Diploma thesis:

The structure of the Chicxulub impact crater at Yucatan/Mexiko - a 3D gravity modelling

Abstract
This study features the Chicxulub impact structure at Yucatan/Mexico. The impact has happened in a shallow water environment 65 Ma ago, i.e. at the K/T-boundary, and caused a structure of at least 180 km diameter. This structure should be well preserved, because of the tertiary sedimentary cover that buried the crater afterwards. Thus it is a good location to study a large terrestrial impact structure. The task is to explore the type of the structure (multiring-basin or peak-ring-crater), its size and morphology. Answering the questions above will help to understand the global effects of the impact. The geological impact structure was modeled by the use of potential field data, seismic profiles, borehole data and 2D stratigraphic models. Reconstructing the palaeotopography visualizes a crater of ~140 km in diameter smoothed by erosion and a deep basin in the north-west due to high energy oceanic wave action after the impact. Comparison of palaeotopography and RTP-magnetics shows a correlation of the positions of the central palaeotopograhic high and the magnetic anomalies, probably due to inherent qualities of the melt pool. The 3D-gravity modeling shows a crater of ~140 km in diameter, as well. Combining the results of gravity and seismic reflection this shows to be part of a larger structure 180 km in diameter, a hint to its origin as a multiring-basin.

Die Struktur des Chicxulub-Impaktkraters in Yucatan/Mexiko - Eine 3D-Schweremodellierung

Zusammenfassung
Thema dieser Arbeit ist die Untersuchung der Impaktstruktur von Chicxulub in Yucatan/Mexiko. Der Impakt fand vor 65 Millionen Jahren an der Kreide-Tertiär-Grenze in einem Flachwassebereich statt und schuf eine Struktur eines Durchmessers von mindestens 180 km. Diese ist gut erhalten, da sie durch tertiäreSedimente bedeckt und erhalten wurde und gibt somit die Möglichkeit einen großen terrestrischen Impaktkrater zu studieren. Die Fragestellungen, die auftreten, sind die Frage nach seiner genauen Größe, nach seiner inneren Struktur (Multiring-Becken oder Peak-Ring-Krater) und seiner Morphologie. Die Beantwortung dieser Fragen kann wichtige Aufschlüsse für die Einschätzung seiner globalen Folgen geben. Über die Ausnutzung von seismischen Profile, Bohrlochdaten, Potentialfeldern und Modellvorstellungen, wurde versucht die Impaktstruktur zu modellieren. Die Rekonstruktion der Paläotopographie des Kraters kommt zu dem Ergebnis, daß der eigentliche Krater einen Durchmesser von ~140 km besitzt und daß sekundäre Effekte im Anschluß an den Impakt zu einer Erosion der topgraphischen Hochs und über einen längeren Zeitraum zur Bildung eines tiefen Beckens, das in den Krater einschneidet , führten. Der Vergleich von Paläotopographie und polreduzierter Magnetik zeigt eine Übereinstimmung in der Lage der zentralen Erhebung und der magnetischen Anomalien, die die Quelle dieser im Schmelzkörper vermuten läßt, der der Erhebung unterliegt. Die 3D-Schweremodellierung deutet ebenfalls auf einen Krater von ~140 km Durchmesser hin, was in Verbindung mit der in der Seismik erkennbaren Versetzung in einem Radius von 90 km, einen Hinweis liefert, daß es sich bei der Chicxulub-Struktur um ein Multiring-Becken liefert.

Presentations:

Ebbing, J., Götze, H.-J. and Jordan, A. (2000). Results of 3D density modelling along the TRANSALP-Traverse. Poster presentation at 2nd International TRANSALP Colloquium, Munich.

