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A model of the Chicxulub impact structure (Yucatan, Mexico) based on its gravity and magnetic signatures
PHYSICS OFTltE EARTH AND PLANETARY INTERIORS ELSEVIER
Physics of the Earth and Planetary Interiors 92 (1995) 271-278
A model of the Chicxulub impact structure (Yucatan, Mexico) based on its gravity and magnetic signatures J.M. Espindola,
M. Mena, M. de La Fuente,
J.O. Campos-Enriquez
*
Instituto de Geo~ica, UNAM, Deleg. Coyoacan, 04510 M~xico D.F., Mexico
Received 20 November 1993; revision accepted 3 March 1995
Abstract
The separating power of the first and second vertical derivatives have enabled us to infer from the Bouguer gravity anomaly the major morphological features of the Chicxulub impact structure. We infer a structural high of a composite nature (a twin peak?). We corroborated the existence of a first concentric ring, which is continuous to the west and SW of the structural high. The existence of more rings is not supported by our analysis. We were also able to constrain the diameter of the structure to about 200 km (this is also supported by the gravity and magnetic modelling). We elaborated a model for the subsurface structure that is robust in the sense that it accounts for the gravity and magnetic anomalies. Our study suggests that the Chicxulub impact structure corresponds to a crater with a central structural high and surrounded by a ring. According to crater formation theories, it would have been formed in a moderately thick lithosphere that only permits the formation of one or few rings.
I. Introduction
The buried geological structure of Chicxulub (Northern Yucatan, Mexico), as inferred from its gravity and magnetic signatures, was originally explained as an impact crater (Penfield and Camargo, 1981). Evidence from recent geochemical studies strongly supports this interpretation (e.g. Hildebrand et al., 1991; Sharpton et al., 1992). Hildebrand et al. (1991) presented a Bouguer anomaly map covering the impact area. They qualitatively compared it with the impact structure of Manicouagan (Canada), but did not present a model of its subsurface structure. Recently, Sharpton et al. (1993) proposed a concep* Corresponding author.
tual model for the buried structure based on gravity data. According to this model, the Chicxulub impact structure corresponds to a 300 km diameter multi-ring (four rings) basin. Because gravity and magnetic data convey masked information related to the impact structure's major morphological features, it is important to analyse further the gravity and magnetic data. As digital processing, including first and second vertical derivatives, can enhance particular features of the gravity and magnetic data associated with the different sources of composite anomalies, such processing can be used to study in more detail the nature of this impact structure. The purpose of this p a p e r is, using the results of such an analysis, to constrain further the nature of this impact structure (i.e. diameter, the existence of
0031-9201/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0031-9201(95)03022-0
272
J.M. Espindola et al. / Physics of the Earth and Planetary Interiors 92 (1995) 271-278
(Continental Margins Study Group (CONMAR), 1989). A circular pattern is conspicuous on the Bouguer gravity anomaly (Fig. 2). Superimposed on a regional gravity trend there is a residual anomaly exhibiting roughly radial symmetry, a maximum negative value of about - 3 0 reGal, and a central peak of about 18 reGal. This central anomaly is also conspicuous on the magnetic anomaly map (with a maximum amplitude of approximately 250 nT) (Fig. 3).
an asymmetrical ring), to present a subsurface model that is consistent with both gravity and magnetic signatures, and to discuss the significance of its major features in the broader context of other terrestrial impact craters (Pilkington and Grieve, 1992).
2. G r a v i t y
and magnetic
data
The gravity data for this study were used by de La Fuente et al. (1992) to construct the Bouguer, free air, and isostatic residual anomaly maps for Mexico. These data were also used in the construction of the gravity anomaly map for North America (Tanner and the D N A G Committee, 1988). Fig. 1 shows the location of the observation points for Chicxulub. The magnetic data were obtained from the total field magnetic anomaly map for Yucatan and adjacent areas
3. D i g i t a l
processing:
subsurface
structure
constraints
on the
Vertical derivatives (Evjen, 1936; Elkins, 1951) are standard, powerful tools used to clarify the horizontal limits and extent of subsurface bodies and structures (i.e. rings, structural highs, and
-89.50
-90.50 22.00
derived
-88.50 22.00
21.00
i:i:;....:...;:).:.:..:j:::.. Z • % Q $ • ": "" ..:.. ~ ..
