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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Tectonic Evolution of the India-Asia Suture Zone since ~40Ma,
South-Central Tibet
Suture zones represent more than the location where
continents have welded together. They are zones that extend
for tens (if not hundreds) of kilometers and often archive complex
geologic histories and episodes of long-term deformation
associated with collision. The discontinuous exposure of
Gangdese magmatic arc rocks has long been recognized along the
India-Asia suture zone. The relatively undeformed Cretaceous-
Tertiary Linzizong volcanics, which cap Gangdese rocks, crop out
mostly from 89.5° to 84° E longitude. Forearc deposits along the
suture are discontinuous. The nature of these discontinuities may
be attributed to the irregularity of the active margin, the nature of
Neotethyan subduction, or trapped oceanic crust within irregular
sections of the margin. The area in the vicinity of 29°54N,
84°24E in south-central Tibet is unique because field relationships
between passive margin rocks (Tethyan sedimentary sequence),
forearc deposits (Xigaze Group), magmatic arc rocks (Gangdese
Batholith), and post-collision deposits can be evaluated. To assess
the structural history of this area, I mapped across the northdirected
Great Counter Thrust system (GCT) and implemented
biotite 40Ar/39Ar thermochronology and zircon U-Pb geochronology
on a granite footwall sample. Results show that a U-Pb zircon age
of ~40 Ma corresponds to the age of emplacement of the granite,
while a younger ~19 Ma 40Ar/39Ar age implies a reset of the biotite
system as the granite passed through the 350°C isotherm. I
correlate the post-collision deposits to the Eocene-Oligocene
Qiuwu Formation based on field relationships, stratigraphy and
petrology. In the mapped area, the Qiuwu Formation (~3 km
thick) comprises three conglomerate units. These range from
cobble/gravel/pebble conglomerates rich in volcanic and plutonic
clasts to a young unit that is made up entirely of sedimentary
clasts. I infer from petrology and clast composition that the
Qiuwu Formation had a northern provenance (Lhasa block and
magmatic arc) that supplied fluvial deposits and alluvial fans to
the south. My observations are consistent with other regions,
where paleocurrent data from other studies suggest south-directed
paleoflow. Previous work in the area suggests that the southdirected
Late Oligocene Gangdese Thrust (GT) crops out north
of the GCT; however, there is no evidence of GT deformation in
the study area. In the structural model, the granite intrudes at
~40 Ma, is exposed, buried, then exposed by the GT ~27-24 Ma,
buried a second time, then exposed by the GCT ~19 Ma. I assume
the GCT cut the GT ≥ 19 Ma because the GT is older than the
GCT. To explain the 20 million year gap between the time of
emplacement (~40 Ma) and the time of exposure (~19 Ma),
I invoke the GT to facilitate exhumation of the 40 Ma granite
~27-24 Ma, thus providing detritus to the Qiuwu conglomerate.
The Qiuwu conglomerate shows evidence for greenschist facies
alteration, requiring it to have been buried to ~12-10 km
(~30°C/km). I suggest a blind
thrust in the GCT system was
responsible for exposing the magmatic arc rocks in this area
from ~12-10 km at ~19 Ma. A Middle Miocene ~N-striking,
W-dipping oblique normal fault system cuts and offsets the
India-Asia suture zone. I interpret that this fault exposes a
shallower structural level to its west (where the GT is not
exposed) compared to the east, where the GT has been previously
interpreted by others to crop out. These results highlight events
that have modified the architecture of the India-Asia suture zone
since ~40 Ma.