Presentation on theme: "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."— Presentation transcript:
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
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
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.
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
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
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)
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.
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.