DUCTILE FAULT ROCKS AND METAMORPHIC ZONATION IN THE DALRADIAN OF THE HIGHLAND BORDER SW OF STONEHAVEN, KINCARDINESHIRE

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.