A fission track and 40Ar/39Ar thermochronology study of plutonic and volcanic rocks from the Gangdese magmatic belt, southern Tibet, gives evidence for average cooling rates of 4-10°C/Ma (54-0 Ma) and unroofing rates of 0.1-0.3 mm/y for this part of the Gangdese batholith. Linzizong volcanics of the Gangdese belt in the Maqu area have experienced deep burial and slow cooling, and an unroofing rate of ~0.3 mm/y was obtained for the period of 40-55 Ma. It is confirmed that there has been a pulse of unroofing (>3 mm/y) at 20-15 Ma in the Quxu pluton of the Gangdese batholith based on fission track and 40Ar/39Ar K-feldspar modeling results. It is evident from fission track data that this pulse was also experienced by plutons north of the Quxu area. These data suggest that onset of intense uplift of southern Lhasa terrane was long after (20 Ma later than) the beginning of collision between India and Asia, and that the uplift has been episodic rather than steady. Therefore, some tectonic models of the India-Eurasia collision, such as Continental Underthrusting, and Continental Injection, are not favored by these data.
New information on the eruption ages of the volcanic rocks in the southern Lhasa terrane has been obtained. An eruptional age of 50 +/- 0.6 Ma of the upper part of Linzizong sequence is indicated by a flat K-feldspar 40Ar/39Ar age spectrum. The lower part of the Linzizong sequence may have been formed 60-65 Ma ago. Miocene age (~15 Ma) volcanic rocks in Majiang area were found to be conformable with conglomerates containing pebbles of Gangdese magmatic rocks, showing that this molasse deposit is middle Miocene in age.
Detailed structural mapping in the Maqu area suggests that the total crustal shortening in the Linzizong Formation is about 15-25%, and the minimum shortening in the Cretaceous Takena Formation and older sediments is about ~40 % along the Maqu and Ganden sections. A biotite sample from a post-deformation diorite stock in the Takena Formation yields a flat age spectrum of ~65 Ma, with an isochron age of 65.6 +/- 1.9 Ma. This requires that the crustal shortening represented by the deformation in the Takena formation occurred prior to the start of the India-Asia collision.
A major low-angle ductile shear zone, containing S-C mylonites, involves metamorphosed granitic rocks at the southeastern edge of the Nyainqentanglha mountain range. Kinematic criteria consistently indicate a top-to-SE sense of shear. FT and 40Ar/39Ar data suggest very young ages (9-0 Ma) and rapid cooling (up to ~200°C/Ma) of the footwall rocks. This shear zone and associated metamorphic rocks are interpreted as a metamorphic core complex structure. The timing of deformation suggests that the extensional tectonics in this area started at 8 +/- 3 Ma, which in turn may indicate that the maximum sustainable crustal thickness was reached in southern Tibet around this time.
A substantial E-W striking and N-dipping ductile shear zone was found within granitic rocks of the Gangdese plutonic belt, in the Quxu area, just north of the Indus-Zangbo suture. Prominent stretching lineation lies on the foliation and is parallel to the dip direction. Quartz c-axis fabrics show strongly asymmetric patterns within the shear zone. Kinematic indicators suggest a top-to-north sense of shear. The lower grade phyllitic rocks in the northern part of this shear zone have been sheared into phyllonites. It is suggested that the shearing deformation occurred between 30-40 Ma, not long after the emplacement of the Quxu pluton. We interpret this shear zone as a N-S extensional fault in the upper crust, and its origin is probably similar to that of the younger, large-scale, E-W trending low-angle normal faults documented in the Higher Himalayas to the south of the suture zone.
Pan, Y., 1993. Unroofing history and structural evolution of the
southern Lhasa Terrane, Tibetan Plateau: implications for the
continental collision between India and Asia.
Unpublished PhD dissertation, State University of New York at Albany. 330pp., +xv
University at Albany Science Library call number: SCIENCE MIC Film QE 40 Z899 1993 P36
Copies of this PhD dissertation can be ordered from Proquest UMI
Front matter (title, table of contents, abstract, acknowledgements) - 0.6MB pdf file
Photo pages in dissertation (colour and greyscale photos with captions): - 22.6MB pdf file
map and cross-section of the Doilung Chu valley, NW of Lhasa
- 0.17MB pdf file
(coloured geological map and cross section; map scale ~1:141,000 - coloured versions of Figures 6.1 and 6.15 in dissertation)
Geological cross-section of the Kyu Chu valley through Ganden and Dagze, NE of Lhasa - 0.11MB pdf file
(uncoloured geological cross section and regional location map - Figures 6.16a and 6.16b in dissertation)
[Map, both cross sections and location map combined] - 0.2MB pdf file
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