Geoscience Reference
In-Depth Information
model in which the Indian plate underthrusts the mountains, is flexed downwards
by and supports the load of the mountains, while being heated, can account
for these differences. The Ganga Basin, which forms in front of the Himalayas
because the Indian plate is flexed downwards there, is filled with sediment eroded
from the mountains. The complex jog in the gravity anomaly between 0 and
100 km is due to the lower-density foreland basin and sub-Himalaya sediments,
which are underthrust beneath the higher-density Lesser Himalaya (Figs. 10.19(b)
and (c)). The model shown in Fig. 10.13 has the Indian plate underthrust as
an intact unit beneath the Himalayas and continuing beneath southern Tibet.
Estimates of the effective elastic thickness of the Indian lithosphere decrease
from south to north, reaching as little as
30 km beneath southern Tibet (see
Section 5.7 for discussion of flexure and elastic thickness). This is consistent
with the overall seismic and thermal picture of the crust and uppermost mantle
beneath the Himalayas and Tibet. The first-order agreement between the Bouguer
gravity anomaly and the isostatic anomaly to the north of the Indus-Tsangpo
suture confirms that the crustal thickness beneath Tibet must be fairly uniform.
However, the upwarping of the Bouguer anomaly there (
∼
30-40 mgal less than
the isostatic anomaly) indicates that the region is under-compensated. This could
be accounted for by the presence of additional mass at some depth (perhaps
formation of eclogite in the Indian lower continental crust) or additional bending
moments acting on the Indian plate.
∼
The Alps
Although our understanding of the Himalayas is still incomplete and details will
change as further work is undertaken, the Alps have been paid much more atten-
tion and are far more accessible. The Alps formed when the Adriatic promontory
on the African plate collided with the southern margins of the Eurasian plate.
The Alps were not the only mountains formed as a result of the convergence of
Africa and Eurasia. Figure 10.20(a) shows the extensive Alpine fold system of the
Mediterranean region. The complex present-day tectonics of the Mediterranean
involves a number of microplates. The main rigid regions are Africa, Eurasia,
Arabia, the Adriatic Sea, central Turkey and central Iran (Figs. 10.20(b) and
(c)). Palaeomagnetic data indicate that the Adriatic block, which was a northern
promontory of the African plate, has been separate from the African plate since
the Cretaceous (estimates are
80-130 Ma) and is now rotating separately. The
clockwise extrusion of the Anatolian block appears to be a consequence of the
northward motions of the African and Arabian plates. The African plate is being
subducted beneath Crete along the Hellenic arc and the back-arc Aegean region
is undergoing intense localized deformation. The Gulf of Corinth is opening at
about 1 cm yr
−
1
,which is about a quarter of the relative motion on the Hellenic
arc. The Hellenic arc itself is retreating southwards. GPS and earthquake data
show that the Anatolian plate is rotating anticlockwise and moving westwards
at
∼
2-3 cm yr
−
1
(Fig. 10.20(d)). GPS measurements yield an estimate for slip
on the 1600-km-long right-lateral North Anatolian fault of 2-3 cm yr
−
1
,whereas
∼
