening has caused a spread of theF1 lineations almost in a plane. Initial divergence in orientation of theF1 lineations due to extreme flattening duringF1 folding has also resulted in a variation in the angle between theF1 andF2lineations in some instances. Upright later folding (F3) with nearly E-W strike of axial planes has led to warps on schistosity, plunge reversals of theF1 andF2 axes, and increase in the angle between theF1 andF2 lineations at some places. Large-scale mapping in the Borikoppalu sector, where the supposed Sargur rocks with ENE ‘trend’ abut against the N-‘trending’ rocks of the Dharwar Supergroup, shows a continuity of rock formations and structures across the hinge of a large-scaleF2 fold. This observation renders the notion, that there is an angular unconformity here between the rocks of the Sargur Group and the Dharwar Supergroup, untenable.
pp 349-371 September 1995
The lead-zinc bearing Proterozoic rocks of Zawar, Rajasthan, show classic development of small-scale structures resulting from superposed folding and ductile shearing. The most penetrative deformation structure noted in the rocks is a schistosity (S1) axial planar to a phase of isoclinal folding (F1). The lineations which parallel the hinges ofF1 folds are deformed by a set of folds (F2) having vertical or very steep axial planes. At many places a crenulation cleavage (S2) has developed subparallel to the axial planes ofF2 folds, particularly in the psammopelitic rocks. The plunge and trend ofF2 folds vary widely over the area.
Deformation ofF2 folds into hook-shaped geometry and development of another set of axial planar crenulation cleavage are the main imprints of the third generation folds (F3) in the region. In addition to these, there are at least two other sets of cleavage planes with corresponding folds in small scales. More common among these is a set of recumbent and reclined folds (F4), developed on steeply dipping early-formed planes. Kink bands and associated sharp-hinged folds represent the other set (F5).
Two major refolded folds are recognizable in the map pattern of the Zawar mineralised belt. The larger of the two, the Main Zawar Fold (MZF), shows a broad hook-shaped geometry. The other large-scale structure is the Zawarmala fold, lying south-west of the MZF. Both the major structures show truncation of lithological units along their respective east ‘limbs’, and extreme variation in the width of formations. The MZF is primarily the result of superimposition ofF3 onF2.F1 folds are relatively smaller in scale and are recognizable in the quartzite unit which responded to deformation mainly by buckle shortening. Large-scale pinching-and-swelling that appears in the outcrop pattern seems to be a pre-F2 feature.
The structural evolutionary model worked out to explain the chronology of the deformational features and the large-scale out-crop pattern envisages extreme east-west shortening following formation ofF1 structures, resulting in the formation of tight and isoclinal antiforms (F2) with pinched-in synforms in between. These latter zones evolved into a number of ductile shear zones (DSZs). The east-west refolding of the large-scaleF2 isoclinal antiforms seems to be the consequence of a continuous deformation and resultant migration of folds along the DSZs. The main shear zone which wraps the Zawar folds followed a curved path.
Because of the penetrative nature of theF2 movement, the early lineations which were at high angles to the later ones (as is evident in the west of Zawarmala), became subparallel to the trend ofF2 folding over a large part of the area. Further, the virtually coaxial nature ofF2 andF3 folds and the refolding ofF3 folds by a new set of N-S folds is an indication of continuous progressive deformation.
pp 373-383 September 1995
During the refolding of an early non-isoclinal fold (say,F1) we may find an offset or side-stepping of the axial surfaces of the later folds (say,F2). The offsets can be seen in both type 2 and type 3 interference patterns. An analysis of the shear fold model shows that there is a maximum limit for the magnitude of side-stepping. The side-stepping is larger for larger interlimb angles ofF1. It decreases with progressive tightening ofF2. By recognizing such side-stepping we can predict on which side the F1 hinge should lie even if the hinge is unexposed or lies outside the domain of observation. The general rule for the sense of side-stepping is the same for shear folds, flexural slip folds and buckling folds. However, the side-stepping in buckling folds should be used with caution, sinceF2 folds on buckled single-layers may show an offset whose sense is opposite to that predicted by the general rule.
pp 385-405 September 1995
The rocks within the Singhbhum shear zone in the North Singhbhum fold belt, eastern India, form a tectonic melange comprising granitic mylonite, quartz-mica phyllonite, quartz-tourmaline rock and deformed volcanic and volcaniclastic rocks. The granitic rocks show a textural gradation from the least-deformed variety having coarse-to medium-grained granitoid texture through augen-bearing protomylonite and mylonite to ultramylonite. Both type I and type II S-C mylonites are present. The most intensely deformed varieties include ultramylonite. The phyllosilicate-bearing supracrustal rocks are converted to phyllonites. The different minerals exhibit a variety of crystal plastic deformation features. Generation of successive sets of mylonitic foliation, folding of the earlier sets and their truncation by the later ones results from the progressive shearing movement. The shear sense indicators suggest a thrust-type deformation. The microstructural and textural evolution of the rocks took place in an environment of relatively low temperature, dislocation creep accompanied by dynamic recovery and dynamic recrystallization being the principal deformation mechanisms. Palaeostress estimation suggests a flow stress within the range of 50–190 MPa during mylonitization.
