Terrestrial Palaeoecology and Global Change

174 Valentin A. Krassilov. Terrestrial Palaeoecology
In this latter model, the Indian plate has collided with the Xizang (Tibet) microplates
sutured by ophiolitic thrust belts. Of them, the Lhasa (Thanghla) plate of southern Tibet
is a Precambrian cratonic block, the sedimenatry cover of which includes, successively,
the Gondwana-type tillites, the non-marine clastic deposits with a mixed Cathaysian/
Gondwana flora, and the Cretaceous (Aptian to Albian) Takena redbeds, folded in the
Late Cretaceous and the overlain by the Palaeogene volcanics. The fossil wood from
Takena shows distinct growth rings indicating a temperate climate, similar to that over
the present-day mid-latitudes (30-40°N).
Since at the northern (Bangong-Nujiang, or Nu Chang) thrust border the Takena
redbeds spread over the Donqiao ophiolite (Xuchang et al., 1980; Girardeau et al., 1984),
the Lhasa plate should have been sutured to the mainland by the mid-Cretaceous. On
evidence of the older Cathaysian flora it might have occurred anywhere close to the
southern Chinese landmass in the Permian already, while an association of the Gondwanic
floristic elements and tillites indicates that Indian plate should also have been not
far from its present-day position relative to Tibet. Their suturing Yarlung–Tsangbo thrust
belt contains the Albian ophiolite assemblage (the Xigase ophiolite: Allegre et al., 1984;
Nicolas et al., 1981; Tapponier et al., 1981) emplaced by the Campanian. In angiosperm
assemblage from Xigaze Group, 71% of species have non-entire leaf margins, about
64% are deciduous (Guo, 1996), which indicates a warm-temperate climate.
The Tsangbo belt is connected to the Kokhistan – Ladakh arc to the west and to
the Naga Hill – Arakan Yoma arc to the east, forming a semicircular suture with a
series of nappes emplaced upon the Tethys-Himalaya in the Late Cretaceous to Palaeogene
and overthrust by the Trans-Himalayan granitoid belt of about the same age
(Honegger et al., 1982; Thakur & Misra, 1984; Schärer et al., 1984). The significance
of these age assignments will be clear in relation to the other parts of the Tethyan
mosaic (Figs. 70 – 75).
Of the better known Tethyan ophiolites, the Troodos of Cypres on the southern border
of Taurian block is radiometrically dated as the mid-Cretaceous. Cenomanian foraminifers
are found in limestones between lava complexes the upper of which is topped
by umbra and overlain by the Maastrichtian to Palaeogene carbonates (Delaloye et al.,
1980; Pantazias, 1980).
In Zagros, between the Taurian and Iranian (Dasht-E-Lût) blocks, an allochthonous
complex is thrust over the Coniacian – Santonian and is overlain by the Maastrichtian
reefal limestones (Berberian & King, 1981). The Samail ophiolite of Oman was similarly
emplaced in the Campanian to Early Maastrichtian, with the shallow-water Maastrichtian
carbonates deposited on a lateritic weathering crust (Smewing, 1980; Tippit et al.,
1981; Coleman, 1981; Woodcock & Robinson, 1982).
The above chronology of ophiolite emplacements indicate that suturing of all continental
fragments in both the Tauro-Omanian and Afgano-Himalayan branches was accomplished
in the Campanian to Maastrichtian time. The sutures then emerged above
sea level exposing their ophiolites to subaerial weathering.