One of the noticeable structural features of the Arabian
shield is the existence of a fault system in a zone 185 miles
(300 km) wide with a length of nearly 750 miles (1,200 km)
extending from the southeastern to the northwestern parts of
the shield. This system was generated just after the end of the
Hijaz tectonic cycle, and it was active from 630 to 530 million
years, making it the last major event of the Precambrian
in the Arabian shield. These faults are left-lateral strike-slip
faults with a 150-mile (250-km) cumulative displacement on
all faults in the system.
The main rock group that was formed during and after
the existence of the Najd fault system is the Ji’balah Group.
This group formed in the grabens that were formed by the
Najd fault system and are the youngest rock group of the Precambrian
Arabian shield. The Ji’balah Group formed
between 600 and 570 million years ago. The Ji’balah Group
is composed of coarse-grained clastic rocks and volcanic
rocks in the lower parts, by stromatolitic and cherty limestone
and argillites in the middle parts, and by fine-grained
clastic rocks in the upper parts. These rocks were probably
deposited in pull-apart basins that developed in extensional
bends along the Najd fault system.
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