POST-LATE MIOCENE THRUSTING IN THE ANNAPURNA RANGE OF CENTRAL NEPAL? CONSTRAINTS FROM TOPOGRAPHIC DATA AND MONAZITE AGE AND TEXTURAL DATA

In active mountain belts around the world, topographic data, particularly steepness data, are routinely used to infer tectonic processes. In the Himalaya, steep slopes are commonly found in the proximal hanging wall of the Main Central thrust (MCT). The presence of steep slopes in the hanging walls of the MCT and other thrusts in the interior of the thrust belt has been used to infer recent, large-magnitude slip on these faults. Such recent motion on these faults would be remarkable because slip has traditionally been thought to end by the Middle Miocene, with post-Middle Miocene slip concentrated on surface-breaking thrusts at the front of the range. Younger, out of sequence slip on the MCT and other interior thrusts is also considered to be supported by Late Miocene to Pliocene Th-Pb ages of monazite from rocks in the footwalls of these thrusts. These ages are taken to represent monazite growth during prograde metamorphism caused by burial of the footwall during thrusting events.

Integration of topographic data with new, isotopically and microstructurally constrained locations of the MCT shows that in detail, the steepest slopes in the Annapurna Range of central Nepal are several kilometers north of the MCT, not spatially associated with it. In situ Th-Pb ages of monazite, chemical and age zoning in monazite and other minerals, and textural relationships between monazite and nearby minerals indicate that both prograde and retrograde monazite grains are preserved in rocks near the MCT. Much of the retrograde monazite probably crystallized from aqueous fluids. Because metasomatic growth of monazite can occur at temperatures as low as 300-400 °C, young Th-Pb ages from these monazites do not necessarily date burial metamorphism caused by slip on faults structurally above the monazite-bearing rocks. Taken together, these interpretations suggest that topographic data and monazite data do not support Late Miocene-Recent slip on the MCT and other thrust faults in the interior of the Himalayan thrust belt in the Annapurna Range.