This project seeks to illuminate the formation, assembly and subsequent evolution of the US mid-continent. Investigators at two universities and their graduate and undergraduate students will examine composition of the mid-continent crust and upper mantle. A novel combination of gravity and magnetic data with seismic velocity ratios vP/vS estimated from seismic receiver function data and velocity structure from surface wave and body wave tomography will be used to establish variations in temperature as well as thickness and bulk composition of multiple layers within the crust. The novel elements of these efforts include development of a new approach to receiver function imaging using parameter-domain cross-correlation and stacking, and coupling of the seismic images to the potential field data via likelihood filters and velocity-density relationships. Magnetic studies will focus also on understanding the physical properties and tectonic implications of magnetically detected boundaries, with particular emphases on a hidden middle Proterozoic geochemical and magnetic boundary dividing the cratonic core of North America and also on the Tennessee-Illinois-Kentucky lineament (or TIKL) and its unusual banded pattern of magnetization. The geophysical model results will be combined with sparsely sampled basement isotope geochemistry and age data to interpret the history of formation and accretion of lithospheric blocks as well as their subsequent tectonomagmatic modification.
Knowledge about physical properties of the crust and its history of tectonism and volcanism is central to a wide array of solid Earth science topics. Assessments of the likelihood of future earthquakes and the potential for economic mineral deposits at depth are just two examples of applications in which such knowledge plays a vital societal role. In order to evaluate better these risks and economic potential, a multi-disciplinary project involving professors and graduate and undergraduate students at two universities will seek to understand key structures, composition and processes that formed the geologic core of the US mid-continent. In mountainous regions of the western US, mapping variations in physical properties and inferring tectonic history are relatively straightforward because geophysical images of the deep crust and uppermost mantle can be corroborated by studies of rocks exposed and/or brought to the surface by geologic processes. In the mid-continent region of the US however, thick sequences of sedimentary rocks cover the ancient rocks produced by episodes of magmatism. Boreholes reaching below the sedimentary cover are few and far between and that has made it challenging to understand the geologic past that may control the locations of earthquakes and hidden mineral deposits. Consequently, tectonic and seismic hazard maps of this region rely more heavily on geophysical lineaments: linear features found in map-views of geophysical data sets signifying geologic boundaries and weak zones. This study seeks to combine several different types of geophysical data sets, including gravity and magnetic anomalies as well as images made possible by EarthScope?s array of seismometers. The previously unknown characteristics of the mid-continent crust derived from these data will be used to better characterize implications of the geologic boundaries for understanding comprehensively the history and properties of the Earth?s outer layers.
