Year

2021

Degree Name

Doctor of Philosophy

Department

School of Earth, Atmospheric and Life Sciences

Abstract

Unravelling the tectonic processes responsible for the uplift of the Himalaya requires in-depth investigation of individual terranes caught up along the Indus Suture Zone that separates the continents of India and Eurasia. The Himalayan Orogen represent the complex amalgamation of, not only the two colliding continents with one plate subducting underneath the other, but also fragments of oceanic crust (ophiolites) and intraoceanic arcs that developed within the vast expanse of the now extinct Neotethyan Ocean. The aim of this thesis was to resolve the tectonic evolution of the Dras-Nindam terrane in the Ladakh Himalaya using standard geological mapping, geochemistry and sophisticated zircon U-Pb geochronology. The Dras-Nindam terrane is sandwiched between Indian and Eurasian continental crust along the Indus Suture Zone in the Ladakh Himalaya, NW India. This tectonostratigraphic unit includes basaltic to andesitic rocks of the Dras Volcanics and associated deep marine, volcaniclastic rocks of the Nindam Formation. Debates exist as to whether this terrane developed as a marginal forearc basin to the larger Ladakh Arc which developed along the southern margin of Eurasia above a single subduction zone or whether it represents a juvenile, intraoceanic island arc that developed above a separate subduction zone much further south within the Neotethyan Ocean. A lack of data relating to the provenance and age-range of the Dras-Nindam terrane further complicates hypotheses related to its early development. Its collision with either India or Eurasia prior to final continent-continent collision was also unresolved, leading to multiple conflicting tectonic reconstructions that required further testing. The increased accessibility to zircon U-Pb dating and easier access to key field localities has provided an opportunity to better constrain the tectonic evolution of a key unit along the Indus Suture Zone that is integral to developing more accurate reconstructions of the world’s largest mountain range.

The overarching aim of this investigation is to test competing hypotheses regarding the tectonic evolution of the Dras-Nindam terrane that occurs along the Indus Suture Zone of the India-Eurasia collision in the Ladakh Himalaya, NW India. This project addresses key tectonic questions by providing robust, new observations, descriptions and geochronology of the Dras-Nindam terrane in Ladakh, using an integrated approach including field relations, petrography, whole rock geochemistry and zircon U-Pb geochronology. The objectives of this investigation are, (i) to determine the maximum age of deposition and provenance of the forearc basin Nindam Formation using detrital zircon U-Pb geochronology to ascertain whether the Dras Arc (a component of the Dras-Nindam terrane) initiated in an intraoceanic setting or was continental-related; (ii) to constrain the timing of initiation and early evolution of the Dras Arc using zircon U-Pb geochronology of extrusive (Dras Volcanics) and intrusive (Kargil Intrusives) magmatic episodes, and; (iii) to establish the youngest magmatic phases within the Dras Arc in order to constrain the maximum age of arc-continent collision. The results of this investigation demonstrate that the Dras and Spong (a component of the Spongtang Ophiolite-Spong Arc complex) arcs developed together as an intraoceanic arc system, that initiated in the Neotethyan Ocean just outboard of the northern margin of India during the Late Jurassic. Some of these earliest low-Mg adakitic felsic volcanic rocks are derived from the partial melting of basaltic ocean crust during incipient stages of arc development. The deep-marine volcaniclastic rocks of the Nindam Formation were deposited in a forearc basin setting with little to no influence from continental rocks and more akin to deposition in an intra-oceanic, island arc setting similar to other Neotethyan ophiolites along the Indus-Yarlung-Tsangpo Suture. Early Eocene (~53 Ma) zircon U-Pb ages obtained from gabbroic blocks within the Sapi-Shergol mélange at the southern thrust contact with the Indian Zanskar Supergroup relate to the youngest phase of magmatism within the Dras Arc. Thus, the Dras Arc was a long-lived, island arc complex spanning some ~108 m.y., between initiation around ~160 Ma, peak arc magmatism between 125-84 Ma and final arc magmatism at ~53 Ma, before the onset of stage 1, or ‘soft’ arc-continent collision at ~50 Ma. Accretion of the Dras Arc onto the northern Indian continental margin extinguished any further arc magmatism and led to partial continental subduction and eclogite metamorphism between 50-47 Ma at Tso Morari. The Dras Arc is geochemically and geochronologically distinct from the continental Ladakh Arc that was active along the southern margin of Eurasia. The Ladakh Arc remained active until at least ~41 Ma, before it collided with the India-Dras margin resulting in terminal continental collision or stage 2 ‘hard’ collision at about the Eocene-Oligocene boundary. This final phase of continental collision led to the uplift of the Himalaya which continues today.

FoR codes (2008)

0403 GEOLOGY, 0402 GEOCHEMISTRY, 040303 Geochronology, 040313 Tectonics

Download

Share

COinS
 

Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.