Thrusting, exhumation and basin fill on the western margin of the South
China block during India-Asian collision
Abstract
The pattern and timing of deformation in the southeast Tibet resulting
from the India-Asian collision remain poorly constrained. Detailed field
mapping, structural analysis and geo-thermochronogic data within a
newly-defined Ludian-Zhonghejiang fold-thrust belt stretching over 120
km between the Diancang Shan and Xuelong Shan metamorphic belt in
western Yunnan, China document Early Cenozoic tectonic evolution of the
conjunction area between the Lanping-Simao and South China blocks. The
study area is cut by two major northwest-striking, southwest-dipping
brittle faults, named Ludian-Zhonghejiang fault and Tongdian fault from
east to west. Kinematic measurements and indicators of S-C fabrics and
striations, as well as juxtaposition of Triassic meta-sedimentary rocks
overlying on Paleocene strata indicate thrusting along the
Ludian-Zhonghejiang fault. Similarly, structural analysis show the
Tongdian fault is also a reverse fault. Between these structures,
fault-bounded Permo-Triassic and Paleocene strata are strongly deformed
by upright southwest-vergent folds with axes that trend nearly parallel
to the traces of the principal faults, consistent with reserve faulting
related to regional NE-oriented compression. Zircon and apatite
(U-Th)/He and apatite fission track ages from a Triassic granitic pluton
in the hanging wall of the Ludian-Zhonghejiang thrust assisted by
inverse modeling reveal a period of accelerated cooling from 50 Ma to 37
Ma, which is interpreted to record the lifespan of the fold-thrust
system collaborated by the intrusive relations between magmas of
~35 Ma dated by zircon U-Pb and the fold and thrust
belt. Since 37 Ma, decreasing cooling rates implies cessation of the
thrusting. Early Cenozoic activity of the deformation system likely
controlled deposition of the Jianchuan-Liming basin evident by coeval
sediments derived from the proximal hanging wall of the fold-thrust
belt. These results, together with tectonic records of contraction in
east Tibet, suggest crustal shortening related to the India-Asian
collision and convergence prevailed the southern and eastern part of the
Tibetan Plateau, which predated Oligo-Miocene onset of extrusion
tectonics in southeast Tibet.