20.00 -90.50
t St ==
,
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i
.~.z:': ...... . ....... -..-..• . ... : . "'..: - : : .~ -..:..'.~?...z.~,',:.:"
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:, :'. .... ,"
•
•
....
~"''
-89.50
"-"
•
" " "
....
:....: I "
"":';
20.00
-88.50
Fig. 1. Location and distribution of gravity data used. Dots represent gravity stations on land, and lines represent the marine gravity traverses offshore of Yucatan where gravity observations were made. (Note the coast-parallel strip devoid of gravity and magnetic measurements.)
J.M. Espindola et al. / Physics of the Earth and Planetary Interiors 92 (1995) 271-278
rims in our case) by separating out the unwanted gravity effects associated with neighbouring bodies from the composite anomaly. Because of this separating power, we obtained the first and second vertical derivatives (Hildenbrand, 1983) of the gravity field (Fig. 2), so as to study details of the Chicxukub structure. Figs. 4 and 5 present the first and second vertical derivatives, respectively. The existence of a first, innermost ring is very clear to the west, SW, and south of the structure centre. To the west, the ring is continuous. The first and second vertical derivatives along the gravity profile of Fig. 2 (Fig. 6; upper and middle panels) enable us to observe clearly the asymmetric nature of this ring. Visible outside this first ring are some other minor isolated features, on the second vertical derivative map for example, but they can hardly be interpreted as a second ring. This point is further supported by the gravity modelling described below.
-90.50 22. O0
-89.50
21.00
Thus first and second vertical derivatives do not confirm the existence of several rings as proposed by Sharpton et al. (1993) on the basis of a qualitative analysis of several gravity profiles. Further, we infer a composite nature (twin peaks) for the source associated with the central gravity high and are able to constrain the diameter of the structure to about 200 km. The latter points are also supported by the gravity modelling.
4. Interpretation A model of the subsurface structure of the Chicxulub impact crater was constructed from a 21-dimensional modelling (Cady, 1977) of a gravity and a magnetic profile (Figs. 2 and 3). As the anomaly is large and roughly symmetric, 2½-D modelling is permissible as a first approximation, as such modelling accounts for the end-effects owing to the limited dimensions of the source-
-88.50 22. O0
21.00
20.00 -90.50
273
-89.50
20.00 -88.50
Fig. 2. Bouguer anomaly map of the study area (after de La Fuente et al. (1992)). (Note that the gravity survey does not cover the shallow-water offshore portion.) Location of gravity profile A-A¢ is also shown.
274
J.M. Espindola et aL/Physics of the Earth and Planetao'Interiors92 (1995)271-278
bodies (Shuey and Pasquale, 1973; Rasmussen and Pedersen, 1979). Data from bore-holes drilled in and around the structure (L6pez-Ramos, 1979) were used to constrain the gravity and magnetic modelling. The lower panel of Fig. 6 presents the gravity section. As can be seen in the figure, the central gravity high is produced by a body exhibiting approximately 9 km of relief that is located in the centre of a depression of 175-180 km diameter. The gravity anomaly is due to the density contrast between the central body and the material filling the crater-like depression. A sedimentary cover of approximately 1 km, as indicated by subsurface well data, is included. The regional gradient observed in the gravity anomaly (Fig. 6, lower panel; Fig. 7, middle panel) is accounted for by a basement tilted at some 4-5 °. There is a direct correlation between the morphology of the impact structure according to our gravity model and the first and second vertical derivatives (Fig. 6). The
-90.50 22.00
model incorporates successfully the morphological elements inferred from our gravity analysis, i.e. the model has also been constrained by the first and second vertical derivatives. The same structural model accounts for the total field magnetic anomaly (Fig. 7, upper panel). Normally, owing to the large variability of the magnetic properties in the rocks involved, the magnetic anomalies of craters show no distinctive signature (Pilkington and Grieve, 1992). However, owing to the low content of magnetic minerals in the carbonate sedimentary rocks of the upper layers and to their relative flatness, the Chicxulub magnetic anomaly stands out clearly from the regional trend. The central magnetic anomaly coincides with the gravity high. This suggests the existence of magnetic material associated with the melting process, a n d / o r of denser and more magnetized rocks uplifted from deeper horizons. However, the fact that the same structural model accounts for the total field magnetic
-89.50
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~
-. . . .
