Geological background
This section provides background on the major lithotectonic units and structures of the Garhwal transect between Helang and Malari along the Alakhnanda-Dhauliganga valley in the Uttarakhand state of northwest India (Fig. 1). The transect begins with the low grade metamorphic rocks of Lesser Himalayan Sequence (LHS) near Helang, extends northward through the Main Central Thrust Zone rocks, high grade metamorphic rocks of the Higher Himalayan Crystallines and finally to the South Tibetan Detachment System and unmetamorphosed sedimentary rocks of the Martoli formation near Malari. The Higher Himalayan Crystallines represent the highest grade, homoclinally north-east dipping metamorphic core of the Himalaya. This belt is bounded along its base by the Main Central Thrust (Heim and Gansser, 1939), now designated as the Munsiari Thrust in geological literature, and the South Tibetan Detachment System (STDS)/Trans-Himadri Fault (Valdiya, 1989, 2005; Valdiya and Pande, 2009) at the top. The mid crustal rocks of the HHC have been extruded out from beneath the Tibetan plateau and thrust southwards along the MCT (Beaumont et al., 2001, Jamieson et al., 2004) over the footwall of low grade metamorphosed and intensely foliated quartzite and phyllonite of the Lesser Himalayan Sequence near Helang. The STDS juxtaposes the unmetamorphosed sedimentary rocks of the Martoli Formation in the hanging wall with the sillimanite grade metamorphic rocks of the HHC near Malari. Based on the metamorphic grade and lithologies, the rocks of the HHC above Munsiari Thrust are divided into two groups: (1) the Munsiari Group in the lower parts and the (2) Vaikrita Group in the upper parts; separated by the Vaikrita Thrust (VT) (Gururajan and Chaudhuri, 1999).
The Main Central Thrust (MCT) was first recognized as the tectonic boundary between the monolithic northerly-dipping high grade High Himalayan Crystalline (HHC) Belt and the Proterozoic Lesser Himalayan (LH) sedimentary belt by Heim and Gansser (1939) and subsequently mapped by various workers along entire length of the Himalaya (Gansser, 1964; Hodges, 2000). Despite its occurrence across the Himalaya, precise identification and location of the MCT has been problematic in the field due to application of the following various criteria (see Searle et al., 2008 for review).
(i) Presence of the ductile sher zones with different kinematics across the whole HHC and its emplacement over the LH along the MCT (Jain and Manickavasagam, 1993; Hubbard, 1996; Davidson, 1997).
(ii) Identification of the MCT along a transition zone between the gneiss of the HHC and LH sediments (Valdiya, 1980; Pecher, 1989).
(iii) Distinct metamorphic break along the MCT (Vannay and Hodges, 1996; Valdiya, 1980).
(iv) Distinct isotopic signatures within the HHC (Ahmad et al., 2000).
Figure 1. Geological map of the Alaknanda-Dhauli Ganga Valleys< Uttarakhand Himalaya
(A) Geological map of the Higher Himalayan Crystalline (HHC) Belt along the Alaknanda-Dhauli Ganga Valleys. Sources: Authors observations and other published literature. (B) Orientation data of 138 main foliation Sm plotted Equal Area Projection using GEOrient. (C) Orientation data of 41 lineations developed on main foliation Sm, plotted on Equal Area Projection using GEOrient. Location of stops shown as latitude and longitude, and original field locations in bracket
Valdiya et al. (1999 and references therein) identified an outermost crustal-scale mylonitized sheared package in the Uttarakhand Himalaya, bounded by the Munsiari Thrust (MT) along its base and the Vaikrita Thrust (VT) at its top. He separated the lower sequence of the Munsiari Formation from the Vaikrita Group by the latter thrust, and recognized the Vaikrita Thrust as the actual MCT (also Ahmad et al., 2000).
It is worthwhile to note that we use the term “formation” (a mainly stratigraphic term) for the deformed and metamorphosed rocks of the HHC with references to the large amount of existing consolidated bibliography even if the more correct term should be “unit” or “metamorphic complex”.
