Geophysical Characterization of the Chicxulub Impact Crater

GEOPHYSICAL CHARACTERIZATION OF THE CHICXULUB IMPACT CRATER S.P.S. Gulick,1 G.L. Christeson,1 P.J. Barton,2 R.A.F. Grieve,3 J.V. Morgan,4 and J. Urrutia-Fucugauchi5 Received 21 August 2012; revised 4 February 2013; accepted 7 February 2013; published 3 April 2013. [1] Geophysical data indicate that the 65.5 million years ago Chicxulub impact structure is a multi-ring basin, with three sets of semicontinuous, arcuate ring faults and a topographic peak ring (PR). Slump blocks define a terrace zone, which steps down from the inner rim into the annular trough. Fault blocks underlie the PR, which exhibits variable relief, due to target asymmetries. The central structural uplift is >10 km, and the Moho is displaced by 1–2 km. The working hypothesis for the formation of Chicxulub is: a 50 km radius transient cavity, lined with melt and impact breccia, formed within 10 s of the impact, and within minutes, weakened rebounding crust rose kilometers above the surface, the transient crater rim underwent localized deformation and collapsed into large slump blocks, resulting in a inner rim at 70–85 km radius, and outer ring faults at 70–130 km radius. The overheightened structural uplift collapsed outward, buried the inner slump blocks, and formed the PR. Most of the impact melt was ultimately emplaced as a coherent <3 km thick melt sheet within the central basin that shallows within the inner regions of the PR. Smaller pockets of melt flowed into the annular trough. Subsequently, slope collapse, ejecta, ground surge, and tsunami waves infilled the annular trough and annular basin with sediments up to 3 km and 900 m thick, respectively. Testing this working hypothesis requires direct observation of the impactites, within and adjacent to the PR and central basin. Citation: Gulick, S. P. S., G. L. Christeson, P. J. Barton, R. A. F. Grieve, J. V. Morgan, and J. Urrutia-Fucugauchi (2013), Geophysical characterization of the Chicxulub impact crater, Rev. Geophys., 51, 31–52, doi:10.1002/rog.20007. 1. INTRODUCTION [2] An extraterrestrial body impacted the Earth in the region that today is the Yucatán Peninsula 65.5 million years ago (Ma), causing worldwide devastation that almost certainly was the principal cause of a mass extinction event [e.g., Alvarez et al., 1980; Hildebrand et al., 1991; Schulte et al., 2010]. Initial evidence for the impact was found at Cretaceous-Paleogene boundary (K-Pg) deposits around the world. This K-Pg layer or event deposit is now known 1 University of Texas Institute for Geophysics, Jackson School of Geosciences, Austin, Texas, USA. 2 Bullard Laboratories, Department of Earth Sciences, University of Cambridge, Cambridge, UK. 3 Department of Earth Sciences, University of Western Ontario, London, Ontario, Canada. 4 Department of Earth Science and Engineering, Imperial College London, London, UK. 5 Instituto de Geofísica, Universidad Nacional Autónoma de México, Ciudad Universitaria, Coyoacán, Mexico. Corresponding author: S. Gulick, University of Texas Institute for Geophysics, Jackson School of Geosciences, J.J. Pickle Research Campus, Mail Code R2200, 10100 Burnet Rd., Austin, TX 78758, USA. (sean@ig. utexas.edu) ©2013. American Geophysical Union. All Rights Reserved. 8755-1209/13/10.1002/rog.20007 to include: relict and glass spherules, formed when hot ejected melt and vapor rapidly cooled in the atmosphere or space [e.g., Smit and Klaver, 1981; Koeberl et al., 1994; Goldin and Melosh, 2009; Artemieva and Morgan, 2009], shocked quartz and zircon, generated by pressures >10 GPa [e.g., Bohor et al., 1993; Krogh et al., 1993; Pierazzo and Melosh, 1999], magnesioferrite spinels [Montanari et al., 1983; Smit and Kyte, 1984], meteoritic Cr isotopic ratios [Shukolyukov and Lugmair, 1998; Trinquier et al., 2006], iridium, and other platinum-group-elements in absolute and relative abundances not otherwise found on Earth [e.g., Alvarez et al., 1980], and, in some places, soot suggesting significant wildfires in the wake of the impact [e.g., Wolbach et al., 1985; Melosh et al., 1990]. Proximal K-Pg event deposits in the Gulf of Mexico-Caribbean Sea are meters to tens of meters thick, with spherule-rich event beds, frequently erosional bases and indicators of highenergy transport from tsunami and gravity flows [Smit et al., 1996; Claeys et al., 2002; Schulte and Kontny, 2005; Schulte et al., 2010; Schulte et al., 2012]. [3] Over a decade after the impact hypothesis was introduced, the Chicxulub impact crater (Figure 1) was Reviews of Geophysics, 51 / 2013 31 Paper number 2012RG000413 GULICK ET AL.: GEOPHYSICAL CHARACTER OF CHICXULUB 30˚ zoic 20˚ sin Ba no Ce ? -100˚ -90˚ -80˚ ? 22˚ Peak Ring ? Crater Center ? Chicxulub Puerto 21˚ Mérida 0 km 50 -91˚ -30-16 -90˚ -10 0 -89˚ 20 10 30 40 50 5464 Gravity (mgal) Figure 1. Bouguer gravity anomaly map (gravity data courtesy of A. Hildebrand and M. Pilkington) over the Chicxulub impact crater. The coastline is displayed with a white line, cenotes and sinkholes [from Connors et al., 1996] with black dots, and the city of Mérida and town of Chicxulub Puerto with maroon stars. The red star marks the nominal position of the crater center from Morgan et al. [1997]. The dotted line marks the extent of the Cenozoic basin [Gulick et al., 2008], the shaded regions mark the areas with mapped ring faults and slump blocks [Gulick et al., 2008], and the white lines mark the mapped inner and outer edge of the peak ring (PR). Inset depicts the regional setting, with red rectangle outlining the region shown in the gravity map. identified in México, based on gravity and magnetic data and drilling results [Hildebrand et al., 1991; Sharpton et al., 1992; Camargo-Zanoguera and Suarez-Reynoso, 1994; Pope et al., 1996; Sharpton et al., 1996]. Subsequently, seismic reflection and refraction data were acquired within and across the crater in 1996 and again in 2005 (Figure 2). The interpretation of these data is consistent with the form of an impact structure, specifically a multi-ring basin (e.g., Figure 3b) that is 180–200 km in diameter. The Chicxulub impact structure includes an intact peak ring (PR) and preserves deformation, at least 30 km deep, deforming the crust-mantle boundary [Morgan et al., 1997; Hildebrand et al., 1998; Morgan and Warner, 1999a; Morgan et al., 2000; Christeson et al., 2001; Gulick et al., 2008; Vermeesch and Morgan, 2008; Christeson et al., 2009; Vermeesch et al., 2009; Barton et al., 2010; Morgan et al., 2011]. [4] The observational record of impact cratering, a ubiquitous process perturbing the crustal structure of silicate planets, is dominated by remotely sensed surface views of impact craters and basins on the Moon, Mars, Venus, and Mercury. The two largest types of impact structures are PR craters, with a clear morphology of crater rim, terrace zone, annular trough, and a topographic PR surrounding a central basin (e.g., Figure 3a), and multi-ring impact basins with a series of concentric topographic rings where the innermost ring may be equivalent to a PR (e.g., Figure 3b). While remotely sensed images of other 0 km 50 Fig. 5A 22˚ Depth (km) 0.0 21˚ C1 S1 Y6 Yax-1 0.5 1.0 1.5 -91˚ -90˚ -89˚ -88˚ Figure 2. Experiment location map. The legend explains the symbols displayed on the map; note that Chicx-03A and Chicx-04A are proposed wells. The dashed rectangle outlines the region shown in the magnetics map of Figure 5a. Cenotes and sinkholes are from Connors et al. [1996]. The city of Mérida and crater center are shown with maroon and red stars, respectively. Inset displays stratigraphy of four drill cores at radial distances (left to right) closest to the crater center [Dressler et al., 2004; Stöffler et al., 2004]. planets and moons provide important information on the morphology of impact craters, such images provide no information on the nature of subsurface structures and 32 GULICK ET AL.: GEOPHYSICAL CHARACTER OF CHICXULUB a. [5] The formation of small craters (< 2–4 km) is fairly well understood from terrestrial studies, laboratory experiments, and nuclear explosions, but the formation of larger impact structures cannot be easily extrapolated from these observations. The cratering process changes with size of the impact: simple bowl-shaped craters evolve to complex central-peak craters, to PR basins, and finally to multi-ring basins. The most common model for the formation of a central-peak impact structure is one where the center of the transient cavity, which is formed directly by the cratering flow field, rebounds upward to form a central uplift, and the uplifted rim collapses inward and downward to form a terrace zone of slumped blocks. Figure 4 shows a cartoon of these processes, as modified from Melosh [1989], that includes the potential effects of asymmetries in the target rocks based on geophysical imaging of the Chicxulub impact structure [Gulick et al., 2008]. In the large PR and multi-ring impact structures, it is believed that an overheightened central uplift subsequently collapses to form a PR [e.g., Collins et al., 2002]. There is, however, no single quantitative consensual model for the formation of topographic PRs, as a result of the current lack of quality ground-truth data [Wünnemann et al., 2005; Grieve et al., 2008]. For example, an alternative model, based largely on planetary observations and the nonlinear scaling of impact melt volumes and crater dimensions with event size, attributes PR formation to the inability of centrally uplifted material to maintain a positive topographic expression due to impact melting [e.g., Baker et al., 2011]. [6] The Chicxulub impact structure, México, is unique. It is: (1) the only known terrestrial impact structure that has been directly linked to a mass extinction event, (2) the only one of the three largest impact structures on Earth that is well-preserved, (3) the only terrestrial impact structure with a global ejecta layer, and (4) the only known terrestrial impact structure with an unequivocal topographic PR. Chicxulub’s role in the K-Pg mass extinction and its exceptional state of preservation make it an important natural laboratory for the study of both large impact structure formation on Earth and other planets, and the effects of large impacts on the Earth’s environment and ecology. [7] Therefore, the Chicxulub multi-ring structure, with its intact terrace zone and PR, presents a unique opportunity to directly examine the geologic record of large-scale impact processes. Here, we present a review of the current high-quality geophysical images and resultant interpretative models over the Chicxulub impact structure. Crater Rim Terrace Zone Peak Ring Central Basin Annular Trough kilometers 90 190 b. kilometers 250 500 Cordillera Inner Ring Inner Rook Outer Rook Figure 3. (a) Image of Lunar Schrödinger Crater from the Lunar Reconnaissance Orbiter’s (LRO) Wide Angle Camera showing a well expressed PR impact structure. The image has 100 m/pixel resolution with an orthographic projection centered at 75,130 . (b) Image of Lunar Orientale Impact Basin from the LRO showing its four concentric topographic rings where the inner ring may be equivalent to a PR. lithologies associated with specific morphometric features. Currently, only terrestrial impact structures can supply this ground-truth information. The Earth, however, is the most geologically active of all the terrestrial planets and consequently has preserved only a small sample (< 200) of its original population of impact structures acquired over geologic time. The steep size-frequency distribution of planetary crater populations results in much fewer large craters than small craters. Sudbury (Canada) and Vredefort (South Africa) are the only other impact structures on Earth of comparable size to the Chicxulub impact structure [Grieve et al., 2008] and both are significantly older (~2 Ga). 