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22.00
~
21.00
21.00
20.00 -90.50
~
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20.00 -88.50
Fig. 3. Total field magnetic anomaly of the study area (after C O N M A R (1989)). (Note that the magnetic survey does not cover the shallow-water offshore portion.) Location of magnetic profile A-AJ is also shown.
J.M. Espindola et al. / Physics of the Earth and Planetary Interiors 92 (1995) 271-278
of greater dimensions. For large craters, the negative anomaly may be modified by the presence of a central gravity high (Pilkington and Grieve, 1992). As we have seen, both of these features are present in the Chicxulub structure. Morphologically, complex craters present a protuding rim followed by a terrace zone, a flat floor and central peak. Multi-ring basins are characterized by at least two asymmetric scarped rings (e.g. Melosh, 1989). Our analysis enables us to infer the presence of a central peak and to confirm the existence of only one approximately well-defined ring. However, it is possible that subsequent rings may not show up in the gravity signature of this buried crater owing to erosion and the low density contrast between infill material and the fractured and brecciated rocks left after impact. Thus, other methods (e.g. seismics a n d / o r drilling) are needed to confirm the existence of more than one ring.
anomaly (Fig. 7, upper panel) confirms the presence of a central structural uplift capped with a melt sheet. The fits to the gravity and magnetic anomalies are fairly good. We obtained r.m.s. deviations of 1.75 mGal and 23.03 nT, respectively.
5. Discussion: Chicxulub structure, impact craters and multi-ring basins Here we discuss the significance of the major elements of the sursurface structure inferred for the Chicxulub impact structure. Complex craters and multi-ring basins show definite geophysical and morphological features. Terrestrial impact craters show negative gravity anomalies, with maximum values roughly proportional to the crater's diameter, for structures less than about 30 kin, and a limiting value of 30 mGal for craters
t
I
0
I0
I
275
I
I
20 50 40 SCALE (Km)
I
50
N
Fig. 4. First vertical derivativeof the Bouguer anomalymap of the continental portion of Fig. 2. (Values are in mGal km- 1,)
276
ZM. Espindola et aL /Physics of the Earth and Planetao, Interiors 92 (199.5) 271 278
that is a constant fraction of the crater's diameter (0.5) independent of the planet's gravity field. The onset of peak ring formation is a function of the planet's gravity field. For the Earth, the onset is found at diameters of about 25 km (Melosh, 1989). Therefore, for a crater with a diameter of 180 km, we would expect a ring of peaks instead of a single central peak. However, for the Chicxulub impact structure gravity only allows one to infer a central structural high composed of twin peaks and one approximately well-defined first ring that is continuous to the west and SW of the structural high. These elements indicate that the Chicxulub buried structure is more complicated than the conceptual model proposed by Sharpton et al. (1993). The deeply located structural high in their model can be easily explained by the averaged gravity profile used in their gravity modelling. In fact, this averaging corresponds to a
The relationship between structural uplift and crater diameter is given by SU = 0.06D L1, where SU is the structural uplift and D is the crater diameter (both in km) (Grieve, 1987). For a diameter of about 180 km this relationship gives an SU of about 18 km, assuming that the crater has not changed its dimensions considerably as a result of erosion. If the central body in the model represents the uplifted basement then the height of the structural uplift of the Chicxulub structure is at least 9 km. This is a lower bound because the limit of major disruption is probably deeper than the base of the body; by simple geometrical reconstruction, as shown in Fig. 6(c), we estimate the structural uplift to be about 14 km, which is of the order of the value given by the above relationship. In the largest complex craters, the central peak is replaced by a ring of peaks with a diameter
I
0
I
10
I
20
I
30
I
40
/
50
SCALE (Km)
N
Fig. 5. Second vertical derivative of the Bouguer anomaly m a p of the continental portion of Fig. 2. (Values are in mGal km 2.1
J.M. Espindola et al. / Physics of the Earth and Planetary Interiors 92 (1995) 271-278
a deeper source. In reality, this anomaly is not smooth but is instead sharp, and therefore corresponds to a shallower source (this point is supported by the fact that the same model accounts for the magnetic anomaly).
