Examples of modern orogenic belts - Introducing Tectonics, Rock Structures and Mountain Belts - page 88

Figure 11.2 The Himalayan orogenic belt. A. Simplified map showing
the main tectonic units and structural lineaments of the Himalayan
belt; the main mountain belt is shown in red, the Asian plate in
shades of yellow; the Indian plate in brown. HXS, Hoh Xil suture;
BNS, Bangong- Nujiang suture; ITS, Indus-Tsangpo suture; HFT,
Himalayan frontal thrust; QTF, Quetta-Chaman strike-slip fault; ATF,
Altyn Tagh strike-slip fault. B. Cartoon cross-sections illustrating
how the Himalayan belt evolved from the subduction phase (1) to
the collision phase (2). ST, South Tibet block; NT, North Tibet block;
T, Tarim block; other symbols as (A). Note sutures BNS and HXS
marking the sites of earlier subduction-collision events. The extent
of Indian plate beneath Tibet is uncertain but probably extends to
the BNS suture (see Figure 11.4).

ATF

central Asia

Pamir
Range

Tarim

HXS

Kun Lun
range

North Tibet

B NS

South Tibet

ITS

Indian plate

Himalayas

500km

Formation , which consists of un-met-
amorphosed clastic sediments formed
in the foredeep basin and derived from
the rising Himalayan mountains.

A

Asia

volc.
arc

BNS HXS

India

NT T

ST

2. The Lesser Himalayan schist belt .
This unit consists of slates and schists
derived from Mid-Proterozoic clastic
sediments, originally laid down on the
Indian passive margin and deformed
into south-directed fold-thrust pack-
ages. The southern boundary is marked
by the Main Boundary thrust (MBT).

1

Himalayan
orogen

Asia

HFT ITS

T

India

ST NT

?

?

2

B

3. The Greater Himalayan crystal-
line complex . This unit consists of
high-grade schists and gneisses derived
from Late Proterozoic clastics, also
from the Indian passive margin. The
metamorphic grade is inverted, becom-
ing greater upwards, from chlorite to
sillimanite at the upper margin of the
belt. Near the upper margin is a zone of
syn-orogenic granite intrusions with a
generally lensoid shape. The southern
boundary of the crystalline complex
is formed by the Main Central thrust
(MCT), which is actually a ductile
shear zone several kilometres wide.

this event, subduction of the oceanic
part of the Indian plate had been
taking place beneath Asia from Cre-
taceous times. The climax of the col-
lision event resulting in the uplift of
the Himalayan range itself occurred
in the Miocene, around 20 Ma ago,
although uplift and thrusting along
the southern border continues today.

Indus-Tsangpo-Yarlung Suture (ITS)
which marks the junction between the
Indian and Asian plates. In the central
sector the suture is offset by a thrust -
the Renbu-Zedong Thrust (RZT). The
southern margin of the belt is defined
by another thrust, the Himalayan
Frontal Thrust (HFT), which marks
the edge of the fold-thrust belt on the
Indian foreland. The central Himala-
yan belt consists of the following four
separate tectonic units, traversing from
south to north (Figures 11.2, 11.3).

The central Himalayan sector
This sector of the Himalayan belt
(Figure 11.2) forms an arc, 1,750 km
long and 250 km across, extending from
north-western India, through Nepal,
to Bhutan, and includes southernmost
Tibet. It is bounded on the north by the

1. The foreland thrust belt . This
unit consists of south-directed fold-
thrust sheets involving the Siwalik

4. The Tethyan shelf belt . This unit
consists of largely un-metamorphosed