The indus clastics: forearc basin sedimentation in the Ladakh Himalaya (India)

The mid-Cretaceous to Early Tertiary clastic sediments of the Indus Group represent the forearc basin succession of the Transhimalayan arc-trench system. In the late Albian to early Eocene, a shallowing-upward megasequence (Tar turbidites to nummulitic limestones) and alluvial to deltaic sediments (Basgò and Temesgam Formations) were deposited respectively on the southern and northern flanks of the basin. Clastic petrography points to provenance from the largely undissected volcanic cover of the massif, with more important plutonic detritus recorded in the northern alluvial units. After collision with the Indian passive margin in the early Eocene, sedimentation in the whole basin was characterized by huge alluvial fans (Nurla Formation) and then by lacustrine deposits formed in an intermontane depression (Nimu Formation). A rapid change in petrographic and geochemical composition is recorded in post-early Eocene units, with sharp increase in plutonic detritus and trace elements typically associated with ophiolites, testifying to the uplift of both arc massif and subduction complex sources after collision with the Indian passive margin. The petrography of the Indus basin clastics and the evolution of detrital modes compare well with volcano-plutonic suites deposited in modern and ancient forearc basins. The composition of sandstone units contained within the accretionary prism instead suggests provenance from an undissected magmatic arc and deposition in abyssal plain and trench settings (Nindam volcaniclastic turbidites). Reworking of arc-derived sandstones took place in small basins perched on the accretionary trench slope. The Asian continental margin of the Neotethys Ocean was thus an active Pacific-type arc-trench system until the Eocene, when it collided with India, underwent polyphase fold-thrust deformation and was finally uplifted several kilometers to form the Indus suture zone of the Himalayas. © 1988.