Paper No. 11
Presentation Time: 4:25 PM
STEEP REACHES OF THE MODI KHOLA DO NOT CORRESPOND TO DIFFERENCES IN APATITE FISSION TRACK AND (U-TH)/HE AGES
MARTIN, Aaron J., Department of Geology, University of Maryland, College Park, MD 20742, WALSH, Lisa S., Department of Geology, University of Maryland, College Park, MD 20742-0001, NADIN, Elisabeth S., Department of Geology and Geophysics, University of Alaska Fairbanks, Fairbanks, AK 99775, OJHA, Tank P., Department of Geosciences, University of Arizona, Tucson, AZ 85721 and FEDENCZUK, Tom M., National Center for Island, Maritime, & Extreme Environment Security, University of Hawaii, Honolulu, HI 96822, martinaj@geol.umd.edu
Detection of active faults is one of the main goals of many studies of the geomorphology of bedrock river systems in mountainous regions. We studied the Modi and Mardi Kholas, located in the western Annapurna Range of central Nepal, near and below the high elevations in the range because other workers, using data from other rivers in central Nepal, have suggested recent motion on faults at this position. We started with elevation contour lines digitized from topographic maps and interpolated using a multiquadric radial basis function to obtain a digital elevation model with 25 meter resolution. We then identified a high concavity zone on the river and steepness transitions and maxima within this high concavity zone. The largest change in steepness overlaps spatially with the Romi fault, a normal fault within Lesser Himalayan rocks. The steepest reach of the Modi Khola is located 1200 m upstream of this transition in Greater Himalayan rocks.
In order to assess whether the largest steepness maximum and transition along the Modi Khola may be caused by active faults, we obtained apatite fission track and (U-Th)/He (AHe) ages from fifteen Greater and Lesser Himalayan pelites and quartzites at nearly constant elevation across these geomorphic features. Fission track ages are mostly c. 0.8 Ma, and large uncertainties mean outliers cannot be distinguished with confidence to have a different age. AHe ages are mostly near 0.5 Ma, also with large uncertainties. Neither type of age shows any apparent difference between the fifteen samples. We modeled cooling paths of the samples using AHe ages and fission track ages and lengths and likewise find no statistically robust difference in cooling histories between any of the samples. Thus at the temporal resolution of the apatite fission track and He data, we find no support for motion on faults in this region of the Modi Khola valley in the past c. 1 million years, and suggest that this steepness maximum and transition on the river likely results from landslide damming of the river. A second, smaller steepness maximum 7 km downstream on the Modi Khola lies at nearly the same latitude as a maximum on the Mardi Khola to the east, and the maxima on both rivers might have formed in response to active faulting. These maxima are obvious targets for future thermochronologic studies.
