Collaborative Research: A Test of the Out-of-Sequence Model for the Main Central Thrust, Western Nepal

University Of Arizona, Tucson AZ

Investigators

Abstract

One of the most interesting data sets to emerge from recent studies of the Himalayan orogenic belt consists of U-Th-Pb ages reported by Harrison et al. (1997) and Catlos et al. (2001a, 2001b) from monazite inclusions within garnet crystals in the metamorphic rocks associated with the Main Central thrust (MCT) in central and eastern Nepal. Some of the monazite inclusions crystallized and were incorporated into the gamets during late Miocenc-Pliocene time. Geothen-nometry and geobarometry data indicate that metamorphic temperatures ranged from 500'-SOO'C and pressures ranged from 8-12 kbar. Because detn'tal monazite in pelitic sediments is destroyed during burial to the depths recorded by the mineral assemblages, the monazite ages most likely record the timing of garnet growth during Himalayan orogenesis (Harrison et al., 1998). Thus, the monazite ages contain infon-nation that is vital for kinematic reconstructions of Himalayan thrust systems, particularly the MCT and its proximal footwall rocks. The interpretation of the monazite ages offered in these previous studies suggests that the MCT was reactivated during late Miocene time, and that rocks in the footwall of the MCT were progressively incorporated into the hanging wall and raised to the surface. A number of independent lines of evidence suggest that this hypothesis may be correct, including 'o Ar/ " Ar cooling ages (Copeland et al., 1991; Macfarlane et al., 1992; Copeland et al., 2001); (2) levelling and GPS studies (Jackson and Bilham, 1994; Bilham et al., 1997; Larsen et al., 1998); and (3) neotectonic and geomorphic studies of the MCT zone in central Nepal (e.g., Bilham et al., 1997). Although reasonable, the MCT reactivation hypothesis incorporates some surprising kinematic processes. Paramount among these is the requirement that approximately 40 km of slip on the MCT occurred during late Miocene-Pliocene time in order to convey the garnets and their monazite inclusions to the surface. If the MCT was indeed reactivated, it would be (by far) the largest out-of-sequence event on a thrust fault ever documented. Whereas out-of-sequence thrusting is now widely accepted in thrust belt models, it generally is restricted to relatively minor displacements (a few km). A reactivation event of the hypothesized magnitude would significantly alter current concepts of how the Himalayan fold-thrust belt operates, and how foldthrust belts in general operate. It is conceivable that the extreme rate of erosion along the MCT in Nepal has shifted the fold-thrust belt into a near terminal state of subcriticality, stalling its forward propagation and completely reorganizing the locus of major thrusting. Thus, the out-ofsequence MCT hypothesis is worthy of careful and critical examination. The key to understanding the young monazite ages lies in the structure of the rocks below the MCT from which the youngest monazite ages were obtained. Unfortunately, the stratigraphy and structure of the rocks below the MCT in central Nepal (where the monazite studies have been executed) are not well documented. Exact placement of the MCT in the field is still hotly debated, such that the tectonostratigraphic context of the samples remains in doubt. Alternatives to out-of-sequence reactivation of the MCT can explain equally well the young monazite ages. In this work, the PI's will implement a critical test of the out-of-sequence hypothesis in western Nepal. They will collect samples for U-Th-Pb monazite dating of gamet-bearing rocks and " Ar/ " Ar dating of micaceous lithologies along north-south transacts from the Main Boundary thrust in the south to the South Tibetan detachment in the north. They have already established the regional stratigraphy, structure, geochronology, and Nd isotope geochemistry of the Lesser Himalayan zone south of the MCT in western Nepal during the past six years (DeCelles et al., 1998a, 1998b, 2000, 2001; Robinson et al., 2001, 2002). They propose to obtain U-Th-Pb ages from monazite inclusions in garnets collected from rocks that span the MCT zone. They will also map the zone in detail and collect samples for U-Pb zircon and Nd-isotopic analysis in order to locate the MCT exactly in the field. The " Ar/ " Ar cooling ages should help to document the regional history of thrust sheet emplacement, which will be needed to support any interpretation of what occurred along the MCT. The proposed work should help to resolve whether the MCT experienced major (several tens of km) slip during late Miocene-Pliocene time. The result of the MCT question will have an impact on general models for orogenic wedges, in particular whether rapid erosion can relocate the locus of major thrusting on a scale required by large-scale reactivation of the MCT. In addition, the proposed " Ar/ " Ar dating should provide an unprecedented level of detail and precision for the timing of thrust sheet emplacement in the Himalaya. Because the Himalaya is intimately related to the growth of the Tibetan Plateau and changes in global ocean chemistry, the PI's results should have applications beyond Himalayan tectonics.