Er. Phillips et Ca. Auton, DUCTILE FAULT ROCKS AND METAMORPHIC ZONATION IN THE DALRADIAN OF THE HIGHLAND BORDER SW OF STONEHAVEN, KINCARDINESHIRE, Scottish journal of geology, 33, 1997, pp. 83-93
High strain zones, characterized by shear fabrics and mylonites, have
been identified within lower amphibolite-grade, coarse granular metasa
ndstones of the Glen Lethnot Grit Formation of the Southern Highland G
roup (Dalradian) along the Highland Border, south-west of Stonehaven.
The rocks comprise part of a polydeformed, regionally metamorphosed, N
eoproterozoic sequence, bounded to the north by the post-orogenic Moun
t Battock Granite (Late Caledonian) and to the south-east by the Highl
and Boundary Fault Zone. Early regional D1 and D2 structures have been
rotated by a major monoform, the Highland Border Downbend (D4), to pr
oduce a steeply dipping, downward facing succession. The mylonitic met
asandstones contain a variety of kinematic indicators, including asymm
etrical pressure shadows, S-C fabrics, shear bands and an extensional
crenulation cleavage, which yield a sense of shear of top-towards-the
SE (present structural position). Regional syn- to post-D2 metamorphis
m resulted in the development of prograde pelitic mineral assemblages
which overgrow earlier D1 structures. Mylonitic fabrics within the hig
h strain zones, developed during D2, both deform and are also overgrow
n by syn- to post-kinematic biotite porphyroblasts. The high strain zo
nes may represent part of a major ductile structure active during the
Dalradian D2-D3 tectonothermal event, which contributed to the develop
ment of strong metamorphic gradients and narrowing of mineral assembla
ge zones within the Highland Border.