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 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 Introduction to the Kohistan Complex Localities of sampled lithologies

3. Overview map

4. Why U/Pb dating in the arc? 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.

5. Indian Plate Diorite 02cm

6. Detailed map Indian Plate

7. Sarangar Gabbro

8. Detailed map Sarangar Gabbro

9. Granite 01cm

10. Detailed map Granitic Rock

11. Diorite

12. Detailed map Diorite

13. Chilas Gabbronorite 01cm2

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: (A)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: (B)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

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

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