Structural Geology of the Southwestern Margin of Nanga Parbat
M.
A. Edwards, W.
S. F. Kidd, (both at: Dept
of Earth and Atmospheric Sciences, University at Albany, Albany, NY
12222
M. Asif Khan (Peshawar University, Pakistan)
D. A. Schneider, P. K. Zeitler, D. Anastasio, (all at: Dept
of Earth and Environmental Sciences, Lehigh University, Bethlehem,
PA 18015, USA).
EOS, Trans. Amer. Geophys. Union, 78, F651. Presented as poster session, 9th December, 1997 Fall Meeting, American Geophysical Union, San Francisco.
Abstract
The Main Mantle Thrust (MMT) is the regional contact between collider India and the overthrust Kohistan-Ladakh series in the Pakistan Himalaya. Early Himalayan-age thrusting and some later (~20 Ma?) normal motion on/near the MMT is modified by very young (e.g. 1.4 Ma leucogranite Th-Pb ages) tectonism at Nanga Parbat-Haramosh Massif (NPHM); the Himalaya's western syntaxis. Our investigations in southwestern NPHM reveal a complex interplay of MMT-related (mostly convergent) structural features followed by those related to uplift and tectonism of NPHM. Across the Diamir/Bunar area, main fabrics trend N to NE. In Diamir Valley, Indian cover passive margin metapelites and carbonates in the MMT footwall are not more than a few 100’s of metres thick. Here, the regionally NW-dipping Indian cover sequences and MMT hanging wall (Kamila amphibolite) are overturned (SE-dipping). These overturned layers are traceable to the Gashit fold, whose hinge line plunges ~N and axial surface dips gently-moderately east. The cover sequence thickness increases markedly to the south; several km structural thickness of carbonates, metapelites and amphibolites are present in the W-E Airl-Nashkin section, only 8 km to the south of Diamir valley. The cover sequence passes east into a dominantly plutonic 5-10 km thick crystalline sequence where coarse to fine grained granite (the Jalhari granite) grades, due to syn- to post-plutonism deformation, into granitic and porphyroclastic gneiss intercalated with gneissic basement. Regionally the cover and crystalline rocks follow the subvertical to steeply E-ESE dipping foliation, and displacement sense is consistently east side (NPHM) up and over west. Within Diamir valley, granitic and gneissic foliation, shear bands, and local fault gouge zones anastamose around less- to un-deformed Jalhari granite lenses of 10-100’s metres width. Plutonism seems to be in part synkinematic, and may provide an older age limit for NPHM tectonism. The E over W sense is consistent with the development of the Gashit fold and the upper limb that includes the overturned cover/MMT layers. The sharp attenuation of the cover sequence in northern Bunar valley could be a result of excision by normal motion along the MMT but we find no compelling evidence for this. We propose that the attenuation is a result of (1) a large frontal ramp in the MMT and an underlying related duplex largely of Indian cover and/or (2) original MMT thrust belt morphology where a lateral ramp-related duplex system has imbricated (and/or infolded) local thin slices of the cover and basement.
Poster
Fig. 1. Regional geological setting of the Nanga Parbat-Haramosh massif [NPHM]
Fig. 2. Outline geological map of the Nanga Parbat-Haramosh Massif
Fig. 2a. Geological map of the southwestern margin of the Nanga Parbat Massif
Fig 3. Geological cross-sections of the Diamir Gah and Nashkin-Airl Gah profiles
Fig 4. Geological cross-section of southern Nanga Parbat massif
Fig 5. Cartoon structure sections prior to Nanga Parbat massif exhumation
Summary captions and access to photos
Acknowledgements: We thank Chris Coath at UCLA for help with
ion microprobe analyses.
This poster is dedicated to the memory of Aslam Mohummad, our great
friend
and logistics-wizard in Pakistan. He died suddenly and unexpectedly in
September 1997.
Contribution to the Nanga Parbat project, funded by the National Science Foundation Continental Dynamics Program.