Near-orthogonal deformation sequence along the Malakand transect, NWFP, northern Pakistan
Abstract
The Malakand transect of northern Pakistan represents a part of the regionally metamorphosed sequence of the Indian plate rocks. Field observations show that these metapelitic rocks are multiply deformed and preserved progressive bulk inhomogeneous shortening resulting in multiple generations of folds. Four near-orthogonal overprinting fabrics have been identified different than those reported previously. D1 structures are rare, both at the meso- and microscale. This is because of the penetrative effects of the later D2 event. D2 not only transposed and obliterated the D1 structures, but also reactivated the S1 foliation. S2 is steeply dipping foliation, dominantly towards north, and E-W striking. S3 makes an orthogonal relationship with the S2 and this geometrical relationship gives rise to L23 intersection lineation, which is shallowly plunging either towards ENE or WSW. S3 is generally sub-horizontal and well developed. The orientation of S4 is very similar to that of S2. However, S4 is sparse and heterogeneously developed. The intersection of S4 with S2 (L24) moderately plunging towards NE or ENE. It is interpreted that both D2 and D4 related fabrics were formed as a result of sub-horizontal curstal shortening with an intervening sub-vertical shortening (D3). The sparse nature of D4 represents waning stages of the orogenesis. Well-developed near-orthogonal fabrics in these rocks revealed that, 1) deformation is always partitioned and can retain original fabric orientations, and 2) orogenic processes follow repeatedly crustal thickening (compression) coupled with decompression unless the equilibrium is attained in the mid- to upper crust.
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