Geophysical images and a crustal model of intrusive structures beneath the Messum ring complex, Namibia

摘要

The Early Cretaceous Messum complex in Namibia is an excellent example of the type of composite mafic and alkaline igneous ring complexes associated with continental rifts worldwide. This study integrates reflection seismic, gravity and aeromagnetic data from a profile across Messum and reveals a heavily intruded root zone extending vertically to Moho depths (40 km). The crustal section to either side of the Messum complex is characterized by a seismically transparent upper crust, to 20 km depth, and a laminar reflective lower crust below. The crust below the Messum complex is seismically transparent at all depths but it shows a combination of high seismic reflection strength and weak coherency relative to the unintruded crust, and P velocities about 0.2-0.3 km/s higher at all depths. The seismic attributes are best explained by a network of mafic intrusions, and this is supported by steep-sided positive magnetic and gravity anomalies centered on the complex. Modelling indicates that about 30% gabbroic material in the crustal column can explain the velocity and potential field data. An increase in reflective strength below 20 km suggests the degree of intrusion is greater in the lower crust but the difference is not resolved in the velocity or gravity models. The new geometric and compositional constraints on the crust below the Messum complex contradict the hypothesis that Messum was the sole eruptive center for an estimated 8600 km~3 of crustally derived rhyodacitic magmas that now form part of the regional Parana-Etendeka volcanic sequences. To our knowledge this is the first study to provide a seismic image of the whole crust below a typical igneous ring complex and our conclusion that the cross-sectional area of the complex is the same in the lower as in the upper crust could not have been predicted from surface studies. The data resolution is insufficient to define intrusion geometry in detail, but the cylindrical 3-D shape throughout the crust and the lack of major breaks in the vertical distribution of intrusions indicate depth-independent controls on the nature of the intruded zone. The suggestion is that the magma transport and emplacement took place in a brittle fashion at all levels of the crust, in response to conditions of high fluid (magma) pressure and a high magma flux.