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09-12-2007

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Geologist crushing rocks!

Nanga Parbat

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Structures in the lower units of the Kohistan arc (NW Pakistan):

preliminary results

G. Zeilinger1, L. Arbaret1, J.-P. Burg1, N. Chaudhry2, H. Dawood3 & S. Hussain3

1 Geologisches Institut, ETH-Zentrum, Sonneggstr. 5, CH-8092 Zürich, Switzerland

2 Institute of Geology, Punjab University, Quaid-e-Azam Campus, Lahore 54590, Pakistan

3 Museum of Natural History, Garden Avenue, Shakaraparian, Islamabad 44000, Pakistan

In NW Pakistan, the Kohistan paleo-arc, which was developed as an island arc above the northward subduction zone of the Tethyan ocean during the Mesozoic, separates the Indian from the Asian plates. The southern boundary of the Kohistan arc is the so called Main Mantle Thrust (MMT), a crustal scale, northward dipping thrust on which the southern parts of the Kohistan arc have been thrust over India (Fig. 1, overview map).

Preliminary results of detailed mapping carried out in the uppermost Indian Plate (at Duber-Bazar, Fig. 1) and the low levels of the Kohistan paleo-arc (from Jijal to Kiru) has covered the following lithologies:

Indian units: Granodiorite and strongly foliated gneisses. SW of MMT the gneisses are folded parallel to the SW trending stretching lineation. Kinematic determination shows a dominant southwestward sense of shear.

Kohistan units: The lowest levels of the Kohistan paleo-arc are dominated by dunites, pyroxenites and peridotites. Dikes of greenish chromite-clinopyroxenites cut this ultramafic sequence. Upwards, meta-gabbros (age 101± 5 Ma., R. Anczkiewicz, oral com.) with high magmatic garnet content complement the so called Jijal Complex . Ca. 700 m SW of Patan, anastomosing shear zones appear in these garnet rich rocks. The density of the anastomosing shear deformation increases northward in this ca. 3 km wide zone, up to 2 km NE of Patan (Fig. 1). This shear zone is described in more details in a companion poster . To the NE of this anastomosing shear zones, rocks are variable in composition, but mainly comprise of gabbros, hornblendite gabbros, diorites, norites, amphibolites and smaller bodies of nearly pure hornblendite (Kamila Belt). Pegmatites intruded this sequence during and after ductile deformation and correspond to a late magmatic event.

In this poster we present structural data on the anastomosed shear zone. The anastomosed shear zone, developed during finite magmatic stage, shows all deformation facies from undeformed protolith to ultra-mylonite (for details: ). The sense of shear on the dispersed, but in average flat to NE dipping shear zones shows dominantly SW directed relative movement of the hanging wall and minor NE directed shear. To the NE of these zone of anastomosing pattern, other shear zones form parallel W-E striking units of amphibolite facies (age 83± 1 Ma., ). These regular shear zones, containing porphyroclastic garnet, reach a thickness of up to 20 m. Lineations strike NE-SW. The foliation of the shear zones dips more regularly to the NNE and the relation between normal and reverse shearing is balanced. The difference in the features and age (ca. 18 Ma.) between the anastomosed shear zone and the amphibolitic shear zone let us to separate the Kamila shear zone s.s. , to the NE, from the Patan ductile shear zone, which is the anastomosed system described here and in the companion poster .

The region underwent late, pervasive faulting. Striations record essentially strike-slip and normal faulting. Results of principal stress calculation show three different stress fields: a poorly recorded one has a SW-NE maximum compressive principal stress (s 1), a second one has a NW-SE s 1, dominantly identified by strike-slip movements. Preliminary analysis of superimposed striations indicates the SW-NE s 1 as the older one. The youngest, best represented stress field shows a SW-NE minimum principal stress and steep s 1, responsible for normal faulting. The NE dipping Patan-fault is a prominent fault-structure (Fig. 1) which cuts the anastomosed shear zones. Minor offsets in lithology, recent seismic activity , rotated blocks (indicated by the change of foliation attitude from mainly W-E to NNE-SSW, south of Patan-fault) and the smooth topography in the widened Indus valley at Patan indicate a still active Patan-fault, but it seems to be a recent and regionally minor fault-contact.

The Patan ductile shear zone must be separated from the Kamila shear zones. Further work on these structures will give more details about the evolution of the "Patan shear zone" and the relationship with the Kamila shear zone.

 

References:

[1] M.Q. Jan and R.A. Howie, The Mineralogy and Geochemistry of the Metamorphosed Basic and Ultrabasic Rocks of the Jijal Complex, Kohistan, NW Pakistan, Journal of Petrology 22(1), 85-126, 1981.

[2] J.-P. Bard, Metamorphic evolution of an obducted island arc: Example of the Kohistan Sequence (Pakistan) in the Himalayan collided range, Geol. Bull. Univ. Peshawar 16, 105-184, 1983.

[3] L. Arbaret, J.-P. Burg, N. Chaudhry, H. Dawood, S. Hussein and G. Zeilinger, Different sets of anastomosing shear zones in the "Kamila Belt", Kohistan, in: 13. HKTIW, Peshawar, 1998.

[4] P.J. Treloar, K.H. Brodie, M.P. Coward, M.Q. Jan, M.A. Khan, R.J. Knipe, D.C. Rex and M.P. Wi lliams, The evolution of the Kamila shear zone, Kohistan, Pakistan, in: Exposed Cross-Sections of the Continental Crust, M.H. Sallisburry and D.M. Fountain, eds., pp. 175 - 214, Kluwer Academic Press, Amsterdam, 1990.

[5] J. Jackson and G. Yielding, The seismicity of Kohistan, Pakistan; source studies of the Hamran (1972.9.3), Darel (1981.9.12) and Patan (1974.12.28) earthquakes, Tectonophysics 91(1-2), 15-28, 1983.

 

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