Near-orthogonal deformation sequence along the Malakand transect, NWFP, northern Pakistan

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. D-1 structures are rare, both at the meso- and microscale. This is because of the penetrative effects of the later D-2 event. D-2 not only transposed and obliterated the D-1 structures, but also reactivated the S-1 foliation. S-2 is steeply dipping foliation, dominantly towards north, and E-W striking. S-3 makes an orthogonal relationship with the S-2 and this geometrical relationship gives rise to L(3)(2 )intersection lineation, which is shallowly plunging either towards ENE or WSW. S-3 is generally sub-horizontal and well developed. The orientation of S-4 is very similar to that of S-2. However, S-4 is sparse and heterogeneously developed. The intersection of S-4 with S-2 (L-4(2)) moderately plunging towards NE or ENE. It is interpreted that both D-2 and D-4 related fabrics were formed as a result of sub-horizontal curstal shortening with an intervening sub-vertical shortening (D-3). The sparse nature of D-4 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.