Gravity surveys

Gravity anomalies in impact structures may result from quite different processes. Small, simple craters in general show a roughly circular negative anomaly. A low-density breccia lens at the crater floor, post-impact young crater sediments, and fracturing of the rocks beneath and around the crater may contribute to the mass deficit.

In larger, complex craters with central uplifts and/or rings, gravity anomalies may be complex as well. Like in simple craters, rock fracturing and low-density impact melt rocks, suevites and other breccias cause negative anomalies, and post-impact crater sediments may also contrast with the pre-impact target rocks. In addition, relatively positive anomalies are measured, if rocks of higher density were uplifted in the modification stage of the cratering process to form the above-mentioned uplifts and rings. Shock lithification of porous rocks may also lead to locally increased density.

In this context, gravity measurements prove to be a valuable tool in the investigation of impact structures. They are important to detect burried impact structures (as for example the famous giant Chicxulub structure http://dsaing.uqac.uquebec.ca/~mhiggins/MIAC/chicxulub.htm), and they can trace the original size of deeply eroded craters where only relics of impactites point to an impact origin (as for example the Rochechouart impact structure; see below: Examples).

Gravity potential theory tells us that from the integration over a gravity negative anomaly the total mass deficit can be calculated regardless of the specific density distribution. As in impact structures the mass deficit is related with the kinetic energy of the projectile, impact energy considerations and estimates of the displaced masses are possible (see below: Ries impact structure). On the other hand, density modelling of gravity anomalies may show details about the internal structure of impact craters (see below: Steinheim Basin impact structure).

Examples:

Rochechouart impact structure (France)

The Rochechouart impact structure is located in the Massif Central of France. The structure is deeply eroded, and only relics of impact melt rocks, suevites and other impactites are exposed. The Bouguer residual anomaly (modified from Schmidt, T. (1984): Bearbeitung und Auswertung von Schweremessungen im Gebiet der Impakt-Struktur von Rochechouart (Massif Central, Frankreich). - Diploma thesis, University of Munich.) has an amplitude of roughly -10 mgal. From model calculations, Schmidt concludes that the diameter of the original impact structure was between about 25 and 30 km. The mass deficit corresponds to about 40,000 megatons.

From the Bouguer residual anomaly, the 2nd horizontal derivitive has been calculated (see image below).

In this field, NW - SE and NE - SW trending linear contours come to the fore. They suggest that the gravity field of the Rochechouart impact structure is influenced also by regional structural trends (see photolineation map, Lambert 1974; Bischoff & Oskierski 1987; geological map 1 : 50,000, Sheet Rochechouart, Chèvremont & Floc'h 1996).

Ries impact structure (Germany)

The 26 km-diameter 15 m.y. Ries impact structure is located in southern Germany. The crater is well preserved and one of the best investigated impact structures.

The Bouguer negative anomaly of the Ries crater is embedded in a regional field with considerable "relief", which implies several possibilities to construct a reliable residual field.

The Bouguer residual anomaly shown here (modified from Kahle, H.-G. (1969): Abschätzung der Störungsmasse im Nördlinger Ries. - Z. Geophys., 33, 317-345.) amounts to about -19 mgal from which roughly 5 mgal may be attributed to the post-impact lake sediments. In the image below, the horizontal gradient of the residual field is shown clearly documenting the complex structure of the Ries crater with a 12 km-diameter inner ring.

From the residual anomaly (related to 400 m a.s.l.), a total mass deficiency between about 70,000 and 100,000 megatons has been computed. Adding the masses of the lake sediments, the fallback suevite, and the mass of sediments ejected from above 400 m a.s.l., a total of at least 300,000 megatons were excavated, which correspond with an impact energy of roughly 1026 - 1027 erg (Pohl et al. 1977).

Steinheim Basin impact structure (Germany)

The Steinheim Basin with a prominent central uplift is thought to be a smaller companion to the Ries crater. From gravity measurements and morphometric considerations, a much larger (~ 7-8 km) than the commonly mentioned 3.7 km-diameter is very probable (Ernstson, K. (1984): A gravity-derived model for the Steinheim impact structure. - Geol. Rundschau, 73/2, 483-498.).

The central negative anomaly hardly exceeds - 2 mgal, and it is surrounded by a discontinuous ring of slightly positive anomalies (diameter 5 - 6 km). The influence of tectonic structures may have been the cause of stronger deviations from a circular symmetry. Density models for crater-crossing profiles are shown below.