Mapping Chicxulub Crater Structure with Overlapping Gravity and Seismic Surveys

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MAPPING CHICXULUB CRATER STRUCTURE WITH OVERLAPPING GRAVITY AND SEISMIC
SURVEYS. A.R. Hildebrand
1
, M. Pilkington
1
, J. Halpenny
2
, R. Cooper
2
, M. Connors
3
, C. Ortiz-Aleman
4
, R.E.
Chavez
4
, J. Urrutia-Fucugauchi
4
, E. Graniel-Castro
5
, A. Camara-Zi
5
, and R.T. Buffler
6
.
1
Geological Survey of
Canada, Ottawa, Ontario, Canada;
2
Geomatics Canada, Ottawa, Ontario, Canada;
3
Athabasca University,
Athabasca, Alberta, Canada;
4
Instituto de Geofisica, UNAM, Ciudad Universitaria, México, D.F., México;
5
Facultad de Ingeniería, Universidad Autónoma de Yucatán, Mérida, Yucatán, México;
6
Institute for Geophysics,
University of Texas, Austin, Texas
The Chicxulub crater will probably come to
exemplify structure of large complex craters due to
its relatively good preservation and its increasing
exploration (1, 2, 3, 4, 5, 6, 7, 8, 9, 10). Mapping
the crater in plan is possible through tying structure
as revealed by seismic studies to the associated
gravity features.
Since 1994 >1,000 gravity stations have
been collected over the crater half covered by land to
supplement the regional survey data of Petróleos
Méxicanos. In 1996 marine surveys were conducted
(~7,000 stations acquired) to extend gravity coverage
north from the coastline to the nearest offshore
seismic line (5). This line was resurveyed in 1996 to
provide a whole crustal view for this chord across the
crater (9) with velocity information acquired to
correct seismic reflections to true depths (e.g. 11).
Juxtaposing these data sets in Figure 1 establishes
ties between gravity and structural features (that are
consistent with known or plausible density contrasts)
for the peripheral slump fault (the outer edge of the
zone of slumping and the approximate position of
the eroded crater rim), the inner edge of the
seismically resolved down-slumped blocks
(approximate margin of the collapsed disruption
cavity), and the edge of the Tertiary basin
Figure 1: Perspective of horizontal gradient of
gravity anomalies over Chicxulub crater juxtaposed
with crater structure as revealed by nearest offshore
seismic reflection profile. View is looking
southwards; coastline is indicated by dark line.
Vertical exaggeration is ~10 X.
(representing crater-fill stratigraphy and position of
the predominant cenote ring).
Figure 2 presents the mapping of these
structures from the seismic line southwards across
the land gravity coverage. The relatively subtle
gravity features associated with structure within the
slump zone are not generally detected in the offshore
data reflecting the relative imprecision (~1.5 mGals)
in the marine survey. The gravity feature associated
with the Tertiary basin margin (at ~15 mGals one of
the two largest associated with the crater) separates
increasingly from the peripheral slump fault on the
west side. This is interpreted to reflect preferential
infilling of the crater by prompt backwash on the
side closest to the shelf margin/deep ocean. The
peak ring has no apparent gravity expression. Note
that the resolved slumped blocks set only an upper
limit on the size of the collapsed disruption cavity;
gravity suggests that the blocks might extend slightly
farther inwards. The tangential Pemex line 1208
indicates the presence of slumped blocks along its
entire length, in contrast to the last resolved block on
Figure 2: Mapping of gradient features from
offshore seismic lines over southern two thirds of
crater. Crater structure is also outlined in part by
location of cenote rings.
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MAPPING CHICXULUB CRATER STRUCTURE: Hildebrand, A.R. et al.
line 1143 suggesting that tangential lines are better
able to resolve weakly coherent strata. Numerous
survey track artifacts occur in the marine survey data
and previously acquired airborne data. An ~180 km-
diameter buried impact structure is confirmed with
an 80 to 90 km diameter collapsed disruption cavity.