2. DATA AND METHODS 2.1. Seismic Data [8] Two shallow-penetration reconnaissance profiles were acquired by Petróleos Mexicanos in 1992 [CamargoZanoguera and Suarez-Reynoso, 1994], and deep-penetration seismic surveys were acquired in 1996 [Morgan et al., 1997] and 2005 [Gulick et al., 2008]. This review focuses on the deep-penetration datasets, which consist of 2470 km of marine seismic reflection profiles, with air gun shots recorded 33 GULICK ET AL.: GEOPHYSICAL CHARACTER OF CHICXULUB Seds Northwest 0 15 30 Water Future Transient Cavity [9] The seismic reflection data include regional profiles over the northern half of the crater, a constant-radius profile exterior of the crater rim at ~85 km radial distance from the crater center, and a grid over the PR in the northwest quadrant of the crater (Figure 2). The processing sequence of these data is summarized by Snyder and Hobbs [1999] and Gulick et al. [2008], and the processed data image impact features from the seafloor down to the base of the crust at ~35 km depth [Morgan et al., 1997; Gulick et al., 2008]. The seismic profiles have been presented in numerous studies focusing on crater morphology and ring structure [Morgan et al., 1997; Morgan and Warner, 1999a, 1999b; Snyder et al., 1999; Morgan et al., 2002a; Gulick et al., 2008], the PR [Brittan et al., 1999; Morgan et al., 2000; Morgan et al., 2011], the melt sheet [Barton et al., 2010], Moho character [Snyder and Hobbs, 1999], and post-impact basin stratigraphy [Bell et al., 2004; Whalen et al., 2013]. [10] Shots and quarry blasts recorded by seismometers and hydrophones have been used to model crustal velocities at different resolutions and scales. Studies have focused on the upper crust [Brittan et al., 1999; Christeson et al., 1999; Mackenzie et al., 2001], the PR [Morgan et al., 2000; Morgan et al., 2002b; Morgan et al., 2011], the melt sheet [Barton et al., 2010], the central uplift [Morgan et al., 2002b; Vermeesch and Morgan, 2008; Christeson et al., 2009; Vermeesch et al., 2009], and the deep structure [Christeson et al., 2001; Christeson et al., 2009]. This review synthesizes recent results that constrain PR, melt sheet, central uplift, and Moho structure [Vermeesch and Morgan, 2008; Christeson et al., 2009; Vermeesch et al., 2009; Barton et al., 2010; Morgan et al., 2011]. [11] Vermeesch and Morgan [2008] and Vermeesch et al. [2009] inverted traveltimes from shots and receivers within a 110  100 km region near the crater center (Figure 2), using the technique of Zelt and Barton [1998]. Their resolution analysis indicates that they can recover the depth to the top of the central uplift to within 0.5 km. Vermeesch and Morgan [2008] and Vermeesch et al. [2009] also performed a joint inversion using both traveltime and gravity data. The joint inversion results in a rougher model that better fits the gravity data and that provides some constraints for regions with no ray coverage. Vermeesch et al. [2009] favor the traveltime model, where there is sufficient ray coverage, and the joint model, where there is no ray coverage. [12] Christeson et al. [2009] obtained a regional velocity model by inverting traveltimes from all shots and receivers displayed in Figure 2, using the technique of Zelt and Barton [1998]. Christeson et al. [2009] also inverted Moho reflections for Moho interface depth, using the method presented by Zelt et al. [2003]. They concluded that they can constrain Moho depth at the crater center with a resolution of ~0.5 km [Christeson et al., 2009]. [13] Barton et al. [2010] and Morgan et al. [2011] used data recorded by the 6 km streamer to produce velocity models of the shallow crust. Barton et al. [2010] used the velocity-imaging technique of Barton and Barker [2003] to transform first-arrival traveltimes into two-dimensional velocity-depth maps to depths of ~3–3.5 km. They used the Northeast Depth (km) a. Crust 30 Moho b. Water Seds Crust Moho c. Inner Rim Washes Away Incipient Inner Rim Crust Moho d. Inner Rim Peak Ring Melt Sheet Peak Ring Lower Crust (Central ? Uplift) ? Terrace Zone Water ? ? Terrace Zone ? Moho 200 km Figure 4. Stages of crater formation into an asymmetric target modified from Gulick et al. [2008] based on modeling by Collins et al. [2008]. Cross sections represent the northwest and northeast quadrants of the Chicxulub impact structure. (a) Pre-impact target with approximate depths of water, sediments (including dashed line representing evaporite marker horizon), and crust. Of the region within the transient cavity, a few kilometers of this material is vaporized or ejected, and the remainder is displaced outward during the impact. (b) Transient cavity formation stage at ~50 s, where the proximally ejected material in the deeper water northeast quadrant largely consists of water. The transient crater floor likely was lined with melt. (c) Crater modification at ~150 s, with differences in the expression of the crater rim and slumping due to the target asymmetries and the rebounding central uplift collapsing into the transient cavity asymmetrically. (d) Final crater morphology prior to Cenozoic sedimentary infilling, with a submerged central uplift capped by melt sheet, an asymmetric PR, terrace zone up to an inner rim, and ring faults farther outward. by 240 ocean bottom and land seismometers (Figure 2). In addition, a passive 20-element array was deployed over the land portion of the impact structure for ~100 days in 1996 [Maguire et al., 1998; Mackenzie et al., 2001]. 34 GULICK ET AL.: GEOPHYSICAL CHARACTER OF CHICXULUB Continental Drilling Program (ICDP) [Urrutia-Fucugauchi et al., 2004], and the UNAM-Federal Commission of Electricity [Urrutia-Fucugauchi et al., 2008]. Stratigraphy of results to map a high-velocity layer, interpreted as the impact melt sheet, interior of the PR [Barton et al., 2010]. Morgan et al. [2011] used full waveform tomography [Pratt, 1999] to invert for high-resolution velocity models of the PR to depths of 1.5 km. They observed abrupt changes in velocity between lithologies within and outside the PR [Morgan et al., 2011]. 2.2. Potential Field Data [14] The circular gravity and magnetic anomalies over the Chicxulub structure (Figures 1 and 5a) helped lead to its identification as the K-Pg impact crater [e.g., Penfield and Camargo-Zanoguera, 1981; Hildebrand et al., 1991]. The inherent non-uniqueness of potential field modeling, however, initially produced structural models that varied widely in estimates of position or thickness of the impact melt sheet, central uplift, PR, and rim [e.g., Hildebrand et al., 1991; Sharpton et al., 1993; Pilkington et al., 1994; Espindola et al., 1995; Hildebrand et al., 1995; Sharpton et al., 1996; Rebolledo-Vieyra et al., 2010]. In this review, we present results from 3-D gravity [Ebbing et al., 2001; Vermeesch and Morgan, 2008; Vermeesch et al., 2009] and magnetic [Pilkington and Hildebrand, 2000; Ortiz-Alemán and Urrutia-Fucugauchi, 2010] studies that incorporated constraints from seismic studies. [15] The joint traveltime-gravity model of Vermeesch and Morgan [2008] and Vermeesch et al. [2009] is described in section 2.1. Ebbing et al. [2001] conducted 3-D gravity modeling of the Chicxulub crater by first using seismic and well data to reconstruct the Paleogene infill, and then building a 3-D density model, using the seismic interpretation of Morgan and Warner [1999a] and 2-D gravity models of Sharpton et al. [1993] and Pilkington et al. [1994] as a guide. Their 3-D model provided estimates for regions of melt, breccia, and megabreccia [Ebbing et al., 2001]. [16] Pilkington and Hildebrand [2000] carried out 3-D magnetic modeling using a two-layer model. The two layers are separately inverted by dividing the crater’s magnetic field into short-wavelength and long-wavelength components located at average source depths of 2 and 5 km, respectively, and resulted in both the upper and lower source layers located within a radial distance of 50–60 km from the crater center. Ortiz-Alemán and Urrutia-Fucugauchi [2010] conducted 3-D forward modeling to build a model with magnetic sources from 2 to 8 km depth within a radial distance of 45 km of the crater center. Both studies interpreted the shallow magnetic sources as highly magnetized impact melt rock and melt-rich breccias, and the deeper sources as associated with the central uplift [Pilkington and Hildebrand, 2000; Ortiz-Alemán and Urrutia-Fucugauchi, 2010]. 2.3. Well Data [17] Exploration and research wells have been drilled within and close to the Chicxulub impact structure by Petróleos Mexicanos (Pemex) [Lopez-Ramos, 1975; Hildebrand et al., 1991; Sharpton et al., 1992], National Autonomous University of México (UNAM) [Urrutia-Fucugauchi et al., 1996; Rebolledo-Vieyra et al., 2000], the International Figure 5. (a) Aeromagnetic anomaly field over the Chicxulub impact crater from Ortiz-Alemán and Urrutia-Fucugauchi [2010]. Contour curves are given in nT. (b) Magnetic source distributions represented as layer topographies in perspective plot from Pilkington and Hildebrand [2000]. (c) 3-D model of magnetized source bodies from Ortiz-Alemán and Urrutia-Fucugauchi [2010]. 