280 IMPACT STRUCTURES Figure Schematic cross section of complex impact structure Notation as described in the legend to Figure with SU corres ponding to structural uplift and Dcp to the diameter of the central uplift Note preservation of beds in outer annular trough of the structure, with excavation limited to the central area to a series of fault terraces Interior to the rim lies a down-faulted annular trough, which is partially filled by a sheet of impact-melt rock and/or polymict allochthonous breccia (Figure 6) Only in the central area of the crater is there evidence of substantial excavation of target materials This central region is structurally complex and can be occupied by a central peak (Figure 5), which is the topographic manifestation of a much broader and extensive area of structurally uplifted target rocks that occurs beneath the centre of complex craters With increasing diameter, a fragmentary ring of interior peaks appears, marking the transition from complex craters to impact basins There have been claims that the largest known terrestrial impact structures, e.g., Chicxulub, Mexico; Sudbury, Canada; and Vredefort, South Africa, have multiring basin forms Although certain of their geological and geophysical attributes form annuli, it is not clear that these correspond, or are related in origin, to the obvious topographical rings observed, for example, in lunar multiring basins A small number of relatively young, and, therefore, only slightly eroded, complex impact structures (e.g., Haughton, Canada; Ries, Germany; Zhamanshin, Kazakhstan) not have an emergent central peak or other interior topographical expression of a central uplift These structures are in mixed targets of platform sediments overlying crystalline basement This difference in form is probably a target rock effect but it has not been studied in detail Geology of Impact Structures Although an anomalous circular topographic, structural, or geological feature may indicate the presence of an impact structure, other terrestrial geological processes can produce similar features The burden Figure Temperature and pressure range of shock meta morphic effects compared to that of endogenic terrestrial meta morphism Planar features include planar deformation features and planar fractures Note scale is logarithmic of proof for an impact origin for a specific structure generally lies with the occurrence of shock metamorphic effects Only small, young, simple structures, where the impacting body has been slowed by atmospheric deceleration, preserve physical evidence of the impacting body These are generally fragments of iron or stony-iron meteorites Stony meteorites are weaker and small stones are crushed as a result of atmospheric interaction Larger impacting bodies (>100– 150 m in diameter) survive atmospheric passage with undiminished impact velocity On impact, a portion of the impacting body’s kinetic energy is partitioned into kinetic energy in the target rocks, which results in the formation of a craterform The remainder of the impact kinetic energy is partitioned into increasing the internal energy of the target rocks The target rocks are compressed by the passage of a shock wave and experience extremely high transient pressures and temperatures For example, the peak pressure experienced in crystalline target rocks by the impact of a stony body, such as a chondritic asteroid, at 25 km s is approximately 100 GPa During shock compression, considerable pressure–volume work is done On decompression, not all this mechanical work is recovered This excess work is manifest as waste heat, leading to the heating, melting, and even vapourization of part of the target rocks and the impacting body, destroying it as a physical entity The combined effect of this compression and heating is the production of a series of irreversible changes that occur in individual minerals and rocks, which are known collectively as shock metamorphic effects Shock metamorphic effects are produced at pressures and temperatures well beyond those in endogenic terrestrial metamorphism (Figure 7) The physical conditions upon impact are a function of initial impact parameters Projectile and target rock type and impact velocity determine peak pressures on