Formation of mélanges in the Indus Suture Zone, Ladakh Himalaya by successive subduction-related, collisional and post-collisional processes during Late Mesozoic-Late Tertiary time
Authors: Robertson A. H. F. 
Published: April 2008

The classic Indus Suture Zone in western Ladakh includes two zones (southern and northern) of highly dismembered rocks, that in the past were widely interpreted as ophiolitic mélanges, created mainly by subduction/accretion processes. The ‘ophiolitic mélange’ was reported to include chaotically distributed blocks of ophiolitic rocks (e.g. serpentinite, gabbro, basalt) and sedimentary rocks (chert, limestone) set in a matrix of deep-sea clastic sediments. This accretionary hypothesis is tested in this paper and found to be inadequate. Units formed simply by oceanic subduction-accretion (i.e. local mud-matrix mélange) are minimal (<1% by volume). In reality, the southern and northern mélange zones are the end products of complex multi-stage tectonic processes, involving subduction (mid-Late Cretaceous to Early Tertiary), initial emplacement (latest Cretaceous), collision (Palaeocene-Eocene) and post-collisional (Late Tertiary) stages. Important components of the mélange as a whole include thrust sheets and broken formation of relatively coherent volcanic-sedimentary successions related to the North Indian passive margin (Karamba and Lamayuru complexes), also mid-Late Cretaceous oceanic arc-type volcanics and volcaniclastic sediments (Dras arc complex). Dismembered serpentinite thrust sheets, cut by swarms of (subduction influenced) diabase dykes, most likely record detached oceanic basement related to the oceanic Dras arc complex. Associated serpentinite was injected along tectonic contacts and into adjacent units during collisional and post-collisional deformation, locally forming serpentinite mélange. Post-collisional, Early-mid-Tertiary non-marine coarse clastic sediments (Indus Group) unconformably overlie various units (serpentinite, volcaniclastics, blueschists) in different locations, and were later thrusted and folded within the southern and northern mélange zones in response to regional Late Tertiary tightening of the suture. As a result, backthrusting reversed the original thrust stacking order, placing the originally most southerly units at the highest structural level. The Indus Suture Zone ‘ophiolitic mélanges’ in western Ladakh are, thus, mainly not ophiolitic (i.e. dismembered oceanic crust), or mélange in entirety (i.e. chaotic block), but are instead dominated by disrupted thrust sheets and broken formation of the Mesozoic Lamayuru-Karamba continental margin and the Upper Cretaceous oceanic Dras arc complex, with only minor preserved remnants of units formed by subduction/accretion (e.g. mud-matrix mélange; oceanic exotics).

The southern and northern mélange zones reflect the existence of several N-dipping subduction zones active in latest Cretaceous-Palaeogene times. Although only minor volumes of accreted oceanic material (oceanic lithosphere and trench-type sediments) are preserved within the Indus Suture Zone in western Ladakh, additional accreted material was bulldozed further south onto the Zanskar continental shelf and is now preserved as mélanges beneath the Spontang ophiolite. Similar mélange is also preserved in eastern Ladakh. Subduction zones evolved into a major interconnected shear zone (suture) during Early Tertiary (54–50 Ma) collisional deformation, dismembering upper (oceanic arc) and lower plate (continental margin) units to produce most of the southern and northern mélange zones. After suturing and initial deposition of non-marine coarse clastic cover sediments, inherited weakness zones within the Indus Suture Zone were exploited, increasing disruption during Late Tertiary regional backthrusting.

The methods used here in the analysis of the Indus Suture Zone mélange could well prove to be useful elsewhere in the Himalaya and in other orogenic belts. This work also emphasizes the need to distinguish between true mélange v. thrust sheets and broken formation in the field, and also cautions against use of the term ophiolitic mélange unless all the components of a dismembered ophiolite are actually present together.