Unfolding of refolded fabrics using Foliation Intersection Axes (FIA) preserved within the porphyroblasts: an example from the Michni area, Mohmand Agency, NWFP, Pakistan
Abstract
Integrated macro-, meso- and microstructural analysis of the part of the Michni area, located ill the Mohmand Agency of NWFP, Pakistan, revealed at least five fabric- generating deformation events. The study area is characterized by a regional scale NNE-SSW trending synformal structure with moderately dipping pervasive foliation. Detailed microstructural analysis using Foliation Intersection/lnflection Axis technique (FIA) showed that the geometry of this regional fold postdates at least two bulk-shortening 'directions. The youngest FIA set 3 trends parallel to the axis of the synform, whereas FIA set 2 and I trend at high angle to the fold axis. Orientation of the FIA set I suggests early NNE-SSW shortening followed by NNW-SSE shortening. The trend of the FIA set 3 implies ESE-WNW bulk shortening. The orientation of FIA set 1 and 2 were obtained from inclusion trails preserved within the porphyroblasts. This suggests that the matrix foliation was the end product after extensive recycling of foliations, where the remains of early deformations were only preserved within the porphyroblasts. Unfolding of foliations using FIA technique has important implications to determine early bulk shortening directions, which is not possible by any other means. Moreover, the results of this study signify multiple shortening directions prior to or. synchronous with the Himalayan orogenesis, which were not reported previously.
References
Adshead-Bell, N.S. & Bell, T.H., 1999. The progressive development of a macroscopic upright fold pair during five near-orthogonal foliation-producing events: complex microstructures versus a simple macrostructure. Tectonophys., 306, 121-147.
Aerden, D.G.A.M., 1995. Porphyroblast non-rotation during crustal extension in the Variscan Pyrenees. J. Struct. Geol., 17, 709-726.
Baig, M.S., Lawrence, R.D. & Snee, L.W., 1988. Evidence for late Precambrian to Early Cambrian orogeny in northwest Himalaya, Pakistan. Geol. Mag., 125, 83-86.
Beck, R.A., Burbank, D.W., Sercombe, W.J., Khan, A.S. & Lawrence, R.D., 1996. Late Cretaceous ophiolite obduction and Paleocene India-Asia collision in the westernmost Himalaya. Geod. Acta., 9, 114-144.
Bell, T.H. & Rubenach, M.J., 1983. Sequential porphyroblast growth and crenulation cleavage development during progressive deformation. Tectonophys., 92, 171-194.
Bell, T.H. & Welch, P.W., 2002. Prolonged Acadian orogenesis: revelations from foliation intersection axis (FIA) controlled monazite dating of foliations in porphyroblasts and matrix. Am. J. Sci., 302, 549-581.
Bell, T.H., Forde, A. & Wang, J., 1995. A new indicator of movement direction during orogenesis: measurement technique and applications to the Alps. Nerra Nova, 7, 500-508.
Bell, T.H., Hickey, K. A. & Upton, J.G., 1998. Distinguishing and correlating multiple phases of metamorphism across a multiply region using the axes of spiral, staircase and sigmoidal inclusion trails in garnet. Jour. Meta. Geol., 16, 767-794.
Cihan, M., 2004. The drawbacks of sectioning rocks relative to fabric orientations in the matrix. A case study from the Robertson River Metamorphics (Northern Queensland, Australia). J. Struct. Geol., 26, 2157-2174.
Coward, M.P., Butler, R. W. H., Khan, M. A. & Knipe, R. J., 1987. The tectonic history of Kohistan and its implications for Himalayan structure. J. Geol. Soc. Lon., 144, 377-391.
DiPietro, J.A., Pogue, K.R., Hussain, A. & Ahmad, I., 1999. Geologic map of the Indus syntaxis and surrounding area, northwest Himalaya, Pakistan. Geol. Soc. Am. sp. Pap., 328, 159-178.
Fyson, W. K., 1980. Fold fabrics and emplacement of an Archean granitoid pluton, Cleft Lake, Northwest Territories. Canadian J. Earth Sci., 17, 325-332.
Hayward, N., 1990. Determination of early fold axis orientation within multiply deformed rocks using porphyroblasts. Tectonophys., 179, 353-369.
Ikeda, T., Shimobayashi, N., Wallis, S. R. & Tsuchiyama, A., 2002. Crystallagraphic orientation, chemical composition and threedimensional geometry of sigmoidal garnet: evidence for rotation. J. Struct. Geol., 24, 1633-1646.
Iig, B.R. & Karlstrom, K.E., 2000. Porphyroblast inclusion trail geometries in the Grand Canyon: evidence for non-rotation and rotation? J. Struct. Geol., 22, 231-243.
Johnson, S. E., 1990. Lack of porphyroblast rotation in the Otago schists, New Zealand: implications for crenulation cleavage development, folding and deformation partitioning. J. Meta. Geol., 8, 13-30.
Johnson, S.E., 1999. Porphyroblast microstructures: a review of current and future trends. Am. Min., 84, 1711-1726.
Johnson, S.E., Duppee, M.E. & Guidotti, C.V., 2006. Porphyroblasts rotation during crenulation cleavage development: an example from the Mooselookmeguntic pluton, Maine USA. J. Meta. Geol., in press.
Kretz, R., 1983. Symbols for rock-forming minerals. Am. Min., 68, 277-279.
Passchier, C.W. & Trouw, R.A.J., 1998. Micro-tectonics. Springer-Verlag, Berlin.
Passchier, C.W., Trouw, R.A.J., Zwart, H.J. & Vissers, R.L., 1992. Porphyroblast rotation: eppur si muove? J. Meta. Geol., 10, 283-294.
Ramsay, J.G., 1962. The geometry and mechanics of formation of similar type folds. J. Geol., 70, 309-327.
Sadin, M. & Aslam, M., 1987. Minerals Reconnaissance in Mohmand Agency, FATA, Pakistan. Geol. Surv. Pak. Inf. Rel. No., 284.
Sayab, M., 2005. Microstructural evidence for N-S shortening in the Mount Isa Inlier (NW Queensland, Australia): the preservation of early W-E trending foliations in porphyroblasts revealed by independent 3D measurement techniques. J. Struct. Geol., 27, 1445-1468.
Tahirkheli, R.A.K., 1979. Geology of Kohistan and adjoining Eurasian Indo-Pakistan continents, Pakistan. Geol. Bull. Univ. Peshawar, 1-30.
Timms, N.E., 2003. Garnet porphyroblast timing and behaviour during fold evolution: implications from a 3-D geometric analysis of a hand-sample scale fold in a schist. Jour. Meta. Geol. 21, 853-873.
Treloar, P.J., Rex, D.C., Guise, P.G., Coward, M.P., Searle, M. P., Windley, B.F., Petterson, M.G., Jan, M.Q. & Luff, L.A., 1989a. K-Ar and Ar-Ar geochronology of the Himalayan collision in NW Pakistan: Constraints on the timing of suturing, deformation, metamorphism and uplift. Tectonophys., 8, 881-909.
Treloar, P.J., William, M.P. & Coward, M.P., 1989b. Metamorphism and crustal stacking in the north Indian plate, North Pakistan. Tectonophys., 165, 167-184.
