BALANCED CROSS-SECTIONS AND KINEMATIC RECONSTRUCTIONS: AN EXAMPLE FROM WESTERN NEPAL

Geological mapping and balanced regional cross-sections in the Himalayan thrust belt of western Nepal yield minimum shortening estimates between the South Tibetan detachment and the Main Frontal thrust of 485-743 km (Robinson et al., 2006), which suggests that total Himalayan shortening (including the Tibetan thrust belt) may exceed 900 km. These cross-sections highlight the relationship between deformation and exhumation of the Lesser and Greater Himalayan tectonostratographic units of Nepal, and emphasize that the thrust belt in western Nepal behaved as a forward propagating system (with minor out-of-sequence adjustments), involving unmetamorphosed and low-grade metamorphic rocks that are brittley deformed in its frontal region and high-grade metamorphic rocks which are ductilely defomed in its hinterland region. Data used during the construction of the cross-sections include field data, location of the Main Himalayan thrust, and Ujahni well data from India which pins the Lesser Himalayan stratigraphy in the foreland. Once the cross sections adhered to the data, they were balanced and restored using the sinuous bed method.

By focusing on the cross-section from farwestern Nepal that extends through the village of Chainpur, I will show how the cross section was built, retrodeformed, the alternative models, and the integration of these data into forward modeling program of 2-D Move to illustrate the sequential development and evolution of the thrust belt. It is essential to convey a quantification of uncertainty of the shortening values derived from a cross-section. I will show how to understand the certainties and uncertainties inherint and how to present these data along with the cross-sections and reconstructions. A cross-section is a powerful tool but is underutilized when not used in conjunction to timing data. I will show how the timing data and the cross-section reconstructions, when used together, help to produce a kinematic model. Essentially these exercises produce viable and admissable balanced cross-sections, yield insight into the development of the thrust belt, and place limitations on numerical and dynamic models.