35 GULICK ET AL.: GEOPHYSICAL CHARACTER OF CHICXULUB features inside and outside the inner rim but reaches ~1 km thick inside the inner rim forming a Cenozoic sedimentary basin that overlies the annular trough, PR, and central basin of the impact structure. [19] Circular features in the gravity field are centered near the town of Chicxulub Puerto (Figure 1), but estimated coordinates for the crater center differ from this position by ~1–7 km [e.g., Hildebrand et al., 1995; Sharpton et al., 1996; Morgan et al., 1997]. All measurements and figures presented in this review use the crater center of 89.54 W, 21.3 N of Morgan et al. [1997]. [20] Structurally, from the exterior ring to the crater center, there is a zone of ring faults with offsets in the meters to tens of meters and then a zone of slump blocks with offsets in tens to hundreds of meters (Figures 1, 6, and 7) [Morgan et al., 1997; Gulick et al., 2008]. The offshore portion of the Chicxulub impact structure is shown to vary by azimuth, and individual ring faults cannot be correlated around the imaged portion of the crater [e.g., Gulick et al., 2008]. We, therefore, present the overall area of mapped ring faults (Figure 1) and group the faults into exterior (~110–130 km from crater center), outer (~90–120 km from crater center), and inner (~70–85 km from crater center) faults (Figure 7). The innermost fault of the inner ring bounds the terrace zone and generally has the largest offset; this results in the largest surface expression of faulting at the top of the K-Pg event deposit, and thus in Chicxulub this transition from the inner ring to the terrace zone has been referred to as the inner rim [e.g., Gulick et al., 2008]. The diameter of the impact structure as defined by the distance to the outer ring faults can be stated as 180 (minimum diameter of outer ring faults) to 200 km (mean diameter of outer ring faults) although outer ring faults are present at radial distances up to 120 km on Line B on which the exterior ring faults are present from 125–130 km (Figure 7) [Gulick et al., 2008]. the wells is summarized by Ward et al. [1995], Sharpton et al. [1996], Rebolledo-Vieyra et al. [2000], Dressler et al. [2004], and Stöffler et al. [2004]. Of particular interest are four wells located interior of the crater rim (Figure 2). Chicxulub 1 (C1), Yucatán 6 (Y6), and Sacapuc 1 (S1) are Pemex exploration wells with limited interval coring [Lopez-Ramos, 1975; Hildebrand et al., 1991; Sharpton et al., 1992]. ICDP well Yaxcopoil 1 (Yax-1) is located in the annular trough near the crater rim, and had almost complete core recovery from 404–1511 m [Urrutia-Fucugauchi et al., 2004]. All four wells recovered post-impact carbonates, impact melt-bearing breccia, and impact melt rock (Figure 2) [Hildebrand et al., 1991; Sharpton et al., 1992; Ward et al., 1995; Sharpton et al., 1996; Dressler et al., 2004; Stöffler et al., 2004]. 3. GEOPHYSICAL OBSERVATIONS 3.1. Impact Basin [18] The Chicxulub impact structure is interpreted as either a PR [Hildebrand et al., 1998] or multi-ring impact basin [Sharpton et al., 1993; Morgan et al., 1997; Morgan and Warner, 1999a, 1999b]. Morphologically, if the top of the K-Pg event deposits is taken as the post-impact surface of the basin, then the Chicxulub impact structure shows some of the same features observed on remotely sensed impact structures. Chicxulub has an inner rim at the inner limit of the ring faults (Figure 1), a terrace zone, an annular trough, a topographic PR, and the central basin inside the PR (Figure 6); if the inner rim is viewed as equivalent to the crater rim, then all of these features are present in PR craters (Figure 3a). Chicxulub also has faults outside of this inner rim that have been grouped into an inner ring, outer ring, and exterior ring (Figure 7) [Morgan et al., 1997]; such concentric rings are present only in multi-ring impact basins (e.g., Figure 3b). The Cenozoic sediment fill has buried all NW Inner Rim Terrace Zone Annular Trough Crater Center SE 0 Depth (km) 2 4 6 8 10 Figure 6. Time migrated, depth converted using 3-D seismic refraction velocity model, and displayed in true amplitude seismic line B. This profile shows the slump blocks down-stepping into the crater across large normal faults reaching to the region beneath the PR. The possible top of the slump blocks are interpreted along with a section of impact lithologies between the crater floor (top of K-Pg event deposit, labeled as Top K-Pg) and slump blocks. The morphological designations for parts of the impact structure are shown across the top of the figure for reference. Vertical exaggeration (VE) ~3X. 36 GULICK ET AL.: GEOPHYSICAL CHARACTER OF CHICXULUB Figure 7. Interpretation from regional seismic profiles crossing the offshore portion of the Chicxulub impact structure from Gulick et al. [2008]. Profiles are displayed clockwise around the crater from a. to f. and all are radial except Lines A0 (a.) and A1 (f.) which cross the impact structure 20 km north of the crater center. The crustal reflectivity and ring faulting differ from the west and northwest profiles to the east and northeast profiles. Measurements of the specifics of the slump blocks that lie beneath the terrace zone are shown in Table 1. Gray shading marks the post-impact Cenozoic sediments (outlined in Figure 1) that are continuous to the northeast due to a lack of a topographic rim; the region also included a deep Mesozoic sedimentary basin with the top of crust lying > 12 km deep. the rings faults are more numerous, and these faults appear to splay upward from subhorizontal surfaces (Figure 7). These surfaces are likely themselves faults and resemble the low angle detachments (décollement) in simple shear or delamination models [Wernicke, 1985; Lister et al., 1986]. In the east and northeast, the zone of rings faults is narrower (25–30 km), the faults are fewer, and the geometry is that of a series of independent listric (curved, inward facing) normal faults that do not appear to share common décollement. [23] The contact between sediments and crystalline crust in the seismic reflection profiles is not distinct but can be inferred from a transition in continuous reflections to discontinuous reflections (Figure 7). Along the majority of the radial lines, the base of sediments appears to be between 3 and 6 km, which is consistent with the depth to the 6 km/s contour in the refraction profiles [Christeson et al., 2001]. Along Line C, the sediment thickness is much greater, with the top of crystalline crust being as deep as 12 km [Christeson et al., 2009]. However, the thickness of sedimentary cover did not apparently drive the geometry [21] The area of ring faults to the west and northwest of the center of the impact structure is considerably larger than the area to the north, northeast, or eastern portions of impact structure (Figures 1 and 7). In contrast, the distance between the innermost ring fault, or inner rim, and the innermost mapped faults of the slump blocks is less variable with azimuth (Figure 1). A concentration of cenotes and sinkholes lie onshore at a radial distance of ~70 km in the west and up to 85 km in the east, which correlate with the region outside of the inner rim and within the area offset by the inner ring faults in the west and within the entire zone of ring faults in the east (Figures 1 and 7). There appears to be a correlation, therefore, between the modern hydrogeology of the Yucatán and the deformation patterns of the Chicxulub impact structure [Pope et al., 1996]. [22] The asymmetries in the extent of ring faulting appear to be related to the underlying crustal structure, either due to how it deformed as a result of the impact or to the existence of preexisting asymmetries [Gulick et al., 2008]. In the west and northwest, the area of ring faults is wider (50–60 km), 37 GULICK ET AL.: GEOPHYSICAL CHARACTER OF CHICXULUB cavity toward the crater center during the final crater collapse (Figure 4). The evaporite layers are clearly offset across as series of normal faults with up to 100 s of meters of throw defining slump blocks >5 km wide (Figures 6 and 7) [Morgan et al., 1997; Morgan et al., 2000; Gulick et al., 2008]. However, these layers should not be interpreted as the full extent of the slump blocks and, on some seismic profiles, there is reflectivity above the evaporite layers that also appears to be downdropped in sync with the large normal faults that bound the slump blocks (e.g., Figure 6). These reflectors appear to represent the top of the slump blocks. [27] There is a chaotic zone largely devoid of reflections below the top of K-Pg event deposit that is outside the PR and above the interpreted top of slump blocks (Figure 6). Drilling into the K-Pg event deposits in sites between the inner rim and PR encountered impact melt rocks and breccia (Figure 2) [Stöffler et al., 2004], and, thus, we interpret this chaotic facies to represent impactites that likely were deposited above the slump blocks by processes during the impact, possibly including proximal ejecta (e.g., Figure 4b) and outward surge of the central uplift (e.g., Figure 4c) and immediately following the impact, possibly including ground surge, slope collapse, and/or tsunami waves. In the seismic profiles, we have mapped the base of the post-impact Cenozoic sediments as the top of the K-Pg deposit. Although the formal boundary between the Cretaceous and the Paleogene is at the base of all impact-related deposits [Molina et al., 2006] that depth within the crater is difficult to determine precisely, and thus following the Schulte et al. [2012] discussion of a proximal impact sequence in Mexico, we refer to the entire thickness of impact deposits below the post-impact Paleogene sediments as the K-Pg event deposit. Additionally, in ICDP drillsite Yax-1 located close to the crater rim but within the terrace zone (Figure 2), Whalen et al. [2013] observed significant sediment deformation structures for ~100 m above the top of the K-Pg event deposit which lies at ~795 m. These sedimentary structures above the K-Pg deposits in the core data suggest that sediment deformation was common and then episodic during the early Paleogene history within the terrace zone [Whalen et al., 2013]. [28] The geometry of the slump blocks, as ascertained from the evaporite layers, varies by azimuth but all seismic profiles collected show that the slump blocks reach beneath the PR (Figures 1, 6, and 7). In Table 1, we summarize the amount of cumulative fault throw, as measured from the top of the evaporite layers in the inner rim on each radial of the ring faults, given that Line C in the northeast has the thickest sediment cover, due to the deepest basement (>12 km) and a preexisting Mesozoic basin in this azimuth [Gulick et al., 2008], yet the pattern of ring faulting on this profile resembles Lines R1 and A1 in the east and east-northeast, where the top of crystalline crust is at ~5 km depth (Figure 7). [24] As pointed out in Gulick et al. [2008] and shown in Figure 7, crustal reflectivity differs between the radial seismic lines A, R3, B to the west and northwest and lines C, R1, and A1 to the east and northeast. The reflective crust lies largely below the ring faults in the west and northwest, while in the east and northeast directions, the crustal reflectivity is shallower and largely lies outside the area of rings faults, but some reflectivity is present within the ring faults (Figure 7). It is unclear if this potential correlation in the presence of reflective zones in the crust and the geometry of the ring faults at depth is causal. The observation that the depth to crustal reflectivity is shallower to the east than to the west-northwest might suggest that the difference in rheology of the target rock influenced the brittle faulting stage of crater collapse. 