ABSTRACT The southeast Ladakh (India) area displays one of the best-preserved ophiolite sections in this planet, in places up to 10 km thick, along the southern bank of the Indus River. Recently, in situ, ultrahigh-pressure (UHP) mineralogical evidence from the mantle transition zone (MTZ; ~410–660 km) with diamond and reduced fluids were discovered from two peridotite bodies in the basal mantle part of this Indus ophiolite. Ultrahigh-pressure phases were also found by early workers from podiform chromitites of another coeval Neo-Tethyan ophiolite in southern Tibet. However, the MTZ phases in the Indus ophiolite are found in silicate peridotites, but not in metallic chromitites, and the peridotitic UHP phases show systematic and contiguous phase transitions from the MTZ to shallower depth, unlike the discrete UHP inclusions, all in Tibetan chromitites. We observe consistent change in oxygen fugacity ( f O 2 ) and fluid composition from (C-H + H 2 ) to (CO 2 + H 2 O) in the upwelling peridotitic mantle, causing melting to produce mid-ocean-ridge basalt (MORB). At shallow depths (<100 km) the free water stabilizes into hydrous phases, such as pargasitic amphibole, capable of storing water and preventing melting. Our discoveries provide unique insights into deep sub-oceanic-mantle processes, and link deep-mantle upwelling and MORB genesis. Moreover, the tectonic setting of Neo-Tethyan ophiolites has been a difficult problem since the birth of the plate-tectonics concept. This problem for the origin of ophiolites in mid-ocean-ridge versus supra-subduction zone settings clearly confused the findings from Indus ophiolites. However, in this contribution, we provide arguments in favor of mid-ocean-ridge origin for Indus ophiolite. In addition, we venture to revisit the “historical contingency” model of E.M. Moores and others for Neo-Tethyan ophiolite genesis based on the available evidence and have found that our new results strongly support the “historical contingency” model.

ABSTRACT We defined the timing of surface abandonment for 10 alluvial and debris-flow fans across contrasting climatic settings in the NW Himalaya of northern India using cosmogenic 10 Be surface exposure dating. Debris-flow fans in the Garhwal, Kullu, and Lahul-Spiti regions of the monsoon-influenced Greater Himalaya were largely abandoned during the Mid- to Late Holocene. Large alluvial fans and smaller debris-flow fans in the semiarid Ladakh region of the Greater and Tethyan Himalaya have surface ages that extend throughout the last glacial. Regional events of landform abandonment and incision were defined for the monsoon-influenced western Himalaya ranges and the semiarid western Himalaya ranges over the past ~120 k.y. In the monsoon-influenced and semiarid western Himalaya ranges, these regional events were limited to the Holocene and from ca. 40 ka, respectively. The timing of fan surface abandonment and regional landform abandonment events coincided with periods of weakening monsoon strength and cooling, and local and regional glacier advances. Regional incision events from the monsoon-influenced and semiarid western Himalaya regions were recognized across various climatic conditions due to the ubiquitous nature of erosion in mountain settings. This study showed that climate-driven processes and glaciation were important drivers in fan sedimentation, catchment sediment flux, and the topographic evolution of the NW Himalaya during the late Quaternary.