Original paper

Insights on the deep roots of Mesozoic ring complexes in Namibia from aeromagnetic and gravity modelling of the Messum and Brandberg complexes

[On the methodological structure of the principle of actualism.]

Vietor, Tim; Trumbull, Robert B.; Nowaczyk, Norbert R.; Hutchins, David G.; Emmermann, Rolf

Kurzfassung

Diese Arbeit unternimmt den Versuch einer gemeinsamen Interpretation der geologischen und geophysikalischen Daten der oberkretarzischen Intrusivkomplexe von Messum-Krater und Brandberg. Beide Komplexe sind durch starke magnetische Anomalien gekennzeichnet. Für Messum vorliegende Schweredaten und refraktionsseismische Profile zeigen im Bereich des Kraters eine positive Schwereanomalie und anomal hohe P-Wellengeschwindigkeiten in der mittleren und unteren Kruste. Der Messum-Komplex besteht vorwiegend aus einer konzentrischen Abfolge von Gabbro- und Syenitintrusionen, die nur ein geringes topographisches Relief aufweisen. Dagegen erhebt sich der ...

Abstract

This study attempts a joint interpretation of geologic and geophysical data from the early Cretaceous Messum and Brandberg intrusive complexes to construct possible crustal models of their roots. Both complexes are characterized by strong positive magnetic anomalies. Additional gravity and seismic refraction data from Messum show a positive gravity anomaly and anomalously high p-wave velocities in the lower and mid crust below it. The Messum complex consists mostly of concentric sets of gabbro and syenite intrusions with little topographic relief whereas the Brandberg is a 1500 meter-high massif made up almost entirely of granite with a small precursor intrusion of monzonite. The crustal model proposed for the Messum complex envisages a near-vertical intruded zone whose width is close to that of the complex at surface. The density and seismic velocity contrast, compared with non-intruded crust on either side, is nearly constant at all levels which suggests a uniform degree of intrusion throughout. The magnetic and gravity modelling suggests that the crust beneath Messum consists of at least 30% gabbroic material down to the Moho. This contradicts the often-cited proposal that Messum is the eruptive center for some 8500 km3 of felsic volcanics in the Paraná-Etendeka province. A crustal model which satisfies the Brandberg magnetic anomaly suggests a layered intrusion with a gabbroic base. The model is consistent with the petrologic evidence that the Brandberg granite is derived ultimately from mantle-sourced basaltic magma. The geometry and layer thicknesses of the magnetic model are not well constrained but several features are certain. First, neither the Brandberg main granite nor the monzonite are sufficiently magnetic to produce the observed anomaly. There must be a much more magnetic body under both the granite and monzonite, which we suggest is mantle-derived gabbro and crystal cumulates resulting from the fractionation which produced the granitoids. Second, the shape of the magnetic anomaly demands that the gabbroic intrusion be in the upper crust and that the margins of the intrusion be wider than the exposed massif and dip outward.

Keywords

actualismtheorymethodsinterpretation