3.2. Terrace Zone [25] Complex craters and impact basins on planetary surfaces typically exhibit morphologically defined terrace zones expressed as steps downward from the crater rim to the crater floor (e.g., Figure 3a) [e.g., Melosh, 1989; French, 1998]. These terrace zones are thought to be part of the final crater collapse process (Figure 4) but only through subcrater floor images can the complexities of these processes be observed. In the Chicxulub impact structure, the crater floor between the inner rim and topographic PR effectively includes a muted expression of the terrace zone and the annular trough (Figure 6). There is significant variation in the morphology of this portion of the impact structure, with some seismic profiles showing almost no morphologic expression of a terrace zone in the top of the K-Pg event deposits (Figure 7) and others, such as Line B, showing a large region that is elevated with respect to the annular trough (Figure 6). [26] Evaporites within the Mesozoic sediments form bright flat lying reflections outside of the inner rim (e.g., Figure 6) [Camargo-Zanoguera and Suarez-Reynoso, 1994; Sharpton et al., 1996; Morgan et al., 1997; Snyder and Hobbs, 1999]. These strata serve as excellent marker horizons to allow tracking of the slumping of the transient TABLE 1. Azimuthal Comparison of Slump Block Observations Measurements From Depth Converted Profiles Distance to inner slump block (km from center) Distance to outer slump block (km from center) Width of zone of slump blocks (km) Fault throw at top of evaporitesfrom crater rim to outer block (m) from crater rim to inner block (m) Line R3 (WNW) Line B (NW) Line R7 (N) Line C (NNE) Line R1 (NE) <43.5 38.8 43.3 35.8 <53.0 70.4 81.1 84.2 69.5 84.0 >26.9 360 42.3 950 40.9 320 33.7 520 >31.0 300 3340 6450 38 2820 2280 2920 GULICK ET AL.: GEOPHYSICAL CHARACTER OF CHICXULUB and less prominent (0.2–0.3 km above crater floor) to the north, east, and northeast (Figure 7) [Morgan et al., 1997; Gulick et al., 2008]. The PR is roughly symmetric about the crater center, with an average radius of 39–40 km measured to the highest peak (Figure 10a) [Morgan et al., 1997; Gulick et al., 2008]. However, the depth to the top of the PR is asymmetric and varies from ~0.62 km in the west to ~1.06 km in the east (Figure 10a) [Gulick et al., 2008]. The PR lies above or close to the slump blocks (Figures 1, 8, and 9a), and bright inward-dipping reflectors run from the outer edge of the PR to the inner edge of the slump blocks (e.g., Figure 8) [Brittan et al., 1999; Morgan et al., 2000; Gulick et al., 2008; Morgan et al., 2011]. [30] High-resolution velocity models [Morgan et al., 2011] constrain a 0.1–0.2 km thick low-velocity zone in the uppermost part of the PR, with velocities ~0.6–0.8 km/s lower seismic profile to the top of the evaporites in the outermost and innermost interpreted slump blocks on the profile. We also record the radial distance of this zone of slump blocks in Table 1 and show it in map view in Figure 1. Note that the western and northwestern profiles R3 and B (Figure 7) show significantly greater fault throw and greater depth beneath the PR; the northern, northeastern, and eastern profiles R7, C, and R1 show lesser amounts of fault throw and shallower depths beneath the PR (Table 1). 3.3. PR [29] The Chicxulub PR is imaged as an irregular and rugged feature that stands several hundred meters above the annular trough (Figures 8 and 9a) [Morgan et al., 1997; Morgan et al., 2000]. It is more prominent (0.4–0.6 km above crater floor) in the west and northwest, a. 0 4 km W VE~5X Toward Crater Center E Cenozoic seds Cenozoic seds Top K-Pg 1000 Impactites Depth (m) Peak Ring 2000 Impactites Top of Melt dipping reflectivity 3000 4000 5000 Line 10 Slump Blocks b. VE~2.5X NW 1.25 km Toward Crater Center SE 0 Cenozoic seds Depth (m) 1000 Top K-Pg Peak Ring Impactites 2000 dipping reflectivity 3000 Slump Blocks Line R3 4000 Figure 8. Two example images of the PR in the northwestern quadrant of Chicxulub. (a) Line 10 is shown at 5-to-1 VE and is time migrated and depth converted. Note the clear dipping reflectivity at the outer edge of the PR, the elevated PR relative to the surround top of the K-Pg deposit, the low-frequency reflector toward the crater center interpreted to be top of melt rock, and the chaotic interval above this reflector interpreted to be rapidly emplaced post-impact sediments from proximal ejecta, slope collapse, ground surge, and tsunamis. (b) Line R3 shown at a 2.5-to-1 VE is time migrated and depth converted. Note the dipping reflectivity, slump block reflections lying beneath the PR, impact-related deposits above the slump blocks, and lack of internal reflections within the PR. 39 GULICK ET AL.: GEOPHYSICAL CHARACTER OF CHICXULUB a) W Chicx-03A 4 km Toward Crater Center E 0 Cenozoic seds Cenozoic seds Top K-Pg 1 Impactites Peak Ring Impactites Depth (km) 2 Top of Melt 3 4 5 Line 10 b) Slump Blocks 0 2.5 3.0 3.5 1 4.0 4.5 5.0 Depth (km) 2 5.5 3 4 5 0 10 20 30 Distance (km) 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 Velocity (km/s) Figure 9. (a) Interpretation of seismic reflection data across the PR from Line 10. (b) Seismic velocity model of the PR from two-dimensional velocity model of Line A/A1, which is located ~0.65 km to the north. Contour interval is 0.5 km/s. White lines display the Line 10 seismic reflection data interpretation. dipping toward the crater center [Morgan et al., 2000]; (Figure 9b). Figure 10b displays a slice through the Vermeesch and Morgan [2008] and Vermeesch et al. [2009] traveltimeonly velocity model at 2.5 km depth. Average velocities at this depth are 5.6 km/s, but a ring with velocities <5.4 km/s is observed beneath or inward of the mapped PR (Figure 10b). The lower-velocity ring continues to the south in the joint traveltime-gravity model [Vermeesch and Morgan, 2008; Vermeesch et al., 2009] (Figure 10c). The low-velocity material would be expected to have low densities, and, indeed, is coincident with a gravity low (Figure 1). [32] One model for Chicxulub PR formation, based largely on numerical modeling, is that it formed by interaction between an inwardly collapsing transient cavity rim and an outwardly collapsing central structural uplift (e.g., Figure 4) [e.g., Brittan et al., 1999; Morgan et al., 2000; Collins et al., 2002; Collins et al., 2008; Morgan et al., 2011]. In this model, the PR is composed of overturned basement rocks that are highly fractured and brecciated, which results in the observed than the overlying Cenozoic sediments and underlying PR material (Figure 11). Morgan et al. [2011] interpret the low-velocity zone as a thin layer of highly porous allogenic impact breccias. On some lines, the base of the low-velocity zone is associated with a low-frequency reflector (Figures 11b and 11c) [Morgan et al., 2011]. The high-resolution models map high-velocity zones (>5.5 km/s) in the annular trough adjacent to the PR (Figure 11); the relatively abrupt decrease in velocity at the location of the dipping reflectors suggests that the rocks that form the PR are lithologically distinct from the surrounding rocks [Morgan et al., 2011]. Otherwise, the uppermost 0.5 km of PR material has average velocities comparable to that of the adjacent Cenozoic sediments and to the impact breccia immediately beneath the top of the K-Pg deposits (Figures 9b and 11) [Brittan et al., 1999; Morgan et al., 2011]. [31] Deeper in the PR (~0.5–2.5 km beneath top of the K-Pg deposits), velocities are depressed compared to adjacent material, with the region of lowered velocities 40 GULICK ET AL.: GEOPHYSICAL CHARACTER OF CHICXULUB a) 2.00 1.75 21°30' 1.25 Crater Center Depth (km) 1.50 1.00 0.75 21°00' Merida K-Pg Surface b) -90°00' 0 km 50 0.50 -89°30' 6.5 6.0 21°30' 5.6 5.4 Velocity (km/s) 5.8 5.2 21°00' 5.0 Z=2.5 km 4.5 -90°00' -89°30' c) 6.5 6.0 21°30' 5.6 5.4 Velocity (km/s) 5.8 5.2 21°00' 5.0 Z=2.5 km 4.5 -90°00' -89°30' Figure 10. (a) Top of K-Pg event deposit structure contour map from Gulick et al. [2008]. The white lines mark the mapped inner and outer edge of the PR. The city of Mérida and crater center are shown with maroon and red stars, respectively. (b) Slice through the traveltime-only 3-D velocity model of Vermeesch and Morgan [2008] and Vermeesch et al. [2009] at 2.5 km depth. The model is masked where not constrained by raypaths. (c) Slice through the joint traveltime-gravity 3-D velocity model of Vermeesch and Morgan [2008] and Vermeesch et al. [2009] at 2.5 km depth. 