Yarlung river, flowing at the geological boundary between Pre-Eocene Asia and India

Douwe in Venice

By Douwe van Hinsbergen

In the summer of 2013, an international team of Earth Scientists undertook a field trip to the Indus-Yarlung suture zone, which demarcates the fault zone where since early Cretaceous time the Neotethyan Ocean subducted into the mantle. In the field, this suture zone exposes relics of oceanic crust – so-called ophiolites – that are thrusted over thrust nappes of carbonates that were once overlying continental crust of the Indian plate – the Tibetan Himalaya. On top of the ophiolites, a thick sequence of sandstones and shales is exposed. Previous research has shown that the sand in these rocks was eroded from the Asian continent, and is interpreted to reflect the ‘Xigaze’ forearc of Asia that formed immediately above the Neotethyan subduction zone.

The aim of the field trip was to constrain what the relationship is between the fragments of oceanic lithosphere preserved as ophiolites, and the Xigaze forearc sediments, and at which latitude the ophiolites and forearc formed. In addition, we studied the tectonic and chemical evolution of the ophiolites, and subduction-related metamorphic rocks that are found in chaotic mélanges below the ophiolites. With these constraints, we hope to better reconstruct the initiation of subduction below Asia, the amount of subduction zones that played a role in subducting the Neotethys ocean, and the tectonic evolution of the Asian forearc. To this end, we carried out paleomagnetic, structural geological, stratigraphic, metamorphic, and petrological and geochemical fieldwork and sample collection.

This research exercise occurred within context of an NSF-Continental Dynamics program led by Paul Kapp (University of Arizona at Tucson), and consisted of several crews spread out over southern Tibet. Our field crew further included Marco Maffione and Wentao Huang, Utrecht University, the Netherlands; Carl Guilmette, Universiy of Waterloo, Canada; Kip Hodges, Mary Schultz, and Nate Borneman, Arizona State University, USA, and Jared Butler, Dalhousie University, Canada, and Shun Li of Beijing University, China.

Below you will find some impressions of the trip!

Group picture of the suture zone field team 2013

Section across the forearc

Shun Li, standing on the contact between pillow lavas and radiolarian cherts of the Xigaze ophiolite

Marco Maffione, studying peridotites in float

Field lunch with Carl Guilmette and Jared Butler

Marco, drilling paleomag samples in the Xigaze sandstones

Checking out new outcrops in the sub-ophiolitic mélange

Field crew checking out the Sang Sang ophiolite at 5100 m elevation