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Fig. 6. Vertical derivatives of gravity profile A-A~ of Fig. 2, and the subsurface model of the Chicxulub impact structure corresponding to the gravity and magnetic profiles of Figs. 2 and 3. Upper panel, first vertical derivative; middle panel, second vertical derivative; lower panel, structural model. Numbers in lower panel indicate density contrast and susceptibility. 1, Basement (+0.04 g cm-3; 500× 10 -5 c.g.s.); 2 and 5, Mesozoic sedimentary sequence (+ 0.02 g cm-3; 200 × 10-5 c.g.s); 3, filling material (e.g. breccias and fractured basement) (-0.1 g cm-3; 300x 10 -5 c.g.s.); 4, impact melt sheet and/or uplifted basement (0.04 g cm-3; 900×10 -5 c.g.s.). Density contrasts are relative to the density of the Tertiary sedimentary cover (2.70 g cm-3).
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s m o o t h i n g o f t h e gravity a n o m a l y . T h i s p r o c e s s has r e s u l t e d in a s m o o t h e r a n d b r o a d e r central gravity h i g h w h i c h can b e i n t e r p r e t e d in t e r m s o f
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Fig. 7. Modelled and observed gravity and magnetic effects. Lower panel, gravity anomaly; upper panel, magnetic anomaly. The subsurface model of Fig. 6 is here reproduced for comparative purposes.
278
,LM. Espindola et aL / Physics of the Earth and Planetary Interiors 92 (1995) 271-278
According to our study, the Chicxulub impact structure corresponds to a crater with a central peak and only one ring. This kind of structure is indicative of a moderately thick lithosphere underlying the impact site. This lithosphere would produce one or few lunar-type rings (Melosh, 1989).
6. Conclusions Based on the separating power of first and second vertical derivatives we are able to infer the existence of a central structural high of a composite nature (twin peak?). The magnetic modelling strongly supports this interpretation. We also confirm the existence of an asymmetric ring. This ring is a continuous feature west, SW and south of the structural high (an interesting fact in relation to the nature of this ring, i.e. a peak-ring?). The existence of more concentric rings was not corroborated by the first and second vertical derivatives. S o m e m i n o r f e a t u r e s are o b s e r v e d in the s e c o n d vertical derivative m a p , for example, outside the first ring, but they can
hardly be interpreted as a second ring (a fact further supported by our gravity and magnetic modelling). Our analysis constrains the diameter of the buried structure to about 200 kin. Based on geology and well data, we constructed a structural model that incorporates the constraints obtained from the gravity analysis. This structural model is robust in the sense that it also accounts for the magnetic anomaly. According to our study, the Chicxulub buried structure would correspond to a crater with a composite peak (twin peak?) and only one ring. However, in its details it presents more complexities than the conceptual model advanced by Sharpton et al. (1993).
Acknowledgements This is a contribution from the Geophysical Institute, National University. Critical reviews by two anonymous referees greatly improved this paper. Comments by Gustavo To!son and William Bandy were also helpful.
