1. Structural character of the terrace zone and implications for crater formation: Chicxulub impact crater David L. Gorney Sean Gulick Gail Christeson GSA Annual Meeting 2004

2. Chicxulub impact crater Occurred ~65 Ma Buried by up to 1 km of carbonates Multi-ring basin, ~190 km diameter Significance K/T boundary mass extinction? Largest well preserved impact structure on Earth -SRTM topography

3. Significance of Study Geophysical data can be used to model subsurface crater structure Structural data constrain numerical modeling of impact event The goal of this study is to reprocess the seismic reflection data, then use those data to image crater structure 1996 BIRPS seismic reflection/refraction survey with Bouger gravity anomaly overlay

4. Idealized Chicxulub Structure Chicxulub exhibits multi-ring basin morphology Final crater morphology is a function of gravitational collapse Variation of widths of slump blocks can indicate impact angle narrow terrace zone indicates more intense deformation Morgan et al., 2000

5. Objectives Use existing seismic reflection data set to examine structural character:Use existing seismic reflection data set to examine structural character: –Reprocess reflection profiles to improve structural constraints –Focus processing scheme and interpretations on slump block/terrace zone features Questions: 1)Are there signs of oblique impact? - Oblique impacts in SW-NE direction has been suggested based on gravity data - SE-NW direction based on tsunami deposits in North America 2) Can various models proposed for peak ring formation be constrained? - over-thrust model: collapsing central uplift interacts with collapsing crater rim (Morgan et al., 2000) - peak ring produced by subvertically uplifted basement (Sharpton et al., 1994) - narrow central uplift, peak ring consists of breccia (Pilkington et al., 1994)

6. Reprocessing To remove coherent noise: Caused by reverberation, as a result of shallow water (~20 m), hard water bottom In response to guided waves, another noise issue in shallow water How? Velocity analysis F/K filter Improve the images: Post-stack time migration

7. Reprocessed Profile: Chicx-B before after Towards crater center 10 km VE ~3x peak ring Terrace zone Tertiary basin 2- way time (sec) 1 2 3 3 2 1 SE NW

8. Terrace Zone 40 to 80 km radial distance no consistent gravity signature

9. ~15 km depth twt (s) 1 2 3 4 5 Chicx-A Base of Tertiary sediments Top of slumps flat slump tops vertical offset – 4 km max, < 1 km minimum widths: 5 km and 10 km slumps underlie peak ring peak ring VE ~2x Radial distance (km) 90 80 70 60 50

10. Chicx-B inward dipping ( < 5°) tops widths 8 – 12 km slumps underlie peak ring individual blocks less distinct - secondary collapse? Base of Tertiary sediments Top of slumps Radial distance (km) 80 70 60 50 40 twt (s)1 2 3 4 5 ~15 km depth peak ring VE ~2x

11. Chicx-C Base of Tertiary sediments Top of slumps Radial distance (km) 80 70 60 50 40 twt (s) 1 2 3 4 5 ~15 km depth irregular dimensions – width and vertical offset width of blocks: 4 – 7 km - narrower locus of deformation – oblique impact? peak ring VE ~2x

12. Radial distance (km) 80 70 60 50 40 Chicx-A1 Base of Tertiary sediments Top of slumps ~15 km depth twt (s) 1 2 3 4 5 slump tops: tilted inward 1 - 2° uniform widths - 9 -11 km vertical offsets generally increasing inward: <1 km at outer blocks, up to 2.5 km towards the peak ring peak ring VE ~2x

13. - narrow zone of deformation in NE direction 20 km VE ~2x A A1 C B A B C

14. Conclusions (1) Variability of terrace zone Geometry of slump tops Width of blocks ranges from 4 km to 12 km Profiles A1 and B show relatively uniform widths (within 2 km) Profiles A and C vary (up to 5 km) Slump tops dip inward (B), outward (A1), or flat (A, C) Normal Faulting Height of scarps varies from 100’s m up to 4 km Schrodinger - 320 km diameter lunar crater

15. Conclusions (2) Impact angle –Some variations likely reflect heterogeneities in target material, but may also suggest oblique impact –If so, the impact direction is SW to NE Venusian Crater – Proclus LTO Topography - Proclus (Venusian) displays a compressed zone of deformation downrange Direction of impact Narrow zone of deformation to NE 15 km From Herrick and Forsberg- Taylor, 2003 10 km Chicx-C

16. Conclusions (3) Crater collapse and peak ring formation: Slumps blocks underlie peak ring, implying component of horizontal motion Interaction of inwardly collapsing crater rim and outwardly collapsing central uplift (Collins et al., 2002; Morgan et al., 2000) From Morgan et al., 2000 [terrace zone]

17. Future Work 2005 seismic survey2005 seismic survey –Peak ring grid –Radial profile –Site survey for IODP drill sites