The Manicouagan Impact Structure: An analysis of its original dimensions and form

Abstract

Manicouagan, Quebec (51°23'N; 68°42'W) is the most intensively studied large complex terrestrial impact structure in a predominantly crystalline target. The ground truth data available from Manicouagan have considerable potential for interpreting the subsurface characteristics of lunar impact structures of comparable morphology and size. Two contrasting hypotheses, however, exist for the preerosional form of Manicouagan: (1) a multi‐ring basin with a final diameter of 75 km and a transient cavity diameter of 30–45 km [Floran and Dence, 1976], and (2) an endogenically modified peak‐ring basin with a final diameter of 100 km and a transient cavity diameter of 80 km [Orphal and Schultz, 1978]. Both hypotheses are based largely on topographic data and interpret the prominent 65 km diameter annular moat as a graben‐like feature formed either (1) by collapse of the outer slope of the transient cavity rim or (2) by uplift of the final crater floor through post‐impact intrusive activity.The present analysis suggests that this annular moat is primarily an erosional feature and that the original form of the impact structure cannot be determined unequivocally on the basis of present topography. If all the available topographic, geologic, and geophysical data are considered, however, then an internally consistent interpretation of original dimensions is possible. The distribution of shock deformation effects in the basement rocks of the present crater floor suggests an original transient cavity diameter of 60 km. The 55 km‐diameter, impact melt‐covered inner plateau provides a minimum estimate for the diameter of the final floor of the crater and the annular moat, with its outliers of downdropped Ordovician limestone, is interpreted as marking the contact between the floor and the innermost slump blocks of the final rim. There is no compelling evidence to regard the annular moat as a graben. By analogy with fresh lunar and mercurian structures, a floor width of 55 km suggests a final rim diameter of 86–95 km. This is consistent with the topographic data from outside the annular moat and with the residual peripheral gravity low, which suggests a final rim diameter of 85–95 km. Due to erosion, it is difficult to assign Manicouagan to a particular morphological class of impact structure, but on the basis of the available data it is most likely that the preerosional form was that of a central peak crater or possibly a peak‐ring basin. At the time of its formation, Manicouagan may have been dimensionally and possibly morphologically similar to the 96 km diameter lunar crater Copernicus.