GEOPHYSICAL IMAGES OF THE FORMATION AND DEFORMATION OF SOUTHERN LAURENTIA

The assembly of Southern Laurentia began in the Mesoproterozoic and culminated in the Grenville orogeny. The record of this assembly can be documented in the Southern Rocky Mountains both geologically and geophysically. However to the east, it must be inferred from geophysical and sparse drilling data. Geophysical data show that the subsequent intraplate deformation of the region occurred along structures that largely do not follow inferred Proterozoic structural trends. This is particularly true of the extensive Mid-Continent rift system that geophysical data show is associated with massive magmatic modification of the crust in the Lake Superior region. This is also true of the Cambrian rifting that broke Southern Laurentia apart and formed the structural framework of the sinuous Ouachita orogen. This Cambrian rifting formed a classic triple junction that created the Southern Oklahoma aulacogen, which also was associated with major magmatic modification of the upper crust. The intraplate deformation that was approximately synchronous with the late Paleozoic Ouachita orogeny caused extensive intraplate deformation (primarily the Ancestral Rocky Mountains) that is very well documented by geophysical and drilling data. Given that geophysical evidence presently available suggests that the Ouachita orogeny was due to a soft collision with an arc to the south, the origin of this deformation is still an open question. The nature, timing, and extent of the Grenville orogeny also poses many unanswered questions as does its relationship to the Mid-Continent rift. For example, prominent geophysical anomalies in Indiana, Ohio, Kentucky, and Tennessee have been interpreted variously as being due to extensions of the Mid-Continent rift system or the signature of the Grenville Front. With EarthScope’s USArray starting to leapfrog into the Southern Laurentia region, the new results that will emerge will shed important new light on the tectonic history of this region.