ABSTRACT
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
Geological
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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