STRUCTURAL CHARACTER OF THE TERRACE ZONE AND IMPLICATIONS FOR CRATER FORMATION, CHICXULUB IMPACT CRATER

The 65 Ma Chicxulub impact structure spans approximately 195 km over the Yucatan Platform and is preserved beneath several hundred meters of carbonate sediments. Seismic reflection profiles reveal a complex morphology characterized by a peak ring, terrace zone, and multiple outer rings. Here we examine the nature of the terrace zone and its role in the modification stage of crater formation. This zone is composed of distinct slump blocks that form a number of inward facing scarps.

Four reflection profiles image the slump blocks, revealing a remarkable variability in these structures along the crater circumference. By compiling a detailed inventory of terrace width and fault locations, we attempt to quantify this variability to detect signs of an oblique impact event, which has previously been suggested by gravity modeling. In addition, the location of the slump blocks in relation to the peak ring and crater rim can provide insight on the gravitational collapse of the transient crater.

The terrace zone is defined as the area between the last undeformed pre-impact strata and the innermost edge of slumping. Minor variations are observed in the outer limit of slumping and width of the terrace zone, but the profiles are consistent within 15 km. Notable variations are seen when comparing the geometry of the slump blocks. Profiles A, A1 and C display flat-topped slump blocks flanked by inward facing normal faults. Profile B exhibits a gently sloping terrace zone, and individual slump blocks are difficult to discern. The vertical throw of the normal faults that facilitate slumping ranges dramatically. While not indicative of an oblique impact event, this along-strike variation underscores the importance of accounting for three-dimensionality when modeling crater structure, whether it is due to post-impact erosion, pre-impact topography, or angle of impact.

In each profile, the terrace zone is present inward of the crater rim and extends toward the crater center to underlie the peak ring. This spatial relationship implies a component of horizontal motion, or inward collapse, in the gravitational modification of the transient cavity. This observation strengthens existing models of peak ring formation as an interaction between an outwardly collapsing central uplift and an inwardly collapsing crater rim.