41 GULICK ET AL.: GEOPHYSICAL CHARACTER OF CHICXULUB a) Annular Trough Peak ring Chicx-03A N Depth (km) 0.2 0.6 Dipping reflectors 1.0 1.4 S Top K-Pg HVZ Line 17 b) W Annular Trough Peak ring Chicx-03A E 0.2 Depth (km) Low-frequency reflector Dipping reflectors 0.6 HVZ Top K-Pg 1.0 1.4 Line 10 c) Annular Trough NW Peak ring Chicx-04A SE Chicx-03A 0.2 Depth (km) Low-frequency reflector 0.6 Dipping reflectors HVZ 1.0 1.4 Line R3 15.6 km 2 Velocity (km/s) 5.7 Figure 11. Full wavefield inverted velocity models and reflection stacks from Morgan et al. [2011] for (a) Line 17, (b) Line 10, and (c) Line R3. Migrated reflection stacks are converted to depth using the inverted velocity model. The locations of the dipping reflectors have been extrapolated from deeper in the section, where the reflections are clearer. HVZ indicates locations of high-velocity zones in rocks in the annular trough adjacent to the PR. impact melt thinning out toward its outer edge topped with 0.1–0.13 km of melt-bearing breccia (wells Y6 and Yax-1, Figure 2). Alternatively, the sampled melt rocks may represent local pods of melt, possibly within a thick melt breccia unit, rather than a thick coherent melt sheet [Sharpton et al., 1996]. [34] Chicxulub basement, melt-bearing breccia, and impact melt rocks have remanent magnetizations that are three to four orders of magnitude higher than near-surface target rocks [Hildebrand et al., 1991; Urrutia-Fucugauchi et al., 1996; Pilkington and Hildebrand, 2000; Ortiz-Alemán and Urrutia-Fucugauchi, 2010], and hence magnetic data can be used to map these lithologies. Three-dimensional magnetic models of the Chicxulub structure include shallow source bodies with reverse magnetic polarity, consistent with remanent magnetization acquired 65.5 Ma during geomagnetic chron 29r [Pilkington and Hildebrand, 2000; inward-dipping region of lowered velocities [Morgan et al., 2000; Morgan et al., 2011]. There are no wells to validate this PR formation model, but two 1.5-km-deep IODP drill holes that would penetrate the PR have been proposed (Figure 2). 3.4. Impact Melt Sheet [33] Impact melt rocks have been sampled at the Chicxulub structure by wells C1, S1, and Y6 at depths of ~1.2–1.6 km (Figure 2) [Hildebrand et al., 1991; Sharpton et al., 1992; Ward et al., 1995; Sharpton et al., 1996; Stöffler et al., 2004]. Using the stratigraphy and thickness of lithological units of all available drill cores across the Chicxulub impact structure, Stöffler et al. [2004] interpret a coherent impact melt layer of unknown thickness topped with 0.175–0.36 km of melt-bearing breccia in the central basin (wells C1 and S1, Figure 2), while the annular trough is filled with coherent 42 GULICK ET AL.: GEOPHYSICAL CHARACTER OF CHICXULUB in the crater’s melt sheet. They further argue that the magnetized zones result from hydrothermal systems that produce magnetic phases during alteration of the melt sheet and that the zones map out regions of faults and fractures favorable to fluid flow [Pilkington and Hildebrand, 2000]. [35] Measurements on cores from well Y6 (V.L. Sharpton, 1998, personal communication to J.V. Morgan) show P-wave velocities of 3.9 km/s in the melt-bearing breccia and 5.8 km/s in the impact melt rock. Seismic reflection data image an intermittent low-frequency reflector within the central basin (Figures 8a, 9a, and 12a) across which velocities increase rapidly to >5.5 km/s; Morgan et al. [2000] interpret this reflector as the contact between breccia and the melt sheet. Barton et al. [2010] shows a detailed seismic image that Ortiz-Alemán and Urrutia-Fucugauchi, 2010]. In the Pilkington and Hildebrand [2000] model, two circular zones with radii of ~20 km and ~45 km are observed (Figure 5b). The zones have widths <5 km, with an average value of 2–3 km [Pilkington and Hildebrand, 2000]. The Pilkington and Hildebrand [2000] inversion places these sources at an average depth of 2 km and models greater relief for the inner zone sources than those of the outer zone (Figure 5b). The Ortiz-Alemán and Urrutia-Fucugauchi [2010] model contains source bodies between 1.5 and 3.5 km depth that extend to a radial distance of 45 km, with no clear pattern of an inner and outer circular zone (Figure 5c). Pilkington and Hildebrand [2000] interpret the shallow magnetic sources as highly magnetized zones a) 0 Chicx-04A W 8 km Towards Crater Center E Cenozoic seds Cenozoic seds Peak Ring Top K-Pg Top of Melt Impactites 2 Impactites ing pp Di 3 rs cto fle Re Depth (km) 1 4 5 Slump Blocks Slump Blocks Line 9 Distance (km) b) 0 0 10 20 30 40 50 60 70 80 90 Peak Ring Peak Ring Depth (km) Central Basin 1 2 3 1.8 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 7.0 Velocity (km/s) 21˚45' c) km 0 10 20 Peak Ring 21˚30' elt Top M -90˚00' 0.0 0.2 -89˚30' 0.4 0.6 0.8 -89˚00' 1.0 1.2 1.4 Depth Below K-Pg Surface (km) Figure 12. (a) Interpretation of seismic reflection data cross the PR and central basin from Line 9. (b) Velocity image for Line 9 created from streamer data first-arrival refraction picks transformed into velocity-distance-depth space [Barton et al., 2010]. White lines mark display the Line 9 seismic reflection data interpretation. (c) Mapped low-frequency reflector, interpreted as the top of the melt sheet, shown in depth below top of K-Pg event deposit. The white lines mark the mapped inner and outer edge of the PR. 43 GULICK ET AL.: GEOPHYSICAL CHARACTER OF CHICXULUB the interpretation of Sharpton et al. [1996], perhaps fed by sills connecting to a deeper coherent melt sheet as imaged by Barton et al. [2010]. reveals that the reflector is topped by upwardly concave events interpreted as sills and underlain by a zone of layered reflections 2–3 km thick. Barton et al. [2010] also present high-resolution velocity models of the shallow crust that show a strong correlation between the more horizontal portions of the low-frequency reflector and an increase to velocities >5.5 km/s (Figure 12b). The high-velocity layer and low-frequency reflector often truncate near the inner edge of the PR [Barton et al., 2010] (Figures 12a–b). High velocities are also observed at similar depths in the annular trough, but are not strongly correlated with features in the seismic reflection data [Barton et al., 2010; Morgan et al., 2011] (Figures 11 and 12b). [36] Using the velocity models of Barton et al. [2010] and Morgan et al. [2011] as a guide, we have mapped locations (Figure 12c) where the low-frequency reflector (Figure 12a) is correlated with the high-velocity layer (Figure 12b). The low-frequency reflector (Figure 12a) is located at an average depth of 0.685  0.175 km below the top of the K-Pg event deposits (~1.9 km below seafloor). The inferred top of impact melt rocks (Figure 12c) is largely absent from beneath the PR, but is observed near its inner and outer edges (Figure 12c) where it shallows toward the PR (Figure 12a). The mapping suggests a mostly continuous melt sheet layer in the central basin, with discontinuous melt-rich patches in the annular trough near the outer edge of the PR (e.g., Figures 6 and 8b). This interpretation is generally consistent with well data and magnetic models, although it places the top of the melt sheet at ~1.9 km depth, which is deeper than the melt rocks encountered at depths of 1.2–1.6 km (Figure 2). Thus, the seismic data would suggest that the wells sample local pods of melt, similar to 3.5. Deep Structure [37] A prominent feature in the gravity field is a twin-peaked gravity high associated with structural uplift of the Chicxulub structure (Figure 1). Velocity models image the structural uplift as a high-velocity zone with velocities of 6.3–6.4 km/s observed at depths of 4.5–5.0 km, and a width of 35–40 km [Vermeesch and Morgan, 2008; Christeson et al., 2009; Vermeesch et al., 2009] (Figure 14). The twin-peaked gravity high extends south nearly to Mérida, and hence the joint traveltime-gravity inversion model shows the high-velocity zone continuing to the south with a width of ~60 km [Vermeesch and Morgan, 2008; Vermeesch et al., 2009] (Figure 14, right panels). The top of the high-velocity zone is highly asymmetric and has a complex 3-D shape [Vermeesch and Morgan, 2008; Vermeesch et al., 2009] (Figure 14). The center of the structural uplift, interpreted at the approximate center of the 6.1 km/s contour in Figure 14a, is located ~10 km west of the crater center at the intersection of the crossing profiles in Figure 14. Vermeesch and Morgan [2008] divide the material composing the structural uplift into upper crustal (<6.0 km/s), midcrustal (>6.0 and < 6.3 km/s), and lower crustal (>6.3 km/s) rocks, based on comparisons with crustal velocities outside the inner rim. They estimate that the width of structural uplift is 15–25 km, 40–60 km, and ~ 80 km for the lower, middle, and upper crustal rocks, respectively. Magnetic models include a central uplift with a width of ~60 km [Pilkington and Hildebrand, 2000; Figure 13. True amplitude, time migrated, and depth converted seismic Lines A0 and A1 spliced together to show a profile across the impact structure. The profile is located approximately 20 km north of the actual crater center and just north of the central uplift. While this image is not optimum for examining shallow features in the impact structure, it does show the presence of reflectivity dipping through the lower 15 km of the crust in the west and 20–25 km of the crust in the east. These zones of reflectivity are consistent with the deeper continuations of the ring faults as shown in Figure 7. The reflectivity that dips away from the center of the profile may be related to the central uplift south of this profile. VE ~6.5X. 44 GULICK ET AL.: GEOPHYSICAL CHARACTER OF CHICXULUB where the volume of acoustic fluidization shrinks after transient cavity formation allowing for both deformation regimes to occur simultaneously. [41] PR formation is not a well-understood process. Collins et al. [2002] model the PR as the result of an outward collapsing fluidized material originating from the rebounding center (Figure 4c). In numerical simulations of the Ries crater, Wünnemann et al. [2005] showed that the strength of the target rocks had a significant effect on the formation of inner rings. For a strong target, an inner ring was formed from overturned shallow basement whereas, for a weaker target, the ring was formed from rocks originating from deeper depths. Head [2010] instead suggests that the PR consists of shocked shallow lithologies at the outer edge of a nested melt cavity. The emplacement of the PR on top of the innermost imaged slump blocks, and the observation of a dipping reflector separating the PR material from the slump blocks as well as from the impact breccias, together imply that the PR did not originate from the same depth as the slump blocks. Rather, like in the Wünnemann et al. [2005] weaker target case or the Collins et al. [2002] model, the PR originated from deeper sourced material that must have dynamically rebounded