Crustal structure across a continental suture zone: a zone of focused crustal thickening, diffuse seismicity, and epiorogenic features in the mid continent of North America
Wednesday, 17 December 2014
Continents grow as crustal terranes of various ages and origins accreted together. Suture zones mark the boundaries of these terranes and provide a discrete marker to investigate the process of continental growth. Investigating continental structure, we can identify whether properties such as crustal thickness or seismic wavespeed changes across suture zones. Further comparison between the locations of sutures and the distribution of tectonic events provides insight into the degree that sutures serve to localize regions of tectonism. In this study we investigate the mid continent of North America, where crust of the Mazatzal and Granite Rhyolite terranes accreted together to make up the Proterozoic basement. Using seismic data from the EarthScope Transportable Array and OIINK FlexArray, we inspect receiver functions and Rayleigh wave phase velocities to investigate lithospheric structures. Through this analysis, we identify a zone where the crust thickens from an average near 40 km, to more than 50 km in an area that encompasses the Illinois-Missouri border and lies along the suture zone between these two Proterozoic terranes. However, the wavespeeds of the crust do not appear to vary between the terranes or in the zone of crustal thickening. The boundary between the Ozark Plateau and Illinois Basin is located within the area of thickened crust and may indicate a potential role played by the suture zone in the formation of these two epiorogenic features. The dense mafic intrusions that were added to the crust during extension of the failed Reelfoot Rift, and now exhibit high seismic wavespeeds, may have provided the additional load that allowed the Illinois Basin to exploit the already weakened suture zone and subside. Diffuse seismicity in the St. Genevieve Fault Zone, which is located to the north of the New Madrid Seismic Zone, lies near the western edge of the area of thickened crust and suggests that ancient suture zones influence modern deformation within continental interiors.