FABRIC SIGNATURE OF SUBGLACIAL TILL DEFORMATION, BREIDAMERKURJOKULL, ICELAND

The foreland of Breidamerkurjokull, Iceland, is the only locality wher e tills known to have undergone subglacial deformation are exposed. Ti ll on the foreland has a two-tiered structure, consisting of a dilatan t upper horizon c. 0.5 m thick and a compact lower till; these horizon s correspond to the ductile deforming A horizon and the brittle-ductil e B horizon observed below the glacier by G. S. Boulton and co-workers . The relationship between known strain history and a variety of macro fabric elements is examined for these two genetic facies of deformatio n till. The upper horizon exhibits variable a-axis fabrics and abundan t evidence for clast re-alignment, reflecting ductile flow and rapid c last response to transient strains. In contrast, the lower horizon has consistently well organized a-axis fabrics with a narrow range of dip values, recording clast rotation into parallel with strain axes durin g brittle or brittle-ductile shear. The data indicate that till strain history imparts identifiable macrofabric signatures, providing import ant analogues to guide the interpretation of Pleistocene tills.