Abstract STSG2000

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Petro-structural and tectonic history of the Lower Kohistan Complex

Gerold Zeilinger¹, J.-P. Burg¹ & L. Arbaret¹ 

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

In NW Pakistan, the Kohistan (Fig. 1) complex separates the Indian and Asian plates. It was formed as an Island Arc during Mesozoic times. Accretion to Asia and subsequent thrusting over the northern margin of the Indian plate along the northward dipping Indus Suture (also called Main Mantle Thrust = MMT) constitute the collisional history. Earlier history involves arc related deformation like splitting the arc in a volcanic and remnant arc and SW-thrusting expressed by anastomosing shear zones in the southern, i.e. lower part of the arc, probably imposed by subduction of the Tethys oceanic crust.

Mapping of the Dubair-Patan-Kiru region and detailed section along the Indus and Dubair Valley has covered, from S to N - i.e. from the bottom to the top of the bulk structure - the following lithologies:

1) The Indian units which are composed of granodiorite and granite intruding strongly foliated gneisses and metasediments.

2) The Jijal Complex, forming the lowest levels of the Kohistan complex. The ultramafic part is formed by layered peridotite, clinopyroxenite, dunite and websterite. Upwards, granulite facies metagabbros complement this complex.

3) The southern part of the Kamila Belt is mainly composed of retrograde metagabbros with locally preserved igneous layering intruded by hornblendite bodies and plagioclase-quartz±amphibole-rich veins. The metagabbro is deformed by numerous anastomosing shear zones with variable spacing and size (see Arbaret et al., this abstract volume). To the N the Kamila Belt comprise sills of gabbroic to dioritic composition and ortho-amphibolites intruded in a crustal sequence and are intruded by deformed granitic and pegmatitic sheets.

We focus on the petro-structural and geochemical evolution of the southern Kamila Belt. Field observations as well as petrological/geochemical data show, that the definition of the Kamila belt as a single homogeneous unit is inadequate. We propose to separate this belt into 3 different zones, from S to N:

a) Sheared gabbro-diorites (Patan region)

Retrograde metagabbros with locally preserved igneous layering showing a heterogeneously distributed deformation forming an anastomosing pattern of shear zones.

b) Kiru amphibolites

More than hundred meters thick sills all derived from imbricate intrusions of gabbroic to dioritic composition intensely deformed along East-West trending shear zones.

c) Kamila amphibolites s.s.

Amphibolitised sequence composed of ortho-amphibolites intruded in a crustal sequence represented by metapelites, meta volcano-clastics, carbonates and metavolcanites. The Kamila amphibolites are intruded by deformed granitic and pegmatitic sheets and were strongly deformed during contact metamorphism along their northern contact with the younger gabbro-noritic Chilas Complex.

Fig. 1: Studied area (squared) located in the structural framework of Northern Pakistan, with the Kohistan Complex, main sutures and syntaxes.