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

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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

• 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

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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 Diorite
0
2
cm
6
Detailed map Indian Plate
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Sarangar Gabbro
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Detailed map Sarangar Gabbro
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Granite
0
1
cm
10
Detailed map Granitic Rock
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Diorite
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Detailed map Diorite
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Chilas Gabbronorite
0
1
cm
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
• 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
• 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 FT
apatite
zircon
in Ma (2? errors)
7.32
5.62.3
Chilas
192
181.5
Indian Plate
111
3.70.5
Dasu
283
231
NE
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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