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Transpressional uplift of middle and deep crustal rocks by constraining bends along
the Karakoram Fault in the K2 - Gasherbrum range
Mike Searle1 and Richard Phillips2
1. Department of Earth Sciences, Oxford University, Parks Rd., Oxford OX1 3PR, UK
2. Berkeley Geochronology Center and Department of Earth & Planetary Sciences, University of
California, Berkeley
The Karakoram fault is a major NW-SE aligned dextral strike-slip that has accommodated a
minor amount of eastward extrusion of thickenend Tibetan crust following the India-Asia
collision ca 50 m.y. ago1,2. Minimum - maximum dextral offsets are 40-150 km and long-term
slip rates are between 2.7-10.2 mm/yr3. U-Pb zircon and monazite ages of undeformed Baltoro
granites that crystallized before initiation of the dextral strike-slip fault, span 25-15 Ma4,5,6. P-T-t
data combined with 40Ar/39Ar and Fission track ages suggest that 10-25 km of material has been
eroded from the central Karakoram since the early Eocene, and <3 km since Plio-Pleistocene7,8,9.
Ductile shearing is constrained as occurring between 15.68 ±0.3 and 13.73±0.5 Ma from U-Pb
ages of sheared leucogranites containing strong dextral C-S fabrics and cross-cutting unsheared
leucogranite dykes3. Transpresional uplift of middle crustal rocks (amphibolites, meta-diorites,
calc-silicates and marbles) occurred before 7 Ma from 40Ar/39Ar hornblende and mica ages10.
Middle crustal rocks have been uplifted along the K2 range adjacent to a major restraining bend
along the Shaksgam section of the Karakoram fault. The K2 orthogneiss has an older, precollisional U-Pb zircon age of 115±3 Ma and 40Ar/39Ar hornblende ages of 90.6±1.8 Ma11. Zircon
Fission track ages from the Gasherbrum diorite, immediately SW of the Karakoram fault vary
from early Cretaceous to middle Tertiary, with no consistent pattern between age and elevation12.
The Gasherbrum diorite has never been at depths greater than 6 km since the early Cretaceous
and less than 3 km of material has been eroded off since then. The higher exhumation rates of the
K2 orthogneiss are attributed to localised transpressional deformation during active right-lateral
motion during the Plio-Pleistocene. The Karakoram fault is restricted to the upper crust, does not
extend into the lower crust, and accommodates only a minor amount of dextral slip.
1. Searle, M.P. 1996. Tectonics, 15, 171-186.
2. Searle, M.P., Weinberg, R. & Dunlap, W.J. 1998. In: geol. Soc. London Special
publication 135, 307-26.
3. Phillips, R.J., Parrish, R.R. & Searle, M.P. 2004. Earth & Planetary Science Letters, 226,
305-19.
4. Parrish, R.R. & Tirrul, R. 1989. Geology, 17, 1076-9.
5. Schärer, U., Copeland, P., Harrison, T.M. & Searle, M.P. 1990. J. Geology, 98, 233-251.
6. Searle, M.P., Crawford, M.B. & Rex, A.J. 1992. Trans. Royal Soc. Edinburgh, Earth
Sciences, 83, 519-38.
7. Searle, M.P., Rex, A.J., Tirrul, R., Rex, D.C., Barnicoat, A. & Windley, B. 1989. Geol.
Soc. America, Spec. Pub. 232, 47-73.
8. Searle, M.P. & Tirrul, R. 1991. J. Geol. Soc. London, 148, 65-82.
9. Krol, M.A., Zeitler, P.K., Poupeau, G. & Pecher, A. 1996. Tectonics, 15, 403-15.
10. Dunlap, W.J., Weinberg, R. & Searle, M.P. 1998. J. Geol. Soc. London, 155, 519-38.
11. Searle, M.P., Parrish, R.R., Tirrul, R. & Rex, D. 1990. J. Geol. Soc. London, 147, 603-6.
12. Cerveny, P.F., Naeser, C., Keleman, P.B., Lieberman, J. & Zeitler, P.K. 1989. Geology,
17, 1044-8.