BIOSTRATIGRAPHICAL CONTROL OF THRUST MODELS FOR THE SOUTHERN UPLANDS OF SCOTLAND

Graptolite biostratigraphy affords a robust and relatively accurate me ans of correlating Ordovician and Silurian hemipelagite and turbidite sequences and has been used to establish the structural development of the regional thrust belt in the Southern Uplands of Scotland. The ove rall structural pattern has long been recognised: greywackes within in dividual thrust slices, deposited within a relatively short time-inter val, become sequentially younger southwards; each overlies the basal M offat Shale Group which was deposited over a longer time. However, rec ent refinement of the graptolite biozonal scheme has allowed the bette r assessment of along-strike variations within the thrust belt which a re here illustrated by two transects; one, based on work in the Rhins of Galloway and the Kirkcudbright areas (SW Southern Uplands), and the other in the Peebles-Hawick area (NE Southern Uplands). The SW transe ct most closely approximates to the regular pattern wherein a southwar d-propagating thrust-front incorporated sequentially younger greywacke units. The uniform geometry is interrupted only locally, towards the southern margin of the thrust belt, by a system of back-thrusts produc ing structural pop-ups. The NE transect departs from this regular mode l: a northern sector shows the orderly initiation of the thrust belt, but towards the SE a more irregular distribution of the thrust-slice a ges can be best explained by out-of-sequence movement. This transect a lso shows more repetitive imbrication of the same biostratigraphic int erval than is apparent farther SW. In both transects the fundamental c hanges in thrust-belt geometry took place from mid-llandovery times on wards, with a reversion to forward-breaking, in-sequence thrusting at the beginning of the Wenlock. The cause is a matter for speculation, b ut may be linked with the accommodation of an obstacle to forward-thru st propagation. However it is recognised that such variations in thrus t geometry are a fundamental feature of most thrust belts and do not r equire a single regionally significant cause.