Abstract EUG 10

Fault systems in the Indus Suture (Kohistan, NW Pakistan)

Gerold Zeilinger¹

Jean-Pierre Burg¹

Nawaz Chaudhry²

Hamid Dawood³

Shahid Hussain³

¹ Earth Sciences Dep., ETH-Zentrum, Zurich, Switzerland

² Institute of Geology, Punjab University, Lahore, Pakistan

³ Museum of Natural History, Islamabad, Pakistan

In NW Pakistan the Kohistan paleo-arc separates the Indian and Asian plates. The southern part of the arc has been thrust over the northern margin of the Indian plate along the northward dipping Indus Suture (also called Main Mantle Thrust). Ductile structures have been much investigated in order to understand collisional processes. However, faulting is an important, pervasive part of the long-lived deformation history that produced the present day suture zone. We present an analysis of fault-striations that document hypercollision in this part of the Himalayas.

Results of best fitting principal stress tensor calculation (Software FSA, B. Célérier, Montpellier) and data set separation yield four stress fields. Crosscutting striations give evidence for relative timing (from old to young) of these faulting events:

1. SSE-NNW maximum compressive principal stress (s 1), dominantly identified from strike-slip movements;
2. W-E directed s 1, expressed by thrust and strike-slip faults;
3. WNW-ESE minimum principal stress and subvertical s 1 that produced widespread normal faults;
4. SSW-NNE directed s 1.

Analysis of regional fault-framework gives evidence for changes in stress fields after collision and subsequent brittle deformation. Stage 1 strike-slip faults were formed during southward thrusting of the Kohistan Arc at the end of the early Miocene ductile-brittle conditions (ca. 20 Ma.). We relate stage 2, E-W compression to the formation of the Nanga Parbat crustal antiform (since 5 Ma.), which stands to the E of our study area. Stage 3 extension took place shortly afterwards, probably as a consequence of stress release in upper crustal levels. Stage 4 compression fits the present stress field, which produced the Patan earthquake in December 1974. It records ongoing southward thrusting of the Kohistan paleo-arc onto the Indian Plate.