Kinematics of the Greater Himalayan sequence, Dhaulagiri Himal ...

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$Fayek & Kyser 2000 uraninite fractionations.pdf - Queen's University$

18 Abstract 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 In the Dhaulagiri region of central Nepal quartz-rich specimens sampled from the Greater Himalayan sequence yield well-defined quartz c-axis fabrics with a dominant top-to-thesouthwest sense of shear. These fabrics reflect pervasive crystal-plastic deformation extending more than 8 km structurally below previously mapped locations of the Main Central thrust fault. Quartz c-axis fabric opening angles suggest deformation temperatures of c.500 ± 50°C within the lower portion of the Greater Himalayan sequence and up to c.670 ± 50°C within the migmatitic upper portion. These temperatures generally plot within error of garnet-biotite temperature estimates for metamorphic assemblages and are interpreted to reflect Tertiary deformation during extrusion of the mid-crust. The quartz c-axis data, a new, detailed tectonostratigraphic map, constrain the position of the Ramgarh thrust in the Dhaulagiri region to be coincident with the Main Central thrust at the base of the pervasively deformed exhumed mid-crustal core. Mean kinematic vorticity numbers (W m ) measured in specimens sampled from the lower portion of the Greater Himalayan sequence range between 0.49 and 0.80 (c.66-41% pure shear) with an average value of 0.67 (c.53% pure shear). These data indicate that extrusion of the mid-crust was facilitated by a significant component of pure shear strain. 34 35 36 37 38 2
39 1. Introduction 40 41 42 43 44 45 46 47 Recently proposed hypotheses of gravitationally-driven mid-crustal flow have influenced the direction of research on orogens (e.g., Grujic et al. 1996, 2002; Beaumont et al. 2001, 2004; Jamieson et al. 2004; White et al. 2004; Searle & Szulc 2005; Williams & Jiang 2005; Law et al. 2006[and references therein]; Lee & Whitehouse 2007). In the Himalaya, where the concept of mid-crustal flow has largely been developed (Nelson et al. 1996; Grujic et al. 1996; Beaumont et al. 2001), the exhumed migmatitic mid-crustal rocks of the Greater Himalayan sequence have been interpreted to represent a formerly-ductile mid-crustal layer that was extruded laterally from beneath the Tibetan plateau above the Main Central thrust fault (MCT). 48 49 50 51 52 53 54 55 56 57 58 59 60 61 The Greater Himalayan sequence is characterized by greenschist to granulite-grade metamorphism and associated anatectic melt. It is separated from the structurally-overlying unmetamorphosed Tethyan sedimentary sequence by the South Tibetan detachment system (STDS), a network of top-to-the-north-sense normal faults that extends the length of the orogen (Figure 1a; Burchfiel et al. 1992). Along its base, the Greater Himalayan sequence is thrust over unmetamorphosed to low grade metamorphic rocks of the Lesser Himalayan sequence across the top-to-the-south-sense MCT (Figure 1b). Contemporaneous Miocene displacement along the top-to-the-north sense STDS and the top-to-the-south sense MCT (see review of age constraints in Godin et al. 2006a) extruded the Greater Himalayan sequence from beneath the Tibetan plateau and facilitated its intrusion into the rapidly eroding Himalaya during the Miocene. While the general concept of this extrusive mid-crustal flow has been applied to various orogens (e.g., Law et al. 2006), the processes involved have been the subject of numerous orogen-scale thermal-mechanical, numerical, and structural models (Burchfiel et al. 1992; Grujic et al. 1996, 2002; Grasemann et al. 1999; Vannay & Grasemann 2001; Beaumont et al. 2001, 2004; 3
Page 1: 1 2 3 4 5 Kinematics of the Greater
Page 5 and 6: 84 85 86 87 88 provide new insight
Page 7 and 8: 129 130 131 132 133 134 135 The Dha
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Page 11 and 12: 218 219 220 221 222 8 cm apart. In
Page 13 and 14: 263 quartzite and, locally, more im
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Page 17 and 18: 352 4.1 Quartz petrofabric analysis
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Page 21 and 22: 443 444 445 446 447 448 449 maximum
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Page 31 and 32: 663 6.3 Flow kinematics 664 665 666
Page 33 and 34: 709 710 711 712 713 714 715 716 717
Page 35 and 36: 753 754 better understand the proce
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Page 39 and 40: 830 831 832 833 834 835 836 837 838
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Page 45 and 46: 1039 1040 1041 1042 1043 1044 1045
Page 47 and 48: 1107 1108 1109 1110 1111 1112 1113
Page 49 and 50: 1163 1164 1165 1166 1167 a leucogra
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1247 1248 1249 1250 1251 1252 1253
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83˚ 30'E South Tibetan Detachment
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A Previous Studies (Central Nepal)
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SW NE NW SE A B W E W E C D
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018 015 grain shape foliation 038 2
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SW A S Qtz C Qtz Ep Am Am Am Ms 200
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10 percent pure shear 80 70 60 50 4
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(A) percent pure shear 60 50 40 30
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