UNDERSTANDING THE PRESERVATION OF AN INVERTED METAMORPHIC PROGRESSION, SCOTTISH CALEDONIDES: INTEGRATING QUANTITATIVE STRUCTURAL, KINEMATIC, AND METAMORPHIC DATA

In the Scottish Caledonides, new thermobarometric and deformation temperature estimates are used to characterize the inverted metamorphic progression at the base of the Scandian (435-420 Ma) orogenic wedge and to test whether it is a relatively intact sequence or whether it represents a composite product of multiple structural/thermal events. At the leading edge of the Moine nappe, subhedral garnets with prograde compositional growth zoning yield peak temperatures (grt-bio) of 428-459 °C at 4.0-6.0 kbar. At similar structural positions, opening angles of quartz c-axis fabrics yield deformation temperatures of 420-460 °C. At the structurally highest position in the Moine nappe, garnets with prograde compositional zoning profiles yield P-T estimates of 560-584 °C (grt-bio) and 4.5-5.1 kbar. Quartz fabrics of samples collected at similar structural positions yield deformation temperature estimates of 490-565 °C. In amphibolite of the structurally higher Ben Hope nappe, garnet rims record P-T conditions of 655-672 °C at 3.9-5.1 kbar. Correlation of quartz c-axis derived deformation T estimates of known Scandian age with broadly similar T estimates calculated from garnet-biotite pairs across the entire structural thickness of the Moine and Ben Hope nappes indicates that the inverted metamorphic sequence from the Moine to the Naver thrust is mostly intact and likely developed during the Scandian phase of the Caledonian orogeny. We interpret formation of this inverted sequence to be due to a combination of progressive accretion of successively lower grade thrust sheets onto the base of the Scandian wedge and heating of the Moine and Ben Hope nappes from above by the relatively higher temperature migmatites of the Naver thrust. Significant vertical ductile thinning (12-40% contribution to overall exhumation), which is indicated by integrated strain and vorticity analyses, likely operated in conjunction with erosion and normal faulting to rapidly exhume the Scandian nappe pile and preserve the inverted metamorphic sequence. 1-D thermal numerical models can test the validity of these interpretations.