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

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466 467 468 Vannay & Hodges [1996]; Khumbu thrust, Searle [1999]), strain localization and contribution of shear heating, or perhaps diachronous metamorphism and peak temperature that varies with structural position. 469 5. Vorticity 470 471 472 473 474 475 Vorticity is defined as the internal rotational component of flow and is equal to the mean angular velocity of material lines with respect to the instantaneous stretching axes (Passchier & Trouw 2005). Vorticity data are critical to understanding flow kinematics in ductily deformed rocks. In the Himalayan orogen recent vorticity studies have begun to empirically-constrain strain within the Greater Himalayan sequence and its bounding shear zones (Grasemann et al. 1999; Law et al. 2004; Jessup et al. 2006; Carosi et al. 2006, 2007). 476 477 478 479 480 481 482 483 484 For plane strain conditions, as indicated by the dominance of type-I cross-girdled quartz c-axis fabrics, the relative proportions of pure shear and simple shear can be expressed in terms of the kinematic vorticity number W k , which relates instantaneous rotation to instantaneous stretching at a point (Bailey et al. 2004). For an entirely pure shear system W k = 0, while in simple shear W k = 1. The ratio of pure shear to simple shear is not a linear relationship; equal contributions of both are made to the instantaneous flow at W k = 0.71 (demonstrated by Law et al., 2004). In natural systems it is rarely possible to quantify rotation rates versus stretch in deformed rocks (Baily et al. 2004), therefore, vorticity of flow is more appropriately approximated by W m a mean vorticity number for plane strain deformation. 485 486 487 Vorticity of flow recorded in specimens collected from the lower-middle portion of the Greater Himalayan sequence has been estimated using the Rigid Grain Net (RGN) of Jessup et al. (2007) and the method of Wallis (1992, 1995). Both methods require data collected within 22
488 489 490 491 492 the X-Z plane of the strain ellipse; therefore, thin sections were cut perpendicular to foliation and parallel to lineation. Quartz c-axis fabrics are interpreted to show that the strain recorded in these rocks is related to the southward transport of the Greater Himalayan sequence. Vorticity analysis of the same rocks, therefore, provides additional kinematic constraints on the same process. 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 W m values were estimated using the RGN (Jessup et al. 2007) for three different samples at various structural positions within the lower portion of the Greater Himalayan sequence (Figure 10). This method requires the same assumptions inherent to all vorticity estimates that are based on the orientation of rigid grains. These assumptions include no mechanical interaction between porphyroclasts, a homogenous matrix, high enough strain to allow grains to reach stable sink positions, and a wide distribution of aspect ratios amongst the porphyroclasts. In sample 053, the structurally highest sample analyzed, 138 amphibole and epidote porphyroclasts that show no internal strain (Figure 10a) and were measured in a ductily deformed quartz matrix yield W m values between 0.61 - 0.73, which corresponds to a pure shear component of 48 - 58% (Figure 10b). 106 porphyroclasts predominantly of amphibole (Figure 10c), also in a ductily deformed quartz matrix, were measured in multiple thin sections of sample 070B. They yield W m values between 0.60 and 0.80, which correspond to a pure shear component of 41 - 59% (Figure 10d). The structurally lowest sample analyzed, 073, is from the Ulleri augen orthogneiss (Figures 2 and 6). Multiple thin sections of this sample were cut to measure a total of 141 feldspar porphyroclasts. The feldspars are locally brittlely fractured but otherwise show no internal strain (Figure 10e). The W m values estimated for sample 073 are interpreted to range between 0.69 and 0.71 corresponding to a pure shear component of 50 - 52% (Figure 10f). 23
Page 1 and 2: 1 2 3 4 5 Kinematics of the Greater
Page 3 and 4: 39 1. Introduction 40 41 42 43 44 4
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
Page 9 and 10: 173 3.3 Greater Himalayan sequence
Page 11 and 12: 218 219 220 221 222 8 cm apart. In
Page 13 and 14: 263 quartzite and, locally, more im
Page 15 and 16: 307 308 309 310 311 312 In this stu
Page 17 and 18: 352 4.1 Quartz petrofabric analysis
Page 19 and 20: 398 399 400 401 402 403 404 405 406
Page 21: 443 444 445 446 447 448 449 maximum
Page 25 and 26: 534 535 through the following equat
Page 27 and 28: 572 573 574 575 576 577 578 579 val
Page 29 and 30: 617 618 619 620 621 622 623 624 625
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
Page 37 and 38: 765 766 References 767 768 769 770
Page 39 and 40: 830 831 832 833 834 835 836 837 838
Page 41 and 42: 900 901 902 903 904 905 906 907 908
Page 43 and 44: 970 971 972 973 974 975 976 977 978
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
Page 51 and 52: 1205 1206 1207 1208 1209 plotted ag
Page 53 and 54: 1247 1248 1249 1250 1251 1252 1253
Page 55 and 56: 83˚ 30'E South Tibetan Detachment
Page 57 and 58: A Previous Studies (Central Nepal)
Page 59 and 60: SW NE NW SE A B W E W E C D
Page 61 and 62: 018 015 grain shape foliation 038 2
Page 63 and 64: SW A S Qtz C Qtz Ep Am Am Am Ms 200
Page 65 and 66: 10 percent pure shear 80 70 60 50 4
Page 67: (A) percent pure shear 60 50 40 30

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