Abstract
A 3D density model of the Eastern and Southern Alps is presented, which was compiled by the use of the velocity model Eschen-38", modified by the MunichTransalp Working Group, and including all available seismic and geological information. It is shown that the simultaneous modelling of the Bouguer gravity field and the geoidal undulation can minimize the ambiguity of the potential field interpretation. For 3D modelling purposes the software IGMAS was used.
The presented model fits the measured Bouguer gravity up to its resolution.  With the exceptions of misfits in outermost regions there are no signs of misfits in the long to medium wavelength range of the observed Bouguer anomaly. Crustal densities vary between 2700-3100 kg/m³ and the mantle was modelled by densities of around 3350 kg/m³. The geoidal undulations were topographically corrected to compare both potential fields to the same density model. The comparison of the modelled and calculated geoidal undulations shows a north-south trending linear misfit of long-wavelengths. At this stage of the project one
may speculate that this misfit is connected with subcrustal or sublithospheric density inhomogeneities, not represented at greater depth of our density model.
The main advantage of this model in opposite to the velocity model is the 3D  geometry of the shallow formations building up the Tauern window, which are causing especially the east-westerly changes in the Boguer anomaly. Our results from density modelling support the idea of the crustal structure of the Eastern Alps of the model "Eschen-38". Further task of this project will be to evolve a density model due to the more dynamic idea of crustal doubling. Comparison of these models will hopefully lead to information on rigidity and isostatic state of the lithosphere in the Eastern Alps.

Ebbing, J. and Götze, H.-J. (2001). 3D Modelling of Crustal Structures in the eastern Alps Under Constraining Conditions. Geophysical Research Abstracts, 3: 1409.

Poster presentation at European Geophysical Society XXVI General Assembly, Nice (France).

Abstract
The new results of the seismic profile TRANSALP initiated a new density modelling of the Eastern Alps. The modelling software IGMAS applied in this study makes use of an interoperable 3D Geoinformations System (IOGIS) and its functionality. It enables us to combine many available constraining data sets directly with the 3D model. These data sets come, in addition to the TRANSALP results, from previous published seismic profiles, tectonics, tomographic studies and geological models.
The integration of different scaled information (e.g. tomography of the upper mantle and structural geology models of the surficial structures) allows us to model the whole lithosphere in the Eastern Alps. The actual density model was optimised for the Bouguer anomaly and topographically reduced geoidal undulations.
The model shows a division into Northern Pre-Alps, central Eastern Alps and Southern Alps. While the northern and southern crustal parts belong to the European and Adriatic crusts, respectively, the central Eastern Alpine area is characterized by a melange of the Adriatic and European crusts.
The potential fields are mainly caused by the density contrast at the crust-mantle boundary (~350 kg/m³). Second order influences to the two fields are of different origin. The upper 10 km of the crust give an amount of up to 30x10-5 m/s² to the modelled Bouguer anomaly, clearly connected to the superficial visible tectonics. In the case of the topographically reduced geoidal undulations the subcrustal density domains are the main additional source.
This modelling of the lithospheric features allows us to calculate the distribution of loads in the study area. These indicate especially high amounts in the area beneath the Tauern window, probably connected to the high-density bodies of the European-Adriatic crustal melange and the tensions they introduce. Knowledge of the load distribution is the first step in calculating the effective elastic thickness of the lithosphere in the Eastern Alps.

Ebbing, J., Götze, H.-J. und Kuder, J. (2001). 3D Inversion von Schwerefeldern mit Hilfe der Euler-Dekonvolution. Posterpräsentation bei der 61. Jahrestagung der DGG, Frankfurt.