References Cady, J.W., 1977. Calculation of gravity and magnetic anomalies along profiles with end corrections and inverse solutions for density and magnetization. Open File Rep. US Geological Survey, Reston, VA. 110 pp. CONMAR (Continental Margins Study Group), 1989. Total field anomaly map. Yucatan and adjacent areas. Oregon State University, Corvallis, OR. de La Fuente, M., Mena, M. and Aiken, C.LV., 1992. Cartas gravimfitricas de la Rept]blica Mexicana, 1. Carta de anomali'a de Bouguer. INEGI, Mexico. Elkins, T.A., 1951. The second derivative method of gravity interpretation. Geophysics, 16: 29-50. Evjen, H.A., 1936. The place of the vertical gradient ill gravitational interpretation. Geophysics, 1: 127-136. Grieve, R.A.F., 1987. Terrestrial impact structures. Annu. Rev. Earth Planet. Sci., 15: 245-270. Hildenbrand, T.G., 1983. FFTFIL. A filtering program based on two-dimensional Fourier analysis of geophysical data. US Geol. Surv. Open File Rep. 83-237. Hildebrand, A.R., Penfield, G.T., Pilkington, D.A., Camargo, A.. Jacobsen, S.B. and Boyton, W.V., 1991. Chicxulub crater: a possible Cretaceous/Tertiaw boundary impac! crater on the Yucatan Peninsula, Mexico. Geology, 19: 867 87 I. L6pez-Ramos, E., 1979. Geok)gla de Mdxico. lnstituto tic Geologla, UNAM, Mexico. Melosh, H.J., 1989. lmpacl Cratering. A Geologic Process. Oxford University Press, New York. Penfield. G.T. and Camargo, Z.A., 1981. Definition of a major igneous zone in the central Yucatan platform with aeromagnelics and gravity. In: Technical Program, Abstracts and Biographies, Society of Exploration Geophysicists, 51st Annual Meeting, Los Angeles. Society of Exploration Geophysicists, Tulsa, OK, 37 pp. Pilkington, M. and Grieve, R.A.F., 1992. The geophysical signature of terrestrial impact craters. Rev. Geophys., 30: 161 181. Rasmussen, R. and Pedersen, L.B., 1979. End corrections in potential field modeling. Geophys. Prospect., 27:749 760. Sharpton, V.L., Dalrymple, G.B., Marin, L.E., Schuraytz, B.C. and Urrutia-Fucugauchi, J., 1992. New links between the Chicxulub impact structure and the Cretaceous/Tertiary boundary. Nature, 359: 819-82t. Sharpton, V i . , Burke, K., Camargo-Zanoguera, A., Hall, S.A., Lee, D.S., Marin, L.E., Sufirez-Reynoso. G., Quezada-Mufieton, J., Spudis, P.D.G. and UrrutiaFucugauchi, J., 1993. Chicxulub multiring impact basin. Size and other characteristics derived from gravity analysis. Science, 261: 1564-1567. Shuey, R.T. and Pasquale, A.S., 1973. End corrections in magnetic profile interpretation. Geophysics, 38:507 512. Tanner, J. and the DNAG Committee, 1988. Gravity anomaly map for North America. The Leading Edge, Vol. 7, No. 11, pp. 15-18.
Physics of the Earth and Planetary Interiors 92 (1995) 271-278
A model of the Chicxulub impact structure (Yucatan, Mexico) based on its gravity and magnetic signatures J.M. Espindola,
M. Mena, M. de La Fuente,
J.O. Campos-Enriquez
*
Instituto de Geo~ica, UNAM, Deleg. Coyoacan, 04510 M~xico D.F., Mexico
Received 20 November 1993; revision accepted 3 March 1995
Abstract
The separating power of the first and second vertical derivatives have enabled us to infer from the Bouguer gravity anomaly the major morphological features of the Chicxulub impact structure. We infer a structural high of a composite nature (a twin peak?). We corroborated the existence of a first concentric ring, which is continuous to the west and SW of the structural high. The existence of more rings is not supported by our analysis. We were also able to constrain the diameter of the structure to about 200 km (this is also supported by the gravity and magnetic modelling). We elaborated a model for the subsurface structure that is robust in the sense that it accounts for the gravity and magnetic anomalies. Our study suggests that the Chicxulub impact structure corresponds to a crater with a central structural high and surrounded by a ring. According to crater formation theories, it would have been formed in a moderately thick lithosphere that only permits the formation of one or few rings.
I. Introduction
The buried geological structure of Chicxulub (Northern Yucatan, Mexico), as inferred from its gravity and magnetic signatures, was originally explained as an impact crater (Penfield and Camargo, 1981). Evidence from recent geochemical studies strongly supports this interpretation (e.g. Hildebrand et al., 1991; Sharpton et al., 1992). Hildebrand et al. (1991) presented a Bouguer anomaly map covering the impact area. They qualitatively compared it with the impact structure of Manicouagan (Canada), but did not present a model of its subsurface structure. Recently, Sharpton et al. (1993) proposed a concep* Corresponding author.
