New U/Pb and Fission track ages and their implication for the tectonic history of the Lower Kohistan Arc Complex, Northe

1. New U/Pb and Fission track ages and their implication for the tectonic history of the Lower Kohistan Arc Complex, Northern Pakistan G. ZEILINGER, J.-P. BURG, U. SCHALTEGGER & D. SEWARD Department of Earth Science, ETH Zurich, Switzerland

2. Outline of presentation • Introduction to the Kohistan Complex • Localities of sampled lithologies • U/Pb dating • Defining the intrusions ages related to early arc history • Results • Two-stage model interpretation • Fission track • Defining the post-collisional exhumation • Results • Back sliding along ISZ since 23 Ma

3. Overview map

4. The Kohistan Arc Complex exposes the mantle-crust transition and mantle-derived magmatism during initial and more evolved stages of the arc. To investigate time-resolved melt formation in an island arc. Why U/Pb dating in the arc?

5. Indian Plate Diorite 0 cm 2

6. Detailed map Indian Plate

7. Sarangar Gabbro

8. Detailed map Sarangar Gabbro

9. Granite 0 cm 1

10. Detailed map Granitic Rock

11. Diorite

12. Detailed map Diorite

13. Chilas Gabbronorite 0 cm 1 2

14. Detailed map Chilas Gabbronorite 00/40/03

15. Swat Ky-dyke

16. Detailed map Swat dyke 38/02/01

17. Interpretation U/Pb Two-stage model for the Kohistan Complex: • initial arc-building stage starting pre-100 Ma and lasting to at least 90 Ma, characterized by the emplacement of gabbroic to granodioritic melts

18. Interpretation U/Pb Two-stage model for the Kohistan Complex: • extensional intra-arc rifting stage at ca. 85 Ma, characterized by the emplacement of gabbronorites. Subsequently felsic dykes generated by anatexis at the base of the arc intruded into the overlying pile. S

19. Raw data plot FT

20. Raw data plot FT

21. Interpretation FT apatite zircon in Ma (2 errors) 7.3±2 5.6±2.3 Chilas 19±2 18±1.5 Indian Plate 11±1 3.7±0.5 Dasu 28±3 23±1 NE

22. Interpretation FT • Younging from north to south (upper plate  lower plate). • could represent a tilting effect, or • a discrete event across the ISZ. • If the latter is the case (to be tested with more data) then there is: • evidence for back sliding of the hanging wall (Kohistan Arc Complex) along the ISZ since at least 23 Ma. • Trend is also recorded with apatite FT data which point to a maximum exhumation rate of ca. 0.8 mm/a in the footwall over the last 3.7 Ma and of ca. 0.3 mm/a for the last 11 Ma in the hanging wall of the ISZ. • As a preliminary interpretation this indicates a normal sense of movement along the ISZ since 23 Ma in support of the structural evidence presented elsewhere.

23. Acknowledgement Supported by Swiss NF project grant Nr. 2000-49372.96. We thank Th. Bär, I. Ivanov and A. von Quadt for technical help, Pakistani officials S. Hussain and H. Dawood for access to the area.Discussions with J.-L. Bodinier, O. Müntener, P. Ulmer are highly appreciated.

24. Acknowledgement

25. Indus Profile

26. Swat Profile