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

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

640 641 642 643 644 645 in western Nepal and eastern Nepal (Figure 4b) and can alternately be interpreted as a succession of the Fagfog, Dandagaon, Nourpul, Dhading, Benighat, and Malekhu formations. A similar lithotectonic succession of units above the Kunchha Formation, has been mapped during this study in the Kali Gandaki valley (Figures 2, 5). The tectonostratigraphy in the Modi and Kali Gandaki river drainages matches known tectonostratigraphy and does not require repitition across a thrust fault. 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 In addition to the tectonostratigraphic data, pervasive high-temperature deformation, as documented by the quartz c-axis fabrics throughout the lower Greater Himalayan sequence of central Nepal, is also inconsistent with the location of the Ramgarh thrust proposed by Pearson & DeCelles (2005). While high-strain recrystallization of quartz has been recognized along the interpreted Ramgrah thrust in central Nepal (Pearson & DeCelles 2005), the associated strain is interpreted to be confined to the lower Kunchha (equivalent to the Kushma Formation; Figure 4b) in the proximal hanging wall and immediately adjacent rocks in the footwall of the inferred thrust. In the Kali Gandaki valley, high temperature (~500˚C), ductile, top-to-the-south deformation extends downwards more than 8 km below the previously mapped MCT and for more than 6 km farther south of the inferred position of the Ramgarh thrust (Figure 1c). Similar deformation profiles across most of central Nepal are apparent from the quartz c-axis data of Bouchez & Pêcher (1981). Thus, the Ramgarh thrust, which was originally defined on the basis of intense mylonitization above the fault and a lack of intense shearing below the fault (Pande 1950; Valdiya 1980) is here reinterpreted in central Nepal to be coincident with the MCT at the base of the Greater Himalayan sequence (Figure 13). It separates the transposed Greater Himalayan sequence above from the less-deformed non-transposed Lesser Himalayan sequence below (Figures 1c, 13). 30
663 6.3 Flow kinematics 664 665 666 667 668 669 670 Detailed mapping in the Kali Gandaki valley, supplemented by microstructural data, shows that the lower 14 km of the Greater Himalayan sequence is pervasively deformed and exhibits dominant top-to-the-south sense of shear. Comparison of deformation temperatures derived from quartz c-axis fabric opening angles to metamorphic equilibrium temperatures from the same structural level suggests that the c-axis fabrics reflect Neohimalayan deformational conditions. Thus, the top-to-the-south sense of shear is interpreted to reflect deformation during southward extrusion of the mid-crust. 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 The vorticity values reported in this study are compatible with structural models of midcrustal flow and extrusion (e.g., Grasemann et al. 1999; Williams et al. 2006) as well as thermomechanical finite-element models (e.g., Beaumont et al. 2001, 2004, 2006; Jamieson et al. 2004, 2006) that predict significant pure shear strain near the base of the mid-crustal rocks. These data are also consistent with deformation in models of critical taper put forth to explain the evolution of the Himalaya (e.g., Bollinger et al. 2006; Kohn 2008) as significant pure shear is implied within the deforming wedge (e.g., Platt 1986). Vorticity data from this study complement data from Bhutan (Carosi et al. 2006), the Everest region (Law et al. 2004; Jessup et al. 2006), westcentral Nepal (Carosi et al. 2007), and northwestern India (Grasemann et al. 1999) and demonstrate that pure shear deformation is evident throughout the Greater Himalayan sequence. Specifically, the range of vorticity values from this study, 0.49 – 0.80 (66 - 41% pure shear) with an average value of 0.67 (53% pure shear) for the lower Greater Himalayan sequence, corroborates other studies that have examined similar structural levels. Data for the adjacent Dolpo region of central Nepal range from 0.66-0.77 (54 – 44% pure shear) with an average vorticity of 0.74 (47% pure shear; Carosi et al. 2007). Data from the lower Greater Himalayan 31
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 and 22: 443 444 445 446 447 448 449 maximum
Page 23 and 24: 488 489 490 491 492 the X-Z plane o
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: 617 618 619 620 621 622 623 624 625
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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