Dome formation and extension in the Tethyan Himalaya, Leo Pargil, northwest India

Abstract

Metamorphic dome complexes occur within the internal structures of the northern Himalaya and southern Tibet. Their origin, deformation, and fault displacement patterns are poorly constrained. We report new fi eld mapping, structural data, and cooling ages from the western fl ank of the Leo Pargil dome in the northwestern Himalaya in an attempt to characterize its post-middle Miocene structural development. The western fl ank of the dome is characterized by shallow, west-dipping pervasive foliation and WNW-ESE mineral lineation. Shear-sense indicators demonstrate that it is affected by east-west normal faulting that facilitated exhumation of highgrade metamorphic rocks in a contractional setting. Sustained top-to-northwest normal faulting during exhumation is observed in a progressive transition from ductile to brittle deformation. Garnet and kyanite indicate that the Leo Pargil dome was exhumed from the mid-crust. 40 Ar/ 39 Ar mica and apatite fi ssion track (AFT) ages constrain cooling and exhuma-tion pathways from 350 to 60 °C and suggest that the dome cooled in three stages since the middle Miocene. 40 Ar/ 39 Ar white mica ages of 16-14 Ma suggest a fi rst phase of rapid cooling and provide minimum estimates for the onset of dome exhumation. AFT ages between 10 and 8 Ma suggest that ductile fault displacement had ceased by then, and AFT track-length data from high-elevation samples indicate that the rate of cooling had decreased signifi cantly. We interpret this to indicate decreased fault displacement along the Leo Pargil shear zone and possibly a transition to the Kaurik-Chango normal fault system between 10 and 6 Ma. AFT ages from lower elevations indicate accelerated cooling since the Pliocene that cannot be related to pure fault displacement, and therefore may refl ect more pronounced regionally distributed and erosion-driven exhumation.

Climate-driven environmental change in the Zhada basin, southwestern Tibetan Plateau

“…1B). The Zhada basin is located in a zone of active arc-parallel extension (Ni and Barazangi, 1985;Zhang et al, 2000;Murphy et al, 2002;Thiede et al, 2006;Valli et al, 2007;Murphy et al, 2009). It is bounded by the South Tibetan detachment system to the southwest, the Indus suture to the northeast, and the Leo Pargil and Gurla Mandhata gneiss domes to the northwest and southeast, respectively (Fig.…”

“…Exhumation of the Leo Pargil and Gurla Mandhata gneiss domes (Fig. 1B) by normal faulting began 9-10 Ma Murphy et al, 2002;Thiede et al, 2006) and continues today.…”

Middle-Miocene transformation of tectonic regime in the Himalayan orogen

“…At this time, the regime of N-S extension was dominant. When the India-Asia convergence speeded up, the Himalaya experienced a rapid deformation, resulting in Figure 8 Statistical diagram of the U-Pb/U-(Th)-Pb ages of leucogranites and 40 Ar/ 39 Ar ages of leucogranites and metasedimentary rocks in Himalayan orogen (data from [1,[4][5][6][7][8][9][10][11][12][13][15][16][17][18][22][23][24]30,). the thrusting and shorting.…”

“…Since 13 Ma, the regime of N-S contraction was dominant. The U-Pb/U-(Th)-Pb ages of zircon/monazite from leucogranites and 40 Ar/ 39 Ar ages of mica from leucogranites and meta-sedimentary rocks in Himalayan orogen (Figure 8 and Table S1) [1,4-13,15-18, 22-24,30,34-83] show that the peak ages of leucogranites are roughly between 24 Ma and 13 Ma, while the peak 40 Ar/ 39 Ar ages are around 18-13 Ma. This means the emplacement of the leucogranites culminated at the same time as the 40 Ar/ 39 Ar cooling ages during ca.…”

“…This confirms the view that NSTR is probably the result of N-S contraction [29] and provides unequivocal evidence for the transformation of tectonic regime. In order to accurately constrain the time of the transformation of tectonic regime from N-S extension to N-S contraction, we summarize and analyze the ages of STDS and NSTRs in Gyirong and other areas in Himalayan orogen ( Figure 7) [4][5][6][7][8][9][10][12][13][14][15][16][17]22,28,[30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48]. The result shows that the STDS was active from ca.…”