Observations
Though top-to-SW shear sense indicators are very common within the HHC, kinematic indicators having only the top – to – SW shear sense are observed within the Munsiari and Joshimath Formations and continue till the upper parts of the Suraithota Formation. Structures showing top – to – NE shear sense first start appearing in the middle parts of the Suraithota Formation somewhere between Suraithota and Tamak at Location I (N30034’12”: E79044’56”) (Fig. 33). However, shear structures showing the top – to – SW structures remain the dominant shear fabric and are prominent up to Tamak village. Between Tamak (Location II ─ N30035’49”:E79047’31”) and the next village Juma, the zone contains many folds and flow structures along with the first appearance of leucogranites. However, shear sense indicators are absent throughout this narrow zone. After crossing Juma (Location III – N30036’7”:E79048’18”), kinematic indicators with top – to – NE shear sense become dominant with the first appearance of migmatite. A large migmatite zone is observed after crossing the next village Jelam. The migmatites are ubiquitously distributed up to Malari but they are observed to be dominant only in the upper parts of the Bhapkund Formation. A few relict structures of top – to – SW shear sense are also observed along with the dominant top – to – NE shear fabric between Bhapkund and Malari. Beyond Malari (Location IV ─ N30040’52”:E79053’20”), i.e. in the Martoli Formation, we observed exclusively top – to – NE downward shear sense indicators.
On the basis of our field observations and analysis of the shear sense indicators, the whole HHC has been subdivided into the following five structural zones between Helang and Ghamsali – Niti, including parts of the LHS and the THS (Fig. 33). The above classification is based on our shear sense analysis and the simple fact that older phase ductile shear fabric (DS1) or the top – to – SW shear sense indicators represents a compressional/contractional environment, while the younger ductile shear phase fabric (DS2) or the top – to – NE shear sense indicators represent an extensional environment. These zones have been mapped as A to E in Figure 33, while observational points of their changes in the field are indicated as I to IV.
(A) Pure contractional zone (PCZ), exhibiting only the DS1 top – to – SW shear phase fabric.
(B) Dominant contractional zone (DCZ), DS2 movement planes with top – to – NE shear sense make their first appearance, but the DS1 fabric remains dominant.
(C) Transition zone (TZ) having no DS1 and DS2 shear sense indicators, but is marked by considerable rock flowage.
Figure 33. Distribution of contractional DS1and extensional DS2 phases in the HHC and adjoining units
Geological map of the Alaknanda and Dhauli Ganga Valleys showing distribution of contractional DS1and extensional DS2 phases of shear structures and their transitions (red arrows) on the basis of shear sense analysis, Uttarakhand Himalaya. Black arrows represent lithological boundaries within the Vaikrita Group. JF Joshimath Formation. SF Suraithota Formation. BF Bhapkund Formation. Note domain boundaries of the DS1and DS2 shear structures and their transitions transgress the lithological boundaries. Change of domains indicated by I to IV. Locations of photographs used in this paper are also shown
(D) Dominant extensional zone (DEZ), in which the DS2 phase shear fabric is dominant over the DS1 phase shear fabric, and
(E) Extensional zone (EZ) with only the DS2 phase shear fabric within the THS.
According to the above classification, the PCZ is the zone from the upper parts of the LHS up to the middle parts of the Suraithota Formation; DCZ is the zone accommodating the upper parts of Suraithota Formation up to the village Tamak. This zone extends up to approximately 18 km (map distance) from the Vaikrita Thrust (VT); TZ is the zone between the villages Tamak and Juma. It starts at a distance of 18 km approximately from the VT and is around 2 km wide; is the zone starting after the village Juma up to Malari. This zone accommodates upper parts of the Suraithota Formation and the entire Bhapkund Formation. It starts at a distance of 20 km from the VT; EZ is the zone which accommodates the entire Martoli Formation and starts at a distance of about 30 km from the VT (Fig. 33).
Observation of numerous shear sense indicators along these valleys revealed two phases DS1 and DS2 of ductile shear deformation. The later phase DS2 showing top – to – NE downward shear sense was superposed over the older phase DS1 showing top – to – SW upward shear sense. The DS1 indicates that the HHC has been deformed within a broad non-coaxial ductile shear zone of the overthrust type with a consistent top – to – SW sense of movement. It also suggests an early deformation history of the HHC involving large scale SW – directed ductile shearing within the HHC belt, associated with the India – Asia continental convergence and compressional tectonics (Jain and Manickavasagam, 1993; Patel et al., 1993). Though it is difficult to envisage precise relationship between the ductile shearing within the HHC and its movement along the MCT, it is likely that the latter represents a zone of very high ductile strain. The STDS, on the contrary, is associated with extensional deformation.
The present work highlights the distribution pattern of various kinematic indicators along the Alaknanda – Dhauli Ganga section and is useful in understanding the evolution of the Himalayan metamorphics within a framework of various recent tectonic models like (i) ductile shearing (Jain and Manickavasagam, 1993), (ii) channel flow (Beaumont et al., 2001, 2006) and (iii) critical wedge/extrusion (Grasemann, et al., 1999; Webb et al., 2007). The ductile shear model postulates consistent top – to – SW thrust shear sense from various kinematic indicators within a broad ductile shear zone, the HHC belt, where mm- to dm-scale ductile shearing along S-C shear fabric caused the inverted metamorphism and decompression melting in the upper parts (Jain and Manickavasagam, 1993); top – to – NE shear sense near its upper margin remained not so well explained. The channel flow model and its different variants visualizes either (i) a pure Couette (or linear) flow between rigid plates where these move relative to one another and produce uniform simple shear across the channel, (ii) a pure Poiseuille (or parabolic) flow between stationary rigid plates where horizontal gradients in lithostatic pressure and frictional resistance along channel boundaries produce greatest velocities in its center with decreasing velocities toward the margins, leading to development of opposing shear senses, or (iii) a combination of the two (Grujic, 2006; Godin et al., 2006 and references therein; Langille et al., 2010). The critical wedge/extrusion models postulate southward extruding metamorphic belt, bounded by the MCT and the STDS at the base and the top, respectively (Burchfiel et al., 1992; Langille et al., 2010).
Shear sense indicators from the HHC belt of the Alaknanda and Dhauli Ganga Valleys could be explained by a combination of the pure Couette (or linear) flow and pure Poiseuille (or parabolic) flow within either a ductile shear zone or a channel, bounded by the MCT and the STDS where top – to – SW shear indicators are superposed by the top – to – NE directed structures over a wide zone near the STDS, assuming a contemporaneous activity of the MCT and STDS. The Couette (or linear) flow possibly provided top – to – SW shearing throughout the shear zone/channel to start with, and was subsequently superposed by the Poiseuille (or parabolic) flow at a later stage; both the flows would remain indistinguishable from each other in the lower parts of the shear zone/channel. It is only after crossing the Transition Zone that the structures of the Poiseuille flow will start showing in the shear zone/channel due to their opposite vergence. However in Western Nepal, the contemporaneous activity of MCT and STDS has beeen discussed by Carosi et al. (2013) and Montomoli et al. (2013). Moreover, Montomoli et al. (2013) demonstrated that ductile shearing within the HHC was not contemporaneous but becomes younger from north to south, where it is localized along discrete shear zones.