pp 407-417 September 1995
Recrystallized grain size was measured from quartzite mylonite specimens collected from parts of Singhbhum shear zone in eastern India. The specimens were collected along five traverses (Mushabani, Pathargora, Surda, Rakha and Jadugoda) across the elongation of the shear zone. The sheared quartzites range from protomylonite through mylonite to ultramylonite. The microstructural studies of the specimens reflect that dynamic recrystallization was the main deformation process. Estimation of flow stresses were derived from these specimens using empirical equations relating to flow stress and recrystallized grain size. The calculated stresses range from 12–28 MPa (Mercieret al 1977), 23–49 MPa (Twiss 1977), 20–68 MPa (Christie and Ord 1980), considering all the traverses. The results show that these values can only be used semiquantitatively.
pp 419-431 September 1995
In low temperature deformation of polymineralic rocks the constituent minerals often show contrasting deformation mechanisms. In naturally deformed arkoses, feldspathic quartzites and grits under greenschist to almandine-amphibolite fades condition, feldspar deforms by microboudinage (rigid-brittle behaviour), while quartz flows by a combination of dislocation creep, pressure solution and solution transfer. Boudin segments develop and separate in a phased sequential manner while quartz matrix flows in a ductile manner, indicating a brittle-ductile toggle during progressive deformation.
Both the pressure solution and dislocation creep flows are volume-conservative. Therefore, a net volume increase during the above deformations is a necessity, unless compensated by a solution-transfer process. Hydrofracturing probably played a role in microboudinage formation as the ambient level of differential stress is estimated to be low around 45–75 MPa.
To develop a synthetic flow law for the above type of deformation in arkoses, one needs to consider the significance of different rate-controlling mechanisms. As feldspar pull-aparts are syntectonically filled with quartz or metamorphic minerals crystallizing during progressive deformation, successive microboudin segmentation will depend on how fast/slow the matrix quartz moves to the open crack or the sealing takes place by transfer of appropriate solute components by pressure solution or solution transfer, the real rate-controlling process.
pp 433-446 September 1995
Drag patterns of foliation are graphically constructed around very competent dykes under bulk strain of pure shear, simple shear and a combination of pure shear and simple shear. Four different types of drag patterns may be produced, depending on the nature of the bulk deformation and the initial orientations of the dyke and the foliation. The drag pattern can be symmetric or asymmetric, inward curving or outward curving. Both the magnitude and the sense of drag may vary along a dyke wall. A uniform sense of drag develops all along a dyke wall only in certain special situations. The type of foliation drag near a dyke may give us a rough idea of the nature of bulk deformation and the relative orientations of the dyke and the foliation with respect to the bulk strain axes.
pp 447-451 September 1995
The internal fabric of a deformed rock represents the state of finite strain. In some special cases the fabrics also record the strain history of the deformed body. These special cases can profitably be utilized to compare the predictions of dynamic models and strain paths in natural deformations. In this contribution, the concept of deformation path in the study of ductile shear zones has been demonstrated.
pp 453-464 September 1995
The solution of stress distribution for a multicrack system and model experiments confirm that en echelon cracks mutually interact with each other during their growth. Such a mechanical interaction deviates the crack-tip stress axes orientations from that of the bulk stress field and leads to a continuous change in propagation direction of tension cracks, initially at a right angle to the bulk tension direction. The sigmoidal shape of en echelon fractures evolve through rotation and crack length increments with changing orientations. The theoretical analysis shows that the instantaneous fracture-tip stress orientation is a function of initial crack spacing, orientation of crack array with respect to the principal axes of far-field stress.
pp 465-488 September 1995
This paper attempts to show that the quartz reefs forming the principal fabric of a large number of granitoid diapirs in the central Indian Bundelkhand batholith are dominated by numerous sigmoidal tensile fissures generated as a result of an EW sinistral brittle-ductile inhomogeneous simple shear but the secretion of quartz veins within the reefs occurred under an approximately EW subhorizontal extension. The paper also discusses the disruption along the reefs and of the reefs themselves by later faulting under a rotational subhorizontal or gently plunging maximum principal compressive stress that seems to be intimately related to the diapinc rise of more and more acidic magmas at relatively deeper levels within the crust, the relationship with the deformation within the supracrustals and suggests that the overall prolate strain within the diapirs was built up gradually, with initial NE-SW shortening, followed by meridional shortening and finally culminating into the NW-SE shortening that opened a large number of tensile cracks occupied by late dyke swarms. The slight reverse component of strike-slip faults, of both dextral and sinistral, corroborates the overall constriction at the centres of individual diapirs at depth. The palaeostress analysis using slickensided striae corroborates the general conclusions presented. Indeed, the paper tries to demonstrate that the prolate strain in the central part of a diapir is not something that occurs simultaneously from all sides radially inwards but is a phased one. It depends upon the size and shape of the initial diapir, and the part of the supracrustals in which the deformation begins first and is controlled by pre-existing planes of weakness. The paper tries to demonstrate how the prolate strain at the centres of granitoid diapirs might express itself at higher crustal levels under a relatively less ductile or brittle-ductile and even brittle regime.