to a height above the slump blocks in order to be emplaced on top of them. This requirement is consistent with the Vermeesch and Morgan [2008] suggestion that the PR originated from at least the middle crust (e.g., Figure 16). [42] The central uplift collapse model for PR formation [Collins et al., 2002] predicts: (1) the PR is formed from fluidized material that translated outward from the center, (2) that impact melt could be present inside or outside of the PR, and (3) that the target heterogeneities may influence the morphology and thickness of the PR [e.g., Collins et al., 2008]. The nested melt cavity model [Head, 2010] predicts the PR formed largely in place with the impact melt being limited to inside the PR and any influence of the target being minor. The low velocities within the PR (e.g., Figure 10) are predicted by both models due to the likelihood of shock induced porosity and brecciation. However, we observe: (1) a heterogeneous PR (e.g., Figures 6–9) where relief and thickness of the PR appears to correlate with target asymmetries [Gulick et al., 2008], (2) impact melts outside of the PR (e.g., Figure 12), and (3) PR geometries including a steeper outer surface and inward dipping reflector bounding the PR lithologies (e.g., Figure 8) that imply outward collapse. These observations all appear to favor the central uplift collapse model. Ortiz-Alemán and Urrutia-Fucugauchi, 2010] (Figure 5), which would correlate with the upper and middle crustal rocks of Vermeesch and Morgan [2008]. Outside of the inner rim velocities of 6.3 km/s are observed at depths of ~15 km [Christeson et al., 2001], and hence the observation of 6.3 km/s material at 4.5–5 km depth implies a structural uplift of at least 10 km [Christeson et al., 2001; Vermeesch and Morgan, 2008]. [38] Modeling of the Moho interface indicates that crustal deformation extends to the base of the crust [Christeson et al., 2009]. The Moho is upwarped by ~1.5–2 km near the center of the Chicxulub crater and depressed by ~0.5–1.0 km at a distance of ~30–55 km from the crater center [Christeson et al., 2009] (Figure 15). The center of the Moho upwarping is located near and slightly north of the crater center (Figure 15), in contrast to the center of the structural uplift, which is located ~10 km to the west of the crater center (Figure 14). Christeson et al. [2009] speculate that uplift of both the Moho and crust are linked and that the asymmetry was produced during the collapse of the transient cavity. Whether this asymmetric collapse was a consequence of oblique impact, lateral asymmetry in crustal strength, or random instabilities during crater modification remains an open question [Christeson et al., 2009]. [39] Where visible, the Moho appears as a band of reflectivity within the seismic reflection profiles at >30 km depth. Within the middle to lower crust and terminating at the Moho are some dipping bands of reflections that have been interpreted as faults [Morgan et al., 2000; Christeson et al., 2001]. These reflective bands are present in the lower 15 km in the west on the western and northwestern profiles (e.g., Figure 13, Line A0) and in the lower 20–25 km in the eastern and northeastern profiles (e.g., Figure 13, Line A1). The radial distances are consistent with these bands being the continuation of a basal normal fault (décollement) in the west and the listric normal faults in the east (Figure 7) into the middle to lower crust. On Line A0, which lies only 20 km north of the crater center but does not cross the central uplift, there is an additional high-amplitude westward dipping reflective band (Figure 13) that may be faulting related to the final collapse downward of the central uplift. These crustal reflectors may mark whole crustal faulting during crater collapse that is preserved and imageable due acoustic properties of the fault planes. 4. DISCUSSION AND CONCLUSIONS 4.1. Terrace Zone and PR Interactions [40] On all seismic images crossing from the terrace zone to the PR, slump blocks are imaged that underlie the PR lithologies (e.g., Figures 6–9). This observation requires that the transient cavity rim (Figure 4b) collapse inward prior to the PR collapsing outward. Interestingly, the terrace zone is clearly deforming in the brittle regime along large offset faults between mostly intact slump blocks, while the PR is undergoing pervasive brittle deformation such that it behaves in a fluid-like manner. This observation is predicted by the acoustic fluidization model of Melosh [1989], 4.2. Melt Sheet Dimensions [43] Integration of seismic, magnetic, and drilling data (section 3.4) suggests that a coherent melt sheet is present in the central basin inward of the PR, that it is largely absent from beneath the PR, and that it is present as discontinuous melt-rich patches in the annular trough near the outer edge of the PR. There is no geophysical evidence for a coherent melt sheet being present exterior of the inner rim. The top of the melt sheet truncates and appears to shallow into the inner edge of the PR; it has an average depth below the 45 GULICK ET AL.: GEOPHYSICAL CHARACTER OF CHICXULUB Within the central basin, the event deposit could be up to 4 km thick including the melt sheet. [47] The sedimentary portion of the K-Pg event deposit overlying the melt sheet inward of the PR are thinner than the sedimentary event deposit in the annular trough. These sediments are 400–900 m thick and include some reflections (e.g., Figures 6, 8a, and 9a) that may suggest a slightly lower, but still extremely high, energy environment during deposition than that which existed within the annular trough. This difference possibly reflects the restricted entrance of water into the central basin due to the barrier of the PR. Tsunami waves likely entered the annular trough primarily through the northeastern quadrant due to the lack of an inner rim in that azimuth (e.g., Figures 4c and 7) and then made it into the central basin by overtopping local low-relief locations within the topographic PR. The reduced thickness of impactites within the central basin could also reflect a lack of proximal ejecta being deposited interior to the PR. These tsunami backwash, airfall sediments, and possible local ground surge were deposited on top of a hot melt sheet in the central basin and likely served to rapidly cool the top of the melt sheet, contributing to the mappable top of melt seismic reflection (e.g., Figures 6 and 8a). However, the remnant high heat flow likely persisted for some time post-impact and, thus, a vigorous hydrothermal system can be expected to persist within these central basin sediments and possibly within the PR into the post-impact Paleogene; a similar post-impact hydrothermal system has been suggested for the Sudbury impact structure and used to explain the large volume of hydrothermal deposits within that impact structure [Ames et al., 2006]. top of the K-Pg event deposit of ~0.7 km (Figure 12c) and below the seafloor of ~2.0 km. [44] The thickness of the melt sheet is poorly constrained. Seismic reflection data image a low-frequency reflector, across which velocity rapidly increases to >5.5 km/s, that is interpreted as the top of a coherent melt sheet [Morgan et al., 2000; Barton et al., 2010]. However, no low-frequency reflector or velocity contrast is imaged deeper that might correspond to the base of the melt sheet. Theoretically, the expected volume of impact melt for the ~180 km diameter Chicxulub structure is ~104 km3 [Grieve and Cintala, 1992; Pierazzo et al., 1997]. Therefore, if the outer extent of the coherent melt sheet is the inward edge of the PR at an average diameter of ~65 km, its expected thickness is ~3 km [Barton et al., 2010]. This thickness might be considered an upper bound, as the observations indicate that some discontinuous melt patches are present within the annular trough, and that additional melt mixes with other impact material to form melt-rich breccias. A 3 km thickness will place the base of the melt sheet ~5 km below the seafloor, which near the crater center would correspond to the top of the structural uplift [Morgan et al., 2000; Vermeesch and Morgan, 2008; Barton et al., 2010]. 4.3. K-Pg Event Deposit Within Chicxulub [45] The thickness of the K-Pg event deposit expands with proximity to the Chicxulub impact structure [e.g., Schulte et al., 2010; Schulte et al., 2012]. Within the impact structure, the event deposit strictly speaking also includes the PR and melt sheet itself. Above and external to these features there is a sedimentary component to the event deposit that is imaged within the annular trough, across the PR, and in the central basin. In the annular trough, the event deposit consists of all the impactites (impact breccias, melt-bearing breccias, and proximal ejecta) that lie between the top of the slump blocks and the base of the normal Paleogene sediments (e.g., Figures 6 and 8b), and appears to include some impact melt proximal to the PR (e.g., Figure 12). On top of the PR, the sedimentary component of the event deposit thins to the width of a single seismic reflection (< ~30 m), but then expands again within the central basin to include all impact material from the melt sheet to the base of the post-impact Paleogene sediments. [46] In both the annular trough and central basin, the seismic facies of this sedimentary component of the event deposit is chaotic with few mappable internal reflectors but with occasional bright spots. This unit is likely equivalent to the chaotic layer with an erosional base mapped by Gulick et al. [2008] at ~85 km from the crater center, which was attributed to ground surge; there, the emplacement of the K-Pg event deposit sediment eroded into the underlying Mesozoic Basin. Schulte et al. [2012] discuss, in detail, similar observations of erosional bases to emplacement of tsunami-produced sediment associated with Chicxulub in the La Popa Basin, México. Therefore, we suggest the K-Pg event deposit within the annular trough is 1–3 km thick (e.g., Figure 6) and consists of a chaotic assemblage of tsunami backwash, ground surge, slope collapse, and proximal ejecta deposits. 