ALBERT

Description: The geometry of the entire crust from the northern part of the Tarim Basin to the southwestern Tian Shan east of Kashi is imaged on a N-S–directed explosive-source deep seismic-reflection profile. The profile reflects the sedimentary formations in the northern part of the Tarim Basin and the fold-and-thrust belt of the southern Tian Shan. N-dipping reflectors of the lower crust, as well as fluctuations in Moho depth, below which several mantle reflectors were observed, reveal the fine crustal structure beneath the junction of the southwest Tian Shan and the Tarim Basin. Mesozoic–Cenozoic shortening of the southwestern Tian Shan occurred at a crustal scale involving detachment-related folding in the basin directed northward toward the mountains and reverse faulting in the mountains directed toward the basin. In addition, a crocodile fabric developed within the lower crust beneath the basin area. The lithospheric structure revealed by the seismic-reflection section between the Tarim Basin and the Tian Shan Mountains reflects a process of intracontinental collision.

Description: Field, structural, kinematic, and deformation temperature analyses were conducted on rocks from the Lhagoi Kangri gneiss dome (southern Tibet) in order to establish the geologic history of the dome, identify major phases of deformation within the dome, and to relate these phases of deformation to the tectonic evolution of the Himalayan middle crust. The Lhagoi Kangri dome, one of a series of gneiss-cored domes in southern Tibet, records stratigraphy and structural features similar to previously studied north Himalayan gneiss domes. Field mapping reveals a sequence of rocks that comprise a cover of unmetamorphosed to amphibolite-grade siliciclastic and minor carbonate rocks overlying a core predominantly composed of foliated and lineated orthogneiss intruded by relatively undeformed granite, which also intrudes the cover rocks both concordantly and discordantly. Field observations and microstructural analyses suggest that the contact between the core and cover rocks was originally a nonconformity, but we do not rule out the possibility of subsequent slip along the surface, as has been reported for correlative structures in other domes. Lhagoi Kangri rocks were pervasively deformed during at least two major tectonic phases. The earliest deformation event (D1) resulted in shortening and thickening of crust, the record of which is largely eliminated, particularly in lower structural levels, by transposition and recrystallization during the second phase of deformation (D2). Ductile deformation during D2 is characterized at higher structural levels by crenulation cleavage that tightens with depth, while at lower structural levels D2 manifests as a distributed shear zone that records some evidence of both plane strain and coaxial flattening, possibly indicating overall heterogeneous general shear. The shear zone is ~3 km thick and contains rocks with mostly symmetrical top-to-north and top-to-south shear sense indicators with a dominant top-to-north component at lower structural levels. Microstructural analyses and quartz c-axis fabrics indicate a range of D2 shear zone temperatures from 200 to 300 °C at the upper boundary to ≥630 °C at the lowest structural levels sampled with minimal evidence of lower temperature overprinting. The interpreted temperatures define a wide range in thermal field gradients (18–90 °C/km) that suggest that temperature indicators were locked in at relatively late stages of D2. The structural framework and kinematic history of the Lhagoi Kangri dome are similar to previously studied north Himalayan gneiss domes as well as to transects through the South Tibetan detachment system, which supports previous interpretations of structural continuity between the north Himalayan gneiss domes and other middle crustal exposures in the Himalaya. The Lhagoi Kangri distributed shear zone, in particular, may represent a deeper ductile manifestation of the South Tibetan detachment system.

Description: Most of the Himalayan Cenozoic leucogranites are products of partial melting of metapelite sources. In the Malashan-Gyirong area (southern Tibet), the geochemical compositions of leucogranites define two groups with distinct whole-rock major elements, large ion lithophile elements, rare earth elements, high field strength elements, and Sr and Hf isotope ratios. Based on published experimental results that define generalized melting reactions of metapelitic sources, we infer that these leucogranites are the products of two different types of crustal anatexis: fluid-fluxed melting and fluid-absent melting of muscovite in metasedimentary sources. As compared to the leucogranites derived from fluid-absent melting, those from fluid-fluxed melting have relatively higher Ca, Sr, Ba, Zr, Hf, Th, and light rare earth element concentrations, and Zr/Hf, Eu/Eu*, and Nd/Nd*, but lower Rb, Nb, Ta, and U concentrations, Rb/Sr and 87 Sr/ 86 Sr ratios, and Hf (t). The geochemical differences can be explained by melting behaviors of major (muscovite, feldspar) and accessory minerals (zircon and monazite) during different modes of crustal anatexis. The systematic elemental and isotopic signatures of different types of crustal anatexis and, in particular, the coupling of major and trace elements that results from common influences on rock-forming and accessory mineral behaviors provide tools with which to refine our understanding of the nature of crustal anatexis.

Description: Field, structural, kinematic, and deformation temperature analyses were conducted on rocks from the Lhagoi Kangri gneiss dome (southern Tibet) in order to establish the geologic history of the dome, identify major phases of deformation within the dome, and to relate these phases of deformation to the tectonic evolution of the Himalayan middle crust. The Lhagoi Kangri dome, one of a series of gneiss-cored domes in southern Tibet, records stratigraphy and structural features similar to previously studied north Himalayan gneiss domes. Field mapping reveals a sequence of rocks that comprise a cover of unmetamorphosed to amphibolite-grade siliciclastic and minor carbonate rocks overlying a core predominantly composed of foliated and lineated orthogneiss intruded by relatively undeformed granite, which also intrudes the cover rocks both concordantly and discordantly. Field observations and microstructural analyses suggest that the contact between the core and cover rocks was originally a nonconformity, but we do not rule out the possibility of subsequent slip along the surface, as has been reported for correlative structures in other domes. Lhagoi Kangri rocks were pervasively deformed during at least two major tectonic phases. The earliest deformation event (D1) resulted in shortening and thickening of crust, the record of which is largely eliminated, particularly in lower structural levels, by transposition and recrystallization during the second phase of deformation (D2). Ductile deformation during D2 is characterized at higher structural levels by crenulation cleavage that tightens with depth, while at lower structural levels D2 manifests as a distributed shear zone that records some evidence of both plane strain and coaxial flattening, possibly indicating overall heterogeneous general shear. The shear zone is ~3 km thick and contains rocks with mostly symmetrical top-to-north and top-to-south shear sense indicators with a dominant top-to-north component at lower structural levels. Microstructural analyses and quartz c-axis fabrics indicate a range of D2 shear zone temperatures from 200 to 300 °C at the upper boundary to ≥630 °C at the lowest structural levels sampled with minimal evidence of lower temperature overprinting. The interpreted temperatures define a wide range in thermal field gradients (18–90 °C/km) that suggest that temperature indicators were locked in at relatively late stages of D2. The structural framework and kinematic history of the Lhagoi Kangri dome are similar to previously studied north Himalayan gneiss domes as well as to transects through the South Tibetan detachment system, which supports previous interpretations of structural continuity between the north Himalayan gneiss domes and other middle crustal exposures in the Himalaya. The Lhagoi Kangri distributed shear zone, in particular, may represent a deeper ductile manifestation of the South Tibetan detachment system.