Paper No. 3
Presentation Time: 2:00 PM
THE INTERIOR GRENVILLE OROGEN AS A COLLAPSED OROGENIC PLATEAU
RIVERS, Toby, Earth Sciences, Memorial University of Newfoundland, Room 4063, Alexander Murray Building, 300 Prince Philip Drive, St. John's, NF A1B 3X5, Canada, trivers@mun.ca
Integration of the seismic structure of the interior Grenville Orogen with peak pressure determinations of Ottawan (~1090-1020 Ma) metamorphism suggests a horst and graben architecture due to collapse of a former orogenic plateau. Horsts are mid-crustal core complexes up to 100 km or more across with sub-horizontal foliations in Ottawan high-grade gneisses (peak
P-T conditions ~1000 ± 100 MPa, 850-900 °C). In contrast, graben represent the remnants of an orogenic lid in which pre-Ottawan structures and mineral assemblages are preserved, foliations are steep, and peak Ottawan
P-T conditions were ≤ 400 MPa and 500 °C. Intervening crust with peak metamorphic pressures between 1000 and 400 MPa is under-represented at the erosion surface. Juxtaposition of the Ottawan mid crust and orogenic lid along normal-sense shear zones was ongoing by ~1050 Ma, i.e., ~40 Ma after the onset of crustal thickening. The orogenic plateau was largely collapsed by ~1020 Ma, when a major early Ottawan thrust-sense boundary in the interior Grenville Province was reworked as a normal-sense shear zone.
Available data point to a temporal progression of peak Ottawan metamorphism from ~1090-1050 Ma in the mid crust, through ~1050-1020 Ma in the upper crust, to <1020 Ma at the margins of the orogenic lid. This implies that metamorphism of the upper crust and heating of the lid took place during collapse as a result of juxtaposition with exhumed hot mid crust. Coeval thickening of the upper crust and exhumation/thinning of the mid crust were accommodated by different geometries of bulk strain suggesting the two levels were decoupled. Crustal-scale triaxial boudinage led to the rise of ductile mid crust into boudin necks and core-complex formation. The presence of juxtaposed mid and upper crustal levels along late normal-sense shear zones is a signature of a collapsed orogen, the fundamentally triaxial nature of bulk strain implying collapse is not amenable to modelling as a 2-D plane-strain process.
