HR: 1340h
AN: T23D-1649    [Abstracts]
TI: Preliminary Stratigraphy and Structure of the Lesser Himalayan Portion of the Himalayan Fold-Thrust Belt, Eastern Bhutan
AU: * Long, S P
EM: slong@princeton.edu
AF: Princeton University, Department of Geosciences,Guyot Hall, Princeton, NJ 08544, United States
AU: McQuarrie, N
EM: nmcq@princeton.edu
AF: Princeton University, Department of Geosciences,Guyot Hall, Princeton, NJ 08544, United States
AU: Tobgay, T
EM: ttobgay@princeton.edu
AF: Princeton University, Department of Geosciences,Guyot Hall, Princeton, NJ 08544, United States
AU: Gehrels, G
EM: ggehrels@email.arizona.edu
AF: University of Arizona, Department of Geosciences, Gould-Simpson Building #77, 1040 E 4th St., Tucson, AZ 85721, United States
AB: Documenting the magnitude and geometry of shortening across the Himalayan orogen is a critical step in testing competing hypotheses of systematic variation along strike due to possible changes in convergence magnitude, observed changes in the width of the Tibetan Plateau, or the possible influence of precipitation gradients or stratigraphic variation on strain accommodation. While balanced cross sections allow for shortening estimates in the western and central parts of the orogen, the eastern portion remains understudied. Thus, the acquisition of data here is necessary to document along-strike changes within the Himalayan orogen. We present new mapping and U-Pb detrital zircon (DZ) ages within Lesser Himalayan (LH) rocks of eastern Bhutan, between the Main Central Thrust (MCT) and Main Boundary Thrust (MBT), and describe the first-order stratigraphy of this portion of the northern Indian passive margin. We use the stratigraphic data to construct balanced cross- sections that illustrate the geometry of this portion of the fold-thrust belt, and allow for preliminary estimates of minimum shortening. Preliminary U-Pb DZ analyses divide the LH section into 2 groups, 1) lower LH units, with the youngest DZ peak at ~1.8 Ga, and 2) younger, upper LH units with DZ spectra between ~1000-500 Ma. The oldest zircons (1.8-1.9 Ga peak) are in the Shumar and Daling Formations; this signature is characteristic of correlative LH units along strike in Nepal. Younger LH zircons were found in the Baxa Formation in the frontal portion of the fold-thrust belt (as young as ~500 Ma), and within 2 samples collected just beneath the MCT (~1000-500 Ma), in rocks mapped in stratigraphic contact directly above the Daling Formation. These data indicate that Proterozoic LH units identified in Nepal are continuous along strike, but argue for the discontinuous presence of important Paleozoic basins. Also included in the upper LH formations are the Diuri Formation (predominantly diamictite), and the sandstone, shale and coal of the Gondwana Sequence. Above the LH strata are up to 6 km of foreland basin deposits of the Neogene Siwalik Group. Major structural features within LH units in eastern Bhutan include 1) a duplex within upper LH units, immediately north of the MBT, and 2) a northern duplex of lower LH units. We propose that the Shumar Thrust, which separates the two domains, is the roof thrust of the upper LH duplex while the MBT is the floor thrust. A preliminary, composite balanced cross-section of the fold-thrust-belt estimates minimum shortening at 326 km, or 72%. This estimate is much less than those in Nepal (~500-700 km), indicating significant along-strike variation.
DE: 8104 Continental margins: convergent
DE: 8108 Continental tectonics: compressional
SC: Tectonophysics [T]
MN: 2007 Fall Meeting