tual model for the buried structure based on gravity data. According to this model, the Chicxulub impact structure corresponds to a 300 km diameter multi-ring (four rings) basin. Because gravity and magnetic data convey masked information related to the impact structure's major morphological features, it is important to analyse further the gravity and magnetic data. As digital processing, including first and second vertical derivatives, can enhance particular features of the gravity and magnetic data associated with the different sources of composite anomalies, such processing can be used to study in more detail the nature of this impact structure. The purpose of this p a p e r is, using the results of such an analysis, to constrain further the nature of this impact structure (i.e. diameter, the existence of
0031-9201/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0031-9201(95)03022-0
272
J.M. Espindola et al. / Physics of the Earth and Planetary Interiors 92 (1995) 271-278
(Continental Margins Study Group (CONMAR), 1989). A circular pattern is conspicuous on the Bouguer gravity anomaly (Fig. 2). Superimposed on a regional gravity trend there is a residual anomaly exhibiting roughly radial symmetry, a maximum negative value of about - 3 0 reGal, and a central peak of about 18 reGal. This central anomaly is also conspicuous on the magnetic anomaly map (with a maximum amplitude of approximately 250 nT) (Fig. 3).
an asymmetrical ring), to present a subsurface model that is consistent with both gravity and magnetic signatures, and to discuss the significance of its major features in the broader context of other terrestrial impact craters (Pilkington and Grieve, 1992).
2. G r a v i t y
and magnetic
data
The gravity data for this study were used by de La Fuente et al. (1992) to construct the Bouguer, free air, and isostatic residual anomaly maps for Mexico. These data were also used in the construction of the gravity anomaly map for North America (Tanner and the D N A G Committee, 1988). Fig. 1 shows the location of the observation points for Chicxulub. The magnetic data were obtained from the total field magnetic anomaly map for Yucatan and adjacent areas
3. D i g i t a l
processing:
subsurface
structure
constraints
on the
Vertical derivatives (Evjen, 1936; Elkins, 1951) are standard, powerful tools used to clarify the horizontal limits and extent of subsurface bodies and structures (i.e. rings, structural highs, and
-89.50
-90.50 22.00
derived
-88.50 22.00
21.00
i:i:;....:...;:).:.:..:j:::.. Z • % Q $ • ": "" ..:.. ~ ..
20.00 -90.50
t St ==
,
"
"
i
.~.z:': ...... . ....... -..-..• . ... : . "'..: - : : .~ -..:..'.~?...z.~,',:.:"
"..'.~". "":':""':', .. . .._!:.: :: :'.-:;:.-'-: .'..:...: ... - . .
:, :'. .... ,"
•
•
....
~"''
-89.50
"-"
•
" " "
....
:....: I "
"":';
20.00
-88.50
Fig. 1. Location and distribution of gravity data used. Dots represent gravity stations on land, and lines represent the marine gravity traverses offshore of Yucatan where gravity observations were made. (Note the coast-parallel strip devoid of gravity and magnetic measurements.)
J.M. Espindola et al. / Physics of the Earth and Planetary Interiors 92 (1995) 271-278
rims in our case) by separating out the unwanted gravity effects associated with neighbouring bodies from the composite anomaly. Because of this separating power, we obtained the first and second vertical derivatives (Hildenbrand, 1983) of the gravity field (Fig. 2), so as to study details of the Chicxukub structure. Figs. 4 and 5 present the first and second vertical derivatives, respectively. The existence of a first, innermost ring is very clear to the west, SW, and south of the structure centre. To the west, the ring is continuous. The first and second vertical derivatives along the gravity profile of Fig. 2 (Fig. 6; upper and middle panels) enable us to observe clearly the asymmetric nature of this ring. Visible outside this first ring are some other minor isolated features, on the second vertical derivative map for example, but they can hardly be interpreted as a second ring. This point is further supported by the gravity modelling described below.
-90.50 22. O0
-89.50
21.00
Thus first and second vertical derivatives do not confirm the existence of several rings as proposed by Sharpton et al. (1993) on the basis of a qualitative analysis of several gravity profiles. Further, we infer a composite nature (twin peaks) for the source associated with the central gravity high and are able to constrain the diameter of the structure to about 200 km. The latter points are also supported by the gravity modelling.