pp 489-498 September 1995
Experiments on extensional faulting were performed with semi-brittle talc-sand beds resting on a ductile clay base. The experiments show that the development of graben in the talc-sand beds is controlled by the deformation in the ductile basement. Graben-like structures form only when there is a non-uniform stretching in the basement. Uniform extension at the basement level fails to produce any such structures. Grabens initiate as large synclinal structures (sag). The sag is generated either by a downward flexing of the talc-sand bed on a ductile basement or by non ****-uniform thinning of beds. Listric master faults bounding the grabens intersect the basement at high angles. The master faults that initiate as curved shear planes rotate further with continued extension. At the initial stage, the graben structures are associated with normal drags, and with progressive deformation, drag patterns change from normal to a reverse one.
pp 499-508 September 1995
Normal faults on mesoscopic scale are observed in the Panjal Thrust Zone in the Dalhousie area of western Htmachal. The boundary between the southern margin of the Higher Himalaya Crystalline (HHC) of Zanskar and the Chamba syncline sequence is also described as a normal fault, referred to as Bhadarwah Normal Fault in the Bhadarwah area of Doda district on the basis of field mapping and shear sense criteria using S-C fabric and porphyroblast rotation. The occurrence of these normal faults suggests that the extensional tectonic regime was not restricted only to the Zanskar shear zone area but that it also occurs south of the Higher Himalayan range. This suggests NE-directed subhorizontal extension and exhumation of deeper level rocks of Higher Himalaya Crystallines.
pp 509-521 September 1995
In the Lesser Garhwal Himalaya, the North Almora Thrust separates the overlying medium-grade Dudatoli-Almora crystallines of Precambrian age from the unmetamorphosed to partly metamorphosed rocks of the Garhwal Group of Late Precambrian age. The crystalline nappe sheet consists of flaggy to schistose quartzites, granite gneisses and garnetiferous mica schist members in an ascending order. In different localities. different members of the Dudatoli-Almora crystallines are exposed along the thrust plane. Southwest of Adbadri fine-grained mylonitized schistose quartzites of Dudatoli-AImora crystallines are in contact with the underlying metabasites of the Garhwal Group. The mylonitized schistose quartzites consist of alternating thick (1 to 2m) quartzite and thin (10 to 20cm) micaceous quartzite bands. The micaceous quartzites can be further differentiated into alternating quartz-rich (0-5 to 2.0 cm thick) and mica-rich (0.2 to 1.0 cm thick) layers. In the quartzites the C-surfaces are parallel to the S-surfaces defined by the alternating quartz-rich and mica-rich layers. Further, the S-surfaces exhibit almost similar folds with multiple wavelengths where the axial planes are nearly parallel and enveloping surfaces are oblique to the lithological layering. The evolution of these folds has been envisaged in three phases of deformation on the basis of field evidence, fold geometry and microstructures.
During the first phase buckle folds (F1) developed in thin micaceous quartzite layers. whereas thick quartzite bands underwent only layer parallel shortening. During the second phase the stress orientation changed and the limbs ofF1 folds were folded (F2). During the third phase of deformation which coincided with thrusting, the rocks were sheared, mylonitized and developed microstructures exhibiting dynamic recrystallization by the processes of subgrain rotation, and continual and discontinuai grain boundary migration. This phase was also responsible for the development of C-surfaces parallel to the lithological layering. Further, in the folded micaceous quartzite layers shearing resulted in the development of C-surfaces parallel to the axial planes ofF2 folds.
pp 523-537 September 1995
The Dating rocks and Darjeeling gneisses, which constitute the Sikkim dome in eastern Himalaya, as well as the Gondwana and Buxa rocks of ‘Rangit Window’, disclose strikingly similar sequences of deformation and metamorphism. The structures in all the rocks belong to two generations.
The structures of early generation are long-limbed, tight near-isoclinal folds which are often intrafolial and rootless. These intrafolial folds are associated with co-planar tight folds with variably oriented axes and sheath folds with arcuate hinges. Penetrative axial plane cleavage and mineral lineation are related structures; transposition of bedding is remarkable. This early phase of deformation (D1) is accompanied by constructive metamorphism. The structures of later generation are open, asymmetrical or polyclinal; a crenulation cleavage or discrete fracture may occur. The structures of early generation are distorted by folds of later generation and recrystallized minerals are cataclastically deformed. Recrystallization is meagre or absent during the later phase of deformation (D2).
The present discussion is on structures of early generation and strain environment during theD1 phase of deformation. The concentration of intrafolial folds in the vicinity of ductile shear zones and decollement or detachment surface (often described as ‘thrust’) may be considered in this context. The rocks of Darjeeling-Sikkim Himalaya display minor structures other than intrafolial folds and variably oriented co-planar folds. The state of finite strain in the rocks, as observed from features like flattened grains and pebbles, ptygmatic folds and boudinaged folds indicate combination of flattening and constrictional type strain. The significance of the intrafolial folds in the same rocks is discussed to probe the environment of strain during progressive deformation (D1).