Figure 3 presents the updated gravity map
over the crater. We have been unable to detect any
extension of a gravity low to ~140 km radius as
advocated by (4), and do not see concentric structure
extending past ~90 km radius, although a weak
positive anomaly may be associated with the crater’s
rim uplift from 90 to 100 km in some places.
Regional anomalies (interpreted as due to variation
in crystalline basement), the imprecision and sparse
coverage of marine surveys, and the extension of the
Tertiary basin to the NE result in poor expression of
the northern third of the impact structure. The
concentrated survey along the coast better mapped
Figure 3: Gravity anomaly map over Chicxulub
crater (Bouguer anomaly on land, free air anomaly
offshore). Line of bilateral symmetry for internal
structures and impact direction indicated by dashed
line and arrow, respectively.
the large-amplitude anomalies of the central high
(~20 mGals) and surrounding low (5-10 mGals).
The former is interpreted as representing the central
structural uplift which is revealed to have
symmetrical “twin peaks”, the Progreso and
Chicxulub Maximos. The latter is interpreted as the
expression of the filling of the collapsed disruption
cavity. The necessary mass deficiency might be
contained in a silicate melt sheet with a negative
density contrast of 0.1-0.2 gcm
-3
(relative to the
surrounding carbonate-anhydrite breccias) of 1.3 to
2.5 km thickness.
The central uplift’s centre is revised to
21.24
E
N, 89.58
E
W vs. the revised centre of the
crater margin of 21.29
E
N, 89.52
E
W. The
magnitude of this offset in the crater’s central
structures indicates an impact direction towards
~045
E
with an elevation angle of ~60
E
(Figure 3).
This impact direction is supported by the alignment
of the “twin peaks”, asymmetry in the collapsed
cavity wall and peak ring, thrusting in the zone of
slumping, and possible asymmetry in the crater’s rim
uplift. This impact trajectory is also consistent with
the stronger development of compressional shearing
on the NE side of the crater. The whole crustal
seismic study of (9) revealed concentric deep seated
reflectors surrounding the crater to the north, with
apparent whole crustal displacement developed to
the NE. This shearing is interpreted as the result of
compression generated during the transient rim
uplift; such compression was apparently greatest to
the NE and weakest to the west (9). In the NE
slumping of a broad block has occurred along this
shear which may represent incipient development of
multiring basin morphology, although this would be
somewhat puzzling in the context of other large
impact structures as preserved on the other terrestrial
planets and moons.
Acknowledgements: We are grateful to Petróleos
Méxicanos, Instituto Méxicano del Petróleo,
National Imaging and Mapping Agency and G.
Kinsland for provision of gravity data, and to M.
Arreguin, M. Villasuso, Y. Nakamura and G.
Christeson for assistance with gravity surveys. A.
Camargo, J. Morgan, M. Warner, G. Christeson and
D. Snyder discussed and supplied seismic data. The
Panamerican Institute of Geography and History and
the National Geographic Society (US) supported
acquisition of gravity data; the British Institutions’
Reflection Profiling Syndicate (UK) and the National
Science Foundation (US) supported seismic surveys.
References: (1) Penfield, G.T. & Camargo Z., A.,
1981, Abstr., 51st Ann. Inter. Mtg, SEG. 37; (2)
Hildebrand et al., 1991, Geology, 19: 867-871; (3)
Pope et al., 1993, Earth, Moon and Planets, 63:93-
104; (4) Sharpton et al., 1993, Science, 261:1564-
1567; (5) Camargo Zanoguera, A. & Suárez
Reynoso, G., 1994, Bol. Asoc. Mex. Géof. Explor.
XXXIV:1-28; (6) Pilkington et al., 1994, JGR,
99:13,147-13,162; (7)Hildebrand et al., 1995,
Nature, 376:415-417; (8)Urrutia-Fucugauchi et al.,
1996, GRL, 23: 1565-1568; (9) Morgan et al., 1997,
Nature, 472-476; (10) Campos-Enriquez et al., 1997,
The Leading Edge, Dec., 1774-1777; (11) Christeson
et al., 1997, Eos, 78:F399.
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