1 - Cochin University of Science and Technology

along with the microtextural evidences of mylonites and
occurrence of augen gneisses also indicate a major shear
control in their generation. It conforms we II with the
established relationships between shear zones, major faults
and lineaments and igneous activity (Murthy & Venkataraman,
1964; Bowden, 1974; Jayananda & Mahabaleswar, 1991; Santosh &
Thara, 1984) which can also explain the extensive
of concordant igneous bodies present in the Gap.
Recent investigations pointing to
correspondence between P-T data on rocks
the
lying
326
occurrence
lack of
south and
north of the Palghat-Cauvery shear zone (Harris et aI, 1982;
Janardhan et ai, 1982; Raith et ai, 1983; Chacko et ai, 1987)
has facilitated arguments in favour of its being a tectonic
boundary between two distinct terrains, within the
southern
indian shield (Drury et ai, 1984; Ramakrishnan, 1988). The
basement highs located beneath Nilgiris and the basement low
located in the Pal ghat Gap, also speak in favour of up I if t
or down fault of distinct blocks against each other,
along
deep crustal breaks (Reddi et ai, 1988). Presence of a thick
crust south of Palghat-Tiruchi region, in the high grade
terra.ins of south India, as shown by higher bouger anomaly
indications or 30-40 mgls (Mishra, 1988) as compared to the
magnetic lows
explained as
underplating
the electrical
on either sides of the Gap has also
a result of crustal accretion or as due
along a shear zone, which is sUbstantiated
been
conductivity measurements carried out across
to
by