SYN-EXTENSIONAL TOPOGRAPHY, BASIN EVOLUTION AND MAGMATISM: THREE OVERLOOKED ASPECTS OF METAMORPHIC CORE COMPLEXES

Extensional detachment faulting has long been recognized as a fundamental mechanism in the process continental rifting leading to the separation of continents. Despite the fact that several numerical models have been generated to explain the formation of metamorphic core complexes, there are still three basic observations that are not collectively predicted in these models:

1. Often, the cores of metamorphic core complexes are the topographically most elevated locations in regional transects along to the extension direction. This is at odds with the fact that metamorphic core complexes represent the regions that have undergone infinite upper crustal extension and in theory they should be topographic lows. Lower crustal flow and flexural-isostatic bending can account for some but not for the full uplift and the observed topography.
2. In many cases in the North American Cordillera, and in other MCCs around the world, there is apparent extensional basin inversion (a term proposed by Elizabeth Miller). This is expressed as the uplift and exposure of the earliest syn-extensional strata and the basement unconformity as well as the development of time-transgressive regional erosional surfaces between the syn-extensional and the post-extensional “sag” deposits. These observations are not predicted by any stretching model where the basins subside through time at decreasing rate.
3. A global compilation of metamorphic core complexes indicates that a large proportion (>93%) of metamorphic core complexes have associated syn-extensional plutons in their lower plate and either epizonal plutons or volcanic rocks in the upper plate. The ratio of aerial extend of exposed lower plate rocks to that of syn-extensional plutons varies from <5% to >50%. These observations indicate that either metamorphic core complexes represent regions of focused magmatism or that large portions of the deep extended crust is made up of syn-extensional igneous rocks, or a combination of both.

As proposed before by Elizabeth Miller and her research group, I argue that magmatism is a fundamental process in the formation of metamorphic core complexes. The presence of magma and/or partial melts beneath the core complexes helps explain the heat-flow and density anomalies required for uplift and basin inversion during the evolution of the core complex.