Journal of the Geological Society , London, Vol. 162, 2005, pp. 135–146. Printed in Great Britain. 135 Subsidence history of the north Indian continental margin, Zanskar–Ladakh Himalaya, NW India R. I. CORFIELD 1,2 , A. B. WATTS 1 & M. P. SEARLE 1 1 Department of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, UK (e-mail: tony@earth.ox.ac.uk) 2 Present address: BP Exploration, Farburn Industrial Estate, Dyce, Aberdeen AB21 7PB, UK Abstract: Detailed geological field mapping has allowed the restoration of two full stratigraphic sections through the highly deformed Mesozoic and Early Tertiary fold and thrust belt of the north Indian continental margin. The two sections, which are representative of a proximal and a distal facies on the margin, have been backstripped using standard techniques. Profiles of the tectonic subsidence and uplift through the pre- collisional history of the margin have been constructed and compared with the predictions of simple thermal and mechanical models. The pre-collisional history can be explained by a thermal model with an initial age of rifting of 270 Ma and a stretching factor, â, of c. 1.2. This model accounts for the general exponential decrease in the backstripped tectonic subsidence. The model fails, however, to completely explain the subsidence and uplift history of the margin since the late Cretaceous. The history during this time is characterized by uplift at the most proximal location and an increase in subsidence at the most distal location. We attribute these differential vertical movements to flexural loading of the north Indian margin by obduction of the Spontang ophiolite. The best fit model is one in which a 70 km wide wedge-shaped load, tapering from 10 to 0 km thick, is emplaced on rifted lithosphere with an elastic thickness, T e , of 5–10 km. These results, which are in accord with the late Cretaceous timing of obduction and the structure of the Spontang ophiolite, provide new constraints on the T e structure of extended continental lithosphere 120–150 Ma after a rifting event. Keywords: Himalaya, continental margin, backstripping, flexure, ophiolites. The tectonic subsidence and uplift history of a sedimentary basin constrain the vertical movements that have modified it during geological time. Backstripping techniques applied to modern basins such as those in passive margin settings have been remarkably successful in determining the pattern of the tectonic subsidence and uplift. There have been comparatively few applications of these techniques, however, to highly deformed margins in the rock record. In the Zanskar–Ladakh Himalaya, NW India (Fig. 1), the Spontang ophiolite is a thrust slice of oceanic crust and upper mantle that lies at present structurally above a continental margin carbonate sequence and its neo-autochthonous early Tertiary limestone cover. Controversy exists as to whether the ophiolite was obducted onto the northern passive margin of the Indian plate during the Eocene (Keleman & Sonnenfield 1983; Reuber 1986; Colchen et al. 1987) or during the late Cretaceous (Searle 1986; Searle et al. 1988, 1997; Corfield & Searle 2000; Corfield et al. 2001). The Spontang ophiolite was formed at 177  1 Ma (mid- Jurassic), as suggested by the U–Pb zircon age of diorite segregations in high-level gabbros (Pederson et al. 2001). An andesitic island arc sequence above the Spontang ophiolite basalts was dated at 88  5 Ma, which constrains the minimum age of subduction beneath the ophiolite. Structural studies and detailed mapping around the ophiolite have shown that a major phase of folding affected all Mesozoic units of the margin shelf sequence before deposition of the unconformably overlying Palaeocene–Eocene limestones (Searle 1986; Searle et al. 1988, 1997; Corfield & Searle 2000). Following the early Eocene (c. 50 Ma) collision between India and Asia, a renewed thrusting event placed the Spontang ophiolite over these limestones. Previous studies, using estimates of subsidence rates for given stratigraphic intervals of the Zanskar passive margin, suggest the timing of ophiolite obduction as either in the late Cretaceous (Searle 1986; Corfield & Searle 2000) or in the Eocene (Garzanti et al. 1987; Premori Silva et al. 1991). Gaetani & Garzanti (1991) presented a geohistory analysis for the passive margin sequence, but included no discussion of the assumptions and uncertainties involved. This paper presents the first attempt to quantitatively derive the tectonic subsidence and uplift history of the north Indian continental margin throughout its entire history, from the rifting and passive margin formation in the late Palaeozoic to the time of last marine sedimentation in the Eocene. We use these tectonic movements to better constrain the timing of ophiolite obduction and the thermal and mechanical structure of the north Indian continental margin. Stratigraphy We have used in this study stratigraphic sections that have been compiled from field mapping across a highly deformed and eroded terrain. Restoration of the post-depositional structural evolution is therefore required so that stratigraphic units can be placed in their correct relative palaeogeographical positions. Furthermore, the construction of a stratigraphic section through the entire Mesozoic sequence (e.g. Fig. 2) requires data from exposures covering a sizeable area. The detailed summary of the stratigraphy given by Gaetani & Garzanti (1991) and the map and restorations presented by Corfield & Searle (2000) can be used for this purpose. The Permian–Mesozoic shelf carbonate succession exposed in the Zanskar Range extended northward to the time-equivalent passive margin and slope deposits (Lamayuru complex) and