Stratigraphic Keys to the Timing of Thrusting in Terrestrial Foreland Basins: Applications to the Northwestern Himalaya

Precise determination of the timing and nature of deformational events in foreland basins is critically dependent on the interpretation of stratigraphic indicators of tectonic activity within a detailed chronologic framework. Frequently data from numerous stratigraphic sections must be synthesized in order to generate a clear definition of a structural event and its response within a sedimentary sequence. In terrestrial deposits, magnetostratigraphic studies can provide the temporal control necessary to establish a reliable and coherent synthesis of tectonism that is based on an amalgamation of the diverse responses to, and indicators of, tectonic activity found in geographically separated localities. Useful indicators in this type of analysis include: 1) classical indicators of tectonism, such as the timing of changes in paleocurrents, provenance, and facies, and the ages of unconformities; and 2) indicators based on magnetic data or precise chronologic control, such as the history of syn- and post-depositional tectonic rotation and changes in the rate of sediment accumulation. Three examples based on the interpretation of such stratigraphic data from the northwestern Himalayan foreland basin are used to illustrate diverse thrusting events during the past 5 million years, including initial thrust propagation, later episodes of reactivation, and large-scale out-of-sequence thrusting. These examples illustrate the high degree to which reliable specification of the sequence of thrust events that are closely spaced in time but are distributed across a broad region is critically dependent on the availability of detailed, time-controlled stratigraphic records.