Structural analysis, origin and deformation conditions of ductile shear zones in Dehnow Granitoied Pluton – west of Mashhad

Abstract

The Mashhad ophiolitic and metamorphic complex was intruded by Mashhad granitoids during different episodes of magmatisms. Dehnow pluton with diorite-granodiorite composition was intruded at earliest stage of magmatism during Late Triassic (Norian, 215± 4My). This pluton is cut by the NW-SE trending ductile shear zones which are dominated by an intense ductile deformation. The rocks of Dehnow pluton along the shear zones are converted into protomylonite, mylonite and ultra-mylonite with steeply dipping mylonitic foliation and gently plunging stretch lineation on it. Kinematic analysis of shear sense indicators such as S-C fabrics, asymmetric folds, asymmetric porphyroclasts, mica fish and domino-type fragmented porphyroclasts reveal that the ductile deformation is related to right lateral reverse slip that may describe by a transpressional deformation regime. the minerals in the mylonitic rocks show variable microstructures such as patchy to crosshatch undulose extinction, shear fracture, deformation lamellae, subgrains formation, bulging  dynamic recrystalization (BLG) and subgrain rotation recrystalization (SGR) in quartz grains, also subgrain formation, deformation twins, flame-shaped prethities, and dynamic recrystalization (BLG) in feldspars, these indicated that the ductile deformation have occured in 300-500 °C (upper green schist and lower amphibolites facies conditions). Age and cross cutting relations between various lithological units in the study area revealed that the ductile shear zones were formed during Cimmerian orogeny between Norian to Early Jurassic age.

Keywords


[1] Ramsay J.G., Graham R.H., "Strain variation in shear belts", Canadian Journal of Earth Sciences, 7 (1970) 786-813.

[2] Ramsay J.G., "Shear zone geometry: a review". Journal of Structural Geology, 2 (1980)83–101

[3] Isik V., "The ductile shear zone in granitoid of the Central Anatolian Crystalline Complex, Turkey: Implications for the origins of the Tuzgölü basin during the Late Cretaceous extensional deformation",Journal of Asian Earth Sciences, 34 (2008), 507-521.

[4] Passchier C.W., Trouw R.A.J., "Microtectonics", second ed., Springer, Berlin (2005).

[5] Sibson R.H., "Fault rocks and fault mechanisms", Journal of the Geological Society, London 133 (1977), 191–213.

[6] Bowden P.B., "A criterion for inhomogeneous plastic deformation". Philosophical Magazine 22 (1970), 455-462.

[7] Poirier J.P., "Shear localization and shear instability in materials in the ductile field". Journal of Structural Geology, 2 (1980), 135-142.

[8] Mancktelow N.S., Pennacchioni G., "The control of precursor brittle fracture and fluid-rock interaction on the development of single and paired ductile shear zones", Journal of Structural Geology, 27 (2005), 645-661.

[9] Segall P., Pollard D.D., "Nucleation and growth of strike slip faults in granite", Journal of Geophysical Research, 88 (1983), 555-568.

[10] Tullis J., Dell’Angelo L., Yund R.A., "Ductile shear zones from brittle precursors in feldspathic rocks: the role of dynamic recrystallization", In: Duba, A., Durham, W., Handin, J., Wang, H. (Eds.), "The Brittle-Ductile Transition", The Heard volume. American Geophysical :union: Monograph, 56 (1990), 67-82.

[11] Pennacchioni G., " Control of the geometry of precursor brittle structures on the type of ductile shear zone in the Adamello tonalites, Southern Alps (Italy)", Journal of Structural Geology 27 (2005), 627-644.

[12] Majidi B., "The geochemistry of ultra basic and basic lava flows occurrences in northeastern Iran, In Geodynamic project in Iran". Geological Survey of Iran, 51 (1983), 463-477

[13] Alavi M., "Sedimentary and structural characteristics of the Paleo-Tethys remnants in northeastern Iran". Geological Society of America Bulletin, 103 (1991): 983-992.

[14] Ghazi M., Hassanipak A.A., Tucker P.J., Mobasher K., 2001, "Geochemistry and 40Ar-39Ar ages of the Mashhad Ophiolite, NE Iran". Abstracts as: Eos. Trans. AGU, 82 (47), (2001) Fall Meet.

[15] Karimpour M.H., Farmer L., Ashori C., Saadat S, "Major, Trace and REE geochemistry of Paleo-Tethys Collision-Related Granitoids from Mashhad, Iran", Journal of Science,Islamic Republic of Iran,17 (2006),127-145.

[16] Karimpour M.H., Stern C.R., Farmer G.L., "Zircon U–Pb geochronology, Sr–Nd isotope analyses, and petrogenetic study of the Dehnow diorite and Kuhsangi granodiorite (Paleo-Tethys), NE Iran", Journal of Asian Earth Sciences, 37 ( 2010), 384-393.

[17] Hirth G, Tullis J., "Dislocation creep regimes in quartz aggregates", Journal of Structural Geology, 14 (1992), 145–159.

[18] Pryer L.L., Robin P.Y.F., "Retrograde metamorphic reactions in deforming granites and the origin of flame perthite", Journal of Metamorphic Geology, 14 (1995), 645–658.

[19] Ten Grotenhui,s S.M, Trouw R.A.J., Passchier C.W., "Evolution of mica fish in mylonitic rocks". Tectonophysics 372 (2003) 1–21.

[20] Nyman M.W., Law R.D., Smelik, E., "Cataclastic deformation mechanism forthe development of core-mantle structures in amphibole", Geology, 20 (1992), 455–458.

[21] Stipp M., Stünitz H., Heilbronner R., Schmid S.M., "The eastern Tonale fault zone: a ‘‘natural laboratory” for crystal plastic deformation of quartz over a temperature range from 250° to 700°C ". Journal of Structural Geology 24 (2002), 1861–1884.