4. Interpretation A model of the subsurface structure of the Chicxulub impact crater was constructed from a 21-dimensional modelling (Cady, 1977) of a gravity and a magnetic profile (Figs. 2 and 3). As the anomaly is large and roughly symmetric, 2½-D modelling is permissible as a first approximation, as such modelling accounts for the end-effects owing to the limited dimensions of the source-
-88.50 22. O0
21.00
20.00 -90.50
273
-89.50
20.00 -88.50
Fig. 2. Bouguer anomaly map of the study area (after de La Fuente et al. (1992)). (Note that the gravity survey does not cover the shallow-water offshore portion.) Location of gravity profile A-A¢ is also shown.
274
J.M. Espindola et aL/Physics of the Earth and Planetao'Interiors92 (1995)271-278
bodies (Shuey and Pasquale, 1973; Rasmussen and Pedersen, 1979). Data from bore-holes drilled in and around the structure (L6pez-Ramos, 1979) were used to constrain the gravity and magnetic modelling. The lower panel of Fig. 6 presents the gravity section. As can be seen in the figure, the central gravity high is produced by a body exhibiting approximately 9 km of relief that is located in the centre of a depression of 175-180 km diameter. The gravity anomaly is due to the density contrast between the central body and the material filling the crater-like depression. A sedimentary cover of approximately 1 km, as indicated by subsurface well data, is included. The regional gradient observed in the gravity anomaly (Fig. 6, lower panel; Fig. 7, middle panel) is accounted for by a basement tilted at some 4-5 °. There is a direct correlation between the morphology of the impact structure according to our gravity model and the first and second vertical derivatives (Fig. 6). The
-90.50 22.00
model incorporates successfully the morphological elements inferred from our gravity analysis, i.e. the model has also been constrained by the first and second vertical derivatives. The same structural model accounts for the total field magnetic anomaly (Fig. 7, upper panel). Normally, owing to the large variability of the magnetic properties in the rocks involved, the magnetic anomalies of craters show no distinctive signature (Pilkington and Grieve, 1992). However, owing to the low content of magnetic minerals in the carbonate sedimentary rocks of the upper layers and to their relative flatness, the Chicxulub magnetic anomaly stands out clearly from the regional trend. The central magnetic anomaly coincides with the gravity high. This suggests the existence of magnetic material associated with the melting process, a n d / o r of denser and more magnetized rocks uplifted from deeper horizons. However, the fact that the same structural model accounts for the total field magnetic
-89.50
-88.50
~
-. . . .
-4OO~'~
22.00
~
21.00
21.00
20.00 -90.50
~
~ -89.50
20.00 -88.50
Fig. 3. Total field magnetic anomaly of the study area (after C O N M A R (1989)). (Note that the magnetic survey does not cover the shallow-water offshore portion.) Location of magnetic profile A-AJ is also shown.
J.M. Espindola et al. / Physics of the Earth and Planetary Interiors 92 (1995) 271-278
of greater dimensions. For large craters, the negative anomaly may be modified by the presence of a central gravity high (Pilkington and Grieve, 1992). As we have seen, both of these features are present in the Chicxulub structure. Morphologically, complex craters present a protuding rim followed by a terrace zone, a flat floor and central peak. Multi-ring basins are characterized by at least two asymmetric scarped rings (e.g. Melosh, 1989). Our analysis enables us to infer the presence of a central peak and to confirm the existence of only one approximately well-defined ring. However, it is possible that subsequent rings may not show up in the gravity signature of this buried crater owing to erosion and the low density contrast between infill material and the fractured and brecciated rocks left after impact. Thus, other methods (e.g. seismics a n d / o r drilling) are needed to confirm the existence of more than one ring.
anomaly (Fig. 7, upper panel) confirms the presence of a central structural uplift capped with a melt sheet. The fits to the gravity and magnetic anomalies are fairly good. We obtained r.m.s. deviations of 1.75 mGal and 23.03 nT, respectively.