Abstract
Eine Möglichkeit direkt aus vorhandenen Potentialfeldern Aussagen über die Lage und Tiefe der Quellen abzuleiten, bietet die Euler-Dekonvolution. Sie markiert die Oberflächen der Störkörper durch Angabe der Quelltiefen des verursachten Potentialfeldes. Dies kann insbesondere in Gebieten in denen wenig Informationen über die Struktur des Untergrundes vorliegen (z. B. Anden, Alpen) eine nützliche Methode sein.
Hier soll die Euler-Dekonvolution an zwei realistische Modellen getestet werden, um die Leistungsfähigkeit dieser Methode zu überprüfen. Als Testgebiete dienen das nordostdeutsche Becken und die Ostalpen, zwei sehr unterschiedliche tektonische Situationen, für die durch Ausnutzung sämtlicher verfügbarer Randbedingungen je ein 3D Modelle angefertigt wurde. Die Euler-Dekonvolution wird nun auf die Modellschwerefelder dieser Gebiete angewandt, und die Lösungen hiervon werden mit den Modellkörpern verglichen. Erste Ergebnisse zeigen, dass mit dieser Methode insbesondere oberflächennahe Strukturen gut aufgelöst werden können.

Ebbing, J., Braitenberg, C., Götze, H.-J. and Meurers, B. (2001). The density structure of the Eastern Alpine crust. Poster presentation at 5th Workshop of Alpine Geological Studies, Obergurgl (Austria).

Abstract
Recent results of the seismic profile TRANSALP initiated new investigations of the density structure in the Eastern Alps. This new experiment and results of previous western Alpine profiles lead to new ideas, which are tested by the use of inverse and forward modelling of potential fields.
The modelling indicates that the Bouguer gravity field in the Eastern Alps is mainly caused by two sources: the density contrast at the crust-mantle boundary (350 kg/m³), and the density inhomogeneities in the upper 10 km of the crust, which contributes to the overall gravity field by amounts of approximately 30 *10-5 m/s² .These uppermost 10 km of the model are well constrained by both observations from geology and seismic and clearly connected with the obvious near-surface tectonic regime.
This means that near surface geologic formations can be rather easily identified in the short wavelengths of the Bouguer anomaly, e.g. the uplift of the Tauern window. The small scaled structures cause gravity anomalies that superpose and interfere with the regional gravity field caused by the Moho interface. Therefore, it is impossible to separate the small and long wavelengths of the gravity field in such complex environment by a simple low-pass filtering. Forward modelling under constraining conditions seems to be the only possibility to eliminate gravity effects of near-surface structures with the consequence that a regional (deeper) field can be calculated on the base of constrained information. This kind of regional field eases the construction of deeper located density structures.
Aside of the "gravity stripping method" two other methods have been used to investigate lower crust and upper mantle density distributions, the first was an interactive 3D forward model matching. In this, a starting model was constructed on base of the seismic results and findings by the TRANSALP profile, which was stepwise interactively modified, and the second was gravity inversion.
The inversion procedure provides detailed insight into the crust mantle interface that is independent from the pre-existing seismic velocity model. It is of value that uncertainties, if present in the velocity model, are not propagated into the density model. Essentially, the two methods agree in the resulting Moho.
Important features of the Moho are the crustal roots along the central part of the Alpine arch, that achieves a depth of 55 km. In the southwest a shallowing of the Moho is found that coincides with the gravity high of the Vicenza area. Moho depth here is significantly less than in the seismic results. The deepening of the Moho towards the central part of the Alps is relatively steeper in the Adriatic domain than in the Alpine domain, indicating an asymmetry with respect to the Alpine Arch. To the North of the Alps the two models agree to a gradual shallowing of the Moho depth to 30 km.

Articles:

Ebbing, J., and Götze, H.-J. (2001). Preliminary 3D density modeling along the TRANSALP-profile. Österreichische Beiträge zu Meteorologie und Geophysik, Heft 26, 125-136.

Abstract
The density structure of the Eastern and Southern Alps is investigated by 3D model matching the Bouguer gravity field and the geoidal undulation. Geoidal undulations were topographically reduced to compare the 3D density model with the Bouguer anomaly and the reduced geoid. This model was compiled by the use of the available seismic and geological information and shows that the main influence of the potential fields is given in the density contrast at the crust-mantle boundary and the uplift of the formations of the Tauern window. This uplift and the connection with the occurrence of Helvetic and Penninic nappes is mainly east-west orientated. This shows the advantages of a 3D model.