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The distinct appearance of the Crisium basin relative to other multiring basins on the Moon illustrates how two fundamental variations in the impact process at low impact angles can affect the formation of multiring impact structures. First, both laboratory cratering experiments and the planetary cratering record indicate that the shape and profile of a transient impact cavity depend on impact angle. Because the point of deepest penetration and excavation migrates uprange of the cavity center at low impact angles, the center of transient cavity collapse in basin-forming impacts also shifts uprange of the cavity center, thereby offsetting both the central mantle uplift and the centers of basin ring formation from the center of basin-filling volcanism. Second, peak shock pressures decrease at lower impact angles. Consequent variations in the distribution of dynamically displaced material around the transient cavity, therefore, may modify the nature of dynamic rebound during cavity collapse and reduce lithospheric failure outside the transient cavity at lower impact angles. Such reduced cavity collapse can account for both the low relief of the outer basin scarp and the topographically high, unusually broad massif ring at Crisium. Also if the reduction in cavity collapse limits isostatic equilibration of the basin cavity, basin-centered lithospheric stress fields during post-impact isostatic uplift may allow widespread early basin volcanism, further enhancing the structural variations in basin appearance as a function of impact angle.

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