4.4. Asymmetries and Their Causes [48] Many of the features at the Chicxulub structure are observed to be asymmetric. In the west and northwest, the area of ring faults is wider (50–60 km), the rings faults are more numerous, and these faults appear to sole into subhorizontal surfaces; in the east and northeast, the zone of rings faults is narrower (25–30 km), the faults are fewer, and the geometry is that of a series of independent listric normal faults (e.g., Figures 1 and 7). The reflective crust lies largely below the ring faults in the west and northwest, while in the east and northeast directions the crustal reflectivity is shallower and largely lies outside the area of rings faults but some reflectivity is present within the ring faults (Figure 7). Within the terrace zone, the western and northwestern profiles show significantly greater fault throw and greater depth beneath the PR (Figures 6–8), while the northern, northeastern and eastern profiles show lesser amounts of fault throw and shallower depths beneath the PR (Figure 7 and Table 1). The PR is more prominent and shallower in the west and northwest, and less prominent and deeper to the north, east, and northeast (Figures 8 and 10). Structural uplift of deep crustal rocks is offset by ~10–20 km west of the crater center (Figure 14), while at the crust-mantle interface, the Moho is upwarped near the crater center (Figure 15). 46 GULICK ET AL.: GEOPHYSICAL CHARACTER OF CHICXULUB a) B’ 21°30' 6.1 6.1 6.1 A’ A 21°00' B Z=5 km -90°00' Z=5 km -89°30' 0 km Z=5 km 50 -89°30' -90°00' -90°00' -89°30' b) Depth (km) 0 A’ A A’ A 5 A’ A 6.1 6.1 6.1 10 -20 c) 0 Depth (km) 0 20 -20 Distance (km) B’ B 0 20 -20 Distance (km) B’ B 0 20 Distance (km) B’ B 5 10 -20 0 20 -20 Distance (km) 2.00 5.50 5.60 0 20 -20 Distance (km) 5.70 5.80 5.90 6.00 0 20 Distance (km) 6.10 6.20 6.30 6.75 Velocity (km/s) Figure 14. (a) Slice at 5 km depth through (left) 3-D regional velocity model [Christeson et al., 2009]; (center) 3-D traveltime-only velocity model [Vermeesch and Morgan, 2008; Vermeesch et al., 2009]; (right) 3-D joint traveltime-gravity velocity model [Vermeesch and Morgan, 2008; Vermeesch et al., 2009]. Contour interval is 0.2 km/s (black lines), with 6.1 km/s contour displayed in white. The city of Mérida and crater center are shown with black and red stars, respectively. Thick yellow lines show the position of the profiles plotted in Figures 14b and c. (b) Models left to right as for Figure 13a, except showing slice along A-A’. (c) Models left to right as for Figure 14a, except showing slice along B-B’. [49] Studies of central peak craters and PR craters on Venus show no relationship between impact angle and location of topographic expressions of central peaks or PRs [Ekholm and Melosh, 2001; McDonald et al., 2008]; hence, asymmetries in these features cannot be used to infer impact direction or angle. Gulick et al. [2008] use seismic data to identify a preexisting Mesozoic basin and increasing late Cretaceous water depth to the north and northeast that correlates with some of the crater asymmetries. Collins et al. [2008] use numerical modeling to conclude that variable water depth and sediment thickness at the time of impact will produce the observed terrace zone asymmetries: a 2 km water layer in the north, northeast, and east results in lesser amounts of fault throw and shallower depths beneath the PR of the slump blocks, while little or no water layer in the northwest results in greater fault throw and greater depth beneath the PR of the slump blocks. Asymmetries in the ring faults and reflective crust may be correlated; the observation that the depth to crustal reflectivity is shallower to the east than to the west-northwest (Figure 7) might suggest that the difference in rheology of the target rock influenced the brittle faulting stage of crater collapse. Alternatively, asymmetries in the ring faults may result from an asymmetric transient cavity formed by an oblique impact [Gulick et al., 2008]. The structural uplift observed in the upper crust at Chicxulub is offset from the center of the Moho upwarping (e.g., Figure 4d); Christeson et al. [2009] speculate that uplift of both the Moho and crust are linked and that the asymmetry was produced during the collapse of the transient cavity. However, whether this asymmetric collapse was a consequence of oblique impact, lateral asymmetry in crustal strength, or random instabilities during crater modification remains an open question [Christeson et al., 2009]. 47 GULICK ET AL.: GEOPHYSICAL CHARACTER OF CHICXULUB a) morphology of Chicxulub to that of Schrödinger and Orientale (Figure 3). We caution, however, against all but the most general of comparisons. As noted, the morphologies of Chicxulub and Schrödinger are derived from different datasets (geophysics and remote-sensed optical imagery, respectively), which reflect different target rock properties and resolution scales. One of the fundamental differences in the morphometry of impact structures on the terrestrial planets is the effect of planetary gravity on the cratering process. The net result of the lower surface gravity on the moon is to reduce the relative amount of impact melt in structures of comparable size occurring in the terrestrial environment [Cintala and Grieve, 1998]. The lower lunar surface gravity results also in the transitional diameters for the various types of crater forms being larger on the moon than in the terrestrial environment; for instance, the lunar impact basin Orientale has rings at ~930, ~620, ~480, and ~320 km, while Chicxulub has ring faults grouped into rings with apparent diameters of 220–260 km, 180–220 km, and 140–170 km and a PR the exterior of which has a diameter from 80–110 km. More importantly, the lower lunar surface gravity also results in relatively greater impact-produced topography of the various morphological elements of lunar compared to terrestrial impact structures, for impact structures of comparable form. [52] It would appear, therefore, inter-planetary comparisons of the nature of Chicxulub, based almost entirely on geophysical data, with impact structures of similar form on the other terrestrial planets must be made with some caution. Chicxulub, however, does serve to illustrate some of the problems associated with comparing morphological parameters between impact structures. The initial problems with defining its diameter based on geophysics underscores the observation that different datasets, and their variations in scale with respect to their ability to determine a particular morphologic feature, e.g., rim diameter, can lead to differing results for the same structure (see Turtle et al., 2005 for thorough discussion of these difficulties and potential problems). 38 36 35 34 33 Crater Center 21° b) Merida -90° 0 km 50 32 Depth (km) 37 22° 31 30 -89° 1.0 0.5 -0.5 -1.0 -1.5 -2.0 21° 0 -90° km 50 Depth Anomaly (km) 1.5 22° -2.5 -89° Figure 15. (a) Moho interface beneath the Chicxulub structure [Christeson et al., 2009]. Interface is only displayed at reflection points. The city of Mérida and crater center are shown with maroon and red stars, respectively. (b) Depth anomaly after removal of regional trend from Moho interface [Christeson et al., 2009]. 4.5. Comparison With Other Large Impacts in the Inner Solar System [50] The main structural and morphologic elements at Chicxulub have been compared to the two other large terrestrial impact structures: Sudbury, Canada and Vredefort, South Africa [e.g., Grieve et al., 2008]. These comparisons involve considerable interpretation, as the three impact structures are of similar size, but vary significantly in their current appearance. Vredefort has been eroded 5–10 km below its original surface and has undergone minor tectonism; virtually all of the allochthonous impactites have been removed leaving the parautochthous rocks of the original floor of the structure exposed. At Sudbury erosion has been less severe, with exposed allochthonous impact melt and various breccia lithologies remaining, but it has been severely deformed to the extent that it no longer expresses a circular form. The nature and occurrence of specific impact melt lithologies at Sudbury and the interpretation of similar lithologies at Chicxulub has been made by Barton et al. [2010], particularly with respect to the interpretation that the impact breccias overlying the impact melt sheet at Chicxulub contain intrusive bodies of impact melt rocks. Additional morphological and structural comparisons between these structures are unwarranted here, as significant new geoscience knowledge has not been added to the understanding of the Sudbury and Vredefort impact structures, in terms of their original morphology and structure; although, there have been recent advances in understanding the nature and extent of post-impact tectonism at Sudbury [e.g., Lenauer and Riller, 2012]. [51] It is tempting to compare the interpreted morphology and nature of Chicxulub, as reviewed here, with that of impact structures on other terrestrial or silicate planetary bodies. Indeed, we have done so in comparing the 4.6. Summary of Features and Relative Timing [53] Based on available geophysical data and as depicted in Figures 4d and 16, we summarize the key features of the Chicxulub impact structure. The impact structure includes a series of ring faults reaching distances of up to 130 km from the crater center (e.g., Figures 1 and 7). These ring faults appear to root in subhorizontal detachment(s) in the west and northwest quadrants while resembling concentric listric faults in the east and northeast (e.g., Figure 4d). The inner rim lies at the inner edge of the ring faults 70–85 km from crater center except where it is absent to the northeast (e.g., Figures 4b and 7); in the deep water that was present in this quadrant, the transient cavity rim would have consisted entirely of water and thus during crater collapse, it simply washed away (Figure 4c). The terrace zone is underlain by wide, intact slump blocks offset by large faults with significant throw (e.g., Table 1 and Figure 6). These slump blocks underlie the PR and are separated from them by a dipping reflector that may be a lithologic or structural boundary, or 48 GULICK ET AL.: GEOPHYSICAL CHARACTER OF CHICXULUB Width of structural uplift Peak ring Yax-1 Y-6 Impact breccia 0 Post-impact sediments Cretaceous rocks Impact breccia Depth (km) Impact melt rocks Upper crust Upper crust 20 0 20 40 60 Mid crust Mid crust Lower crust 80 100 Upper crust 120 Mid crust 140 160 180 Distance (km) Figure 16. Geophysically constrained cross section of upper 20 km of the Chicxulub impact structure. See Figure 4b for a cross section deeper into the impact structure. Drill sites Yax-1 and Y-6 placed in their representative radial