Low-grade metamorphism in the Scottish Southern Uplands terrane: deciphering the patterns of accretionary burial, shearing and cryptic aureoles

Systematic studies of metapelitic grade linked with the geological re-surve y of the Southern Uplands have been used to generate a contoured metamorphi c map currently covering nearly two-thirds of the terrane. These studies, b ased on approximately one pelite sample per 2.5 km(2), have used XRD measur ements of clay mineral reaction progress, particularly illite crystallinity , to delineate zones of diagenesis and low-grade metamorphism in the imbric ated Ordovician and Silurian strata. The regional pattern revealed by metap elitic zonal sequences does not agree with earlier observations that grade increases across the strike, from SE to NW. Instead, the map shows consider able variations in metamorphic trends. including patterns of grade increasi ng from older into younger strata, indicative of accretionary burial. Other patterns identified include those generated by high strain rates in the Mo niaive Shear Zone (MSZ), and by extensive low-temperature cryptic aureoles associated with late granitic intrusions. The present pattern is the result of uplift that generated normal movement on reactivated thrust faults and differential block movement on NW-trending faults. Regional metamorphic patterns were generated by burial and underplating in an accretionary thrust stack. Subduction was initiated in the early Caradoc and probably ceased in the early Wenlock. Metapelitic patterns suggest tha t two levels of accretion are exposed in the terrane. Strata accreted to th e toe of the prism and stacked above the decollement zone are typically at late diagenetic grade. Underplated strata below the decollement are typical ly at anchizonal grades with moderate to well-developed slaty cleavage. Coh erent thrust-bounded tracts of strata at both levels were rotated and burie d to produce a syntectonic depth-controlled pattern of meta-morphism. Shear zone metamorphism at depths of 12 km or more was probably confined to the underplated lower level of the thrust stack, and Devonian granitic intrusio ns were also emplaced mainly within the underplated strata.