Munsiari Group
Low- to medium- grade Munsiari Group is essentially made up of a highly imbricated and mylonitized Paleoproterozoic low to medium grade Munsiari Group rocks of dominant mylonitic augen gneiss [207Pb/206Pb zircon age of 1848 ± 5 Ma from Jharkula ± about 10 km west of Joshimath on main Rishikesh-Joshimath road and 1830 ± 6 Ma age from Tapovan (Wiedenbeck et al., 2014; Stop 8)] and fine grained biotite-rich gneiss, garnetiferous mica schist, phyllonite, sheared amphibolite, and a persistent imbricated horizon of sericite-bearing sheared and foliated quartzite; the latter bears a very strong lithological resemblance with the Lesser Himalayan Berinag Group quartzite. On the whole the rocks of this group display intense penetrative shearing. It is likely to have been incorporated within the basement Munsiari-type rocks due to imbricated Lesser Himalayan rocks. The pelitic samples within the Munsiari Group have mineral assemblage: quartz + biotite + plagioclase ± garnet ± staurolite + muscovite ± graphite ± andalusite (Spencer et al., 2012), while amphibolitic bodies have zoned calcic amphibole, plagioclase, quartz, biotite and minor epidote, opaque, chlorite and calcite. “Peak” pressure-temperature (P-T) conditions recorded by pelitic rocks increase structurally upward from 500 °C/0.5 GPa up to c. 580 °C/1.1 GPa (Spencer et al., 2012). According to Célérier et al. (2009) the Munsiari Group rocks have cooled to the white mica Ar-Ar closure temperature (350 ± 50 °C) at c. 9 Ma.
Vaikrita Group
The overlying Vaikrita Group sequence of the HHC within the lower and upper amphibolite facies is divided into three formations between the VT and the STDS: (1) the Joshimath Formation, (2) the Suraithota Formation, and (3) the Bhapkund Formation.
The Joshimath Formation comprises garnet-biotite-muscovite schist and psammitic/pelitic gneiss. Numerous quartz veins are seen within this unit, many of which have been isoclinaly folded. Geothermobarometric data by Spencer et al. (2012) indicated highest “Peak P-T” conditions with respect to the previous formation with temperatures in spanning from c. 700 °C up to 830 °C and pressure of 1.0 to 1.4 GPa”.
The Suraithota Formation comprises kyanite-garnet-biotite schist/psammitic gneiss and quartzites with thin amphibolite intercalations. Depositional features such as cross-beds are seen in less deformed rocks and indicate that many parts of this formation have been overturned. Pressure-Temperature values reported in the literature for this formation (referred by the previous authors as the Pandukeshwar formation) are in the range of c. 800-860 °C and c. 1.4 GP (Spencer et al., 2012).
The Bhapkund Formation comprises sillimanite (fibrolite)-garnet-biotite psammitic gneiss/schist, small tourmaline-rich leucogranite lenses/dykes and the Malari leucogranite body. Migmatization is pervasive within this formation. Phase equilibria constraints and geothermobarometry calculations by Spencer et al. (2012) reveal a pressure decreases from 1.05 GP to 0.85 GP towards the STDS in the the Bhapkund Formation (referred as the Badrinath formation), while the temperature is roughly constant (810-860 °C). A clear metamorphic break is present between the high-grade rocks of the Bhapkund Formation in the footwall of the STDS and the hanging wall low-grade to unmetamorphosed rocks of the Martoli Formation, where the maximum T of c. 450 °C has been qualitative estimated using phase equilibria arguments by Sachan et al. (2010).
Within the Vaikrita Group, metamorphic grade increases up the structural section (Jain et al., 2013), a phenomena called inverted metamorphism. This places the sillimanite–K-feldspar gneiss and migmatite of the Bhapkund Formation, which were metamorphosed under amphibolite facies at >650 °C, at the top (Sachan et al., 2010). Overall, the HHC trends NW-SE in this valley with dips of the main foliation Sm towards NE- to E. Various shear sense indicators proliferate between Helang and Malari. Asymmetric structures such as S-C and S-C’ fabric, boudins, sigma structures, folds etc. provide valuable information regarding direction of tectonic transport. Top-to-south or southwest shear sense indicators are observed within the lower and middle parts of the HHC and top-to-north or northeast shear sense indicators are observed in the upper parts near the STDS.
Tethyan Himalayan Sequence
Martoli Formation
The basal Tethyan Himalayan Sequence (THS) is best exposed around the Malari village as the Martoli Formation, which dominantly contains greyish green slate and quartzite. Overall dip of the Martoli is towards N-to-NE. Other Paleo-Mesozoic formations of this sequence can be observed along the upper reaches of Dgauli Ganga and Birthi Rives.