new u/pb and fission track ages and their implication for the tectonic history of the lower kohistan...
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![Page 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](https://reader036.vdocuments.site/reader036/viewer/2022062511/5518b8db550346991f8b525e/html5/thumbnails/1.jpg)
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
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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
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Overview map
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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.
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Indian Plate Diorite0 2cm
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Detailed map Indian Plate
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Sarangar Gabbro
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Detailed map Sarangar Gabbro
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Granite0 1cm
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Detailed map Granitic Rock
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Diorite
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Detailed map Diorite
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Chilas Gabbronorite
0 1cm 2
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Detailed map Chilas
Gabbronorite
00/40/03
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Swat Ky-dyke
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Detailed map Swat dyke
38/02/01
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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
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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
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Raw data plot FT
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Raw data plot FT
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Interpretation FT3.7±0.5 11±1
23±1 28±3
19±218±1.5
5.6±2.3 7.3±2
Chilas
Dasu
Indian Plate
NE
apatite zirconin Ma (2 errors)
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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.
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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.
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Acknowledgement
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Indus Profile
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Swat Profile