positions. Original figure from [Vermeesch and Morgan, 2008], but modified here for the upper ~5 km based on this review. thicknesses of sediments to finish the infilling of the annular trough and central basin (e.g., Figures 6, 8b, 9, and 16); finegrained airfall likely continued for years. Further into the early Paleogene, a gradual return to normal sedimentary processes ensued within the annular trough and central basin with the earliest stages likely including significant hydrothermal processes as suggested by elevated metal concentrations in the lower Paleogene carbonates [Ames et al., 2004]. Sedimentary processes within the impact basin continued to be affected by the presence of the impact-related topography for tens of millions of years [Whalen et al., 2013]. may even mark the conduit of a post-impact hydrothermal system (e.g., Figure 8). The PR exhibits variable relief and bounds the coherent melt sheet within the central basin, although some impact melt is present within the annular trough outside of the PR (e.g., Figures 6, 8b, 9, 12, and 16). Proximal ejecta and impact breccias emplaced rapidly by a number of dynamic processes lie within the annular trough above the slump blocks of the terrace zone and within the central basin above the melt sheet (e.g., Figures 6, 8b, 9, and 16). Beneath the melt sheet by < 3 km is the top of the central uplift which has been permanently displaced by >10 km vertically and an upwarped Moho which is displaced by 1–2 km (e.g., Figure 4d and 14). The ring faults are in places observed to reach through the entire crust, and there is some potential faulting within the lower crust related to the central uplift (e.g., Figures 4d and 13). [54] In terms of timing, superposition, and based on models of impact process [e.g., Melosh, 1989; Pierazzo et al., 1997; Pierazzo and Melosh, 1999; Collins et al., 2002; Collins et al., 2008], we suggest the following sequence of events formed the Chicxulub impact structure. A bolide, likely a carbonaceous chrondritic meteorite [Kyte, 1998; Trinquier et al., 2006], struck a carbonate platform of variable water depth in the Yucatan 65.5 Ma forming a 100 km wide (50 km radius) transient cavity within seconds (e.g., Figure 4b). Within minutes, the acoustically fluidized rebounding crust rose above the surface, the transient crater rim deformed along discrete faults at several kilometer spacing with 100 s of meter of throw and collapsed into large slump blocks moving the inner rim of the final crater outward by kilometers (Figure 4c), and ring faults formed farther outward for tens of kilometers up to a maximum observed distance of 130 km. Also, within minutes, the rebounding crustal material collapsed outward, but asymmetrically due to target heterogeneities, and buried the inner slump blocks (e.g., Figures 4c and 6). As the central portion of the rebounded crust submerged to a final depth of ~5 km, an up to 3 km thick melt sheet formed and lapped up onto the PR with some material reaching the annular trough (Figure 16). Within tens of minutes, proximal ejecta, slope collapse, and ground surge started infilling the annular trough and to a lesser extent the central basin. Within hours to days, tsunami waves brought in large 4.7. Next Stage of Research Into the Chicxulub Impact Structure [55] The details of the impact processes that occurred at Chicxulub now can be expressed as a working hypothesis. In order to test key aspects of this hypothesis direct observation and measurements on the impact materials are required, especially within and adjacent to the PR and central basin. Two proposed drill sites (Figure 2) that would represent a logical next step include Chicx-3A to sample the PR lithologies (Figure 9) and Chicx-4A (Figure 12) to sample an expanded section of the post-impact Paleogene sediments and the possible impact melt and/or dipping reflector at the outer edge of the PR. A clear future target should be to sample the coherent impact melt sheet and overlying impact-related sediments. Physical properties, geochemistry, lithology, and structural geology of these samples combined with downhole logging will then provide data to update the existing models of large impact processes to further our understanding of these planetary events. GLOSSARY Annular trough: Depressed portion of the crater floor lying between the crater rim and the peak ring. Central uplift: Zone of structural uplift near the crater center that is formed during collapse of transient cavity by the inward and upward movement of material from below the cavity floor. Crater floor: Surface of crater lying within the crater rim after completion of transient cavity collapse and impactite deposition. 49 GULICK ET AL.: GEOPHYSICAL CHARACTER OF CHICXULUB Baker, D. M. H., J. W. Head, C. I. Fassett, S. J. Kadish, D. E. Smith, M. T. Zuber, and G. A. Neumann (2011), The transition from complex crater to peak-ring basin on the Moon: New observations from the Lunar Orbiter Laser Altimeter (LOLA) instrument, Icarus, 214, 377–393, doi:10.1016/j.icarus.2011.05.030. Barton, P., and N. Barker (2003), Velocity imaging by tau–p transformation of refracted seismic traveltimes, Geophys. Prospect., 51, 195–203, doi:10.1046/j.1365-2478.2003.00365.x. Barton, P. J., R. A. F. Grieve, J. V. Morgan, A. T. Surendra, P. M. Vermeesch, G. L. Christeson, S. P. S. Gulick, and M. R. Warner (2010), Seismic images of Chicxulub impact melt sheet and comparison with the Sudbury structure, in Large Meteorite Impacts and Planetary Evolution IV, Geol. Soc. Amer. Spec. Pap. 465, edited by R. L. Gibson and W. U. Reimold, pp. 103–113, doi:10.1130/2010.2465(07). Bell, C., J. V. Morgan, G. J. Hampson, and B. Trudgill (2004), Stratigraphic and sedimentological observations from seismic data across the Chicxulub impact basin, Meteorit. Planet. Sci., 39, 1089–1098. Bohor, B. F., W. J. Betterton, and T. E. Krogh (1993), Impact-shocked zircons: Discovery of shock-induced textures reflecting increasing degrees of shock metamorphism, Earth Planet. Sci. Lett., 119, 419–424. Brittan, J., J. Morgan, M. Warner, and L. Marin (1999), Near-surface seismic expression of the Chicxulub impact crater, in Large Meteorite Impacts and Planetary Evolution II, vol. Geol. Soc. Am. Spec. Pap. 339, edited by B. O. Dressler and V. L. Sharpton, pp. 269–279, Geological Society of America, Boulder, Colorado. Camargo-Zanoguera, A., and G. Suarez-Reynoso (1994), Evidencia sismica del crater impacto de Chicxulub [Seismic evidence of the Chicxulub impact crater], Boletín de la Asociación Mexicana de Geofisicos de Exploracion, 34, 1–28. Christeson, G. L., R. T. Buffler, and Y. Nakamura (1999), Upper crustal structure of the Chicxulub impact crater from wide-angle ocean bottom seismograph data, in Large Meteorite Impacts and Planetary Evolution II, Geol. Soc. Am. Spec. Pap. 339, edited by B. O. Dressler and V. L. Sharpton, pp. 291–298, Geological Society of America, Boulder, Colorado. Christeson, G. L., Y. Nakamura, R. T. Buffler, J. Morgan, and M. Warner (2001), Deep crustal structure of the Chicxulub impact crater, J. Geophys. Res., 106, 21751–21769. Christeson, G. L., G. S. Collins, J. V. Morgan, S. P. S. Gulick, P. J. Barton, and M. R. Warner (2009), Mantle deformation beneath the Chicxulub impact crater, Earth Planet. Sci. Lett., 284, 249–257, doi:10.1016/j.epsl.2009.04.033. Cintala, M. J., and R. A. F. Grieve (1998), Scaling impact melting and crater dimensions: Implications for the lunar cratering record, Meteorit. Planet. Sci., 33, 889–912, doi:10.1111/j.1945-5100.1998.tb01695.x. Claeys, P., W. Kiessling, and W. Alvarez (2002), Distribution of Chicxulub ejecta at the Cretaceous-Tertiary boundary, in Catastrophic Events and Mass Extinctions: Impacts and Beyond, vol. Geol. Soc. Am., Spec. Pap. 356, edited by C. Koeberl and K. G. MacLeod, pp. 55–68. Collins, G. S., H. J. Melosh, J. V. Morgan, and M. R. Warner (2002), Hydrocode simulations of Chicxulub crater collapse and peak-ring formation, Icarus, 157, 24–33. Collins, G. S., J. V. Morgan, P. J. Barton, G. L. Christeson, S. P. S. Gulick, J. Urrutia-Fucugauchi, M. R. Warner, and K. Wünnemann (2008), Dynamic modeling suggests terrace zone asymmetry in the Chicxulub crater is caused by target heterogeneity, Earth Planet. Sci. Lett., 270, 221–230. Connors, M., A. R. Hildebrand, M. Pilkington, C. Ortiz-Aleman, R. E. Chavez, J. Urrutia-Fucugauchi, E. Graniel-Castro, A. CamaraZi, J. Vasquez, and J. F. Halpenny (1996), Yucatán karst features and the size of Chicxulub crater, Geophys. J. Int., 127, F11–F14. Dressler, B. O., V. L. Sharpton, C. S. Schwandt, and D. Ames (2004), Impactites of the Yaxcopoil-1 drilling site, Chicxulub impact structure: Petrography, geochemistry, and depositional environment, Meteorit. Planet. Sci., 39, 857–878, doi:10.1111/ j.1945-5100.2004.tb00935.x. Creataceous-Paleogene (K-Pg) event deposit: The sequence of deposits formed during the impact that within the impact structure include the impact melt sheet, peak ring, and impact-related sedimentary units emplaced by a range of processes. Impact melt sheet: Coherent mass of shock-melted target material produced by a hypervelocity impact. Inner rim: Edge of the target rocks that stands topographically above surrounding terrain and marks the inner limit of ring faults leading to the crater floor in Chicxulub. Multi-ring impact basin: Largest class of impact structures which exhibit multiple concentric topographic rings. Peak ring: Concentric ring of irregular topographic peaks that rise above the crater floor. Ring faults: Extensional faults that form concentric rings surrounding the crater center. Slump blocks: Down-dropped target material that forms the terrace zone. Terrace zone: Region inward of crater rim formed by collapse of uplifted crater rim. Transient cavity: Conceptual cavity produced by excavation and displacement of target rocks as a direct result of the cratering flow field in an impact event before its gravitational collapse that leads to formation of the final crater. [56] ACKNOWLEDGMENTS. We thank Martin Connors, Athabasca University, for providing his cenote and sinkhole files, and Mark Pilkington, Geological Survey of Canada, for providing his original graphics file shown in Figure 5B. We recognize Matthew MacDonald and Keren Mendoza for earlier seismic interpretation efforts while graduate students at the University of Texas and Universidad Nacional Autónoma de México, respectively. 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