Prograde evolution of the Scottish Caledonides and tectonic implications

Kyle T. Ashley, J. Ryan Thigpen, Richard D. Law

Research output: Contribution to journalArticlepeer-review

21 Scopus citations

Abstract

Recent thermometric analyses of samples collected in thrust-parallel structural transects across the Scandian (435-415 Ma) orogenic wedge in northwest Scotland provide a comprehensive characterization of the synorogenic retro-wedge thermal architecture. However, the paucity of petrologically-important metamorphic mineral phases (e.g., staurolite, Al-silicates) has limited investigation of pressure-temperature (P- T) histories, which hinders our ability to examine the nature of orogen-scale kinematic and thermal coupling. New data collected along a foreland-to-hinterland transect from the Moine to the Naver thrust sheets provides additional constraints for characterizing the prograde metamorphic evolution. In addition, we characterized Ti diffusion profiles in quartz inclusions in garnet to constrain duration of metamorphic heating. These results are used to develop coupled kinematic-thermal models of Scandian orogenic evolution. Early garnet core growth conditions are constrained by isopleth intersections, with peak P- T estimates determined by conventional exchange and net transfer thermobarometry and thermodynamic calculations. Most samples follow normal prograde heating and burial profiles, with peak conditions of 450. °C and 5.0. kbar in the immediate hanging wall to the Moine thrust, increasing in temperature and pressure to 733. °C and 9.5. kbar in the immediate hanging wall to the Naver thrust. These normal prograde pressure trajectories are interpreted to reflect burial of incipient thrust sheets beneath the overriding wedge at the leading edge of the orogen. Prograde heating coeval with burial is interpreted to result from surface-directed isotherm perturbation due to thrust-related advection in the overriding wedge. One exception to this is a sample from the top of the Moine thrust sheet, where prograde heating occurs during decompression (540. °C and 8.1. kbar to 590. °C and 7.0. kbar). In this case, the short lag times between motion on the Moine and Ben Hope thrusts may have limited advectionary heating until after exhumation associated with motion on the underlying Moine thrust was underway. Ti diffusion profiles in quartz inclusions in garnet suggest the near-peak thermal evolution of these rocks occurred over very short time scales (< 200,000 years). While most of the garnets are inferred to be Scandian in age, we document evidence for pre-Scandian garnet cores in structurally higher (more hinterland positioned) samples that must have grown under higher temperatures. In the hanging wall of the Moine thrust, high grossular garnets with estimated formation conditions > 9 kbar are probably of detrital origin.

Original languageEnglish
Pages (from-to)160-178
Number of pages19
JournalLithos
Volume224-225
DOIs
StatePublished - May 1 2015

Bibliographical note

Funding Information:
Reviews by Rob Strachan and Daniel Viete greatly improved the quality of this manuscript. We thank Frank Spear, Mark Caddick and Donald Stahr for thoughtful discussions pertaining to this study. Frank Spear also graciously shared his program to model Ti diffusion profiles for geospeedometry purposes. Luca Fedele and Bob Tracy are thanked for assistance with microprobe analysis. This paper is based on work supported by the National Science Foundation under grant No. EAR-1220345 (awarded to R.D. Law) and by a graduate research grant by the Department of Geosciences, Virginia Tech (awarded to K.T. Ashley).

Publisher Copyright:
© 2015 Elsevier B.V.

Keywords

  • Forward stability model
  • Prograde evolution
  • Tectonic model
  • Thermal profiles

ASJC Scopus subject areas

  • Geology
  • Geochemistry and Petrology

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