5. Discussion: Chicxulub structure, impact craters and multi-ring basins Here we discuss the significance of the major elements of the sursurface structure inferred for the Chicxulub impact structure. Complex craters and multi-ring basins show definite geophysical and morphological features. Terrestrial impact craters show negative gravity anomalies, with maximum values roughly proportional to the crater's diameter, for structures less than about 30 kin, and a limiting value of 30 mGal for craters
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Fig. 4. First vertical derivativeof the Bouguer anomalymap of the continental portion of Fig. 2. (Values are in mGal km- 1,)
276
ZM. Espindola et aL /Physics of the Earth and Planetao, Interiors 92 (199.5) 271 278
that is a constant fraction of the crater's diameter (0.5) independent of the planet's gravity field. The onset of peak ring formation is a function of the planet's gravity field. For the Earth, the onset is found at diameters of about 25 km (Melosh, 1989). Therefore, for a crater with a diameter of 180 km, we would expect a ring of peaks instead of a single central peak. However, for the Chicxulub impact structure gravity only allows one to infer a central structural high composed of twin peaks and one approximately well-defined first ring that is continuous to the west and SW of the structural high. These elements indicate that the Chicxulub buried structure is more complicated than the conceptual model proposed by Sharpton et al. (1993). The deeply located structural high in their model can be easily explained by the averaged gravity profile used in their gravity modelling. In fact, this averaging corresponds to a
The relationship between structural uplift and crater diameter is given by SU = 0.06D L1, where SU is the structural uplift and D is the crater diameter (both in km) (Grieve, 1987). For a diameter of about 180 km this relationship gives an SU of about 18 km, assuming that the crater has not changed its dimensions considerably as a result of erosion. If the central body in the model represents the uplifted basement then the height of the structural uplift of the Chicxulub structure is at least 9 km. This is a lower bound because the limit of major disruption is probably deeper than the base of the body; by simple geometrical reconstruction, as shown in Fig. 6(c), we estimate the structural uplift to be about 14 km, which is of the order of the value given by the above relationship. In the largest complex craters, the central peak is replaced by a ring of peaks with a diameter
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Fig. 5. Second vertical derivative of the Bouguer anomaly m a p of the continental portion of Fig. 2. (Values are in mGal km 2.1
J.M. Espindola et al. / Physics of the Earth and Planetary Interiors 92 (1995) 271-278
a deeper source. In reality, this anomaly is not smooth but is instead sharp, and therefore corresponds to a shallower source (this point is supported by the fact that the same model accounts for the magnetic anomaly).
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s m o o t h i n g o f t h e gravity a n o m a l y . T h i s p r o c e s s has r e s u l t e d in a s m o o t h e r a n d b r o a d e r central gravity h i g h w h i c h can b e i n t e r p r e t e d in t e r m s o f
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278
,LM. Espindola et aL / Physics of the Earth and Planetary Interiors 92 (1995) 271-278
According to our study, the Chicxulub impact structure corresponds to a crater with a central peak and only one ring. This kind of structure is indicative of a moderately thick lithosphere underlying the impact site. This lithosphere would produce one or few lunar-type rings (Melosh, 1989).
6. Conclusions Based on the separating power of first and second vertical derivatives we are able to infer the existence of a central structural high of a composite nature (twin peak?). The magnetic modelling strongly supports this interpretation. We also confirm the existence of an asymmetric ring. This ring is a continuous feature west, SW and south of the structural high (an interesting fact in relation to the nature of this ring, i.e. a peak-ring?). The existence of more concentric rings was not corroborated by the first and second vertical derivatives. S o m e m i n o r f e a t u r e s are o b s e r v e d in the s e c o n d vertical derivative m a p , for example, outside the first ring, but they can
hardly be interpreted as a second ring (a fact further supported by our gravity and magnetic modelling). Our analysis constrains the diameter of the buried structure to about 200 kin. Based on geology and well data, we constructed a structural model that incorporates the constraints obtained from the gravity analysis. This structural model is robust in the sense that it also accounts for the magnetic anomaly. According to our study, the Chicxulub buried structure would correspond to a crater with a composite peak (twin peak?) and only one ring. However, in its details it presents more complexities than the conceptual model advanced by Sharpton et al. (1993).
Acknowledgements This is a contribution from the Geophysical Institute, National University. Critical reviews by two anonymous referees greatly improved this paper. Comments by Gustavo To!son and William Bandy were also helpful.
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