Rate of Neogene depositional and deformational processes, north-west Himalayan foredeep margin, Pakistan

Abstract

The dynamics of molasse accumulation and the subsequent structural displacement of a foredeep basin during orogenesis have rarely been chronometrically defined. The Himalayan foothill belt of northern Pakistan and India constitutes a detailed record of molasse sedimentation in a classical foredeep setting. Subsequent deformation of this sedimentary sequence has resulted in the exposure of over 3000 metres of the Neogene and Quaternary aged Siwalik Group. Recently, the development of a magnetic polarity stratigraphy constrained by the fission-track dating of numerous intercalated volcanic ashes has permitted a precise chronostratigraphic statement to be made concerning the accumulation and deformational history of this non-marine sedimentary sequence. Data from numerous localities in northern Pakistan and India document the onset, development, and termination of various events in this sedimentation history with a precision of from ~20000 to ~50000 yrs. Sediment accumulation rates associated with the progressive migration of the foredeep depocentre have been determined to range from -20 to -50 cm/1000 yrs. The southward progradation of lateral facies changes at rates of up to 30 m/1000 yrs together with the southward advance of the basin depocentre at rates of over 20 m/1000 yrs suggest a rough equilibrium between the modelled northward plate motion of the Indian subcontinent and the southward displacement of depositional processes within the foredeep. This illustration of the dynamic involvement of proximal foredeep terraine in the continuing Himalayan orogenesis yields a model potentially useful in similar tectonic settings.