Paper No. 9
Presentation Time: 4:05 PM


BIREN, Marc B., Planetary and Space Science Centre, Department of Earth Sciences, University of New Brunswick, 2 Bailey Drive, Fredericton, NB E3B 5A3, Canada,

Thin (< 2.5 mm) white shock veins that penetrate the anorthositic central peaks of the 90 km, 214 Ma-old Manicouagan impact structure in Quebec offer key structural insights regarding central uplift formation in crystalline rocks.

Field mapping, petrographic examination (optical microscopy and scanning electron microscopy), and geochemical comparison (energy dispersive spectroscopy) of the shock veins and host rocks have been used to estimate shock pressures and temperatures recorded in the veins. The information obtained has been further used to quantify how much uplift occurred and to characterize the coherence of the uplifted peaks.

Shock excursions up to 30 GPa are indicated by the presence of stishovite and maskelynite within the veins and along their margins, while temperature excursions up to ~2000˚ C, are indicated by the presence of plagioclase-rich melt veins and the various partially melted silicate phases contained therein. Bulk shock effects up to 12 GPa are defined by the presence of shatter cones and planar fractures and planar deformation features in various minerals.

Shock attenuation calculations (down axis) based on these bulk pressures suggest that the anorthositic peaks rose ~10 km during crater modification, which is in good agreement with an uplift estimate of ~7 to 10 km, based on low-temperature thermochronology and geothermal considerations.

Four groups of linear veins have been identified at various locations within the uplifted peaks and the majority of veins in each group exhibit radial symmetry with respect to the geometric center of the structure. When viewed as a crude marker, this radial arrangement suggests that the anorthositic rocks underwent minimal disruption and/or rotation during uplift despite the significant distance traveled.

Shock veins within the central uplift at Manicouagan offer insight into the shock processes that formed them, afford an important spatial context with which to better understand similar veins observed in many meteorites, and also provide vital clues to the formation of central uplifts in crystalline target rocks.