Djordje Grujic

Ductile extrusion of the Higher Himalayan Crystalline in Bhutan: evidence from quartz microfabrics

Quartz textures measured from deformed quartz tectonites within the Lesser Himalaya and Higher Himalaya Crystalline of Bhutan show similar patterns. Orientation and distribution of the quartz crystallographic axes were used to confirm the regional shear sense: the asymmetry of c-axis and a-axis patterns consistently indicates top-to-the-south shearing. The obliquity of the texture and the inferred finite strain (plane strain to moderately constrictional), suggest the strain regime had a combination of rotational and irrotational strain path. In most of the samples from the Bhutan Himalaya, the inferred deformation mechanisms suggest moderate- to high-temperature conditions of deformation that produced the observed crystallographic preferred orientation. Much higher temperature of deformation is indicated in the quartz veins from a leucogranite. The observed ductile deformation is pervasively developed in the rocks throughout the investigated area. The intensity of deformation increases only slightly in the vicinity of the Main Central Thrust. Simultaneous southward shearing within a large part of the Higher Himalaya Crystalline near and above the Main Central Thrust and normal faulting across the South Tibetan Detachment, is explained by the tectonically induced extrusion of a ductily deforming wedge. The process of extrusive flow suggested here can be approximated quantitatively by channel flow models that have been used to describe subduction zone processes. Channel flow accounts for some observed phenomena in the Himalayan orogen such as inverted metamorphic sequences near the Main Central thrust, not related to an inversion of isotherms, and the syntectonic emplacement of leucogranites into the extruding wedge, locally leading to an inversion of isotherms due to heat advection.

The influence of initial fold geometry on Type 1 and Type 2 interference patterns: an experimental approach

Abstract Experiments on superposed folding were performed in a plane strain-pure shear rig using paraffin wax as an analogue for rocks. A series of pre-formed cylindrical folds were refolded with the compression direction acting parallel to the initial fold hinge direction, the intermediate axis perpendicular to the first fold axial plane and the extension direction parallel to the first fold axial plane and perpendicular to its axis (Type 1 interference geometry). Roundness, tightness and amplitude were varied to investigate the influence of first-fold geometry on the interference patterns. Experimental folding of an initially planar layer oriented parallel to the first-fold envelope provides a reference geometry for comparison. The results suggest that hinge roundness has less influence on the interference pattern than other factors. Close initial folds refold into Type 2 interference patterns, which become even more pronounced as the tightness increases; open folds produce Type 1 interference patterns. Folds with the same interlimb angle and the same roundness produce different interference patterns depending on relative amplitude: folds with relatively large amplitude are refolded into Type 2 patterns, whereas folds with small amplitude give clear dome and basin structures. In a second set of experiments, the compression direction also acted parallel to the first-fold hinge, but the orientations of the intermediate and extension axes were interchanged. The interference patterns obtained are very similar to those in Type 1 interference geometry, suggesting that the major factor in determining whether Type 1 or Type 2 interference patterns develop is the initial fold geometry and not the kinematics of the second deformation.

MELT-BEARING SHEAR ZONES : ANALOGUE EXPERIMENTS AND COMPARISON WITH EXAMPLES FROM SOUTHERN MADAGASCAR

Abstract Analogue model experiments were conducted to investigate the influence of irregularly distributed weak sites in localising strain, as an aid to understanding shear zone development in partially molten rocks. The very weak inclusions consisted of Vaseline in a homogeneous matrix of paraffin wax, which has a power-law viscous rheology. Boundary conditions were those of pure shear at constant natural strain rate and confining stress σ3. The inclusions were initially perfect cylinders with axes parallel to the intermediate bulk strain axis Y. Conjugate shear zones nucleate on the inclusions and link up to form an anastomosing pattern of high strain zones of concentrated shear surrounding much more weakly deformed pods of near coaxial strain. The zones initiate at angles near 45° to the bulk shortening axis Z but stretch and rotate towards the X axis with increasing bulk strain. All inclusions nucleate shear zones, so that with increasing development of the anastomosing pattern, weak material occurs only within the high strain zones. The restriction of migmatite leucosomes to shear zones in natural examples could also reflect a corresponding control of melt on the sites of shear zone nucleation, rather than implying accumulation from the surrounding wall-rock. The model geometry is very similar to that observed in small-scale shear zones in migmatites of southern Madagascar. Elongate zones rich in weak inclusions, originally either perpendicular or at 45° to the Z axis, were also modelled for direct comparison with the regional-scale geometry of the Pan-African high-grade ‘shear zones’ on Madagascar.

Folds with axes parallel to the extension direction: an experimental study

Experimental studies of single- and multilayer folding have generally considered shortening of layers oriented perpendicular to the maximum extension direction X (i.e. layers parallel to YZ), or in a more limited number of cases, oblique layers still containing the intermediate Y axis. Few experimental studies have considered the case where the extension direction X lies within the layer itself, although in nature folds with axes parallel to X are quite commonly seen. These folds have often been ascribed to passive rotation of fold axes during continued shear, but it has been shown both theoretically and experimentally that active buckle folds can also develop with axes parallel to X. Single- and multilayer analogue model experiments were performed on planar layers oriented initially perpendicular to the intermediate Y axis, and with the extension direction X lying within the layer itself. All experiments were conducted in plane strain—either in pure shear or simple shear. Paraffin waxes of different melting ranges were used as analogues for rocks with a power-law rheology (stress exponent around 2–3). With a viscosity ratio of ca. 30:1, no measurable fold amplification was discernible for shortening of 36% or shear strain of 3.6. Neither domed initial perturbations with circular sections parallel to the layer nor cylindrical perturbations elongate parallel to the initial stretching direction were significantly amplified. Only at much higher viscosity ratios (ca. 600:1) did active buckle folding develop. This folding at high viscosity ratio was associated with flow of the matrix in the X direction around the layer, developing a strong linear fabric parallel to X in the matrix immediately adjacent to the layer. The development of this flow discontinuity between matrix and layer may be characteristic of active buckling of layers parallel to XZ, with fold axes parallel to X.

Shape and structure of (analogue models of) refolded layers

The visualisation of the intricate three-dimensional shapes that arise in non-coaxial refolding is an ongoing problem in the study of multiply deformed terrains. Such complex forms also allow space for subjective interpretation of folding sequences. Analogue models offer a solution to the problem of visualisation because the entire surface of a refolded layer can be examined. In addition, analogue models may permit a quantitative geometric study by digitising models in three dimensions. This paper will examine a method for geometric analysis of multiply folded surfaces. We used the results of analogue experiments with paraffin wax where pre-formed single layer folds were deformed to produce dome-basin and dome-crescent-mushroom interference patterns. The surfaces of buckled layers in these analogue models were digitised to produce virtual models from which accurate attitude measurements of layers and fold hinges were made. To achieve three-dimensional digitising we developed an optical triangulation system based on structured light measurement by the double-scan technique. We applied standard analytical tools used by structural geologists for the analysis of the obtained data and we compared experimental structures with natural examples.